IFAE 2006

Incontri di Fisica delle Alte EnergieItalian Meeting on High Energy Physics

G. Montagna · O. Nicrosini · V. Vercesi (Eds.)

IFAE 2006Incontri di Fisica delle Alte EnergieItalian Meeting on High Energy Physics

Pavia, 19–21 April 2006

123

Guido Montagna,Oreste Nicrosini,Valerio Vercesi

INFN – Sezione di Pavia andDipartimento di Fisica Nuclearee Teorica – Università di Pavia(Italy)

Library of Congress Control Number: 2006938540

ISBN 978-88-470-0529-7 Springer Berlin Heidelberg New York

This work is subject to copyright. All rights are reserved, whether thewholeor part of the material is concerned,specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproductionon microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof ispermitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version,and permission for use must always be obtained from Springer. Violations are liable to prosecution under theGerman Copyright Law.

Springer is a part of Springer Science+Business Media

springer.com

© Springer-Verlag Italia 2007

The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply,even in the absence of a specific statement, that such names are exempt from the relevant protective laws andregulations and therefore free for general use.

Cover concept: Simona Colombo, Milano

Typesetting: LE-TEX Jelonek, Schmidt, Vöckler GbR, Leipzig

Printing and binding: Grafiche Porpora, Segrate, Milano

Printed on acid-free paper 57/3141/NN - 5 4 3 2 1 0

Printed in Italy Springer-Verlag Italia Srl, Via Decembrio 28, I-20137 Milano

Preface

The 2006 edition of the IFAE (“Incontri di Fisica delle Alte Energie”)Workshop reviews the recent and most important advancements in High-Energy Physics and Astroparticle Physics, including reports as well on multi-disciplinary applications of detector developments and on Grid computing.The Workshop (http://www.pv.infn.it/ifae2006/) was held in Pavia fromApril 19th to 21st, in the beautiful medieval frame of the “San Tommaso”Congress Centre and saw the participation of more than 150 researchers.

Presentations, both theoretical and experimental, addressed the status ofStandard Model and Flavour physics, Neutrino and Cosmological topics, newinsights beyond the present understanding of fundamental particle physicsand cross-fertilization in areas such as medicine, biology, technological spin-offs and computing. Special emphasis was given to the expectations of theforthcoming Large Hadron Collider, due in operation next year, the status ofits experiments and the possibilities offered by this new energy frontier. Thevenue of plenary sessions interleaved with parallel ones allowed for a rich ex-change of ideas, presented in these Proceedings, that form a coherent pictureof the findings and of the open questions in our field.

We are happy to have had the opportunity of organizing such an event inPavia, and we are pleased of the enthusiastic response of the community.

We acknowledge the financial contributions of STMicroelectronics, IBM,CAEN, Banca Intermobiliare and the sponsorship of Comune di Pavia. Weare deeply indebted to INFN and University of Pavia, whose support hasbeen fundamental to accomplish the high standards of the meeting. We aregrateful to Diana A. Scannicchio and Carlo M. Carloni Calame for theirextensive and generous help. And we warmly thank all the speakers and par-ticipants, who have been the real engine behind the success of this Workshop.

Pavia, November 2006 The Editors

IFAE 2006: the participants

Contents

INVITED TALKS

Particle Physics: a Progress ReportGuido Altarelli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Getting Ready for Physics at the LHCFabiola Gianotti . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Lattice QCD and Numerical SimulationsRaffaele Tripiccione . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

The double life of the X mesonA.D. Polosa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Physics with Neutrino BeamsMauro Mezzetto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Status and perspectives of Dark Matterand Astroparticle searchesOliviero Cremonesi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Future Perspectives of High Energy Experimental Physicsand the Role of INFNUmberto Dosselli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

REVIEW TALKS OF PARALLEL SESSIONS

Status of the Standard ModelPatrizia Azzi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

New physicsAndrea Perrotta, Alessandro Strumia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

VIII Contents

Flavour PhysicsStefano Giagu and Luca Silvestrini . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Neutrinos and Cosmic Rays: Session SummaryEligio Lisi, Laura Patrizii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Detectors and New TechnologiesA. Cardini, M. Michelotto, V. Rosso . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

PARALLEL SESSION: Standard Model Physics(P. Azzi and F. Piccinini, conveners)

Theoretical progress in the muon g-2M. Passera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Beyond leading-log approximationin the Parton Shower approach to Bhabha processG. Balossini, C. M. Carloni Calame, G. Montagna, O. Nicrosini,F. Piccinini, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Status and prospects of the σe+e−→hadrons measurementFederico Nguyen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Recent Results from HERAAndrea Parenti . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Twistors and UnitarityPierpaolo Mastrolia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Resummations in QCD: recent developmentsAndrea Banfi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

Resummation of Drell–Yan rapidity distributionsPaolo Bolzoni . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Recent jet measurements at the TevatronSofia Vallecorsa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

Vector Boson Production Associatedwith Jets @ LHC (ATLAS)Monica Verducci on behalf of ATLAS Collaboration . . . . . . . . . . . . . . . . . 153

Recent developments on precise electroweak observablesSandro Uccirati . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

W and Z bosons physics at LHC at low luminositySara Bolognesi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

Contents IX

Electroweak correctionsto the charged-current Drell–Yan processC.M. Carloni Calame, G. Montagna, O. Nicrosini, A. Vicini . . . . . . . . . 167

Single Top at Hadron CollidersSimona Rolli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Top physics at the LHCAndrea Dotti . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

QCD corrections to Higgs physics at the LHCGiuseppe Bozzi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

Standard Model Higgs Boson Searchesat the Large Hadron ColliderStefano Rosati . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

PARALLEL SESSION: New Physics(A. Perrotta and A. Strumia, conveners)

Searching for extra-SUSY signals at LHCLorenzo Menici . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

Search for New Physics at the TevatronSimona Rolli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

New physics searches in B meson decaysS. Vecchi for the LHCb collaboration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

Fermion mass in E6 GUTwith discrete family permutation symmetry S3

Francesco Caravaglios, Stefano Morisi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

Search for Supersymmetry with early ATLAS dataT. Lari, for the ATLAS Collaboration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Detection methods for long lived particles at the LHCSara Viganò, Alberto De Min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

A holographic composite Higgs modelRoberto Contino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

New Physics in Top Events at the LHCMarina Cobal-Grassmann . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

Searching for micro black holes at LHCG.L. Alberghi, R. Casadio, D. Galli,D. Gregori, A. Tronconi, V. Vagnoni, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

X Contents

PARALLEL SESSION: Flavour Physics(S. Giagu and L. Silvestrini, conveners)

Lepton Flavor Violation and Rare Kaon DecaysParide Paradisi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

Neutral Kaon Physics at KLOEMarco Dreucci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

Charged kaons and Vus at KLOEKLOE collaboration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

Rare decays at the B-FactoriesConcetta Cartaro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

CP violation and CKM parameters determinationin BaBarNicola Neri . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

Semileptonic and nonleptonic decays of Bc

Mikhail A. Ivanov, Jürgen G. Körner, Pietro Santorelli . . . . . . . . . . . . . . 251

Aspects of non leptonic Bs decaysR. Ferrandes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

Measurement of the Bs oscillation frequency at CDFGiuseppe Salamanna on behalf of the CDF Collaboration . . . . . . . . . . . . . 261

Prospects for heavy flavor physics at LHCG. Passaleva . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

QED corrections and the B → Kπ puzzleElisabetta Baracchini . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

Status of the Unitarity Triangle analysisM. Bona, M. Ciuchini, E. Franco, V. Lubicz, G. Martinelli, F. Parodi,M. Pierini, P. Roudeau, C. Schiavi, L. Silvestrini, V. Sordini,A. Stocchi, V. Vagnoni . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

Non-perturbative inputs for Flavour PhysicsFederico Mescia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

Bounds on the supersymmetric flavour spaceValentina Porretti . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

Contents XI

PARALLEL SESSION: Neutrinos and Cosmic Rays(E. Lisi and L. Patrizii, conveners)

Neutrino oscillations with artificial sourcesMaximiliano Sioli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291

Future measurements of neutrinosfrom the Sun, Earth and SupernovaeAldo Ianni . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

Cosmology and Neutrinos, of fixed and variable massMarco Cirelli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

Double Beta Decay ExperimentsMaura Pavan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

Some Trends in Theoretical Models for Neutrino MassesMichele Frigerio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

Supernova neutrino burst as a probeof shock waves and matter density fluctuationsGian Luigi Fogli, Eligio Lisi, Alessandro Mirizzi, Daniele Montanino . . 313

Leptonic CP violationDavide Meloni . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

Neutrino astronomy with km3 underwater and under iceGiorgio Riccobene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

Ultra High Energy Cosmic Rays:Observations and Theoretical AspectsDaniel De Marco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

Recent Results in Gamma Ray Astronomy with IACTsVincenzo Vitale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

Gamma-ray Astronomy with full coverage experimentsPaola Salvini . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

PARALLEL SESSION: Detectors and New Technologies(A. Cardini, M. Michelotto and V. Rosso, conveners)

The Liquid Xenon calorimeter of the MEG experimentFabrizio Cei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

The Silicon Vertex Trigger Upgrade at CDFAlberto Annovi for the CDF Collaboration . . . . . . . . . . . . . . . . . . . . . . . . . . 345

XII Contents

Monolithic Active Pixel Sensorsin a 130 nm Triple Well CMOS TechnologyV. Re, C. Andreoli, M. Manghisoni, E. Pozzati, L. Ratti, V. Speziali,G. Traversi, S. Bettarini, G. Calderini, R. Cenci, F. Forti, M. Giorgi,F. Morsani, N. Neri, G. Rizzo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349

The external scanning proton microprobe in Florence:set-up and examples of applicationsLorenzo Giuntini . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353

Infrastructure of the ATLAS Event FilterAndrea Negri . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

The CMS High-Level TriggerPietro Govoni . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361

WLCG Service Challenges and Tiered architecturein the LHC eraDaniele Bonacorsi, Tiziana Ferrari (on behalf of INFN SC group) . . . . . 365

List of Participants

Abbiendi GiovanniINFN, Sezione di Bologna

Altarelli GuidoUniversità di Roma Tre

Ambroglini FilippoUniversità di Perugia

Andreazza AttilioUniversità di Milano

Annovi AlbertoLaboratori Nazionali di Frascati

Antinori FedericoINFN, Sezione di Padova

Antonelli VitoUniversità di Milano

Azzi PatriziaINFN, Sezione di Padova

Balossini GiovanniUniversità di Pavia

Banfi AndreaUniversità di Milano Bicocca

Baracchini ElisabettaUniversità di Roma “La Sapienza”

Barbieri RiccardoScuola Normale Superiore, Pisa

Bellomo MassimilianoINFN, Sezione di Pavia

Bocci AndreaScuola Normale Superiore, Pisa

Boffi SigfridoUniversità di Pavia

Bolognesi SaraUniversità di Torino

Bolzoni PaoloUniversità di Milano

Bonacorsi DanieleINFN-CNAF, Sezione di Bologna

Bozzi ConcezioINFN, Sezione di Ferrara

Bozzi GiuseppeLPSC, Grenoble

Brambilla MassimoUniversità di Pavia

Brigliadori LucaINFN, Sezione di Bologna

XIV List of Participants

Cadeddu SandroINFN, Sezione di Cagliari

Caffo MicheleINFN, Sezione di Bologna

Cardelli EdoardoINFN, Sezione di Cagliari

Cardini AlessandroINFN, Sezione di Cagliari

Carloni Calame Carlo MichelINFN, Sezione di Pavia

Cartaro ConcettaUniversità di Trieste

Casadio RobertoUniversità di Bologna

Cei FabrizioUniversità di Pisa

Cherubini RobertoLaboratori Nazionali di Legnaro

Chiavassa AndreaUniversità di Torino

Ciafaloni PaoloINFN, Sezione di Lecce

Cirelli MarcoYale University

Cobal MarinaUniversità di Udine

Conta ClaudioINFN, Sezione di Pavia

Contino RobertoINFN, Sezione di Roma 1

Cozzi MichelaINFN, Sezione di Bologna

Cremonesi OlivieroINFN, Sezione di Milano

Crosetti GiovanniUniversità della Calabria

De Marco DanielUniversity of Delaware

De Sanctis UmbertoUniversità di Milano

Devecchi FedericaUniversità di Pavia

Di Pompeo FrancescoLaboratori Nazionali del Gran Sasso

Dolce DonatelloINFN, Sezione di Firenze

Dosselli UmbertoINFN, Sezione di Padova

Dotti AndreaINFN, Sezione di Pisa

Dreucci MarcoLaboratori Nazionali di Frascati

Eulisse GiulioNortheastern University, Boston

Fabbri FabrizioINFN, Sezione di Bologna

Ferrandes RossellaUniversità di Bari

Ferrari RobertoINFN, Sezione di Pavia

Ferri FedericoINFN, Sezione di Milano

Ferroni FernandoINFN, Sezione di Roma 1

List of Participants XV

Franchino SilviaINFN, Sezione di Pavia

Frigerio MicheleSPhT, CEA/Saclay

Furcas SaraINFN, Sezione di Cagliari

Gambino PaoloINFN, Sezione di Torino

Gatti FlavioINFN, Sezione di Genova

Gaudio GabriellaINFN, Sezione di Pavia

Giagu StefanoUniversità di Roma “La Sapienza”

Gianotti FabiolaCERN, Ginevra

Giuntini LorenzoINFN, Sezione di Firenze

Goggi GiorgioUniversità di Pavia

Govoni PietroINFN, Sezione di Milano

Gresele AmbraUniversità di Trento

Ianni AldoLaboratori Nazionali del Gran Sasso

Introzzi GianlucaINFN, Sezione di Pavia

Lanza AgostinoINFN, Sezione di Pavia

Lari TommasoINFN, Sezione di Milano

Lisi EligioINFN, Sezione di Bari

Livan MicheleUniversità di Pavia

Lombardo Maria PaolaLaboratori Nazionali di Frascati

Maltoni FabioUniversité de Louvain

Maltoni MicheleICTP, Trieste

Marconi UmbertoINFN, Sezione di Bologna

Marzuoli AnnalisaUniversità di Pavia

Massarotti PaoloINFN, Sezione di Napoli

Mastrolia PierpaoloUniversity of Zurich

Meloni DavideINFN, Sezione di Roma 1

Menici LorenzoUniversità di Roma “La Sapienza”

Mescia FedericoLaboratori Nazionali di Frascati

Mezzetto MauroINFN, Sezione di Padova

Michelotto MicheleINFN, Sezione di Padova

Mila GiorgiaINFN, Sezione di Torino

Mirizzi AlessandroUniversità di Bari

XVI List of Participants

Montagna GuidoUniversità di Pavia

Montanari ClaudioINFN, Sezione di Pavia

Morello MichaelScuola Normale Superiore, Pisa

Morisi StefanoUniversità di Milano

Navarria Francesco L.Università di Bologna

Negri AndreaUniversity of California, Irvine

Neri NicolaINFN, Sezione di Pisa

Nervo MarcoUniversità di Torino

Nguyen FedericoUniversità di Roma Tre

Nicrosini OresteINFN, Sezione di Pavia

Paganoni MarcoUniversità di Milano Bicocca

Paradisi ParideUniversità di Roma Tor Vergata

Parenti AndreaUniversità di Padova

Passaleva GiovanniINFN, Sezione di Firenze

Passera MassimoINFN, Sezione di Padova

Patrizii LauraINFN, Sezione di Bologna

Pavan MauraUniversità di Milano Bicocca

Perrotta AndreaINFN, Sezione di Bologna

Piai MaurizioUniversity of Washington

Piazzoli AdalbertoUniversità di Pavia

Piccinini FulvioINFN, Sezione di Pavia

Piemonte ClaudioITC-IRST, Trento

Polesello GiacomoINFN, Sezione di Pavia

Polosa Antonio DavideINFN, Sezione di Roma 1

Porretti ValentinaUniversità di Roma Tre

Pullia AntoninoUniversità di Milano Bicocca

Rappoldi AndreaINFN, Sezione di Pavia

Raselli Gian LucaINFN, Sezione di Pavia

Ratti SergioUniversità di Pavia

Rebuzzi DanielaINFN, Sezione di Pavia

Re ValerioUniversità di Bergamo

Riccardi CristinaUniversità di Pavia

List of Participants XVII

Riccobene GiorgioLaboratori Nazionali del Sud

Rimoldi AdeleUniversità di Pavia

Rolli SimonaTufts University, Naperville

Roncadelli MarcoINFN, Sezione di Pavia

Rosati StefanoINFN, Sezione di Roma 1

Rossi SandroFondazione CNAO, Milano

Rosso ValeriaUniversità di Pisa

Salamanna GiuseppeUniversità di Roma “La Sapienza”

Salvatore DanielaUniversità della Calabria

Salvini PaolaINFN, Sezione di Pavia

Sannino FrancescoNBI, Copenhagen

Santorelli PietroUniversità di Napoli

Scannicchio DianaINFN, Sezione di Pavia

Scannicchio DomenicoUniversità di Pavia

Silvestrini LucaINFN, Sezione di Roma 1

Sioli MaximilianoUniversità di Bologna

Slavich PietroLAPTH, Annecy

Strumia AlessandroUniversità di Pisa

Tancini ValentinaUniversità di Milano Bicocca

Torre PaolaUniversità di Pavia

Trentadue LucaUniversità di Parma

Tripiccione RaffaeleUniversità di Ferrara

Ubiali MariaUniversità di Genova

Uccirati SandroINFN, Sezione di Torino

Ullio PieroSISSA, Trieste

Vagnoni VincenzoINFN, Sezione di Bologna

Vallecorsa SofiaUniversità di Ginevra

Vecchi StefaniaINFN, Sezione di Bologna

Vercesi ValerioINFN, Sezione di Pavia

Verducci MonicaCERN, Ginevra

Vicini AlessandroUniversità di Milano

Viganò SaraUniversità di Milano Bicocca

Vitale VincenzoUniversità di Udine

List of Not Received Contributions

The transparencies of the related talks are available at the web pagehttp://www.pv.infn.it/ifae2006/talks/

F. AntinoriStatus and perspectivesof research in ultrarelativisticnucleus-nucleus collisions

R. BarbieriFuture perspectives of elementaryparticles phenomenology

M.P. LombardoPhase transitionsin the Standard Model

P. CiafaloniElectroweak correctionsat the TeV scale

F. AmbrogliniMinimum Bias and UnderlyingEvent studies at the LHC

A. GreseleTop mass and cross sectionat the Tevatron

A. Boccib-tagging for the Higgs searchat the LHC

F. MaltoniMonteCarlo for new physics at LHC

P. SlavichSplit Susy and the LHC

P. UllioDark matter and LHC

F.R. NavarriaSearches for extra dimensionsfrom LEP to LHC to NLC

F. SanninoDynamical EW breaking: a classic

M. PiaiLittle(st) Higgs and LHC

P. GambinoInclusive radiative B decays:an update

M. MorelloRare and charmless decays at CDFII

F. GattiThe neutrino massfrom β-decay: Mβ

XX List of Not Received Contributions

M. MaltoniSynergies between acceleratorand atmospheric neutrinos searches

A. ChiavassaHigh and ultra-highenergy cosmic rays

S. RossiThe CNAO project

R. CherubiniHadron radiobiology and itsimplications in hadrotherapyand radioprotection

F. Di PompeoWArP: a double phase argondetector for direct searchof dark matter

S. CadedduMicroelectronics for time calibrationof the muon system in LHCb

C. PiemonteDevelopment of 3D silicon detectorsat ITC-irst

M. BriscoliniHigh-performance computingand data managementarchitectures in HEP

G. EulisseInteractive Web-based AnalysisClients using AJAX: examplesfor CMS, ROOT and GEANT4

INVITED TALKS

Particle Physics: a Progress Report

Guido Altarelli

Dipartimento di Fisica ‘E. Amaldi’, Università di Roma Tre and INFN, Sezione diRoma Tre, I-00146 Rome, Italy andCERN, Department of Physics, Theory Division, CH-1211 Geneva 23, Switzerlandguido.altarelli@cern.ch

1 Introduction

I would like to present a concise review of where we stand in particle physicstoday. First I will discuss QCD, then the electroweak sector and finally themotivations and the avenues for new physics beyond the Standard Model.

2 QCD

QCD stands as a main building block of the Standard Model (SM) of parti-cle physics. For many years the relativistic quantum field theory of referencewas QED, but at present QCD offers a much more complex and intriguingtheoretical laboratory. Indeed, due to asymptotic freedom, QCD can be con-sidered as a better defined theory than QED. The statement that QCD isan unbroken renormalisable gauge theory with six kinds of triplets quarkswith given masses completely specifies the form of the Lagrangian in termsof quark and gluon fields. From the compact form of its Lagrangian onemight be led to think that QCD is a “simple” theory. But actually this simpletheory has an extremely rich dynamical content, including the property ofconfinement, the complexity of the observed hadronic spectrum (with lightand heavy quarks), the spontaneous breaking of (approximate) chiral sym-metry, a complicated phase transition structure (deconfinement, chiral sym-metry restauration, colour superconductivity), a highly non trivial vacuumtopology (instantons, U(1)A symmetry breaking, strong CP violation, . . . ),the property of asymptotic freedom and so on.

How do we get predictions from QCD? There are non perturbative meth-ods: lattice simulations (in great continuous progress), effective lagrangiansvalid in restricted specified domains [chiral lagrangians, heavy quark effectivetheories, Soft Collinear Effective Theories (SCET), Non Relativistic QCD. . . ]and also QCD sum rules, potential models (for quarkonium) and so on. But

4 Guido Altarelli

the perturbative approach, based on asymptotic freedom and valid for hardprocesses, still remains the main quantitative connection to experiment.

Due to confinement no free coloured particles are observed but only coloursinglet hadrons. In high energy collisions the produced quarks and gluonsmaterialize as narrow jets of hadrons. Our understanding of the confinementmechanism has much improved thanks to lattice simulations of QCD at finitetemperatures and densities [1]. The potential between two colour chargesclearly shows a linear slope at large distances (linearly rising potential). Theslope decreases with increasing temperature until it vanishes at a criticaltemperature TC. Above TC the slope remains zero. The phase transitions ofcolour deconfinement and of chiral restauration appear to happen togetheron the lattice. Near the critical temperature for both deconfinement andchiral restauration a rapid transition is observed in lattice simulations. Inparticular the energy density ε(T ) is seen to sharply increase. The criticalparameters and the nature of the phase transition depend on the number ofquark flavours Nf and on their masses. For example, for Nf = 2 or 2 + 1(i. e. 2 light u and d quarks and 1 heavier s-quarks), TC ∼ 175 MeV andε(TC) ∼ 0.5−1.0 GeV/fm3. For realistic values of the masses ms and mu,d thephase transition appears to be a second order one, while it becomes first orderfor very small or very large mu,d,s. The hadronic phase and the deconfinedphase are separated by a crossover line at small densities and by a criticalline at high densities. Determining the exact location of the critical point inT and µB is an important challenge for theory which is also important forthe interpretation of heavy ion collision experiments. At high densities thecolour superconducting phase is probably also present with bosonic diquarksacting as Cooper pairs.

A large investment is being done in experiments of heavy ion collisionswith the aim of finding some evidence of the quark gluon plasma phase.Many exciting results have been found at the CERN SPS in the past yearsand more recently at RHIC. At the CERN SPS some experimental hintsof variation with the energy density were found in the form, for example,of J/Ψ production suppression or of strangeness enhancement when goingfrom p–A to Pb–Pb collisions. Indeed a posteriori the CERN SPS appearswell positioned in energy to probe the transition region, in that a markedvariation of different observables was observed. The most impressive effectdetected at RHIC, interpreted as due to the formation of a hot and densebubble of matter, is the observation of a strong suppression of back-to-backcorrelations in jets from central collisions in Au–Au, showing that the jet thatcrosses the bulk of the dense region is absorbed. The produced hot mattershows a high degree of collectivity, as shown by the observation of elliptic flow(produced hadrons show an elliptic distribution while it would be sphericalfor a gas) and resembles a perfect liquid with small or no viscosity. However,for quark gluon plasma, it is fair to say that the significance of each singlepiece of evidence can be questioned and one is still far from an experimentalconfirmation of a phase transition. The experimental programme on heavy

Particle Physics: a Progress Report 5

ion collisions will continue at RHIC and then at the LHC where ALICE,a dedicated heavy ion collision experiment, is in preparation.

As we have seen, a main approach to non perturbative problems in QCDis by simulations of the theory on the lattice, a technique started by K. Wil-son in 1974 which has shown continuous progress over the last decades. Onerecent big step, made possible by the availability of more powerful dedicatedcomputers, is the evolution from quenched (i. e. with no dynamical fermions)to unquenched calculations. In doing so an evident improvement in the agree-ment of predictions with the data is obtained. For example [2], modern un-quenched simulations reproduce the hadron spectrum quite well. Calculationswith dynamical fermions (which take into account the effects of virtual quarkloops) imply the evaluation of the quark determinant which is a difficult task.How difficult depends on the particular calculation method. There are sev-eral approaches (Wilson, twisted mass, Kogut-Susskind staggered, Ginsparg–Wilson fermions), each with its own advantages and disadvantages (includ-ing the time it takes to run the simulation on a computer). Another areaof progress is the implementation of chiral extrapolations: lattice simulationis limited to large enough masses of light quarks. To extrapolate the resultsdown to the physical pion mass one can take advantage of the chiral effectivetheory in order to control the chiral logs: log(mq/4πfπ). For lattice QCD oneis now in an epoch of pre-dictivity as opposed to the post-dictivity of thepast. And in fact the range of precise lattice results currently includes manydomains: the QCD coupling constant (the value αs(mZ) = 0.1170(12) hasbeen recently quoted [3]: the central value is in agreement with other deter-minations but one would not trust the stated error as the total uncertainty),the quark masses, the form factors for K and D decay, the B parameterfor kaons, the decay constants fK , fD, fDs, the Bc mass, the nucleon axialcharge gA (the lattice result [4] is close to the experimental value gA ∼ 1.25and well separated from the SU(6) value gA = 5/3) and many more.

Recently some surprising developments in hadron spectroscopy have at-tracted the general attention. Ordinary hadrons are baryons, B ∼ qqq andmesonsM ∼ qq̄. For a long time the search for exotic states was concentratedon glueballs, gg bound states, predicted at M >∼ 1.5 GeV by the lattice. Aswell known, experimentally glueballs were never clearly identified, probablybecause they are largely mixed with states made up of quark-antiquark pairs.Hybrid states (qq̄g or qqqg) have also escaped detection. Recently a numberof unexpected results have revamped the interest for hadron spectroscopy.Several experiments have reported new narrow states, with widths belowa few MeV(!!): Θ+(1540) with the quantum numbers of nK+ or pK0

S or,in terms of quarks, of uudds̄; D+

sJ (2317) ∼ Dsπ, D+sJ (2460) ∼ D∗

sπ, . . .and X0(3872) ∼ ππJ/Ψ . The interpretations proposed are in terms of pen-taquarks ([ud][ud]s̄ for Θ+ for example), tetraquarks ([qq][q̄q̄]) vs. meson-meson molecules for low lying scalar mesons or for X0 and also in termsof chiral solitons. Tetraquarks and pentaquarks are based on diquarks: [qq]of spin 0, antisymmetric in colour, 3̄ of SU(3)colour, and antisymmetric in

6 Guido Altarelli

flavour, 3̄ of SU(3)flavour. Tetraquarks were originally proposed for scalarmesons by Jaffe [5]. It is well known that there are two clusters of scalarmesons: one possible nonet at high mass, around 1.5 GeV, and a low lyingnonet below 1 GeV. The light nonet presents an inversion in the spectrum:the mesons that would contain s-quarks in the conventional qq̄ picture andwould hence be heavier are actually lighter. In the tetraquark interpretationthis becomes clear because the s-quarks with index “3” of the conventionalpicture is now replaced be the diquark [ud]. However, one can still formulatedoubts about the existence of so many scalar states [6]. The tetraquark in-terpretation for the doubly charmed X0(3872) has been proposed recently byMaiani et al. [7] as opposed to that in terms of a D–D∗ molecule by Braatenand Kusunoki [8]. Both models appear to face difficulties with the data. Forputative pentaquark states like the Θ+ doubts on their existence have muchincreased recently. Not only there are mass inconsistencies among differentexperiments, evident tension between a small width and large productionrates and the need of an exotic production mechanism to explain the lack ofevidence at larger energies. But the most disturbing fact is the absence of thesignal in some specific experiments where it is difficult to imagine a reasonfor not seeing it [9].

We now discuss perturbative QCD [10]. In the QCD Lagrangian quarkmasses are the only parameters with dimensions. Naively (or classically) onewould expect massless QCD to be scale invariant so that dimensionless ob-servables would not depend on the absolute energy scale but only on ratiosof energy variables. While massless QCD in the quantum version, after reg-ularisation and renormalisation, is finally not scale invariant, the theory isasymptotically free and all the departures from scaling are asymptoticallysmall, logarithmic and computable in terms of the running coupling αs(Q2).Mass corrections, present in the realistic case together with hadronisationeffects, are suppressed by powers. The QCD beta function that fixes the run-ning coupling is known in QCD up to 4 loops in the MS or MS definitionsand the expansion is well behaved. The 4-loop calculation by van Ritbergen,Vermaseren and Larin [11] involving about 50.000 4-loop diagrams is a greatpiece of work. The running coupling is a function of Q2/Λ2

QCD, where ΛQCD

is the scale that breaks scale invariance in massless QCD. Its value in MS,for 5 flavours of quarks, from the PDG’06 is ΛQCD ∼ 222(25)MeV. Thisfundamental constant of nature, which determines the masses of hadrons, isa subtle effect arising from defining the theory at the quantum level. Thereis no hierarchy problem in QCD, in that the logarithmic evolution of therunning makes the smallness of ΛQCD with respect to the Planck mass MPlnatural: ΛQCD ∼MPl exp [−1/2bαs(M2

Pl)].The measurements of αs(Q2) are among the main quantitative tests of the

theory. The most precise and reliable determinations are from e+e− colliders(mainly at LEP: inclusive hadronic Z decay, inclusive hadronic τ decay, eventshapes and jet rates) and from scaling violations in Deep Inelastic Scattering(DIS). Z decay widths are very clean: the perturbative expansion is known

Particle Physics: a Progress Report 7

to 3-loops, power corrections are controlled by the light-cone operator ex-pansion and are very suppressed due to mZ being very large. For measuringαs(Q2) [12] the basic quantity is Γh the Z hadronic partial width. It en-ters in Rl, σh, σl and ΓZ (the width ratio of hadrons to leptons, the hadroncross section at the peak, the charged lepton cross section at the peak andthe total width, respectively) which are separately measured with largelyindependent systematics. From combining all these measurements one ob-tains αs(m2

Z) = 0.1186(27) [13]. The error is predominantly theoretical andis dominated by our ignorance on mH and from higher orders in the QCDexpansion (the possible impact of new physics is very limited, given the re-sults of precision tests of the SM at LEP). The measurement of αs(mZ) fromτ decay is based on Rτ , the ratio of the hadronic to leptonic widths. Rτ

has a number of advantages that, at least in part, tend to compensate forthe smallness of mτ . First, Rτ is maximally inclusive, more than Re+e−(s),because one also integrates over all values of the invariant hadronic squaredmass. Analyticity is used to transform the integral into one on the circleat |s| = m2

τ . Also, a factor (1 − sm2

τ)2 that appears in the integral kills the

sensitivity of the region Re(s) = m2τ where the physical cut and the associ-

ated thresholds are located. Still the quoted result (PDG’06) looks a bit tooprecise: αs(m2

Z) = 0.120(3). This precision is obtained by taking for grantedthat corrections suppressed by 1/m2

τ are negligible. This is because, in themassless theory, no dim-2 Lorentz and gauge invariant operators exist thatcan appear in the light cone expansion. In the massive theory, the coefficientof 1/m2

τ does not vanish but is proportional to light quark mass-squared m2.This is still negligible if m is taken as a Lagrangian mass of a few MeV. Butwould not at all be negligible, actually would much increase the theoreticalerror, if it is taken as a constituent mass of order m ∼ ΛQCD. Most peoplebelieve the optimistic version. I am not convinced that the gap is not filledup by ambiguities of O(Λ2

QCD/m2τ ) e. g. from ultraviolet renormalons. In any

case, one can discuss the error, but it is true and remarkable, that the centralvalue from τ decay, obtained at very small Q2, when evolved at Q2 = m2

Z ,is in perfect agreement with all other precise determinations of αs(m2

Z) atmore typical LEP values of Q2. The measurements of αs from event shapesand jet rates are affected by non perturbative hadronic corrections which aredifficult to precisely assess. The combined result gives αs(m2

Z) = 0.120(5)(PDG’06). By measuring event shapes at different energies in the LEP1 andLEP2 ranges one also directly sees the running of αs.

In DIS QCD predicts the Q2 dependence of a generic structure functionF (x,Q2) at each fixed x, not the x shape. But the Q2 dependence is relatedto the x shape by the QCD evolution equations. For each x-bin the dataallow to extract the slope of an approximately straight line, the log slope:d logF (x,Q2)/d logQ2. For most x values the Q2 span and the precision ofthe data are not much sensitive to the curvature. A single value of ΛQCD mustbe fitted to reproduce the collection of the log slopes. The QCD theory of scal-ing violations, based on the renormalization group and the light-cone operator

8 Guido Altarelli

expansion, is crystal clear. Recently (’04) the formidable task of computingthe splitting functions at NNLO accuracy has been completed by Moch, Ver-maseren and Vogt, a really monumental, fully analytic calculation [14]. Forthe determination of αs the scaling violations of non-singlet structure func-tions would be ideal, because of the minimal impact of the choice of inputparton densities. Unfortunately the data on non-singlet structure functionsare not very accurate. For example, NNLO determinations of αs from theCCFR data on F3νN with different techniques have led to the central val-ues αs(m2

Z) = 0.1153 [15]), αs(m2Z) = 0.1174 [16], αs(m2

Z) = 0.1190 [17],with average and common estimated error of αs(m2

Z) = 0.117(6) which I willuse later. When one measures αs from scaling violations on F2 from e orµ beams, the data are abundant, the errors small but there is an increaseddependence on input parton densities and especially a strong correlation be-tween the result on αs and the input on the gluon density. There are severalmost complete and accurate derivations of αs from scaling violations in F2

with different, sophisticated methods (Mellin moments, Bernstein moments,truncated moments. . . ). We quote here the result at NNLO accuracy fromMRST’04 (see PDG’06): αs(m2

Z) = 0.1167(40).More measurements of αs could be listed: I just reproduced those which

I think are most significant and reliable. There is a remarkable agreementamong the different determinations. If I directly average the five values listedabove from inclusive Z decay, from Rτ , from event shapes and jet rates ine+e−, from F3 and from F2 in DIS I obtain αs(m2

Z) = 0.1187(16) in goodagreement with the PDG’06 average αs(m2

Z) = 0.1176(20).The importance of DIS for QCD goes well beyond the measurement of αs.

In the past it played a crucial role in establishing the reality of quarks andgluons as partons and in promoting QCD as the theory of strong interactions.Nowadays it still generates challenges to QCD as, for example, in the domainof structure functions at small x or of polarized structure functions or ofgeneralized parton densities and so on.

The problem of constructing a convergent procedure to include the BFKLcorrections at small x in the singlet splitting functions, in agreement with thesmall-x behaviour observed at HERA, has been a long standing puzzle whichhas now been essentially solved. The naive BFKL rise of splitting functionsis tamed by resummation of collinear singularities and by running couplingeffects. The resummed expansion is well behaved and the result is close tothe perturbative NLO splitting function in the region of HERA data at smallx [18, 19].

In polarized DIS one main question is how the proton helicity is dis-tributed among quarks, gluons and orbital angular momentum: 1/2∆Σ +∆g+Lz = 1/2 [20]. The quark moment ∆Σ was found to be small: typically,at Q2 ∼ 1 GeV2, ∆Σexp ∼ 0.2 (the “spin crisis”). Either ∆g + Lz is largeor there are contributions to ∆Σ at very small x outside of the measuredregion. ∆g evolves like ∆g ∼ logQ2, so that eventually should become large(while ∆Σ and ∆g + Lz are Q2 independent in LO). It will take long before

Particle Physics: a Progress Report 9

this log growth of ∆g will be confirmed by experiment! ∆g can be measuredindirectly by scaling violations and directly from asymmetries, e. g. in cc̄ pro-duction. Existing direct measurements by Hermes, Compass, and at RHICare still very crude and show no hint of a large ∆g. The perspectives of bettermeasurements are good at Compass and RHIC in the near future.

Another important role of DIS is to provide information on parton densityfunctions (PDF) which are instrumental for computing cross-sections of hardprocesses at hadron colliders via the factorisation formula. The predictionsfor cross sections and distributions at pp or pp̄ colliders for large pT jets orphotons, for heavy quark production, for Drell–Yan,W and Z production areall in very good agreement with experiment. There was an apparent problemfor b quark production at the Tevatron, but the problem appears now to besolved by a combination of refinements (log resummation, B hadrons insteadof b quarks, better fragmentation functions. . . ) [21]. The QCD predictionsare so solid that W and Z production are actually considered as possibleluminosity monitors for the LHC.

A great effort is being devoted to the preparation to the LHC. Calculationsfor specific processes are being completed. A very important example is Higgsproduction via g+g→ H . The amplitude is dominated by the top quark loop.Higher order corrections can be computed either in the effective lagrangianapproach, where the heavy top is integrated away and the loop is shrunk downto a point [the coefficient of the effective vertex is known to α4

s accuracy [22]],or in the full theory. At the NLO [23] the two approaches agree very well forthe rate as a function of mH . Rapidity and pT distributions have also beenevaluated at NLO [23]. The [log(pT/mH)]n have been resummed in analogywith what was done long ago for W and Z production. Recently the NNLOanalytic calculation for the rate has been completed in the effective lagrangianformalism [23, 24].

The activity on event simulation also received a big boost from the LHCpreparation. General algorithms for performing NLO calculations numerically(requiring techniques for the cancellation of singularities between real andvirtual diagrams), for example the dipole formalism by Catani, Seymour etal. [25], have been developed. The matching of matrix element calculationof rates together with the modeling of parton showers has been realised inpackages, as for example in the MC@NLO based on HERWIG. The matrixelement calculation, improved by resummation of large logs, provides the hardskeleton (with large pT branchings) while the parton shower is constructedby a sequence of factorized collinear emissions fixed by the QCD splittingfunctions. In addition, at low scales a model of hadronisation completes thesimulation. The importance of all the components, matrix element, partonshower and hadronisation can be appreciated in simulations of hard eventscompared with the Tevatron data.

Before closing I would like to mention some very interesting developmentsat the interface between string theory and QCD, twistor calculus. A precur-sor work was the Parke-Taylor result in 1986 [27] on the amplitudes for n

10 Guido Altarelli

gluons (all taken as incoming) with given helicities. Inspired by dual models,they derived a compact formula for the maximum non vanishing helicity vi-olating amplitude (with n− 2 plus and 2 minus helicities) in terms of spinorproducts. Using the relation between strings and gauge theories in twistorspace Witten [28] developed in ’03 a formalism in terms of effective verticesand propagators that allows to compute all helicity amplitudes. The method,much faster than Feynman diagrams, leads to very compact results. Sincethen rapid progress followed [23]: for tree level processes powerful recurrencerelations were established (Britto, Cachazo, Feng; Witten), the method wasextended to include massless fermions (Georgiu, Khoze) and also externalEW vector bosons (Bern et al.) and Higgs particles (Dixon, Glover, Khoze,Badger et al.). The level already attained is already important for multijetevents at the LHC. And the study of loop diagrams has been started. Insummary, this road looks very promising.

A different string connection is the attempt at obtaining results on QCDfrom the AdS correspondence, pioneered by Maldacena [29]. The startingpoint is the holographic correspondence between D = 10 string theory andthe N = 4 SUSY Yang-Mills in four dimensions at large Nc. From there toget to real life QCD the way looks impervious, but a number of results foractual processes have been advocated and the perspective is exciting [30].

In conclusion, I think that the domain of QCD appears as one of greatmaturity but also of robust vitality with many rich branches and plenty ofnew blossoms. The physics content of QCD is very large and our knowledge,especially in the non perturbative domain, is still very limited but progressboth from experiment (LEP, HERA, Tevatron, RHIC, LHC. . . ) and from the-ory is continuing at a healthy rate. And all the QCD predictions that we wereable to formulate and to test are in very good agreement with experiment.

3 The Physics of Flavour

In the last decade great progress in different areas of flavour physics has beenachieved. In the quark sector, the amazing results of a generation of frontierexperiments, obtained at B factories and at accelerators, have become avail-able. QCD has been playing a crucial role in the interpretation of experimentsby a combination of effective theory methods (heavy quark effective theory,NRQCD, SCET), lattice simulations and perturbative calculations. The hopeof these experiments was to detect departures from the CKM picture of mix-ing and CP violation as signals of new physics. Finally, B mixing and CPviolation agree very well with the SM predictions based on the CKM ma-trix [31]. The recent measurement of ∆ms by CDF and D0, in fair agreementwith the SM expectation, has closed another door for new physics. It is onlyin channels that are forbidden at tree level and occur through penguin loops(as is the case for B → πK modes) that some deviation could be hidden.The amazing performance of the SM in flavour changing transitions and for

Particle Physics: a Progress Report 11

CP violation in K and B decays poses a strong constraint on all proposedmodels of new physics.

In the leptonic sector the study of neutrino oscillations has led to the dis-covery that at least two neutrinos are not massless and to the determinationof the mixing matrix [32]. Neutrinos are not all massless but their massesare very small. Probably masses are small because νs are Majorana particles,and, by the see-saw mechanism, their masses are inversely proportional tothe large scale M where lepton number (L) violation occurs (as expectedin GUT’s). Indeed the value of M ∼ mνR from experiment is compatiblewith being close to MGUT ∼ 1014 − 1015 GeV, so that neutrino masses fitwell in the GUT picture and actually support it. It was realized that decaysof heavy νR with CP and L violation can produce a B–L asymmetry. Therange of neutrino masses indicated by neutrino phenomenology turns out tobe perfectly compatible with the idea of baryogenesis via leptogenesis [33].This elegant model for baryogenesis has by now replaced the idea of baryo-genesis near the weak scale, which has been strongly disfavoured by LEP. Itis remarkable that we now know the neutrino mixing matrix with good ac-curacy. Two mixing angles are large and one is small. The atmospheric angleθ23 is large, actually compatible with maximal but not necessarily so: at 3σ:0.31 ≤ sin2 θ23 ≤ 0.72 with central value around 0.5. The solar angle θ12 islarge, sin2 θ12 ∼ 0.3, but certainly not maximal (by more than 5σ). The thirdangle θ13, strongly limited mainly by the CHOOZ experiment, has at presenta 3σ upper limit given by about sin2 θ13 ≤ 0.08. While these discoveries aretruly remarkable, it is somewhat depressing that the detailed knowledge ofboth the quark and the neutrino mixings has not led so far to a compellingsolution of the dynamics of fermion masses and mixings: our models can re-produce, actually along different ways, the observed values, but we do notreally understand their mysterious pattern.

4 Precision Tests of the Standard Electroweak Theory

The results of the electroweak precision tests as well as of the searches forthe Higgs boson and for new particles performed at LEP and SLC are nowavailable in final form. Taken together with the measurements of mt, mW

and the searches for new physics at the Tevatron, and with some other datafrom low energy experiments, they form a very stringent set of precise con-straints [13] to compare with the Standard Model (SM) or with any of itsconceivable extensions. When confronted with these results, on the whole theSM performs rather well, so that it is fair to say that no clear indication fornew physics emerges from the data [34]. The main lesson of precision testsof the standard electroweak theory can be summarised as follows. The cou-plings of quark and leptons to the weak gauge bosons W± and Z are indeedprecisely those prescribed by the gauge symmetry. The accuracy of a fewper-mille for these tests implies that, not only the tree level, but also the

12 Guido Altarelli

structure of quantum corrections has been verified. To a lesser accuracy thetriple gauge vertices γWW and ZWW have also been found in agreementwith the specific prediction of the SU(2)

⊗U(1) gauge theory. This means

that it has been verified that the gauge symmetry is unbroken in the verticesof the theory: all currents and charges are indeed symmetric. Yet there isobvious evidence that the symmetry is otherwise badly broken in the masses.This is a clear signal of spontaneous symmetry breaking. The practical im-plementation of spontaneous symmetry breaking in a gauge theory is via theHiggs mechanism. The Higgs sector of the SM is still very much untested.What has been tested is the relation M2

W =M2Z cos2 θW , modified by small,

computable radiative corrections. This relation means that the effective Higgs(be it fundamental or composite) is indeed a weak isospin doublet. The Higgsparticle has not been found but in the SM its mass can well be larger thanthe present direct lower limitmH

>∼ 114GeV obtained from direct searches atLEP-2. The radiative corrections computed in the SM when compared to thedata on precision electroweak tests lead to a clear indication for a light Higgs,not too far from the present lower bound. The exact upper limit for mH inthe SM depends on the value of the top quark mass mt (the one-loop radia-tive corrections are quadratic in mt and logarithmic in mH). The measuredvalue of mt went down recently (as well as the associated error) accordingto the results of Run II at the Tevatron. The CDF and D0 combined valueis at present [35] mt = 171.4 ± 2.1 GeV (it went slightly down with respectto the value from Run I). As a consequence the present limit on mH is morestringent [36]: mH < 199 GeV (at 95% c. l., after including the informationfrom the 114 GeV direct bound).

5 Outlook on Avenues beyond the Standard Model

No signal of new physics has been found neither in electroweak precision testsnor in flavour physics. Given the success of the SM why are we not satisfiedwith that theory? Why not just find the Higgs particle, for completeness, anddeclare that particle physics is closed? The reason is that there are both con-ceptual problems and phenomenological indications for physics beyond theSM. On the conceptual side the most obvious problems are that quantumgravity is not included in the SM and the related hierarchy problem. Amongthe main phenomenological hints for new physics we can list coupling uni-fication, dark matter, neutrino masses, baryogenesis and the cosmologicalvacuum energy.

The computed evolution with energy of the effective SM gauge couplingsclearly points towards the unification of the electro-weak and strong forces(Grand Unified Theories: GUT’s) at scales of energyMGUT ∼ 1015−1016 GeVwhich are close to the scale of quantum gravity,MPl ∼ 1019 GeV. One is led toimagine a unified theory of all interactions also including gravity (at presentsuperstrings provide the best attempt at such a theory). Thus GUT’s and

Particle Physics: a Progress Report 13

the realm of quantum gravity set a very distant energy horizon that modernparticle theory cannot ignore. Can the SM without new physics be valid upto such large energies? One can imagine that some obvious problems could bepostponed to the more fundamental theory at the Planck mass. For example,the explanation of the three generations of fermions and the understandingof fermion masses and mixing angles can be postponed. But other problemsmust find their solution in the low energy theory. In particular, the structureof the SM could not naturally explain the relative smallness of the weakscale of mass, set by the Higgs mechanism at µ ∼ 1/

√GF ∼ 250GeV with

GF being the Fermi coupling constant. This so-called hierarchy problem isdue to the instability of the SM with respect to quantum corrections. Thisis related to the presence of fundamental scalar fields in the theory withquadratic mass divergences and no protective extra symmetry at µ = 0.For fermion masses, first, the divergences are logarithmic and, second, theyare forbidden by the SU(2)

⊗U(1) gauge symmetry plus the fact that at

m = 0 an additional symmetry, i. e. chiral symmetry, is restored. Here, whentalking of divergences, we are not worried of actual infinities. The theory isrenormalisable and finite once the dependence on the cut off Λ is absorbed ina redefinition of masses and couplings. Rather the hierarchy problem is one ofnaturalness. We can look at the cut off as a parameterization of our ignoranceon the new physics that will modify the theory at large energy scales. Thenit is relevant to look at the dependence of physical quantities on the cut offand to demand that no unexplained enormously accurate cancellations arise.

The hierarchy problem can be put in very practical terms: loop correctionsto the higgs mass squared are quadratic in Λ. The most pressing problem isfrom the top loop. With m2

h = m2bare + δm2

h the top loop gives

δm2h|top ∼ − 3GF

2√

2π2m2

tΛ2 ∼ −(0.2Λ)2 . (1)

If we demand that the correction does not exceed the light Higgs massindicated by the precision tests, Λ must be close, Λ ∼ o (1 TeV). Similarconstraints arise from the quadratic Λ dependence of loops with gauge bosonsand scalars, which, however, lead to less pressing bounds. So the hierarchyproblem demands new physics to be very close (in particular the mechanismthat quenches the top loop). Actually, this new physics must be rather special,because it must be very close, yet its effects are not clearly visible (the “LEPParadox” [37]). Examples of proposed classes of solutions for the hierarchyproblem are:Supersymmetry. In the limit of exact boson-fermion symmetry the quadraticdivergences of bosons cancel so that only log divergences remain. However,exact SUSY is clearly unrealistic. For approximate SUSY (with soft break-ing terms), which is the basis for all practical models, Λ is replaced by thesplitting of SUSY multiplets, Λ ∼ mSUSY −mord. In particular, the top loopis quenched by partial cancellation with s-top exchange, so the s-top cannotbe too heavy.