XLVITH Rencontres de MoriondQCD and High Energy Interactions
Theoretical Summary
Lance Dixon (CERN & SLAC)
La Thuile, 26 March 2011
Lots to Review!
• We heard an amazing number of excellent talks on different theoretical ideas and approaches
• And, especially with the advent of the LHC, a flood of new data from the energy frontier, which heavily impacts many theories of new physics at the electroweak scale
• Can’t possibly cover everything• Apologies in advance if I missed your favorite topic or
talk, or if I distorted it
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 2
Some General Themes
• Searching for new physics – new terms in the Lagrangian of Nature – in many different channels
• Searching for, or further exploring, new phenomena in QCD, e.g. QGP in heavy ion collisions, or diffraction in new kinematical regimes
• Improving the precision of theoretical predictions in systematic approximations like pQCD, SCET, other resummations
• Establishing new frameworks/models/pictures where no fully systematic approximation exists
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 3
New Physics the big picture
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H. Murayama
S. Todorova
But still lots to do in understanding “old physics”
Where is Higgs?
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 5
• Been looking for him for decades• Know everything but Higgs mass• Search really heating up now, bothfrom last stages of Tevatron operation,first stages of LHC
Precisionelectroweak
LEP2directsearch Look for now mostly in WW, ZZ,
and bb (with associated W or Z)
M. Cooke
SM Higgs at Tevatron
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 6
• Expect 10-12 fb-1 total by September shutdown.• Could “fail to exclude” at 2s • Hard to get to 3 s evidence in SM unless analysisimprovements are greater than projected
M. Cooke
SM Higgs at Tevatron (cont.)
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• Low mass region more difficult, at ~ 1.5-2 x SM now, with 5.9 fb-1, even with many channels, improved acceptances, hadronic t’s, MVAs, …
M. Cookenot yet updated for new channels
• High mass region also difficult, many channels, but most sensitive is
• Now CDF/D0 each can exclude a range of masses
gg H WW lnln
R. Lysak
SM Higgs at Tevatron (cont.)
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New high mass combination of CDF and D0
J. Hays
But Wait!
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• Theoretical uncertainties leave (high mass)Higgs in a “half dead, half alive state” - A. Djouadi
Which uncertainties?
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Biggest ones:1) gg H has large higher order corrections
A. Djouadi
2) gg H strongly dependent on g(x) for x~0.1. Different pdf sets give quitedifferent cross sections: - MSTW, CTEQ, NNPDF have Tevatron jet data, g(x) larger- ABKM09, HERAPDF do not, g(x) smaller.
However, partial N3LO results and NNLL resummations suggest (to me) that final cross section will not dip to lowest value in k=3 range.
But Wait!
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• Brand new NNLO fit to HERA data: HERAPDF1.5• Gives large x gluon much closer to MSTW!
V. Radescu
• NNLO Higgs cross sectionnot yet shown, but clearly itwill rise a lot closer to thatusing MSTW.
High Mass
How to improve sH uncertainty?
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• Very large logs due to soft-gluons. To go beyondNNLO, can resum “eikonal logs”: NNLL Catani,de Florian, Grazzini, Nason
also Ahrens et al., arXiv:1008.3162
C. White
H
• Maybe go to next-to-eikonal approximation before long.
• Would still want at least one N3LO hard term: a3 d(x)
Improve uncertainties from cuts
• Effect of jet vetos. Need to resum large logs here, ~ ln(mH/qT), from soft and collinear ISR. Done at Tevatron (as I understand it) by reweighting MC to an NNLL resummation of Higgs qT spectrum
(HQT, Bozzi et al.; and/or RESBOS).• SCET recently applied to resum another variable, “beam
thrust”. Reweight to this, check consistency… Berger et al.
• Subtle “collinear anomaly” in SCET applied to
Drell-Yan qT resummation (where there is a ton of data).
Stay tuned for
final predictions.Moriond QCD 26 March 2011L. Dixon Theoretical Summary 13
T. Becher
Where is SUSY?
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• Been looking for her for decades• Know nothing (theoretically) about any masses (no upperbounds, anyway, except for lightest Higgs)• Search really heating up now, from first stages of LHC,but also Tevatron, progress in flavor physics too
?
“Generic” SUSY Cascade Searches at LHC
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• Already surpassed Tevatron in many cases!
• Will discover or really constrain overall mass scale over next two years
N. Barlow
M. Chiorboli
Many more specialized SUSY Searches
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• Just one example: top squark might be much lighter than• other quarks, because of running effects, large tL-tR splitting):
Tevatron, Ph. Gris
~ ~
SUSY Higgs
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• Relatively simple sector: only depends on ~ masses of heavy
Higgses, H, A, H±
• Light Higgs h has mass < MZ cos b + radiative corrections• LEP2 lower bound want tan b somewhat large• Unification suggests very large tan ~ b mt/mb ~ 40• If MA small, coupling of h to b,s,d, ,t m,e all enhanced by
tanb
• However, if MA large – decoupling limit – effect shuts off
~ tanb
Direct SUSY Higgs Search
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Cross section tan2β enhanced Tevatron, St. Denis
(roughly, due to SUSY radiative corrections!)
watchthis space?
SUSY in Flavor
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• Muon g-2 anomaly likes large tanβ• Rare B decays can also get huge
enhancements• However, both also depend on masses, flavor mixings of other superpartners Bs,d m+m-
M.-O. Bettler, R. Fleischer
Bs m+m- (cont.)
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• Exciting because LHCb has already reached ~ Tevatron sensitivity:
M.-O. Bettler
• Will cover much of remaining distance to SM in next year or two
Theory input for Bs m+m-
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R. Fleischer
Predict using
“U spin” s d
U spin for CP Asymmetries too
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R. Fleischer
Enough information
to solve for g
At LHCb
Other New Physics Theories
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• 4th generation• New W’s or Z’s• Vector-like quarks• Axial-like gluons• Universal (Flat) Extra Dimensions (UED)• Warped Extra Dimensions (RS)• String Excitations• Black Holes (only the safe kind?)• Many searched for in di-lepton, di-photon, or di-jets Particle masses excluded up to around 1 TeV• Other searches often harder, more model specific
TaylorStrogolasHarel ThompsonChoBrandt
Strings at the LHC?
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T. Taylor
Possibly, if there are also large extra dimensions, and stringmass scale M is also very low.
gg g* ggqg q* qg …
BIG di-jet bump from
So big that
pQCD for colliders
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New physics Higgs physics
LHC
@ 7
TeV • Every process shown
also occurs with one more jetat ~ 1/5 the rate• Very important to understand
not only Standard Model
production of X but also of
X + n jetswhere
X = W, Z, tt, WW, H, … n = 1,2,3,…
_
If you don’t know exactly what to look for, need to understand all major SM processes.
pQCD for hadron colliders
• Great advancement of last 5 years:• Monte Carlo programs that match leading order QCD exact matrix
elements to parton showers (LO ME+PS)
– ALPGEN, MadGraph, SHERPA. • Describe event “shapes” pretty well• But, we need to do better, especially with normalizations.• For this we need NLO QCD.• Even better would be NLO QCD matched to a parton shower
(NLOPS).• Even better than that, merging samples with different numbers of
jets, as in LO ME+PS but at NLO MENLOPS
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(Approximate) MENLOPS• Merging samples with different numbers of jets at NLO = MENLOPS• Can get very close with a recipe combining available NLOPS
(POWHEG) and LO ME+PS tools, at least for Drell-Yan (W,Z) and also top-quark pair production K. Hamilton
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New NLO methods and results
• Unitarity-based method applied to one-loop 6-point amplitudes
entering pp W+W+ + 2 jets T. Melia
• Same sign dilepton background• Preparation for full pp WW + 2 jets,
QCD background to WW electroweak scattering• Unitarity recycles trees into loops
• Greatly improved theory uncertainty
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New NLO methods (cont.)
• All NLO calculations need one-loop amplitudes, but they also need to integrate tree cross sections with one extra parton over a singular phase space region.
• Catani-Seymour dipole subtraction method for dsA is standard, but requires lots of different terms – may be computationally limiting step.
• Method proposed by Nagy-Soper recently implemented. T. Robens• Promising for more complex processes if integrals of subtraction
terms dsA can be tabulated numerically.
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Approximate (N)NLO method• LoopSim – method to approximate higher order corrections
– like an approximate Sudakov factor S. Sapeta• Works best when leading-order (LO) QCD is anomalously small –
i.e. when there are “giant K factors”, as in Z + 1 jet production at LHC
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Top is here
Based on
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M. Cristinziani
O. Brandt
Parameter in PYTHIA
• Yukawa coupling is parameter in Lagrangian (related to MS mass)• Given this exceptional precision,theorists need to do a better jobof relating the two masses (via pole mass, presumably)
___
Z. Ye
Z. Ye
Helps, but now onlysensitive at 5 GeV level
Top mostly looks normal so far
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 32
S. Amerio
M. CristinzianiJ. Gruschke
Except for maybe AFB
• In SM,
AFB = 0 at O(as2). MC is O(as
3), first order in which AFB ≠ 0
• Would like O(as4), to be sure no large corrections.
• People working on it, in conjunction with s(t t) at NNLO.• What about measuring AFB with an extra jet?
• Known at second order already [Dittmaier, Uwer, Weinzierl]Moriond QCD 26 March 2011L. Dixon Theoretical Summary 33
_
O. Brandt
New physics for AFB?
• s-channel heavy color-octet,
Lorentz vector state• Candidates: top-color, technicolor,
universal and warped extra dimensions, chiral color,
axi-gluons, … dijet and Mtt bumps
• t-channel: flavor-changing interactionssame sign tops, especially at LHC
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Bai, Hewett, Kaplan, Rizzo, 1101.5203
QCD in extreme or nonperturbative environments
• Self-consistent nonperturbative solution to coupled gluon-quark quasi-classical equations: currents generated by fermions and gauge field exactly compensate each other
• Proposed application to heavy ion collisions as alternative to Glasma model [McLerran ,Venugopalan]
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A. Koshelkin
A. Tang
• Study jet quenching via (quasi)perturbative approach,
Bethe-Heitler gluon radiation + collisional losses
off scattering centers in quark-gluon plasma, • Technically challenging calculation.• At the end, very sensitive to non-perturbative
distance scales r ~ 1/Q, reflected in dependence on
as freezing point used
Extreme QCD (cont.)
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B. Zakharov
• Another approach to understanding jet quenching,
also the bulk properties of quark-gluon plasma.• AdS/CFT maps a strongly-coupled 3+1 (conformal) gauge theory
“holographically” to a semi-classical 4+1 gravitational theory in Anti-de Sitter space
AdS/CFT
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Finite temperature black hole present(Hawking radiation)
• Not really QCD, but maybe it is “close enough” at strong coupling
AdS/CFT & bulk flow
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• Very strong coupling very small viscosity: h/s = 1/4p Kovtun, Starinets, Son
• Using this value in hydro codes gives second moment of azimuthal
particle correlations, v2, consistent with observations first at RHIC,
and now at LHC
M. Nicassio , ALICE
• Comparison with RHIC results 30 % higher
Hydrodynamic model predictions with viscous corrections in agreement with observed increase
|h|<0.8 0.2<pt<5GeV/c
Chen
• Third moment, v3, recently measured at RHIC, and in perfect agreement too! Chen
AdS/CFT & jet quenching
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 39
• Energy loss in strongly-coupled plasma is large – so largeit can’t be reliably computed for light quark – need heavy one • But that’s harder to measure, hasn’t been done (quite) yet,though open charm has now been found in heavy ions
parton
hot and dense mediumA. Mischke
g + c as energy loss probe
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 40
F. Arleo
Study, e.g.
Preliminary computation using a perturbative energy loss framework
b
c
• Can use a different model to put in quarks Sugimoto
AdS/CFT for light hadron spectrum
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Closed strings
Open strings on D8
D4 wrapped onD4-brane
D8-brane
Nc strings are attached
glueballs
mesons
baryons
Meson & baryonmasses, form factors, etc., agree surprisinglywell with data,with only 2 freeparameters
Nonperturbative QCD in B Physics
• Into the brown muck
• For mQ >> LQCD, a heavy quark is almost
static within its meson, and its spin decouples
• Nowadays, want to go to high order in LQCD/ mQ
• Have to classify all local operators at a given order
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b
q
T. Mannel
Nonperturbative QCD in B’s (cont.)
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T. Mannel
Relates matrix elements to better known matrix elements of lower dimension operators.• Apply method to inclusive B c l n decay width G
inclusive
Also reviewed new lattice value
Nonperturbative QCD in B’s (cont.)
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 44
G. Paz
• CP asymmetry in B s g once thought to beperturbative, precise SM test.• But now long-distance effects have beenestimated using SCET, found to be large
vs.exp
From b to c
• Life gets more difficult for theorists:
LQCD/ mc is not << 1.
• More model dependence, e.g. pole dominance model for the various topological amplitudes entering D decays C.-D. Lu
• Life (sometimes) gets easier for experimentalists: • Particle yields can be much larger – consider for example
the rich cornucopia of exotic c-c containing hadrons from 3872 to 4660 GeV D. Liventsev
• No analogs discovered yet in the b-b system
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_
_
Want new particles?
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 46
D. Liventsev
What are they?
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 47
D. Liventsev
or baryon-antibaryon?? C. Qiao
dd
Compute two-pion exchange (loop level)potential between baryons. Might be abound state, but answerdepends on cutoff ineffective theory, soinconclusive
Soft & diffractive physics
• New implementation of a helical variation on the Lund string model
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S. Todorova
Proposed test for it:
The underlying event at LHC
• Harder than expected• The “hard tail” of the
underlying event is double
parton scattering.• Suggests that DPS
should be visible at LHC soon,
whether in bbjj E. Berger
gjjj as at Tevatron N. Skachkov
or maybe even Wjj or Zjj
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C.-E. Wulz
Soft diffraction at LHC
• MC strategy proposed to remedy this. K. Goulianos
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Hard diffraction at LHC
• New results for p0p0
• Two curious features:
1) Amplitude preferred by `Jz=0 selection rule’, g+g+ p0p0, vanishes exactly at LO.
2) Other amplitude, g+g- p0p0, has a radiation zero at LO.Moriond QCD 26 March 2011L. Dixon Theoretical Summary 51
L. Harland-Lang
g
g
Conclusions• Standard Model is still holding up, though some tensions need
follow-up: D0 like-sign muon asymmetry, Tevatron top-quark forward backward asymmetry, muonic hydrogen Lamb shift measurement, …
• Theory is coming along nicely in multiple regimes: pQCD for colliders, HQET in B physics, SCET now for colliders, AdS/CFT as
a qualitative model for heavy ion strongly coupled quark-gluon plasma and hadron spectra
• Some areas are still difficult and may need new frameworks: for example X, Y, Z spectroscopy
• All in all a very exciting week to be in Moriond/La Thuile!
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 52
Extra Slides
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 53
AdS/CFT & gluon scattering
• Alday, Maldacena showed how to compute gluon scattering at strong coupling as a minimal area problem for a string world-sheet with a polygonal boundary (strong-coupling Wilson loop)
• Can also compute it at weak coupling. Numerical results are remarkably similar, for no good reason (at present): V. Del Duca
• Can this circle ideas or techniques help with NNLO QCD? Time will tell…
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 54
Extreme QCD (cont.)
• Unitary exclusive quark combination models. Also applied to heavy-ion collisions – J/y pT spectrum
(after fitting model to electron pT spectrum)
• Basic principles:
- ALL quarks goes into hadrons (confinement)
- Unitarity (without gluons!)
“If your model isn’t unitary, I can’t help you”
Moriond QCD 26 March 2011L. Dixon Theoretical Summary 55
Li S.-Y.
General Uncertainty Issues
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• Should one ever add independent sources of errorlinearly, even if theoretical? No! Implies 100% correlation.
A. Djouadi, J. Hays
• What is the “right” distributionfor theoretical uncertainties?Flat or Gaussian?• I would say neither:- pQCD theorist rarely has any idea which part of her/his bandis preferred (I know I don’t)- won’t swear that answer won’tever fall outside band (I wouldn’t).• Envelope of Flat+Gaussian?