physics of the large hadron collider lecture 3: …physics of the large hadron collider lecture 3:...

Physics of the Large Hadron Collider

Lecture 3: Simulation, Signatures and Backgrounds at the LHC

Johan Alwall, SLAC

Michelson lectures at Case Western ReserveApril 13-16, 2009

Johan Alwall - Simulation at the LHC 2

Outline

● Elements of an LHC event simulation● Simulation tools

– General-purpose tools, Matrix Element tools

● Standard Model backgrounds● New Physics signatures

– Supersymmetry-like signatures

– Analyzing SUSY-like events; leptons, jets

● Summary● Summary of lecture series


Simulating physics at the LHC

Elements of an LHC event simulation



1. Hard interaction



1. Hard interaction

2. Partonshowers



1. Hard interaction

2. Partonshowers

3. Hadronization,hadron decay



1. Hard interaction

2. Partonshowers


4. Underlying event / multiple interactions



5. Detector simulation

1. Hard interaction

2. Partonshowers


4. Underlying event / multiple interactions


Simulation tools

General-purpose event simulators

Most widely used: PYTHIA, HERWIG, SHERPA

– Simple hard processes (2→1, 2→2, some 2→3)

– Parton showering / QCD radiation

– Hadronization

– Underlying event

– Many parameters, tuned to LEP and Tevatron

NEW, since 2004


Simulation tools

Matrix element generators – for hard process

2

Diagrams for by MadGraph


Simulation tools

Matrix element generators

– Matrix element expressions for multi-particle final states very complicated

– Time for implementation and risk of mistakes increase exponentially with complexity

– Tree-level amplitudes built up following simple rules

→ Solution: Automatized Matrix Element generators


Simulation tools

Matrix element generators

– Based on Feynman diagrams/Helicity amplitudes:CompHep, MadGraph, Grace, Amegic++

– Based on recursion relations:AlpGen, Whizard, Helac, Comix

– Automatic generation of leading order processes in the Standard Model and models of New Physics

– Tools for automatic inclusion of new models from Lagrangians: LanHep, FeynRules

Johan Alwall - QCD and New Physics at the LHC 13

MadGraph/MadEvent

● MadGraph/MadEvent – an automatized Matrix element and event generator

● On-demand simulation of (almost) any process in the SM or beyond (at tree level)

● Web-based or local simulation

● Interfaces to parton showers and detector simulations – full simulation chain online!

Welcome to visit us at http://madgraph.hep.uiuc.edu !

Model FeynRules MadGraph

Detector Pythia MadEvent


Simulation tools

Detector simulation– Fast general-purpose detector simulators:

PGS (“Pretty good simulations”), AcerDet, Delphes● Specify parameters to simulate different experiments

– Experiment-specific fast simulation● Detector response parameterized● Run time ms-s/event

– Experiment-specific full simulation● Full tracking of particles through detector using GEANT● Run time several minutes/event


Standard Model backgrounds

Most important Standard Model backgrounds:

Process: Cross section (pb)QCD jet production (> 100 GeV) 106 Vector boson (Z/W) production 104-105 b-quark production (>100 GeV) 5000Top quark pair production: 800Single top quark production 400Double vector boson production 100

New Physics 1 fb-100 pb

Expectednewphysics



Hard QCD jet production– Background to (pretty much) all processes due to

enormous cross section

– Jets can be mistagged as tau leptons, photons, electrons, even muons

– Decays to neutrinos and mismeasurement of jets give real and fake missing energy

– Cross section falls rapidly when asking for multiple hard jets or large missing energy

– Difficult to simulate rare events

– Estimated from data using side-band analyses



Vector boson production (Z/W)– Z decays to charged leptons pairs (10% BR),

neutrinos (missing energy) (20%) or jets (70%)

– W decays to lepton+neutrino (33% BR) or jets

– Extra jets from QCD radiation

– Cross section falls steeply when asking for multiple hard jets (see later)



Top quark pair production– Top quark only quark which decays without

hadronizing

– Decays to b quark and W

– Decay modesclassified accordingto W decay:

● Hadronic (45%)● Semileptonic (44%)● Double leptonic

(11%)



Top quark pair production

Main background to many searches for New Physics, in particular searches involving jets, leptons and missing energy

– Large cross section

– Large scale of process (top mass 175 GeV)

– Large decay branching ratios to leptons and missing energy

– Jet veto often necessary for searches for weakly interacting particles, e.g. Higgs and charged Higgs


New physics discovery

● Discovery of New Physics depends on– Cross section of New Physics processes

– Amount of Standard Model backgrounds

● Discovery defined as a 5σ deviation from Standard Model expectations

● Standard Model backgrounds fall off quickly with energy – most searches focus on high-energy signatures


New physics signatures

Possible New Physics signatures at the LHC:– New resonances decaying to two visible particles

(new gauge bosons, KK-modes from extra dimensions, Higgs bosons)

– Long-lived charged massive particles (CHAMPs)

– Missing energy signatures (SUSY, Little Higgs, UED, ADD)

– Multi-particle signatures: TeV-scale black holes, Hidden valleys


New physics signatures

Possible New Physics signatures at the LHC:– New resonances decaying to two visible particles

(new gauge bosons, KK-modes from extra dimensions, Higgs bosons)

– Long-lived charged massive particles (CHAMPs)

– Missing energy signatures (SUSY, Little Higgs, UED, ADD)

– Multi-particle signatures: TeV-scale black holes, Hidden valleys, R-parity violating SUSY

Signature I have worked most on


Supersymmetry-like signatures

muonmuon

jet

jetjet Missing energy

jet

jet

jet

Missing energy

ATLAS detector, side view Front view



● Complicated final states including missing transverse momentum, multiple hard jets, leptons and/or top and bottom quarks

● Similar signatures for any model with a discrete symmetry giving a dark matter candidate (SUSY, Little Higgs, UED)

● Main Standard Model backgrounds:– Top pair production

– Vector boson production + jets

– QCD multi-jet production



Associated gluino-squark production in SUSY


Supersymmetry-like signaturesJets from decay




Jets from QCD rad




Jets from QCD rad

Same-sign leptons




Jets from QCD rad

Same-sign leptons

Invisible particles/Missing energy




Missing energy distributions:CMS benchmark points

LM1 (600 GeV) and HM1 (1800 GeV)


Difficulties with SUSY-like signatures

● Difficult to measure jets and missing energy– Will be among the last objects to be

well-understood

– Will never get very high precision (up to few GeV)

● No part of event fully reconstructed– No resonances

– Direct info only on mass differences, not absolute mass scales

● Large jet combinatorics in event reconstruction– Additional hard jets from QCD radiation

Johan Alwall - Hunting for New Physics at the LHC 33

Earliest signals

● First few months of LHC running: cleanest signals in muon and electron channels (with jet and/or missing energy cuts to suppress background)

– Excess of high-energy leptons from cascade decays

– Often some (large) fraction of lepton pairs have the same sign (++ or --), while same-sign leptons in the Standard Model are rare


Information from SUSY-like events

Information from leptons:– Edges and endpoints in opposite-sign same-flavor

events

– Position of edge:



Information from leptons:– Relation between rate of same-sign and opposite-

sign events

OSO

F

OSSF

Z cand

SSOF

SSSF

J.A. et al,arXiv:0810.3921



Total transverse energy of event

– Peak position related to mass of produced particles




Information from jets– Distinguish between squark and gluino production



Information from jets– Distinguish between squark and gluino production

– Difficulty 1: If mass splitting between gluino and squark is small, soft jet from gluino → squark decay

– Difficulty 2: Weak bosons/Higgs/top in decay chain→ Additional jets which complicate jet counting

– Difficulty 3: Hard jets from QCD radiation→ Might look like extra decay jets if not properly simulated

– Precision simulation necessary to get both jet numbers and energy variables right



Number of jets in 2-lepton events in SPS1a, LHC 1 fb-1


Need of precision simulations

Range of predictions in SUSY scenario without jet matching



Need of precision simulations

Range of predictions in SUSY scenario with jet matching



Analyzing the excesses

● How to distinguish between scenarios?– Additional signatures (e.g. single-produced top

partners in Little Higgs)

– Mass spectrum (expected mass hierarchy in SUSY, semi-degenerate in extra dimentions)

– Cross sections (vary by factor ~10 between scalars, fermions and vectors)

– Spin determinations by angular observables (needs very high statistics and very clean signatures)

● Model distinction in general difficult; needs high-statistics data



● Model-independent first characterization of data– Before full-scale models - focus on basic questions:

● Character of produced particles● Basic decay chains (leptons, weak bosons, b-quarks)● Mass scales, cross sections, branching ratios

– Avoid parameters that cannot be constrained

– Avoid dependences between parameters that might not be present in data

(Propaganda slide for J.A. et al, arXiv:0810.3921)



● Model-independent first characterization of data

Lep(Q)

Q

Q

Q

q

ET

Q

q

ET

W/Z(*)

Q

q

ET

l/νl/ν* *on- or off-shell

g

g

NI/CI NI/CI

One of four Simplified models for characterization of early data

J.A. et al, arXiv:0810.3921


Summary

Today I have talked about:– Elements of simulation of LHC events

– Simulation tools

– Standard Model backgrounds● QCD, Weak bosons, Top quark pairs

– New Physics signatures, esp. Supersymmetry-like signatures (missing energy, leptons and jets)

– Ideas for analysis of excesses


Summary of lecture series

● The Standard Model – an amazing construction– Explains pretty much everything to date

– Problems: Hierarchy problem, Dark matter

● Several classes of ideas for new physics– Important ideas have similar signatures:

Missing energy, leptons and jets

● Distinguishing New Physics at the LHC, and analyzing the excesses, needs high-precision event simulation– Great advances in recent years, still much to do


Recommended reading

● Simulation tools:– Pythia 6.4 manual (also excellent physics manual):

Sjöstrand, Mrenna, Skands, JHEP 0605:026,2006

– MadGraph/MadEvent 4, manual and physics:Alwall et al., JHEP 0709:028,2007

● SM backgrounds and New Physics searches at the LHC

– The CMS Physics TDR: http://cmsdoc.cern.ch/cms/cpt/tdr

– The ATLAS Physics TDR (outdated):http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/TDR/TDR.html


Recommended reading

● My two most important recent contributions:– “QCD radiation in the production of heavy colored

particles at the LHC”, Alwall, de Vissher, Maltoni, JHEP 0902:017,2009

– “Simplified Models for a first characterization of New Physics at the LHC”, Alwall, Schuster, Toro,arxiv:0810.3921 (accepted for publication in PRD)


Backup slides


Simulating QCD radiation

● For any New Physics signature involving hard jets, it is crucial to correctly simulate hard jet production in Standard Model backgrounds

● Besides top quark decay, only source of hard jets in the SM is QCD radiation. Examples:– W/Z production plus jets

– Hard photon plus jets

– Top quark pairs plus extra jets (esp. > 4 jet or dilepton + > 2 jet signals)

– QCD multijet production



● For soft jets, and jets at large rapidity (small angle with beam), the Parton Shower approach is excellent

– Step-by-step subsequent QCD emissions

– Fast, computationally cheap (1→2 splittings)

– No limit on particle multiplicity● However, only formally valid in the soft and

collinear regions of phase space

– Can be tuned to give reasonable description also in a wider region, but not clear if tuning can be extrapolated to higher energies/other processes



● For high-pT, central and widely separated jets, full matrix element calculations necessary– Includes subleading, non-logarithmic terms

– Includes interference between diagrams

– Describes jet production away from the soft and colllinear region

– Fixed parton multiplicity

– Slow, large computer resources needed

● Diverges in the soft and collinear region (due to non-resummation of logarithms)



Parton showers can get multiple hard jet production from QCD radiation wrong by orders of magnitude

ʃ pT(N-th jet) > x (GeV)Jet def. cutoff

Cro

ss s

ectio

n (

pb)



Goal: Simultaneously simulate jets throughout both the hard and soft/collinear regions, for several jet multiplicities (e.g. Z+0,1,2,3,4 jets)– Without double counting between samples

+

0-jet ME event

1-jet ME event




+

0-jet ME event+ PS

1-jet ME event

Double counting!




– Without discontinuities in distributions

PS ME MatchedME+

PS




– Without discontinuities in distributions

– Without large dependence on the highest multiplicity available



Solution: “Jet matching” between ME and PS– Separate “hard jet” and “soft/collinear jet” regions

using phase-space cutoff

– Allow ME jets to populate only “hard” region and PS emissions only “soft” region

– Modify ME description to mimick the parton shower near the cutoff

– Schemes: Catani, Krauss, Kuhn, Webber [2001],M.L. Mangano [2002, 2006]Multiple variants (J.A. et al, 2007)



W+jets production at the TevatronMadEvent+Pythia (k

T-jet MLM scheme)

Cutoff

log(Jet resolution scale for 1 → 2 radiated jets ~ pT(2nd jet))

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