Physics of the Large Hadron Collider Lecture 3: …Physics of the Large Hadron Collider Lecture 3:...
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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
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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
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Simulating physics at the LHC
Elements of an LHC event simulation
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Simulating physics at the LHC
1. Hard interaction
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Simulating physics at the LHC
1. Hard interaction
2. Partonshowers
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Simulating physics at the LHC
1. Hard interaction
2. Partonshowers
3. Hadronization,hadron decay
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Simulating physics at the LHC
1. Hard interaction
2. Partonshowers
3. Hadronization,hadron decay
4. Underlying event / multiple interactions
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Simulating physics at the LHC
5. Detector simulation
1. Hard interaction
2. Partonshowers
3. Hadronization,hadron decay
4. Underlying event / multiple interactions
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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
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Simulation tools
Matrix element generators – for hard process
2
Diagrams for by MadGraph
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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
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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
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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
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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
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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
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Standard Model backgrounds
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
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Standard Model backgrounds
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
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Standard Model backgrounds
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)
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Standard Model backgrounds
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%)
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Standard Model backgrounds
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
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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
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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
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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
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Supersymmetry-like signatures
muonmuon
jet
jetjet Missing energy
jet
jet
jet
Missing energy
ATLAS detector, side view Front view
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Supersymmetry-like signatures
● 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
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Supersymmetry-like signatures
Associated gluino-squark production in SUSY
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Supersymmetry-like signaturesJets from decay
Associated gluino-squark production in SUSY
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Supersymmetry-like signaturesJets from decay
Jets from QCD rad
Associated gluino-squark production in SUSY
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Supersymmetry-like signaturesJets from decay
Jets from QCD rad
Same-sign leptons
Associated gluino-squark production in SUSY
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Supersymmetry-like signaturesJets from decay
Jets from QCD rad
Same-sign leptons
Invisible particles/Missing energy
Associated gluino-squark production in SUSY
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Supersymmetry-like signatures
Missing energy distributions:CMS benchmark points
LM1 (600 GeV) and HM1 (1800 GeV)
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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
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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
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Information from SUSY-like events
Information from leptons:– Edges and endpoints in opposite-sign same-flavor
events
– Position of edge:
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Information from SUSY-like events
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
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Information from SUSY-like events
Total transverse energy of event
– Peak position related to mass of produced particles
J.A. et al,arXiv:0810.3921
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Information from SUSY-like events
Information from jets– Distinguish between squark and gluino production
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Information from SUSY-like events
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
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Information from SUSY-like events
Number of jets in 2-lepton events in SPS1a, LHC 1 fb-1
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Need of precision simulations
Range of predictions in SUSY scenario without jet matching
J.A. et al,arXiv:0810.5350
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Need of precision simulations
Range of predictions in SUSY scenario with jet matching
J.A. et al,arXiv:0810.5350
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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
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Analyzing the excesses
● 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)
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Analyzing the excesses
● 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
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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
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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
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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
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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)
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Backup slides
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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
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Simulating QCD radiation
● 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
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Simulating QCD radiation
● 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)
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Simulating QCD radiation
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)
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Simulating QCD radiation
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
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Simulating QCD radiation
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+ PS
1-jet ME event
Double counting!
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Simulating QCD radiation
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
– Without discontinuities in distributions
PS ME MatchedME+
PS
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Simulating QCD radiation
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
– Without discontinuities in distributions
– Without large dependence on the highest multiplicity available
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Simulating QCD radiation
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)
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Simulating QCD radiation
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))