hard core protons soft-physics at hadron colliders
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Hard Core Protons soft-physics at hadron colliders. Craig Buttar University of Sheffield With Arthur Moraes and Ian Dawson. Outline. Brief introduction to LHC and ATLAS Soft-physics processes Why study soft processes Available models for soft physics processes - PowerPoint PPT PresentationTRANSCRIPT
Hard Core Protonssoft-physics at hadron colliders
Craig Buttar University of Sheffield
With Arthur Moraes and Ian Dawson
Outline
• Brief introduction to LHC and ATLAS
• Soft-physics processes
• Why study soft processes
• Available models for soft physics processes
• Comparison of models to data
• Final tuning of the model
• Extrapolation to the LHC
• Outlook
LHC and ATLAS
7 TeV protons on 7 TeV protons25ns bunch crossing rateLow luminosity=1033cm-2s-1
High luminosity=1034 cm-2s-1
Cross-sections
ediffractivnonSDDDelasticpp
inelastic
At the LHC 101mb 23mb 78mb
55mb
Soft physics processesAn event where there is no observable high-pt signature eg jetPhysically a combination of several physical processes: mainly non-diffractive inelastic double diffractiveExperimentally depends on the experiment-trigger: Collider expts usually measure non-single diffractive(NSD)
Soft physics
Minimum bias
Underlying event
Associated with high PT events:Beam remnantsISRMore difficult to define experimentally and theoretically
How are minimum bias and underlying events related ?
Forward productionLow multiplicityLarge E-flow
Central productionHigh multiplicitySmall E-flow
A minimum bias event
Charged particle flow and Charged energy flow at the LHC~77% of charged particles in ||<5
~6% of charged energy flow in ||<5
ATLAS covers ||<5->ATLAS is a central detector !(miss all that lovely diffractionPhysics eg diffractive Higgs)
dN/deta~6=>30ch tracks in ID and 60 tracks in ATLAS per event At high luminosity ~ 15/events crossing !
Non-diffractive inelastic
Pythia6.2
ATLAS
How well is the min-bias understood ?
• Compare predictions of PYTHIA and PHOJETQCD+multi-parton vs DPM+multi-chain fragmentation
• Results agree at the level of 20%
Radiation Background
A crucial aspect of this is the calculation of the particle levels in ATLAS
• Radiation levels• Background contributions to
trigger rates• Occupancy
PHOJET/PYTHIA neutrons photons e+e- protons muons pionstotal >10MeV >30keV >0.5MeV >10MeV >10MeV >10MeV
Inner-detector regions 1.08->1.11 1.06->1.11 0.99->1.15 1.0->1.1 1.1 1.1 1.1Muon-barrel regions 1.09->1.12 1.09->1.21 1.09->1.18 0.9->1.2 1.0->1.2 1.0->1.2 low-statsMuon-forward regions 1.09->1.15 1.05->1.14 1.06->1.13 0.9->1.4 1.0->1.2 1.1->1.6 low-stats
Effect of soft-physics on events
Hbb high luminosity~ 15 minimum bias events/crossing
Hbb low luminosity~ 1.5 minimum bias events/crossing
The underlying event
High PT scatter
Beam remnants
ISR
Studying the underlying event
How to measure the properties of the underlying event
CDF analysis
Multi-jet cross-sections: HERA and D0
Important for central jet-veto
Modelling soft physics
How to describe low-pt behaviour ?
• Different approaches but all lead to multi-parton scattering
Evidence for multi-parton interactions-UA5
Look at minimum biasEvents
Violation of KNO scaling-Multiparton interactions ?
CDF analysis of underlying event
The underlying event cannot be explained by single partonParton scattering
ZEUS Multijet analysis
CDF evidence for multi-parton interactions
mbeff7.13.27.15.14
Xjetspp 3/ 0
eff
BADP m
2
Related to distribution of partons in transverse space
Modelling multi-parton interactions
pp@40TeV
QCD 22 cross-section increasesRapidly with Pt-minExceeds the total pp xsect ??????
s
utpQxfQxfutstddxdx
dp
dTji
kij
kjiT ˆ
ˆˆ),(),()ˆ,ˆ,ˆ(ˆˆ 22
122
11
2,,
12
4/
min
2
2
s
p
t
t
hard
T
dpdp
d
pp
hardn
Solution ?Introduce Multi-partonscattering
PYTHIA model
• pT-min • Impact parameter
• energy dependence
Multiple interactions solve total xsect problemNeed to tame the PT divergenceParameters of the model:
Control divergenceAbrupt vs smooth cut-off
Defines number of interactionsSmall i.p.high probability of interactionMatter distribution
Pt-min
PT
σ
Pt-min
Abrupt cut-off typcially givesSmaller cross-sections than Abrupt cut-off Leads to few multi-parton interactions
2
2
2
32
21
2
31
expexp1
a
r
aa
r
ar
d
Impact parameter
Double Gaussian-hard core
Greater overlap gives greaterProbability of interaction
PYTHIA tuningAbrupt vs smooth cut-off scenario-abrupt cut-of does not produce enough parton-parton interactions, has a low multiplicity cut-off
Pt-min is ~1.9GeV default value
The underlying event requires less activity => higher ptLose ‘unification’ of min-bias and underlying event
Increasing Pt-min
Double gaussian with defaultCore size = 0.2
Alternatively we can tune the matter distribution
Small coreCore x2 default
Hard core
Default Pt-min=1.9
Increasing core size
The min-bias are insensitive to the matter distribution
Pt-min vs matter distribution
• Can we decide which is the correct tuning for the underlying event ?
• Use other data, in this case HERA datarequire agreement with precision high ET-jet data
HERA Data2/df High ET 2/df Low ET
Default MI model (abrupt cut-off) 2.23 3.72Defaults with double gaussian core=0.2 (default) 2.83 3.44Defaults with double gaussian core=0.4 2.09 5.04Defaults with single gaussian Pt-min =1.6 (R.Field/CDF) 3.17 4.93
Extrapolation to LHC energies
4.4
)()(
)()(
CDFMBCDFUE
LHCMBLHCUE
PYTHIA tuning
PYTHIA’s model parameters
Minimum bias Underlying event “Universal”
MSTP(81) 1 1 1 (D) √
MSTP(82) 4 4 4 √
PARP(82) 1.9 2.2 / 1.9(?) 1.9 (D) (BUT need to change core)
PARP(83) 0.5 0.5 0.5 (D) √
PARP(84) 0.2 / 0.4 (?) 0.2 (PARP(82)=2.2) / 0.4 (PARP(82)=1.9)
0.4 (investigate minimum bias!)
PARP(90) 0.16 0.16 0.16 (D) √
Summary and outlook
• Study of soft interactions is important for experiment design and analysis
• Multi-parton interactions are a good model for soft interactions
• Comparing PYTHIA model to min-bias data from ISR, SppS, Tevatron; and underlying event data from HERA and Tevatron, we can define parameters
• There is ambiguity in what is the best way to fit the datapt-min vs matter distribution vs ISR
• Need to look at other data to try resolve ambiguities
• Need to determine energy dependence to allow extrapolation to the LHC
• Put data into JETWEB to make quantitative comparisons
Why
Why: the hadronic event environment
• Prediction of radiation levels
• Pile-up and pattern recognition issues-detector occupancy, pedestal effect
• Low-pt physics and connection to underlying event in ‘interesting high-pt events
• Higgs studies eg H• Central jet veto
Tuning pythia
• Pythia is the standard MC in ATLASbut some work with phojet-compare different physics pQCD vs dual-parton model
• Pythia models minimum bias using multiple-interaction formalism
• Main parameters are:pt-min cut-off scale for pQCD-physically motivated by screeningmodel of the parton density-double gaussianstring drawing and nearest neighbour(and pdfs)
• Use ISR+UA5+Tevatron data
pp
QCDn
Low x_gamma 2nd jet forward
r19822.1/6
R36518.3/6
R38818.7/7
R28463.3/6
Low x_gamma 2nd jet central
R36515.3/7
R38819.3/7
R28448.7/7
R19812.2/7
R36512.8/8
R38816.4/8
R28421.9.8
R19821.9/8
Min-bias also relatively insensitive to pt-min