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 Comparison of models to data Final tuning of the model Extrapolation to the LHC Outlook

LHC and ATLAS 7 TeV protons on 7 TeV protons 25ns bunch crossing rate Low luminosity=1033cm-2s-1 High luminosity=1034 cm-2s-1

Cross-sections inelastic At the LHC 101mb 23mb 78mb 55mb

Soft physics processes An event where there is no observable high-pt signature eg jet Physically a combination of several physical processes: mainly non-diffractive inelastic double diffractive Experimentally 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 remnants ISR More difficult to define experimentally and theoretically How are minimum bias and underlying events related ?

A minimum bias event Forward production Low multiplicity Large E-flow Central production High multiplicity Small E-flow Forward production Low multiplicity Large E-flow

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 diffraction Physics 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 PHOJET QCD+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

Effect of soft-physics on events Hbb low luminosity ~ 1.5 minimum bias events/crossing Hbb high luminosity ~ 15 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 bias Events Violation of KNO scaling Multiparton interactions ?

CDF analysis of underlying event The underlying event cannot be explained by single parton Parton scattering

ZEUS Multijet analysis

CDF evidence for multi-parton interactions Related to distribution of partons in transverse space

Modelling multi-parton interactions spp@40TeV QCD 22 cross-section increases Rapidly with Pt-min Exceeds the total pp xsect ?????? Solution ? Introduce Multi-parton scattering

PYTHIA model Multiple interactions solve total xsect problem Need to tame the PT divergence Parameters of the model: Control divergence Abrupt vs smooth cut-off pT-min Impact parameter energy dependence Defines number of interactions Small i.p.high probability of interaction Matter distribution

Pt-min σ Abrupt cut-off typcially gives Smaller cross-sections than Leads to few multi-parton interactions

Greater overlap gives greater Probability of interaction d Impact parameter Greater overlap gives greater Probability of interaction Double Gaussian-hard core

PYTHIA tuning Abrupt 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 pt Lose ‘unification’ of min-bias and underlying event Double gaussian with default Core size = 0.2   Increasing Pt-min

Alternatively we can tune the matter distribution Small core Default Pt-min=1.9 Core x2 default Hard core 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 data require agreement with precision high ET-jet data

Extrapolation to LHC energies

1.9 (D) (BUT need to change core) 0.4 (investigate minimum bias!) PYTHIA tuning PYTHIA’s model parameters Minimum bias Underlying event “Universal” MSTP(81) 1 1 (D) √ MSTP(82) 4 4 √ PARP(82) 1.9 2.2 / 1.9(?) 1.9 (D) (BUT need to change core) PARP(83) 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 (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 data pt-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 ATLAS but 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 screening model of the parton density-double gaussian string drawing and nearest neighbour (and pdfs) Use ISR+UA5+Tevatron data Why TeV-scale There are a number of very general arguments for the Higgs and possibly new physics to be found at or below 1 TeV Higgs mass As the Higgs mass increases the self-coupling and W+Z coupling grow. At about 1TeV require new physics to explain the ‘strongly interacting Higgs’. Naturalness Fundamental scalars are ‘inelegant’. Therefore theorists propose that the EW theory is a low energy representation of physics at large mass scale eg the Planck mass at 1019GeV. If this is true then the radiative corrections to the Higgs mass force the Higgs mass to this energy scale. To avoid this requires unaturally precise cancellations. The cancellations can be avoided by introducing new physics at the 1TeV scale, either strongly interacting eg compositness or SUSY. No-lose theorem: Something must happen, even a null result would be interesting. And if all else fails we can do B-physics.

Low x_gamma 2nd jet forward 18.3/6 R388 18.7/7 R284 63.3/6 r198 22.1/6

Low x_gamma 2nd jet central 15.3/7 R388 19.3/7 R284 48.7/7 R198 12.2/7

R365 12.8/8 R388 16.4/8 R284 21.9.8 R198 21.9/8

Min-bias also relatively insensitive to pt-min