1. Hard Core Protons soft-physics at hadron colliders
Craig Buttar
University of Sheffield
With Arthur Moraes and Ian Dawson

2. 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

3. 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

4. Cross-sections
inelastic
At the LHC 101mb mb mb
55mb

5. 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 ?

6. 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

7. 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

8. 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%

9. Radiation Background
A crucial aspect of this is the calculation of the particle levels in ATLAS
Radiation levels
Background contributions to trigger rates
Occupancy

11. 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

12. The underlying event
High PT scatter
Beam remnants
ISR

13. 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 

14. Modelling soft physics
How to describe low-pt behaviour ?
Different approaches but all lead to multi-parton scattering

15. Evidence for multi-parton interactions-UA5
Look at minimum bias
Events
Violation of KNO scaling
Multiparton interactions ?

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

17. ZEUS Multijet analysis

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

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

20. 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

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

22. Greater overlap gives greater Probability of interactiond
Impact parameter
Greater overlap gives greater
Probability of interaction
Double Gaussian-hard core

23. PYTHIA tuning
Abrupt vs smooth cut-off scenario-abrupt cut-of does not produce
enough parton-parton interactions, has a low multiplicity cut-off

24. Pt-min is ~1.9GeV default value

25. 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

26. Alternatively we can tune the matter distribution
Small core
Default Pt-min=1.9
Core x2 default
Hard core
Increasing core size

27. The min-bias are insensitive to the matter distribution

29. 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

30. Extrapolation to LHC energies

31. 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) √

32. 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

33. 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

34. 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.

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

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

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

38. Min-bias also relatively insensitive to pt-min