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Models for minimum bias and the underlying event

Models for minimum bias and the underlying event. A. Moraes, C. Buttar and I. Dawson University of Sheffield. HERA and the LHC. CERN, 26 th March 2004. Experimental definition : depends on the experiment trigger ! “Minimum bias” is usually associated to

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Models for minimum bias and the underlying event

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  1. Models for minimum bias and the underlying event A. Moraes, C. Buttar and I. Dawson University of Sheffield HERA and the LHC CERN, 26th March 2004

  2. Experimental definition: depends on the experiment trigger! “Minimum bias” is usually associated to non-single-diffractive events (NSD), e.g. ISR, UA5, E735, CDF,… Underlying event in charged jet evolution(CDF style analysis) Minimum bias events • It is not only minimum bias event! • The underlying event is everything except the two outgoing hard scattered jets. • In a hard scattering process, the underlying event has a hardcomponent (initial + final-state radiation and particles from the outgoing hard scattered partons) and a softcomponent (beam-beam remnants). • CDF analysis: • charged particles: • pt>0.5 GeV and |η|<1 • cone jet finder: Df = f - fljet σtot ~ 102 - 118 mb σNSD ~ 65 - 73mb (PYTHIA) (PHOJET) (PHOJET) (PYTHIA)

  3. Defines two interaction regions in pt scale: hard and soft processes. Uses perturbative QCD to describe high-pt parton-parton scattering. Model for hadron-hadron collisions (Pythia): • Attempts to extend perturbative (high-pt) picture down to low-pt region. • Considers the possibility that multiple parton scattering takes place in hadron-hadron collisions. n ~ σint ↓pt0↑n (and vice-versa) pt0 • Different options for hadronic matter distributions. ↓d↑probability of hard-scattering (and vice-versa) d

  4. Comparing different PYTHIA tunings to the data: dNchg/dη at η=0 F(z) = <nchg > P(nchg) LHC z = nchg /<nchg > √s (GeV) High-multiplicity events are described differently by each tuning

  5. Comments: Central Region (data dNchg/dη ~ 4) Transverse < Nchg > • ATLAS – TDR: it hasn’t been tuned to UE data; doesn’t include double diffraction for • minimum bias events; uses a model with a small hadronic core size; doesn’t correct for particle decays which affect minimum bias distributions and doesn’t include • a pTmin energy dependence. dNchg/dη ~ 15 • CDF tuning: it has been tuned particularly to UE data; doesn’t correct for particle decays which affect minimum bias distributions. dNchg/dη ~ 10 Ratio (MC/Data) Pt (leading jet in GeV)

  6. PHOJET • Developed mainly for softand semi-hard particle production. • Implements ideas of Dual Parton Model for low-pT processes. • Multiple Pomeron exchanges (sea-quark multi-chains) enhances the event activity. • Limited to production mechanisms of strong interactions. • However, useful tool for MB and UE studies where jets are involved.

  7. PYTHIA vs PHOJET F(z) = <nchg > P(nchg) z = nchg /<nchg >

  8. PYTHIA vs PHOJET dNchg/dη at η=0 Transverse < Nchg > LHC Pt (leading jet in GeV) √s (GeV) • PYTHIA models favour ln2(s); • PHOJET suggests a ln(s) dependence.

  9. Combined χ2(not the minimum χ2 !): Multiplicity information: ‹nch›, dN/dη, KNO Transverse region (UE): ‹nch› and ‹pTsum› • PHOJET1.12 and PYTHIA6.214 – tuned give the best “global” agreement to the data!

  10. LHC predictions: PYTHIA6.214 – tuned vs. CDF tuning Central Region (min-bias dNchg/dη ~ 7) Transverse < Nchg > LHC dNchg/dη ~ 30 x 3 x 2 dNchg/dη ~ 20 Tevatron Pt (leading jet in GeV)

  11. LHC predictions: PYTHIA6.214 – tuned vs. PHOJET1.12 Central Region (min-bias dNchg/dη ~ 7) LHC Transverse < Nchg > dNchg/dη ~ 30 x 3 dNchg/dη ~ 15 x1.5 Tevatron Pt (leading jet in GeV)

  12. LHC predictions: pp collisions at √s = 14 TeV

  13. Default Pythia Tuned Pythia Jetweb comparisonPreliminary ZEUS precision di-jet Photoproduction data Increasing sensitivity to Underlying event Craig Buttar ISMD 2003

  14. Conclusions: • Current minimum bias and underlying event data for pp and pp can be described with appropriate tunings for PYTHIA and PHOJET. • PYTHIA6.214 – tuned and PHOJET1.12 with its default settings give the best “global” agreement to the data (χ2/407 d.o.f 9.27 and 7.25, respectively). • CDF tuning (“tune A”) is the best model describing UE at the Tevatron, however, fails to reproduce several minimum bias distributions at lower energies. • There are sizeable uncertainties in LHC predictions generated by different models • PYTHIA6.214 – tuned and PHOJET1.12 generate LHC predictions with ~ 30% difference for minimum bias event, and a factor of ~ 2 for underlying event distributions. • Updated results:www.cern.ch/amoraes

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