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Jet fragmentation in p+p collisions at 200 GeV in the STAR experiment.

Jet fragmentation in p+p collisions at 200 GeV in the STAR experiment. Elena Bruna, for the STAR Collaboration Yale University. DNP Meeting, Oct 23-26 2008. Jet Physics in STAR. Jet reconstruction in STAR Jet-p T distributions for different jet algorithms

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Jet fragmentation in p+p collisions at 200 GeV in the STAR experiment.

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  1. Jet fragmentation in p+p collisions at 200 GeV in the STAR experiment. Elena Bruna, for the STAR Collaboration Yale University DNP Meeting, Oct 23-26 2008

  2. Jet Physics in STAR • Jet reconstruction in STAR • Jet-pT distributions for different jet algorithms • Systematic study of jet finding algorithms • Fragmentation functions for charged and identified particles for different jet algorithms Elena Bruna Outline • Measure jet fragmentation in p+p: • test for QCD models at RHIC energies • reference for Au+Au  Study the hadrochemical modifications of jets in the medium

  3. Jets in high-energy collisions • High-pT partons produced in hard scatterings • Full jet (spray of collimated hadrons) after parton fragmentation  access to partonic kinematics hadrons D c, xc Jet cross section: Elena Bruna a, xa b, xb p p σab d, xd D hadrons

  4. Jet Reconstruction Cone Algorithms seed • ‘seed’ (E>Ethreshold) • iterative approach • Primarily used at hadron colliders • Cone • SISCone :all particles as seeds tracks or towers Rcone particles seed [Cacciari, Soyez, arXiv:0704.0292] fragmentation R=√(Δφ2+Δη2) outgoing parton Elena Bruna STAR Detectors: TPC + e.m. calorimeter Recombination Algorithms • Seedless • Not bound to a circular structure • Start from merging particles close in phase-space • kT : pairs of high-pT particles clustered first • Anti-kT :pairs of low-pT particles clustered first [Cacciari, Salam, Soyez, arXiv:0802.1189]

  5. Jet pT spectra Counts FastJet Package [Cacciari,Salam,Soyez http://www.lpthe.jussieu.fr/~salam/fastjet/] R=0.7 |ηjet|<0.3 Elena Bruna STAR preliminary p+p √s=200 GeV JP trigger • Raw measurement, no correction for: • Jet energy resolution • Trigger bias • Track reconstruction efficiency • Agreement of FastJet Finders and cone with low-pT seed • Different onset for the high-pT seed Cone Only the highest energy jet taken per event

  6. Fragmentation functions for charged hadrons 20< pT,jet<30 GeV Charged particles Charged particles (no e-) Elena Bruna • Electrons: • identified with dE/dx and p/E • mainly from p0 and g • few electrons: ≈10% of events have at least 1 e- • No major difference in fragmentation functions w/ & w/o e- p+p √s=200 GeV JP trigger Definition: x=ln (Ejet/phadr) We use: x=ln (pT,jet/pT,hadr) : we do not make assumptions on the particle mass  mpart=0  jet Minv=0

  7. x for different jet energies 20< pT,jet<30 GeV 10< pT,jet<15 GeV pT>=1 GeV/c p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger R=0.7 pT,jet>40 GeV 30< pT,jet<40 GeV p+p √s=200 GeV JP trigger Elena Bruna p+p √s=200 GeV JP trigger • Uncorrected spectra • Different Jet Finders show similar performance for a given R • Mean x increases with jet momentum

  8. Identified particles: L, K0s pT>0.5 GeV/c for all particles 20<Ejet<50 GeV 10<Ejet<15 GeV 15<Ejet<20 GeV Elena Bruna [M. Heinz, Hard Probes 08] Lpp, K0s  p+p- Kinematical range of V0 in STAR: 0.5<pT<8 -10 GeV/c Cone Jet Finder with Eseed=0.5 GeV/c Here: x=ln(Ejet/phadr)

  9. Summary and outlook Elena Bruna • Jet-pT spectra measured up to ~ 50 GeV (correction for energy resolution to be done) • Systematic study of jet finding algorithms in p+p: • No difference in jet pT and x observed for a given R • No preferred choice of jet finders in p+p  Good baseline for jet reconstruction in Au+Au • Fragmentation functions measured for different jet energies and particle species (hadrons, L and K0s). • Electron contamination not significant. • Outlook: • PID at high-pT (relativistic rise of dE/dx) • PID fragmentation functions also for Au+Au

  10. Extra slides Elena Bruna

  11. x and charged multiplicity/jet STAR preliminary R=0.7, JP Elena Bruna STAR preliminary

  12. R=0.7 vs R=0.4 STAR preliminary Elena Bruna STAR preliminary

  13. Energy resolution ERECO vs EPYTHIA Elena Bruna • Jet Finder (Mid-point Cone Algorithm) applied to PYTHIA p+p events: • PYTHIA particles  PYTHIA Jets (no detector effects) • Reconstructed tracks and calorimeter towers  RECO Jets (detector effects) • Resolution: ~25%

  14. Comparison with PYTHIA 20<Ereco<30 GeV 30<Ereco<40 GeV 40<Ereco<50 GeV Elena Bruna [M. Heinz, Hard Probes 2008] Midpoint Cone Algorithm (seed=0.5 GeV) R=0.7 Charged particles x distributions

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