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Jet Fragmentation in STAR going from pp to Au+Au

Jet Fragmentation in STAR going from pp to Au+Au. Elena Bruna, for the STAR Collaboration Yale University. Winter Workshop on Nuclear Dynamics, Big Sky Feb. 1-8 2009. Going from pp…. Topological Jets in pp at RHIC energies p T spectrum up to 50 GeV What we know:

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Jet Fragmentation in STAR going from pp to Au+Au

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  1. Jet Fragmentation in STAR going from pp to Au+Au Elena Bruna, for the STAR Collaboration Yale University Winter Workshop on Nuclear Dynamics, Big Sky Feb. 1-8 2009

  2. Going from pp… • Topological Jets in pp at RHIC energies • pT spectrum up to 50 GeV • What we know: • Good agreement of jet spectrum with NLO/PYTHIA at RHIC energies • Jet spectra and fragmentation functions measured at Tevatron  good agreement with theory • Perspectives in STAR: • Measure jet Fragmentation Functions at RHIC energies • Compare conceptually different jet algorithms (kT, anti-kT, cone) • Energy resolution using PYTHIA simulation • Understand the Trigger Bias • Goal: • compare Fragmentation functions in pp and Au+Au Elena Bruna

  3. ...to Au+Au • Topological jets measured in Au+Au in STAR at RHIC energies! • What we want to measure: • Jet pT spectrum • Jet Fragmentation Functions • What we expect with an unbiased jet population: • pT spectrum Nbin scaling (jet production is a hard process) • Fragmentation functions  modification Elena Bruna

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

  5. seed Jet Reconstruction • Seed Cone: • ‘seed’ (E>Ethreshold) • iterative approach • Seedless Cone (SIS cone): • all the particles used as seeds • Splitting/Merging applied fragmentation tracks or towers outgoing parton Cone Algorithms Rcone seed R=√(Δφ2+Δη2) [Cacciari, Soyez, arXiv:0704.0292] • Seedless, not bound to a circular structure • kT: starts from merging low pT particles close in the phase-space • Anti-kT: starts from merging high pT particles close in the phase-space Recombination Algorithms Elena Bruna [Cacciari, Salam, Soyez, arXiv:0802.1189]

  6. Experimental setup for pp and Au+Au • charged particle pT (TPC) • neutral tower Et 0.05x0.05 (ηxϕ) (EMC) • corrected for hadronic energy. • Electron correction for double counting • EMC provides fast trigger. • Two trigger setups with the EMC: • Jet Patch Trigger (JP): • 1x1(ηxϕ), Et>8 GeV • High Tower Trigger (HT): • tower 0.05x0.05 (ηxϕ) Et> 5.4 GeV (pp) • cluster 0.1x0.1 (ηxϕ) Et> 7.5 GeV (AuAu) h=-1 h=0 h=+1 f : 120 towers in Df=0-2p EMC h : 40 towers Elena Bruna

  7. Going from p+p … • Analyzed STAR data-sets: • p+p (2006) High-Tower (HT) trigger (single tower Et>5.4 GeV) • p+p (2006) Jet-Patch (JP) trigger (ηxϕ=1x1 with sum Et>8 GeV) Elena Bruna

  8. Fragmentation functions for charged hadrons • Definition: x=ln (Ejet/phadr) • We use: x=ln (pT,jet/pT,hadr) : no assumptions on the particle mass 30< pT,jet<40 GeV STAR Preliminary • Fragmentation function in qualitative agreement for all Jet-Finders • Statistical errors only • Systematic studies ongoing p+p √s=200 GeV JP trigger Elena Bruna

  9. Jet-Patch vs High Tower triggers 10< pT,jet<15 GeV 20< pT,jet<30 GeV STAR Preliminary STAR preliminary STAR Preliminary STAR preliminary p+p √s=200 GeV p+p √s=200 GeV kT R=0.7 Uncorrected 30< pT,jet<40 GeV pT,jet>40 GeV STAR Preliminary STAR preliminary STAR Preliminary p+p √s=200 GeV p+p √s=200 GeV STAR preliminary • Low pTjet: Multiplicity of charged particles influenced by Neutral Energy Fraction and high z fragmenting jets  stronger trigger BIAS in the HT sample • High pTjet: JP and HT do not show difference due to a smaller bias of different trigger selections  JP and HT fragmentation functions similar for higher jet energy

  10. x for different jet energies (1) 20< pT,jet<30 GeV 10< pT,jet<15 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger JP trigger R=0.4 pT,jet>40 GeV 30< pT,jet<40 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger • Uncorrected spectra • Different Jet Finders show similar performance for a given R Elena Bruna

  11. x for different jet energies (2) 20< pT,jet<30 GeV 10< pT,jet<15 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger pT>=1 GeV/c pT>=1 GeV/c JP trigger R=0.7 pT,jet>40 GeV 30< pT,jet<40 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger pT>=1 GeV/c pT>=1 GeV/c • Conceptually different Jet Finders  Similar performance for different cone radii and for different jet pT  suggests no significant NLO effects at RHIC energies

  12. p+p Fragmentation function vs. PYTHIA JP trigger R=0.7 |ηjet|<0.3 Increasing Jet Energy 40<Ereco<50 20<Ereco<30 GeV 30<Ereco<40 GeV R<0.4 R<0.7 Increasing Cone R Good agreement with PYTHIA especially at low R Elena Bruna

  13. pp: reference for Au+Au • Jets in pp are a safe baseline for Au+Au • Good agreement with NLO/PYTHIA • Free choice of jet algorithms in pp • Systematic studies ongoing (e- identification, hadronic shower, trigger bias) … to Au+Au • Analyzed STAR data-sets: • Au+Au (2007) High-Tower (HT) trigger (cluster ET>7.5 GeV) • Au+Au (2007) Minimum-Bias (MB) trigger Elena Bruna

  14. CDF preliminary ~ 21 GeV STAR preliminary pt per grid cell [GeV] η ϕ Jet Finding in Heavy-Ion collisions • GOAL: Fully reconstruct the jet in high-multiplicity environment • How to suppress background: • Reduce the jet area • Apply a pTcut on tracks and towers Jet energy fraction outside cone R=0.3 Reconstructed Jet • Background estimation: • Mean energy in out-of-cone areas Out-of-cone area Elena Bruna

  15. Au+Au 0-20% Rc=0.4, no pt cut, out-of-cone area <pt,Bkg> [Gev] Reference multiplicity (~centrality) Background in Au+Au 0-20% • Event-by-event basis: • <pT,Bkg>=mean pT in out-of-cone area • Bkg for Fragmentation Functions = mean FF in out-of-cone area for a given pTjet • Background energy in R=0.4 ~ 45 GeV (no pT cut) • Substantial region-to-region background fluctuations ⇒ significantly reduced by applying a pT cut STAR Preliminary STAR Preliminary Background fluctuations [Gev] Elena Bruna Rc

  16. Simulation • 1) Jet Finder on PYTHIA events • 2) a. PYTHIA event embedded in Au+Au real event b. Jet Finder on PYTHIA + AuAu • 3) Compare (1) and (2) • detector effects neglected Elena Bruna

  17. STAR preliminary Consequences of background fluctuation for FF measurements Simulation STAR Preliminary • To determine the jet energy on an event-by-event basis a ptcut> 2 GeVis necessary to reduce the effect of background fluctuations in Au+Au collisions; • Pythia+Au+Au and Pythia select similar jet population.

  18. Simulation: ξ distribution for 30 GeV (pt,cut>2 GeV) Charged particle FF: Rc(FF)=0.7 and pt>0 GeV 30 GeV mono-jet embedded in 0-20% central Au+Au STAR event Jet energy determination: Rc=0.4 and pt>1(2)GeV STAR preliminary • ξ background subtraction method and jet energy resolution in Au+Au 0-20% causes deviations < 10-20% for ξ<2-2.5 using Pythia fragmentation • Systematic deviations in the ξ shape ratio at low ξ are caused by jet-energy resolution • Only statistical errors STAR Preliminary STAR preliminary

  19. Data: ξ distribution for jet energies > 30 GeV in Au+Au Au+Au HT Et>7.5 GeV STAR preliminary stat. errors only LOCone FastJet kt pthadron~10 GeV Cone and kt algorithm give similar fragmentation-function measurements for ptcut > 2 GeV and reconstructed jet pt above 30 GeV in Au+Au 0-10% Elena Bruna

  20. Fragmentation Function in Au+Au 0-20% and p+p for 30 GeV jets  AuAu jet energies should correspond to 30 GeV pp jets STAR Preliminary • 1) pt,jetrec.(pp) > 30 GeV • 2) pt,jetrec.(Au+Au)>31 GeV for ptcut>2 GeV • 3) pt,jetrec.(Au+Au)>35 GeV for ptcut>1 GeV dominated by uncertainties due to background subtraction for pthadron<2 GeV STAR Preliminary Statistical errors only No apparent modification in the fragmentation function with respect to p+p Elena Bruna

  21. Au+Au results: • Simulation  jet Finding works in high multiplicity environment!  background reasonably under control • Data: • SPECTRA(MinBias data): Binary scaling observed with no pT cut (S. Salur talk) • PYTHIA fragmentation assumed • FRAGMENTATION FUNCTIONS (HT trigger data): -- no apparent modification w.r.t. pp • Corrections for energy resolution (bkg fluctuations) • Corrections based on PYTHIA fragmentation -- we would expect a modification (high-pT hadron suppression) • Measurements consistent: why?

  22. What’s happening! • Effect A:Biased sample of jets due to the High-Tower Trigger: • the HT trigger favors “surface” jets that are not modified by the medium • Ejet (AuAu) = Ejet (pp)  FF unmodified • If this is true  HT jets should not binary scale even without pTcut! • Effect B: Biased sample of jets due to energy loss and pTcut • The jet softens in the medium • Its energy is not recovered with pTcutAND assuming PYTHIA fragmentation • Its energy is UNDERESTIMATED  ξ=ln(ptjet/pt) should be larger • If this is true  Quenching models could address this issue Au+Au p+p dN/dξ ξ Au+Au p+p dN/dξ ξ

  23. Conclusions • Full jet reconstruction feasible at RHIC • Jet Fragmentation Functions: • Measured and under control in pp • Powerful tool to study medium effects in Au+Au • We do not see modification of the Fragmentation Functions • Surface effect on HT events or Softening of jets in the medium or both? Elena Bruna

  24. What’s next • How to address this issue: • High-Tower jet-pT spectrum • di-jets in Min Bias and HT events • Quenching models will help • Systematic studies (detector effect,…) to have more control on our understanding of jet finder in Au+Au • Jets are a precious tool to explore the medium: exciting physics is coming! Thank you ! Elena Bruna

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