1. 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 p T 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 (k T, anti-k T, cone) Energy resolution using PYTHIA simulation Understand the Trigger Bias Goal: compare Fragmentation functions in pp and Au+Au 2Elena Bruna

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

4. Jets in high-energy collisions High-p T partons produced in hard scatterings a, x a b, x b σ ab c, x c d, x d Full jet (spray of collimated hadrons) after parton fragmentation gives access to: partonic kinematics jet cross section s TOOL: Jet-Finding algorithms 4Elena Bruna

5. Jet Reconstruction Seedless, not bound to a circular structure k T : starts from merging low p T particles close in the phase-space Anti-k T : starts from merging high p T particles close in the phase-space R cone seed tracks or towers R=√(Δφ 2 +Δη 2 ) Seed Cone: ‘seed’ (E>Ethreshold) iterative approach Seedless Cone (SIS cone): all the particles used as seeds Splitting/Merging applied Cone Algorithms Recombination Algorithms [Cacciari, Salam, Soyez, arXiv:0802.1189] [Cacciari, Soyez, arXiv:0704.0292] outgoing parton fragmentation seed 5Elena Bruna

6. Experimental setup for pp and Au+Au charged particle p T (TPC) neutral tower E t 0.05x0.05 (ηx ϕ ) (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(ηx ϕ ), E t >8 GeV High Tower Trigger (HT): tower 0.05x0.05 (ηx ϕ ) E t > 5.4 GeV (pp) cluster 0.1x0.1 (ηx ϕ ) E t > 7.5 GeV (AuAu)  40 towers  : 120 towers in  =0- 2  6Elena Bruna  =-1  =0  =+1 EMC

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

8. Fragmentation functions for charged hadrons Definition:  =ln (E jet /p hadr ) We use:  =ln (p T,jet /p T,hadr ) : no assumptions on the particle massξ p T,hadr (GeV/c)(p Tjet =30 GeV) 111 24 31.5 40.5 50.2 30< p T,jet <40 GeV p+p √s=200 GeV JP trigger Fragmentation function in qualitative agreement for all Jet-Finders Statistical errors only Systematic studies ongoing 8Elena Bruna STAR Preliminary

9. Jet-Patch vs High Tower triggers 10< p T,jet <15 GeV 20< p T,jet <30 GeV 30< p T,jet <40 GeV p T,jet >40 GeV Low p Tjet : Multiplicity of charged particles influenced by Neutral Energy Fraction and high z fragmenting jets  stronger trigger BIAS in the HT sample High p Tjet : 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 STAR preliminary k T R=0.7 Uncorrected k T R=0.7 Uncorrected  10GeV jet p Thadr (GeV/c) 20GeV jet p Thadr (GeV/c) 13.77.4 30.51 50.070.1  30GeV jet p Thadr (GeV/c) 40GeV jet p Thadr (GeV/c) 11114.7 31.52 50.20.26 9 STAR Preliminary p+p √s=200 GeV

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

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

12. p+p Fragmentation function vs. PYTHIA Increasing Cone R Increasing Jet Energy Increasing Jet Energy 20<E reco <30 GeV30<E reco <40 GeV 40< E reco <50 R<0.4 R<0.7 Good agreement with PYTHIA especially at low R JP trigger R=0.7 |η jet |<0.3 12Elena 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) Elena Bruna13 … to Au+Au Analyzed STAR data-sets: Au+Au (2007) High-Tower (HT) trigger (cluster E T >7.5 GeV) Au+Au (2007) Minimum-Bias (MB) trigger

14. 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 p Tcut on tracks and towers Elena Bruna 14 Jet energy fraction outside cone R=0.3 CDF preliminary Background estimation: Mean energy in out-of-cone areas ϕ η p t per grid cell [GeV] STAR preliminary ~ 21 GeV Reconstructed Jet Out-of-cone area

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

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 16Elena Bruna

17. Consequences of background fluctuation for FF measurements STAR preliminary 17 Simulation STAR Preliminary

18. Simulation: ξ distribution for 30 GeV (p t,cut >2 GeV) Charged particle FF: R c (FF)=0.7 and p t >0 GeV using Pythia fragmentation 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 30 GeV mono-jet embedded in 0-20% central Au+Au STAR event 18 STAR preliminary STAR Preliminary

19. Data: ξ distribution for jet energies > 30 GeV in Au+Au Cone and k t algorithm give similar fragmentation-function measurements for p t cut > 2 GeV and reconstructed jet p t above 30 GeV in Au+Au 0-10% LOCone FastJet k t STAR preliminary stat. errors only p t hadron ~10 GeV Au+Au HT E t >7.5 GeV 19Elena Bruna

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

21. Au+Au results: Simulation  jet Finding works in high multiplicity environment!  background reasonably under control Data: Binary scaling SPECTRA (MinBias data): Binary scaling observed with no p T 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-p T hadron suppression) Measurements consistent: why? 21

22. What’s happening! 22 Effect A: 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 p Tcut ! Effect B Effect B: Biased sample of jets due to energy loss and p Tcut The jet softens in the medium Its energy is not recovered with p Tcut AND assuming PYTHIA fragmentation Its energy is UNDERESTIMATED  ξ=ln(p t jet /p t ) should be larger If this is true  Quenching models could address this issue dN/dξ ξ Au+Au p+p dN/dξ ξ Au+Au p+p

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 Elena Bruna23 We do not see modification of the Fragmentation Functions Surface effect Softening both Surface effect on HT events or Softening of jets in the medium or both?

24. What’s next How to address this issue: High-Tower jet-p T 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! Elena Bruna24 T h a n k y o u !