1. Elena Bruna for the STAR Collaboration Yale University Quark Matter 09, Knoxville 03/29 -04/04 2009

2. Our approach Investigate Jet Fragmentation Functions in AuAu w.r.t. pp We use di-jets in triggered events: “Trigger” jet: large neutral energy in single tower  jet comes from surface – will test this ! “Recoil” jet: away side of trigger jet  jet travels thru the medium and suffers quenching – will test this! Di-jet rates If we can fully reconstruct jets, the di-jet rate in AuAu should not be suppressed – will test this! If unbiased jet population, quenching  modification of Fragmentation Function – will test this! Elena Bruna for the STAR Collaboration - QM09 2 “recoil” jet “trigger” jet

3. Experimental setup for pp and AuAu Trigger setup with the STAR e.m. calorimeter (EMC): High Tower Trigger (HT): tower 0.05x0.05 (ηx ϕ ) with E t > 5.4 GeV Data Set analyzed: pp (2006): HT trigger events AuAu (2007): HT trigger events, 0-20% central Jet Finder Algorithm: Anti-kT (from FastJet package) R=0.4, |  jet |<1-R charged particle p T (TPC), 0.1<p T <20 GeV/c neutral tower E t 0.05x0.05 (ηx ϕ ) (EMC) Hadronic correction Electron correction for double counting 3 Elena Bruna for the STAR Collaboration - QM09 [M. Cacciari, G. Salam, G. Soyez 0802.1188]

4. Jet Finding in Heavy-Ion collisions GOAL: Fully reconstruct jets in high-multiplicity environment How to suppress background: Reduce the jet area (in pp >80% of p T (Jet) in R<0.4) Apply a p Tcut,particle on tracks and towers before Jet Finding 4 ϕ η p t per grid cell [GeV] STAR preliminary ~ 21 GeV di-jet event Elena Bruna for the STAR Collaboration - QM09 pp √s=200 GeV STAR Preliminary

5. Event Background in AuAu Background fluctuations [Gev] RcRc Event-by-event basis: p T (Jet Measured) ~ p T (Jet) +  A ±  √A  is the background energy per unit area A is the jet area , A  estimated from FastJet algorithm Background energy in R=0.4 ~ 45 GeV Substantial region-to-region background fluctuations Comparable in magnitude from FastJet and naïve random cones ⇒ significantly reduced by applying a p T cut,particle on tracks and towers 5 STAR Preliminary Multiplicity  (GeV/area) Elena Bruna for the STAR Collaboration - QM09 AuAu √s=200 GeV STAR Preliminary

6. Background to di-jets in AuAu Background di-jet rate = “Fake” + Additional Hard Scattering Fake jets: background particles clustered as jets Additional hard scattering contribution in HI Collisions: uncorrelated in  w.r.t. Trigger jet (does not contribute in inclusive jet measurements) Is estimated using “jet” spectrum at 90° to trigger jet Use “jet” spectrum at 90° to correct for “fake” di-jets 6 Trigger jet p T > 10 GeV p Tcut,particle = 0.1 GeVp Tcut,particle =2 GeV Trigger jet p T > 10 GeV di-jet  Trigger jet Elena Bruna for the STAR Collaboration - QM09 STAR Preliminary Au+Au HT 0- 20%

7. Towards Fragmentation Functions GOAL: get a good energy estimate for recoil jet in AuAu Two approaches: 1) Use trigger jet energy as proxy for recoil jet: Trigger jet found with p Tcut,particle on tracks and towers  small background fluctuations Energy of trigger jet used for FF in recoil jet (gamma-jet like approach) 2) The energy of recoil jet used Recoil jet found with no p Tcut,particle  large background fluctuations Use recoil jet energy after correcting for background fluctuations (unfolding) 7 Elena Bruna for the STAR Collaboration - QM09 HT trigger “recoil” jet “trigger” jet

8. Fragmentation Functions In AuAu: FF(Jet)=FF(Jet+Bkg)-FF(bkg) Bkg estimated from charged particle spectra out of jets, rescaling to the area with R=0.7 Elena Bruna for the STAR Collaboration - QM09 8 Charged particle FF: R(FF)=0.7 AuAu (Jet+Bkg) AuAu (Bkg) p T Jet rec (trigger)>20 GeV & p Tcut,particle =2 GeV STAR Preliminary large uncertainties due to background (further systematic evaluation needed)  rec =ln( p T,Jet rec / p T,hadr ) low z high z

9. Assumption: trigger jet in AuAu is equivalent to pp  vacuum fragmentation (no large nuclear effects) Shapes of spectra and FF are similar in pp and AuAu  trigger jets not significantly modified Trigger jet energy can be used as a proxy for recoil jet Trigger Jet Energy as a proxy? 9 Elena Bruna for the STAR Collaboration - QM09 pT(trigger jet)>20 GeV Ptcut=2 GeV Ratio of FF: AuAu/pp STAR Preliminary Normalized spectra above 7 GeV for shape comparison Uncorrected spectra STAR Preliminary z rec =p T,hadr /p T,Jet rec (trigger) p T Jet rec (trigger)>20 GeV p Tcut,particle =2 GeV large uncertainties due to background (further systematic evaluation needed)

10. Recoil Jet FF from 1 st approach 10 Energy of trigger jet used p T Jet rec (trigger)>20 GeV & p Tcut,particle =2 GeV p T Jet rec (recoil)>25 GeV & p Tcutparticle =0.1 GeV CAVEAT: nuclear k T effect not taken into account, expected to be of the order 2-3 GeV  No significant modification of FF of recoil jets with p Trec >25 GeV STAR Preliminary Elena Bruna for the STAR Collaboration - QM09 large uncertainties due to background (further systematic evaluation needed) z rec =p T,hadr /p T,Jet rec (trigger) R=0.4 R=0.7 Trigger jet energy uncertainty

11. Large background fluctuations in AuAu w/o p Tcut,particle Parameterized by Gaussian smearing with  =6 GeV in AuAu 0-20% Solution: unfold background fluctuations and extract “true” spectrum  allows to compare pp and AuAu Data driven – model independent approach 2 nd approach: “unfolding” method Elena Bruna for the STAR Collaboration - QM09 11 Pythia jets Pythia+AuAu MB jets STAR Preliminary Simulation: Effect of bkg fluctuations on true jet spectrum

12. di-jet spectra from unfolding Significant suppression seen Indicates: Energy shifts to larger cone radii (>0.4) Some Jets “absorbed” 12 Elena Bruna for the STAR Collaboration - QM09 STAR Preliminary Biased to extreme path length of recoil jets

13. Recoil Jet FF from unfolding Elena Bruna for the STAR Collaboration - QM09 13 p Trec (trigger) > 10 GeV & p Tcut,particle =2 GeV p Trec (recoil) > 25 GeV & p Tcut,particle =0.1 GeV Energy of recoil jet used  No significant modification of FF of recoil jets with p Trec >25 GeV  Dominated by non-interacting jets? STAR Preliminary R=0.4 R=0.7 p t,rec (AuAu)>25 GeV ⇒ ~ 25 GeV STAR Preliminary large uncertainties due to background (further systematic evaluation needed)

14. Recoil Jet FF: Lower Jet p T 14 Elena Bruna for the STAR Collaboration - QM09 Reducing the jet energy  indication of modification of FF p Trec (trigger) > 10 GeV & p Tcut,particle =2 GeV 20<p Trec (recoil)<25 GeV & p Tcut,particle =0.1 GeV Energy of recoil jet used STAR Preliminary large uncertainties due to background (further systematic evaluation needed) R=0.4 R=0.7 20<p t,rec (AuAu)<25 GeV ⇒ ~ 18 GeV

15. Summary Evidence that di-jet rates are suppressed A. Recover a fraction of the jet energy  shift towards smaller energies B. Do not reconstruct jet  Biased jet population selected p Trec (recoil)>25 GeV No strong modification of FF (two approaches lead to a similar conclusion) High-energy recoil jets are biased (non interacting) 20<p Trec (recoil)<25 GeV di-jet rates less suppressed A. “Feed-down” from high-energy jets B. More complete jet energy recovered Indication of modification of FF Elena Bruna for the STAR Collaboration - QM09 15 STAR Preliminary p t,rec (AuAu)>25 GeV STAR Preliminary 20<p t,rec (AuAu)<25 GeV

16. Outlook di-jets are a promising tool to study Jet Fragmentation Functions Extreme selection: recoil jets have a longer in-medium path Investigate further the systematics Compare to quenching models ( JEWEL, qPYTHIA, … ) How can we recover an unbiased jet population? Look at larger radii Look at di-jets in AuAu Min Bias Change path length bias? Investigate sub-jets / energy flow profile Clustering/re-distribution of energy within the jet Elena Bruna for the STAR Collaboration - QM09 16

17. Extra slides Elena Bruna for the STAR Collaboration - QM09 17

18. Trigger jet: FF ratio Elena Bruna for the STAR Collaboration - QM09 18