Measurements of Jet Structure and Fragmentation from Full

Measurements of Jet Structure and Fragmentation from Full Jet Reconstruction in Heavy Ion Collisions at RHIC Elena Bruna for the STAR Collaboration Yale University Quark Matter 09, Knoxville 03/29 -04/04 2009

Our approach “trigger” jet “recoil” jet Elena Bruna for the STAR Collaboration - QM 09 Investigate Jet Fragmentation Functions in Au. Au 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 travels thru the medium and suffers quenching – will test this! Di-jet rates If we can fully reconstruct jets, the di-jet rate in Au. Au should not be suppressed – will test this! If unbiased jet population, quenching modification of Fragmentation Function – will test this! 2

Experimental setup for pp and Au. Au • Trigger setup with the STAR e. m. calorimeter (EMC): • High Tower Trigger (HT): tower 0. 05 x 0. 05 (ηxϕ) with Et> 5. 4 Ge. V • Data Set analyzed: • Au. Au (2007): HT trigger events, 0 -20% central • Jet Finder Algorithm: Anti-k. T (from Fast. Jet package) • R=0. 4 , |hjet|<1 -R [M. Cacciari, G. Salam, G. Soyez 0802. 1188] • charged particle p. T (TPC), 0. 1<p. T<20 Ge. V/c • neutral tower Et 0. 05 x 0. 05 (ηxϕ) (EMC) • Hadronic correction • Electron correction for double counting Elena Bruna for the STAR Collaboration - QM 09 • pp (2006): HT trigger events 3

Jet Finding in Heavy-Ion collisions pt per grid cell [Ge. V] STAR preliminary ~ 21 Ge. V di-jet event pp √s=200 Ge. V STAR Preliminary η ϕ Elena Bruna for the STAR Collaboration - QM 09 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

r (Ge. V/area) Event Background in Au. Au • Event-by-event basis: • p T (Jet Measured) ~ p. T (Jet) + r A ± s √A Au. Au √s=200 Ge. V STAR Preliminary • Substantial region-to-region background fluctuations • Comparable in magnitude from Fast. Jet and naïve random cones ⇒ significantly reduced by applying a p. T cut, particle on tracks and towers Background fluctuations [Gev] Multiplicity STAR Preliminary Rc Elena Bruna for the STAR Collaboration - QM 09 • r is the background energy per unit area • A is the jet area • r, A estimated from Fast. Jet algorithm • Background energy in R=0. 4 ~ 45 Ge. V 5

Background to di-jets in Au. Au Background di-jet rate = “Fake” + Additional Hard Scattering Fake jets: background particles clustered as jets Additional hard scattering contribution in HI Collisions: p/2 di- Trigger jet t je (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 Trigger jet p. T > 10 Ge. V p. Tcut, particle = 0. 1 Ge. V Au+Au HT 020% STAR Preliminary p. Tcut, particle=2 Ge. V Au+Au HT 020% STAR Preliminary Elena Bruna for the STAR Collaboration - QM 09 uncorrelated in Df w. r. t. Trigger jet 6

Towards Fragmentation Functions GOAL: get a good energy estimate for recoil jet in Au. Au HT er “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) Elena Bruna for the STAR Collaboration - QM 09 1) Use trigger jet energy as proxy for recoil jet: gg Two approaches: tri “trigger” jet 7

Fragmentation Functions Jet energy determination: R=0. 4 large uncertainties due to background (further systematic evaluation needed) STAR Preliminary Au. Au (Jet+Bkg) Charged particle FF: R(FF)=0. 7 hz g i h z w lo In Au. Au: x =ln( p /p ) FF(Jet)=FF(Jet+Bkg)-FF(bkg) p. T Jet rec(trigger)>20 Ge. V & p. Tcut, particle=2 Ge. V Bkg estimated from charged particle spectra out of jets, rescaling to the area with R=0. 7 rec T, Jet rec T, hadr Elena Bruna for the STAR Collaboration - QM 09 Au. Au (Bkg) 8

Trigger Jet Energy as a proxy? Assumption: trigger jet in Au. Au is equivalent to pp vacuum fragmentation (no large nuclear effects) large uncertainties due to background (further evaluation needed) Ratio systematic of FF: Au. Au/pp STAR Preliminary Normalized spectra above 7 Ge. V for shape comparison Uncorrected spectra jet)>20 p. T(trigger Ge. V Jet rec(trigger)>20 p. Tcut, particle Ge. V Ptcut=2=2 Ge. V zrec=p. T, hadr/p. T, Jet rec (trigger) Shapes of spectra and FF are similar in pp and Au. Au Elena Bruna for the STAR Collaboration - QM 09 STAR Preliminary trigger jets not significantly modified Trigger jet energy can be used as a proxy for recoil jet 9

Recoil Jet FF from large uncertainties due to background (further systematic evaluation needed) approach R=0. 4 R=0. 7 Energy of trigger jet used p. T Jet rec(trigger)>20 Ge. V & p. Tcut, particle=2 Ge. V p. T Jet rec(recoil)>25 Ge. V & p. Tcutparticle=0. 1 Ge. V CAVEAT: nuclear k. T effect not taken into account, expected to be of the order 2 -3 Ge. V large uncertainties due to background (further systematic evaluation needed) Trigger jet energy uncertainty zrec=p. T, hadr/p. T, Jet rec (trigger) No significant modification of FF of recoil jets with p. Trec>25 Ge. V STAR Preliminary zrec=p. T, hadr/p. T, Jet rec (trigger) Elena Bruna for the STAR Collaboration - QM 09 STAR Preliminary st 1 10

nd 2 approach: “unfolding” method Large background fluctuations in Au. Au w/o p. Tcut, particle Parameterized by Gaussian smearing with s=6 Ge. V in Au. Au 0 -20% Simulation: Effect of bkg fluctuations on true jet spectrum Pythia jets Pythia+Au. Au MB jets Solution: unfold background fluctuations and extract “true” spectrum allows to compare pp and Au. Au Data driven – model independent approach Elena Bruna for the STAR Collaboration - QM 09 STAR Preliminary 11

di-jet spectra from unfolding STAR Preliminary Elena Bruna for the STAR Collaboration - QM 09 • Significant suppression seen Indicates: • Energy shifts to larger cone radii (>0. 4) • Some Jets “absorbed” • Biased to extreme path length of recoil jets 12

Recoil Jet FF from unfolding R=0. 4 R=0. 7 Energy of recoil jet used p. Trec(trigger) > 10 Ge. V & p. Tcut, particle=2 Ge. V p. Trec(recoil) > 25 Ge. V & p. Tcut, particle=0. 1 Ge. V pt, rec(Au. Au)>25 Ge. V ⇒ < pt, rec(pp)> ~ 25 Ge. V STAR Preliminary Elena Bruna for the STAR Collaboration - QM 09 STAR Preliminary No significant modification of FF of recoil jets with p. Trec>25 Ge. V Dominated by non-interacting jets? 13

Recoil Jet FF: Lower Jet p. T R=0. 4 R=0. 7 p. Trec(trigger) > 10 Ge. V & p. Tcut, particle=2 Ge. V 20<p. Trec(recoil)<25 Ge. V & p. Tcut, particle=0. 1 Ge. V Energy of recoil jet used STAR Preliminary Reducing the jet energy indication of modification of FF Elena Bruna for the STAR Collaboration - QM 09 20<pt, rec(Au. Au)<25 Ge. V ⇒ < pt, rec(pp)> ~ 18 Ge. V 14

Summary Evidence that di-jet rates are suppressed A. B. Recover a fraction of the jet energy shift towards smaller energies Do not reconstruct jet pt, rec(Au. Au)>25 Ge. V STAR Preliminary p. Trec(recoil)>25 Ge. V 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 Ge. V di-jet rates less suppressed A. B. “Feed-down” from high-energy jets More complete jet energy recovered Indication of modification of FF 20<pt, rec(Au. Au)<25 Ge. V STAR Preliminary Elena Bruna for the STAR Collaboration - QM 09 Biased jet population selected STAR Preliminary 15

Outlook di-jets are a promising tool to study Jet Fragmentation Functions Extreme selection: recoil jets have a longer in-medium path Look at larger radii Look at di-jets in Au. Au 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 - QM 09 Investigate further the systematics Compare to quenching models (JEWEL, q. PYTHIA, …) How can we recover an unbiased jet population? 16

Elena Bruna for the STAR Collaboration - QM 09 Extra slides 17

Trigger jet: FF ratio Elena Bruna for the STAR Collaboration - QM 09 18