1 / 18

Elena Bruna for the STAR Collaboration Yale University

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. Investigate Jet Fragmentation Functions in AuAu w.r.t. pp

ata
Download Presentation

Elena Bruna for the STAR Collaboration Yale University

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  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! “trigger” jet Elena Bruna for the STAR Collaboration - QM09 “recoil” jet

  3. Trigger setup with the STAR e.m. calorimeter (EMC): • High Tower Trigger (HT): tower 0.05x0.05 (ηxϕ) with Et> 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 , |hjet|<1-R • charged particle pT (TPC), 0.1<pT<20 GeV/c • neutral tower Et 0.05x0.05 (ηxϕ) (EMC) • Hadronic correction • Electron correction for double counting Experimental setup for pp and AuAu [M. Cacciari, G. Salam, G. Soyez 0802.1188] Elena Bruna for the STAR Collaboration - QM09

  4. ~ 21 GeV STAR preliminary pt per grid cell [GeV] η ϕ 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 pT(Jet) in R<0.4) • Apply a pTcut,particle on tracks and towers before Jet Finding Elena Bruna for the STAR Collaboration - QM09 di-jet event pp √s=200 GeV STAR Preliminary

  5. Event Background in AuAu r (GeV/area) • Event-by-event basis: • pT (Jet Measured) ~ pT (Jet) + r A ± s √A • r is the background energy per unit area • A is the jet area • r, 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 pTcut,particleon tracks and towers AuAu √s=200 GeV STAR Preliminary Multiplicity Elena Bruna for the STAR Collaboration - QM09 STAR Preliminary Background fluctuations [Gev] Rc

  6. Background to di-jets in AuAu Trigger jet • Background di-jet rate = “Fake” + Additional Hard Scattering • Fake jets: • background particles clustered as jets • Additional hard scattering contribution in HI Collisions: • uncorrelated in Df 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 Trigger jet pT > 10 GeV Trigger jet pT > 10 GeV pTcut,particle = 0.1 GeV Au+Au HT 0-20% pTcut,particle=2 GeV Au+Au HT 0-20% Elena Bruna for the STAR Collaboration - QM09 p/2 STAR Preliminary STAR Preliminary di-jet

  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 pTcut,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 pTcut,particle  large background fluctuations • Use recoil jet energy after correcting for background fluctuations (unfolding) Elena Bruna for the STAR Collaboration - QM09 “trigger” jet HT trigger “recoil” jet

  8. Fragmentation Functions Jet energy determination: R=0.4 large uncertainties due to background (further systematic evaluation needed) STAR Preliminary AuAu (Jet+Bkg) • 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 AuAu (Bkg) high z low z Charged particle FF: R(FF)=0.7 Elena Bruna for the STAR Collaboration - QM09 xrec=ln( pT,Jet rec / pT,hadr) • pT Jet rec(trigger)>20 GeV & pTcut,particle=2 GeV

  9. Trigger Jet Energy as a proxy? • 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 large uncertainties due to background (further systematic evaluation needed) Ratio of FF: AuAu/pp STAR Preliminary STAR Preliminary Elena Bruna for the STAR Collaboration - QM09 pT(trigger jet)>20 GeV Ptcut=2 GeV pT Jet rec(trigger)>20 GeV pTcut,particle=2 GeV Normalized spectra above 7 GeV for shape comparison Uncorrected spectra zrec=pT,hadr/pT,Jet rec (trigger) zrec=pT,hadr/pT,Jet rec (trigger)

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

  11. 2nd approach: “unfolding” method • Large background fluctuations in AuAu w/o pTcut,particle • Parameterized by Gaussian smearing with s=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 STAR Preliminary Simulation: Effect of bkg fluctuations on true jet spectrum Pythia jets Pythia+AuAu MB jets Elena Bruna for the STAR Collaboration - QM09

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

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

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

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

  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

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

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

More Related