Elena Bruna Yale University Lessons from STAR Elena Bruna Yale University EMCal Offline Meeting, Frascati May 21st 2009

ALICE vs STAR BEMC: -1<h<1 0<f<2p TPC: -1<h<1 EMCal: -0.7<h<0.7 80<f<190° TPC: -0.9<h<0.9 0<f<2p

Jet Finders in STAR kT, Anti-kT, SISCone (from FastJet package): Anti-kT expected to be less sensitive to background effects in heavy ion collisions. R=0.4, R=0.2 Jet acceptance |hjet|<1-R Recombination scheme: E-scheme with massless particles pT,cut =0 GeV on tracks/towers (pTcut =2 GeV  biased jet sample) Cuts at particle level: 0.1< pT(tracks) <20 GeV/c (upper limit to reduce space-charge effects) E(towers)>150 MeV

Systematic errors / corrections in STAR Hadronic corrections Double counting of electrons BEMC calibration Tracking efficiency, pT resolution Corrections in spectra and fragmentation functions Jet pT resolution (detector effect + missing energy) Fake jets (dominant in Au+Au) Background fluctuations (dominant in Au+Au)

Hadronic and e± corrections in STAR At “Reader” level (i.e. before feeding the Jet =Finders) Hadronic avoid double counting of hadronic energy (pT + hadronic shower) On the BEMC towers that match TPC tracks Possible to set the fractional energy to be subtracted: Etower=Etower – f x ptrack (f =100% used) Electron avoid double counting of e± energy (pT + e.m. shower) On the BEMC towers that match TPC e± candidates tracks Keep only track pT or keep only tower E or Etower=Etower – √(pel2+mel2) e± id: p/E + SMD cluster size + min<dE/dx<max

Event background in Au+Au at STAR Event-by-event basis: pT (Jet Measured) ~ pT (Jet) + r A ± s √A r = median pT per unit area A = jet area rA  estimated and subtracted (by FastJet) Background energy in R=0.4 ~ 45 GeV Substantial region-to-region background fluctuations width = s √A (from FastJet) Comparable in magnitude with naïve random cones ⇒ significantly reduced by applying a pT cut on tracks and towers r (GeV/area) AuAu √s=200 GeV STAR Preliminary Multiplicity STAR Preliminary Background fluctuations [Gev] Rc

Fake jets at STAR Fake jets = background particles clustered as jets Background model dependent In inclusive measurements: Randomize azimuth of each charged track and calorimeter tower Run jet finder Remove leading particle from each jet Re-run jet finder In di-jet analysis: “Fake” + Additional Hard Scattering contribution in HI Collisions Use “jet” spectrum at 90° to correct for “fake” di-jets

Corrections at work Raw spectrum Correction for “fake” jets Unfolding bkg fluctuations (s~6.8 GeV) Correction for jet pT resolution

Lessons from cross sections and RAA R=0.4: significant energy recovered, but visible trend R=0.2: jet energy not fully recovered in small R p+p: jet more collimated with increasing jet pT Au+Au: suggests strong broadening of the energy profile

Lessons from di-jets and FF STAR Preliminary ratio of di-jet spectra AuAu/pp ratio of Fragmentation Functions AuAu/pp STAR Preliminary pt,rec(AuAu)>25 GeV ⇒ < pt,rec(pp)> ~ 25 GeV Biased to extreme path length of recoil (High-Tower triggered ev.) Significant suppression seen Energy shifts to larger cone radii (>0.4) Some Jets “absorbed” No significant modification of FF of recoil jets with pTrec>25 GeV Dominated by non-interacting jets?

FastJet analysis in ALICE Hadronic corrections  done. From STAR: Flexibility, i.e. different correction schemes to study systematic errors (no, MIP, fractional). Can we do this in ALICE? e± double counting  foreseen, in progress. Flexibility in PID cuts? Background: r (magnitude) and s (fluctuations) From STAR: (1) r and s estimated by FastJet on an event-by-event basis. Doable in ALICE with the EMCal acceptance? Use only TPC tracks? Out-of-cone areas? To be studied. From STAR: (2) medium broadens jets, most likely also out of R=0.4 How to deal with possibly spread jets in ALICE-EMCal? To be studied Is a statistical background subtraction needed? Fake jets: From STAR: (1) randomizing azimuth of tracks and towers (2) “jet” spectrum at 90° w.r.t. di-jet axis Can we do (1) and (2) in ALICE-EMCal acceptance? To be studied.

Summary Lessons from STAR: possible bias with pTcut on tracks/towers  analysis done also with no pTcut broadening of jets in Au+Au w.r.t. p+p  need to explore jet energy profile, sub-jets, etc. Useful for the PPR to evaluate the ALICE-EMCal capability in handling the critical aspects found by STAR