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Jet results from STAR: overview

Jet results from STAR: overview. Elena Bruna, for the STAR Collaboration Yale University. Winter Workshop on Nuclear Dynamics, Ocho Rios, Jamaica January 2 - 9, 2010. High-p T at RHIC: towards jets. Suppression of high-p T hadrons in Au+Au at RHIC w.r.t. p+p

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Jet results from STAR: overview

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  1. Jet results from STAR: overview Elena Bruna, for the STAR Collaboration Yale University Winter Workshop on Nuclear Dynamics, Ocho Rios, Jamaica January 2 - 9, 2010

  2. High-pT at RHIC: towards jets • Suppression of high-pT hadrons in Au+Au at RHIC w.r.t. p+p • Evidence for parton energy loss in the medium Elena Bruna

  3. Jets in Heavy-Ion Collisions Motivation Measure properties of QGP via modification (jet quenching) of “calibrated and unbiased” pQCD probes Goal Reconstruct the full jet kinematics of hard scattering in unbiased way, even in presence of (underlying) heavy-ion collision. Jet-finding connects theory and experiment ! Tools Jet-finding algorithms jet energy loss in the medium Elena Bruna

  4. Jets at RHIC STAR preliminary pt per grid cell [GeV] η ϕ p+p pt,jet rec~21 GeV/c Au+Au 0-20% pt,jet rec~21 GeV/c STAR preliminary Jets clearly visible in p+p Full jet reconstruction: a challenge in A+A We have the tools: • Modern jet-finding algorithms • Background fluctuation estimates di-jet event pt per grid cell [GeV] η ϕ di-jet event Elena Bruna

  5. Recombination algorithms • Recombination algorithms: • Seedless - ALL particles are clustered into “jets” • kT: from pairs of low-pT particles. • Not bound to a circular structure • Anti-kT: from pairs of high-pT particles. • Circular shape, radius ~R resolution parameter FastJet M. Cacciari, G. Salam, G. Soyez 0802.1188 collinear safe Infrared safe

  6. Jets are “calibrated” probes PRL 97 (2006) 252001 Jet cross section in p+p (STAR), p+p, DIS, well described by pQCD Jets in p+p are a good reference for Au+Au Elena Bruna

  7. Fragmentation Functions in p+p Preliminary Preliminary Preliminary Preliminary H. Caines QM 09 Reasonable agreement between data and PYTHIA Jets in p+p are a good reference for Au+Au Data not corrected to particle level. “PYTHIA” = PYTHIA +GEANT R=0.4 20 <Jet pTreco< 30 GeV/c 30 <Jet pTreco< 40 GeV/c See A. Timmins’ talk Elena Bruna

  8. Jets in d+Au J. Kapitan EPS HEP 09 σkT,raw (p+p) = 2.8 ± 0.1 GeV/c σkT,raw (d+Au) = 3.0 ± 0.1 GeV/c no strong Cold Nuclear Matter effect on jet kT broadening seen Systematics under investigation Elena Bruna

  9. Jets in A+A • A challenge in high-multiplicity environment • How to suppress background: • Reduce the jet area • -Apply a pTcut on tracks and e.m. towers • Multiple channels: • Inclusive, di-jet, jet-hadron, gamma-jet See A. Hamed’s talk Elena Bruna

  10. Event Background in Au+Au • pT (Jet Measured) ~ pT (Jet) + ρ A ± σ √A • ρ = background energy per unit area • A = jet area • ρ, A: estimated from FastJet algorithm • Background energy in R=0.4 ~ 45 GeV • Substantial region-to-region • background fluctuations (σ) • Comparable in magnitude from FastJet • and naïve random cones • Significantly reduced by applying • a pTcuton tracks and towers r(GeV/area) AuAu √s=200 GeV STAR Preliminary AuAu √s=200 GeV STAR Preliminary Multiplicity Multiplicity STAR Preliminary STAR Preliminary Background fluctuations [GeV] See S. Salur’s talk Elena Bruna Rc

  11. Other background: “fake” jets “trigger” jet • Fake jets = uncorrelatedbackground particles randomly clustered as jets • Different approaches to estimate background jets in Au+Au at STAR: • Event randomization (Inclusive spectra) • Jet spectrum at 90° (di-jet analysis) “recoil” jet pTcut,particle = 0.1 GeV Au+Au HT 0-20% pTcut = 0.1 GeV Au+Au HT 0-20% STAR Preliminary STAR Preliminary EMC trigger Elena Bruna

  12. Corrections at work • Raw spectrum • Correction for “fake” jets • Unfolding bkg fluctuations (s~6.8 GeV) • Correction for jet pT resolution (~ 15%) Elena Bruna

  13. What we expect - for unbiased jet reconstruction - • Jet energy fully recovered even in case of quenching • Jet is a hard process, scales as Ncoll • Inclusive spectra: • RAAjet = 1 • Di-jet analyses: • Ratio of recoil spectra Au+Au/p+p = 1 • Modified fragmentation in case of dense medium Wiedemann, Sapeta arXiv:0707.3494 Elena Bruna

  14. What we observe “trigger” jet - di-jet measurements - “recoil” jet AntiKt R=0.4 pt,trig>10 GeV/c STAR Preliminary Significant suppression of recoil jets Indicates broadening: • Energy shifts to larger cone radii (>0.4) or • Some Jets “absorbed” in the limit E.B. QM09 arXiv:0907.4788 EMC trigger Elena Bruna

  15. What we observe “trigger” jet - di-jet measurements: Fragmentation Functions - “recoil” jet No apparent modification of FF of recoil jets with pTrec>25 GeV would imply non-interacting jets, but: Jet broadeningEnergy shift harder FF Need to better determine the jet energy E.B. QM09 arXiv:0907.4788 EMC trigger Elena Bruna

  16. What we observe - Jet inclusive measurements - M. Ploskon QM09 arXiv:0908.1799 Elena Bruna

  17. What we observe - Jet inclusive measurements: RAA - RAA<1 Full energy NOT recovered, jet broadened Au+Au M. Ploskon QM09 arXiv:0908.1799 Elena Bruna

  18. What we observe - Jet inclusive measurements: 0.2/0.4 - R=0.4 p+p: jets more collimated with increasing pT Au+Au: suggests strong broadening of the energy profile R=0.2 M. Ploskon QM09 arXiv:0908.1799 Elena Bruna

  19. What we observe “trigger” jet - Jet-Hadron correlations - Trigger jet: Anti-kt R=0.4, pt,cut>2 GeV/c, pt,recjet>20 GeV/c J. Putschke RHIC/AGS Associated hadron pt,assoc>2.5 GeV/c 1<pt,assoc<2.5 GeV/c 0.1<pt,assoc<1 GeV/c Significant broadening on the recoil side Observed modification of “Fragmentation Function” STAR Preliminary 0-20% Au+Au STAR Preliminary 0-20% Au+Au STAR Preliminary 0-20% Au+Au Open symbols p+p Open symbols p+p Open symbols p+p flat bkg subtraction by ZYAM - jet v2 under investigation EMC trigger Elena Bruna

  20. Summary • Jet baseline measurements in p+p and d+Au under control • di-jet results in Au+Au: • Recoil jets suppressed in Au+Au • No significant modification of measured FF  Artifacts of broadening! • Inclusive jet results in Au+Au: • Jet suppression at high-pT (RAA<1) • Broadening of jet profile from R=0.2 to R=0.4 • Jet energy not fully recovered (further develop jet finding techniques) • Jet-Hadron correlation results: • Broadening and softening of recoil side STAR Preliminary 0-20% Au+Au Elena Bruna

  21. BACKUP SLIDES Elena Bruna

  22. R matters! The choice of R depends on • The system we are looking at (e+e-, p+p, Cu+Cu, Au+Au, Pb+Pb,…) • Tradeoff: don’t want to lose too much out-of-cone radiation but want to have a small background in the jet area p+p 200 GeV STAR Preliminary In p+p: ~80% of jet energy within R=0.4 for 20 GeV jets Elena Bruna

  23. Towards a consistent picture of jet quenching:-Jet-Hadron Correlations - Out-of-cone energy R>0.4: 10<pt,jettrig <15 GeV/c = 2.47 GeV 15<pt,jettrig <20 GeV/c = 2.98 GeV 20<pt,jettrig <50 GeV/c = 2.99 GeV • low ptassoc : azimuthal width of recoil particle wider in Au+Au than in p+p • Broadening on the recoil side • Out-of-cone energy may “account for” di-jet suppression • low ptassoc : enhancement of yield in Au+Au w.r.t. p+p  “Modification of FF” Elena Bruna

  24. Di-hadron correlations Elena Bruna

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