From yesterday Jet II Full Jet Reconstruction Goal

From yesterday Jet II: Full Jet Reconstruction Goal: set the Jet Energy Scale • Different systematics to take into account (tracking, …) • Background fluctuations: the challenge Jet III: Results p+p and d+Au: reference/control measurements Broadening observed at RHIC Elena Bruna (Yale&INFN Torino)

Today Jet III: Results p+p and d+Au: reference/control measurements Broadening observed at RHIC More on PHENIX vs STAR More quenching observables: di-jets, jet-hadron Jet IV: The Present: from RHIC to LHC Elena Bruna (Yale&INFN Torino) Hard Probes at LHC vs RHIC Results on quenching at LHC

Jet-finding in PHENIX CAVEAT: jet-finder based on unmodified jet-shapes ⇒ veto against modified/quenched jets “Anti-quenching” biases! Elena Bruna (Yale&INFN Torino) 3

Jet-finding in PHENIX Step back: how does jet-finding work in PHENIX? Jet-finding based on a Gaussian kernel (σ=0. 3, 0. 4) Focuses on the core of the jet Direct rejection of fake jets (i. e. jet by jet): May select specific jet fragmentation 1) Sum p. T 2 inside a Gaussian kernel to obtain a discriminant: 2) Keep jets with g 0. 1 > threshold Are those jets that keep a Gaussian shape only minimally interacting? That would explain the suppressed RAA to the level of surface emission Elena Bruna (Yale&INFN Torino) 4

Jets in A+A: possible biases CAVEAT: jet-finder based on unmodified jet-shapes ⇒ veto against modified/quenched jets “Anti-quenching” biases! p. T cut to minimize background ⇒ bias towards less-interacting jets Can we exploit the biases? Elena Bruna (Yale&INFN Torino) 5

Di-jet measurements ger jet rig Ct il Reco EM jet r e g Trigger jets are biased towards the surface. Recoil jets are exposed to a maximum pathlength in the medium. Large energy loss expected. Anti-k. T, R=0. 4 Trigger Jet: p. T, cut=2 Ge. V/c, p. T(trig)>20 Ge. V/c Coincidence rate: how often I measure a recoil jet once the trigger jet is found σ=6. 5 Ge. V/c Elena Bruna (Yale&INFN Torino) 6

di-jet coincidence rate Recoil Jet: R=0. 4, p. T, cut=0. 2 Ge. V/c Trigger Jet: p. T, cut=2 Ge. V/c p. T, cut on trigger jet: allows similar trigger jet population in p+p and Au+Au Recoil jets measured per trigger jet coincidence rate STAR preliminary Significant suppression in di-jet coincidence measurements broadening and/or absorption? Elena Bruna (Yale&INFN Torino) 7

Broadening or absorption? If broadening: Energy shift? p+p Absorption? Au+Au Jet energy spread outside R p. TJet(R)<p. TJet(true) shift of spectrum towards lower p. T If absorption: Jet is so quenched that the jet-finding algorithms do not find it Jet is lost Or both? Elena Bruna (Yale&INFN Torino) 8

Broadening scenario possible interpretation of di-jet suppression How much would the Recoil Au. Au spectra need to be shifted in order to recover unbiased pp? (simple assumption = constant shift of the spectra, i. e. constant energy loss) Au. Au shifted/pp Compatible with a jet p. T shift Δ = 7 -8 Ge. V/c Elena Bruna (Yale&INFN Torino) 9

Broadening or absorption? Energy shift? p+p Absorption? Au+Au If we were able to measure unambiguously the jet energy (even in presence of quenching) we could measure the Fragmentation Functions (FF) and: If absorption: FF(A+A) = FF(p+p) jets that come out are pp-like jets If broadening: FF(A+A) < FF(p+p) jets are modified ! Remark: measure of jet energy (background + possible medium effects) is challenging! Elena Bruna (Yale&INFN Torino) 10

Jet-hadron correlations Towards Fragmentation Functions If tangential (halo) emission: Away side yield in Au+Au similar to p+p, also for low p. T, assoc If energy loss: Decrease of high-p. T, assoc particles Strong enhancement of low p. T, assoc Broadening Elena Bruna (Yale&INFN Torino) 11

Jet-hadron correlations Trigger jet: Anti-kt R=0. 4, pt, cut>2 Ge. V/c, ptjet>20 Ge. V/c 0. 1<pt, assoc<1 Ge. V/c STAR Preliminary 0 -20% Au+Au Open symbols p+p 1<pt, assoc<2. 5 Ge. V/c STAR Preliminary 0 -20% Au+Au pt, assoc>2. 5 Ge. V/c STAR Preliminary 0 -20% Au+Au Open symbols p+p Significant broadening on the recoil side Observed modification of “Fragmentation Function” Remark: flat bkg subtraction by ZYAM - jet v 2 under investigation Elena Bruna (Yale&INFN Torino) 12

Jet-hadron correlations Broadening π Significant (Gaussian) broadening of the away side. Broadening decreases with jet energy. Out-of-cone (R>0. 4) energy ~ 6 -9 Ge. V. in agreement with broadening scenario in di-jet analysis! Elena Bruna (Yale&INFN Torino) 13

Jet-hadron correlations Softening π Softening of “jet fragmentation” Significant enhancement at low p. T (p. T<2 Ge. V) Suppression at low p. T Elena Bruna (Yale&INFN Torino) 14

Jets IV: The Present: From RHIC to LHC… Elena Bruna (Yale&INFN Torino)

LHC: the hard probes factory Elena Bruna (Yale&INFN Torino) 16

Remarks on Jet Kinematics (1) jet 1 Jet production: qq(gq, gg) Energy-momentum conserved. -h q 2 q 1 jet 2 +h jets back-to-back in f Not necessarily back-to-back in h ! Why? Example: q 1 + q 2 j 1 + j 2 q 1=(x 1, 0, 0, x 1) q 2=(x 2, 0, 0, -x 2) If x 1=x 2 Dy 12=0 jets back-to-back in h ! this is more likely for high-p. T jets, where the total energy goes into the transverse plane If x 1 ≠x 2 Dy 12≠ 0 jets not back-to-back in h ! Elena Bruna (Yale&INFN Torino) 17

Remarks on Jet Kinematics (1) jet 1 Simulation: PYTHIA p+p √s. NN= 200 Ge. V -h q 2 q 1 +h jet 2 h range Simulation: PYTHIA p+p √s. NN= 200 Ge. V The higher the jet p. T, the more peaked at mid-rapidity it is Elena Bruna (Yale&INFN Torino) 18

Remarks on Jet Kinematics: RHIC vs LHC 1) For the same x 1 at RHIC and LHC, the higher the √s. NN energy, the larger the rapidity gap between the di-jets! 2) For fixed hadron p. T, different parton energies are sampled at RHIC vs LHC ! Near side has higher p. Tparton than away side Fixed p. Ttrig & p. Tassoc larger p. Tparton at LHC di-hadron p. Ttrig>8 Ge. V Keep in mind (1) and (2) when comparing di-hadron/di-jets at RHIC vs LHC Elena Bruna (Yale&INFN Torino) 19

Hard processes: from RHIC to LHC Large increase of jet x-section from RHIC to LHC! Elena Bruna (Yale&INFN Torino) 20

LHC: the hard probes factory LO p+p y=0 (h++h-)/2 π0 From RHIC to LHC: • fireball hotter, denser, longer lifetime √s = 5500 Ge. V 200 Ge. V 17 Ge. V LHC • huge increase of hard probes! need high-p. T triggers ! RHIC SPS Elena Bruna (Yale&INFN Torino) 21

LHC: the hard probes factory jet cross section in p+p: RHIC vs LHC Simulation: PYTHIA Anti-k. T, R=0. 4 huge increase of hard probes! need high-p. T triggers ! Cross-section falls with a smaller (power-law) exponent less sensitivity to the energy scale: important for background treatment Elena Bruna (Yale&INFN Torino) 22

Hard processes: from RHIC to LHC x. T N. Glover CTEQ, Rhodes, (2006) • Different x. T range: RHIC: 0. 15 – 0. 45 LHC: 0. 02 - 0. 2 • RHIC is quark dominated. LHC is gluon dominated Elena Bruna (Yale&INFN Torino) 23

LHC detectors for Jet analysis tracking ECAL HCAL muon hadron PID counters lumi. Complimentary measurements: • large acceptance for charged hadrons, leptons and neutral energy (ATLAS, CMS) • Hadron PID in ALICE (|h|<1) • ALICE: full tracking to very low p. T • ATLAS, CMS also low p. T with vertex detector (pp) Elena Bruna (Yale&INFN Torino) 24

Jet x-section measurement in ALICE: p+p (PYTHIA) ALICE EMCal PPR (2009) EMCal acceptance: |η|<0. 7, Δφ=110 o EMCal needed for triggering and for neutral jet energy component TPC used for charged tracks Large kinematical reach in 1 year ALICE p+p running Elena Bruna (Yale&INFN Torino) 25

Jet x-section measurement in ALICE: Pb+Pb (q. PYTHIA) ALICE EMCal PPR (2009) EMCal acceptance: |η|<0. 7, Δφ=110 o Large kinematical reach in 1 year ALICE running Precise measurement: Effect of background fluctuations in jet spectrum suppressed due to harder underlying partonic spectrum! Elena Bruna (Yale&INFN Torino) 26

Underlying background at LHC ALICE EMCal PPR (2009) PYTHIA jet spectrum √s=5. 5 Te. V: • embedded in HIJING 0 -10% Pb+Pb • unfolded assuming Gaussian fluctuations with s=12 Ge. V/c • unfolded spectrum within 20% of the input spectrum! background fluctuations under control because of the harder jet spectrum at LHC wrt RHIC ! LHC: background less dangerous because of the harder parton spectrum Elena Bruna (Yale&INFN Torino) 27

Underlying background at LHC Jet p+p x-sec (PYTHIA) Ratio: Jet p+p x-sec / Jet p+p ✕ Bkg fluctuations Simulation: PYTHIA Anti-k. T, R=0. 4 LHC: background less dangerous because of the harder parton spectrum Elena Bruna (Yale&INFN Torino) 28

Let’s look at the data. . Elena Bruna (Yale&INFN Torino) 29

Jet quenching at the LHC ALICE, Phys. Lett. B 696 (2011) 30. Central Pb+Pb suppressed ! Peripheral suppressed less Elena Bruna (Yale&INFN Torino) 30

Jet quenching at the LHC ALICE, Phys. Lett. B 696 (2011) 30. LHC RAA: • sharp rise above 7 Ge. V • minimum at ~ 0. 5 RHIC RAA Next: • PID • increase statistics • take pp reference at 2. 76 Te. V LHC RAA<RHIC RAA RHIC: high-p. T hadrons hadronize from quarks LHC: high-p. T hadrons hadronize from gluons (larger color charge) Elena Bruna (Yale&INFN Torino) 31

Jet quenching at the LHC Di-Jet asymmetry: ar. Xiv: 1011. 6182 Elena Bruna (Yale&INFN Torino) Anti-k. T R=0. 4 32

Jet quenching at the LHC CMS Di-Jet asymmetry: ar. Xiv: 1102. 1957 Anti-k. T Iterative cone R=0. 5 Elena Bruna (Yale&INFN Torino) 33

Quenching or fluctuations? Cacciari, Salam, Soyez, ar. Xiv: 1101. 2878 • Use HYDJET instead of HIJING • Fluctuations might potentially have an impact on the dijet asymmetry From the paper: “It is not our intention to claim that the striking di-jet asymmetry results are an artifact of fluctuations. Nevertheless fluctuations can significantly affect the main observable Aj”. Elena Bruna (Yale&INFN Torino) 34

Quenching or fluctuations? Cacciari, Salam, Soyez, ar. Xiv: 1101. 2878 • Next Use HYDJET steps: instead of HIJING • - Other Fluctuations might potentially impact the dijet asymmetry observables: jet energy have profilean(jet core), on. R-dependence of Aj, jet-hadron From the paper: “It is not our intention to claim that the striking di-jet correlations asymmetry results are an artifact of fluctuations. Nevertheless fluctuations -More exhaustive investigation of different scenarios of fluctuations and quenching can significantly affect the main observable Aj”. Elena Bruna (Yale&INFN Torino) 35

Summary Jet I: Intro & Motivations Jet II: Full Jet Reconstruction Jet III: Results Why jets in heavy ion collisions? Jet Tomography! • Access kinematics of the binary hard-scattering • Characterize the parton energy loss in the hot QCD medium • Study medium response to parton energy loss Jet-finding connects Theory and Experiment Goal: set the Jet Energy Scale • Different systematics to take into account (double counting, …) • Background fluctuations: the challenge p+p and d+Au: reference/control measurements Broadening observed at RHIC • From RHIC to LHC: huge increase of hard probes! Jet IV: • LHC: Less sensitivity to the energy scale: The Present: from RHIC to LHC important for background treatment • First observation of quenching! Elena Bruna (Yale&INFN Torino)

“Science is a way to teach how something gets to be known, what is not known, to what extent things are known (for nothing is known absolutely), how to think about things so that judgments can be made, how to distinguish truth from fraud, and from show”. R. Feynman Thanks for this fruitful school ! Elena Bruna (Yale&INFN Torino)

BACKUP Elena Bruna (Yale&INFN Torino)

Jet Energy resolution with di-jets Particle-Detector jet Res: p. TJet(Part. Lev) – p. TJet(Det. Lev) ~10 -25 % di-jet Res: p. TJet 1– p. TJet 2 (PY Det. Lev. ) ~ good! (dijet data) : But: (dijet PY Det. Lev. ) > (Part-Det) di-jet imbalance includes both energy resolution and k. T (initial state) effect! [k. T=p. Tjet sin. Dfdijet] k. T: good agreement between data and simulation Use PYTHIA to determine the jet energy resolution Elena Bruna (Yale&INFN Torino) 39

Jet-finding and systematics. . Tracking performance Tracking is limited by misalignment, luminosity, resolution… • Rare processes as high-p. T jets are likely to come from high luminosity runs Example of high-luminosity distortion? Space-charge effect accumulation of space charge in the TPC that causes an anomalous transport of drifting electrons in the TPC, affecting the tracking performance by shifting the momentum up or down (depending on the charge) • Tracking resolution at high-p. T is expected to deteriorate need to apply an upper p. T cut on tracks PYTHIA simulation: p+p 200 Ge. V effect of upper p. T cut on jet energy scale Elena Bruna (Yale&INFN Torino) 40

Jet-finding and systematics. . Unobserved neutral energy Experiments like STAR and ALICE do not detect neutral, long-lived particles (neutrons, K 0 L) PYTHIA simulation: p+p at 200 Ge. V • mean missed E ~ 9% • median missed E <0. 3 % • 50% of jets loose no energy • model dependent Elena Bruna (Yale&INFN Torino) 41

Fragmentation Functions large uncertainties due to background (further systematic evaluation needed) Au. Au (Bkg) hz g i h z w lo Charged particle FF: R(FF)=0. 7 STAR preliminary Jet energy determined in R=0. 4 p. T Jet(trig)>20 Ge. V p. Tcut=2 Ge. V Au. Au (Jet+Bkg) xrec=ln( p. T, Jet rec / p. T, hadr) Au. Au: FF(Jet)=FF(Jet+Bkg)-FF(bkg) Bkg estimated from charged particle spectra out of jet cones Bkg dominates at low p. T Elena Bruna (Yale&INFN Torino) 42

Fragmentation Functions EM No apparent modification of FF of recoil jets with“trigger” jet p. Trec>25 Ge. V would imply non-interacting jets, but: “recoil” jet Jet broadening Energy shift harder FF Need to better determine the jet energy C er gg tri Elena Bruna (Yale&INFN Torino) 43

Fake jets in PHENIX Pedestal comes from combinatorics of residual fake jets When 17. 8 (Ge. V/c)2 used as standard fake rejection cut level: < 10% contamination at 7. 5 Ge. V/c Elena Bruna (Yale&INFN Torino) 44

Jet Yields in ALICE Elena Bruna (Yale&INFN Torino) 45

DCal for Di-Jet analysis @ ALICE Elena Bruna (Yale&INFN Torino) 46