Full Jet Reconstruction in Heavy Ion Collisions Sevil Salur

Charge for this talk: 1)Update on the latest results on jet-medium interactions as seen in A+A collisions at RHIC. 2)New techniques for full jet reconstruction. 1)Results that help move our understanding forward, (this is not an experiment-specific talk). 2Sevil SalurQuark Matter 2009, Knoxville TN

Why Pursue Full Jet Reconstruction? Sevil Salur3 Enables study of jet quenching at the partonic level. Uniquely large kinematic reach In A+A much reduced geometric biases, full exploration of quenching. Multiple channels for consistency checks: Inclusive, di-jets, h-jets, gamma-jets Qualitatively new observables: energy flow, jet substructure, fragmentation function Quark Matter 2009, Knoxville TN

Jets: a theorist’s view Beam Remnants Beam Remnants p p ,K,p,n,…} Jet Initial State Radiation Hadronization Final State Radiation JETS: Colored partons from the hard scatter (2  n) Fragmentation via gluon radiation Hadronization: “spray” of colorless hadrons Parton Level: Calculable with pQCD Underlying Event: Beam remnants  Soft Background S.D Drell, D.J.Levy and T.M. Yan, Phys. Rev. 187, 2159 (1969) N. Cabibbo, G. Parisi and M. Testa, Lett. Nuovo Cimento 4,35 (1970) J.D. Bjorken and S.D. Brodsky, Phys. Rev. D 1, 1416 (1970) Sterman and Weinberg, Phys. Rev. Lett. 39, 1436 (1977) … and many more 4 Sevil Salur Detector Quark Matter 2009, Knoxville TN

Jets: an experimentalist’s view Beam Remnants Beam Remnants p p ,K,p,n,…} Jet Initial State Radiation Hadronization Final State Radiation 5 Sevil Salur Detector JETS: Collection of 4-vectors of calorimeter energy clusters and charged track momentum Quark Matter 2009, Knoxville TN

Jet Reconstruction Algorithms: Cone jet Anti-k T jet K T jet Jet outgoing parton Fragmentation process Hard scatter 1. Mid Point Cone: Merging & Splitting 2. SIS CONE 3. Leading Order High Seed Cone (LOHSC) 4. K T (starting point: low p T particles) 5. Anti-K T (starting point: high p T particles) Cone Algorithm: Sequential recombination: Cluster pairs of objects close in relative p T 6Sevil Salur Goal: re-associate hadrons to accurately reconstruct the partonic kinematics. Kinematic ambiguity e.g., E-scheme vs p-scheme: 6. Gaussian filtering. Y. Lai, B. Cole arXiv: See QM 2009 S2A Talk by Y. Lai Quark Matter 2009, Knoxville TN

The FastJet Algorithms Suite of modern Collinear and infrared safe jet algorithms sequential recombination: k T, Cambridge/Aachen, anti-k T cone: SISCone (Seedless Infrared-safe Cone) Motivated by high precision jets in high luminosity p+p at LHC (pileup) but directly applicable to heavy ion collisions Two important algorithmic advances: 1. Large improvements to processing time vs. event multiplicity 2. Rigorous definition of jet area for subtraction of diffuse event background M. Cacciari, G. Salam [hep-ph] M. Cacciari, G. Salam, G. Soyez [hep-ph] FastJet – p T (Jet Measured) ~ p T (Parton) +  × A(Jet) ±  A(Jet) 7Sevil Salur A, ρ, σ are all measurable quantities! STAR Preliminary Au+Au Central A= Jet Area  = Diffuse noise,  =noise fluctuations Quark Matter 2009, Knoxville TN

Heavy Ion Background Discussion p T (Jet Measured) ~ p T (Parton) +  × A(Jet) ±  A(Jet) 8Sevil Salur A= Jet Area  = Diffuse noise,  =noise fluctuations Fundamental Assumption: Two separable components: signal and background. How might it be violated? 1. Biases in background estimation due to presence of a jet. a) Initial state radiation (Expected to be small compared to jet energy). b) Final state “out-of-cone” radiation. 2.Different Algorithms respond differently to background. (k T and Anti-k T ) Quark Matter 2009, Knoxville TN Cone jet Anti-k T jet K T jet

Jets in p+p at the Tevatron Cone Algorithm k T Algorithm Cone and k T jet spectra are consistent 9Sevil SalurQuark Matter 2009, Knoxville TN

Cone Algorithm Inclusive jet cross section over many orders of magnitude consistent with the NLO QCD Jets in p+p at the Tevatron 10Sevil Salur Cone and k T jet spectra are consistent Quark Matter 2009, Knoxville TN

Phys. Rev. Lett. 97 (2006) Jets in p+p at RHIC Reconstructed by a mid-point jet cone algorithm with R = 0.4 Experimental uncertainty ~50% STAR jet reconstruction: neutral energy from Barrel EMC charged hadrons from TPC Agrees with NLO p-QCD 11Sevil SalurQuark Matter 2009, Knoxville TN

Towards Jets in A+A at RHIC Phys. Rev. Lett (2006) Phys. Rev. Lett. 91 (2003) Phys. Rev. Lett. 97 (2006) Phys.Rev.Lett.97:162301,2006 High p T hadron suppression described by pQCD+partonic energy loss Medium seems to be transparent to photons  colored medium. Conclusive evidence for large partonic energy loss in dense matter (final state effect) 12Sevil SalurQuark Matter 2009, Knoxville TN

Jet quenching via inclusive hadrons: Quantitative Understanding? S. A. Bass, C. Gale, A. Majumder, C. Nonaka, G. Qin, T. Renk, J. Ruppert [nucl-th] Good fit of theory to data but limited discrimination of underlying physics. Theory: Modifications of jets in a 3-D hydrodynamic medium All calculations have same initial structure, final vacuum fragmentation, nuclear geometry. Parameters can be adjusted to describe data well: varies between 4-18 GeV/c 2 13Sevil SalurQuark Matter 2009, Knoxville TN TECHQM!

Di-Hadrons : Quantitative Understanding? Sevil Salur14 Di-hadron suppression not yet well-described by NLO theory H. Zhang, J. F. Owens, E. Wang, X.N. Wang Phys. Rev. Lett. 98, (2007) J.L. Nagle arXiv: [nucl-ex] J. Adams, et al Phys Rev. Lett. 97, (2006) See the next talk by J. Nagle recoil yield per trigger Vary energy loss parameter z T =p T recoil /p T trig Quark Matter 2009, Knoxville TN

Sevil Salur15 New developments are in progress! H. Zhang, J. F. Owens, E. Wang, X.N. Wang Phys. Rev. Lett. 98, (2007) J.L. Nagle arXiv: [nucl-ex] J. Adams, et al Phys Rev. Lett. 97, (2006) See the next talk by J. Nagle See the talks in S3A by A. Hamed, M. Connors, A. Hanks ZOWW arXiv: recoil yield per trigger z T =p T recoil /p T trig Di-Hadrons : Quantitative Understanding? Quark Matter 2009, Knoxville TN Vary energy loss parameter  0

Full Jet Reconstruction in Heavy Ion Collisions Sevil Salur16 Goal is Unbiased Jet Reconstruction: Reconstruct partonic kinematics independent of fragmentation details - quenched or unquenched. trigger recoil Why Pursue Full Jet Reconstruction? Enables study of jet quenching at the partonic level. Uniquely large kinematic reach In A+A much reduced geometric biases, full exploration of quenching. Multiple channels for consistency checks: Inclusive, di-jets, h-jets, gamma-jets Qualitatively new observables: energy flow, jet substructure, fragmentation function Multi-hadronic Observables: Geometric Biases: dominated by jets that have not interacted! Limited kinematic reach. Jet energy not constrained. Quark Matter 2009, Knoxville TN

Can we see jets at RHIC? 17Sevil Salur STAR Preliminary p+p   Quark Matter 2009, Knoxville TN QM 2009 Talks by E. Bruna, H. Caines, M. Ploskon, J. Putschke

Can we see jets at RHIC? STAR Preliminary Au+Au Central   18Sevil Salur PHENIX Preliminary Cu+Cu Quark Matter 2009, Knoxville TN QM 2009 Talks by E. Bruna, M. Ploskon, J. Putschke, Y. S. Lai

Reconstructed Spectra in p+p and Cu+Cu Sevil Salur19 QM 2009 Talk by Y. S. Lai Large p T range within restricted PHENIX acceptance! Unfolding of the spectra is last step! Forthcoming soon! Quark Matter 2009, Knoxville TN

Extracting di-jet angular width Sevil Salur20 QM 2009 Talk by Y. S. Lai Quark Matter 2009, Knoxville TN No centrality dependence on the widths!

E T [GeV] dN Jet /dE T (per event) N bin scaled p+p Reconstructed Jet Spectra & Corrections: Au+Au 0-10% STAR Preliminary  MB-Trig R=0.4 p T cut =1 GeV Seed=4.6 GeV LOHSC S. Salur [STAR Collaboration], arXiv: [nucl-ex] 21Sevil Salur MB-Trig: Minimum Bias Trigger Agreement with N bin Scaled p+p (~50%). p+p: Phys. Rev. Lett. 97 (2006) Quark Matter 2009, Knoxville TN

E T [GeV] dN Jet /dE T (per event) N bin scaled p+p Reconstructed Jet Spectra & Corrections: Au+Au 0-10% STAR Preliminary  MB-Trig R=0.4 p T cut =1 GeV Seed=4.6 GeV LOHSC S. Salur [STAR Collaboration], arXiv: [nucl-ex] 22Sevil Salur MB-Trig: Minimum Bias Trigger Suppression of backgrounds in heavy ions: Limit jet resolution parameter R Cut on track/calorimeter p T Agreement with N bin Scaled p+p (~50%). p+p: Phys. Rev. Lett. 97 (2006) Quark Matter 2009, Knoxville TN

E T [GeV] dN Jet /dE T (per event) MB-Trigger: Agreement with N bin Scaled p+p (~50%). HT-Trigger: Bias towards hard fragmentation: Bad for quenching Studies! N bin scaled p+p Reconstructed Jet Spectra & Corrections: Large HT-trigger bias persists at least to 30 GeV. Similar to leading particle bias. Au+Au 0-10% STAR Preliminary  MB-Trig O HT-Trig R=0.4 p T cut =1 GeV Seed=4.6 GeV LOHSC 23Sevil Salur MB-Trig: Minimum Bias Trigger S. Salur [STAR Collaboration], arXiv: [nucl-ex] What about other algorithms? p+p: Phys. Rev. Lett. 97 (2006) Quark Matter 2009, Knoxville TN Suppression of backgrounds in heavy ions: Limit jet resolution parameter R Cut on track/calorimeter p T HT-Trig: Satisfied Minimum Bias and requires a pion/photon with p T >7.5 GeV

P T Cut KT STAR Preliminary Au+Au 0-10%  MB-Trig  N bin Scaled p+p Au+Au 0-10%  MB-Trig  N bin Scaled p+p Au+Au 0-10%  MB-Trig  N bin Scaled p+p P T Cut Dependence Bias Imprecise subtraction of underlying event? Do we introduce a bias with p T -cuts? Sensitivity to fragmentation model? S. Salur [STAR Collaboration], arXiv: [nucl-ex] 24Sevil Salur p+p: Phys. Rev. Lett. 97 (2006) STAR Preliminary Au+Au Central   p T cut Quark Matter 2009, Knoxville TN

Sevil Salur25 Un-Biased Jet Measurements 1) Minimize the kinematic cuts, e.g P T Cut 2) Data driven corrections : a. Experimental characterization of background fluctuations. b. Detailed unfolding of fluctuations.  Correcting for smearing Cross-Section ETET Unfolding bgd from signal Correct via “unfolding” for the “min-bias” jet reconstruction. LOHSC seed=4.6 GeV R=0.4 PyDet PyEmbed PyTrue E T [GeV] dN Jet /dE T (a.u.) STAR Preliminary Quark Matter 2009, Knoxville TN S. Salur [STAR Collaboration], arXiv: [nucl-ex] See QM 2009 S2A Talk by: M. Ploskon

Inclusive jet spectrum: p+p STAR Preliminary Au+Au Anti-k T and k T jet spectra are consistent. BEMC calibration Uncertainty Sevil Salur26 Unfolding Uncertainty Unfolding uncertainty corresponds to a factor of 2 in jet cross-section. Quark Matter 2009, Knoxville TN See QM 2009 S2A Talk by: M. Ploskon

R AA of Jets R = 0.4 A large fraction of jets are reconstructed! (Compare pion R π AA = 0.2) Sevil Salur27 STAR Preliminary BEMC calibration Uncertainty Quark Matter 2009, Knoxville TN See QM 2009 S2A Talk by: M. Ploskon

What happens at high p T ? Sevil Salur28 W. Vogelsang Private Communication p+p Relative contributions of quark and gluon vary. What about quenching dependence on parton species? Relative contribution of sub-processes to inclusive jet production Quark Matter 2009, Knoxville TN

What happens at high p T ? Sevil Salur29 The EMC Effect: Deviation between structure Functions of Au and deuterium. W. Vogelsang Private Communication p+p Initial state effects at large x ~15% σ Α /σ d What about other high x effects? Relative contribution of sub-processes to inclusive jet production Relative contributions of quark and gluon vary. J. Gomez et al., SLAC–PUB–5813 June 7, 2001 D.F. Geesaman et al., Ann. Rev. Nucl. Part. Sci. 45, 337 (1995) B. A. Cole. et al, arXiv:hep-ph/ SLAC E139 Quark Matter 2009, Knoxville TN

See QM 2009 S2A Talk by: M. Ploskon STAR Preliminary Jet Energy Profile: p+p Au+Au BEMC calibration Uncertainty Sevil Salur30 Au+Au: Stronger decrease in yield within R=0.2 as compared to R=0.4 Quark Matter 2009, Knoxville TN

Cross-section ratios in p+p and Au+ Au with R=0.2/R=0.4 p+p Increase in the ratio with increasing p T. more focused cone with increasing jet p T Au+Au Decrease in the ratio with increasing p T. See QM 2009 S2A Talk by: M. Ploskon Sevil Salur31 STAR Preliminary Quark Matter 2009, Knoxville TN Jet Energy Profile: Quantitative differences due to jet resolution parameter R. Evidence of broadening of the jet energy profile due to quenching! Is R=0.4 large enough to reconstruct the jets in an unbiased way?

Nicolas Borghini arXiv: Sevil Salur QM 2009 Talk by K. Zapp JEWEL (Jet Evolution with Energy Loss): K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv: Parton shower with microscopic description of interactions with medium Q-Pythia: N. Armesto, L. Cunqueiro and C. A. Salgado arXiv: [hep-ph] MC implementation in Pythia of medium-induced gluon radiation through an additive term in the vacuum splitting functions. QM 2009 Talks by N. Armesto and Salgado Analytic Calculations: Quantitative analysis of data requires model building… Many more…. PYQUEN (Lokhtin, Snigriev), PQM (Dainese, Loizides, Paic), HIJING (Gyulassy, Wang)… See other QM2009 talks. Quark Matter 2009, Knoxville TN YaJEM: T. Renk arXiv: QM 2009 Talk by T. Renk

Sevil Salur33 We need to confront the calculations with data! Nicolas Borghini arXiv: QM 2009 Talk by K. Zapp K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv: Strong broadening of shower in transverse momentum with respect to jet axis. Angular distribution becomes wider! No strong broadening of shower when P T cut >2 GeV is selected. (limitations of broadening observable) Analytic Calculations vs New Monte Carlos Quark Matter 2009, Knoxville TN

Fragmentation Functions from di-jets Sevil Salur34 “recoil” jet “trigger” jet Quark Matter 2009, Knoxville TN Large HT-trigger bias in FF No-trigger bias in FF

Fragmentation Functions from di-jets Sevil Salur35 “recoil” jet “trigger” jet Quark Matter 2009, Knoxville TN Large HT-trigger bias in FF No-trigger bias in FF large uncertainties due to background (further systematic evaluation needed) 20<p t,rec (AuAu)<25 GeV ⇒ ~ 18 GeV p T (trigger) > 10 GeV & P T cut =2 GeV 20<p T (recoil jet) < 25 GeV & P T cu t=0.1 GeV STAR Preliminary See QM 2009 S2A: Talk by E.Bruna Apparent modification in the z of Au+Au with respect to p+p. But a biased population of jets.

K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv: K. Zapp Talk 2009 QM Significant uncertainties due to the sensitivity to hadronisation: Look for new observables unaffected by the hadronisation. Parton vs Hadron Clear increase in multiplicity due to radiative energy loss Collisional energy loss when recoils are counted toward the jet 36Sevil SalurQuark Matter 2009, Knoxville TN

Sevil Salur37 QCD JET Observables p T cut infrared safe insensitive observables! : number of subjets, thurst … K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv: In vacuum (LEP) data well understood in pQCD QM 2009 Talk by K. Zapp Medium Induced Radiation  More Coarser Jet Structure Quark Matter 2009, Knoxville TN Cone jet Anti-k T jet K T jet

Another way to do it: Jet quenching at the LHC P. Jacobs and M. van Leeuwen Nucl. Phys A774, 237 (2006) High p T Jets well above the background at LHC Pb+Pb at 5.5 TeV: enormous jet energy range  qualitatively new probes 38Sevil Salur N. Grau for ATLAS Copious production of hard probes : Jets, charm & bottom… Quark Matter 2009, Knoxville TN

Sevil Salur39 LHC ALICE EMCAL Physics Performance Update, CD-2 ALICE Pb+Pb background seems to be under control for the reconstructed jet-energy. CMS Eur. Phys. J. 50 (2007) 117  +jet (Z+jet) cleaner means to determine FF QM 2009 Talks by: N. Grau, G. Veres Poster by M. Heinz … Detector Upgrades: 2 super modules are installed for ALICE Full azimuthal calorimetric coverage for ATLAS & CMS ATLAS N. Grau for ATLAS Quark Matter 2009, Knoxville TN

Conclusions: Why Pursue Full Jet Reconstruction? Sevil Salur40 Full jet reconstruction gives access to the full spectrum of fragmentation topologies: Enables study of jet quenching at the partonic level. “ When you have completed 95 percent of your journey, you are only halfway there. ” Japanese Proverb – First full jet reconstruction at RHIC (0-10% central heavy ion collisions - reach is up to 50 GeV). – N bin scaling (50% Syst Uncert.) observed for the least biased case, R=0.4 and p T cut =0.1 GeV Uniquely large kinematic reach – New theory developments FASTJET and New medium-modified shower MC codes… Q-Pythia, JEWEL,… – But beware of biases: data taking and selection of particles (p T cut, R) – Path length and jet radius dependencies. – Session 2A In A+A much reduced geometric biases, full exploration of quenching. Multiple channels for consistency checks: Inclusive, di-jets, h-jets, γ-jets Qualitatively new observables: energy flow, jet substructure, fragmentation function Quark Matter 2009, Knoxville TN

Thank you! Sevil Salur41Quark Matter 2009, Knoxville TN Nestor Armesto Elena Bruna Jana Bielcikova Matteo Cacciari Helen Caines Brian Cole David d'Enterria Nathan Grau John Harris Wolf G. Holzman Peter Jacobs Jan Kapitan Yue Lai Leticia Mendez Mateusz Ploskon Joern Putschke Thorsten Renk Gavin Salam Carlos Salgado Gregory Soyez Gabor Veres Urs Achim Wiedemann Korinna Zapp …

42Sevil Salur

R AA of Jets R = 0.4 R = 0.2 Jets are reconstructed in an unbiased way for the R=0.4 Sevil Salur43 STAR Preliminary See QM 2009 S2A Talk by: M. Ploskon

Sevil Salur44 The FastJet Measurement of the Jet Area 1.Add randomly distributed ghost particles of known density d to the event 2.Run the jet algorithm 3.Count the number n of ghost particles assigned to the jet 4.Jet area A= n/d Accounts for event-wise fluctuations in shape and area of jet Jets are irregular objects! Jet area is non-trivial (≠ πR 2 )

Nuclear Modification Factors of Di-jets Sevil Salur45 See QM 2009 S2A: Talk by E.Bruna Maximum path length for the recoil! p T (recoil jet)  large suppression STAR Preliminary “recoil” jet “trigger” jet Quark Matter 2009, Knoxville TN Large HT-trigger bias in FF No-trigger bias in FF Significant suppression in di-jet coincidence

The FastJet Algorithms: Background Subtraction in Pb+Pb Pb+Pb jet cross-section is recovered after the subtraction M. Cacciari, G. Salam [hep-ph] M. Cacciari, G. Salam, G. Soyez [hep-ph] FastJet – 46Sevil Salur Background subtraction works in the Heavy Ion environment.

47Sevil Salur Anti-k T is resistant to absorption by background of jet energy at large radius M. Cacciari, G. Salam, G. Soyez [hep-ph] The FastJet Algorithms: Background Subtraction in p+p

Sevil Salur48 Modified Fragmentation Function Modified Leading Logarithmic Approximation: - good description of vacuum fragmentation (basis of PYTHIA) - introduce medium effects at parton splitting Fragmentation is strongly modified at p T hadron ~1-5 GeV Borghini and Wiedemann, hep-ph/  =ln(E Jet /p hadron ) Jet quenching: Low p T enhancement p T hadron~ 2 GeV

Fragmentation-function in Au+Au 0-20% and p+p No apparent modification in the ξ of Au+Au with respect to p+p. STAR Preliminary Where is the jet quenching? Biases: Online triggering, P T cut J. Putschke [STAR Collaboration], arXiv: [nucl-ex] See QM 2009 S2A: Talks by E.Bruna, H.Caines 49Sevil Salur LOCone FastJet k T Good agreement between the algorithms!

Quantitative Understanding: Di-Hadrons? Sevil Salur50  vary model parameter  0 Di-hadron suppression not yet well-described by NLO theory H. Zhang, J. F. Owens, E. Wang, X.N. Wang Phys. Rev. Lett. 98, (2007) J.L. Nagle arXiv: [nucl-ex] J. Adams, et al Phys Rev. Lett. 97, (2006) z T =p T recoil /p T trig

 s = 200 GeV  s NN = 200 GeV  s NN = 5.5 TeV A. Pulvirenti q q hadrons leading particle jet production in quark matter Multiple interaction inside the collision region Lose energy through medium induced gluon radiation q q hadrons leading particle leading particle schematic view of jet production hadrons Jets at RHIC & LHC: Early production from parton-parton scatterings Direct probes of partonic phase Early production from parton-parton scatterings Direct probes of partonic phase 51Sevil Salur

52Sevil Salur Jet Jet fragmentation getting more diffuse as jet propagate in the medium?

STAR Preliminary Inclusive jet spectrum: p+p BEMC calibration Uncertainty Sevil Salur53 New analysis (k T & anti-k T ) agrees with published STAR data (Mid-point cone) Au+Au See QM 2009 S2A Talk by: M. Ploskon Quark Matter 2009, Knoxville TN

Di-hadrons? Sevil Salur54 Di-hadrons not yet well-constrained by NLO theory z T =p T recoil /p T trig Zhang et al. (ZOWW) PRL 98, See the next talk by Jamie Nagle

What’s happening! 55 Effect A: Effect A: Biased sample of jets due to the High-Tower Trigger: the HT trigger favors “surface” jets that are not modified by the medium Ejet (AuAu) = Ejet (pp)  FF unmodified If this is true  HT jets should not binary scale even without Ptcut! Effect B Effect B: Biased sample of jets due to energy loss and pTcut The jet softens in the medium Its energy is not recovered with pTcut AND assuming PYTHIA fragmentation Its energy is UNDERESTIMATED  ξ=ln(p t jet /p t ) should be larger If this is true  Quenching models could address this issue dN/dξ ξ Au+Au p+p dN/dξ ξ Au+Au p+p Sevil Salur

Fragmentation Functions Sevil Salur56 See QM 2009 S2A: Talk by E.Bruna p T (trigger) > 10 GeV & P T cut =2 GeV p T (recoil jet) > 25 GeV & P T cu t=0.1 GeV No apparent modification in the z of Au+Au with respect to p+p. Recoil jet Trigger jet STAR Preliminary

Fragmentation Functions Sevil Salur57 See QM 2009 S2A: Talk by E.Bruna p T (trigger) > 10 GeV & P T cut =2 GeV 20<p T (recoil jet) < 25 GeV & P T cu t=0.1 GeV Apparent modification in the z of Au+Au with respect to p+p. Recoil jet Trigger jet STAR Preliminary Jets look more softer and more diffuser!

H – recoil jet coincidences STAR Preliminary 58Mateusz Ploskon, LBNL, STAR, QM'09 Increasing Path Length

q-PYTHIA ( 59 Carlos Salgado Talk 2009 QM Sevil Salur

Q-Pythia 60Sevil Salur

QCD Observables: (Analytic Calculations) Nicolas Borghini arXiv: Increase in the subjet distributions Gluon Jet Number of subjets vs Opening Angle Multiplicity is sensitive to hadronisation.observe a strong broadening of the shower in k T 61Sevil Salur Need theoretical understanding of jet energy loss

Sevil Salur62 QCD JET Observables p T cut infrared safe insensitive observables! : number of subjets, broadening, energy flow… K. Zapp, G. Ingelman, J. Rathsman, J. Stachel, U. A. Wiedemann arXiv: In vacuum (LEP) data well understood in pQCD Need theoretical understanding of jet energy loss 100 GeV Jet

Theory: Jet Quenching – Energy Loss Elastic energy loss: Bjorken ’82 Bremsstrahlung: Gyulassy, Wang, Plumer ’92 jet quenching measures color charge density, plasma transport coefficients But quantitative analysis of data requires model building Current status: large discrepancies (factor~10) in extracted medium parameters (transport coefficients) → ongoing efforts to resolve this Renk: medium increases virtuality of partons during evolution PYQUEN (Lokhtin, Snigriev): PYTHIA afterburner reduces energy of final state partons and adds radiated gluons according to BDMPS expectations. PQM (Dainese, Loizides, Paic): MC implementation of BDMPS quenching weights HIJING (Gyulassy, Wang): jet and mini-jet production with induced splitting JEWEL (Zapp, Ingelman, Rathsman, Stachel, Wiedemann): parton shower with microscopic description of interactions with medium q-PYTHIA (Armesto, Cunquiero, Salgado, Xiang): includes BDMPS-like radiation in modified splitting function See many QM2009 talks… 63Sevil Salur

Jet Reconstruction: connect theory and experiment Goal: re-associate hadrons to accurately reconstruct the partonic kinematics (infrared and collinear safe) pQCD theory calculates partons experiment measures fragments of partons: hadrons and calorimeter towers (clusters of hadrons) Apply “same” jet clustering algorithm to data and theory. hadrons schematic view of jet production hadrons q q θ≈0 64Sevil Salur

Underlying event (UE) & pile-up are distributed uniformly in y and  p T (Jet Measured) ~ p T (Parton) + X A(Jet) 22 pile-up in p+p + Pythia dijet  A j  A j  = Diffuse noise (p T added)  /  Area Definition: Estimate the active area of each jet by filling event with many very soft particles then count how many are clustered into given jet M. Cacciari, G. Salam [hep-ph] Study of P T /A j determine the noise density ρ on an event-by-event basis The FastJet Algorithms 65Sevil Salur

Background in k T Use the same approach for HI Study the p T /A j and remove the contribution  A j The scaled pp cross-section is recovered after the subtraction M. Cacciari, G. Salam [hep-ph] Looks promising: Has to be studied further for RHIC & LHC with experimentalist tools 66Sevil Salur

67 M. Cacciari, G. Salam hep-ph/ k T Algorithms are not so slow after all Geometric nearest neighbor search Voronoi Diagram Divide the plane into cells (one per vertex), N points can be constructed with O (N ln N ) N 3  Nlog(N) Computational Geometry Algorithms Library Orders of magnitude faster Large N region is feasible. Geometrical and minimum-finding of the kt jet-finder require O (N ln N )

Event Characteristics: Jet Area & Fluctuations Jet Area Background Fluctuations Heavy-ion: Reduction in Jet Area & Increase in fluctuations Pythia Jets embedded in real Au+Au background events have the same area and fluctuations with that of Jets in real Au+Au data. MB-trig PyEmbed PyTrue R=0.4 KT p T cut = 0.1 GeV Jet Area MB-trig PyEmbed PyTrue Counts Sigma STAR Preliminary Jet E T > 20 GeV Au+Au 0-10% M. Cacciari, G. Salam, G. Soyez [hep-ph]

69 Energy Resolution LOHSC KTCAMB p T cut =1 GeV R=0.4 Seed=4.6 GeV Counts ∆E Event by event comparison of PyTrue vs PyDet vs PyEmbed. p T cut =1 GeV R=0.4  E = E PyDet - E PyTrue  E = E PyEmbed – E PyDet  E = E PyEmbed - E PyTrue Smearing due to background subtraction in Au+Au. STAR Preliminary Shift of median due to un-measured particles (n, K 0 L ) and the p T cut. Tail at positive ∆E causes a kick in the spectrum. E T =35±5 GeV Sevil Salur

70 Effect of Resolution on Spectrum p T cut =0.1 GeV -Increase p T threshold: Reduce the effect of background fluctuations (jet reconstruction in 0-10% Au+Au is similar in p+p) - The p T cut is expected to produce biases. Similar effects also observed for KT & Cambridge/Aachen dN Jet /dE T (a.u.) LOHSC seed=4.6 GeV R=0.4 PyDet PyEmbed PyTrue PyDet PyEmbed PyTrue PyDet PyEmbed PyTrue p T cut =1.0 GeV LOHSC seed=4.6 GeV R=0.4 p T cut = 2.0 GeV LOHSC seed=4.6 GeV R=0.4 E T [GeV] STAR Preliminary Sevil Salur

71 Resolution and Efficiency & Acceptance Corrections Resolution effect corrected assuming Pythia Fragmentation. Embed Pythia Jets in 0-10% Central Events with MBtrig. E T [GeV] dN Jet /dE T (a.u.) LOHSC E T [GeV] PyEmbed / PyTrue p T cut =0.1 GeV R=0.4 Seed=4.6 GeV pol5 Use the fit functions from the ratio of PyEmbed to PyTrue to correct for energy resolution, efficiency & acceptance. PyTrue PyEmbed P T cut LOHSCKTCAMB 0.1 GeV GeV GeV E T -dependent correction factors STAR Preliminary Sevil Salur

ATLAS Efficiency and ET resolution as a function of jet ET and eta. Gamma-jet measurements possible ! Di-jet correlation expectations, D(z) determination Effective RAA of jets (reco/input) Talk by N. Grau QM Sevil Salur

CMS 73Sevil Salur Talk by G. Veres QM 2009 CMS Eur. Phys. J. 50 (2007) 117  +jet (Z+jet) cleaner means to determine fragmentation function

Fake jet contamination “Fake” jet rate estimation: Central Au+Au dataset (real data) Randomize azimuth of each charged particle and calorimeter tower Run jet finder Remove leading particle from each found jet Re-run jet finder “Fake” jets: signal in excess of background model from random association of uncorrelated soft particles (i.e. not due to hard scattering) STAR Preliminary 74Sevil Salur