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Jet Analysis in Heavy-Ion Collisions

Jet Analysis in Heavy-Ion Collisions. Elena Bruna INFN Torino & Yale University. 5 th International School on QGP. Torino, March 2011.

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Jet Analysis in Heavy-Ion Collisions

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  1. Jet Analysis in Heavy-Ion Collisions Elena Bruna INFN Torino & Yale University 5th International School on QGP. Torino, March 2011

  2. “The same thrill, the same awe and mystery, come again and again when we look at any problem deeply enough. With more knowledge comes deeper, more wonderful mystery, luring one on to penetrate deeper still. With pleasure and confidence we turn over each stone to find unimagined strangeness.” R. Feynman These (experimental) lectures won’t probably tell you everything you would ever wanted to know about jets…but I hope some of the young minds will be inspired and start/continue working on hard probes to turn over more stones… 5th International School on QGP. Torino, March 2011

  3. Past, Present, Future… Pb+Pb @ √sNN=2.76 TeV LHC RHIC LEP e+e- • Full jet reconstruction  energy of the hard scattering but challenging in A+A • New jet-finding tools • Dophysics with jets ! Elena Bruna (Yale&INFN Torino)

  4. Jet I: Intro & Motivations Outline Jet II: Full Jet Reconstruction Jet III: Results Jet IV: The Present: from RHIC to LHC Elena Bruna (Yale&INFN Torino)

  5. Jet I: Intro & Motivations

  6. Jets in high-energy collisions pQCD Factorization: Collins, Soper, Sterman Nucl. Phys. B263 (1986) 37 PDF Partonic x-section Fragmentation function Factorization: assumed between the perturbative hard part and the universal non-perturbative fragmentation (FF) and parton distribution functions (PDF) Universality: fragmentation functions and parton distribution functions are universal (i.e. FF from ee, PDF from ep, use for pp) hadrons c, xc a, xa b, xb p p σab d, xd hadrons Elena Bruna (Yale&INFN Torino)

  7. Jets in high-energy collisions pQCD Factorization: Collins, Soper, Sterman Nucl. Phys. B263 (1986) 37 p + p  p0 PDF Partonic x-section Fragmentation function QCD factorization works! p + p p p + pp p+p √s=200 GeV p+p √s=200 GeV Elena Bruna (Yale&INFN Torino)

  8. Jets in high-energy collisions pQCD Factorization: Collins, Soper, Sterman Nucl. Phys. B263 (1986) 37 PDF Partonic x-section Fragmentation function hadrons c, xc a, xa b, xb p p σab d, xd hadrons PDFs: Probability for a parton a(b) to carry a fraction xa(xb) of the hadron momentum Universal can be measured with fit to experimantal data for one or more processes that can be calculated with perturbative QCD, i.e. deep inelastic scattering DIS (like e-p), Drell-Yan processes (qq  l+l-) and others Many PDFs on the market (CTEQ, GRV, MRST,…) Elena Bruna (Yale&INFN Torino)

  9. Jets in high-energy collisions pQCD Factorization: Collins, Soper, Sterman Nucl. Phys. B263 (1986) 37 PDF Partonic x-section Fragmentation function Hard scattering: dσ/dt = parton cross section calculable in powers of αS LO NLO Elena Bruna (Yale&INFN Torino)

  10. Jets in high-energy collisions pQCD Factorization: Collins, Soper, Sterman Nucl. Phys. B263 (1986) 37 PDF Partonic x-section Fragmentation function z = Fragmentation Functions: probability to find, at scale Q, a hadron h with a fraction z of the parton c momentum universal and measured with fits to experimental data Many D on the market (KKP, AKK, …) hadrons parton p(hadron) p (parton) z Elena Bruna (Yale&INFN Torino)

  11. Jets in Nucleus-Nucleus collisions Self-generated “hard” probes Detector Jet Tomography! Hard processes make perturbative QCD applicable  high momentum transfer Q2 Hard processes scale as Nbin Calibrated LASER/x-ray Elena Bruna (Yale&INFN Torino)

  12. Jets in Nucleus-Nucleus collisions • Questions: • How does the parton lose energy?2) What happens to the radiated energy? • 3) Collisional energy loss? • 4) Does the energy loss depend on the parton type? jet energy loss in the medium Interpretation: Gluon radiation DEloss ~ ρgluon (gluon density) DEloss ~ ΔL2 (medium length) [~ ΔLwith expansion] DEgluon > DEquark, m=0 > DEquark, m>0  Important to measure DE of gluons  light  heavy quarks… Transport coefficient: q = m2 / Lis the <pT2> transferred from the parton to a gluon per unit path length ^ Elena Bruna (Yale&INFN Torino)

  13. Jets in Nucleus-Nucleus collisions Eskola, Honkanen, Salgado, Wiedemann Nucl Phys A747 (2005) 511 ^ q = 5 – 15 GeV2 / fm from RHIC RAA Data Elena Bruna (Yale&INFN Torino)

  14. Borghini and Wiedemann, hep-ph/0506218 Some Predictions: FF Gyulassy et al., nucl-th/0302077 Renk, Phys. Rev. C79:054906,2009 ph z=ph/pjet pjet Energy loss in the medium  softer fragmentation ξ stretches the low z part Elena Bruna (Yale&INFN Torino)

  15. Some Predictions: Jet shapes If energy loss by gluon radiation  broadening of the jet energy profile R = jet radius (on η-ϕ plane) =√(Δϕ2+Δη2) ωmin (pTmin) = minimum pT on particles in the jet Energy loss ratio goes down with larger b. Energy loss ratio becomes smaller with smaller R and larger ωmin. Limit of large R and ωmin=0  no out-of-cone energy ΔEin~E I Vitev, S Wicks, B-W Zhang, JHEP 0811,093 (2008); EPJC 62, 139 (2009). Elena Bruna (Yale&INFN Torino)

  16. Some Predictions: Jet shapes I Vitev, S Wicks, B-W Zhang, JHEP 0811,093 (2008); EPJC 62, 139 (2009). Vitev, Zhang, PRL 104 (2010) 132001, arXiv: 0910.1090 • Limits: • small Rmax and large ωmin  single particle • suppression. • large Rmax and small ωmin  all jet energy recovered  RAAjet=1 ! (jet production is hard process, scales as Nbin) Elena Bruna (Yale&INFN Torino)

  17. Some Predictions: Jet shapes I Vitev, S Wicks, B-W Zhang, JHEP 0811,093 (2008); EPJC 62, 139 (2009). Vitev, Zhang, PRL 104 (2010) 132001, arXiv: 0910.1090 • Limits: • small Rmax and large ωmin  single particle • suppression. • large Rmax and small ωmin  all jet energy recovered  RAAjet=1 ! (jet production is hard process, scales as Nbin) Elena Bruna (Yale&INFN Torino)

  18. Jet quenching from single high-pT hadrons • Observations at RHIC: • Large suppression of high-pT hadrons: factor ~ 5 • Photons are not suppressed • They don’t interact with the medium (good!) • Nbin scaling works Elena Bruna (Yale&INFN Torino)

  19. Jet quenching from single high-pT hadrons • Observations at RHIC: • Large suppression of high-pT hadrons: factor ~ 5 • Photons are not suppressed • They don’t interact with the medium (good!) • Nbin scaling works • Also Heavy Flavor is suppressed at RHIC • same as light quarks • role of bottom? • collisional energy loss/resonant elastic scattering? Elena Bruna (Yale&INFN Torino)

  20. Jet quenching from single high-pT hadrons ALICE, Phys. Lett. B 696 (2011) 30. RHIC suppression < LHC RHIC: high pT hadrons hadronize from quarks LHC: from gluons (larger color charge!) • Prediction: Vitev(hep-ph/050322v1) • GLV – pQCD factorization • medium-induced gluon brems. Elena Bruna (Yale&INFN Torino)

  21. Jet quenching from di-hadrons Start from a high-pt “trigger” particle and look on the away side (in f). Azimuthal correlation function shows ~complete absence of “away-side” jet Partner in hard scatter is absorbed in the dense medium not the case in d+Au  final state effect Elena Bruna (Yale&INFN Torino)

  22. Jet quenching from di-hadrons y x Start from a high-pt “trigger” particle make azimuthal correlation ~complete absence of “away-side” jet Partner in hard scatter is strongly interacting with the dense medium not the case in d+Au  final state effect ! Path-length dependence of di-jet topologies Out-of-plane in-plane Back-to-back suppression out-of-plane stronger than in-plane Elena Bruna (Yale&INFN Torino)

  23. Jet quenching from di-hadrons increasing pTtrig increasing pTassoc STAR, Phys.Rev.C82 024912 (2010) Elena Bruna (Yale&INFN Torino)

  24. Jet quenching from di-hadrons increasing pTtrig • At low trigger pT & low pTassoc: • double bump: • Mach Cone – conical emission? • Cherenkov Radiation? • pure 3D hydro? • [won’t discuss this] increasing pTassoc • At high trigger pT: • re-emergence of away-side jet (punch thru)? • or • tangential jets? STAR, Phys.Rev.C82 024912 (2010) Elena Bruna (Yale&INFN Torino)

  25. Trigger and Surface Biases Trigger particles biased toward the surface Surface bias, as seen in hydro models Experiments online-trigger dependent: Large pT or energy deposition triggers  bias towards hard fragmentation! EM calorimetry  bias towards large EM fraction Elena Bruna (Yale&INFN Torino)

  26. High pT: towards jets ALICE, Phys. Lett. B 696 (2011) 30. • What we have so far: • Suppression of high-pT hadrons in A+A (at RHIC and LHC) w.r.t. p+p • Evidence for parton energy loss in the medium • But: • Geometrical bias: dominated by surface jets • Jet energy not constrained • Limited kinematic reach • What we want: • Precise measurement of the parton energy loss • Measurement of the modified fragmentation function • How? Renk and Eskola, hep-ph/0610059 Elena Bruna (Yale&INFN Torino)

  27. High pT: towards jets How? • g-Jet • Jet energy well constrained • limited kinematic reach (x-sec scales as αSαem) • Difficult to have a clean measurement of photons z = p(h)/pparton Di-Hadron Eγ = pparton p≠ pparton Leading Hadron Courtesy Thomas Ullrich Elena Bruna (Yale&INFN Torino)

  28. Direct g-hadron fragmentation functions STAR, Phys. Rev. C 82 (2010) 34909 Good agreement w/ theory models more assoc h± for p0than for g  different parton energies for p0 and g (p0 come from fragmentation of higher energy parton) Au+Au: different path-length for the recoil jet for p0 and g and triggers Trig particle= g or p0 Assoc particle: h± Elena Bruna (Yale&INFN Torino)

  29. Direct g-hadron fragmentation functions IAA= ratio of associated yield per trigger in Au+Au to that in p+p Trig particle= g or p0 Assoc particle: h± 8<Etrig<16 GeV/c IAA < 1 for zT>0.3 data can distinguish between different theoretical models low zT: expected differences between p0 and g IAA due to path-length dependence of the energy loss Measurements do not indicate path-length or color-charge dependence ! Elena Bruna (Yale&INFN Torino)

  30. High pT: towards jets How? • g-Jet • Jet energy well constrained • limited kinematic reach (x-sec scales as αSαem) • Difficult to have a clean measurement of photons • g-Jet • Jet energy well constrained • limited kinematic reach (x-sec scales as αSαem) • Difficult to have a clean measurement of photons • Full Jet Reconstruction • Larger kinematic reach • large background  complex and challenging ! z = p(h)/pparton Ejet = pparton Eγ = pparton Courtesy Thomas Ullrich Elena Bruna (Yale&INFN Torino)

  31. Jet II: Full Jet Reconstruction Elena Bruna (Yale&INFN Torino)

  32. Jets: Theory vs Experiment Theory (pQCD): jet = High-pT parton produced in hard scatterings, or the closest object to a parton • Experiment: jet = spray of collimated hadrons • GOAL: measure the parton energy in experiments  do jet physics! • Tool: Full jet reconstruction with jet-finding algorithms • for both Theory and Experiment ! Elena Bruna (Yale&INFN Torino)

  33.         Theoretical requirements pT pT cone iteration Jet 1 Jet 1 Jet 2 y y • Collinear safety • replaces one parton by two at the same place • the algorithm should be insensitive to any collinear radiation. • Infrared safety •  a soft emissions that add very soft gluon •  the jet-finding algorithm should not be sensitive to soft radiation Elena Bruna (Yale&INFN Torino)

  34. Experimental requirements CDF • Detector independence: the performance of the jet algorithm should not be dependent on detector segmentation, energy resolution, … • Stability with luminosity: jet finding should not be strongly affected by multiple hard scatterings at high beam luminosities. • Fast • Efficient: the jet algorithm should find as many physically interesting jets as possible, with good energy resolution Elena Bruna (Yale&INFN Torino)

  35. Jet Finding algorithms Review of CDF Jet Algorithms, arXiv:hep-ex/0005012v2 FastJet JHEP 0804:005, arXiv:0802.1188 FastJet JHEP 0804, 063 (2008), arXiv:0802.1189v2 • Particles are combined into jets • the larger experimental coverage, the better • Which particles? The measured ones: • charged tracks (TPC) • neutral towers(EMC) • charged energy (Hcal) • Different ways of combining particles  jet-finding algorithms Elena Bruna (Yale&INFN Torino)

  36. Sequential Recombination kTi,j= particle transverse momentum (pT) kT: p>0 (soft particles merged first) Anti-kT: p<0 (hard particles merged first) R=resolution parameter Example: Anti-kT Blue = highest pT particle 2π If dij<kTi-2  merged ϕ If dij>kTi-2  not merged  call it a jet 0 -1 η +1 Elena Bruna (Yale&INFN Torino)

  37. kT vs anti-kT FastJet M. Cacciari, G. Salam, G. Soyez 0802.1188 • ALL particles are clustered into “jets” • kT not bound to a circular structure • Anti-kT circular shape, “cone” radius ~R parameter • Expected to be less sensitive to background/“back reaction” (it starts from high-pT particles)  ideal choice in heavy-ion collision • Recombination algorithms are collinear and infrared safe Elena Bruna (Yale&INFN Torino)

  38. R matters! Elena Bruna (Yale&INFN Torino)

  39. R matters! Elena Bruna (Yale&INFN Torino)

  40. R matters! Elena Bruna (Yale&INFN Torino)

  41. R matters! Elena Bruna (Yale&INFN Torino)

  42. R matters! p+p 200 GeV STAR Preliminary In pp: ~80% of jet energy within R=0.4 for 20 GeV jets The choice of R depends on • The system we are looking at (e+e-, pp, AuAu, PbPb,…) • Tradeoff: don’t want to loose too much out-of-cone radiation (corrections for hadronization become difficult) but want to have a small background in the jet area Elena Bruna (Yale&INFN Torino)

  43. Jets in Heavy-Ion Collisions at RHIC and LHC Central Au+Au √sNN=200 GeV Central Pb+Pb√sNN=2.76 TeV ALICE tracking data STAR preliminary STAR EMC + tracking data ETjet ~ 21 GeV • Why measure jets in heavy ion collisions? [inclusive, di-jets, jet-hadron, g-jet,..] • Access kinematics of the binary hard-scattering • Characterize the parton energy loss in the hot QCD medium • modified fragmentation, energy flow within jets, quark vs gluon jet difference • flavor and mass dependence • Study medium response to parton energy loss – establish properties of the medium Elena Bruna (Yale&INFN Torino)

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