1. 1/44 From High-Energy Heavy-Ion Collisions to Quark Matter Episode IV : Jet fragmentation in nuclear collisions Carlos Lourenço, CERN CERN, August, 2007

2. 2/44 The little bang according to the Scientific American QGP

3. 3/44 It is not easy to see inside the QGP Question: How can we probe the properties of the hot and dense matter system produced in high-energy nuclear collisions? Answer: Use hard probes, like heavy quarkonia (see previous lecture) or jets Questions: What is a jet? What is jet tomography? What is jet quenching? What is jet fragmentation? Why should we care? Can jet production be accurately calculated? With which ingredients? How well do we know them? Only in pp or also in nuclear collisions? Answers: See next slides... Warning: Do not talk about “jet fragmentation in nuclear collisions” if you are flying on a plane in the USA ! Seriously !!! The FBI will be waiting for you when the plane lands... (it happened to a physicist from RHIC)

4. 4/44 What is a jet? A “blast” of particles, all going in roughly the same direction Calorimeter View Tracking View

5. 5/44 “Tomography” in heavy-ion collisions: The absorption of the jets gives the 3-D density profile of the QCD medium produced in the nuclear collisions What is “jet tomography”? Reminder from first lecture

6. 6/44 What is jet quenching? The jets are initiated by high-energy (hard) quarks or gluons. If there is significant parton energy loss, by multiple gluon radiation, while traversing the dense matter  smaller jet yield (“jet quenching”) Modifications of the jet fragmentation function in nuclear collisions give properties of the medium High p T hadrons are mostly produced from jet fragmentation... Therefore, parton energy loss also results in the suppression of high p T hadron yields Gluons, light quarks and heavy quarks should suffer different levels of energy loss:  E loss (g) >  E loss (q) >  E loss (Q) (color factor) (mass effect) Suppression of high p T leading hadrons → seen at RHIC Disappearance of “away-side” jets → indirectly seen at RHIC Modified energy / particle flow within jet (fragmentation function) → not yet seen

7. 7/44 Can jet production be calculated in pQCD? Yes, through calculations of processes like this one: The data points seem to agree with the pQCD calculation, over 11 orders of magnitude! But... look again, at the high E T tail... on a linear scale, as (data-theory) / theory E T (GeV) ?  A clear indication of quark substructure (compositeness) ! Really ??? g g g g

8. 8/44 What are the protons made of? pQCD calculates partonic processes, like qq → qq, qg → qg, gg → gg But our beams are made of protons, antiprotons, neutrons... not of quarks and gluons! The probability that we find quarks, anti-quarks or gluons inside a proton depends on their fractional momenta and on the “resolution” of our probe: f(x,Q 2 ) gluons sea quarks valence quarks parton distribution functions, PDFs ?

9. 9/44 How do we know what the parton densities are? Parton Density Functions Hard Scatter Calculation Cross Section Calculation Measurement 5 experiments e-e- e-e- qq  DIS l+l+ l-l-  Drell-Yan

10. 10/44 Each class of experiments (DIS, Drell-Yan, etc) gets part of the story No single experiment measures the full picture of the proton (it’s like the tale of the blind men and the elephant) The results from each experiment go into a global fit Not all experiments agree – there is an art to “average” them together Two principal groups do this: → Martin, Roberts, Stirling and Thorne (MRST) → CTEQ (Coordinated Theoretical-Experimental project on QCD) What means MRST, CTEQ6M, etc? “Laws are like sausages. It’s better not to see them being made.” – Otto von Bismark PDFs

11. 11/44 Back to the CDF high-E T jet excess New sets of Parton Distribution Functions were calculated, including the CDF data The excess is gone! The quarks do not have substructure after all... Level of belief that this was “new physics” Graduate student doing analysis Postdoc Graduate student’s advisor Other faculty on experiment Spokesperson Average theorist Big-shot theorist

12. 12/44 PDFs pQCD D(z) PDFs D(z) Fragmentation into the final state hadrons We need to convolute the pQCD hard interaction with (initial state) parton densities and (final state) fragmentation functions, which define how the quarks and gluons hadronise. People operate particle detectors, not parton detectors... The PDFs and the fragmentation functions are (supposed to be) universal: they should be the same for all processes. hadrons partons

13. 13/44 Shadowing Anti-shadowing EKS 98 for Pb Back to the Parton Distribution Functions Is a free proton the same as a proton inside a nucleus? No! There are “nuclear effects” modifying the parton distribution functions. The probabilities of finding partons of given x change when the proton is inside a nucleus. The “EKS 98 model” provides the ratio between the PDFs in a proton of a nucleus of mass number A and in a free proton: “Shadowing” or “anti-shadowing”: decrease or increase of the parton’s density in the nucleus, in a certain kinematic range

14. 14/44 Is this relevant? This implies a ~20% higher charm production cross section in Pb-Pb collisions at the SPS and a ~40% lower value at the LHC, as compared to a linear extrapolation from pp collisions. Remarks: For a given collision energy and a given mass produced, the values of x depend on the rapidity range where the measurement is made. If the pp and Pb-Pb collisions are collected at different energies, the corrections for the nuclear effects are particularly tricky. We cannot directly compare heavy-ion and pp data. It has implications in the analysis of the SPS J/  suppression data.

15. 15/44 Jet quenching: setting the stage with pp collisions Azimuthal correlations show strong back-to-back peaks Azimuthal Angular Correlations

16. 16/44 Jet quenching: discovery mode with central Au-Au collisions Azimuthal correlations show that the “away-side jet” has disappeared... if we only detect high-p T particles, p T > 2 GeV/c Azimuthal Angular Correlations

17. 17/44 Jet quenching: where is the energy gone? The “away-side jet” energy is distributed over many soft particles, no longer collimated into jets, which are seen when the p T threshold is lowered to 200 MeV/c Azimuthal Angular Correlations

18. 18/44 Jet quenching: latest result with much higher statistics The availability of higher statistics reveals a very curious double-peak structure in the azimuthal distribution of the away-side peak, in central Au+Au collisions

19. 19/44 Jet quenching through the “nuclear modification factor” The nuclear modification factor, versus p T, shows a very significant suppression of high-p T hadrons in central Au+Au collisions, with respect to the extrapolation of pp measurements. No suppression is seen for photon production.

20. 20/44 From High-Energy Heavy-Ion Collisions to Quark Matter Episode V : Back to the future; nuclear collisions at the LHC SPS RHIC LHC

21. 21/44 AGS and SPS : 1986 – 1994 : low energy or light ions  properties of the hadronic phase SPS : 1994 – 2003 : Pb-Pb at  s = 20 GeV J/  and  ’ (and  c ?) suppression  deconfinement  compelling evidence for a “new state of matter” with “QGP-like properties” RHIC : 2000 – ?? : Au-Au at  s = 200 GeV parton energy loss (jet quenching) parton flow  compelling evidence for a strongly-coupled QGP (the “perfect fluid”) LHC : 2008 – ?? : Pb-Pb at  s = 5500 GeV heavy quarks (c,b), jets, upsilons, thermal photons precision spectroscopy  continue exploration and discovery of high-density QCD properties Heavy-ion running: from AGS to LHC energies

22. 22/44 Very large cross sections at the LHC Pb-Pb instant. luminosity: 10 27 cm -2 s -1 ∫ Lumi = 0.5 nb -1 (1 month, 50% run eff.) Hard cross sections: Pb-Pb = A 2 x pp  pp-equivalent ∫ Lumi = 20 pb -1  1 event limit at 0.05 pb (pp equiv.) jet Z 0 +jet  +jet  prompt h  /h   pp  s = 5.5 TeV 1 event J  1  b 1 nb 1 pb Hard Probes of QCD matter at LHC energies

23. 23/44 Three LHC experiments will collect Pb-Pb data ATLAS CMS ALICE CMSATLASALICE “Heavy-ion experiment”

24. 24/44 Solenoid magnet 0.5 T Central tracking system: ITS TPC TRD TOF Muon spectrometer: absorbers tracking stations trigger chambers dipole Specialized detectors: HMPID PHOS Forward detectors: PMD FMD, T0, V0, ZDC ALICE

25. 25/44 CMS (with HF, CASTOR, ZDC) + TOTEM: almost full η acceptance at the LHC !  charged tracks and muons: |η| < 2.5, full φ  electrons and photons: |η| < 3, full φ  jets, energy flow: |η| 8.3 for neutrals), full φ  excellent granularity and resolution  very powerful and flexible High-Level-Trigger CASTOR TOTEM HF  = -8 -6 -4 -2 0 2 4 6 8 ZD C 5.2 < |η| < 6.6 η > 8.3 neutrals T1/T2 CASTOR ZDC RP RP Phase space coverage of the CMS detector

26. 26/44 Si Tracker + ECAL + muon-chambers Si Tracker Silicon micro-strips and pixels Calorimeters ECAL PbWO 4 HCAL Plastic Sci/Steel sandwich Muon Barrel Drift Tube Chambers (DT) Resistive Plate Chambers (RPC) h ±, e ±, ,  ± measurement in the CMS barrel (|  | < 2.5)

27. 27/44 Charm cross section at the LHC is higher by a factor ~ 10 w.r.t. RHIC energies and by a factor ~ 1000 w.r.t. SPS energies:  s = 20 GeV   cc pp ~ 5  b  s = 200 GeV   cc pp ~ 600  b  s = 5.5 TeV   cc pp ~ 6600  b Abundance of charm production at the LHC will enable detailed studies of several topics, including charm thermalisation (through elliptic flow measurements) The detection of D and B mesons requires an accurate determination of the collision vertex and of the distance between the extrapolated charged tracks and the vertex, in the transverse plane and in the beam axis Typical impact parameters: a few 100  m for D decays and ~500  m for B mesons Including EKS98 shadowing Charm and beauty yields vs. energy and collision system

28. 28/44 Heavy flavour production at LHC energies Initial state effects: Nuclear shadowing suppresses low-p T heavy flavoured particles in p-A and A-A collisions: ~ 35% reduction of charm production and ~ 15% reduction of beauty (EKS98 dixit)  It must be studied in p-A collisions charm beauty Pb-Pb / pp  s = 5.5 TeV Heavy Quark energy loss: Parton energy loss is expected to occur by: medium-induced gluon radiation collisions in the medium It depends on the properties of the medium: length, energy density, etc. It is also expected depend on the colour factor and on the quark mass: We must probe heavy quark energy loss through ratios of p T distributions, between Pb-Pb and pp, between B and D mesons, etc We should also do these studies using jets tagged by the presence of D or B mesons  E g >  E c~q >  E b  R AA  < R AA D < R AA B  E (L,  QGP )

29. 29/44 Invariant mass analysis Reconstruction of D 0  K    decays in ALICE Large combinatorial background (dN ch / dy ~ 6000 in central Pb-Pb collisions) Main selection cuts: pair of opposite-charge tracks with large impact parameters good pointing of the reconstructed D 0 momentum to the primary vertex D0D0 simulation

30. 30/44 Quarkonia studies in ALICE with dimuons M  (GeV/c 2 ) J/  Y Y’, Y’’ After combinatorial background subtraction : simulation Rapidity window: 2.4–4.0 Resolution: 70 MeV at the J/  100 MeV at the Y

31. 31/44 Quarkonia studies in ALICE with electron-positron pairs simulation Combining the ITS, TPC and TRD data, available for |  | < 0.9, ALICE will have access to vertexing information for the electrons (but not for the muons, contrary to NA60 and CMS)

32. 32/44 prompt J/  J/   from B J/       Measuring beauty yields from displaced J/  production prompt J/  J/   from B vtr (  m) entries / event J/   e  e  A large fraction of the J/  mesons observed at the LHC come from decays of B mesons They can be separated from the “prompt” J/  mesons because they are produced away from the collision vertex simulation

33. 33/44 So far, only the dimuon decay channel has been considered. The physics performance has been evaluated with the 4 T field (2 T in return yoke) and requiring a good track in the muon chambers. The good momentum resolution results from the matching of the muon tracks to the tracks in the silicon tracker. Quarkonia studies in CMS

34. 34/44      dN ch /d  = 3500 Pb-Pb event simulated using the CMS software framework developed for pp Pb-Pb →  + X event in CMS

35. 35/44 barrel + endcaps barrel The material between the silicon tracker and the muon chambers (ECAL, HCAL, magnet’s iron) prevents hadrons from giving a muon tag but impose a minimum muon momentum of 3.5–4.0 GeV/c. This is no problem for the Upsilons, given their high mass, but sets a relatively high threshold on the p T of the detected J/  ’s. The low p T J/  acceptance is better at forward rapidities; total acceptance ~1%. The dimuon mass resolution is 35 MeV, in the full  region. barrel + endcaps p T (GeV/c) J/  Acceptance p T (GeV/c)  J/  →     : acceptances and mass resolutions in CMS simulation

36. 36/44 CMS has a very good acceptance for dimuons in the Upsilon mass region (21% total acceptance, barrel + endcaps) The dimuon mass resolution enables the separation of the three Upsilon states: ~ 54 MeV within the barrel and ~ 86 MeV when including the endcaps Barrel: both muons in |  | < 0.8 Barrel + endcaps: muons in |  | < 2.4 p T (GeV/c)  Acceptance  →     : acceptances and mass resolutions in CMS simulation

37. 37/44 J/   ● produced in 0.5 nb -1 ■ rec. if dN/d  ~ 2500 ○ rec. if dN/d  ~ 5000 Expected rec. quarkonia yields: J/  : ~ 180 000  : ~ 26 000  ’ : ~ 7 300;  ’’ : ~ 4 400  Good statistical accuracy of the expected  ’ /  ratio versus p T Pb-Pb Similar low p T yields for J/  and  p T reach of CMS quarkonia measurements (for 0.5 nb -1 ) simulation with HLT

38. 38/44 E T reach x2 x35 jets Pb-Pb at 5.5 TeV design luminosity CMS High Level Trigger: 12 000 CPUs of 1.8 GHz ~ 50 Tflops ! Processes full events with fast versions of the offline algorithms pp L1 trigger rate (design L): 100 kHz Pb-Pb collision rate: less than 8 kHz  pp L1 trigger rate  Pb-Pb collision rate  the HLT can process all Pb-Pb events Pb-Pb event size: ~2.5 MB (up to ~9 MB) Data storage bandwidth: 225 MB/s  10–100 Pb-Pb events/s HLT reduction factor: 3000 Hz → 100 Hz Average HLT time budget per event: ~10 s The event samples of hard processes are statistically enhanced by very large factors The CMS High Level Trigger

39. 39/44 Nuclear modification factor = AA-yield / pp-yield = “QCD medium” / “QCD vacuum” Pb-Pb 0.5 nb -1 Important measurement to compare with parton energy loss models and derive the initial parton density, dN g /dy, and the medium “transport coefficient” HLT Impact of the CMS HLT on the p T reach of R AA simulation

40. 40/44 Iterative cone method plus background subtraction Jet spatial resolution in pseudo-rapidity and in azimuth: better than 3% for E T > 100 GeV   ○ without background ■ central Pb-Pb 100 GeV jet on a Pb-Pb event, after Bg subtr. Jet reconstruction and spatial resolution in CMS

41. 41/44 The E T resolution is ~10% in pp and ~15% for Pb-Pb collisions Jet reconstruction efficiency, purity and E T resolution in CMS

42. 42/44 min. bias HLT Jet spectra up to E T ~ 500 GeV (Pb-Pb, 0.5 nb -1, HLT-triggered)  Detailed studies of medium-modified (quenched) jet fragmentation functions Gluon radiation: large angle (out-of-cone) vs. small angle emission Jet E T reach and fragmentation functions in CMS simulation

43. 43/44 dimuon trigger associated hadrons g  * away side Unique possibility to calibrate jet energy loss (and FF) with back-to-back gauge bosons (large cross sections and excellent detection capabilities). Heavy quark dimuon (dominant) background can be rejected by a secondary vertex cut. Resolutions: 50  m in radius and 20  m in  Z 0 +jet ,  and Z tagging of jet production in CMS simulation Z0Z0

44. 44/44 Before the measurements are made, theorists often tell us that the interpretation of the data will be easy However, theorists are often wrong... especially before the measurements are made... The good news is that now, based on the knowledge gained at the SPS and at RHIC, we have clear directions concerning which way to go at the LHC... We will find the way out... Lessons from the SPS and RHIC to the LHC I hope that some of you found these lectures a lively beginning and not a dead end...