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Ridges and Jets at RHIC and LHC

Ridges and Jets at RHIC and LHC. Rudolph C. Hwa University of Oregon. Quantifying Hot QCD Matter INT UW June, 2010. Key point of this talk:. Late-time physics can affect our assumption about the nature of early-time physics. What is common between RHIC and LHC:.

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Ridges and Jets at RHIC and LHC

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  1. Ridges and Jets at RHIC and LHC Rudolph C. Hwa University of Oregon Quantifying Hot QCD Matter INT UW June, 2010

  2. Key point of this talk: Late-time physics can affect our assumption about the nature of early-time physics What is common between RHIC and LHC: Partons have to hadronize at the end when density is low, no matter what the initial state may be. Universal approach: parton recombination at all pT at any initial energy What is different: Which partons recombine? Jet-jet reco at LHC. Have to understand RHIC data well, before projecting to LHC.

  3. intermediate ReCo TT TS SS Hadronization Cooper-Frye k1+k2=pT lower ki higher density Introduction Usual domains in pT at RHIC low high pT 2 6 Hydro pQCD GeV/c Fragmentation kT > pT

  4. (fm/c) 1 8 0.6 hadronization rapid thermalization hydro Initial state scattering occurs even earlier. Cronin effect in pA is larger for proton than for ; it implies final-state effect (in ReCo),not hard-scattering+frag, not hydro. Regions in time An example of late-time physics affecting thinking about early-time physics: Cronin effect: --- initial-state or final-state effect? Early-time physics: CGC, P violation, … Pay nearly no attention to hadronization at late times.

  5. PHOBOS PRL 91, 052303 (03) Before understanding that, we should understand single-particle distribution, summed over all charged and integrated over all pT Simpler scenario BRAHMS hasdN/dyat fixed =0.4 GeV/c Large  structure of Ridge --- PHOBOS PHOBOS, PRL104,062301(10)  ~ 4, pTtrig>2.5GeV/c Referred to as “long-range” correlation on the near side

  6. BRAHMS PLB 684,22(10) PHOBOS all charged BRAHMS  only How is this difference to be understood? How much does it contribute to the  distribution? y is commonly identified with  Proton contribution should not be ignored.

  7. Dusling, Gelis, Lappi, Venugopalan arXiv: 0911.2720 Early-time physics: CGC how one goes from initial-state to final state in one step

  8. Ridge without detailed input on early-time physics

  9. Topics to be covered: Hadronization Ridges with or without trigger Jets First, we need to understand single-particle distribution in pT, , Npart, and  ---- before correlation.

  10. Pion at y=0 Recombination function q and qbar momenta, k1, k2, add to give pion pT TT Protonat y=0 TTT same T for partons, , p phase space factor in RF for proton formation Hadron production at low pT in the recombination model It doesn’t work with transverse rapidity yt At low pT empirical evidence

  11. p Same T for , K, p --- in support of recombination. Proton production from recombination PHENIX, PRC 69, 034909 (04) went on to mT plot Slight dependence on centrality --- to revisit later

  12. associated particles Suarez QM08 SS trigger ST peak (J) TT ridge (R)  Mesons:  Baryons: TTT in the ridge B/M in ridge even higher than in inclusive distr. Ridge formation

  13. 6 5 4 3 2 1 Out-of-plane In-plane Feng, QM08 Jet and Ridge Yield STAR Preliminary jet part, near-side ridge part, near-side jet part, near-side ridge part, near-side 20-60% top 5% s 3<pTtrig<4, 1.5<pTassoc<2.0 GeV/c Different s dependencies for different centralities --- important clues on the properties of correlation and geometry

  14. Hard parton directed ats , loses energy along the way, and enhances thermal partons in the vicinity of the path. The medium expands during the successive soft emission process, and carries the enhanced thermal partons along the flow. Flow direction  normal to the surface Reinforcement of emission effect leads to a cone that forms the ridge around the flow direction . s  But parton directionsandflow direction are not necessarily the same. s  If not, then the effect of soft emission is spread out over a range of surface area, thus the ridge formation is weakened. Correlation betweens and

  15. CEM  s Correlated emission model (CEM) Chiu-Hwa, PRC 79, 034901 (09) STAR Feng QM08 3<pTtrig <4 1.5 <pTassoc <2 GeV/c

  16. Region where hydro claims relevance --- requires rapid thermalization 0 = 0.6 fm/c Semi-hard scattering 1<kT<3 GeV/c Copiously produced, but not reliably calculated in pQCD t < 0.1 fm/c That was Ridge associated with a trigger Single-particle distribution at low pT(<2 GeV/c) Something else happens even more rapidly 1. If they occur deep in the interior, they get absorbed and become a part of the bulk. 2. If they occur near the surface, they can get out. --- and they are pervasive.

  17. Base, independent of , not hydro bulk Ridge, dependent on , hadrons formed by TT reco Correlated part of two-particle distribution on the near side ? Putschke, Feng (STAR) Wenger (PHOBOS) trigger assoc part JET RIDGE Ridge can be associated with a semihard parton without a trigger. How is this untriggered ridge related to the triggered ridge on the near side of correlation measurement?

  18. Two events: parton 1 is undetected thermal partons 2 lead to detected hadrons with the same 2 1 2 2 1 If events are selected by trigger(e.g. Putschke QM06, Feng QM08), the ridge yield is integrated over all associated particles 2. untriggered ridge triggered ridge yield Ridge is present whether or not 1 leads to a trigger. Semihard partons drive the azimuthal asymmetry with a  dependence that can be calculated from geometry. (next slide)

  19. For every hadron normalto the surface there is a limited line segment on the surface around 2through which the semihard parton 1can be emitted. 2   elliptical integral of the second kind Ridge due to enhanced thermal partons near the surface Top view: segment narrower at higher b Side view: ellipse (larger b) flatter than circle (b=0) around =0. R(pT,,b)  S(,b) b normalized to RA nuclear density D(b) Hwa-Zhu, PRC 81, 034904 (2010) Geometrical consideration for untriggered Ridge

  20. After average over , Compare with data that show exponential behavior r(pT,b) can be determined;  dependence comes only from S(,b); v2 can be calculated. Single-particle distribution at low pT with Ridge

  21. Feng QM08 Normalization adjusted to fit, since yield depends on exp’tal cuts Normalization is not readjusted. s dependence is calculated Ridge yield with trigger at 1 S(,b) correctly describes the  dependence of correlation

  22. art Summary Ridge R(pT,,b) v2(pT,b)=<cos 2 > yield YR() RAA(pT,,b)  dependencies in are all inter-related --- for pT<2 GeV/c RAA(pT, , b) can be calculated with the  dependence arising entirely from the ridge. Hwa-Zhu, PRC 81, 034904 (2010)

  23. pT  Npart Jets PHENIX 0903.4886 Dependence on  and Npart pT>2 GeV/c Need some organizational simplification. Clearly,  and b are related by geometry.

  24. Nuclear medium that hard parton traverses  Geometrical path length k D(x(t),y(t)) x0,y0 density (Glauber) Dynamical path length Average dynamical path length  to be determined Probability of hard parton creation at x0,y0 Geometrical considerations

  25. centrality looks universal, except for c=0.05 (no  dep at c=0) Define It contains all the information on the relationship between  and b. It suggests that P(,,c) may depend on fewer variables.

  26. we can calculate • PHENIX data gives For every pair of  and c:  We can plot the exp’tal data Define KNO scaling

  27. 5 centralities and 6 azimuthal angles () in one universal curve for each pT Lines are results of calculation in RM. Hwa-Yang, PRC 81, 024908 (2010) • details in geometry • dynamical effect of medium • hadronization Complications to take into account: Scaling behavior in 

  28. hadronization geometrical factors due to medium q  probability of hard parton creation with momentum k k degradation b Nuclear modification factor only adjustable parameter  TS+SS recombination

  29.  is dimensionless Result of calculation in terms of 

  30. At LHC, the densities of hard partons is high. At kT not too large, adjacent jets can be so close that shower partons from two parallel jets can recombine. Two hard partons Two-jet recombination at LHC

  31. Overlap of two jet cones  - probability for overlap of two shower partons from adjacent jets same jet 1 =10-m, m=1, 2, 3 different jets =10-3: 1-jet (S1S’1) =10-1: 2-jet (S1S2) Recombination of two shower partons from two jets

  32. , b are the same for the two jets, but  and ’ are independent  For given , b there is only one (,b) KNO scaling implies ’ Inclusive distribution Go back to

  33. Hwa-Yang, PRC 81, 024908 (2010) 1 jet >1 ! 2 jet Scaling Scaling badly broken modest increase at 50-60% for 1-jet for 2 jets scales The admixture of ruins the scaling behavior. Pion production at LHC Observation of large RAA at pT~10 GeV/c will be a clear signature of 2-jet recombination.

  34. pT~10 GeV/c pT~10 GeV/c  p gluon kT~20 GeV/c (1-j fragmentation) k1 k’i pT=k’1+k’2 (2-j recombination) k2 gluon kT>20 GeV/c (1-j fragmentation) k1 k’i pT=k’1+k’2 +k’3 (2-j recombination) k2 Recombination (2 jets) vs fragmentation (1 jet) more probable even more probable If pT>20 GeV/c, 2-j requires higher ki, whose density is lower; thus smaller  reduces probability of recombination.

  35. Production rates of p and  are separately reduced, as pT is increased, but the p/ ratio is still >1 even up to pT~20 GeV/c 5-20 Hwa-Yang, PRL97,042301 (2006)

  36. If 2-jet dominates single-particle inclusive at pT~10 GeV/c, then there are many such hadrons ( and p) at that pT at all . If higher pTtrig ( > 30 GeV/c), then 1-jet dominates, and ridge is not expected (from RHIC). Ridge Using trigger at pTtrig ~ 10 GeV/c to find ridge would involve subtraction of a huge background. It probably will be hard to find detectable ridge at LHC.  ~ 4 correlation at RHIC

  37. 1/Ntrig dNch/d Jet peak TS reco

  38. Single-particle distribution factorizble Longitudinal: TransVerse: similarly for h=p BRAHMS, PRL 94,162301(05) <pT> essentially independent of y

  39. Chiu-Hwa (preliminary) Two-particle distribution ridge Ridge distribution per trigger correlation in transverse component --- ridge no correlation in 

  40. 1/Ntrig dNch/d PHOBOS  1(trig) Correlation is in the transverse component, (ridge being TT+TTT reco) with negligible correlation between trigger 1 and associated 2 map 1(2) to dN/d: PHOBOS Where is the long-range correlation that requires early-time physics?

  41. Conclusion Hadronization and initial geometry are important to understanding RHIC and LHC physics pT<2GeV/c semihard partons  ridge (TT reco)  dependence pT>2GeV/c (RHIC): TS+SS reco  scaling pT~10GeV/c (LHC): 2j-SS reco  scaling broken Probably no ridge at higher pTtrig and pTassoc at LHC. 1 and dN/d are related with no need for long-range correlation between (trig) and (ridge).

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