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Unresolved issues in hard- scattering at RHIC

Unresolved issues in hard- scattering at RHIC. M. J. Tannenbaum Brookhaven National Laboratory Upton, NY 11973 USA. Early Time Dynamics in Heavy Ion Collisions McGill University Montreal, Canada July 16-19, 2007.

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Unresolved issues in hard- scattering at RHIC

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  1. Unresolved issues in hard- scattering at RHIC M. J. Tannenbaum Brookhaven National Laboratory Upton, NY 11973 USA Early Time Dynamics in Heavy Ion Collisions McGill University Montreal, Canada July 16-19, 2007 ETD-HIC2007

  2. As the expected energy in a typical jet cone is  R2 x1/ 2 x dET/d= R2/2 x dET/d ~ 300 GeV for R=1 at sNN=200 GeV where the maximum Jet energy is 100 GeV, Jets can not be reconstructed in Au+Au central collisions at RHIC. BDMPS 1997-1998 In 1998 at the QCD workshop in Paris, Rolf Baier asked me whether jets could be measured in Au+Au collisions because he had a prediction of a QCD medium-effect on colored partons in a hot-dense-medium with lots of unscreened color charge. But hard-scattering can be well studied by single inclusive and 2-particle correlation measurements as it was discovered at the CERN ISR in the 1970’s: “Everything you want to know about JETS can be measured with two particle correlations.” And it just so happened that the PHENIX detector was designed to trigger, measure and separate  and 0 out to pT> 25 GeV/c ! ETD-HIC2007

  3. Corrected for acceptance N A Ratio of pTa to pTt of away side to trigger particles kT smearing Ratio of jet transverse momenta “Everything” was correct except that in p-p correlations the away side xE distribution does not measure the fragmentation function* It measures the ratio of pT of away jet to trigger jet * contradicts Feynman, Field and Fox, Nucl. Phys. B128(1977)1-65 trigger (pTt) away (pTa) n is the power of invariant pTt spectrum PHENIX PRD74, 072002 (2006) ETD-HIC2007

  4. 19% norm uncertainty p-p collisions at RHIC: 0 production (PHENIX) arXiv:0704.3599v1 [hep-ex] No surprise (to me) that NLO pQCD agrees with data ETD-HIC2007

  5. p-p CCOR A.L.S. Angelis, et al, Phys.Lett. 79B, 505 (1978) ISR 0 vs RHIC p-p  RHIC pp vs AuAu 0 are suppressed in Au+Au eg 200 GeV Nuclear Modification Factor 0 invariant cross section in p-p at s=200 GeV is a pure power law for pT > 3 GeV/c, n=8.100.05. New 62.4 GeV measurement agrees with CCOR shifted up in pT by 3.3% ETD-HIC2007

  6. PHENIX PRL 91(2003) 172301 p/ ratio much larger than from jet fragmentation: The Baryon Anomaly-still not understood ps: If this is ‘recombination’  QGP: Fries,Muller, Nonaka PRL 90 202303 (2003) THE major discoveries at RHIC Discovery-PHENIX PRL 88 (2002) 022301 1) 0 suppressed by a factor of 5 compared to point-like scaling for 3< pT<20 GeV/c 2)h and 0 behave differently at intermediate pT 2--6 GeV/c latest preprint nucl-ex/0611007 ETD-HIC2007

  7. Reaction Plane z y x py px Anisotropic (Elliptic) Transverse Flow--an Interesting complication in AA collisions • spatial anisotropy momentum anisotropy • Perform a Fourier decomposition of the momentum space particle distributions in the x-y plane • v2 is the 2nd harmonic Fourier coefficient Directed flow zero at midrapidity Elliptical flow dominant at midrapidity ETD-HIC2007

  8. Universal Scaling D.Teaney, PRC68, 034913 (2003) Any viscosity damps v2 fpr pT> 1.7 GeV/c Don’t forget large asimuthal isotropy is it a perfect liquid? Nice plots from Arkadij Taranenko-PHENIX-QM2006 ETD-HIC2007

  9. MJT Status of RAA in AuAu at sNN=200 GeV QM05 Direct  are not suppressed. 0 and  suppressed even at high pT Implies a strong medium effect (energy loss) since  not affected. Suppression is flat at high pT. Are data flatter than theory? ETD-HIC2007

  10. = 2(1) 2 1 Comparison with theory including systematic error Model values from C.Loizides hep-ph/0608133v2 Presented at QM2006. Journal article to be submitted soon. ETD-HIC2007

  11. QM05-06: Direct e in Au+Au indicate a theoretical crisis Au+Au p-p p-p beautiful agreement of e with c b production PHENIX PRL97(2006)252002 Au+Au PHENIX PRL 98 (2007)172301  heavy quarks suppressed the same as light quarks, and they flow, but less.  This disfavors the energy loss by gluon bremsstrahlung in medium hypothesis  but brings string theorists into the game. ETD-HIC2007

  12. L RAA0 in AuAu sNN=200 GeV vs. Reaction Plane to probe details of the theory-learn something new! PHENIX nucl-ex/0611007 (submitted to Phys. Rev. C.) L = distance from edge to center of participants calculated in Glauber model Little/no energy loss for Le< 2 fm 3 < pT < 5 GeV/c RAA is absolute, v2 is relative so no hint of this in v2 measurements. This result also suggests that v2 for pT>2GeV/c is due to anisotropic energy loss not flow. RAA() vs. centrality varies density of and distance through medium ETD-HIC2007

  13. Direct  with respect to the reaction plane Turbide, Gale & Fries, PRL 96 (2006) 032303 predict that if jet(parton) suppression is due to g+q-->g+q (+g) in the medium then the reaction g+q--> +q should create a source of direct photons proportional to the distance traversed through the medium-fewer on the mid-plane more vertical, the opposite of 0 and other hadronic jet fragments PHENIX preliminary Not seen to within errors. Same sign of v2 for  and 0 more likely from present data. ETD-HIC2007

  14. “We see how Carlo Rubbia wheels and deals in order to raise multimillion-dollar funding for his work, and to publish his results first so as to maintain his dominance in the field and secure the Nobel Prize. We also witness Rubbia’s anxious futile attempts, once he has won the Nobel Prize, to prove a highly speculative new theory called supersymmetry”. CGC?‘Monojets’ in d+Au? ETD-HIC2007

  15. Brahms RdAu, RCP in d+Au vs rapidity BIG effect in RCP; some tendency in RdAu. BRAHMS PRL93 242303 ETD-HIC2007

  16. away trigger d+Au is it CGC?`Monojets’?  correlations widths and conditional yields the same for triggers in all 3 spectrometers for pp and dAu PHENIX PRL 96 (2006)222301 ETD-HIC2007

  17. STAR Fixed h, as E & pT grows STAR-Correlations in d+Au PRL 97, 152302 (2006) • are suppressed at small <xF> and <pT,p> • Spp-SdAu= (9.0 ± 1.5) % • consistent with CGC picture • are consistent in d+Au and p+p at larger <xF> and <pT,p> • as expected by HIJING STAR Preliminary STAR Preliminary 25<Ep<35GeV STAR Preliminary STAR Preliminary 35<Ep<45GeV Statistical errors only ETD-HIC2007

  18. Correlations-one of the first definitive results PHENIX PRC 71 051902 2.4<pTt<4 GeV/c 1.7< pTa<2.5 GeV/c Trigger mesons and baryons in the region of the baryon anomaly both show the same trigger (near) side and away side jet structure. This ‘kills’ the elegant recombination model of the baryon anomaly ETD-HIC2007

  19. STAR PRL95 showed that the away jet really didn’t vanish--it just lost energy and widened STAR-PRL91(2003)072304 4< pTt< 6 GeV/c 2<pTa<pTt xEpTa/pTt~0.5 STAR-PRL95(2005)152301 4< pTt< 6 GeV/c 0.15<pTa<4 GeV/c xE  pTa/pTt~0.04 ETD-HIC2007

  20. Measured ratio of particles pTa/pTt xE  Ratio of jet transverse momenta 1.0 0.8 0.6 0.4 0.2 Shape of xE distribution depends on and n but not on b PHENIX PRD74, 072002 (2006) ETD-HIC2007

  21. Away jet /trigger jet ( ) decreases with increasing centrality I applied my xE formula to STAR PRL 95 yields 4 < pTt < 6 GeV/c <pTt>=4.56 GeV/c pp, AuAu sNN=200 GeV STAR, J. Adams, Fuqiang Wang, et al PRL 95, 152301 (2005) ETD-HIC2007

  22. Newer STAR data AuAu: PRL 97 Thanks to Dan Magestro for table of data points PRL 97: narrow jets punch through for pTt>8 GeV/c STAR PRL 97 data has much too flat shape, an apparent sharp break, and disagrees in normalization with STAR PRL95. Is it two components? 8 < pTt < 15 GeV/c <pTt>=9.38 GeV/c STAR, J. Adams, D. Magestro, et al PRL 97, 162301 (2006) ETD-HIC2007

  23. If this is punch-through due to tangential emission, why does it depend on pT? Tangential emission We must carefully map out how this effect depends on pTt and pTa and particle type and angle to the reaction plane... ETD-HIC2007

  24. Away side correlations in Au+Au much wider than in p-p D Away side distribution much wider in A+A than p-p in correlation fn. C() Subtraction of v2 (flow?) effect J() causes a dip at 180o which gives 2 peaks at D~1 radian independent of system and centrality for Npart >100. This is also seen for (auto) correlations of low pT particles. Is this the medium reaction to the passage of a color-charged parton? Stay tuned, much more study needed. AuAu 200 0-5% like sign 0.2< pT,1 pT,2< 0.4 GeV/c PHENIX AuAu PRL 98 (2007) 232302 ETD-HIC2007

  25. The biggest result at QM2006??! QM2005 -I wanted to make a T-shirt pp dir  reference is pQCD QM2006- pp dir  reference is run 5 msmt If RAA= RAA the whole concept of energy loss changes: perhaps no effect for pT>20 GeV ETD-HIC2007

  26. Eskola et al. NPA696 (2001) 729 Anti Shadowing Shadowing gluons in Pb / gluons in p 100 xT x For Au+Au min bias direct  RAA is simple Au+Au minimum bias at mid-rapidity Eskola,Kolhinen,Ruuskanen Nucl. Phys. B535(1998)351 Do the structure function ratios actually drop by ~20% from x=0.1 to x=0.2? ETD-HIC2007

  27. 100 xT Central Collisions---no theory counterpart-yet Au+Au Central Collisions at mid-rapidity Theorists, HELP! Very few attempts so far for structure function measurements or theory as a function of impact parameter: E665, ZPC 65, 225 (1995) Li and Wang, PLB 527, 85 (2002) Klein and Vogt PRL 91, 142301 (2003) Emel’yanov, et al. PRC 61, 044904 (2000) and references therein. Experimentalists: RHIC p+A, eRHIC Nobody has seriously measured nor calculated structure function ratios as a function of centrality!!! ETD-HIC2007

  28. What I still don’t understand-I • After 6 runs at RHIC, many discoveries have been made in Au+Au collisions but there is much that is still not known or understood: • Is the nuclear modification factor RAA for 0 really constant at a factor of 5 suppression over the range 3< pT< 20 GeV/c which would occur for a constant-fractional energy loss analogous to bremsstrahlung, or does the suppression tend to vanish at larger pT? Is dE/dx constant or a constant fraction or something else? • Does RAA for direct- really approach that of 0 at large pT~20 GeV/c as indicated by preliminary data? If true this would argue that the suppression due to a medium effect vanishes at large pT> 20 GeV/c and the effect observed is due to the structure function. If this is confirmed, it would be VERY BAD for LHC. • The detailed mechanism of jet suppression due to interaction with the medium is not understood. It is not known whether partons lose energy continuously or discretely, whether they stop in the medium so that the only observed jet fragments are those emitted from the surface or whether partons merely lose energy exiting the medium such that those originating from the interior of the medium with initially higher pT are submerged (due to the steeply falling pT spectrum) under partons emitted at the surface which have not lost energy. In either case, there is a surface bias. ETD-HIC2007

  29. What I still don’t understand-II The reason why heavy quarks appear to lose the same energy as light quarks is not understood. • It is not known whether a parton originating at the center of the medium can exit the medium without losing any energy. • It is not known where the energy from the absorbed jets or parton energy loss goes or how it is distributed.• The surface bias discussed above complicates the use of two-particle correlations of hard-scattered partons to probe the medium since detecting a particle from an away-side parton changes the surface bias of the trigger parton. This means that detection of both a trigger and away side particle is required in order to constrain the hard-scattering kinematics and the position of the origin of the hard-scattered parton-pair within the nuclear matter. Then, the main correlation information with relatively stable kinematics and origin is obtained by studying correlations with an additional 1 or two particles, i.e. a total of 3 or 4 particle correlations, which is much more complicated and requires much more data than the same studies in p+p collisions. ETD-HIC2007

  30. What I still don’t understand-III • The baryon anomaly, the increase of the p±/± ratio in the range 2<pT <6 GeV/c in Au+Au collisions from the value given by parton-fragmentation in this pT range in p+p collisions, is not understood. Elegant recombination models fail to explain the similar jet activity correlated to the p and  triggers in this “intermediate” pT range. • The wide away-side non-identified hadron correlations for triggers in the intermediate range 2<pT <6 GeV/c in Au+Au collisions, with a possible dip at 180o which causes apparent peaks displaced by ~60o, is not understood. It could represent a Mach cone due to the analogy of a sonic-boom of the parton passing through the medium faster than the speed of sound, or it could indicate jets with large deflections. The effect may be related to the baryon anomaly, which occurs in this pT range; or the peaks, which are seen also for much softer trigger particles, may not be a hard-scattering effect. • The ridge is not understood. What causes it? What are its properties? How does it depend on pTt, angle to reaction plane etc? Why isn’t there an away-side ridge? • Finally, J/ suppression, which for more than 20 years has represented the gold-plated signature of deconfinement, is not understood. ETD-HIC2007

  31. NA50 at SPS (0<y<1) PHENIX at RHIC (|y|<0.35) J/ Suppression--RAAPHENIX mid-rapidity (e+e-) the same as NA50!!! Suppression increases at more forward rapidity PHENIX PRL 98, 232301 (2007) Bar: uncorrelated error Bracket : correlated error Global error = 12% is not shown ETD-HIC2007

  32. END ETD-HIC2007

  33. ICHEP-Paris1982: first measurement of QCD subprocess angular distribution using 0-0 correlations-Rutherford Scattering of quarks DATA: CCOR NPB 209, 284 (1982) p-p QCD ETD-HIC2007

  34. p0 RAA 62.4 GeV : NEW!! • “Old”: p+p reference from fit to ISR data-good effort, bad idea. • “New”: p+p reference measured at RHIC! • Suppression in 62.4 GeV is now more similar to that for 200 GeV ETD-HIC2007

  35. = 2(1) Comparison with theory including systematic error-2 I.Vitev, PLB 639, 38 (2006) Presented at QM2006. Journal article to be submitted soon. ETD-HIC2007

  36. = 2(1) Comparison with theory including systematic error-3 Wicks, Horowitz, Djordjevic,Gyulassy NPA783, 493 (2007) Presented at QM2006. Journal article to be submitted soon. ETD-HIC2007

  37. The leading-particle effect a.k.a. trigger bias • Due to the steeply falling power-law spectrum of the scattered partons, the inclusive particle pT spectrum is dominated by fragments biased towards large z. This was unfortunately called trigger bias by M. Jacob and P. Landshoff, Phys. Rep. 48C, 286 (1978) although it has nothing to do with a trigger. Fragment spectrum given pTt is weighted to high zt by ztn-2 ( <z>=1/b ) ETD-HIC2007

  38. Continuing as in PRD 74, 072002 (2006) We can integrate over the trigger jet zt and find the inclusive pion cross section: Bjorken parent-child relation: parton and particle invariant pT spectra have same power n Inclusive high pT particle has n-1 times larger <z> than unbiased fragmentation, <z>=1/b ETD-HIC2007

  39. Prob. that you make a jet with which fragments to a  with zt=pTt/ Prob. that away jet with fragments to a  with za=pTa/ 2 particle Correlations Appears to be sensitive to away jet Frag. Fn. (1) ETD-HIC2007

  40. PHENIX-compared measured xE distribution to numerical integral using LEP fragmentation functions-not sensitive! ISR-CCOR 1979 LEP bq=8.2 bg=11.4 PHENIX PRD 74 (2006) 072002 xE ~ pTa/pTt ~z/<ztrig> <ztrig>=0.85 measured Dq(z)~e-6z • independent of pTt ETD-HIC2007

  41. Amazingly, I got a neat analytical result (1) Take: (2) Using: Where (a,0)= (a)=(a-1) (a) ETD-HIC2007

  42. The final result Where B/b<m>b is the mean charged multiplicity in the jet ETD-HIC2007

  43. Why dependence on the Frag. Fn. vanishes • The only dependence on the fragmentation function is in the normalization constant B/b which equals <m>, the mean multiplicity in the away jet from the integral of the fragmentation function. • The dominant term in the xE distribution is the Hagedorn function … so that at fixed pTt the xE distribution is predominantly a function only of xE and thus exhibits xE scaling, as observed. • The reason that the xE distribution is not sensitive to the shape of the fragmentation function is that the integral over zt in (1, 2) for fixed pTt and pTa is actually an integral over jet transverse momentum .. . However since the trigger and away jets are always roughly equal and opposite in transverse momentum (in p+p), integrating over … simultaneously integrates over … . The integral is over zt, which appears in both trigger and away side fragmentation functions in (1). ETD-HIC2007

  44. Corrected for acceptance N A Oh yes--PHENIX o - hcorrelation functionsp+p s=200 GeV: PRD 74, 072002 (2006) d+Au N jT jet fragmentation transverse momentum-measure directly F  kTparton transverse momentum-more complicated. ETD-HIC2007

  45. PHENIX ISR Results RMS kT in p+p @ 200 GeV Main contribution to the systematic errors comes from unknown ratio gluon/quark jet => D(z) slope =><m> ETD-HIC2007

  46. A very interesting formula Ratio of jet transverse momenta measured ETD-HIC2007

  47. Does the formula work? PHENIX p+p PRD 74, 072002 (2006) ETD-HIC2007

  48. xh=0.8 due to kT smearing It works for PHENIX p+p PRD 74, 072002 ETD-HIC2007

  49. Measured ratio of particle pTa/pTt xE Ratio of jet transverse momenta in Au+Au indicates that away jet has lost energy relative to trigger jet. Formula works in Au+Au: Away-side yield vs pTa/pTt is steeper in Au+Au than p-p indicating energy loss The away side pTa/pTt xE distribution triggered by a leading particle with pTt was thought to be equal to the fragmentation function but we found that it is NOT sensitive to the shape of the fragmentation function but only to the shape of the inclusive pTt spectrum with power n (=8.1). Formula derived in PRD 74 (2006) 072002 works for pp and AA h(3<pTt<4 GeV/c)--h ETD-HIC2007

  50. q  Compton g q q  Annihilation g q Direct  - hcorrelations in p+p s=200 GeV New PHENIX preliminary result p0 Direct g In order to understand whether away side xE distribution from a direct  is the (quark) fragmentation function must understand kT smearing for direct . Next step is to measure kT for direct  ETD-HIC2007

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