Relativistic Heavy-Ion Physics: A Minority Report. Lanny Ray UT Austin, April 7, 2014

Transcription

1 Relativistic Heavy-Ion Physics: A Minority Report Lanny Ray UT Austin, April 7, 014 1

2 Outline Pre-RHIC summary and discovery claims First years at RHIC and the sqgp UT-Austin & U. Washington alternative analysis Challenges to the discovery claims Alternate ideas What new physics IMHO is accessible in relativistic HI

3 Pre-RHIC: summary Tandem Van de Graaff HI (1970s) few MeV per nucleon; probing nuclear structure and transfer reaction mechanisms. Bevatron/BEVALAC HI at Berkeley (start in 197) 100s MeV per nucleon, fixed targets, nuclei are blown apart, probe high nucleon densities, many-body forces, nuclear EoS; nucleon transport, hydrodynamics, TDHF made sense and worked well. AGS at BNL (1990s) -11 GeV per nucleon, fixed targets, meson & strangeness production, quark or nucleon DoF? Hadronic transport and hydro continue to be used. Super-proton-synchrotron (SPS) at CERN (1980s-000) fixed targets, CM energy per NN [.6,19]GeV, partonic or hadronic DoF; discovery of quark-gluon plasma (QGP) claimed. s NN BEVALAC AGS 3

4 Pre-RHIC: summary More emission Collective flow of nuclear matter (nucleons) Reaction plane spectators Reaction plane Beam direction Mid-rapidity Gutbrod, Kamper, Kolb Phys. Rev. C 4, 640 (1990): collective flow of nuclear matter squeeze-out Yield a v cos 1 Yield vs Au+Au Less emission beam energy per nucleon v : amplitude of cos anisotropy 4

5 Yield(Pb Pb)/ N Yield part Be p Be/ N part Pre-RHIC: SPS results J/y / Drell-Yan vs path length Pb Au e e X W X L N part More emission Increasing (positive) v ; in-plane emission Less emission Reaction plane QGP discovery claimed: Nature 403, 581 (000) A classic example of affirming the consequent logical fallacy 5

6 The RHIC era begins in 000 s NN 7 00 GeV (CM energy per NNpair) p p,d Au, Au Au STAR (UT group) PHOBOS PHENIX BRAHMS 6

7 First, some explanations: collision centrality Caveat: There is no unique definition STAR Trigger detectors Minimum-bias trigger: ZDCs+CTB/TOF ZDC Minbias event collection Zero-degree calorimeters Central trigger barrel (CTB) (now TOF, VPD not shown) Alternate trigger cut definitions CTB 7

8 (barn) STAR min-bias data: Collision centrality track cuts trigger cuts replot on log-log slope 3/ 4 C. Adler et al., PRL 87, (001) d dn h d dn 1/ 4 h N 3/ 4 h 4N 3/ 4 h d dn h constant linear plot: centrality made easy! d 1/ 4 dn h STAR data: Au-Au 130 GeV 1/ 4 N h end point 8

9 0-10% Centrality Monte Carlo Glauber NN interaction range (Track cut centrality definition) r inel NN / b random A+A impact parameter Sample impact parameter, b Sample Woods-Saxon densities for N positions Count number of participating nucleons: N part Count number of N+N interacting pairs: N bin N part, Nbin (nucleons) King Glauber I Calculate mean multiplicity Kharzeev and Nardi Phys. Lett. B 507, 11 (001) dnch (1 x) n d where n pp pp dn ch N part xn pp N / d for p p bin Sample NBD for event-wise N ch Fit MCG model to data end-point Estimate trigger/vertex finding inefficiencies Subdivide distribution, e.g. 0-10% Calculate : N ch Define centrality measure :, b, N part, N bin N N bin part / Au+Au 130 GeV Fractions of total reaction cross section 9

10 What is v and how is it measured? Conventional event plane method; intended to measure anisotropy wrt reaction plane: Project tracks onto transverse plane f Correct for tracking inefficiencies, fit event-wise distributions: 3 d N E 3 d p ch d Nch 1 v cos p dp dy t t f EP Determine the event-plane angle: Q N ch i i1 1 EP 1 tan u(f ), sum all unit vectors Project raw v : v cos f, raw Correct for EP resolution:, EP v v cos EP, a EP, Q Q y x, raw b b EP where a, denote subsetsof tracks (many choices of sub-events) But, this is approximately equal to averaging over all particle pairs, event plane is irrelevant. v { } cos ( fi f j ) v i j, EP Jets bias the EP, and strongly affect these highly biased measures. 10

11 Hydro for A+A collisions Initial conditions: Rapid thermalization (< 1 fm/c) essential for v ; thermal energy based on nuclear overlap adjusted to fit yields Time evolution: Ideal or viscous +1 and now 3+1 hydrodynamic evolution/expansion; shear/bulk viscosities and EOS [E(r)] adjusted; v decreases with time Hadronization: Cells g hadrons at arbitrary energy density; hadron cascade Rapid isotropization: <1 fm/c Increasing proper time, system expands Tunable at each stage Falsifiable theory or elaborate parametrization of data? Hundreds of papers and talks: e.g. Peter Kolb and Ulrich Heinz, nucl-th/ (003); U. Heinz, Proc. of Science, PoS(CERP010)010, 11

12 QGP Victory Declared at RHIC Jet Quenching Miklos Gyulassy - Columbia Jet Quenching R AA Y A A N Y p bin p Gyulassy at Quark Matter 004, Oakland, CA My conclusion: overwhelming evidence at QM04 that QGP Bulk Matter is made in AuAu at 00 AGeV and he concludes that the field is at: The END of searching for the QGP and is at the: The BEGINNING of measuring its properties 4<p t,trig <6 GeV/c; <p t,assoc <p t,trig STAR, PRL 91, (003) These statements have defined the HI field ever since. 1

13 Not (weak) QGP but (strong) QGP Perfect Liquid D. Teaney, Effect of shear viscosity on spectra, elliptic flow, and Hanbury Brown-Twiss radii, Phys. Rev. C 68, (003) viscosity ~ 0 Paul and Ulrike Romatschke, Viscosity Information from Relativistic Nuclear Collisions: How Perfect is the Fluid Observed at RHIC? PRL 99,17301 (007) STAR data CERN Courier April, 008 Vary shear viscosity/entropy in hydro fits to v g viscosity ~ 0 g strong interactions. More on these later 13

14 Alternative data analysis: UT-Austin & UW The philosophy behind our analysis approach is simple: Analyze p+p data and try to understand in terms of pqcd, PDFs, FFs. Then examine p+au (d+au) and understand those data (initial state effects, multiple scattering) Then examine Au+Au and see if the data require anything else using, an unbiased survey of spectra and correlations, with minimal data modeling Then, interpret data with new physics mechanisms, beyond that in p+p, p+a only when the data demand it. University of Washington collaborators (co-conspirators): Thomas Trainor Duncan Prindle 14

15 Begin with p+p spectra Two-component soft + (semi)hard model: (STAR)PRD 74, (nucl-ex/060608) s 00 GeV p t spectra for increasing N ch replot on transverse rapidity mt pt y ln t m0 Data S pp N Gaussian ch n n ch h n n s s n n ch h dnch lim Nch ytdy t S Nch 0 Two-component decomposition of p+p data. pp y t 15

16 From spectra to correlations 00 GeV p+p Peak y t =.66, p t ~1 GeV/c Δρ y t =.66 ρ ref y t p t ~.0 p t ~ 1.0 p t ~ 0.5 y t1 y t mt pt y ln t m Soft and semi-hard components in spectra have corresponding counter-parts in correlations p t (GeV/c)

17 UT-UW correlation measure definition ρ(p 1,p ) = particle density Event 1 ρ sibling (p 1,p ) ρ reference (p 1,p ) Fill D histograms (a,b), e.g. (f 1,f ), ( 1, ), (f 1 -f, 1 - ), (p t1,p t ), etc. Event r r ref dnch ddf r r sib ref ( a, b) ( a, b) 1 measures number of correlated pairs per final state particle Motivated by p-p superposition null hypothesis square-root of r ref (a,b); (for,f space) normalized ratio of D binned histograms; acceptance cancellation; two-track ineff. corrections 17

18 Angular Correlations: 00 GeV minbias NSD p+p Start with 6D correlations: r r ref Φ Φ 1 =Φ -π On f 1 vs f On 1 vs Φ Φ Δ r r ref η Φ 1 = Φ Φ 1 =Φ +πφ 1 (radians) η 1 Jet example Φ 1 - Φ π Φ 1 Φ Six dimensions reduce to four without any loss of information: f, p,, f, p, f, p p 1, 1 t1 t t1, t Define rotated coordinates reflecting data symmetry: 1 f f f 1 18

19 Model decomposition (,f ) 00 GeV minbias NSD p+p cut low y t pairs and project onto relative,f: 4Δρ 4Δρ ρ ref cut larger y t pairs and project onto relative,f: ρ ref φ Δ φ Δ η Δ η Δ Soft fragmentation plus HBT, e For A+A the data require an additional quadrupole: A Q cos(f ) A Q ~ N ch v bkgs Same-side D Gaussian plus away-side ridge classic jet structure With D correlations we can cleanly separate the quadrupole from the others. 19

20 Jet cross section (Mini)jets in 00 GeV p+p at STAR STAR PRL 97, 5001 (006) HIJING (PYTHIA) jets r r ref r r ref f f Jets on Jets off 4Δρ We conjecture that this angular correlation structure is generated by the fragmentation of semi-hard scattered partons, i.e. minijets. ρ ref Jet algorithm: EM Cal E seed > 0.5 GeV R cone = 0.4 p t > 0. GeV/c Minijets are jets with no low p t cut-off for its fragments, typically ~ 1 GeV/c 0

21 proton-proton Centrality evolution: 00 GeV Au+Au Analyzed 1.M minbias 00 GeV Au+Au events; included all tracks with p t > 0.15 GeV/c, η < 1, full φ STAR, Phys. Rev. C 86, (01) 84-93% Δρ ρ ref 74-84% 64-74% 55-64% 46-55% φ Δ η Δ 8-38% 18-8% 9-18% 5-9% 0-5% Δρ ρ ref From M. Daugherity s Ph.D Thesis (008) We observe the evolution of several correlation structures including the same-side ridge φ Δ η Δ 1

22 Centrality evolution: 00 GeV Au+Au Fit Parameters vs. centrality: N bin / N part amplitude width f width ST ST same-side D Gaussian Minijet binary scaling to mid-centrality away-side ridge quad ~ N ch v Remove non-jet elements from data; the ridge : 64-74% 55-64% 46-55% 38-46% 8-38% φ Δ η Δ STAR Preliminary Growth of the ridge(s) same and away-side

23 STAR Preliminary r r ref y t p t evolution: 00 GeV Au+Au Au-Au 00 GeV 18-8% E. Oldag UT Austin Ph.D. thesis Define y t cut bins Project D angular correlations Fit each with D model Obtain p t dependence of jet-ridge and quadrupole y t1 3

24 Conceptual Problems & Counter Evidence For hydro to produce enough v at RHIC, the system must isotropize/equilibrate in ~0.5 fm/c. This is impossible based on pqcd and non-perturbative parton-instanton estimates [Kovchegov, Nucl. Phys. A764, 476 (006)]. Saturation models do not help. There are no known mechanisms for reaching even, approximate equilibrium (momentum isotropization) this quickly; maybe the system does not need to. Angular correlations jets, dijets (away-side) not dissipated, Glauber superposition of minijets (transparency) is evident almost to mid-central collisions. For those same collisions and for particles having about the same p t we see a large quadrupole; v is at its maximum! AS dipole (dijet) amplitude If partonic scattering is strong enough for hydro conditions, then why aren t there medium effects on the jets in peripheral A+A? but follow, then exceed binary scaling. ST 4

25 S. Voloshin, J. Phys. Conf. Ser. 9, 76 (005) Trainor, arxiv: (008) Counter Evidence Hydro advocates concede that their models are not reliable at higher p t where they strongly overestimate v. The applicable range for hydro is claimed to be p t < GeV/c. As p t g 0 directional dependence becomes meaningless. Let s divide out this trivial p t dependence and look closely at some viscous hydro predictions at lower momentum. v / p t vs yt Teaney (curve A) Romatschke (curve B): viscous hydro fits for pions; often cited as evidence for Perfect Liquid Plotting on transverse rapidity reveals a universal shape (fits to data) and dramatic failure of hydro, but... Number of constituent quark scaling: Empirically, v {EP} vs p t scales with n q = or 3; cited as evidence of partonic flow. This may be another manifestation of the common transverse rapidity offset. 5

26 From: Miklos Gyulassy s QM04 talk Conceptual Problems To be fair, some hydro models can describe v at lower p t with parameter adjustments. But there is a wide range of hydro predictions. Can hydro be tested? Can it be falsified? 6

27 Conceptual Problems Also to be fair, event-by-event hydro with fluctuating initial energy density, thermalized jets, lumpy final states, produce angular correlations similar to the data with only collective flow: [NexSPHerio code: Y. Hama, et al. Braz. J. Phys. 35, 4 (005)] Initial energy density transverse & longitudinal NexSPHerio predictions for Au+Au 00 GeV 60-80% 40-60% 0-30% 0-10% If this idea is correct, then p+p jets must thermalize in peripheral collisions because elliptic flow (v ) begins already in those collisions. This requires that the hydro bumps conspire to promptly and seamlessly replace the pqcd minijets in peripheral A+A collisions an amazing conspiracy! 7

28 ALICE,.76 TeV Pb+Pb, 40-50% A new scaling rule for v Combine v {D} results from SPS to RHIC (LHC result not included yet). We find a simple scaling given by: N ch opt v {D} y y x x initial A A overlap eccentricity opt, log s NN 13.5 GeV N bin d Nch v ddf { D} opt The y t distributions of pairs contributing to minijets and quadrupole are the same. same-side jet peak quadrupole The binary scaling and y t dependence might be expected for an elementary pqcd process operating at the partonic level. More on this later. 8

29 Counter Evidence: Jet quenching (?) Alice Ohlson, STAR arxiv: Recoil jet in Au+Au compared with p+p (same trigger jet p t ); angular broadening and p t -softening in more-central collisions. Jet does not lose energy; redistributed to softer modes; FF modification. D AA PTOT, AA PTOT, pp Softening of the FF induced by medium (e.g. gluon radiation); phenomenological modification of splitting functions, Borghini, Wiedemann hep-ph/ (005) Softened FF ln E Jet p 9

30 Two Big surprises from the LHC 7 TeV p+p CMS Collaboration, JHEP 1009,091(010). 5.0 TeV p+pb ATLAS, PRL 110, 1830 (013) Same-side ridges everywhere! 5.0 TeV p+pb ALICE, Phys.Lett.B 719, 9 (013) Subtract low multiplicity correl. from high multiplicity correl. Well described as quadrupoles. Is this evidence of elliptic flow in p+p, p+pb or something else? 30

31 Meanwhile, back at RHIC Quadrupole in 00 GeV p+p A Quad ~ N ch Prindle (STAR) ISMD-013 Quadrupole in 00 GeV d+au [PHENIX, arxiv: ]; subtracted low N ch from high N ch correlations: (blue points and fits) STAR Preliminary Quadrupoles are everywhere! 31

32 Back to QCD: BFKL* Pomerons - gluon interference E. Levin and A. H. Rezaeian, Phys. Rev. D 84, (011) N N Multiple gluon emission from or more Pomerons interfere producing azimuth anisotropy wrt momentum transfer Q T Two-BFKL Pomeron exchange with two-gluon emission & interference Resulting correlation: cos( f ) Random emission results in uniform dependence. A pqcd, gluon saturation model prediction for a quadrupole correlation, or v. *(Balitsky, Fadin, Kuraev, Lipatov) f 3

33 BFKL glasma* model fits to LHC p+p, p+pb Dusling and Venugopalan, Phys. Rev. D 87, (013) 7 TeV p+p (CMS) 5 TeV p+pb (ATLAS) *Gluon saturated, coherent condensate (next slides) f f f Could v in A+A be caused by gluon interference? So far the only model which reproduces v in A+A is hydro, but gluon interference has not been explored yet. 33

34 Gluon saturation Parton effective area ( Q ) / Q Nuclear area in transverse plane R R If #partons N ~ A A, then partons begin to overlap and g g g dominates. S A, Q is parton p t R A Parton area Gluon density saturates at Q S ~ ( Q S ) N A R A ~ A 1/3, saturation scale Low-x partons projected onto transverse plane A+A overlap may increase Q S, leading to observable effects probed at higher s NN 34

35 Color Glass Condensate* (an effective theory of QCD at high energy) ln 1 x Bzdak and Skokov, PRL 111, (013); CGC prediction for p+pb at LHC CGC *(see arxiv: for a recent review and 35

36 Outlook Ten years ago leading theorists in the HI community declared QGP victory. Today, after collision energy dependent correlation measurements, and the p+pb (LHC) and d+au (RHIC) results, and the BFKL saturation CGC results, Miklos Gyulassy (recall QM 004) is saying (paraphrased)*: Maybe there is no flow, no QGP, no Perfect Liquid at RHIC; maybe CGC emitted gluon interference is the dominant mechanism underlying correlations instead of pressure driven flow. He calls for radical re-evaluation of our past (flow dominated) paradigms. However, these views are not (yet) mainstream in the HI community. Nevertheless I believe that we can learn much more about QCD by paying attention to the data trends accessible with p+p, p+a and A+A at RHIC and LHC, e.g. 1) High density gluon initial-states, accessible with A+A overlap ) Gluon saturation, condensation (?) 3) Modified fragmentation functions 4) Multi-parton interactions and novel gluon interference * 36

37 Conclusions The preponderance of the data disfavor the sqgp and perfect liquid scenario at RHIC and probably at the LHC also. pqcd minijets/dijets are produced at binary scaling rates or greater and are not diminished by a medium. pqcd g 4 interference processes may explain the remaining, large correlations beyond minijets and are worth exploring further. The unique physics which HI should focus on include gluon saturation effects where unprecedented gluon densities may be achieved in A+A, possible CGC, and novel pqcd diagrams which are possible in the multi-parton environment. There is new and interesting QCD physics accessible with the RHIC and LHC A+A programs, but I am concerned that those communities will not exploit this opportunity. 37