International CCAST Summer School and Workshop on QCD and RHIC Physics Where did the energy go? – OUTLINE – motivation analysis results summary Fuqiang Wang Purdue University International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Lattice QCD prediction: Physics motivation Quantum Chromodynamics predicts phase transition between hadrons and Quark-Gluon Plasma at high energy density. The goal of RHIC is to create QGP – a state of deconfined, thermalized quarks and gluons Lattice QCD prediction: F. Karsch, Nucl. Phys. A698, 199c (2002) (1) energy density? (2) thermalization? TC ~ 170  8 MeV eC ~ 0.5 GeV/fm3 International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

To probe energy density self-generated, penetrating probe: large pT hadrons, jets Gluon bremsstrahlung Jet quenching: X.-N. Wang et al. ? they are generated early (by hard-scatterings); they need time to escape the collision zone; during this time, a QGP (or whatever medium) is formed; they interact with the medium, losing energy, thus giving us information about the medium. Can also come from initial state suppression? e.g. PDF different in Au and deuteron International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics High pT suppression suppression of high pT hadron yield suppression of back side angular correlation no suppression in d+Au  Final state interaction in Au+Au, consistent with energy loss by high pT particles – jet quenching. strong absorption International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Energy density inferred from models X.-N. Wang, PLB 579 (04) 299 pQCD calculations: x30 gluon density x100 energy density in central Au+Au collisions Bjorken estimate: 4 - 20 GeV/fm3 (t=0.2-1 fm/c) High pT particles (partons/hadrons) lose energy. Where did the energy do? International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics To measure energy loss … by going to low pT. S. Pal, S. Pratt, PLB574 (2003) 21. X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036. C.A. Salgado, U.A. Wiedemann, hep-ph/0310079. M. Gyulassy, I. Vitev, X.-N. Wang, B.-W. Zhang, nucl-th/0302077. …… Pal, Pratt, PLB 574 (2003) 21 How is energy distributed? amount of energy loss? contribution from medium? International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Reconstructing low pT associated particles Df (1/Ntrig) dN/d(Df) STAR Preliminary background Signal p+p Jet-like structures Au+Au top 5% Select a leading particle 4<pT<6 GeV/c, |h|<0.75. Associate other particles (0.15<pT<4 GeV/c,|h|<1.1) with the leading particle. Form Df,Dh correlations. Background from mix- events. v2 modulation on background. Normalize in 0.9<|Df|<1.3. Efficiency corrections are applied to associated particles. Take difference and normalize per trigger. High pT particle p+p High pT particle Au+Au International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Azimuth angular correlations High pT: 1/Ntrigger dN/d(Df) D f (radians) Low pT: M.G. Albrow et al. NPB145, 305 (1978) p+p, 53 GeV near side: |Df|<1.1, |Dh|<1.4 away side: |Df-p|<2, |h|<1.1 International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics “Jet” sizes STAR Preliminary near: |Df|<1.1, |Dh|<1.4 away: |Df-p|<2, |h|<1.1 Au+Au top 5% bkgd subt. RMS (1/Ntrig) dN/d(Df) s Df near (1/Ntrig) dN/d(Dh) With increasing centrality: Near side broadens in h but not f. Away side modest increase in size. Dh International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Longitudinal broadening N.Armesto, C.A.Salgado, U.A.Wiedemann, hep-ph/0405301 long. flow S.A. Voloshin, nucl-th/0312065 p + p radial flow International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

“Jet” charge multiplicity and “energy” STAR Preliminary p+p With the same final leading particle, we are selecting a larger energy jet in central AA than in pp. International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics Jet quenching model X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036 with energy loss without energy loss DE = 1.4 – 2.2 GeV } Caution: cannot be readily compared to data yet International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics Medium contribution? Total scalar pT: Initial parton energy + medium contribution? TPC acceptance of away side partner? International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Thermal-shower recombination Hwa, Yang, nucl-th/0401001 In this model, the thermal- shower recombination is the largest contribution to high pT particles. One mechanism for energy contribution from medium. International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

To probe thermalization Put two sources of particles together: one from jet fragmentation that are initially hard. the other from bulk medium that are soft. and see how they become at end of the day. Leading hadrons Medium jet medium International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Associated particles pT distributions STAR Preliminary: Quark Matter 2004 Away side: energy from the initial parton is redistributed to low pT energy loss in medium! Near side: Overall enhancement from pp to AA larger initial parton energy (and modest energy loss) Away syst. error Near softening in spectra: partial equilibration with medium International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics Raito of d+Au spectra 6<pT<10 GeV/c 4<pT<6 GeV/c Associated pT (GeV/c) d+Au spectra ratio Associated pT (GeV/c) Au+Au / p+p STAR Preliminary STAR Preliminary Initial parton energy was larger in AA than pp  a hardening of spectra is expected. No hardening is observed in AA  contribution of soft hadrons from energy loss. International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Partial approach toward thermalization STAR Preliminary: QM 2004 Leading hadrons Medium In central Au+Au, the balancing hadrons are greater in number, softer in pT, and distributed broadly in angle, relative to pp or peripheral Au+Au.  away-side products seem to approach equilibration with bulk medium traversed, making thermalization of the bulk itself quite plausible. International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Summary: where did the energy go? High pT suppressed. High pT partons/particles lose energy. The lost energy is redistributed at low pT. high enough energy density? Models require x30 normal nuclear gluon density. parton thermalization? Broader angular distribution, larger multiplicity, softer pT. Partial thermalization with bulk medium. High degree of thermalization in medium itself plausible. A new form of matter created at RHIC: dense, strongly interacting, opaque. QGP? International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Are leading particles from jets? PHENIX, PRL 91, 172301 (2003) p / p ~ 0.9 in central p / p ~ 0.3 in peripheral p+p @ ISR QM’04 non-frag. p / p ~ 0.6 non-frag. p / Nch ~ 0.3 pT=3-4 GeV/c: ~30% are probably from other sources. B. Alper, NPB 87 (1975) 41 International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

Coalescence / recombination models Fries et al, PRC 68 (03) 44902 Greco et al, PRC 68 (03) 34904 Hwa et al, nucl-th/0401001 Coalescence / recombination models predict a range of non-fragmentation contributions. All predict a rapid drop of non-fragmentation contribution above 4 GeV/c. pT>4 GeV/c: may mainly come from jets, or related to jets. International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics Compare to pp, pA pT (GeV/c) dN/dpT2 [a.u.] PRELIMINARY p+p STAR Preliminary p+p pT (GeV/c) <ztrigger> G. Boca et al. ZPC49, 543 (1991) p+A International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics AA/pp vs IAA Peripheral: AA/pp=1.01 IAA=0.84 - broadening in h - pp reference - v2 - model difference Central: AA/pp=1.66 IAA=1.25 Df Df International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics IAA < 1 in peripheral ? STAR Preliminary pp reference International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019

International CCAST Summer School and Workshop on QCD and RHIC Physics Hijing (no quenching) 4 < pTtrig < 6 GeV/c 2 < pT < 4 GeV/c 0.15 < pT < 4 GeV/c Df Df International CCAST Summer School and Workshop on QCD and RHIC Physics 2/23/2019