Quark–gluon plasma and color glass condensate at RHIC? The perspective from the BRAHMS experiment

Abstract

We review the main results obtained by the BRAHMS collaboration on the properties of hot and dense hadronic and partonic matter produced in ultrarelativistic heavy ion collisions at RHIC. A particular focus of this paper is to discuss to what extent the results collected so far by BRAHMS, and by the other three experiments at RHIC, can be taken as evidence for the formation of a state of deconfined partonic matter, the so called quark-gluon-plasma (QGP). We also discuss evidence for a possible precursor state to the QGP, i.e. the proposed Color Glass Condensate.

Melting hadrons, boiling quarks

“…17 shows the four BNL experiments operating at the time: BRAHMS, PHOBOS, PHENIX, and STAR, which reported on the QGP physical properties that have been discovered in the first three years of RHIC operations. These four experimental reports were later published in an issue of Nuclear Physics A [140][141][142][143].…”

Novel subjet observables for jet quenching in heavy-ion collisions

“…Considerations of the RSD distributions show that the R IQR[ΔS 12 ] carries the largest discrimination power for highp T jets, although there is an associated uncertainty of [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]% due to hadronization effects. For low-p T jets ( p T 120 GeV/c) and/or recoil-jets in dijet systems the use of the R Q 1 [ΔS 12 ] and/or the median R Q 2 [ΔS 12 ] may prove more advantageous, with hadronization uncertainties that are smaller than 5% for the chosen subjet parameters.…”

“…Experiments at RHIC and the LHC observed a strong suppression of high transverse momentum particle yields [9][10][11][12][13][14][15], suppression of inclusive and semi-inclusive yields of fully reconstructed jets [16][17][18][19][20], and, more recently, the internal structure of the jets [21][22][23][24][25] for detailed studies of jet quenching. However, in all these measurements the treatment of the background originating from the copiously produced particles not associated to hard scatterings poses an experimental challenge for precise and unbiased measurements.…”

CMS Physics Technical Design Report: Addendum on High Density QCD with Heavy Ions

“…On the other hand, measurements in the forward region can be used to study baryon density effects on particle production, essentially changing the chemistry of the produced quark-gluon system [145,146]. Thermal and chemical analyses of the current data in different rapidity slices indicate that the system has larger baryo-chemical potential, less transverse flow and fewer degrees of freedom at forward rapidity [147]. Based on extrapolations from BRAHMS data (figure 1.20-right), the maximum net baryon density at the LHC is expected in the pseudorapidity region η ≈ 5-6.…”

“…Measurements at RHIC suggest that the initial state described by the concept of parton saturation is directly reflected in the multiplicity of produced hadrons and their phase-space distribution. These global features of multiparticle production, measured by PHOBOS and others [64,68,147,159], exhibit great simplicity, such as factorisation into separate dependencies on energy and collision geometry as well as the general feature of extended longitudinal scaling over a large fraction of the rapidity range [141,142]. In CMS, the high tracking efficiency and low rate of fake tracks, together with a large calorimetric coverage, provide a precise measurement of global event characteristics, event by event.…”