Stopping and Baryon Transport in Heavy Ion Reactions

Abstract

Abstract. In this report I will give an experimental overview on nuclear stopping in hadron collisions, and relate observations to understanding of baryon transport. Baryon number transport is not only evidenced via net-proton distributions but also by the enhancement of strange baryons near mid-rapidity. Although the focus is on high-energy data obtained from pp and heavy ions from RHIC, relevant data from SPS and ISR will be considered. A discussion how the available data at higher energy relates and gives information on baryon junction, quarkdiquark breaking will be made. IntroductionA major goal for relativistic heavy ion reactions is to form hot nuclear matter at energy densities in clear excess over the value (≈ 1GeV/fm 3 ) predicted from Lattice QCD needed to make the phase transition or cross over to matter dominated by degrees of freedom of quarks and gluons rather than of hadrons. The next goal is to study and quantify the properties to confront those with properties of non-perturbative QCD. Such experiments has been carried out at RHIC with considerable progress and remarkable results from the first 3 years of experiments. Please refer to the white-papers from the 4 experiments in Refs. [1]. The deposited energy is essential to understand the formation of this medium. Because baryon number is conserved, and rapidity distributions are only slightly affected by rescattering in the later stages of the collisions, the measured baryon distribution retains information about energy loss and allows the degree of nuclear stopping to be determined. Such measurements may also help distinguish between different mechanism for transporting baryons to mid-rapidity.At very low energies 1 − 15 AGeV the hadrons preserve their identity with multiple collisions and excitation to resonances being important ingredient in the description of both stopping, transverse momentum spectra as well as strange particle production. At the higher energies partonic degrees become important, and many features can be described using a string picture. At these higher energies it have long be thought that the dominant mechanism for transport of baryon number is that of quark-diquark breaking (of the strings) where the baryon number is carried(associated) with the valence quarks. I.e. the distribution will reflect the distribution q(x) −q(x). Such a mechanism is not able to move the net-baryon number over a large range of x . These distribution are flat in x ( e −y in rapidity ) for a single collision as observed at SPS energies. Already an analysis of the ISR data in pp collisions [2,3], and later data from HERA that shows a non-zero baryon asymmetry of ≈ 8% in γp reaction at more than 7 units of rapidity from the incident baryon [4] has demonstrated that additional mechanisms with a slower x and

Stopping Power from SPS to LHC energies

“…[1]. The predicted values for RHIC and LHC energies, clearly depart from the linear extrapolation for constant relative rapidity loss [8], which seems to be valid only at lower energies ( √ s NN ≤ 20 GeV).…”

“…The net-baryon rapidity distribution measured at RHIC is both qualitatively and quantitatively very different from those at lower energies indicating that a significantly different system is formed near mid rapidity [8]. Fig.…”

“…This picture, corroborated with an increase of the ratio p/p to ≈ 1 suggests that the reaction at the LHC is more transparent in contrast to the situation at lower energy. [8] and from BRAHMS [10] .…”

Estimation of stopped protons at energies relevant for a beam energy scan at the BNL Relativistic Heavy Ion Collider

“…is the beam rapidity with m p being the mass of proton. Therefore, the net-baryon number at mid-rapidity is the measure of baryon stopping [19][20][21]. To quantify the baryon stopping, we have used Eq.…”

“…As the net-baryon number is conserved and rapidity distribution is modified due to re-scattering of the particles after the collision, the net-baryon rapidity distribution becomes a useful probe to give information about the baryon transport and baryon stopping. Since, the neutrons are not measured mostly in heavy-ion experiments, the net-proton rapidity distributions are used to quantify the baryon stopping [19][20][21][22][23][24] .…”