Mass generation in coalescence - effects on hadron spectra

Different scenarios for the creation of constituent mass in the hadron formation process are discussed. Effects of these may be observable in hadron momentum spectra.Comment: Poster Proceedings of Quark Matter 2005, 4 page

Forward photon measurements in ALICE as a probe for low-x gluons

The low-x gluon density in the proton and, in particular, in nuclei is only very poorly constrained, while a better understanding of the low-x structure is crucial for measurements at the LHC and also for the planning of experiments at future hadron colliders. In addition, deviations from linear QCD evolution are expected to appear at low x, potentially leading to gluon saturation and a universal state of hadronic matter, the color-glass condensate. However, these effects have not been unambiguously proven to date. Fortunately, data from the LHC can be used directly to provide better constraints of the parton distribution functions (PDFs). In this context, a Forward Calorimeter (FoCal) is proposed as an addition to the ALICE experiment, to be installed in the Long Shutdown 3. The main goal of the FoCal proposal is to measure forward direct photons in pp and p-Pb collisions to obtain experimental constraints on proton and nuclear PDFs in a new region of low x. Based on the current knowledge from DIS experiments and first results from LHC, we will discuss the physics case for this proposed detector. While open charm measurements do provide important constraints, a photon measurement would provide additional unique information. The direct photon measurement requires a new electromagnetic calorimeter with extremely high granularity. The corresponding innovative design principle of a high-resolution Si-W sandwich calorimeter is discussed.Comment: Proceedings of Hard Probes 2018, 30 September - 5 October, Aix-Les-Bains, Franc

Direct Photons from Relativistic Heavy-Ion Collisions

Direct photons have been proposed as a promising signature for the quark-gluon plasma (QGP) formation in relativistic heavy-ion collisions. Recently WA98 presented the first data on direct photons in Pb+Pb-collisions at SPS. At the same time RHIC started with its experimental program. The discovery of the QGP in these experiments relies on a comparison of data with theoretical predictions for QGP signals. In the case of direct photons new results for the production rates of thermal photons from the QGP and a hot hadron gas as well as for prompt photons from initial hard parton scatterings have been proposed recently. Based on these rates a variety of different hydrodynamic models, describing the space-time evolution of the fireball, have been adopted for calculating the direct photon spectra. The results have been compared to the WA98 data and predictions for RHIC and LHC have been made. So far the conclusions of the various models are controversial. The aim of the present review is to provide a comprehensive and up-to-date survey and status report on the experimental and theoretical aspects of direct photons in relativistic heavy-ion collisions.Comment: 91 pages, 44 figures, revised version to be published in Phys. Re

Hydrodynamic description of spectra at high transverse mass in ultrarelativistic heavy ion collisions

Transverse mass spectra of pions and protons measured in central collisions of heavy ions at the SPS and at RHIC are compared to a hydrodynamic parameterization. While the chemical temperature needed at RHIC is significantly higher compared to SPS, the spectra may be described using kinetic freeze-out parameters which are similar for both beam energies. At RHIC either the temperature or the flow velocity is higher, but the data provide no unambiguous proof for much stronger transverse flow. The contribution of such hydrodynamic emission at high transverse momenta is investigated in detail. It is shown that hydrodynamics may be relevant up to relatively high transverse momenta. The importance of the velocity profile used in this context is highlighted.Comment: 10 pages, 11 figures, accepted in Eur. Phys. J. C, data from more experiments used in this versio

Studying Intermediate pT Hadron Production with Fluctuations

Mechanisms for particle production at intermediate pT in nuclear collisions at RHIC are discussed, emphasizing the differences in associated jet-like correlations between color-neutral and colored production. An alternative production mechanism involving both recombination and fragmentation is suggested, which might simultaneously lead to an enhancement of baryons and to jet-like correlations. To gain more insight into the relative importance of different mechanisms a study of constrained distributions of associated multiplicity is proposed. In a simple model it is shown that these multiplicity distributions may change significantly, if the nature of the production mechanism fluctuates from event to event.Comment: 7 pages, 4 figures, talk at Hot Quarks 2004 conferenc

Summary of Results on Hard Probes

Results on observables related to hard scattering processes as presented at this conference are reviewed. This includes in particular measurements related to jet quenching, a phenomenon which has been predicted as a signature for the hot and dense early phase of strongly interacting matter.Comment: Summary talk at Quark Matter 2002, Nantes, Proceedings to be published in Nucl. Phys. A, 10 pages, 6 figures, (one figure added in this version

The rapidity dependence of the proton-to-pion ratio in Au+Au and p+p collisions at sqrt(sNN) = 62.4 and 200 GeV

The BRAHMS measured proton-to-pion ratios in Au+Au and p+p collisions at \rootsnn{62.4} and \rootsnn{200} are presented as a function of transverse momentum and collision centrality within the pseudo-rapidity range 0 < eta < 3.8 The results for Au+Au at sqrt(sNN) = 200 GeV are compared with predictions from models which incorporate hydro-dynamics, hadron rescattering and jet production, in the eta interval covered. In Au+Au collisions at sqrt(sNN) = 200 GeV, eta ~ 2.2, and at sqrt(sNN) = 62.4 GeV, eta = 0, the bulk medium can be characterized by the common value of \mi_B ~ 65 MeV. The p/pi ratios measured for these two selections display a striking agreement in the pT range covered (up to 2.2 GeV/c). At a collision energy of 62.4 GeV and at forward pseudo-rapidity we found a crossing point of p/pi+ ratios measured in central and semi-peripheral Au+Au and in p+p reactions. The crossing occurs in the narrow eta bin around value of 3.2, simultaneously in the whole covered pT range (0.3 GeV/c < pT < 2 GeV/c).Comment: 4 pages, 4 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee Included corrections indicated in internal review report. Minor correction to Fig.

Influence of hydrodynamics on the interpretation of the high pTp_{T} hadron suppression at RHIC

A hybrid parameterization including contributions of hydrodynamics and of expectations from the spectral shape observed in p+p collisions is introduced. This parameterization can successfully describe identified hadron spectra over a wide range of $p_{T}$ in Au+Au reactions at $\sqrt{s_{NN}}$ = 200 GeV for all centralities. The parameters of the hydrodynamic source compare reasonably well to other attempts to describe the spectra. The description is obtained using one universal suppression factor of the hard scattering component independent of $p_{T}$ and hadron species. For the fit results obtained the observed nuclear modification factor for the different particles converges to a universal suppression behavior for $p_{T} > 6 \mathrm{GeV}/c$.Comment: updated version (only typos), 16 pages, 10 figure