We investigate the collision energy dependence of deuteron and antideuteron emission in the RHIC-BES low- to mid-energy range 
$$\sqrt{s_{NN}}$$

 = 4.6–200 GeV where the formation rate of antinuclei compared to nuclei is strongly suppressed. In the coalescence picture, this can be understood as bulk emission for nuclei in contrast to surface emission for antinuclei. By comparison with experimental data on the coalescence parameter 
$$B_2$$

, we are able to extract the respective source geometries. This interpretation is further supported by results from the UrQMD transport model, and establishes the following picture: At low energies, nucleons freeze out over the total fireball volume, while antinucleons are annihilated inside the nucleon-rich fireball and can only freeze out on its surface. Towards higher energies, this annihilation effect becomes less significant because of the enhanced meson production in the reaction. Thus, the nucleon and antinucleon freeze-out distributions become similar with increasing energy.

https://link.springer.com/content/pdf/10.1140/epja/s10050-020-00269-8.pdf

Deuteron and antideuteron coalescence in heavy-ion collisions: energy dependence of the formation geometry

Besides the dedicated search for strangelets NA52 measures light (anti)particle and (anti)nuclei production over a wide range of rapidity. Compared to previous runs the statistics has been increased in the 1998 run by more than one order of magnitude for negatively charged objects at different spectrometer rigidities. Together with previous data taking at a rigidity of -20 GeV/c we obtained 10^6 antiprotons 10^3 antideuterons and two antihelium3 without centrality requirements. We measured nuclei and antinuclei (p,d,antiprotons, antideuterons) near midrapidity covering an impact parameter range of b=2-12 fm. Our results strongly indicate that nuclei and antinuclei are mainly produced via the coalescence mechanism. However the centrality dependence of the antibaryon to baryon ratios show that antibaryons are diminished due to annihilation and breakup reactions in the hadron dense environment. The volume of the particle source extracted from coalescence models agrees with results from pion interferometry for an expanding source. The chemical and thermal freeze-out of nuclei and antinuclei appear to coincide with each other and with the thermal freeze-out of hadrons.

Antimatter and Matter Production in Heavy Ion Collisions at CERN (The NEWMASS Experiment NA52)

We study (anti)deuteron formation rates in heavy-ion collisions in the framework of a wave-function based coalescence model. The main feature of our model is that nucleons are emitted from the whole spherically symmetric fireball volume, while antinucleons are emitted only from a spherical shell close to the surface. In this way, the model accounts for nucleon-antinucleon annihilations in the center of the reaction at lower beam energies. Comparison with experimental data on the coalescence parameter in the range $\sqrt{s_{NN}}= 4.7-200$ GeV allows us to extract radii of the respective source geometries. Our results are qualitatively supported by data from the UrQMD transport model which shows a comparable trend in the geometric radii as a function of beam energy. In line with our expectations, we find that at lower energies, the central region of the fireball experiences stronger annihilation than at higher energies.

Modeling (anti)deuteron formation at RHIC with a geometric coalescence model

Strangeness enhancement in heavy-ion collisions is studied at the parton level by examining the partition of the new sea quarks generated by gluon conversion into the strange and non-strange sectors. The CTEQ parton distribution functions are used as a baseline for the quiescent sea before gluon conversion. By quark counting simple constraints are placed on the hadron yields in different channels. The experimental values of particle ratios are fitted to determine the strangeness enhancement factor. A quantitative measure of Pauli blocking is determined. Energy dependence between CERN-SPS and relativistic heavy-ion collider energies is well described. No thermal equilibrium or statistical model is assumed.

Strangeness Enhancement in the Parton Model

We are presenting new results on K± and π± production in minimum bias lead-lead collisions at 158 A GeV/c. The measurements were performed at zero degree production angle and for a wide rapidity range. The analysis method used the full particle identification capabilities (time of flight, Cerenkov counters and hadronic calorimeter) of the spectrometer. We show K/π and π−/π+ ratios as a function of rapidity, as well as invariant particle production cross sections. As a particularity we observe near projectile rapidity an enhancement in the number of π− with respect to π+.

New results from NA52 on particle production in Pb-Pb collisions at 158 A GeV/c

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New results from NA52 on particle production in Pb-Pb collisions at 158 A GeV/c