Baryon-antibaryon dynamics in relativistic heavy-ion collisions

The dynamics of baryon-antibaryon annihilation and reproduction ($B\overline{B}\leftrightarrow 3M$) is studied within the Parton-Hadron-String Dynamics (PHSD) transport approach for Pb+Pb and Au+Au collisions as a function of centrality from lower Super Proton Synchrotron (SPS) up to Large Hadron Collider (LHC) energies on the basis of the quark rearrangement model. At Relativistic Heavy-Ion Collider (RHIC) energies we find a small net reduction of baryon-antibaryon ($B\overline{B}$) pairs while for the LHC energy of $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ a small net enhancement is found relative to calculations without annihilation (and reproduction) channels. Accordingly, the sizable difference between data and statistical calculations in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV for proton and antiproton yields [ALICE Collaboration, B. Abelev et al., Phys. Rev. C 88, 044910 (2013)], where a deviation of 2.7 $\sigma$ was claimed by the ALICE Collaboration, should not be attributed to a net antiproton annihilation. This is in line with the observation that no substantial deviation between the data and statistical hadronization model (SHM) calculations is seen for antihyperons, since according to the PHSD analysis the antihyperons should be modified by the same amount as antiprotons. As the PHSD results for particle ratios are in line with the ALICE data (within error bars) this might point towards a deviation from statistical equilibrium in the hadronization (at least for protons and antiprotons). Furthermore, we find that the $B\overline{B}\leftrightarrow 3M$ reactions are more effective at lower SPS energies where a net suppression for antiprotons and antihyperons up to a factor of 2–2.5 can be extracted from the PHSD calculations for central Au+Au collisions.

Figure 1

The reaction rates of the $B\overline{B}\to 3M$ reactions (solid line) as a function of time in 5% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV (a) and Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV (b) integrated over rapidity. The solid blue lines denote the rates for $B\overline{B}$ annihilation when discarding the reproduction channels; the red solid lines stand for the $B\overline{B}$ annihilation rate when including the backward channels whereas the dashed lines display the reproduction rate in the latter case.

Figure 2

The transverse-momentum spectra for protons and positive and negative pions as well as for kaons and antikaons from PHSD at midrapidity in comparison to the data from the PHENIX Collaboration [48] for 5% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. The full red lines show the results of calculations with the $2\leftrightarrow 3$ reactions included while dashed lines correspond to calculations with the $2\leftrightarrow 3$ reactions discarded.

Figure 3

The transverse-momentum spectra for protons and positive and negative pions as well as for kaons and antikaons from PHSD at midrapidity in comparison to the data from the ALICE Collaboration [19, 49, 50, 51, 52] for 5% central Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. The full red lines show the results of calculations with the $2\leftrightarrow 3$ reactions included while dashed lines correspond to calculations with the $2\leftrightarrow 3$ reactions discarded.

Figure 4

The transverse-momentum spectra for $\overline{p},\overline{\mathrm{\Lambda }}+{\overline{\mathrm{\Sigma }}}^0,{\mathrm{\Xi }}^{-},{\overline{\mathrm{\Xi }}}^{+}$, and ${\mathrm{\Omega }}^{-}+{\overline{\mathrm{\Omega }}}^{+}$ from PHSD at midrapidity in comparison to the data from the PHENIX and STAR Collaborations [48, 54, 55] for 5% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. The full red lines show the results of calculations with the $2\leftrightarrow 3$ reactions included while dashed lines correspond to calculations with the $2\leftrightarrow 3$ reactions discarded.

Figure 5

The transverse-momentum spectra for $\overline{p},\overline{\mathrm{\Lambda }}+{\overline{\mathrm{\Sigma }}}^0,{\mathrm{\Xi }}^{-},{\overline{\mathrm{\Xi }}}^{+}$, and ${\mathrm{\Omega }}^{-}+{\overline{\mathrm{\Omega }}}^{+}$ from PHSD at midrapidity in comparison to the data from the ALICE Collaboration [19, 50, 51] for 5% central Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. The full red lines show the results of calculations with the $2\leftrightarrow 3$ reactions included while dashed lines correspond to calculations with the $2\leftrightarrow 3$ reactions discarded.

Figure 6

Time evolution of the midrapidity yields of the protons [(a) and (b)] and antiprotons [(c) and (d)] in 0–5% central Pb+Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ from HSD [(a) and (c)] and PHSD [(b) and (d)]. The dashed red lines show the results of calculations with only $B\overline{B}$ annihilation, the solid black lines show results with the $2\leftrightarrow 3$ reactions included while the dotted blue lines correspond to calculations with the $2\leftrightarrow 3$ reactions discarded.

Figure 7

The rapidity density of baryons and antibaryons from PHSD at midrapidity in comparison to data from the PHENIX and STAR Collaborations [48, 54, 55] for 5% central Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV. The dashed red lines show the results of calculations with only $B\overline{B}$ annihilation, the solid black lines show results with the $2\leftrightarrow 3$ reactions included, while dotted blue lines correspond to calculations with the $2\leftrightarrow 3$ reactions discarded.

Figure 8

The rapidity density of baryons and antibaryons from PHSD at midrapidity in comparison to the data from the ALICE Collaboration [19, 49, 50, 51, 52] for 5% central Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. The dashed red lines show the results of calculations with only $B\overline{B}$ annihilation, the solid black lines show results with the $2\leftrightarrow 3$ reactions included, while dotted blue lines correspond to calculations with the $2\leftrightarrow 3$ reactions discarded.

Figure 9

The midrapidity yields of mesons (a), baryons (b), and antibaryons (c) from PHSD as a function of the invariant energy $\sqrt{s_{NN}}$ for central heavy-ion collisions in comparison with the experimental data taken from Refs. [19, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68]. The solid lines refer to calculations including the $B\overline{B}\leftrightarrow 3M$ channels while the dashed lines display calculations without these channels. The particle yields from PHSD are connected by lines to draw the eye although experimental data and calculations do not always correspond to the same centrality selection (and system) for different bombarding energies.

Figure 10

Ratios of 0–5% central midrapidity yields from calculations with the full $B\overline{B}\leftrightarrow 3M$ reactions to calculations without them for the antibaryons as a function of the invariant energy $\sqrt{s_{NN}}$.

Figure 11

Ratios of 0–5% central midrapidity yields from PHSD calculations with the full $B\overline{B}\leftrightarrow 3M$ reactions to calculations with only annihilation as a function of the invariant energy $\sqrt{s_{NN}}$.

Figure 12

The midrapidity yields from PHSD4.0 including the $B\overline{B}\leftrightarrow 3M$ reactions (solid lines) for 0–10% central Pb+Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. The data points are taken from Ref. [18].