$Σ(1385)^{\pm}$ resonance production in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV

Hadronic resonances are used to probe the hadron gas produced in the late stage of heavy-ion collisions since they decay on the same timescale, of the order of 1 to 10 fm/$c$, as the decoupling time of the system. In the hadron gas, (pseudo)elastic scatterings among the products of resonances that decayed before the kinetic freeze-out and regeneration processes counteract each other, the net effect depending on the resonance lifetime, the duration of the hadronic phase, and the hadronic cross sections at play. In this context, the $\Sigma(1385)^{\pm}$ particle is of particular interest as models predict that regeneration dominates over rescattering despite its relatively short lifetime of about 5.5 fm/$c$. The first measurement of the $\Sigma(1385)^{\pm}$ resonance production at midrapidity in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}= 5.02$ TeV with the ALICE detector is presented in this Letter. The resonances are reconstructed via their hadronic decay channel, $\Lambda\pi$, as a function of the transverse momentum ($p_{\rm T}$) and the collision centrality. The results are discussed in comparison with the measured yield of pions and with expectations from the statistical hadronization model as well as commonly employed event generators, including PYTHIA8/Angantyr and EPOS3 coupled to the UrQMD hadronic cascade afterburner. None of the models can describe the data. For $\Sigma(1385)^{\pm}$, a similar behaviour as ${\rm K}^{*} (892)^{0}$ is observed in data unlike the predictions of EPOS3 with afterburner.

 

Submitted to: EPJC
e-Print: arXiv:2205.13998 | PDF | inSPIRE
CERN-EP-2022-104
Figure group

Figure 1

Left:Invariant mass distribution of Λπ pairs for Σ(1385)+ after subtraction of the mixed-event background in 0–10% central Pb–Pb collisions and 3.5 < pT < 4.5 GeV/c. Right:Invariant mass distribution of Λπ pairs for Σ(1385)− after subtraction of the mixed-event background in 30–50% centrality class and 2.5 < pT < 3.5 GeV/c. The black curves represent the combined fit using a signal (continuous blue line) plus residual background (red dashed line) model, as described in the text.

Figure 2

The product of efficiency$\times$BR for $\Sigma(1385)^{+}$ (left) and $\Sigma(1385)^{-}$ (right) in the three centrality classes used for the analysis. The branching ratio, BR, is included in the correction. The error bars represent the statistical uncertainties.

Figure 3

$p_{\rm T}$ spectra of $\Sigma(1385)^{+}$ (left) and $\Sigma(1385)^{-}$ (right) resonances in \PbPb collisions at \fivenn in three centrality classes. Empty and shaded boxes depict the total and uncorrelated uncertainties, respectively. The Blast-Wave fit functions are plotted up to 7 \GeVc. The measurements in \pp collisions at \seven~ and in \pPb collisions at \five (NSD)~ are quoted for comparisons.

Figure 4

Ratio of the measured \pt distributions of summed $\Sigma(1385)^{\pm}$ to model predictions from \Angantyr (green) and EPOS with (blue) and without (red) the UrQMD afterburner. Each panel corresponds to one of the three centrality classes in \PbPb collisions at \fivenn. Shaded bands represent the model statistical uncertainty, while bars correspond to the data total uncertainties.

Figure 5

Σ(1385)± to pion yield ratio measured in ALICE [24, 28] together with the STAR measurements [34, 35] in various collision systems and energies are reported as a function of the charged-particle multiplicity density at midrapidity, 〈dNch/dηlab〉|ηlab< 0.5|. Data are compared with model calculations for LHC energies from the GSI– Heidelberg grand canonical statistical hadronization model [1], PYTHIA 8.2 [33], PYTHIA8/Angantyr [32] and EPOS3 with and without UrQMD [31].