$\overlineΣ^{\pm}$ production in pp and p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with ALICE

TL;DR

The paper reports the first measurement of anti-$\Sigma^{\pm}$ production in $pp$ and $p$--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV using the decay channel $\overline{\Sigma}^{\pm} \to \overline{n}\pi^{\pm}$ and a novel antineutron reconstruction with the PHOS calorimeter. It presents transverse-momentum spectra in $0.5<p_T<3$ GeV/$c$, compares them to several models (notably EPOS LHC/EPOS4, PYTHIA 8, PHOJET, DPMJET), and discusses integrated yields via Thermal-FIST and dynamical models. The results show that MPI-enabled EPOS models describe the data well, while models without MPI underperform at high $p_T$, and the $R_{pPb}$ factors are consistent with unity, with no strong strangeness dependence. The findings constrain strangeness production mechanisms and multiparton interaction effects in small collision systems and provide benchmarks for hadronization models.

Abstract

The transverse momentum spectra and integrated yields of anti-$Σ$ hyperons ($\overlineΣ^{\pm}$) have been measured in pp and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ALICE experiment. Measurements are performed via the newly accessed decay channel $\overlineΣ^{\pm} \rightarrow {\rm\overline{n}}π^{\pm}$. A new method of antineutron reconstruction with the PHOS electromagnetic spectrometer is developed and applied to this analysis. The $p_{\rm T}$ spectra of $\overlineΣ^{\pm}$ are measured in the range $0.5 < p_{\rm T} < 3$ GeV/$c$ and compared to predictions of the PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4 models. The EPOS LHC and EPOS4 models provide the best descriptions of the measured spectra both in pp and p-Pb collisions, while models which do not account for multiparton interactions provide a considerably worse description at high $p_{\rm T}$. The total yields of $\overlineΣ^{\pm}$ in both pp and p-Pb collisions are compared to predictions of the Thermal-FIST model and dynamical models PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4. All models reproduce the total yields in both colliding systems within uncertainties. The nuclear modification factors $R_\mathrm{pPb}$ for both $\overlineΣ^{+}$ and $\overlineΣ^{-}$ are evaluated and compared to those of protons, $Λ$ and $Ξ$ hyperons, and predictions of EPOS LHC and EPOS4 models. No deviations of $R_\mathrm{pPb}$ for $\overlineΣ^{\pm}$ from the model predictions or measurements for other hadrons are found within uncertainties.

Figures (11)

• Figure 1: Shower shape parameters $\lambda_\mathrm{long}$ vs. $\lambda_\mathrm{short}$ for $\gamma$ (left) and ${\rm \overline{n}}$ (right) simulated in p--Pb collisions simulated with the DPMJET 3.0-5 dpmjet:2000he event generator.
• Figure 2: Fraction of different types of particles which produce clusters in PHOS before (left) and after the application of a default set of selections (right) as a function of cluster energy in p--Pb collisions simulated with the DPMJET 3.0-5 dpmjet:2000he event generator.
• Figure 3: Decay topology of the $\overline{\Sigma}^{\pm} \to {\rm \overline{n}} \pi^\pm$ decay. In the reconstruction procedure, the direction of the ${\rm \overline{n}}$ momentum is approximated by the vector pointing from the the PV to the cluster in the PHOS. DCA of $\pi^\pm$ is not used in the analysis.
• Figure 4: Same event, mixed event, signal from data, and MC true signal invariant mass distributions of ${\rm \overline{n}}\pi^+$ (top row) and ${\rm \overline{n}}\pi^-$ (bottom row) pairs in pp (left column) and p--Pb (right column) collisions for two selected $p_{\rm{T}}$ bins. Solid and dashed vertical lines represent integration ranges at 1/4 and 1/3 of the maximum, respectively. The PYTHIA 8 pythia and DPMJET 3.0-5 dpmjet:2000he event generators were used for Monte-Carlo simulations.
• Figure 5: Comparison of peak position (left column) and full width at half maximum, FWHM, (right column) in pp (top row) and p--Pb collisions (bottom row) in data and MC simulations. The PYTHIA 8 pythia and DPMJET 3.0-5 dpmjet:2000he event generators were used for Monte-Carlo simulations. The PDG mass values of $\overline{\Sigma}^{+}$ and $\overline{\Sigma}^{-}$ParticleDataGroup:2024cfk are shown with short and long dashed lines, respectively.