Measurement of charged Anti-Sigma hyperons spectra with electromagnetic calorimeter PHOS of ALICE

This thesis presents the results of the first measurement of differential anti-Σ± hyperon production spectra based on transverse momentum
in proton–proton and proton–nucleus collisions at energies of √𝑠NN = 5.02 TeV in the range 0.5 < 𝑝T < 3 GeV/𝑐 in the central rapidity region.
The measurements were conducted using the decay channel Σ± → $\bar{n}$𝜋±, in which antineutrons are detected by a signal in the electromagnetic calorimeter
PHOS, and charged pions are detected in the internal tracking system and the time-projection chamber. A method for identifying antineutrons based on electromagnetic-hadron shower profile analysis and time-of-flight measurements in the PHOS calorimeter was developed and experimentally verified. This study is the first measurement in the ALICE experiment, where antineutrons are reconstructed using electromagnetic calorimeter data.

The obtained Σ± spectra are compared in detail with calculations from the following Monte Carlo generators:
– for pp collisions: PYTHIA 8, EPOS LHC, EPOS4, and PHOJET;
– for p–Pb collisions: DPMJET, EPOS LHC, and EPOS4.

It was found that the EPOS LHC and EPOS4 models satisfactorily reproduce both the shape of the Σ± spectra and their absolute yield in pp and p–Pb
collisions within the limits of the total systematic and statistical uncertainties. The PYTHIA 8 and PHOJET models significantly underestimate the yields of anti-Σ hyperons at high pT (by a factor of approximately three), while DPMJET overestimates the yield at low pT and underestimates it by a factor of approximately four at high pT in p–Pb collisions. The observed discrepancies indicate the need to take into account collective effects and multiparton interactions included in the EPOS family of models, even for small and intermediate collision systems. The integrated Σ± yields per unit rapidity in p–p and p–Pb collisions were determined. To account for phase space regions inaccessible for direct measurements, spectra were extrapolated using various parameterizations. The contribution of the extrapolated regions is under
control and is approximately 16–24% in the low-pT region and 1–3% in the high-pT region in pp collisions, and 8–14% and 5–12%, respectively, in p–Pb collisions.

The measured integral yields are compared with the predictions of the Thermal-FIST statistical thermodynamic model and dynamic generators (PYTHIA 8, EPOS LHC, EPOS4, PHOJET, and DPMJET). Both the thermodynamic model and the dynamic generators agree with the experimental values of the integral Σ± yields within the measurement uncertainties for both pp and p–Pb collisions. The ratios of the integral yields of anti-Σ± to the yields of strange and non-strange hadrons are investigated. It has been demonstrated that current dynamical models have difficulty describing the absolute yields of Λ-hyperons and systematically overestimate the Σ/Λ ratio, while the ratios to kaons are reproduced at an acceptable level. The statistical Thermal-FIST model, in contrast, shows good agreement with the data for the Σ/Λ ratio, but slightly underestimates the ratios to kaons. This difference indicates the sensitivity of the relative yields of strange baryons to hadron formation mechanisms and can be used for further testing of dynamical and statistical models in high-multiplicity events in pp and p–Pb collisions. 

The nuclear modification factor 𝑅pPb for anti-Σ+ and anti-Σ− is calculated and compared with similar quantities for protons, Λ-, and Ξ-hyperons. The 𝑅pPb values for Σ± agree within the uncertainty limits with data for other baryons, indicating the absence of a pronounced dependence of nuclear effects on strangeness in the studied range
𝑝T. The EPOS LHC and EPOS4 models correctly describe the experimental 𝑅pPb values for anti-Σ hyperons. 

The method for reconstructing antineutrons in the PHOS electromagnetic calorimeter, developed and tested in the thesis, creates a fundamental opportunity for the systematic study of a wide range of observables. The obtained experimental data on the Σ± spectra in pp and p–Pb collisions form a new basis for validating and improving
theoretical models of strange baryon production in relativistic collisions and lay the foundation for future high-precision measurements in future runs at the Large Hadron Collider and in other high-energy experiments.