Measurement of differential cross-section of pi0 mesons in pp collisions at $\sqrt{s}$ = 13 TeV with ALICE

In this thesis, a method for reconstruction of π0 mesons at high transverse momenta using merged clusters of the PHOS calorimeter was developed. For the PHOS calorimeter, the key parameter for selecting merged clusters is the parameter M02. To select merged clusters from π0-mesons, a 𝑝T-dependent function was constructed for the threshold value of the parameter M02. This is necessary to preserve statistics at high transverse momenta. Merged cluster selection is effective starting from 𝑝T = 30 GeV/𝑐. The mPHOS method does not allow for identifying π0-mesons, so to reconstruct the final spectrum, the purity of the π0-meson spectrum was calculated; after all corrections, it is 70–80%.

Using the developed method, the differential cross section for the production of π0-mesons in 𝑝𝑝 collisions at √𝑠 = 13 TeV was measured in the central rapidity region in the transverse momentum range of 30 < 𝑝T < 100 GeV/𝑐. The measured cross section agrees within the statistical and systematic errors with other methods for measuring π0-meson spectra in the ALICE experiment and allows for improved accuracy in measuring the π0-meson cross section in this transverse momentum range. The differential cross section for π0-meson production in 𝑝𝑝 collisions at √𝑠 = 13 TeV was obtained in the transverse momentum range 0.2 < 𝑝T < 200 GeV/𝑐 by combining the results of all π0-meson spectral measurement methods in the ALICE experiment. The obtained cross section is consistent with the averaged spectrum
of charged π-mesons measured in the same system of 𝑝𝑝 collisions at √𝑠 = 13 TeV. Calculations in the PYHIA8 generator with the Monash and Ropes tunes
overestimate the obtained cross section by 30–50% and do not reproduce the spectrum shape. On the other hand, calculations in the LHC EPOS generator also overestimate the experimental data by 10–20% for 𝑝T > 2 GeV/𝑐. Calculations within the framework of the pQCD collinear factorization theorem within the NLO pQCD
using the CT18 PDF and NNFF1.0 FF sets reproduce the measured cross section over a wide range of 𝑝T within the theoretical uncertainties, however, the DSS14 FF set overestimates the experimental data. Calculations of pQCD in the NLO approximation using the CT18 PDF and the updated BDSS FF show better agreement with the obtained cross section for 𝑝T ≳ 50 GeV/𝑐 compared to the earlier DSS14 FF set.

The results of the 𝑥T-scaling of the π0-meson production cross sections at √𝑠 = 0.9, 2.76, 7, 8, and 13 TeV are presented. The scaling exponent 𝑛(𝑥T,√𝑠1,√𝑠2) (where √𝑠1 >
√𝑠2) is calculated for all possible π0-meson production cross sections at √𝑠 measured by the ALICE experiment. The resulting value 𝑛π0eff = 5.01 ± 0.05 is consistent with the exponent determined from the charged π-meson spectra: 𝑛π±eff = 5.04 ± 0.02. The pT-dependent π0-meson production cross sections scaled with respect to the exponent 𝑛π0eff at various √𝑠 are in good agreement with each other.

A violation of the mT-scaling of the η-meson production cross section, derived from the measured π0-meson production cross section, was detected in
𝑝𝑝 collisions at √𝑠 = 13 TeV. Regarding the η- to π0-meson yields, a significant discrepancy is observed between the experimental data and the results of the mT-scaling model in the range of  𝑝T < 4 GeV/𝑐; the model overestimates the η/π0 ratio by up to 60% at 𝑝T = 0.55 GeV/𝑐.