Measurement of $ω$ meson production in pp and p$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}} = 5.02}$ TeV

TL;DR

This study presents the first measurement of the ω meson production cross sections in pp and p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with ALICE, using the decay $\omega \rightarrow \pi^{+}\pi^{-}\pi^{0}$ and three complementary $\pi^{0}$ reconstruction methods to reduce systematics. It reports the nuclear modification factor $R_{\text{pPb}}$ for ω mesons, finding unity within uncertainties across $2.2$–$20$ GeV/$c$, implying no CNM effects in this momentum range, and compares the ω yield to predictions from PYTHIA, EPOS LHC, and DPMJET, highlighting significant model shortcomings. The analysis also examines the ω/$\pi^{0}$ production ratio in pp and p$-$Pb, observing no strong system-size dependence and hints of an energy-dependent trend at $p_{\rm T}>4$ GeV/$c$, consistent with simulation trends. Overall, the results emphasize tension between experimental data and contemporary MC models and provide essential benchmarks for understanding meson production and hadronization at LHC energies.

Abstract

We present the measurement of the $p_{\rm T}$-differential production cross section of $ω$ mesons in pp and p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV at midrapidity by ALICE. In addition, the first measurement of the nuclear modification factor $R_{\text{pPb}}$ for $ω$ mesons at LHC energies is presented, complementing the existing measurements of lighter neutral mesons such as the $π^0$ and $η$. Within the measured $p_{\rm T}$-range, the $R_{\text{pPb}}$ of $ω$ mesons is compatible with no cold nuclear matter effects within the uncertainties, consistent with previous measurements at lower energies. The $ω/π^0$ ratio is presented for both collision systems, showing no collision system dependence within the uncertainties. The comparison to previously published $ω/π^0$ ratios at lower and higher collision energies in pp collisions suggests a decreasing trend of the ratio above $p_{\rm T}=~4 $ GeV/$c$ with increasing collision energy. The data in both collision systems are compared to predictions from PYTHIA, EPOS LHC and DPMJET event generators, revealing significant shortcomings in these models' ability to describe the production of $ω$ mesons.

Figures (7)

• Figure 1: Invariant mass distributions of $\pi^{+}\pi^{-}\pi^{0}$ triplets in the indicated $p_{\rm T}$ ranges for $\pi^{0}$'s reconstructed with PCM (top), PCM-EMCal (middle), and EMCal (bottom) in pp (left) and p--Pb (right) collisions. Subtraction of the third-order polynomial background description (blue) from the $\pi^{+}\pi^{-}\pi^{0}$ candidates (black) results in the signal of $\omega$ candidates (red). In p--Pb collisions, this signal is scaled by a factor of three for better visibility. The vertical dashed lines denote the invariant mass region where the raw yield was obtained through bin counting, as described in \ref{['sec:Omega']}.
• Figure 2: Correction factors $\epsilon$ applied to the raw $\omega$ yields for each $\pi^{0}$ reconstruction method in pp and p--Pb collisions. The correction factor comprises the geometrical acceptance ($A$), the reconstruction efficiency ($\epsilon_\text{rec}$), and normalization to azimuthal and rapidity ranges.
• Figure 3: Ratio of the $\omega$ meson production cross sections extracted using the three different $\pi^{0}$ reconstruction methods to a combined Lévy-Tsallis parametrization. Both in pp (left) and in p--Pb (right), agreement between the reconstruction methods is observed within the statistical (bars) and systematic (boxes) uncertainties.
• Figure 4: Lorentz-invariant $\omega$ meson production cross section in pp (open markers) and p--Pb (closed markers). Statistical uncertainties are represented by vertical error bars, while boxes show the systematic uncertainties. Furthermore shown are Lévy-Tsallis parametrizations of the two cross sections and two predictions from simulations of the $\omega$ meson production per collision system. The lower panels contain the ratios of the data and simulations to the parametrization of the data points in the respective collision system. A gray box in each ratio panel depicts the normalization uncertainty of the visible minimum bias cross section.
• Figure 5: Production ratio $\omega$/$\pi^{0}$ in pp and p--Pb collisions calculated using the $\omega$ meson production measurements and the corresponding charged ALICE:2019hno and neutral pion ALICE:2018vhm references, respectively. The charged pion reference was scaled up by 3.3% (see text for details). Also shown are two predictions from simulations per collision system, with bands representing their statistical uncertainties, as well as a $m_\text{T}$ scaling curve, which converges towards a high-$p_{\rm T}$ constant (C) fitted to the ratio for $p_{\rm T}$$>$ 4 GeemV/$c$.