Multi-strange baryon production in pp collisions at $\sqrt{s}$ = 7 TeV with ALICE

Abstract: A measurement of the multi-strange $\Xi^-$ and $\Omega^-$ baryons and their antiparticles by the ALICE experiment at the CERN Large Hadron Collider (LHC) is presented for inelastic proton-proton collisions at centre of mass energy of 7 TeV. The transverse momentum ($p_{\rm T}$) distributions were studied at mid-rapidity (|y| < 0.5) in the range of 0.6 < $p_{\rm T}$ < 8.5 GeV/$c$ for $\Xi^-$ and $\Xi^+$ baryons, and in the range of 0.8 < $p_{\rm T}$ < 5 GeV/$c$ for $\Omega^-$ and $\Omega^+$. Baryons and antibaryons were measured as separate particles and we find that the baryon to antibaryon ratio of both particle species is consistent with unity over the entire range of the measurement. The statistical precision of the current LHC data has allowed us to measure a difference between the mean $p_{\rm T}$ of $\Xi^-$ ($\Xi^+$) and $\Omega^-$ ($\Omega^+$). Particle yields, mean $p_{\rm T}$, and the spectra in the intermediate $p_{\rm T}$ range are not well described by the PYTHIA Perugia 2011 tune Monte Carlo event generator, which has been tuned to reproduce the early LHC data. The discrepancy is largest for $\Omega^-$ ($\Omega^+$). This PYTHIA tune approaches the $p_{\rm T}$ spectra of $\Xi^-$ and $\Xi^+$ baryons below $p_{\rm T}$ < 0.85 GeV/$c$ and describes the $\Xi^-$ and $\Xi^+$ spectra above $p_{\rm T}$ > 6.0 GeV/$c$. We also illustrate the difference between the experimental data and model by comparing the corresponding ratios of ($\Omega^{-}+\Omega^+)/(\Xi^-+\Xi^+)$ as a function of transverse mass.

• The paper presents a detailed measurement of multi-strange baryon production using transverse momentum spectra from approximately 130 million pp collision events.
• The analysis employs ALICE's inner tracking system and time projection chamber to measure Ξ and Ω baryons across a wide pT range at mid-rapidity.
• The study finds that PYTHIA underestimates low to intermediate pT yields, emphasizing the need for improved models of strangeness production.

Multi-Strange Baryon Production in Proton-Proton Collisions at 7 TeV with ALICE

The presented paper by the ALICE Collaboration provides a detailed analysis of multi-strange baryon production in proton-proton (pp) collisions at a center-of-mass energy of 7 TeV, utilizing data from the ALICE experiment at CERN's Large Hadron Collider (LHC). The study focuses on the production of multi-strange baryons, particularly the $\Xi$ and $\Omega$ species, and their antiparticles. These baryons are compelling due to their heavy strange quark content and provide insights into particle production mechanisms in high-energy collisions.

Data and Methodology

The dataset includes approximately 130 million minimum bias pp collision events. The analysis focuses on the transverse momentum ($p_T$) spectra and yields of $\Xi$ and $\Omega$ baryons at mid-rapidity ($|y| < 0.5$). The $p_T$ spectra were recorded over the range of 0.6 to 8.5 GeV/$c$ for the $\Xi$ baryons and 0.8 to 5 GeV/$c$ for the $\Omega$ baryons. These measurements utilized ALICE's capabilities, primarily using its inner tracking system and time projection chamber for high-precision particle reconstruction.

A key finding is that the baryon-to-antibaryon ratio remains consistent with unity, indicating an even production of particles and antiparticles across the measured momentum range. This symmetry is essential for understanding baryon formation processes in pp collisions where initial states lack strange valence quarks.

Results and Comparisons

Detailed Tsallis and power-law fits to the $p_T$ distributions reveal nuanced differences in the production dynamics between the various baryon species. The fit parameters suggest discrepancies between the experimental $p_T$ spectra and those predicted by the PYTHIA Perugia 2011 Monte Carlo event generator. The PYTHIA model inaccurately predicts the yield and spectral shape of multi-strange baryons, particularly underestimating production in the low to intermediate $p_T$ range. These discrepancies highlight the limitations of current perturbative QCD models in capturing non-perturbative aspects and strangeness production in pp collisions.

The paper also presents the ratio of $(\Omega+\overline{\Omega}) / (\Xi+\overline{\Xi})$ as a function of transverse mass, which demonstrates an increase up to a specific range, followed by a potential saturation. This pattern suggests differential production mechanisms or suppression effects that are not fully replicated by the PYTHIA model, emphasizing the need for refined theoretical approaches or model tuning to better account for strange quark dynamics.

Implications and Future Directions

These results provide critical benchmarks for improving theoretical models of particle production. The observed differences between experimental data and existing theoretical predictions underscore the complexity of multi-strange baryon production mechanisms and the necessity for models that more accurately describe the underlying physics of strangeness production.

Future experiments and refined models must consider the interplay of soft and hard QCD processes to enhance our understanding of multi-strange baryon production. The ALICE experiment's ongoing upgrades and future runs at higher luminosities will likely yield higher precision data, essential for resolving current theoretical challenges and refining our understanding of strangeness dynamics in high-energy particle collisions.