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

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.


Figure 1

The invariant mass distributions of $\Xi^-$ (a) and $\Omega^-$ (b) baryoncandidates (solid histograms) and their antiparticles (dashed histograms). Also marked (in shaded blocks) background sampling regions used in the signal extraction. The entire measured $p_{\rm T}$ range is presented.
[png]   [pdf]   [eps]

Figure 2

(a) $\Xi^-$ and $\Omega^-$ baryon (solid circles and squares, respectively)and their antiparticle (open symbols) spectra, shown with Tsallis fits (b) Experimental data to Monte Carlo (PYTHIA Perugia 2011) comparison. The errors are added in quadrature. The normalization uncertainty is shown as a black band.
[png]   [pdf]   [eps]

Figure 3

(a) d$N$/d$y$ and (b) $\langle p_\mathrm{T}\rangle$ of $\Xi^{\pm}$ and $\Omega^{\pm}$ as a function of collision energy The STAR and CMS data are normalized to NSD (see text) events, STAR $\Xi^{\pm}$ and $\Omega^{\pm}$ are representedby open rhombuses and stars, respectively. CMS $\Xi^{\pm}$ measurements are shown as open triangles,and ALICE $\Xi^{\pm}$ and $\Omega^{\pm}$ as filled circles and squares Multi-strange baryons produced using PYTHIA Perugia 2011 simulation ($\Xi^{\pm}$ baryons as a long-dashed curveand $\Omega^{\pm}$ baryons as a dashed curve) are plotted for reference The uncertainties are added in quadrature.
[png]   [pdf]   [eps]

Figure 4

($\Omega^-$+$\overline{\Omega}^+$) to ($\Xi^-$+$\overline{\Xi}^+$) ratio in $\sqrt{s}~=~7$~TeV pp events as a function of (${m_{\mathrm{T}} - m_0}$) Experimental data (closed symbols: data points; dashed curve: ratio of Tsallis fits),and PYTHIA Perugia 2011 simulation (solid curve). Errors on experimental points were added in quadrature.
[png]   [pdf]   [eps]