Hyperon Production in pp Collisions at √s = 7 TeV at the LHC with ALICE

The Σ0(Σ̄0) baryon is studied in the transverse momentum range between 1 and 10 GeV/c using the unique capability of the ALICE detector to measure low energy photons. The production of the strange and double-strange baryon resonances (Σ(1385)±,Ξ(1530)0) has been measured at mid-rapidity (|y| < 0.5) in proton-proton collisions at √ s = 7 TeV. Transverse momentum spectra are compared to QCD-inspired models, which in general underpredict the data.


Introduction
The study of strange baryons and their resonances in proton-proton (pp) collisions contributes to the understanding of hadron production mechanisms and provides a reference for tuning QCD-inspired event generators [1]. The strange-quark content makes these baryons a valuable tool in understanding production mechanisms, since the initial colliding projectiles contain no strange valence quarks and therefore all strange particles are created in the collision.
In addition, a measurement of resonance production in the pp system serves as a reference for understanding resonance production in heavy-ion collisions. Many resonances have lifetimes of a few fm/c, comparable to the lifetime of the hadronic phase [1,2] and are sensitive probes of the dynamical evolution of the fireball. Previous measurements at a collision energy √ s = 0.2 TeV with the STAR detector at RHIC have shown that the yields of Σ(1385) ± in Au-Au in comparison to pp collisions indicate the presence of rescattering and regeneration in the time span between chemical and kinetic freezeout [2]. A forthcoming analysis of strange baryon resonances in Pb-Pb collisions by the ALICE collaboration will further explore those effects at higher energy and density of the colliding system. The published ALICE results in pp collisions [1] will therefore serve as benchmark for the Σ(1385) ± and Ξ(1530) 0 baryons 1 . While yields of Σ 0 have been measured in many different colliding systems at low and intermediate/moderate energies, only one measurement exists in high energy nuclear collisions. The STAR detector reconstructed the electromagnetic Σ 0 → Λ+γ decay via the weak decay of the Λ → p+π − and γ conversions into e + e pairs in the detector material [3]. There, the cross section ratio Σ 0 /Λ = 0.16 +0.41 −0.09 was measured in minimum bias 0.2 TeV d+Au collisions. High energy nuclear collisions are of particular interest due to the possibility of measuring isospin degeneracy factors from Σ 0 and Λ yields and of opening new channels of hyperon production via partonic degrees of freedom [4]. a e-mail: Alexander.Borissov@cern.ch 1 Antiparticles here and below are not listed for brevity.  Figure 1. Invariant mass of Λγ andΛγ pairs, where the Λ orΛ is detected in the central tracking system and the photon is detected using the conversion method (left) or PHOS (right). The red curves are fits to the data using Gaussian peaks plus fourth-order polynomials to describe the background. The invariant mass distribution after the background subtraction is presented on the bottom part of the plot.

Detection of Σ 0 Hyperon
The data analysis is carried out using a sample of ∼250 million minimum-bias pp collisions at √ s = 7 TeV collected during 2010 run at the LHC. The Σ 0 is reconstructed through the decay Σ 0 → Λ + γ. The Λ is identified through its decay to a proton and a π − , while the photon is detected through its conversion to an e + e − pair. Tracks from the secondary vertex (V0) are reconstructed in the ALICE tracking system, which consists of the Inner Tracking System (ITS) and the Time Projection Chamber (TPC) [5]. The combined acceptance of the ITS and TPC is |η| < 0.9. The pairs of tracks are selected using a secondary vertex finding algorithm which is described in detail in Refs. [1,5,6].
Further refinment of the sample of converted photons and rejection of weak decays, mainly from K 0 s and Λ, is performed using electron identification via dE/dx and the restriction of the opening angle between electron and positron. This method of reconstruction is called the photon conversion method (PCM). The probability of photon conversion in the central tracking system is ∼ 0.08 and the reconstruction efficiency is about 0.67 [7]. To reduce the combinatorial background, a restriction on the Σ 0 phase space (an Armenteros-Podolanski cut) was used: p i l is the longitudinal momentum of a particle i (γ or Λ) with respect to the Σ 0 momentum direction, and p t corresponds to transverse momentum of Λ with respect to the Σ 0 momentum. A clean Σ 0 invariant mass peak reconstructed using this technique is presented in the left panel of Fig. 1.
The combined Σ 0 +Σ 0 signals are presented in Fig. 1 for the two independent photon detection methods: photon-conversion method (left) and photon detection in PHOS (right). The peaks are fitted using a Gaussian peak added to a polynomial that describes the background. These fits have χ 2 /nd f approximately equal to 1.   Table 1. Parameters extracted from the Levy-Tsallis fits to the transverse momentum spectra [1]. The values of dN/dy are calculated using the spectra in the measured range and the extrapolation of the fitted Levy-Tsallis function outside the measured range. First the statistical and second the systematic uncertainties are quoted here. Systematic uncertainties are the ones derived from the Levy-Tsallis fits only.
The reconstructed width of the Σ 0 is around 2.5 MeV and its value is in agreement with the value from simulations, see Fig. 2 right panel. These results allow the continuation of the Σ 0 analysis towards the extraction of its p T spectra and an evaluation of Σ 0 /Λ(p T ) cross section ratio from pp collisions at √ s = 7 TeV, as well as the analysis of ALICE data from proton-Pb and Pb-Pb collisions.
The transverse momentum spectra of both Σ(1385) ± are compared to standard tunes of PYTHIA 6 [9], PYTHIA 8 [10], HERWIG [11] and SHERPA [12] in Fig. 3. For all applied generators the overall   Figure 3. The transverse momentum spectrum of Σ(1385) + is compared to standard tunes of PYTHIA 6 [9] and PYTHIA 8 [10], the latest release of HERWIG (6.521) [11], and SHERPA release 1.4.6 [12]. The MC data are binned according to the data. Spectra points are plotted at the center of the p T interval. The lower panel shows the ratio data/MC. p T -independent uncertainties are not shown.
production cross sections are largerly underestimated. The same is true for the Ξ(1530) 0 when its p T spectrum is compared to the same set of generators [1]. Note, that upcoming ALICE data will allow more detailed investigation of the hyperon production at LHC, including its study in the underlying soft events and within jets.