Abstract
The first measurements of the transverse momentum (pT) spectra and integrated yields of Σ0 and Σ−0 baryons in proton-proton (pp) collisions at √s = 7 TeV are reported. The Σ0(Σ−0) signal is reconstructed via the Λ (Λ−) + γ decay channel by invariant mass analysis. The Λ (Λ−) baryon is reconstructed by its weak decay into p + π−(p− + π+), while the photon is detected exploiting the unique capability of the ALICE detector to measure low energy photons via conversion into e+e− pairs. A significant disagreement between the data and the predictions from the QCD-inspired Monte Carlo event generator PYTHIA is observed for the pT spectrum. The yield ratio of Σ0(Σ−0) to Λ is shown as a function of pT . Finally, the new data point for the integrated yield ratio of Σ0(Σ−0) to Λ from pp collisions at 7 TeV is found to be similar to the results obtained in different elementary collison systems (e+e−, pp) measured at collision energies below 100 GeV.
Highlights
In pp collisions at TeV center-of-mass energies, a significant disagreement [1] has been observed between the pT spectra of measured Λ and the spectra given by PYTHIA6 [2]
The spatial resolution of the Inner Tracking System (ITS) enables one to measure the distance of closest approach (DCA) to the primary vertex (PV) of charged tracks with pT > 1 GeV/c with a resolution of 75 μm in the transverse plane
The V0 detectors consist of two sets of scintillator tiles installed on both sides of the interaction point and covering -3.7 < η < -1.7 and 2.8 < η < 5.1. They are used for the interaction trigger of the experiment and for the rejection of beam-gas background interactions. √ The data sample analyzed in this paper was recorded in 2010 during the LHC pp run at s = 7 TeV using a minimum-bias (MBOR) trigger [7]
Summary
In pp collisions at TeV center-of-mass energies, a significant disagreement [1] has been observed between the pT spectra of measured Λ and the spectra given by PYTHIA6 [2]. The study of the strange baryons and of their resonances in such collisions provides an important reference for the tuning of the event generators. Since Σ0 and Λ have different isospin values (respectively 1 and 0) but have the same quark content (uds) and similar masses (mΣ0 − mΛ ∼ 77 MeV) [3], measuring their production and comparing them with PYTHIA model [2] could help to understand the strangeness production mechanisms, in particular those of baryons with non-zero isospin. Σ±,0 production measurements are important to quantify their feed-down contribution to the momentum spectra of their decay particles. Σ0 induces a sizable feed-down on the Λ [4] and contributes to the contamination from decay photons to the low momentum part of the direct photon spectrum [5]
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