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KS0 production in p+p interactions measured by NA61/SHINE by:0 (Source: Crossref)
    This article is part of the issue:


    The NA61/SHINE collaboration measured the production of KS0 in p+p interactions at two beam momenta: 80 and 158GeV/c. The π++π decay of KS0 (branching ratio of 69.2%) was measured via the invariant mass method. The rapidity and transverse momentum distributions of KS0 are presented and compared to transport model predictions. The measured mean multiplicities of KS0 mesons equal to KS0=0.120±0.001(stat.)±0.005(sys.) (80GeV/c) and KS0=0.162±0.001(stat.)±0.011(sys.) (158GeV/c) are compared with the available data in the energy range sNN=3–32GeV.

    ∗ Talk presented at BPU11 Congress.

    PACS: 13.25.Es

    1. Introduction

    NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) is a fixed-target experiment located at the CERN Super Proton Synchrotron. Its research program is focused on the systematic measurement of hadron production in proton–proton, proton–nucleus, and nucleus–nucleus interactions. The aim of these measurements is to study the phase diagram of strongly interacting matter and search for the critical point.

    The production of strange particles is particularly interesting, because enhanced strange particle production has been proposed as a signal of Quark–Gluon Plasma (QGP) formation more than 50 years ago.1 Therefore, studying the production of KS0 mesons in p+p collisions is important as a reference for possible modifications of strangeness production in nucleus–nucleus collisions, as well as for understanding strangeness production in elementary interactions.

    Since 2009, NA61/SHINE2 has collected data on p+p, Be+Be, Ar+Sc, Xe+La and Pb+Pb collisions in the beam momentum range of 13–158AGeV/c. This unique two-dimensional scan allows for a systematic investigation of KS0 production in the few GeV energy range, where the onset of deconfinement is predicted to occur.3 In this contribution, we present the results of KS0 production in p+p collisions at 80 and 158GeV/c.

    This paper is organized as follows. Section 2 describes the experimental facility of the NA61/SHINE detector, while Sec. 3 explains the analysis workflow. Section 4 is devoted to the experimental results, and Sec. 5 provides a summary to conclude this paper.

    2. NA61/SHINE Detector

    The results presented in this contribution are based on datasets containing 5 million and 58 million p+p events taken at beam momenta of 80 and 158GeV/c, respectively. These datasets were recorded by the NA61/SHINE fixed target large acceptance hadron detector at the CERN SPS accelerator complex.4 Figure 1 illustrates the layout of the detector, which includes four large volume time projection chambers (TPCs). Two of the TPCs (VTPC1 and VTPC2) are situated within superconducting dipole magnets, while two larger TPCs (MTPC-R and MTPC-L) are placed downstream of the magnets to provide acceptance at high momenta. A fifth smaller TPC (GAP-TPC) is located on the beamline between VTPC1 and VTPC2.

    Fig. 1.

    Fig. 1. The schematic layout of the NA61/SHINE experiment at the CERN SPS during p+p 80 and 158GeV/c data taking.

    A set of beam position detectors located upstream of the target provides precise information about beam trajectory. The liquid hydrogen target (LHT), composed of a cylindrical vessel of 20.29cm length and 3cm diameter filled with liquid hydrogen, is situated upstream of the entrance window of VTPC-1. Particle identification is performed basing on the measurement of specific energy loss (dEdx) in the TPC gas and of velocity obtained from Time of Flight (ToF) detectors positioned symmetrically to the beamline.

    3. Analysis

    The KS0 mesons were identified through their charged decay channel: KS0π++π with a branching ratio of 69.2%.5 The decay particles form the so-called V0 topology. The first step of the analysis involves processing the raw measured data using the reconstruction chain. During this step, the trajectories and momenta of tracks are reconstructed, as well as the position of the main vertex. Subsequently, V0 candidates were searched for, assuming oppositely charged daughter particles emerging from the same vertex. The V0 candidates could either be real particles, which would generate a peak in the invariant mass spectra, or a combinatorial background. Several selection criteria were applied to reduce the background in the invariant mass spectra. To determine the raw KS0 yield in different rapidity-pT (ypT) bins, a Lorentz distribution and a second-order polynomial describing the background were fit to the data (see Fig. 2). The resulting signal yields were then normalized to the number of analyzed events and the area of a single ypT bin. To correct for losses due to trigger bias, geometrical acceptance, reconstruction efficiency and selection criteria applied in the analysis, the generated simulation data underwent the same reconstruction and analysis procedure.

    Fig. 2.

    Fig. 2. (Color online) The invariant mass distributions of KS0 candidates for experimental data (left) and MC (right). The dashed vertical lines indicate the regions over which the KS0 signal was integrated. The signal data points and the fitted background are shown in black, the fitted signal in blue and the total fit results in red. These plots are obtained in p+p interactions at 80GeV/c.

    4. Results

    The KS0 yields in different rapidity bins were obtained from the corresponding measured transverse momentum distributions. Extrapolation to the high pT region outside of the acceptance was performed using the function f(pT)=ApTepT2+m02T, where m0 is the mass of the KS0 and T is the inverse slope parameter. The transverse momentum distributions at mid rapidity (y0) are shown in Fig. 3.

    Fig. 3.

    Fig. 3. The transverse momentum distributions at mid rapidity (y0) of KS0 mesons in inelastic p+p interactions at 80GeV/c (left) and 158GeV/c6 (right).

    Kaon yields in each rapidity bin were obtained from the corresponding measured transverse momentum distributions. The resulting dndy spectrum of KS0 mesons in inelastic p+p interactions at 80 and 158GeV/c6 is plotted in Fig. 4.

    Fig. 4.

    Fig. 4. Rapidity distributions of KS0 mesons in inelastic p+p interactions at 80GeV/c (left) and 158GeV/c (right).

    The mean multiplicity of KS0 mesons was calculated as the sum of measured points in Fig. 4 and the integral below linear functions through the last two measured points on both sides representative for the unmeasured region (for the beam momenta of 158GeV/c). Since there is not the same number of measured points for the beam momenta at 80GeV/c, in order to calculate the mean multiplicity of KS0 mesons, last two measured points are mirrored (in respect to y=0) and the procedure was repeated as described above. Calculated mean multiplicities of KS0 mesons in p+p interactions at 80 and 158GeV/c6 compared with results from other prominent experiments are shown in Fig. 5.

    Fig. 5.

    Fig. 5. Comparison of the mean multiplicities of KS0 mesons in p+p interactions at 80 and 158GeV/c with results from other prominent experiments.

    The comparison of the rapidity distributions with results obtained in other experiments such as results from Brick et al. at FNAL7 and from Ammosov et al. at SERPUKHOV8 as well as with predictions obtained from K+ and K yields published by NA61/SHINE for inelastic p+p interactions at 80 and 158GeV/c9 are shown in Fig. 6. These predictions are based on valence-quark counting arguments10 and lead to the formula 14(NK++3NK).

    Fig. 6.

    Fig. 6. Comparison of the rapidity distributions of KS0 mesons in p+p interactions at 80GeV/c (left) and 158GeV/c6 (right) with world results.

    Rapidity distributions are compared with theoretical models prediction such as EPOS 1.99,11,12 PHSD,13,14 SMASH 2.015 and UrQMD 3.416,17 in Fig. 7. The experimental data are best described with SMASH 2.0 (80GeV/c) and EPOS 1.99 (158GeV/c). All other models overpredict the KS0 yield.

    Fig. 7.

    Fig. 7. Comparison of the rapidity distributions of KS0 mesons in p+p interactions at 80GeV/c (left) and 158GeV/c6 (right) and theoretical models prediction.

    5. Conclusion

    The rapidity distributions are in agreement with results from other experiments at nearby beam momenta. Mean multiplicities from model calculations deviate by up to 20% from the measurements. The results of KS0 production in proton–proton interactions presented in this paper significantly improve, with their high statistical precision, the knowledge of strangeness production in elementary interactions.


    This work has been partially supported by the ICTP-SEENET-MTP NT-03 Program.


    Marjan Ćirković


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