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We look for fluctuations expected for the QCD critical point using an intermittency analysis in the transverse momentum phase space of protons produced around midrapidity in the 12.5 % most central C+C, Si+Si and Pb+Pb collisions at the maximum SPS energy of 158 A  GeV. We find evidence of power-law fluctuations for the Si+Si data. The fitted power-law exponent ϕ 2 = 0 . 96 - 0.25 + 0.38 (stat.) ± 0.16 (syst.) is consistent with the value expected for critical fluctuations. Power-law fluctuations had previously also been observed in low-mass π + π - pairs in the same Si+Si collisions.

A measurement of charged hadron pair correlations in two-dimensional$$\\Delta \\eta \\Delta \\phi $$Δ η Δ ϕ   space is presented. The analysis is based on total 30 million central Be + Be collisions observed in the NA61/SHINE detector at the CERN SPS for incident beam momenta of 19 A , 30 A , 40 A , 75 A , and 150 A  $$\\text {Ge} \\text {V}/c$$Ge / c . Measurements were carried out for unlike-sign and like-sign charge hadron pairs independently. The$$C(\\Delta \\eta ,\\Delta \\phi )$$C ( Δ η , Δ ϕ ) correlation functions were compared with results from a similar analysis on p + p interactions at similar beam momenta per nucleon. General trends of the back-to-back correlations are similar in central Be + Be collisions and p + p interactions, but are suppressed in magnitude due to the increased combinatorial background. Predictions from the Epos and UrQMD models are compared to the measurements. Evolution of an enhancement around$$(\\Delta \\eta ,\\Delta \\phi ) = (0,0)$$( Δ η , Δ ϕ ) = ( 0 , 0 ) with incident energy is observed in central Be + Be collisions. It is not predicted by both models and almost non-existing in proton–proton collisions at the same momentum per nucleon.

The measurement of$$K^{*}(892)^0$$K ∗ ( 892 ) 0 resonance production via its$$K^{+}\\pi ^{-}$$K + π - decay mode in inelastic p+p collisions at beam momentum 158 $$\\text{ Ge }\\text{ V }\\!/\\!c$$Ge V / c ($$\\sqrt{s_{NN}}=17.3$$s NN = 17.3  $$\\text{ Ge }\\text{ V }$$Ge V ) is presented. The data were recorded by the NA61/SHINE hadron spectrometer at the CERN Super Proton Synchrotron. The template method was used to extract the$$K^{*}(892)^0$$K ∗ ( 892 ) 0 signal and double-differential transverse momentum and rapidity spectra were obtained. The full phase-space mean multiplicity of$$K^{*}(892)^0$$K ∗ ( 892 ) 0 mesons was found to be$$(78.44 \\pm 0.38 \\mathrm {(stat)} \\pm 6.0 \\mathrm {(sys)) \\cdot 10^{-3}}$$( 78.44 ± 0.38 ( stat ) ± 6.0 ( sys ) ) · 10 - 3 . The NA61/SHINEresults are compared with the Epos1.99 and Hadron Resonance Gas models as well as with world data from p+p and nucleus–nucleus collisions.

New results on the production of charged pions in p+p interactions are presented. The data come from a sample of 4.8 million inelastic events obtained with the NA49 detector at the CERN SPS at 158 GeV/c beam momentum. Pions are identified by energy loss measurement in a large TPC tracking system which covers a major fraction of the production phase space. Inclusive invariant cross sections are given on a grid of nearly 300 bins per charge over intervals from 0 to 2 GeV/c in transverse momentum and from 0 to 0.85 in Feynman x. The results are compared to existing data in overlapping energy ranges.

Measurements of inclusive spectra and mean multiplicities of π ± , K ± , p and p ¯ produced in inelastic p + p interactions at incident projectile momenta of 20, 31, 40, 80 and 158  GeV / c ( s = 6.3, 7.7, 8.8, 12.3 and 17.3  GeV , respectively) were performed at the CERN Super Proton Synchrotron using the large acceptance NA61/SHINE hadron spectrometer. Spectra are presented as function of rapidity and transverse momentum and are compared to predictions of current models. The measurements serve as the baseline in the NA61/SHINE study of the properties of the onset of deconfinement and search for the critical point of strongly interacting matter.

This paper reports measurements of two-pion femtoscopic correlations in Be+Be collisions at a beam momentum of 150 A GeV / c  (energy available in the center-of-mass system for nucleon pair s NN = 16.84 GeV) by the NA61/SHINE experiment at the CERN SPS accelerator. The obtained momentum space correlation functions can be well described by a Lévy distributed source model. The transverse mass dependence of the Lévy source parameters is presented, and their possible theoretical interpretations are discussed. The results show that the Lévy exponent α is approximately constant as a function of m T  , and far from both the Gaussian case of α = 2 or the conjectured value at the critical endpoint, α = 0.5 . The radius scale parameter R shows a slight decrease in m T  , which can be explained as a signature of transverse flow. Finally, an approximately constant trend of the intercept parameter λ as a function of m T  was observed, similar to previous NA44 S + Pb results (obtained with a Gaussian approximation, but unlike RHIC results).

The NA61/SHINE experiment at the CERN Super Proton Synchrotron studies the onset of deconfinement in strongly interacting matter through a beam energy scan of particle production in collisions of nuclei of varied sizes. This paper presents results on inclusive double-differential spectra, transverse momentum and rapidity distributions and mean multiplicities of π ± , K ± , p and p ¯ produced in 40 Ar+ 45 Sc collisions at beam momenta of 13 A , 19 A , 30 A , 40 A , 75 A and 150 A   Ge V / c . The analysis uses the 10% most central collisions, where the observed forward energy defines centrality. The energy dependence of the K ± / π ± ratios as well as of inverse slope parameters of the K ± transverse mass distributions are placed in between those found in inelastic p + p and central Pb + Pb collisions. The results obtained here establish a system-size dependence of hadron production properties that so far cannot be explained either within statistical or dynamical models.

Strong interactions preserve an approximate isospin symmetry between up ( u ) and down ( d ) quarks, part of the more general flavor symmetry. In the case of K meson production, if this isospin symmetry were exact, it would result in equal numbers of charged ( K + and K − ) and neutral ( K 0 and K ¯ 0 ) mesons produced in collisions of isospin-symmetric atomic nuclei. Here, we report results on the relative abundance of charged over neutral K meson production in argon and scandium nuclei collisions at a center-of-mass energy of 11.9 GeV per nucleon pair. We find that the production of K + and K − mesons at mid-rapidity is (18.4 ± 6.1)% higher than that of the neutral K mesons. Although with large uncertainties, earlier data on nucleus-nucleus collisions in the collision center-of-mass energy range 2.6 < s N N < 200 GeV are consistent with the present result. Using well-established models for hadron production, we demonstrate that known isospin-symmetry breaking effects and the initial nuclei containing more neutrons than protons lead only to a small (few percent) deviation of the charged-to-neutral kaon ratio from unity at high energies. Thus, they cannot explain the measurements. The significance of the flavor-symmetry violation beyond the known effects is 4.7 σ when the compilation of world data with uncertainties quoted by the experiments is used. New systematic, high-precision measurements and theoretical efforts are needed to establish the origin of the observed large isospin-symmetry breaking. Strong interaction is blind to quark flavor, so collisions of nuclei with the same number of protons and neutrons should generate the same number of charged and neutral kaons. Here, instead, the authors show a significant excess of charged over neutral kaon production in Ar+Sc nuclei collisions, compatibly with earlier measurements which however suffered from larger uncertainties, and show that known effects cannot explain the result.

The critical point of strongly interacting matter is searched for at the CERN SPS by the NA61/SHINE experiment in central 40 Ar +  45 Sc collisions at 13  A , 19  A , 30  A , 40  A , and 75  A  GeV/ c . The dependence of the second-order scaled factorial moments of proton multiplicity distributions on the number of subdivisions in transverse momentum space is measured. The intermittency analysis uses statistically independent data sets for every subdivision in transverse and cumulative-transverse momentum variables. The results obtained do not indicate the searched intermittent pattern. An upper limit on the fraction of correlated protons and the intermittency index is obtained based on a comparison with the Power-law Model.

The yields of K S 0 mesons have been measured in inelastic p+p interactions at incident projectile momenta of 31, 40 and 80  Ge V / c ( s NN = 7.7 , 8.8 and 12.3  Ge V , respectively). The data were recorded by the NA61 / SHINE spectrometer at the CERN Super Proton Synchrotron and the K S 0 mesons identified via their decays into π + π - pairs. Double-differential distributions are presented as function of transverse momentum and rapidity. The mean multiplicities of K S 0 mesons were determined to be ( 5.95 ± 0.19 ( s t a t ) ± 0.30 ( s y s ) ) × 10 - 2 at 31  Ge V / c , ( 7.61 ± 0.13 ( s t a t ) ± 0.43 ( s y s ) ) × 10 - 2 at 40  Ge V / c and ( 11.58 ± 0.12 ( s t a t ) ± 0.55 ( s y s ) ) × 10 - 2 at 80  Ge V / c . The results on K S 0 production are compared with the production of charged kaons in corresponding reactions and with model calculations ( Epos1.99 , SMASH 2.0 and PHSD) as well as with published data from other experiments.