Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200A GeV S 1 S and 158A GeV Pb 1 Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions. The purpose of studying ultrarelativistic heavy-ion collisions is to understand the nature of hadronic matter under extreme conditions. Specifically, we are interested in a new form of matter, quark-gluon plasma, which may be produced in such collisions. Transverse momentum distributions are one of the most common tools used in studying high energy collisions. This is because the transverse motion is generated during the collision and hence is sensitive to the dynamics. More than 45 years ago, Fermi proposed a statistical method [1] to understand the results of high energy hadron-hadron collisions. Because of saturation of the phase space, the multiparticle production resulting from the high energy elementary collisions is consistent with a thermal description [1][2][3]. In heavy-ion collisions hydrodynamical behavior, that is, local thermal equilibrium and collective motion, may be expected because of the large number of secondary scatterings.It is now possible to identify and quantitatively measure the collective motion by systematic studies of results from different collision systems, using light (Si at BNL and S at CERN) and heavy (Au at BNL and Pb at CERN) ion beams [4][5][6]. A high degree of nuclear stopping and a strong Coulomb effect (also due to the high stopping) have already been reported in Pb 1 Pb central collisions [7,8]. In this Letter, we present transverse momentum distributions of pions, kaons, and protons, measured in the NA44 spectrometer, from Pb 1 Pb and S 1 S collisions. Results of calculations from a hydrodynamical model [5] will be used to aid in this analysis.The NA44 magnetic focusing spectrometer consists of two room-temperature dipoles and three superconducting quadruples. Particles originating from the target are focused at a plane about ten meters downstream and detected by a tracking system consisting of a pad chamberstrip chamber-scintillator hodoscope complex. Particle identification is done with two threshold Cherenkov counters and two highly segmented TOF hodoscopes. The phase-space coverage (transverse momentum p T vs rapidity y) is determined by the combination of the spectrometer angle (relative to the beam direction) and the nominal momentum setting of the magnets. The momentum resolution is typically s p ͞p # 0.2% and the TOF counters have an average time resolution of 100 ps. More details of the spectrometer can be found elsewhere [9].The spectrometer momentum range is 620% around the nominal values of 4 and 8 GeV͞c. For kaons and protons, the 8 GeV͞c setting was used and the rapidity coverage is (2.5-3.4) and (2.4-2.8) for kaons and protons, respectively. Two angular settings (44 and 130 mrad) were utilized in order ...

The NA62 experiment reports the branching ratio measurement $$ \mathrm{BR}\left({K}^{+}\to {\pi}^{+}\nu \overline{\nu}\right)=\left({10.6}_{-3.4}^{+4.0}\left|{}_{\mathrm{stat}}\right.\pm {0.9}_{\mathrm{syst}}\right)\times {10}^{-11} $$ BR K + → π + ν ν ¯ = 10.6 − 3.4 + 4.0 stat ± 0.9 syst × 10 − 11 at 68% CL, based on the observation of 20 signal candidates with an expected background of 7.0 events from the total data sample collected at the CERN SPS during 2016–2018. This provides evidence for the very rare K+→$$ {\pi}^{+}\nu \overline{\nu} $$ π + ν ν ¯ decay, observed with a significance of 3.4σ. The experiment achieves a single event sensitivity of (0.839 ± 0.054) × 10−11, corresponding to 10.0 events assuming the Standard Model branching ratio of (8.4 ± 1.0) × 10−11. This measurement is also used to set limits on BR(K+→ π+X), where X is a scalar or pseudo-scalar particle. Details are given of the analysis of the 2018 data sample, which corresponds to about 80% of the total data sample.

Two-pion correlations from Pb+Pb collisions at 158 GeV/c pernucleon are measured by the NA44 experiment at CERN. Multidimensional ts characterize the emission volume, which is found to be larger than in S-induced collisions. Comparison with the RQMD model is used to relate the t parameters to the actual emission volume.

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Search for production of an invisible dark photon in π 0 decays The NA62 collaboration