The extreme energy densities generated by ultra-relativistic collisions between heavy atomic nuclei produce a state of matter that behaves surprisingly like a fluid, with exceptionally high ...temperature and low viscosity. Non-central collisions have angular momenta of the order of 1,000ћ, and the resulting fluid may have a strong vortical structure that must be understood to describe the fluid properly. The vortical structure is also of particular interest because the restoration of fundamental symmetries of quantum chromodynamics is expected to produce novel physical effects in the presence of strong vorticity. However, no experimental indications of fluid vorticity in heavy ion collisions have yet been found. Since vorticity represents a local rotational structure of the fluid, spin-orbit coupling can lead to preferential orientation of particle spins along the direction of rotation. Here we present measurements of an alignment between the global angular momentum of a non-central collision and the spin of emitted particles (in this case the collision occurs between gold nuclei and produces Λ baryons), revealing that the fluid produced in heavy ion collisions is the most vortical system so far observed. (At high energies, this fluid is a quark-gluon plasma.) We find that Λ and hyperons show a positive polarization of the order of a few per cent, consistent with some hydrodynamic predictions. (A hyperon is a particle composed of three quarks, at least one of which is a strange quark; the remainder are up and down quarks, found in protons and neutrons.) A previous measurement that reported a null result, that is, zero polarization, at higher collision energies is seen to be consistent with the trend of our observations, though with larger statistical uncertainties. These data provide experimental access to the vortical structure of the nearly ideal liquid created in a heavy ion collision and should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the strong force.
The DANSS detector is located directly under the nuclear reactor at the Kalinin nuclear power plant. Such a position ensures about 50 m.w.e. shielding from cosmic rays in the vertical direction; as a ...result, the detector occupies an intermediate position between surface and underground detectors in the shielding from cosmic rays. The sensitive volume of the detector consisting of a 1-m
3
plastic scintillator is surrounded by the multilayer passive shielding and muon veto. The main aim of the DANSS experiment is to measure the antineutrino spectrum at various distances from the source. To this end, the detector is placed on a lifting platform in order to record data at three positions of 10.9, 11.9, and 12.9 m from the reactor core. The detector can reconstruct muon tracks passing through its sensitive volume. The pressure, temperature, and decay coefficients for muons in various regions of the zenith angle
have been determined from the muon data collected during four years using the effective generation level method.
Global polarization of Λ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at √SNN = 200 GeV, with no significant difference between Λ and Λ¯. These new ...results reveal the collision energy dependence of the global polarization together with the results previously observed √SNN = 7.7 – 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy.
Nonmonotonic variation with collision energy (sqrts_{NN}) of the moments of the net-baryon number distribution in heavy-ion collisions, related to the correlation length and the susceptibilities of ...the system, is suggested as a signature for the quantum chromodynamics critical point. We report the first evidence of a nonmonotonic variation in the kurtosis times variance of the net-proton number (proxy for net-baryon number) distribution as a function of sqrts_{NN} with 3.1 σ significance for head-on (central) gold-on-gold (Au+Au) collisions measured solenoidal tracker at Relativistic Heavy Ion Collider. Data in noncentral Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of sqrts_{NN}.
The Λ (Λ¯) hyperon polarization along the beam direction has been measured in Au+Au collisions at sNN=200 GeV, for the first time in heavy-ion collisions. The polarization dependence on the ...hyperons' emission angle relative to the elliptic flow plane exhibits a second harmonic sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction, expected due to elliptic flow. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum (pT) dependence at pT greater than 1 GeV/c. The magnitude of the signal is about 5 times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, the kinematic vorticity calculations in the blast-wave model tuned to reproduce particle spectra, elliptic flow, and the azimuthal dependence of the Gaussian source radii measured with the Hanbury Brown–Twiss intensity interferometry technique reproduce well the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the polarization signal.
First results on
-dependence measurements in inclusive
meson production in
interactions (
, Al, Cu, Sn, and W) are presented at
GeV/
c
. The ratios of the differential cross sections of four of these ...nuclei to the differential cross section on aluminum were measured and the dependence of these cross sections on the atomic of the nucleus was studied. The measurements were carried out in the kinematic region of the Feynman variable
and transverse momentum
GeV/
c
.
mesons were detected in the decay mode
at the SPASCHARM experimental setup using negative charged beams at beamline 14 of the U-70 accelerator complex.
The detection equipment of the DANSS setup includes 2500 silicon photomultipliers and approximately 100 photomultiplier tubes (PMTs). The system of data acquisition from these photosensors is based ...on waveform digitization, with which it is possible to simultaneously obtain both amplitude and timing information. The modules of waveform digitizers (WFDs) are made to the VME standard and allow parallel digitization of 64 differential signals at a frequency of 125 MHz with a 12-bit amplitude resolution. The programmable logic of the WFDs provides production of the system trigger based on the analysis of the PMT signals and its propagation without any additional hardware. Owing to the extremely low analog noise, it is possible to use the full dynamic range of the digitization. These WFD modules are superior to other similar modules in the throughput and may find wide application to perform similar tasks of waveform digitization.
Global polarization of Ξ and Ω hyperons has been measured for the first time in Au + Au collisions at √sNN = 200 GeV. The measurements of the Ξ− and ... hyperon polarization have been performed by ...two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay Ξ → Λ + π, as well as by measuring the polarization of the daughter Λ hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over Ξ− and ... is measured to be ⟨PΞ⟩ = 0.47 ± 0.10(stat) ± 0.23(syst)% for the collision centrality 20%–80%. The ⟨PΞ⟩ is found to be slightly larger than the inclusive Λ polarization and in reasonable agreement with a multiphase transport model. The ⟨PΞ⟩ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of Ω, ⟨PΩ⟩ = 1.11 ± 0.87(stat) ± 1.97(syst)% was obtained by measuring the polarization of daughter Λ in the decay Ω → Λ + K, assuming the polarization transfer factor CΩΛ = 1.
We report the first measurement of rapidity-odd directed flow (v1) for D0 and D0¯ mesons at midrapidity (|y|<0.8) in Au+Au collisions at sNN=200 GeV using the STAR detector at the Relativistic Heavy ...Ion Collider. In 10–80% Au+Au collisions, the slope of the v1 rapidity dependence (dv1/dy), averaged over D0 and D0¯ mesons, is −0.080±0.017(stat)±0.016(syst) for transverse momentum pT above 1.5 GeV/c. The absolute value of D0 meson dv1/dy is about 25 times larger than that for charged kaons, with 3.4σ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.
The paper presents the performance studies of the scintillation detector prototype with SiPM readout using different types of the front-end electronics. The timing and position resolutions with LED ...have been obtained. These results can be applied for the proton and deuteron beams polarimetry at the Internal Target Station at Nuclotron.
PDF
