M. Cherney

Global Λ hyperon polarization in nuclear collisions

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 extreme temperatures and energy densities generated by ultra-relativistic collisions between heavy nuclei produce a state of matter with surprising fluid properties1. Non-central collisions have angular momentum on the order of 1000~, and the resulting fluid may have a strong vortical structure2–4 that must be understood to properly describe the fluid. It 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 vorticity15. However, no experimental indications of fluid vorticity in heavy ion collisions have so far been found. Here we present the first measurement of an alignment between the angular momentum of a non-central collision and the spin of emitted particles, revealing that the fluid produced in heavy ion collisions is by far the most vortical system ever observed. We find that Λ and Λ hyperons show a positive polarization of the order of a few percent, consistent with some hydrodynamic predictions5. A previous measurement6 that reported a null result at higher collision energies is seen to be consistent with the trend of our new observations, though with larger statistical uncertainties. These data provide the first experimental access to the vortical structure of the “perfect fluid”7 created in a heavy ion collision. They should prove valuable in the development of hydrodynamic models that quantitatively connect observations to the theory of the Strong Force. Our results extend the recent discovery8 of

Bulk Properties of the Medium Produced in Relativistic Heavy-Ion Collisions from the Beam Energy Scan Program

© 2017 American Physical Society. We present measurements of bulk properties of the matter produced in Au+Au collisions at sNN=7.7,11.5,19.6,27, and 39 GeV using identified hadrons (π±, K±, p, and p) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity (|y| < 0.1) results for multiplicity densities dN/dy, average transverse momenta (pT), and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.

Dijet imbalance measurements in Au+Au and pp collisions at sNN =200 GeV at STAR

We report the first di-jet transverse momentum asymmetry measurements from Au+Au and p+p collisions at RHIC. The two highest-energy back-to-back jets reconstructed from fragments with transverse momenta above 2 GeV/c display a significantly stronger momentum imbalance in heavy-ion collisions than in the p+p reference. When re-examined with correlated soft particles included, we observe that these di-jets then exhibit a unique new feature -- momentum balance is restored to that observed in p+p for a jet resolution parameter of R=0.4, while re-balancing is not attained with a smaller value of R=0.2.We report the first dijet transverse momentum asymmetry measurements from Au+Au and pp collisions at RHIC. The two highest-energy back-to-back jets reconstructed from fragments with transverse momenta above 2u2009u2009GeV/c display a significantly higher momentum imbalance in heavy-ion collisions than in the pp reference. When reexamined with correlated soft particles included, we observe that these dijets then exhibit a unique new feature-momentum balance is restored to that observed in pp for a jet resolution parameter of R=0.4, while rebalancing is not attained with a smaller value of R=0.2.

A unified control system for the STAR experiment

The control system for the STAR experiment at RHIC is presented. The VME-based architecture is described. Reasons for the hardware and software choices are discussed. A significant new application of a slow control system (EPICS) to a run control setting is discussed. Interfaces to the detector subsystems are described. The initial implementation of the control systems for the baseline STAR detector is summarized. >

Results from CERN experiment NA44

A preliminary analysis is presented, of proton-nucleus collisions at projectile energy 450 GeV and sulphur-lead interactions at projectile energy 200 A·GeV. The focussing spectrometer is designed to measure single and double-particle cross sections with high statistics, at central and forward rapidities. Inclusive spectra for π±, K±, p and p emitted from p + Be, p + Pb, S + S, and S + Pb collisions are presented, as well as the two-particle correlation function for minimum bias S + Pb → 2π+ + X collisions.

Results from CERN experiment NA36 on strangeness production

Author(s): Andersen, E.; Barnes, P.D.; Blaes, R.; Braun, H.; Brom, J.M.; Cherney, M.D.; Cohler, M.; Cruz, B. de la; Diebold, G.E.; Dunin, B.; Escobales, B.; Fang, R.; Fernandez, C.; Franklin, G.; Garabatos, C.; Garzon, J.A.; Geist, W.M.; Gomez, A.; Greiner, D.E.; Gruhn, C.; Hafidouni, M.; Hrubec, J.; Lacquot, J.L.; Jegham, E.; Jones, P.G.; Kuipers, J.P.; Ladren, M.; Guevara, P. Ladron de; Liko, D.; Lopez-Ponte, S.; Govhoiden, G.; MacNaughton, J.; Michalon, A.; Michalon-Mentzer, M.E.; Mosquera, J.; Natkaniec, Z.; Nelson, J.M.; Neuhofer, G.; Ogle, W.; Heros, C. Perez de los; Plo, M.; Porth, P.; Powell, B.; Quinn, B.; Ramil, A.; Riester, J.L.; Rohringer, H.; Sakrejda, I.; Thorsteinsen, T.; Traxler, J.; Voltolini, C.; Yanez, A.; Ye, Y.; Zybert, R.

Upsilon production in U+U collisions at 193 GeV with the STAR experiment

We present a measurement of the inclusive production of Upsilon mesons in U+U collisions at 193 GeV at mid-rapidity (|y| < 1). Previous studies in central Au+Au collisions at 200 GeV show a suppression of Upsilon(1S+2S+3S) production relative to expectations from the Upsilon yield in p+p collisions scaled by the number of binary nucleon-nucleon collisions (Ncoll), with an indication that the Upsilon(1S) state is also suppressed. The present measurement extends the number of participant nucleons in the collision (Npart) by 20% compared to Au+Au collisions, and allows us to study a system with higher energy density. We observe a suppression in both the Upsilon(1S+2S+3S) and Upsilon(1S) yields in central U+U data, which consolidates and extends the previously observed suppression trend in Au+Au collisions.

Hardware controls for the STAR experiment at RHIC

A hardware controls system has been implemented for the STAR experiment at RHIC. Approximately 10000 parameters governing experiment operation are currently controlled and monitored. The system is based on the Experimental Physics and Industrial Control System (EPICS). The architecture of STAR hardware controls are presented as well as the results of operation of the integrated baseline system. Novel features of the system include a specialized field bus (High-level Data Link Control-HDLC), new EPICS record support, control DEVice (CDEV) interfaces to accelerator and magnet control systems, and C++ based communication between STAR online and hardware controls and their associated databases.