Wed·Jan 25, 2017Wednesday·January 25, 2017

1. 01*

   Global $\Lambda$ hyperon polarization in nuclear collisions: evidence for the most vortical fluid

   STAR Collaboration

   The extreme temperatures and energy densities generated by ultra-relativistic collisions between heavy nuclei produce a state of matter with surprising fluid properties. Non-central collisions have angular momentum on the order of 1000$\hslash$, and the resulting fluid may have a strong vortical structure 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 vorticity. 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 $\Lambda$ and $\overline{\Lambda }$ hyperons show a positive polarization of the order of a few percent, consistent with some hydrodynamic predictions. A previous measurement 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" 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 discovery of hydrodynamic spin alignment to the subatomic realm.

   nucl-exNature(2017)·1113 citations
2. 02*

   High $p_{\mathrm{T}}$ spectra and anisotropy of light and heavy hadrons

   Peter Christiansen🇸🇪

   Data driven studies of heavy-ion results has played a big role in highlighting interesting features of these complex systems. In this proceeding, a simple QGP-brick interpretation of the $R_{\mathrm{AA}}$ and $v_2$ at high $p_{\mathrm{T}}$ ($p_{\mathrm{T}}\approx 10$ GeV/c) is presented. This interpretation draws attention to two fundamental questions: is there an effect of the asymmetric QGP expansion on the $v_2$ at high $p_{\mathrm{T}}$? is there an energy loss difference between quarks and gluons? Finally, it is discussed how these studies can be extended using Event-Shape Engineering and how they can be applied to compare the energy loss of light and heavy quarks.

   ↳ hep-phnucl-exNucl.Part.Phys.Proc.(2017)·0 citations
3. 03*

   Momentum anisotropy effects for quarkonium in a weakly-coupled quark-gluon plasma below the melting temperature

   Simone Biondini🇨🇭 · Nora Brambilla🇩🇪 · Miguel Angel Escobedo🇫🇮 · Antonio Vairo🇩🇪

   In the early stages of heavy-ion collisions, the hot QCD matter expands more longitudinally than transversely. This imbalance causes the system to become rapidly colder in the longitudinal direction and a local momentum anisotropy appears. In this paper, we study the heavy-quarkonium spectrum in the presence of a small plasma anisotropy. We work in the framework of pNRQCD at finite temperature. We inspect arrangements of non-relativistic and thermal scales complementary to those considered in the literature. In particular, we consider temperatures larger and Debye masses smaller than the binding energy, which is a temperature range relevant for presently running LHC experiments. In this setting we compute the leading thermal corrections to the binding energy and the thermal width induced by quarkonium gluo-dissociation.

   ↳ hep-phhep-thnucl-exnucl-thPRD(2017)·10 citations
4. 04*

   Impacts of nuclear-physics uncertainty in stellar temperatures on the s-process nucleosynthesis

   N. Nishimura🇬🇧 · G. Cescutti · R. Hirschi · T. Rauscher🇬🇧 · J. Den Hartogh · A. St. J. Murphy🇬🇧

   We evaluated the uncertainty relevant to s-process nucleosynthesis using a Monte-Carlo centred approach. We are based on a realistic and general prescription of temperature dependent uncertainty for the reactions. We considered massive stars for the weak s-process and AGB stars for the main s-process. We found that the adopted uncertainty for (n,$\gamma$) rates, tens of per cent on average, affect the production of s-process nuclei along the $\beta$-stability line, while for $\beta$-decay, for which contributions from excited states enhances the uncertainty, has the strongest impact on branching points.

   ↳ astro-ph.SRnucl-exnucl-thJPS Conf.Proc.(2017)·0 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.