Tue·Jun 21, 2016Tuesday·June 21, 2016

1. 01*

   Higher harmonic flow coefficients of identified hadrons in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV

   ALICE Collaboration

   The elliptic, triangular, quadrangular and pentagonal anisotropic flow coefficients for ${\pi }^{\pm }$, ${\mathrm{K}}^{\pm }$ and p+$\overline{\mathrm{p}}$ in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV were measured with the ALICE detector at the Large Hadron Collider. The results were obtained with the Scalar Product method, correlating the identified hadrons with reference particles from a different pseudorapidity region. Effects not related to the common event symmetry planes (non-flow) were estimated using correlations in pp collisions and were subtracted from the measurement. The obtained flow coefficients exhibit a clear mass ordering for transverse momentum ($p_{\mathrm{T}}$) values below $\approx$ 3 GeV/$c$. In the intermediate $p_{\mathrm{T}}$ region ($3<p_{\mathrm{T}}<6$ GeV/$c$), particles group at an approximate level according to the number of constituent quarks, suggesting that coalescence might be the relevant particle production mechanism in this region. The results for $p_{\mathrm{T}}<3$ GeV/$c$ are described fairly well by a hydrodynamical model (iEBE-VISHNU) that uses initial conditions generated by A Multi-Phase Transport model (AMPT) and describes the expansion of the fireball using a value of 0.08 for the ratio of shear viscosity to entropy density ($\eta /s$), coupled to a hadronic cascade model (UrQMD). Finally, expectations from AMPT alone fail to quantitatively describe the measurements for all harmonics throughout the measured transverse momentum region. However, the comparison to the AMPT model highlights the importance of the late hadronic rescattering stage to the development of the observed mass ordering at low values of $p_{\mathrm{T}}$ and of coalescence as a particle production mechanism for the particle type grouping at intermediate values of $p_{\mathrm{T}}$ for all harmonics.

   nucl-exhep-exJHEP(2016)·111 citations
2. 02*

   Impact of residual contamination on inclusive and direct photon flow

   F. Bock (Heidelberg University/LBNL)🇺🇸 · C. Loizides (LBNL)🇺🇸 · T. Peitzmann (Utrecht University/Nikhef)🇳🇱 · M. Sas (Utrecht University/Nikhef)🇳🇱

   Direct photon flow is measured by subtracting the contribution of decay photon flow from the measured inclusive photon flow via the double ratio $R_{\gamma }$, which defines the excess of direct over decay photons. The inclusive photon sample is affected by a modest contamination from different background sources, which is often ignored in measurements. However, due to the sensitivity of the direct photon measurement even a residual contamination may significantly bias the extracted direct photon flow. In particular, for measurements using photon conversions, which are very powerful at low transverse momentum, these effects can be substantial. Assuming three different types of correlated background contributions we demonstrate using the Therminator2 event generator that the impact of the contamination on the magnitude of direct photon flow can be on the level of $50\%$, even if the purity of the inclusive photon sample is about $97\%$. Future measurements should attempt to account for the contamination by measuring the background contributions and subtracting them from the inclusive photon flow.

   nucl-exhep-phJ.Phys.G(2017)·4 citations
3. 03*

   Evidence for collectivity in pp collisions at the LHC

   CMS Collaboration

   Measurements of two- and multi-particle angular correlations in pp collisions at sqrt(s) = 5, 7, and 13 TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 1.0 inverse picobarn (5 TeV), 6.2 inverse picobarns (7 TeV), and 0.7 inverse picobarns (13 TeV), were collected using the CMS detector at the LHC. The second-order (v[2]) and third-order (v[3]) azimuthal anisotropy harmonics of unidentified charged particles, as well as v[2] of K0 short and Lambda/anti-Lambda particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For high-multiplicity pp events, a mass ordering is observed for the v[2] values of charged hadrons (mostly pions), K0 short, and Lambda/anti-Lambda, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pt of about 2 GeV/c. For 13 TeV data, the v[2] signals are also extracted from four- and six-particle correlations for the first time in pp collisions, with comparable magnitude to those from two-particle correlations. These observations are similar to those seen in pPb and PbPb collisions, and support the interpretation of a collective origin for the observed long-range correlations in high-multiplicity pp collisions.

   nucl-exhep-exPLB(2017)·641 citations
4. 04*

   The GlueX DIRC Project

   Justin Stevens🇺🇸 · Fernando Barbosa🇺🇸 · Jason Bessuille🇺🇸 · Eugene Chudakov🇺🇸 · Roman Dzhygadlo🇩🇪 · Cristiano Fanelli🇺🇸 · John Frye🇺🇸 · John Hardin🇺🇸 · Jim Kelsey🇺🇸 · Maria Patsyuk🇺🇸 · Carsten Schwartz🇩🇪 · Jochen Schwiening🇩🇪 and 3 other authors

   The GlueX experiment was designed to search for and study the pattern of gluonic excitations in the meson spectrum produced through photoproduction reactions at a new tagged photon beam facility in Hall D at Jefferson Laboratory. The particle identification capabilities of the GlueX experiment will be enhanced by constructing a DIRC (Detection of Internally Reflected Cherenkov light) detector, utilizing components of the decommissioned BaBar DIRC. The DIRC will allow systematic studies of kaon final states that are essential for inferring the quark flavor content of both hybrid and conventional mesons. The design for the GlueX DIRC is presented, including the new expansion volumes that are currently under development.

   ↳ physics.ins-detnucl-exJINST(2016)·23 citations
5. 05*

   Uncertainties in the production of $p$ nuclei in massive stars obtained from Monte Carlo variations

   T. Rauscher🇬🇧 · N. Nishimura🇬🇧 · R. Hirschi · G. Cescutti · A. St.J. Murphy🇬🇧 · A. Heger🇦🇺

   Nuclear uncertainties in the production of $p$ nuclei in massive stars have been quantified in a Monte Carlo procedure. Bespoke temperature-dependent uncertainties were assigned to different types of reactions involving nuclei from Fe to Bi. Their simultaneous impact was studied in postprocessing explosive trajectories for three different stellar models. It was found that the grid of mass zones in the model of a 25 $M_{\odot }$ star, which is widely used for investigations of $p$ nucleosynthesis, is too crude to properly resolve the detailed temperature changes required for describing the production of $p$ nuclei. Using models with finer grids for 15 $M_{\odot }$ and 25 $M_{\odot }$ stars with initial solar metallicity, it was found that most of the production uncertainties introduced by nuclear reaction uncertainties are smaller than a factor of two. Since a large number of rates were varied at the same time in the Monte Carlo procedure, possible cancellation effects of several uncertainties could be taken into account. Key rates were identified for each $p$ nucleus, which provide the dominant contribution to the production uncertainty. These key rates were found by examining correlations between rate variations and resulting abundance changes. This method is superior to studying flow patterns, especially when the flows are complex, and to individual, sequential variation of a few rates.

   ↳ astro-ph.HEastro-ph.SRnucl-exnucl-thMNRAS(2016)·68 citations
6. 06*

   Examination of the sensitivity of the thermal fits to heavy-ion hadron yield data to the modeling of the eigenvolume interactions

   Volodymyr Vovchenko🇩🇪 · Horst Stoecker🇩🇪

   The hadron-resonance gas (HRG) model with the mass-proportional eigenvolume (EV) corrections is employed to fit the hadron yield data of the NA49 collaboration for central Pb+Pb collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=6.3,7.6,8.8,12.3,$ and $17.3$ GeV, the hadron midrapidity yield data of the STAR collaboration for Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV, and the hadron midrapidity yield data of the ALICE collaboration for Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2760$ GeV. At given bombarding energy, for a given set of radii, the EV HRG model fits do not just yield a single $T-{\mu }_B$ pair, but a whole range of $T-{\mu }_B$ pairs, each with similarly good fit quality. These pairs form a valley in the $T-{\mu }_B$ plane along a line of nearly constant entropy per baryon, $S/A$, which increases nearly linearly with bombarding energy $E_{\mathrm{lab}}$. The entropy per baryon values extracted from the data at the different energies are a robust observable: it is almost independent of the details of the modeling of the eigenvolume interactions and of the specific $T-{\mu }_B$ values obtained. These results show that the extraction of the chemical freeze-out temperature and chemical potential is extremely sensitive to the modeling of the short-range repulsion between the hadrons. This implies that the ideal point-particle HRG values are not unique. The wide range of the extracted $T$ and ${\mu }_B$ values suggested by the eigenvolume HRG fits, as well as the approximately constant $S/A$ at freeze-out, are consistent with a non-equilibrium scenario of continuous freeze-out, where hadrons can be chemically frozen-out throughout the extended space-time regions during the evolution of the system. Even when the EV HRG fits are restricted to modest temperatures suggested by lattice QCD, the strong systematic effects of EV interactions are observed.

   ↳ hep-phnucl-exnucl-thPRC(2017)·57 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.