Tue·Aug 29, 2017Tuesday·August 29, 2017

1. 01*

   Direct Photon Production and Azimuthal Anisotropy at Low Transverse Momentum measured in PHENIX

   Wenqing Fan (for PHENIX collaboration)🇺🇸

   The PHENIX experiment discovered a large excess of low-$p_T$ direct photons in Au+Au collisions at 200 GeV compared to reference p+p collisions, which has been attributed to thermal radiation from the medium produced in the collisions. At the same time the excess photons show a large azimuthal anisotropy, expressed as Fourier coefficients $v_2$ and $v_3$. These surprising results have not yet been fully described by theoretical models. We will present the results obtained from real photons in the electromagnetic calorimeter and photons converted on the outer shell of the Hadron Blind Detector. PHENIX has also developed a new technique to identify conversion photons without assuming the radius where the conversion happened. This method greatly increases the available statistics and reduces systematic uncertainties.

   nucl-exNucl.Part.Phys.Proc.(2017)·1 citation
2. 02*

   Freezeout systematics due to the hadron spectrum

   Sandeep Chatterjee🇵🇱 · Debadeepti Mishra🇮🇳 · Bedangadas Mohanty🇮🇳 · Subhasis Samanta🇮🇳

   We investigate systematics of the freezeout surface in heavy ion collisions due to the hadron spectrum. The role of suspected resonance states that are yet to be confirmed experimentally in identifying the freezeout surface has been investigated. We have studied two different freezeout schemes - unified freezeout scheme where all hadrons are assumed to freezeout at the same thermal state and a flavor dependent sequential freezeout scheme with different freezeout thermal states for hadrons with or without valence strange quarks. The data of mean hadron yields as well as scaled variance of net proton and net charge distributions have been analysed. We find the freezeout temperature $T$ to drop by $\sim 5\%$ while the dimensionless freezeout parameters ${\mu }_B/T$ and $V{T}^3$ (${\mu }_B$ and $V$ are the baryon chemical potential and the volume at freezeout respectively) are insensitive to the systematics of the input hadron spectrum. The observed hint of flavor hierarchy in $T$ and $V{T}^3$ with only confirmed resonances survives the systematics of the hadron spectrum. It is more prominent between $\sqrt{s_{NN}}\sim 10-100$ GeV where the maximum hierarchy in $T\sim 10\%$ and $V{T}^3\sim 40\%$. However, the uncertainties in the thermal parameters due to the systematics of the hadron spectrum and their decay properties do not allow us to make a quantitative estimate of the flavor hierarchy yet.

   ↳ nucl-thhep-lathep-phnucl-exPRC(2017)·39 citations
3. 03*

   Alpha Clustering with a Hollow Structure --- Geometrical Structure of Alpha Clusters from Platonic Solids to Fullerene Shape

   Akihiro Tohsaki🇯🇵 · Naoyuki Itagaki🇯🇵

   We study $\alpha$-cluster structure based on the geometric configurations with a microscopic framework, which takes full account of the Pauli principle, and which also employs an effective inter-nucleon force including finite-range three-body terms suitable for microscopic alpha-cluster models. Here, special attention is focused upon the $\alpha$ clustering with a hollow structure; all the $\alpha$ clusters are put on the surface of a sphere. All the Platonic solids (five regular polyhedra) and the fullerene-shaped polyhedron coming from icosahedral structure are considered. Furthermore, two configurations with dual polyhedra, hexahedron-octahedron and dodecahedron-icosahedron, are also scrutinized. As a consequence, we insist on the possible existence of stable $\alpha$-clustering with a hollow structure for all the configurations. Especially, two configurations, that is, dual polyhedra of dodecahedron-icosahedron and fullerene, have a prominent hollow structure compared with other six configurations.

   ↳ nucl-thnucl-exPRC(2018)·4 citations
4. 04*

   Collectivity from interference

   Boris Blok🇮🇱 · Christian D. Jäkel🇧🇷 · Mark Strikman🇺🇸 · Urs Achim Wiedemann🇨🇭

   In hadronic collisions, interference between different production channels affects momentum distributions of multi-particle final states. As this QCD interference does not depend on the strong coupling constant, it is part of the no-interaction baseline that needs to be controlled prior to searching for other manifestations of collective dynamics. Here, we introduce a model that is based on the QCD theory of multi-parton interactions and that allows one to study interference effects in the production of $m$ particles in hadronic collisions with $N$ parton-parton interactions ("sources"). In an expansion in powers of $1/(N_c^2-1)$ and to leading order in the number of sources $N$, we calculate interference effects in the $m$-particle spectra and we determine from them the second and fourth order cumulant momentum anisotropies $v_n$. Without invoking any azimuthal asymmetry and any density dependent non-linear dynamics in the incoming state, and without invoking any interaction in the final state, we find that QCD interference alone can give rise to values for $v_n\{2\}$ and $v_n\{4\}$, $n$ even, that persist unattenuated for increasing number of sources, that may increase with increasing multiplicity and that agree with measurements in proton-proton (pp) collisions in terms of the order of magnitude of the signal and the approximate shape of the transverse momentum dependence. We further find that the non-abelian features of QCD interference can give rise to odd harmonic anisotropies. These findings indicate that the no-interaction baseline including QCD interference effects can make a sizeable if not dominant contribution to the measured $v_n$ coefficients in pp collisions. Prospects for analyzing QCD interference contributions further and their possible relevance for proton-nucleus and nucleus-nucleus collisions are discussed shortly.

   ↳ hep-phhep-exnucl-exnucl-thJHEP(2017)·56 citations
5. 05*

   Observables for possible QGP signatures in central pp collisions

   Michelangelo L. Mangano🇨🇭 · Benjamin Nachman🇺🇸

   Proton-proton (pp) data show collective effects, such as long-range azimuthal correlations and strangeness enhancement, which are similar to phenomenology observed in heavy ion collisions. Using simulations with and without explicit existing models of collective effects, we explore new ways to probe pp collisions at high multiplicity, in order to suggest measurements that could help identify the similarities and differences between large- and small-scale collective effects. In particular, we focus on the properties of jets produced in ultra-central pp collisions in association with a Z boson. We consider observables such as jet energy loss and jet shapes, which could point to the possible existence of an underlying quark-gluon plasma, or other new dynamical effects related to the presence of large hadronic densities.

   ↳ hep-phhep-exnucl-exnucl-thEPJC(2018)·30 citations
6. 06*

   Nucleon Axial Radius and Muonic Hydrogen - A New Analysis and Review

   Richard J. Hill🇨🇦 · Peter Kammel🇺🇸 · William J. Marciano🇺🇸 · Alberto Sirlin🇺🇸

   Weak capture in muonic hydrogen ($\mu$H) as a probe of the chiral properties and nucleon structure predictions of Quantum Chromodynamics (QCD) is reviewed. A recent determination of the axial-vector charge radius squared, $r_A^2(zexp.)=0.46(22){\mathrm{fm}}^2$, from a model independent $z$ expansion analysis of neutrino-nucleon scattering data is employed in conjunction with the MuCap measurement of the singlet muonic hydrogen capture rate, ${\Lambda }_{\mathrm{singlet}}^{\mathrm{MuCap}}=715.6(7.4){\mathrm{s}}^{-1}$, to update the induced pseudoscalar nucleon coupling: ${\bar{g}}_P^{\mathrm{MuCap}}=8.23(83)$ derived from experiment, and ${\bar{g}}_P^{\mathrm{theory}}=8.25(25)$ predicted by chiral perturbation theory. Accounting for correlated errors this implies ${\bar{g}}_P^{\mathrm{theory}}/{\bar{g}}_P^{\mathrm{MuCap}}=1.00(8)$, confirming theory at the 8% level. If instead, the predicted expression for ${\bar{g}}_P^{\mathrm{theory}}$ is employed as input, then the capture rate alone determines $r_A^2(\mu \mathrm{H})=0.46(24){\mathrm{fm}}^2$, or together with the independent $z$ expansion neutrino scattering results, a weighted average $r_A^2(ave.)=0.46(16){\mathrm{fm}}^2$. Sources of theoretical uncertainty are critically examined and potential experimental improvements are described that can reduce the capture rate error by about a factor of 3. Muonic hydrogen can thus provide a precise and independent $r_A^2$ value which may be compared with other determinations, such as ongoing lattice gauge theory calculations. The importance of an improved $r_A^2$ determination for phenomenology is illustrated by considering the impact on critical neutrino-nucleus cross sections at neutrino oscillation experiments.

   ↳ hep-phhep-latnucl-exnucl-thRept.Prog.Phys.(2018)·85 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.