Fri·May 11, 2018Friday·May 11, 2018

1. 01*

   Prompt and non-prompt $J/\psi$ and $\psi (2\mathrm{S})$ suppression at high transverse momentum in 5.02 TeV Pb+Pb collisions with the ATLAS experiment

   ATLAS Collaboration

   A measurement of $J/\psi$ and $\psi (2\mathrm{S})$ production is presented. It is based on a data sample from Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV and $pp$ collisions at $\sqrt{s}$ = 5.02 TeV recorded by the ATLAS detector at the LHC in 2015, corresponding to an integrated luminosity of $0.42{\mathrm{nb}}^{-1}$ and $25{\mathrm{pb}}^{-1}$ in Pb+Pb and $pp$, respectively. The measurements of per-event yields, nuclear modification factors, and non-prompt fractions are performed in the dimuon decay channel for $9<p_T^{\mu \mu }<40$ GeV in dimuon transverse momentum, and $-2.0<y_{\mu \mu }<2.0$ in rapidity. Strong suppression is found in Pb+Pb collisions for both prompt and non-prompt $J/\psi$, as well as for prompt and non-prompt $\psi (2\mathrm{S})$, increasing with event centrality. The suppression of prompt $\psi (2\mathrm{S})$ is observed to be stronger than that of $J/\psi$, while the suppression of non-prompt $\psi (2\mathrm{S})$ is equal to that of the non-prompt $J/\psi$ within uncertainties, consistent with the expectation that both arise from \textit{b}-quarks propagating through the medium. Despite prompt and non-prompt $J/\psi$ arising from different mechanisms, the dependence of their nuclear modification factors on centrality is found to be quite similar.

   nucl-exhep-exEPJC(2018)·161 citations
2. 02*

   $J/\psi$ production cross section and its dependence on charged-particle multiplicity in $p+p$ collisions at $\sqrt{s}$ = 200 GeV

   STAR Collaboration: J. Adam🇺🇸 · L. Adamczyk🇵🇱 · J. R. Adams🇺🇸 · J. K. Adkins🇺🇸 · G. Agakishiev🇷🇺 · M. M. Aggarwal🇮🇳 · Z. Ahammed🇮🇳 · N. N. Ajitanand🇺🇸 · I. Alekseev🇷🇺 · D. M. Anderson🇺🇸 · R. Aoyama🇯🇵 · A. Aparin🇷🇺 and 337 other authors

   We present a measurement of inclusive $J/\psi$ production at mid-rapidity ($|y|<1$) in $p+p$ collisions at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The differential production cross section for $J/\psi$ as a function of transverse momentum ($p_T$) for $0<p_T<14$ GeV/$c$ and the total cross section are reported and compared to calculations from the color evaporation model and the non-relativistic Quantum Chromodynamics model. The dependence of $J/\psi$ relative yields in three $p_T$ intervals on charged-particle multiplicity at mid-rapidity is measured for the first time in $p+p$ collisions at $\sqrt{s}$ = 200 GeV and compared with that measured at $\sqrt{s}$ = 7 TeV, PYTHIA8 and EPOS3 Monte Carlo generators, and the Percolation model prediction.

   ↳ hep-exnucl-exPLB(2018)·67 citations
3. 03*

   Longitudinal fluctuations and decorrelations of anisotropic flows at the LHC and RHIC energies

   Xiang-Yu Wu🇨🇳 · Long-Gang Pang🇺🇸 · Guang-You Qin🇨🇳 · Xin-Nian Wang🇨🇳

   We perform a systematic study on the decorrelation of anisotropic flows along the pseudorapidity in relativistic heavy-ion collisions at the LHC and RHIC energies. The dynamical evolution of the QGP fireball is simulated via the CLVisc (ideal) (3+1)-dimensional hydrodynamics model, with the fully fluctuating initial condition from A-Multi-Phase-Transport (AMPT) model. Detailed analysis is performed on the longitudinal decorrelations of elliptic, triangular and quadrangular flows in terms of flow vectors, flow magnitudes and flow orientations (event planes). It is found that pure flow magnitudes have smaller longitudinal decorrelation than pure flow orientations, and the decorrelation of flow vectors is a combined effect of both flow magnitudes and orientations. The longitudinal decorrelation of elliptic flow has a strong and non-monotonic centrality dependence due to the initial elliptic collision geometry: smallest decorrelation in mid-central collisions. In contrast, the decorrelations of triangular and quadrangular flows have weak centrality dependence, slightly larger decorrelations in more peripheral collisions. Our numerical results for Pb+Pb collisions at the LHC are in good agreement with the ATLAS data, while our RHIC results predict much larger longitudinal decorrelations as compared to the LHC. We further analyze the longitudinal structures of the AMPT initial conditions and find that the final-state longitudinal decorrelation effects are strongly correlated with the lengths of the initial string structures in the AMPT model. The decorrelation effects are typically larger at lower collision energies and in more peripheral collisions due to shorter lengths of the string structures in the initial states.

   ↳ nucl-thhep-phnucl-exPRC(2018)·61 citations
4. 04*

   Tests of the Standard Model in Neutron Beta Decay with Polarized Neutron and Electron and Unpolarized Proton

   A. N. Ivanov🇦🇹 · R. Höllwieser🇦🇹 · N. I. Troitskaya🇦🇹 · M. Wellenzohn🇦🇹 · Ya. A. Berdnikov🇷🇺

   We analyse the electron--energy and angular distribution of the neutron beta decay with polarized neutron and electron and unpolarized proton, calculated in Phys. Rev. C 95, 055502 (2017) within the Standard Model (SM), by taking into account the contributions of interactions beyond the SM. After the absorption of vector and axial vector contributions by the axial coupling constant and Cabibbo-Kobayashi-Maskawa (CKM) matrix element (Bhattacharya et al., Phys. Rev. D 85, 054512 (2012) and so on) these are the contributions of scalar and tensor interactions only. The neutron lifetime, correlation coefficients and their averaged values, and asymmetries of the neutron beta decay with polarized neutron and electron are adapted to the analysis of experimental data on searches of contributions of interactions beyond the SM. Using the obtained results we propose some estimates of the values of the scalar and tensor coupling constants of interactions beyond the SM. We use the estimate of the Fierz interference term "b_F = - 0.0028 +/- 0.0026" by Hardy and Towner (Phys. Rev. C 91, 025501 (2015)), the neutron lifetime "tau_n = 880.2(1.0)s"(Particle Data Group, Chin. Phys. C 40, 100001 (2016)) and the experimental data "N_{\exp} = 0.067 +/- 0.011_{\rm stat.} +/- 0.004_{\rm syst.}" for the averaged value of the correlation coefficient of the neutron-electron spin-spin correlations, measured by Kozela et al. (Phys. Ref. C 85, 045501 (2012)). The contributions of G-odd correlations are calculated and found at the level of 10^{-5} in agreement with the results obtained by Gardner and Plaster (Phys. Rev. C 87, 065504 (2013)).

   ↳ hep-phastro-ph.COnucl-exPRC(2018)·18 citations
5. 05*

   Low-momentum direct photon measurement in Cu$+$Cu collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV

   A. Adare · S. Afanasiev · C. Aidala · N.N. Ajitanand · Y. Akiba · H. Al-Bataineh · J. Alexander · M. Alfred · K. Aoki · L. Aphecetche · R. Armendariz · S.H. Aronson and 440 other authors

   We have measured direct photons for $p_T<5$GeV/$c$ in minimum bias and 0\%--40\% most central events at midrapidity for Cu$+$Cu collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV. The $e^{+}{e}^{-}$ contribution from quasi-real direct virtual photons has been determined as an excess over the known hadronic contributions in the $e^{+}{e}^{-}$ mass distribution. A clear enhancement of photons over the binary scaled $p$$+$$p$ fit is observed for $p_T<4$ GeV/$c$ in Cu$+$Cu data. The $p_T$ spectra are consistent with the Au$+$Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the $p$$+$$p$ baseline are 285$\pm$53(stat)$\pm$57(syst)~MeV/$c$ and 333$\pm$72(stat)$\pm$45(syst)~MeV/$c$ for minimum bias and 0\%--40\% most central events, respectively. The rapidity density, $dN/dy$, of photons demonstrates the same power law as a function of $d{N}_{\mathrm{ch}}/d\eta$ observed in Au$+$Au at the same collision energy.

   ↳ hep-exnucl-exPRC(2018)·33 citations
6. 06*

   Opacity dependence of elliptic flow in kinetic theory

   Aleksi Kurkela🇨🇭 · Urs Achim Wiedemann🇨🇭 · Bin Wu🇨🇭

   The observation of large azimuthal anisotropies $v_n$ in the particle spectra of proton-proton (pp) and proton-nucleus (pA) collisions challenges fluid dynamic interpretations of $v_n$, as it remains unclear how small collision systems can hydrodynamize and to what extent hydrodynamization is needed to build up $v_n$. Here, we study in a simple kinetic theory how the same physics that leads to hydrodynamization in large systems represents itself in small systems. We observe that one third to one half of the elliptic flow signal seen in fully hydrodynamized systems can be built up in collisions that extend over only one mean free path $l_{\mathrm{mfp}}$ and that do not hydrodynamize. This is qualitatively in line with observing a sizeable $v_2$ in $pp$ collisions for which other characteristics of soft multi-particle production seem well-described in a free-streaming picture. We further expose a significant system size dependence in the accuracy of hybrid approaches that match kinetic theory to viscous fluid dynamics. The implications of these findings for a reliable extraction of shear viscosity are discussed.

   ↳ hep-phhep-exnucl-exnucl-thEPJC(2019)·88 citations
7. 07*

   Beam-energy and centrality dependence of direct-photon emission from ultra-relativistic heavy-ion collisions

   A. Adare · S. Afanasiev · C. Aidala · N.N. Ajitanand · Y. Akiba · R. Akimoto · H. Al-Bataineh · J. Alexander · M. Alfred · A. Al-Jamel · H. Al-Ta'ani · A. Angerami and 662 other authors

   The PHENIX collaboration presents first measurements of low-momentum ($0.4<p_T<3$ GeV/$c$) direct-photon yields from Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}$=39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}$=200. Analyzing the photon yield as a function of the experimental observable $d{N}_{\mathrm{ch}}/d\eta$ reveals that the low-momentum ($>$1\,GeV/$c$) direct-photon yield $d{N}_{\gamma }^{\mathrm{dir}}/d\eta$ is a smooth function of $d{N}_{\mathrm{ch}}/d\eta$ and can be well described as proportional to $(d{N}_{\mathrm{ch}}/d\eta {)}^{\alpha }$ with $\alpha \approx 1.25$. This scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and the Large Hadron Collider, for centrality selected samples, as well as for different, $A$$+$$A$ collision systems. At a given beam energy the scaling also holds for high $p_T$ ($>5$\,GeV/$c$) but when results from different collision energies are compared, an additional $\sqrt{s_{{}_{NN}}}$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.

   ↳ hep-exnucl-exPRL(2019)·72 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.