Fri·Mar 16, 2018Friday·March 16, 2018

1. 01*

   Neutral pion and $\eta$ meson production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV

   ALICE Collaboration

   Neutral pion and $\eta$ meson production in the transverse momentum range 1 < $p_T$ < 20 GeV/$c$ have been measured at mid-rapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semi-central Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. These results were obtained using the photon conversion method as well as the PHOS and EMCal detectors. The results extend the upper $p_T$ reach of the previous ALICE ${\pi }^0$ measurements from 12 GeV/$c$ to 20 GeV/$c$ and present the first measurement of $\eta$ meson production in heavy-ion collisions at the LHC. The $\eta /{\pi }^0$ ratio is similar for the two centralities and reaches at high $p_T$ a plateau value of 0.457 $\pm$ 0.013${}^{stat}$ $\pm$ 0.018${}^{syst}$. A suppression of similar magnitude for ${\pi }^0$ and $\eta$ meson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of NLO pQCD predictions and hydrodynamic models. The measurements show a stronger suppression with respect to what was observed at lower center-of-mass energies in the $p_T$ range 6 < $p_T$ < 10 GeV/$c$. At $p_T$ < 3 GeV/$c$, hadronization models describe the ${\pi }^0$ results while for the $\eta$ some tension is observed.

   nucl-exPRC(2018)·54 citations
2. 02*

   The ${}^7$Be(n,p)${}^7$Li reaction and the Cosmological Lithium Problem: measurement of the cross section in a wide energy range at n_TOF (CERN)

   L. Damone · M. Barbagallo · M. Mastromarco · A. Mengoni · L. Cosentino · E. Maugeri · S. Heinitz · D. Schumann · R. Dressler · F. Käppeler · N. Colonna · P. Finocchiaro and 146 other authors

   We report on the measurement of the ${}^7$Be($n,p$)${}^7$Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the Big Bang Nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and showed a large discrepancy between each other. The measurement was performed with a Si-telescope, and a high-purity sample produced by implantation of a ${}^7$Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low-energy, relative to current evaluations, in the region of BBN interest the present results are consistent with the values inferred from the time-reversal ${}^7$Li($p,n$)${}^7$Be reaction, thus yielding only a relatively minor improvement on the so-called Cosmological Lithium Problem (CLiP). The relevance of these results on the near-threshold neutron production in the p+${}^7$Li reaction is also discussed.

   nucl-exastro-ph.CO
3. 03*

   Positrons at JLab - Advancing Nuclear Science in Hall B

   Volker D. Burkert🇺🇸

   In this talk I address two high impact physics programs that require the use of polarized and unpolarized positron beams in addition to using electron beams of the same energy. First, I address what will be gained from using positron beams in addition to electron beams in the extraction of the Compton Form Factors (CFFs) and generalized parton distributions (GPDs) from Deeply Virtual Compton Scattering (DVCS) on a proton target. As a second high impact science program I discuss an experimental scenario using unpolarized positrons to measure elastic scattering on protons in an effort to determine definitively the 2-photon exchange contributions in order to resolve a longstanding discrepancy in the determination of the proton's electric and magnetic form factors.

   nucl-exhep-exnucl-thAIP Conf.Proc.(2018)·0 citations
4. 04*

   Practical considerations for measuring global spin alignment of vector mesons in relativistic heavy ion collisions

   A. H. Tang · B. Tu · C. S. Zhou

   Global spin alignment of vector mesons is a sensitive probe of system vorticity and particle production mechanism in relativistic heavy ion collisions. The measurement of global spin alignment is gaining increasing interest and deserves careful considerations. In this paper, we lay out a few practical issues that need to be taken care of when measuring global spin alignment of vector mesons. They are, the correction for event plane resolution, reconciling measurements made with different event planes, the correction for the effect of finite acceptance in pseudorapidity, and the consideration for measuring the azimuthal angle dependence. Insights and methodologies offered in this paper will help experiments to measure the global spin alignment properly and accurately.

   nucl-exnucl-thPRC(2018)·34 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.