Thu·Mar 12, 2015Thursday·March 12, 2015

1. 01*

   Measurement of dijet $k_{\mathrm{T}}$ in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV

   ALICE Collaboration

   A measurement of dijet correlations in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, k_{\rm Ty} = p_\rm{T,jet}^\rm{ch+ne} \; \rm{sin}(\Delta\varphi_{\rm{dijet}}) with $\Delta {\phi }_{\mathrm{dijet}}$ the azimuthal angle between a full and charged jet and p_\rm{T,jet}^\rm{ch+ne} the transverse momentum of the full jet, is used to study nuclear matter effects in p-Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p-Pb collisions with respect to pp collisions. Measurements of the dijet $k_{\mathrm{Ty}}$ as a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of $k_{\mathrm{Ty}}$ due to nuclear matter effects in p-Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.

   nucl-exhep-exPLB(2015)·44 citations
2. 02*

   Collectivity in the light radon nuclei measured directly via Coulomb excitation

   L. P. Gaffney🇧🇪 · A. P. Robinson🇬🇧 · D. G. Jenkins🇬🇧 · A. N. Andreyev🇧🇪 · M. Bender🇫🇷 · A. Blazhev🇩🇪 · N. Bree🇧🇪 · B. Bruyneel🇩🇪 · P. A. Butler🇬🇧 · T. E. Cocolios🇬🇧 · T. Davinson🇬🇧 · A. N. Deacon🇬🇧 and 40 other authors

   Background: Shape coexistence in heavy nuclei poses a strong challenge to state-of-the-art nuclear models, where several competing shape minima are found close to the ground state. A classic region for investigating this phenomenon is in the region around $Z=82$ and the neutron mid-shell at $N=104$. Purpose: Evidence for shape coexistence has been inferred from $\alpha$-decay measurements, laser spectroscopy and in-beam measurements. While the latter allow the pattern of excited states and rotational band structures to be mapped out, a detailed understanding of shape coexistence can only come from measurements of electromagnetic matrix elements. Method: Secondary, radioactive ion beams of ${}^{202}$Rn and ${}^{204}$Rn were studied by means of low-energy Coulomb excitation at the REX-ISOLDE facility in CERN. Results: The electric-quadrupole ($E2$) matrix element connecting the ground state and first-excited $2_1^{+}$ state was extracted for both ${}^{202}$Rn and ${}^{204}$Rn, corresponding to $B(E2;2_1^{+}\to 2_1^{+})=29_{-8}^{+8}$ W.u. and $43_{-12}^{+17}$ W.u., respectively. Additionally, $E2$ matrix elements connecting the $2_1^{+}$ state with the $4_1^{+}$ and $2_2^{+}$ states were determined in ${}^{202}$Rn. No excited $0^{+}$ states were observed in the current data set, possibly due to a limited population of second-order processes at the currently-available beam energies. Conclusions: The results are discussed in terms of collectivity and the deformation of both nuclei studied is deduced to be weak, as expected from the low-lying level-energy schemes. Comparisons are also made to state-of-the-art beyond-mean-field model calculations and the magnitude of the transitional quadrupole moments are well reproduced.

   nucl-exPRC(2015)·10 citations
3. 03*

   Experimental level densities of atomic nuclei

   M. Guttormsen🇳🇴 · M. Aiche🇫🇷 · F.L. Bello Garrote🇳🇴 · L.A. Bernstein🇺🇸 · D.L. Bleuel🇺🇸 · Y. Byun · Q. Ducasse🇫🇷 · T.K. Eriksen🇳🇴 · F. Giacoppo🇳🇴 · A. Görgen🇳🇴 · F. Gunsing🇫🇷 · T.W. Hagen🇳🇴 and 15 other authors

   It is almost 80 years since Hans Bethe described the level density as a non-interacting gas of protons and neutrons. In all these years, experimental data were interpreted within this picture of a fermionic gas. However, the renewed interest of measuring level density using various techniques calls for a revision of this description. In particular, the wealth of nuclear level densities measured with the Oslo method favors the constant-temperature level density over the Fermi-gas picture. From the basis of experimental data, we demonstrate that nuclei exhibit a constant-temperature level density behavior for all mass regions and at least up to the neutron threshold.

   nucl-exEPJA(2015)·43 citations
4. 04*

   Are narrow unflavoured mesons a signature of new physics?

   Boris Tatischeff🇫🇷 · Egle Tomasi-Gustafsson🇫🇷

   New data of narrow low mass unflavoured mesonic structures are presented. A table of these exotic masses is obtained adding previously published data. The mass sequence shows a significant coupling of some of these clusters with stable hadrons: pion, nucleon, and deuteron. Indeed this coupling allows to reproduce rather well the masses of exotic narrow baryons and dibaryons. A discussion is presented to suggest a possible interpretation of these exotic hadronic structures.

   nucl-exhep-phPoS(2015)·0 citations
5. 05*

   Tomography of the Quark-Gluon-Plasma by Charm Quarks

   Taesoo Song🇩🇪 · Hamza Berrehrah🇩🇪 · Daniel Cabrera🇩🇪 · Juan M. Torres-Rincon🇫🇷 · Laura Tolos🇩🇪 · Wolfgang Cassing🇩🇪 · Elena Bratkovskaya🇩🇪

   We study charm production in ultra-relativistic heavy-ion collisions by using the Parton-Hadron-String Dynamics (PHSD) transport approach. The initial charm quarks are produced by the Pythia event generator tuned to fit the transverse momentum spectrum and rapidity distribution of charm quarks from Fixed-Order Next-to-Leading Logarithm (FONLL) calculations. The produced charm quarks scatter in the quark-gluon plasma (QGP) with the off-shell partons whose masses and widths are given by the Dynamical Quasi-Particle Model (DQPM) which reproduces the lattice QCD equation-of-state in thermal equilibrium. The relevant cross section are calculated in a consistent way by employing the effective propagators and couplings from the DQPM. Close to the critical energy density of the phase transition, the charm quarks are hadronized into $D$ mesons through coalescence and/or fragmentation depending on transverse momentum. The hadronized $D$ mesons then interact with the various hadrons in the hadronic phase with cross sections calculated in an effective lagrangian approach with heavy-quark spin symmetry. Finally, the nuclear modification factor ${\mathrm{R}}_{\mathrm{AA}}$ and the elliptic flow $v_2$ of $D^0$ mesons from PHSD are compared with the experimental data from the STAR Collaboration for Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ =200 GeV. We find that in the PHSD the energy loss of $D$ mesons at high $p_T$ can be dominantly attributed to partonic scattering while the actual shape of ${\mathrm{R}}_{\mathrm{AA}}$ versus $p_T$ reflects the heavy quark hadronization scenario, i.e. coalescence versus fragmentation. Also the hadronic rescattering is important for the ${\mathrm{R}}_{\mathrm{AA}}$ at low $p_T$ and enhances the $D$-meson elliptic flow $v_2$.

   ↳ nucl-thhep-phnucl-exPRC(2015)·233 citations
6. 06*

   Mid-rapidity charged hadron transverse spherocity in pp collisions simulated with Pythia

   Eleazar Cuautle🇲🇽 · Antonio Ortiz🇲🇽 · Guy Paic🇲🇽

   The pp collisions have been studied for a long time, however, there are still some effects which are not completely understood, such as the long range angular correlations and the flow patterns in high multiplicity events, which were recently discovered at the LHC. In a recent work it was demonstrated that in Pythia 8, multi-parton interactions and color reconnection can give some of the observed effects similar to the collective flow well known from heavy-ion collisions. Now using the same model, a study based on mid-rapidity charged hadron transverse spherocity is presented. The main purpose of this work is to show that a differential study combining multiplicity and event shapes opens the possibility to understand better the features of data, specially at high multiplicity.

   ↳ hep-phnucl-exNPA(2015)·80 citations
7. 07*

   A Threshold Gas Čerenkov Detector for the Spin Asymmetries of the Nucleon Experiment

   Whitney R. Armstrong🇺🇸 · Seonho Choi🇰🇷 · Ed Kaczanowicz🇺🇸 · Alexander Lukhanin🇺🇸 · Zein-Eddine Meziani🇺🇸 · Brad Sawatzky🇺🇸

   We report on the design, construction, commissioning, and performance of a threshold gas Čerenkov counter in an open configuration, which operates in a high luminosity environment and produces a high photo-electron yield. Part of a unique open geometry detector package known as the Big Electron Telescope Array, this Čerenkov counter served to identify scattered electrons and reject produced pions in an inclusive scattering experiment known as the Spin Asymmetries of the Nucleon Experiment E07-003 at the Thomas Jefferson National Accelerator Facility (TJNAF) also known as Jefferson Lab. The experiment consisted of a measurement of double spin asymmetries $A_{\parallel }$ and $A_{\perp }$ of a polarized electron beam impinging on a polarized ammonia target. The Čerenkov counter's performance is characterised by a yield of about 20 photoelectrons per electron or positron track. Thanks to this large number of photoelectrons per track, the Čerenkov counter had enough resolution to identify electron-positron pairs from the conversion of photons resulting mainly from ${\pi }^0$ decays.

   ↳ physics.ins-detnucl-exNucl.Instrum.Meth.A(2015)·5 citations
8. 08*

   Probing Dark Energy models with neutrons

   G. Pignol🇫🇷

   There is a deep connection between cosmology -- the science of the infinitely large --and particle physics -- the science of the infinitely small. This connection is particularly manifest in neutron particle physics. Basic properties of the neutron -- its Electric Dipole Moment and its lifetime -- are intertwined with baryogenesis and nucleosynthesis in the early Universe. I will cover this topic in the first part, that will also serve as an introduction (or rather a quick recap) of neutron physics and Big Bang cosmology. Then, the rest of the manuscript will be devoted to a new idea: using neutrons to probe models of Dark Energy. In the second part, I will present the chameleon theory: a light scalar field accounting for the late accelerated expansion of the Universe, which interacts with matter in such a way that it does not mediate a fifth force between macroscopic bodies. However, neutrons can alleviate the chameleon mechanism and reveal the presence of the scalar field with properly designed experiments. In the third part, I will describe a recent experiment performed with a neutron interferometer at the Institut Laue Langevin that sets already interesting constraints on the chameleon theory. Last, the chameleon field can be probed by measuring the quantum states of neutrons bouncing over a mirror. In the fourth part I will present the status and prospects of the GRANIT experiment at the ILL.

   ↳ astro-ph.COhep-phnucl-exphysics.ins-detInt.J.Mod.Phys.A(2015)·39 citations
9. 09*

   The beam energy dependence of collective flow in heavy ion collisions

   Hannah Petersen🇩🇪 · Jan Steinheimer🇩🇪 · Jussi Auvinen🇺🇸 · Marcus Bleicher🇩🇪

   The major goals of heavy ion research are to explore the phase diagram of quantum chromodynamics (QCD) and to investigate the properties of the quark gluon plasma (QGP), a new state of matter created at high temperatures and/or densities. Collective anisotropic flow is one of the most promising observables to gain insights about the properties of the system created in relativistic heavy ion reactions. The current status of the beam energy dependence of the first three Fourier coefficients of the azimuthal distribution of the produced particles $v_1$ to $v_3$ within hybrid transport plus hydrodynamics approaches are summarized.

   ↳ nucl-thhep-phnucl-exPoS(2015)·2 citations
10. 10*

    Bottomonium production at forward rapidity with ALICE at the LHC

    Massimiliano Marchisone (for the ALICE collaboration)🇮🇹

    Bottomonium production is a powerful tool to investigate hadron collisions and the properties of the medium created in heavy-ion collisions. According to the color-screening model, these mesons give important information about the deconfined medium called Quark-Gluon Plasma (QGP) produced in ultrarelativistic heavy-ion collisions. Cold nuclear matter (CNM) effects can modify the bottomonium production even in absence of deconfined matter: the study of proton-nucleus collisions is therefore essential to disentangle these effects from the hot ones. Last but not least, measurement in pp collisions serve as crucial test of different QCD models of quarkonium hadroproduction and provide the reference for the study in nucleus-nucleus collisions. In ALICE, bottomonium is measured at forward rapidity ($2.5<y<4$) down to zero transverse momentum, exploiting the dimuon decay channel. The latest results in pp, Pb-Pb and p-Pb collisions are discussed and compared to theoretical calculations.

    ↳ hep-exnucl-ex0 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.