Tue·Dec 23, 2014Tuesday·December 23, 2014

1. 01*

   Centrality dependence of particle production in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$= 5.02 TeV

   ALICE Collaboration

   We report measurements of the primary charged particle pseudorapidity density and transverse momentum distributions in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, and investigate their correlation with experimental observables sensitive to the centrality of the collision. Centrality classes are defined using different event activity estimators, i.e. charged particle multiplicities measured in three disjunct pseudorapidity regions as well as the energy measured at beam rapidity (zero-degree). The procedures to determine the centrality, quantified by the number of participants ($N_{\mathrm{part}}$), or the number of nucleon-nucleon binary collisions ($N_{\mathrm{coll}}$), are described. We show that, in contrast to Pb-Pb collisions, in p-Pb collisions large multiplicity fluctuations together with the small range of participants available, generate a dynamical bias in centrality classes based on particle multiplicity. We propose to use the zero-degree energy, which we expect not to introduce a dynamical bias, as an alternative event-centrality estimator. Based on zero-degree energy centrality classes, the $N_{\mathrm{part}}$ dependence of particle production is studied. Under the assumption that the multiplicity measured in the Pb-going rapidity region scales with the number of Pb-participants, an approximate independence of the multiplicity per participating nucleon measured at mid-rapitity of the number of participating nucleons is observed. Furthermore, at high-$p_{\mathrm{T}}$ the p-Pb spectra are found to be consistent with the pp spectra scaled by $N_{\mathrm{coll}}$ for all centrality classes. Our results represent valuable input for the study of the event activity dependence of hard probes in p-Pb collision and, hence, help to establish baselines for the interpretation of the Pb-Pb data.

   nucl-exPRC(2015)·388 citations
2. 02*

   Quantification of nuclear uncertainties in nucleosynthesis of elements beyond Iron

   T. Rauscher🇬🇧

   Nucleosynthesis beyond Fe poses additional challenges not encountered when studying astrophysical processes involving light nuclei. Generally higher temperatures and nuclear level densities lead to stronger contributions of transitions on excited target states. This may prevent cross section measurements to determine stellar reaction rates and theory contributions remain important. Furthermore, measurements often are not feasible in the astrophysically relevant energy range. Sensitivity analysis allows not only to determine the contributing nuclear properties but also is a handy tool for experimentalists to interpret the impact of their data on predicted cross sections and rates. It can also speed up future input variation studies of nucleosynthesis by simplifying an intermediate step in the full calculation sequence. Large-scale predictions of sensitivities and ground-state contributions to the stellar rates are presented, allowing an estimate of how well rates can be directly constrained by experiment. The reactions 185W(n,gamma) and 186W(gamma,n) are discussed as application examples. Studies of uncertainties in abundances predicted in nucleosynthesis simulations rely on the knowledge of reaction rate errors. An improved treatment of uncertainty analysis is presented as well as a recipe for combining experimental data and theory to arrive at a new reaction rate and its uncertainty. As an example, it is applied to neutron capture rates for the s-process, leading to larger uncertainties than previously assumed.

   nucl-exastro-ph.HEastro-ph.SRnucl-thPoS(2015)·0 citations
3. 03*

   Observation of the chiral magnetic effect in ZrTe5

   Qiang Li🇺🇸 · Dmitri E. Kharzeev🇺🇸 · Cheng Zhang🇨🇳 · Yuan Huang🇺🇸 · I. Pletikosic🇺🇸 · A. V. Fedorov🇺🇸 · R. D. Zhong🇺🇸 · J. A. Schneeloch🇺🇸 · G. D. Gu🇺🇸 · T. Valla🇺🇸

   The chiral magnetic effect is the generation of electric current induced by chirality imbalance in the presence of magnetic field. It is a macroscopic manifestation of the quantum anomaly in relativistic field theory of chiral fermions (massless spin $1/2$ particles with a definite projection of spin on momentum) -- a dramatic phenomenon arising from a collective motion of particles and antiparticles in the Dirac sea. The recent discovery of Dirac semimetals with chiral quasi-particles opens a fascinating possibility to study this phenomenon in condensed matter experiments. Here we report on the first observation of chiral magnetic effect through the measurement of magneto-transport in zirconium pentatelluride, ZrTe_5. Our angle-resolved photoemission spectroscopy experiments show that this material's electronic structure is consistent with a 3D Dirac semimetal. We observe a large negative magnetoresistance when magnetic field is parallel with the current. The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect. The observed phenomenon stems from the effective transmutation of Dirac semimetal into a Weyl semimetal induced by the parallel electric and magnetic fields that represent a topologically nontrivial gauge field background.

   ↳ cond-mat.str-elhep-phnucl-exnucl-thNat.Phys.(2016)·599 citations
4. 04*

   Formation of hypermatter and hypernuclei within transport models in relativistic ion collisions

   A.S. Botvina (1,2)🇩🇪 · J. Steinheimer (1)🇩🇪 · E. Bratkovskaya (1)🇩🇪 · M. Bleicher (1)🇩🇪 · J. Pochodzalla (3,4) ((1) Frankfurt Institute for Advanced Studies, Frankfurt am Main, Germany (2) Institute for Nuclear Research RAS, Moscow, Russia (3) Helmholtz-Institut Mainz, Mainz, Germany (4) Institut fuer Kernphysik and PRISMA Cluster of Excellence, J.Gutenberg-University Mainz, Germany)🇩🇪

   Within a combined approach we investigate the main features of the production of hyper-fragments in relativistic heavy-ion collisions. The formation of hyperons is modelled within the UrQMD and HSD transport codes. To describe the hyperon capture by nucleons and nuclear residues a coalescence of baryons (CB) model was developed. We demonstrate that the origin of hypernuclei of various masses can be explained by typical baryon interactions, and that it is similar to processes leading to the production of conventional nuclei. At high beam energies we predict a saturation of the yields of all hyper-fragments, therefore, this kind of reactions can be studied with high yields even at the accelerators of moderate relativistic energies.

   ↳ nucl-thhep-exhep-phnucl-exPLB(2015)·80 citations
5. 05*

   A novel method for the line-of-response and time-of-flight reconstruction in TOF-PET detectors based on a library of synchronized model signals

   P. Moskal🇵🇱 · N. Zoń🇵🇱 · T. Bednarski🇵🇱 · P. Białas🇵🇱 · E. Czerwiński🇵🇱 · A. Gajos🇵🇱 · D. Kamińska🇵🇱 · Ł. Kapłon🇵🇱 · A. Kochanowski🇵🇱 · G. Korcyl🇵🇱 · J. Kowal🇵🇱 · P. Kowalski🇵🇱 and 17 other authors

   A novel method of hit time and hit position reconstruction in scintillator detectors is described. The method is based on comparison of detector signals with results stored in a library of synchronized model signals registered for a set of well-defined positions of scintillation points. The hit position is reconstructed as the one corresponding to the signal from the library which is most similar to the measurement signal. The time of the interaction is determined as a relative time between the measured signal and the most similar one in the library. A degree of similarity of measured and model signals is defined as the distance between points representing the measurement- and model-signal in the multi-dimensional measurement space. Novelty of the method lies also in the proposed way of synchronization of model signals enabling direct determination of the difference between time-of-flights (TOF) of annihilation quanta from the annihilation point to the detectors. The introduced method was validated using experimental data obtained by means of the double strip prototype of the J-PET detector and ${}^{22}$Na sodium isotope as a source of annihilation gamma quanta.The detector was built out from plastic scintillator strips with dimensions of 5 mm x 19 mm x 300 mm, optically connected at both sides to photomultipliers,from which signals were sampled by means of the Serial Data Analyzer.Using the introduced method, the spatial and TOF resolution of about 1.3 cm ($\sigma$) and 125 ps ($\sigma$) were established, respectively.

   ↳ physics.ins-detnucl-exNucl.Instrum.Meth.A(2015)·88 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.