Mon·Feb 6, 2017Monday·February 6, 2017

1. 01*

   First measurement of jet mass in Pb-Pb and p-Pb collisions at the LHC

   ALICE Collaboration

   This letter presents the first measurement of jet mass in Pb-Pb and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV and 5.02 TeV, respectively. Both the jet energy and the jet mass are expected to be sensitive to jet quenching in the hot Quantum Chromodynamics (QCD) matter created in nuclear collisions at collider energies. Jets are reconstructed from charged particles using the anti-$k_{\mathrm{T}}$ jet algorithm and resolution parameter $R=0.4$. The jets are measured in the pseudorapidity range $|{\eta }_{\mathrm{jet}}|<0.5$ and in three intervals of transverse momentum between 60 GeV/$c$ and 120 GeV/$c$. The measurement of the jet mass in central Pb-Pb collisions is compared to the jet mass as measured in p-Pb reference collisions, to vacuum event generators, and to models including jet quenching. It is observed that the jet mass in central Pb-Pb collisions is consistent within uncertainties with p-Pb reference measurements. Furthermore, the measured jet mass in Pb-Pb collisions is not reproduced by the quenching models considered in this letter and is found to be consistent with PYTHIA expectations within systematic uncertainties.

   nucl-exhep-exPLB(2018)·126 citations
2. 02*

   Measurement of $\hat{q}$ in Relativistic Heavy Ion Collisions using di-hadron correlations

   M. J. Tannenbaum🇺🇸

   The azimuthal width of the di-hadron correlations in p$+$p collisons, beyond the fragmentation transverse momentum, $j_T$, is dominated by $k_T$, the so-called intrinsic transverse momentum of a parton in a nucleon, which can be measured. The predicted azimuthal broadening in A$+$A collisions should produce a larger $k_T$ than in p$+$p collisions. The present work introduces the observation that the $k_T$ measured in p$+$p collisions for di-hadrons with $p_{Tt}$ and $p_{Ta}$ must be reduced to compensate for the energy loss of both the trigger and away parent partons when comparing to the $k_T$ measured with the same di-hadron $p_{Tt}$ and $p_{Ta}$ in Au$+$Au collisions. This idea is applied to a recent STAR di-hadron measurement, with result $⟨\hat{q}L⟩=2.1\pm 0.6$ GeV${}^2$. This is more precise but in agreement with a theoretical calulation of $⟨\hat{q}L⟩=14_{-14}^{+42}$ GeV${}^2$ using the same data. Assuming a length $⟨L⟩\approx 7$ fm for central Au$+$Au collisions the present result gives $\hat{q}=0.30\pm 0.09$ GeV${}^2$/fm, in fair agreement with the JET collaboration result of $\hat{q}\approx 1.2\pm 0.3$ GeV${}^2$/fm at initial time ${\tau }_0=0.6$ fm/c in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.

   nucl-exPLB(2017)·17 citations
3. 03*

   Study of jet quenching with Z+jet correlations in PbPb and pp collisions at sqrt(s[NN]) = 5.02 TeV

   CMS Collaboration

   The production of jets in association with Z bosons, reconstructed via the mu+mu- and e+e- decay channels, is studied in pp and, for the first time, in PbPb collisions. Both data samples were collected by the CMS experiment at the LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The PbPb collisions were analyzed in the 0-30% centrality range. The back-to-back azimuthal alignment was studied in both pp and PbPb collisions for Z bosons with transverse momentum p[tz] > 60 GeV/c and a recoiling jet with p[tj] > 30 GeV/c. The pt imbalance, x[jz] = p[tj] / p[tz], as well as the average number of jet partners per Z, R[jz], were studied in intervals of p[tz], in both pp and PbPb collisions. The R[jz] is found to be smaller in PbPb than in pp collisions, which suggests that in PbPb collisions a larger fraction of partons, associated with the Z bosons, lose energy and fall below the 30 GeV/c p[tj] threshold.

   nucl-exhep-exPRL(2017)·121 citations
4. 04*

   Measurements of $e^{+}{e}^{-}$ pairs from open heavy flavor in $p$+$p$ and $d$+$A$ 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 · N. Apadula · L. Aphecetche · J. Asai and 498 other authors

   We report a measurement of $e^{+}{e}^{-}$ pairs from semileptonic heavy-flavor decays in $p$+$p$ collisions at $\sqrt{s_{NN}}=200$~GeV. The $e^{+}{e}^{-}$ pair yield from $b\bar{b}$ and $c\bar{c}$ is separated by exploiting a double differential fit done simultaneously in dielectron invariant mass and $p_T$. We used three different event generators, {\sc pythia}, {\sc mc@nlo}, and {\sc powheg}, to simulate the $e^{+}{e}^{-}$ spectra from $c\bar{c}$ and $b\bar{b}$ production. The data can be well described by all three generators within the detector acceptance. However, when using the generators to extrapolate to $4\pi$, significant differences are observed for the total cross section. These difference are less pronounced for $b\bar{b}$ than for $c\bar{c}$. The same model dependence was observed in already published $d$+$A$ data. The $p$+$p$ data are also directly compared with $d$+$A$ data in mass and $p_T$, and within the statistical accuracy no nuclear modification is seen.

   nucl-exPRC(2017)·28 citations
5. 05*

   B-meson production at forward and backward rapidity in $p$+$p$ and Cu+Au collisions at $\sqrt{s_{{}_{NN}}}$=200 GeV

   C. Aidala · N.N. Ajitanand · Y. Akiba · R. Akimoto · J. Alexander · M. Alfred · V. Andrieux · K. Aoki · N. Apadula · H. Asano · E.T. Atomssa · T.C. Awes and 449 other authors

   The fraction of $J/\psi$ mesons which come from B-meson decay, ${\text{F}}_{B\to J/\psi }$, is measured for J/$\psi$ rapidity \mbox{$1.2<|y|<2.2$} and $p_T>0$ in $p$+$p$ and Cu+Au collisions at $\sqrt{s_{{}_{NN}}}$=200 GeV with the PHENIX detector. The extracted fraction is ${\text{F}}_{B\to J/\psi }$ = 0.025 $\pm$ 0.006(stat) $\pm$ 0.010(syst) for $p$+$p$ collisions. For Cu+Au collisions, ${\text{F}}_{B\to J/\psi }$ is 0.094 $\pm$ 0.028(stat) $\pm$ 0.037(syst) in the Au-going direction ($-2.2<y<-1.2$) and 0.089 $\pm$ 0.026(stat) $\pm$ 0.040(syst) in the Cu-going direction ($1.2<y<2.2$). The nuclear modification factor, $R_{\mathrm{CuAu}}$, of B mesons in Cu+Au collisions is consistent with binary scaling of measured yields in $p$+$p$ at both forward and backward rapidity.

   nucl-exPRC(2017)·25 citations
6. 06*

   Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

   STAR Collaboration: L. Adamczyk · J. K. Adkins · G. Agakishiev · M. M. Aggarwal · Z. Ahammed · N. N. Ajitanand · I. Alekseev · D. M. Anderson · R. Aoyama · A. Aparin · D. Arkhipkin · E. C. Aschenauer and 337 other authors

   The STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Charged jets are reconstructed with the anti-kT algorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff of track constituents ($p_T>0.2$ GeV/c). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at mid-rapidity, for charged-jet transverse momentum $p_T^{\mathrm{jet},\mathrm{ch}}<30$ GeV/c. Comparison is made to similar measurements for Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV, to calculations for p+p collisions at $\sqrt{s}$ = 200 GeV based on the PYTHIA Monte Carlo generator and on a Next-to-Leading Order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of $2.8\pm 0.2(\mathrm{stat})\pm 1.5(\mathrm{sys})$ GeV/c, for $10<p_T^{\mathrm{jet},\mathrm{ch}}<20$ GeV/c and R = 0.5. No medium-induced change in jet shape is observed for $R<0.5$. The azimuthal distribution of low-$p_T^{\mathrm{jet},\mathrm{ch}}$ recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasi-particles in the hot QCD medium. Measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central Au+Au collisions of $50\pm 30(\mathrm{sys})\%$ of the large-angle yield in p+pcollisions predicted by PYTHIA.

   nucl-exhep-exPRC(2017)·147 citations
7. 07*

   Rosenbluth separation of the ${\pi }^0$ Electroproduction Cross Section off the Neutron

   M. Mazouz🇹🇳 · Z. Ahmed🇺🇸 · H. Albataineh🇺🇸 · K. Allada🇺🇸 · K. A. Aniol🇺🇸 · V. Bellini🇮🇹 · M. Benali🇫🇷 · W. Boeglin🇺🇸 · P. Bertin🇫🇷 · M. Brossard🇫🇷 · A. Camsonne🇺🇸 · M. Canan🇺🇸 and 86 other authors

   We report the first longitudinal/transverse separation of the deeply virtual exclusive ${\pi }^0$ electroproduction cross section off the neutron and coherent deuteron. The corresponding four structure functions $d{\sigma }_L/dt$, $d{\sigma }_T/dt$, $d{\sigma }_{LT}/dt$ and $d{\sigma }_{TT}/dt$ are extracted as a function of the momentum transfer to the recoil system at $Q^2$=1.75 GeV${}^2$ and $x_B$=0.36. The $ed\to ed{\pi }^0$ cross sections are found compatible with the small values expected from theoretical models. The $en\to en{\pi }^0$ cross sections show a dominance from the response to transversely polarized photons, and are in good agreement with calculations based on the transversity GPDs of the nucleon. By combining these results with previous measurements of ${\pi }^0$ electroproduction off the proton, we present a flavor decomposition of the $u$ and $d$ quark contributions to the cross section.

   ↳ hep-exnucl-exPRL(2017)·24 citations
8. 08*

   A measurement of the ionization efficiency of nuclear recoils in silicon

   F. Izraelevitch · D. Amidei · A. Aprahamian · R. Arcos-Olalla · G. Cancelo · C. Casarella · A. E. Chavarria · P. Collon · J. Estrada · G. Fernández Moroni · Y. Guardincerri · G. Gutiérrez and 17 other authors

   We have measured the ionization efficiency of silicon nuclear recoils with kinetic energy between 1.8 and 20 keV. We bombarded a silicon-drift diode with a neutron beam to perform an elastic-scattering experiment. A broad-energy neutron spectrum was used and the nuclear recoil energy was reconstructed using a measurement of the time of flight and scattering angle of the scattered neutron. The overall trend of the results of this work is well described by the theory of Lindhard et al. above 4 keV of recoil energy. Below this energy, the presented data shows a deviation from the model. The data indicates a faster drop than the theory prediction at low energies.

   ↳ physics.ins-detastro-ph.IMnucl-exJINST(2017)·32 citations
9. 09*

   Production of ${\pi }^0$ and $\eta$ mesons up to high transverse momentum in pp collisions at 2.76 TeV

   ALICE Collaboration

   The invariant differential cross sections for inclusive ${\pi }^0$ and $\eta$ mesons at midrapidity were measured in pp collisions at $\sqrt{s}=2.76$ TeV for transverse momenta $0.4<p_{\mathrm{T}}<40$ GeV/$c$ and $0.6<p_{\mathrm{T}}<20$ GeV/$c$, respectively, using the ALICE detector. This large range in $p_{\mathrm{T}}$ was achieved by combining various analysis techniques and different triggers involving the electromagnetic calorimeter (EMCal). In particular, a new single-cluster, shower-shape based method was developed for the identification of high-$p_{\mathrm{T}}$ neutral pions, which exploits that the showers originating from their decay photons overlap in the EMCal. The measured cross sections are found to exhibit a similar power-law behavior with an exponent of about $6.3$. Next-to-leading-order perturbative QCD calculations differ from the measured cross sections by about $30$% for the ${\pi }^0$, and between $30$-$50$% for the $\eta$ meson, while generator-level simulations with PYTHIA 8.2 describe the data to better than $10$-$30$%, except at $p_{\mathrm{T}}<1$ GeV/$c$. The new data can therefore be used to further improve the theoretical description of ${\pi }^0$ and $\eta$ meson production.

   ↳ hep-exnucl-exEPJC(2017)·73 citations
10. 10*

    Four-body calculation of ${}^2\mathrm{H}(d,p){}^3\mathrm{H}$ and ${}^2\mathrm{H}(d,n){}^3\mathrm{He}$ reactions above breakup threshold

    A. Deltuva🇱🇹 · A. C. Fonseca🇵🇹

    Nucleon transfer reactions in deuteron-deuteron collisions at energies above the three- and four-body breakup threshold are described using exact four-body equations for transition operators that are solved in the momentum-space framework. Differential cross sections, analyzing powers, polarizations, and spin transfer coefficients are obtained using realistic two-nucleon potentials and including the Coulomb repulsion between protons. Overall good agreement between predictions and experimental data is found. Most remarkable discrepancies are seen around the minima of the differential cross section at higher energies and in the outgoing nucleon polarization at lower energies.

    ↳ nucl-thnucl-exPRC(2017)·31 citations
11. 11*

    On the upper bound of entropy production rate from particle multiplicity in heavy ion collisions

    Mateusz Ploskon (LBNL, Berkeley)🇺🇸 · Martin Veselsky (SAOS, Bratislava)🇸🇰

    We provide a simple derivation for particle production in heavy-ion collisions that is proportional to the rate of entropy production. We find that the particle production depends only on the power of the centre-of-mass collision energy $\sqrt{s_{\mathrm{NN}}}$ and the effective phase-space/volume (e.g. geometry of the collision approximated by the number of nucleons participating in the collision $N_{\mathrm{part}}$). We show that at low-energies the pseudo-rapidity density of particles per participating nucleon pair scales linearly with $\sqrt{s_{\mathrm{NN}}}$ while at high-energies with ${\sqrt{s_{\mathrm{NN}}}}^{1/3}$. The ${\sqrt{s_{\mathrm{NN}}}}^{1/3}$ region is directly related to sub-nucleon degrees of freedom and creation of a quark-gluon plasma (QGP). This picture explains experimental observation that the shape of the distributions of pseudorapidity-density per nucleon pair of charged particles does not depend on $\sqrt{s_{\mathrm{NN}}}$ over a large span of collision energies. We provide an explanation of the scaling and connect it with the maximum rate per unit time of entropy production. We conclude with remarks on the hadron-parton phase transition. In particular, our considerations suggest that the pseudo-rapitidy density of the produced particles per $N_{\mathrm{part}}/2$ larger than approximately 1 (excluding particles from jet fragmentation) is a signature of a QGP formation.

    ↳ hep-phhep-exnucl-exnucl-th2 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.