Mon·May 14, 2018Monday·May 14, 2018

1. 01*

   Cross section measurements of ${}^{155,157}$Gd(n,$\gamma$) induced by thermal and epithermal neutrons

   M. Mastromarco🇮🇹 · A. Manna🇮🇹 · O. Aberle🇨🇭 · S. Amaducci · J. Andrzejewski🇵🇱 · L. Audouin🇫🇷 · M. Bacak🇦🇹 · J. Balibrea🇪🇸 · M. Barbagallo🇮🇹 · F. Becvar🇨🇿 · E. Berthoumieux🇫🇷 · J. Billowes🇬🇧 and 112 other authors

   Neutron capture measurements on ${}^{155}$Gd and ${}^{157}$Gd were performed using the time-of-flight technique at the n\_TOF facility at CERN. Four samples in form of self-sustaining metallic discs isotopically enriched in ${}^{155}$Gd and ${}^{157}$Gd were used. The measurements were carried out at the experimental area (EAR1) at 185 m from the neutron source, with an array of 4 C${}_6$D${}_6$ liquid scintillation detectors. The capture cross sections of ${}^{155}$Gd and ${}^{157}$Gd at neutron kinetic energy of 0.0253 eV have been estimated to be 62.2(2.2) kb and 239.8(9.3) kb, respectively, thus up to 6\% different relative to the ones reported in the nuclear data libraries. A resonance shape analysis has been performed in the resolved resonance region up to 180 eV and 300 eV, respectively, in average resonance parameters have been found in good agreement with evaluations. Above these energies the observed resonance-like structures in the cross section have been tentatively characterised in terms of resonance energy and area up to 1 keV.

   nucl-exEPJA(2019)·27 citations
2. 02*

   Suppression of $\Lambda (1520)$ resonance production in central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV

   ALICE Collaboration

   The production yield of the $\Lambda (1520)$ baryon resonance is measured at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV with the ALICE detector at the LHC. The measurement is performed in the $\Lambda (1520)\to {\mathrm{pK}}^{-}$ (and charge conjugate) hadronic decay channel as a function of the transverse momentum ($p_{\mathrm{T}}$) and collision centrality. The $p_{\mathrm{T}}$-integrated production rate of $\Lambda (1520)$ relative to $\Lambda$ in central collisions is suppressed by about a factor of 2 with respect to peripheral collisions. This is the first observation of the suppression of a baryonic resonance at the LHC and the first 3$\sigma$ evidence of $\Lambda (1520)$ suppression within a single collision system. The measured $\Lambda (1520)/\Lambda$ ratio in central collisions is smaller than the value predicted by the statistical hadronisation model calculations. The shape of the measured $p_{\mathrm{T}}$ distribution and the centrality dependence of the suppression are reproduced by the EPOS3 Monte Carlo event generator. The measurement adds further support to the formation of a dense hadronic phase in the final stages of the evolution of the fireball created in heavy-ion collisions, lasting long enough to cause a significant reduction in the observable yield of short-lived resonances.

   nucl-exhep-exPRC(2019)·79 citations
3. 03*

   Production of the $\rho$(770)${}^0$ meson in pp and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV

   ALICE Collaboration

   The production of the $\rho$(770)${}^0$ meson has been measured at mid-rapidity $(|y|<0.5)$ in pp and centrality differential Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The particles have been reconstructed in the $\rho$(770)$\to {\pi }^{+}{\pi }^{-}$ decay channel in the transverse momentum ($p_T$) range $0.5-11$ GeV/$c$. A centrality dependent suppression of the ratio of the integrated yields $2\rho$(770)${}^0/({\pi }^{+}+{\pi }^{-})$ is observed. The ratio decreases by $\sim 40\%$ from pp to central Pb-Pb collisions. A study of the $p_T$-differential $2\rho$(770)${}^0/({\pi }^{+}+{\pi }^{-})$ ratio reveals that the suppression occurs at low transverse momenta, $p_T<2$ GeV/$c$. At higher momentum, particle ratios measured in heavy-ion and pp collisions are consistent. The observed suppression is very similar to that previously measured for the $K^{\ast }$(892)${}^0/K$ ratio and is consistent with EPOS3 predictions that may imply that rescattering in the hadronic phase is a dominant mechanism for the observed suppression.

   nucl-exhep-exPRC(2019)·92 citations
4. 04*

   Azimuthal anisotropy of heavy-flavour decay electrons in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

   ALICE Collaboration

   Angular correlations between heavy-flavour decay electrons and charged particles at mid-rapidity ($|\eta |<0.8$) are measured in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. The analysis is carried out for the 0-20% (high) and 60-100% (low) multiplicity ranges. The jet contribution in the correlation distribution from high-multiplicity events is removed by subtracting the distribution from low-multiplicity events. An azimuthal modulation remains after removing the jet contribution, similar to previous observations in two-particle angular correlation measurements for light-flavour hadrons. A Fourier decomposition of the modulation results in a positive second-order coefficient ($v_2$) for heavy-flavour decay electrons in the transverse momentum interval $1.5<p_{\mathrm{T}}<4$ GeV/$c$ in high-multiplicity events, with a significance larger than $5\sigma$. The results are compared with those of charged particles at mid-rapidity and of inclusive muons at forward rapidity. The $v_2$ measurement of open heavy-flavour particles at mid-rapidity in small collision systems could provide crucial information to help interpret the anisotropies observed in such systems.

   nucl-exhep-exPRL(2019)·67 citations
5. 05*

   Measurements of low-$p_{\mathrm{T}}$ electrons from semileptonic heavy-flavour hadron decays at mid-rapidity in pp and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV

   ALICE Collaboration

   Transverse-momentum ($p_{\mathrm{T}}$) differential yields of electrons from semileptonic heavy-flavour hadron decays have been measured in the most central (0-10%) and in semi-central (20-40%) Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. The corresponding production cross section in pp collisions has been measured at the same energy with substantially reduced systematic uncertainties with respect to previously published results. The modification of the yield in Pb-Pb collisions with respect to the expectation from an incoherent superposition of nucleon-nucleon collisions is quantified at mid-rapidity ($|y|$ $<$ 0.8) in the $p_{\mathrm{T}}$ interval 0.5-3 GeV/$c$ via the nuclear modification factor, $R_{\mathrm{AA}}$. This paper extends the $p_{\mathrm{T}}$ reach of the $R_{\mathrm{AA}}$ measurement towards significantly lower values with respect to a previous publication. In Pb-Pb collisions the $p_{\mathrm{T}}$-differential measurements of yields at low $p_{\mathrm{T}}$ are essential to investigate the scaling of heavy-flavour production with the number of binary nucleon-nucleon collisions. Heavy-quark hadronization, a collective expansion and even initial-state effects, such as the nuclear modification of the Parton Distribution Functions, are also expected to have a significant effect on the measured distribution.

   nucl-exhep-exJHEP(2018)·36 citations
6. 06*

   Inclusive J/$\psi$ production at forward and backward rapidity in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV

   ALICE Collaboration

   Inclusive J/$\psi$ production is studied in p-Pb interactions at a centre-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\mathrm{NN}}}=8.16$ TeV, using the ALICE detector at the CERN LHC. The J/$\psi$ meson is reconstructed, via its decay to a muon pair, in the centre-of-mass rapidity intervals $2.03<y_{\mathrm{cms}}<3.53$ and $-4.46<y_{\mathrm{cms}}<-2.96$, where positive and negative $y_{\mathrm{cms}}$ refer to the p-going and Pb-going direction, respectively. The transverse momentum coverage is $p_{\mathrm{T}}<20$ GeV/$c$. In this paper, $y_{\mathrm{cms}}$- and $p_{\mathrm{T}}$-differential cross sections for inclusive J/$\psi$ production are presented, and the corresponding nuclear modification factors $R_{\mathrm{pPb}}$ are shown. Forward results show a suppression of the J/$\psi$ yield with respect to pp collisions, concentrated in the region $p_{\mathrm{T}}\lesssim 5$ GeV/$c$. At backward rapidity no significant suppression is observed. The results are compared to previous measurements by ALICE in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and to theoretical calculations. Finally, the ratios $R_{\mathrm{FB}}$ between forward- and backward-$y_{\mathrm{cms}}$ $R_{\mathrm{pPb}}$ values are shown and discussed.

   nucl-exhep-exJHEP(2018)·63 citations
7. 07*

   Inclusive J/$\psi$ production in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV

   ALICE Collaboration

   Inclusive J/$\psi$ production is studied in Xe-Xe interactions at a centre-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV, using the ALICE detector at the CERN LHC. The J/$\psi$ meson is reconstructed via its decay into a muon pair, in the centre-of-mass rapidity interval $2.5<y<4$ and down to zero transverse momentum. In this Letter, the nuclear modification factors $R_{\mathrm{AA}}$ for inclusive J/$\psi$, measured in the centrality range 0-90% as well as in the centrality intervals 0-20% and 20-90% are presented. The $R_{\mathrm{AA}}$ values are compared to previously published results for Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV and to the calculation of a transport model. A good agreement is found between Xe-Xe and Pb-Pb results as well as between data and the model.

   nucl-exhep-exPLB(2018)·24 citations
8. 08*

   ${\rm Y}$ suppression at forward rapidity in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

   ALICE Collaboration

   Inclusive ${\rm Y}$(1S) and ${\rm Y}$(2S) production have been measured in Pb-Pb collisions at the centre-of-mass energy per nucleon-nucleon pair $\sqrt{s_{{}_{\mathrm{NN}}}}=5.02$ TeV, using the ALICE detector at the CERN LHC. The ${\rm Y}$ mesons are reconstructed in the centre-of-mass rapidity interval $2.5<y<4$ and in the transverse-momentum range $p_{\mathrm{T}}<15$ GeV/$c$, via their decays to muon pairs. In this Letter, we present results on the inclusive ${\rm Y}$(1S) nuclear modification factor $R_{\mathrm{AA}}$ as a function of collision centrality, transverse momentum and rapidity. The ${\rm Y}$(1S) and ${\rm Y}$(2S) $R_{\mathrm{AA}}$, integrated over the centrality range 0-90%, are $0.37\pm 0.02(\mathrm{stat})\pm 0.03(\mathrm{syst})$ and $0.10\pm 0.04(\mathrm{stat})\pm 0.02(\mathrm{syst})$, respectively, leading to a ratio $R_{\mathrm{AA}}^{{\rm Y}(2S)}/R_{\mathrm{AA}}^{{\rm Y}(1S)}$ of $0.28\pm 0.12\text{(stat)}\pm 0.06\text{(syst)}$. The observed ${\rm Y}$(1S) suppression increases with the centrality of the collision and no significant variation is observed as a function of transverse momentum and rapidity.

   nucl-exhep-exPLB(2019)·82 citations
9. 09*

   Anisotropic flow of identified particles in Pb-Pb collisions at $\sqrt{{\text{s}}_{\mathrm{NN}}}=5.02$ TeV

   ALICE Collaboration

   The elliptic ($v_2$), triangular ($v_3$), and quadrangular ($v_4$) flow coefficients of ${\pi }^{\pm }$, ${\mathrm{K}}^{\pm }$, $\mathrm{p}+\overline{\mathrm{p}}$, $\Lambda +\overline{\Lambda }$, ${\mathrm{K}}_{\mathrm{S}}^0$, and the $\varphi$-meson are measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. Results obtained with the scalar product method are reported for the rapidity range $|y|<$ 0.5 as a function of transverse momentum, $p_{\text{T}}$, at different collision centrality intervals between 0-70%, including ultra-central (0-1%) collisions for ${\pi }^{\pm }$, ${\mathrm{K}}^{\pm }$, and $\mathrm{p}+\overline{\mathrm{p}}$. For $p_{\text{T}}<3$ GeV$/c$, the flow coefficients exhibit a particle mass dependence. At intermediate transverse momenta ($3<p_{\text{T}}<$~8-10 GeV$/c$), particles show an approximate grouping according to their type (i.e., mesons and baryons). The $\varphi$-meson $v_2$, which tests both particle mass dependence and type scaling, follows $\mathrm{p}+\overline{\mathrm{p}}$ $v_2$ at low $p_{\text{T}}$ and ${\pi }^{\pm }$ $v_2$ at intermediate $p_{\text{T}}$. The evolution of the shape of $v_{\mathrm{n}}(p_{\mathrm{T}})$ as a function of centrality and harmonic number $n$ is studied for the various particle species. Flow coefficients of ${\pi }^{\pm }$, ${\mathrm{K}}^{\pm }$, and $\mathrm{p}+\overline{\mathrm{p}}$ for $p_{\text{T}}<3$ GeV$/c$ are compared to iEBE-VISHNU and MUSIC hydrodynamical calculations coupled to a hadronic cascade model (UrQMD). The iEBE-VISHNU calculations describe the results fairly well for $p_{\text{T}}<2.5$ GeV$/c$, while MUSIC calculations reproduce the measurements for $p_{\text{T}}<1$ GeV$/c$. A comparison to $v_{\mathrm{n}}$ coefficients measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV is also provided.

   nucl-exhep-exJHEP(2018)·198 citations
10. 10*

    Transverse momentum spectra and nuclear modification factors of charged particles in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV

    ALICE Collaboration

    Transverse momentum ($p_{\mathrm{T}}$) spectra of charged particles at mid-pseudorapidity in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV measured with the ALICE apparatus at the Large Hadron Collider are reported. The kinematic range $0.15<p_{\mathrm{T}}<50$ GeV/$c$ and $|\eta |<0.8$ is covered. Results are presented in nine classes of collision centrality in the 0-80% range. For comparison, a pp reference at the collision energy of $\sqrt{s}$ = 5.44 TeV is obtained by interpolating between existing \pp measurements at $\sqrt{s}$ = 5.02 and 7 TeV. The nuclear modification factors in central Xe-Xe collisions and Pb-Pb collisions at a similar center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, and in addition at 2.76 TeV, at analogous ranges of charged particle multiplicity density $⟨\mathrm{d}{\mathrm{N}}_{\mathrm{ch}}/\mathrm{d}\eta ⟩$ show a remarkable similarity at $p_{\mathrm{T}}>10$ GeV/$c$. The centrality dependence of the ratio of the average transverse momentum $⟨p_{\mathrm{T}}⟩$ in Xe-Xe collisions over Pb-Pb collision at $\sqrt{s}$ = 5.02 TeV is compared to hydrodynamical model calculations.

    nucl-exhep-exPLB(2019)·158 citations
11. 11*

    Global polarization of $\Lambda$ hyperons in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV

    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

    Global polarization of $\Lambda$ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at $\sqrt{s_{{}_{NN}}}$ = 200 GeV, with no significant difference between $\Lambda$ and $\bar{\Lambda }$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $\sqrt{s_{{}_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity ${\eta }^H$ within $|{\eta }^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2\sigma$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.

    nucl-exPRC(2018)·451 citations
12. 12*

    Direct photon elliptic flow in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV

    ALICE Collaboration

    The elliptic flow of inclusive and direct photons was measured at mid-rapidity in two centrality classes 0-20% and 20-40% in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV by ALICE. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the detector material with the $e^{+}{e}^{-}$ pairs reconstructed in the central tracking system. The results of the two methods were combined and the direct photon elliptic flow was extracted in the transverse momentum range $0.9<p_{\mathrm{T}}<6.2$ GeV/$c$. A comparison to RHIC data shows a similar magnitude of the measured direct-photon elliptic flow. Hydrodynamic and transport model calculations are systematically lower than the data, but are found to be compatible.

    nucl-exhep-exPLB(2019)·91 citations
13. 13*

    Two particle differential transverse momentum and number density correlations in p-Pb and Pb-Pb at the LHC

    ALICE Collaboration

    We present measurements of two-particle differential number correlation functions $R_2$ and transverse momentum correlation functions $P_2$, obtained from p-Pb collisions at 5.02 TeV and Pb-Pb collisions at 2.76 TeV. The results are obtained using charged particles in the pseudorapidity range $|\eta |<$ 1.0, and transverse momentum range $0.2<p_{\mathrm{T}}<2.0$ GeV/$c$ as a function of pair separation in pseudorapidity, $|\Delta \eta |$, azimuthal angle, $\Delta \phi$, and for several charged-particle multiplicity classes. Measurements are carried out for like-sign and unlike-sign charged-particle pairs separately and combined to obtain charge-independent and charge-dependent correlation functions. We study the evolution of the width of the near-side peak of these correlation functions with collision centrality. Additionally, we study Fourier decompositions of the correlators in $\Delta \phi$ as a function of the pair separation $|\Delta \eta |$. Significant differences in the dependence of their harmonic coefficients on multiplicity classes are found. These differences can be exploited, in theoretical models, to obtain further insight into charged-particle production and transport in heavy-ion collisions. Moreover, an upper limit of non-flow contributions to flow coefficients $v_n$ measured in Pb-Pb collisions based on the relative strength of Fourier coefficients measured in p-Pb interactions is estimated.

    nucl-exhep-exPRC(2019)·41 citations
14. 14*

    Centrality and pseudorapidity dependence of the charged-particle multiplicity density in Xe-Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV

    ALICE Collaboration

    In this Letter, the ALICE Collaboration presents the first measurements of the charged-particle multiplicity density, $\mathrm{d}{\mathrm{N}}_{\mathrm{ch}}/\mathrm{d}\eta$, and total charged-particle multiplicity, $N_{\mathrm{ch}}^{\mathrm{tot}}$, in Xe-Xe collisions at a centre-of-mass energy per nucleon--nucleon pair of $\sqrt{s_{\mathrm{NN}}}$ = 5.44 TeV. The measurements are performed as a function of collision centrality over a wide pseudorapidity range of $-3.5<\eta <5$. The values of $\mathrm{d}{\mathrm{N}}_{\mathrm{ch}}/\mathrm{d}\eta$ at mid-rapidity and $N_{\mathrm{ch}}^{\mathrm{tot}}$ for central collisions, normalised to the number of nucleons participating in the collision ($N_{\mathrm{part}}$) as a function of $\sqrt{s_{\mathrm{NN}}}$, follow the trends established in previous heavy-ion measurements. The same quantities are also found to increase as a function of $N_{\mathrm{part}}$, and up to the 5% most central collisions the trends are the same as the ones observed in Pb-Pb at a similar energy. For more central collisions, the Xe-Xe scaled multiplicities exceed those in Pb-Pb for a similar $N_{\mathrm{part}}$. The results are compared to phenomenological models and theoretical calculations based on different mechanisms for particle production in nuclear collisions. All considered models describe the data reasonably well within 15%.

    nucl-exhep-exPLB(2019)·162 citations
15. 15*

    Resolving the $\Lambda$ hypernuclear overbinding problem in pionless effective field theory

    Lorenzo Contessi🇮🇱 · Nir Barnea🇮🇱 · Avraham Gal🇮🇱

    We address the $\Lambda$-hypernuclear `overbinding problem' in light hypernuclei which stands for a 1--3 MeV excessive $\Lambda$ separation energy calculated in ${}_{\Lambda }^5$He. This problem arises in most few-body calculations that reproduce ground-state $\Lambda$ separation energies in the lighter $\Lambda$ hypernuclei within various hyperon-nucleon interaction models. Recent pionless effective field theory nuclear few-body calculations are extended in this work to $\Lambda$ hypernuclei. At leading order, the $\Lambda N$ low-energy constants are associated with $\Lambda N$ scattering lengths and the $\Lambda NN$ low-energy constants are fitted to $\Lambda$ separation energies ($B_{\Lambda }^{\exp }$) for $A\le 4$. The resulting pionless-EFT interaction reproduces in few-body stochastic variational method calculations the reported value $B_{\Lambda }^{\exp }{(}_{\Lambda }^5$He)=3.12$\pm$0.02 MeV within a fraction of MeV over a broad range of momentum-space cut-off parameters. Possible consequences and extensions to heavier hypernuclei and to neutron-star matter are discussed.

    ↳ nucl-thhep-phnucl-exPRL(2018)·43 citations
16. 16*

    Tetraneutron: Rigorous continuum calculation

    A. Deltuva🇱🇹

    The four-neutron system is studied using exact continuum equations for transition operators and solving them in the momentum-space framework. A resonant behavior is found for strongly enhanced interaction but not a the physical strength, indicating the absence of an observable tetraneutron resonance, in contrast to a number of earlier works. As the transition operators acquire large values at low energies, it is conjectured that this behavior may explain peaks in many-body reactions even without a resonance.

    ↳ nucl-thcond-mat.quant-gasnucl-exPLB(2018)·45 citations
17. 17*

    Measurement of the inclusive J/$\psi$ polarization at forward rapidity in pp collisions at $\sqrt{s}=8$ TeV

    ALICE Collaboration

    We report on the measurement of the inclusive J/$\psi$ polarization parameters in pp collisions at a center of mass energy $\sqrt{s}=8$ TeV with the ALICE detector at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 1.23 pb${}^{-1}$. J/$\psi$ resonances are reconstructed in their di-muon decay channel in the rapidity interval $2.5<y<4.0$ and over the transverse-momentum interval $2<p_{\mathrm{T}}<15$ GeV/$c$. The three polarization parameters (${\lambda }_{\theta }$, ${\lambda }_{\phi }$, ${\lambda }_{\theta \phi }$) are measured as a function of $p_{\mathrm{T}}$ both in the helicity and Collins-Soper reference frames. The measured J/$\psi$ polarization parameters are found to be compatible with zero within uncertainties, contrary to expectations from all available predictions. The results are compared with the measurement in pp collisions at $\sqrt{s}=7$ TeV.

    ↳ hep-exnucl-exEPJC(2018)·67 citations
18. 18*

    Dielectron production in proton-proton collisions at $\sqrt{s}$ = 7 TeV

    ALICE Collaboration

    The first measurement of e${}^{+}$e${}^{-}$ pair production at mid-rapidity ($|{\eta }_{\mathrm{e}}|$ $<$ 0.8) in pp collisions at $\sqrt{s}=7$ TeV with ALICE at the LHC is presented. The dielectron production is studied as a function of the invariant mass ($m_{\mathrm{ee}}$ $<$ 3.3 GeV/$c^2$), the pair transverse momentum ($p_{\mathrm{T},\mathrm{ee}}$ $<$ 8 GeV/$c$), and the pair transverse impact parameter (DCA${}_{\mathrm{ee}}$), i.e., the average distance of closest approach of the reconstructed electron and positron tracks to the collision vertex, normalised to its resolution. The results are compared with the expectations from a cocktail of known hadronic sources and are well described when PYTHIA is used to generate the heavy-flavour contributions. In the low-mass region (0.14 $<$ $m_{\mathrm{ee}}$ $<$ 1.1 GeV/$c^2$), prompt and non-prompt e${}^{+}$e${}^{-}$ sources can be separated via the DCA${}_{\mathrm{ee}}$. In the intermediate-mass region (1.1 $<$ $m_{\mathrm{ee}}$ $<$ 2.7 GeV/$c^2$), a double-differential fit to the data in $m_{\mathrm{ee}}$ and $p_{\mathrm{T},\mathrm{ee}}$ and a fit of the DCA${}_{\mathrm{ee}}$ distribution allow the total $\mathrm{c}\overline{\mathrm{c}}$ and $\mathrm{b}\overline{\mathrm{b}}$ cross sections to be extracted. Two different event generators, PYTHIA and POWHEG, can reproduce the shape of the two-dimensional $m_{\mathrm{ee}}$ and $p_{\mathrm{T},\mathrm{ee}}$ spectra, as well as the shape of the DCA${}_{\mathrm{ee}}$ distribution, reasonably well. However, differences in the $\mathrm{c}\overline{\mathrm{c}}$ and $\mathrm{b}\overline{\mathrm{b}}$ cross sections are observed when using the generators to extrapolate to full phase space. Finally, the ratio of inclusive to decay photons is studied via the measurement of virtual direct photons in the transverse-momentum range 1 $<$ $p_{\mathrm{T}}$ $<$ 8 GeV/$c$. This is found to be unity within the statistical and systematic uncertainties and consistent with expectations from next-to-leading order perturbative quantum chromodynamic calculations.

    ↳ hep-exnucl-exJHEP(2018)·61 citations
19. 19*

    Dielectron and heavy-quark production in inelastic and high-multiplicity proton-proton collisions at $\sqrt{s}$ = 13 TeV

    ALICE Collaboration

    The measurement of dielectron production is presented as a function of invariant mass and transverse momentum ($p_{\mathrm{T}}$) at midrapidity ($|y_{\mathrm{e}}|<0.8$) in proton-proton (pp) collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV. The contributions from light-hadron decays are calculated from their measured cross sections in pp collisions at $\sqrt{s}=7$ TeV or 13 TeV. The remaining continuum stems from correlated semileptonic decays of heavy-flavour hadrons. Fitting the data with templates from two different MC event generators, PYTHIA and POWHEG, the charm and beauty cross sections at midrapidity are extracted for the first time at this collision energy: $\mathrm{d}{\sigma }_{\mathrm{c}\bar{\mathrm{c}}}/\mathrm{d}y{|}_{y=0}=974\pm 138(stat.)\pm 140(syst.)\mu \mathrm{b}$ and $\mathrm{d}{\sigma }_{\mathrm{b}\bar{\mathrm{b}}}/\mathrm{d}y{|}_{y=0}=79\pm 14(stat.)\pm 11(syst.)\mu \mathrm{b}$ using PYTHIA simulations and $\mathrm{d}{\sigma }_{\mathrm{c}\bar{\mathrm{c}}}/\mathrm{d}y{|}_{y=0}=1417\pm 184(stat.)\pm 204(syst.)\mu \mathrm{b}$ and $\mathrm{d}{\sigma }_{\mathrm{b}\bar{\mathrm{b}}}/\mathrm{d}y{|}_{y=0}=48\pm 14(stat.)\pm 7(syst.)\mu \mathrm{b}$ for POWHEG. These values, whose uncertainties are fully correlated between the two generators, are consistent with extrapolations from lower energies. The different results obtained with POWHEG and PYTHIA imply different kinematic correlations of the heavy-quark pairs in these two generators. Furthermore, comparisons of dielectron spectra in inelastic events and in events collected with a trigger on high charged-particle multiplicities are presented in various $p_{\mathrm{T}}$ intervals. The differences are consistent with the already measured scaling of light-hadron and open-charm production at high charged-particle multiplicity as a function of $p_{\mathrm{T}}$. Upper limits for the contribution of virtual direct photons are extracted at 90% confidence level and found to be in agreement with pQCD calculations.

    ↳ hep-exnucl-exPLB(2019)·65 citations
20. 20*

    Baryon clustering at the critical line and near the hypothetical critical point in heavy-ion collisions

    Edward Shuryak🇺🇸 · Juan M. Torres-Rincon🇺🇸

    We study clustering of baryons at the freeze-out point of relativistic heavy-ion collisions. Using a Walecka-Serot model for the nucleon-nucleon (NN) interaction we analyze how the modified/critical $\sigma$ mode---responsible for the NN attraction---allows for clustering of nucleons when the system is close to a possible critical point of QCD. We investigate clusters of few nucleons, and also the internal cluster configuration when the system is long lived. For realistic heavy-ion collisions we study to how extend such clusters can be formed in a finite time, and perform the statistical analysis of cumulants and higher-order moments (skewness and kurtosis) for collisions at the Beam Energy Scan of RHIC.

    ↳ hep-phnucl-exnucl-thPRC(2019)·50 citations
21. 21*

    S- and p-wave structure of $S=-1$ meson-baryon scattering in the resonance region

    D. Sadasivan🇺🇸 · M. Mai🇺🇸 · M. Doring🇺🇸

    We perform a simultaneous analysis of s- and p-waves of the $S=-1$ meson-baryon scattering amplitude using all low-energy experimental data. For the first time, differential cross section data are included for chiral unitary coupled-channel models. From this model s- and p-wave amplitudes are extracted and we observe both well-known $I(J^P)=0(1/2^{-})$ s-wave states as well as a new $I(J^P)=1(1/2^{+})$ state absent in quark models and lattice QCD results. Multiple statistical and phenomenological tests suggest that, while the data clearly require an $I=1$ p-wave resonance, the new state just accounts for the absence of the decuplet $\Sigma (1385)3/2^{+}$ in the model.

    ↳ nucl-thhep-phnucl-exPLB(2019)·31 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.