Mon·Jul 26, 2021Monday·July 26, 2021

1. 01*

   Polarization of $\Lambda$ and $\overline{\Lambda }$ hyperons along the beam direction in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

   ALICE Collaboration

   The polarization of the $\Lambda$ and $\overline{\Lambda }$ hyperons along the beam ($z$) direction, $P_{\mathrm{z}}$, has been measured in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to $P_{\mathrm{z}}$ comes from elliptic flow induced vorticity and can be characterized by the second Fourier sine coefficient $P_{\mathrm{z},s2}=⟨P_{\mathrm{z}}\sin (2\phi -2{\Psi }_2)⟩$, where $\phi$ is the hyperon azimuthal emission angle, and ${\Psi }_2$ is the elliptic flow plane angle. We report the measurement of $P_{\mathrm{z},s2}$ for different collision centralities, and in the 30-50% centrality interval as a function of the hyperon transverse momentum and rapidity. The $P_{\mathrm{z},s2}$ is positive similarly as measured by the STAR Collaboration in Au-Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV, with somewhat smaller amplitude in the semi-central collisions. This is the first experimental evidence of a non-zero hyperon $P_{\mathrm{z}}$ in Pb-Pb collisions at the LHC. The comparison of the measured $P_{\mathrm{z},s2}$ with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.

   nucl-exhep-exPRL(2022)·112 citations
2. 02*

   K${}_{\mathrm{S}}^0$- and (anti-)$\Lambda$-hadron correlations in pp collisions at $\sqrt{s}=13$ TeV

   ALICE Collaboration

   Two-particle azimuthal correlations are measured with the ALICE apparatus in pp collisions at $\sqrt{s}=13$ TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the transition regime between bulk and hard production, probed with the condition that a strange meson (K${}_{\mathrm{S}}^0$) or baryon ($\Lambda$) with transverse momentum $p_{\mathrm{T}}>3$ GeV/c is produced. Azimuthal correlations between kaons or $\Lambda$ hyperons with other hadrons are presented at midrapidity for a broad range of the trigger ($3<p_{\mathrm{T}}^{\mathrm{trigg}}<20$ GeV/$c$) and associated particle $p_{\mathrm{T}}$ (1 GeV/$c$ $<p_{\mathrm{T}}^{\mathrm{assoc}}<p_{\mathrm{T}}^{\mathrm{trigg}}$), for minimum-bias events and as a function of the event multiplicity. The near- and away-side peak yields are compared for the case of either K${}_{\mathrm{S}}^0$ or $\Lambda$($\overline{\Lambda }$) being the trigger particle with that of inclusive hadrons (a sample dominated by pions). In addition, the measurements are compared with predictions from PYTHIA 8 and EPOS LHC event generators.

   nucl-exhep-exEPJC(2021)·14 citations
3. 03*

   Measurements of the groomed and ungroomed jet angularities in pp collisions at $\sqrt{s}=5.02$ TeV

   ALICE Collaboration

   The jet angularities are a class of jet substructure observables which characterize the angular and momentum distribution of particles within jets. These observables are sensitive to momentum scales ranging from perturbative hard scatterings to nonperturbative fragmentation into final-state hadrons. We report measurements of several groomed and ungroomed jet angularities in pp collisions at $\sqrt{s}=5.02$ TeV with the ALICE detector. Jets are reconstructed using charged particle tracks at midrapidity ($|\eta |<0.9$). The anti-$k_{\mathrm{T}}$ algorithm is used with jet resolution parameters $R=0.2$ and $R=0.4$ for several transverse momentum $p_{\mathrm{T}}^{\text{ch jet}}$ intervals in the 20$-$100 GeV/$c$ range. Using the jet grooming algorithm Soft Drop, the sensitivity to softer, wide-angle processes, as well as the underlying event, can be reduced in a way which is well-controlled in theoretical calculations. We report the ungroomed jet angularities, ${\lambda }_{\alpha }$, and groomed jet angularities, ${\lambda }_{\alpha \text{,g}}$, to investigate the interplay between perturbative and nonperturbative effects at low jet momenta. Various angular exponent parameters $\alpha =1$, 1.5, 2, and 3 are used to systematically vary the sensitivity of the observable to collinear and soft radiation. Results are compared to analytical predictions at next-to-leading-logarithmic accuracy, which provide a generally good description of the data in the perturbative regime but exhibit discrepancies in the nonperturbative regime. Moreover, these measurements serve as a baseline for future ones in heavy-ion collisions by providing new insight into the interplay between perturbative and nonperturbative effects in the angular and momentum substructure of jets. They supply crucial guidance on the selection of jet resolution parameter, jet transverse momentum, and angular scaling variable for jet quenching studies.

   nucl-exhep-exJHEP(2022)·77 citations
4. 04*

   Bayesian uncertainty quantification for nuclear matter incompressibility

   Jun Xu🇨🇳 · Zhen Zhang🇨🇳 · Bao-An Li🇺🇸

   Within a Bayesian statistical framework using the standard Skyrme-Hartree-Fock model, the maximum {\it a posteriori} (MAP) values and uncertainties of nuclear matter incompressibility and isovector interaction parameters are inferred from the experimental data of giant resonances and neutron-skin thicknesses of typical heavy nuclei. With the uncertainties of the isovector interaction parameters constrained by the data of the isovector giant dipole resonance and the neutron-skin thickness, we have obtained $K_0=223_{-8}^{+7}$ MeV at 68% confidence level using the data of the isoscalar giant monopole resonance in ${}^{208}$Pb measured at the Research Center for Nuclear Physics (RCNP), Japan, and at the Texas A&M University (TAMU), USA. Although the corresponding ${}^{120}$Sn data gives a MAP value for $K_0$ about 5 MeV smaller than the ${}^{208}$Pb data, there are significant overlaps in their posterior probability distribution functions.

   ↳ nucl-thnucl-exPRC(2021)·29 citations
5. 05*

   Finding signatures of the nuclear symmetry energy in heavy-ion collisions with deep learning

   Yongjia Wang🇨🇳 · Fupeng Li🇨🇳 · Qingfeng Li · Hongliang Lü🇨🇳 · Kai Zhou🇩🇪

   A deep convolutional neural network (CNN) is developed to study symmetry energy $E_{\mathrm{sym}}(\rho )$ effects by learning the mapping between the symmetry energy and the two-dimensional (transverse momentum and rapidity) distributions of protons and neutrons in heavy-ion collisions. Supervised training is performed with labelled data-set from the ultrarelativistic quantum molecular dynamics (UrQMD) model simulation. It is found that, by using proton spectra on event-by-event basis as input, the accuracy for classifying the soft and stiff $E_{\mathrm{sym}}(\rho )$ is about 60% due to large event-by-event fluctuations, while by setting event-summed proton spectra as input, the classification accuracy increases to 98%. The accuracy for 5-label (5 different $E_{\mathrm{sym}}(\rho )$) classification task are about 58% and 72% by using proton and neutron spectra, respectively. For the regression task, the mean absolute error (MAE) which measures the average magnitude of the absolute differences between the predicted and actual $L$ (the slope parameter of $E_{\mathrm{sym}}(\rho )$) are about 20.4 and 14.8 MeV by using proton and neutron spectra, respectively. Fingerprints of the density-dependent nuclear symmetry energy on the transverse momentum and rapidity distributions of protons and neutrons can be identified by convolutional neural network algorithm.

   ↳ nucl-thnucl-exPLB(2021)·24 citations
6. 06*

   Opening angle and dineutron correlations in knockout reactions with Borromean two-neutron halo nuclei

   J. Casal🇮🇹 · M. Gómez-Ramos🇩🇪

   Background. Knockout reactions with proton targets provide an invaluable tool to access the properties of two-neutron halo nuclei. Recently, experimental results for the average opening angle as a function of the intrinsic neutron momentum in 11Li have shown a localization of dineutron correlations on the nucleus surface. Purpose. Study the model dependence and the effect of distortion and absorption on the opening angle distributions to assess the reliability of this observable to extract properties of Borromean two-neutron halo nuclei. Method. A quasifree sudden model is used to describe the knockout process, where absorption effects are modeled by the eikonal S-matrix between the proton target and the core of the Borromean nucleus. Final states in momentum space are built within a three-body model for the projectile, which enables the description of momenta and opening angle distributions. Results. A strong dependence on absorption effects is found for the opening angle at large intrinsic momenta, while the region of lower momenta is mostly insensitive to them. Reasonable agreement with the available data is obtained for 11Li at low momenta with weights for s and p waves different from those previously reported, showing a model dependence in their extraction. For 19B, test calculations show marked sensitivity to small p-wave components. Conclusions. The opening angle for (p,pn) knockout reactions on Borromean nuclei at small intrinsic momenta is a reliable observable mostly sensitive to the structure of the Borromean nucleus. For larger momenta, the reaction mechanism leads to a larger distortion of the distribution. In the case of nuclei with small components of opposite parity to the dominant ones, this observable can be used to explore them. The relation between dineutron in coordinate space and opening angle in momentum space is found to be model-dependent.

   ↳ nucl-thnucl-exPRC(2021)·12 citations
7. 07*

   Shadow generalized parton distributions: a practical approach to the deconvolution problem of DVCS

   V. Bertone🇫🇷 · H. Dutrieux🇫🇷 · C. Mezrag🇫🇷 · H. Moutarde🇫🇷 · P. Sznajder🇵🇱

   Deeply virtual Compton scattering (DVCS) attracts a lot of interest due to its sensitivity to generalized parton distributions (GPDs) which provide a rich access to the partonic structure of hadrons. However, the practical extraction of GPDs for this channel requires a deconvolution procedure, whose feasibility has been disputed. We provide a practical approach to this problem based on a next-to-leading order analysis and a careful study of evolution effects, by exhibiting shadow GPDs with arbitrarily small imprints on DVCS observables at current and future experimental facilities. This shows that DVCS alone will not allow for a model independent extraction of GPDs and a multi-channel analysis is required for this purpose.

   ↳ hep-phhep-exhep-latnucl-exSciPost Phys.Proc.(2022)·4 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.