Nuclear Matter Effects Research Articles

Effects of nuclear matter and composition in core-collapse supernovae and long-term proto-neutron star cooling

Abstract We study the influence of hot and dense matter in core-collapse supernovae by adopting an up-to-date nuclear equation of state (EOS) based on the microscopic nuclear many-body frameworks. We explore effects of the EOS based on the Dirac–Brückner–Hartree–Fock theory through comparisons with those based on the variational method. We also examine effects of the differences in the composition of nuclei and nucleons by using the same EOS as the variational method but employing two different treatments in computations of nuclear abundances. We perform numerical simulations of core-collapse supernovae adopting the three EOSs. We also perform numerical simulations of the long-term evolution over 70 s of the proto-neutron star cooling. We show that the impacts of different modeling of the composition are remarkable as in those due to different treatments of uniform matter in the gravitational collapse, bounce and shock propagation. The cooling of a proto-neutron star and the resulting neutrino emission are also affected by the compositional difference even if the same treatment is used in computing uniform matter of the EOS.

Nuclear matter effects on jet production at electron-ion colliders

In these proceedings we report recent progress on understanding hadron and jet production in electron-nucleus collisions at the future Electron-Ion Collider [1,2]. These processes will play an essential role in the exploration of the partonic structure of nuclei and the study of parton shower evolution in strongly-interacting matter. We employ the framework of soft-collinear effective theory, generalized to include in-medium interactions, to present the first theoretical results for inclusive hadron and jet cross sections, as well as the jet charge modification in deep inelastic scattering on nuclei. We further demonstrate how to separate initial-state and final-state effects.

Gluon EMC effects in nuclear matter

We investigate the gluonic structure of nuclei within a mean-field model of nuclear structure based upon the modification of the structure of a bound nucleon, with the nucleon described by the Nambu–Jona-Lasinio model. This approach has been shown to reproduce the European Muon Collaboration (EMC) effect, involving the ratio of the spin-independent structure functions of a heavier nucleus to that of the deuteron. It also predicts a significant nuclear modification for the spin structure functions, known as the polarized EMC effect. Here we report sizeable nuclear modifications of the gluon distributions (a ‘gluon EMC effect’) for the ratios of both the unpolarized and polarized gluon distributions in nuclear matter to those of a free nucleon.

Centrality dependence of J/\u03c8 and \u03c8(2S) production and nuclear modification in p-Pb collisions at sqrt{s_{mathrm{NN}}} = 8.16 TeV

The inclusive production of the J/ψ and ψ(2S) charmonium states is studied as a function of centrality in p-Pb collisions at a centre-of-mass energy per nucleon pair sqrt{s_{mathrm{NN}}} = 8.16 TeV at the LHC. The measurement is performed in the dimuon decay channel with the ALICE apparatus in the centre-of-mass rapidity intervals −4.46 < ycms< −2.96 (Pb-going direction) and 2.03 < ycms< 3.53 (p-going direction), down to zero transverse momentum (pT). The J/ψ and ψ(2S) production cross sections are evaluated as a function of the collision centrality, estimated through the energy deposited in the zero degree calorimeter located in the Pb-going direction. The pT-differential J/ψ production cross section is measured at backward and forward rapidity for several centrality classes, together with the corresponding average 〈pT〉 and leftlangle {p}_{mathrm{T}}^2rightrangle values. The nuclear effects affecting the production of both charmonium states are studied using the nuclear modification factor. In the p-going direction, a suppression of the production of both charmonium states is observed, which seems to increase from peripheral to central collisions. In the Pb-going direction, however, the centrality dependence is different for the two states: the nuclear modification factor of the J/ψ increases from below unity in peripheral collisions to above unity in central collisions, while for the ψ(2S) it stays below or consistent with unity for all centralities with no significant centrality dependence. The results are compared with measurements in p-Pb collisions at sqrt{s_{mathrm{NN}}} = 5.02 TeV and no significant dependence on the energy of the collision is observed. Finally, the results are compared with theoretical models implementing various nuclear matter effects.

Open and hidden beauty production in pPb collisions at LHCb

In these proceedings, the measurement of open beauty hadrons and ϒ production in pPb data at nucleon-nucleon centre-of-mass energy 8.16 TeV is presented. Nuclear matter effects, including the modification of gluon PDF and beauty hadronisation in nuclear environment are quantified via nuclear modification factors and forward-backward production ratios. The production of ϒ excited states is also used to investigate final state effects in pPb collisions.

Properties of the nuclear symmetry energy studied by the radioactivity of unstable nuclei

In this paper, the properties of the symmetry energy in asymmetric nuclear matter, including the quadratic and quartic symmetry energies as well as their density slopes at the saturation density, are studied by the radioactivity of unstable nuclei. The symmetry energy was initially introduced as a quantum correction energy in the Weizsacker-Bethe liquid-drop model, which was proposed to describe the binding energies of finite nuclei. In order to systematically study the properties of the symmetry energy, the concept of the nuclear matter was introduced, which consists of neutrons and protons with finite neutron and proton densities but infinite neutron and proton numbers. By comparing the properties of the isospin symmetric nuclear matter, in which the neutron density is equal to the proton density, and those of the isospin asymmetric nuclear matter, in which the two densities are unequal, the symmetry energy is found to approximately represent the difference of the energy per nucleon in them. That is to say, the symmetry energy is an evaluation of the energy cost to convert protons in symmetric nuclear matter to neutrons in asymmetric nuclear matter. Since the symmetry energy characterizes the isospin asymmetric effect in nuclear matter, it plays critical roles in not only nuclear physics but also astrophysics. For example, the symmetry energy is closely related to the nuclear masses, the structures and properties of nuclei far away from stability line and near drip lines, the mechanism of heavy-ion reactions, the structures and component of neutron stars, the masses and radii of neutron stars, the cooling process of neutron stars, and so on. Besides of the quadratic symmetry energy, the quartic symmetry energy can have obvious effects on the properties of neutron stars as well. The quartic symmetry energy can significantly affect both the proton fraction in β-stable neutron stars and the critical density for the direct Urca process, which leads to faster cooling of neutron stars. In addition, the quartic symmetry energy is found to be very important for the location of the inner edge of crusts and the core-crust transition density and pressure in neutron stars. So the quartic symmetry energy can also have an influence on the structure of neutron stars. Because of the very important roles of the symmetry energy playing in not only nuclear physics but also astrophysics, it is of great significance to study the properties of both the quadratic and quartic symmetry energies. Based on the fundamental Hugenholtz-Van Hove (HVH) theorem, the properties of the symmetry energy are found to be closely related to the isoscalar and the isovector parts of the nucleon single-particle potentials, where the latter two parts can be extracted from the nuclear radioactivity. So by linking both the symmetry energy and the nuclear radioactivity to the nucleon single-particle potentials, the symmetry energy can be directly studied by nuclear radioactivity. In this paper, both the heavy-cluster radioactivity and the proton radioactivity are employed to constrain the properties of not only the quadratic symmetry energy but also the quartic symmetry energy. Besides, the effect of both the spectroscopic factor of proton radioactivity and the deformation in the daughter nucleus on the extracted results are also discussed, which are subsequently found to be very limited.

Nuclear Physics at the Energy Frontier: Recent Heavy Ion Results from the Perspective of the Electron Ion Collider

Quarks and gluons are the fundamental constituents of nucleons. Their interactions rather than their mass are responsible for 99 % of the mass of all visible matter in the universe. Measuring the fundamental properties of matter has had a large impact on our understanding of the nucleon structure and it has given us decades of research and technological innovation. Despite the large number of discoveries made, many fundamental questions remain open and in need of a new and more precise generation of measurements. The future Electron Ion Collider (EIC) will be a machine dedicated to hadron structure research. It will study the content of protons and neutrons in a largely unexplored regime in which gluons are expected to dominate and eventually saturate. While the EIC will be the machine of choice to quantify this regime, recent surprising results from the heavy ion community have begun to exhibit similar signatures as those expected from a regime dominated by gluons. Many of the heavy ion results that will be discussed in this document highlight the kinematic limitations of hadron–hadron and hadron–nucleus collisions. The reliability of using as a reference proton–proton (pp) and proton–ion (pA) collisions to quantify and disentangle vacuum and Cold Nuclear Matter (CNM) effects from those proceeding from a Quark Gluon Plasma (QGP) may be under question. A selection of relevant pp and pA results which highlight the need of an EIC will be presented.

Properties of Light Mesons in Interactions between Heavy Nuclei in the PHENIX Experiment

Observations of collective effects in (A + A) collisions of heavy nuclei confirm the formation of quark–gluon plasma. The systematic study of collective effects of hot nuclear matter associated with the quark–gluon medium and their separation from the effects of cold nuclear matter associated with nucleon medium is one of the main aims of modern high-energy physics. Such studies are conducted using different systems of colliding nuclei. The results are presented from measuring the nuclear modification factors of neutral pions in Cu + Au, p + Au, d + Au, and 3He + Au collisions at 200 GeV.

Nuclear modification of prompt and non-prompt J/Ψ production in p-Pb collisions at √SNN = 5.02 TeV with ALICE

Various Cold Nuclear Matter (CNM) effects, such as nuclear shadowing or partonic energy loss, can modify the production of J/Ψ in heavy-ion collisions with respect to what is measured in elementary colliding systems. The study of p-Pb collisions at the Large Hadron Collider (LHC) energy scale represents a crucial tool to assess the influence of Cold Nuclear Matter on J/Ψ production in order to achieve a more correct interpretation of Pb-Pb collision results. The ALICE detector at the LHC is capable of reconstructing J/Ψ mesons at central rapidity through their e+e- decay channel down to zero transverse momentum (pT), and has measured the fraction of J/Ψ produced from the decay of beauty-flavoured hadrons (non-prompt J/Ψ) in p-Pb collisions down to pT = 1:3 GeV/c. In this paper, the results obtained by ALICE from the measurement of the prompt and non-prompt J/Ψ yields at mid-rapidity in p-Pb collisions at √SNN = 5.02 TeV will be discussed in comparison to different theoretical predictions including CNM effects.

Recent results from the ALICE Experiment at LHC

ALICE (A Large Ion Collider Experiment) at the LHC performed high statistics measurements in Pb-Pb collisions at the top LHC energy, complemented with large recent reference datasets in elementary proton-proton collisions at the same energy. Elementary pp collisions are serving as baseline for testing QCD properties and allow the study of the changes induced by the hot and dense medium produced in heavy ion collisions. Key observables like nuclear modification factors, jet production, flow phenomena and spectra for identified particles, related to the different stages of collision evolution, are presented and compared with the most recent results from p-Pb and Xe-Xe collisions, thus allowing to probe both initial cold nuclear matter and final state effects, combined with the system size dependence of the measurements.

Charmed baryon Λc in nuclear matter

Density dependences of the mass and self-energies of ${\mathrm{\ensuremath{\Lambda}}}_{c}$ in nuclear matter are studied in the parity projected QCD sum rule. Effects of nuclear matter are taken into account through the quark and gluon condensates. It is found that the four-quark condensates give dominant contributions. As the density dependences of the four-quark condensates are not known well, we examine two hypotheses. One is based on the factorization hypothesis (F-type) and the other is derived from the perturbative chiral quark model (QM-type). The F-type strongly depends on density, while the QM-type gives a weaker dependence. It is found that, for the F-type dependence, the energy of ${\mathrm{\ensuremath{\Lambda}}}_{c}$ increases as the density of nuclear matter grows, that is, ${\mathrm{\ensuremath{\Lambda}}}_{c}$ feels repulsion. On the other hand, the QM-type predicts a weak attraction ($\ensuremath{\sim}20$ MeV at the normal nuclear density) for ${\mathrm{\ensuremath{\Lambda}}}_{c}$ in nuclear matter. We carry out a similar analysis of the $\mathrm{\ensuremath{\Lambda}}$ hyperon and find that the F-type density dependence is too strong to explain the observed binding energy of $\mathrm{\ensuremath{\Lambda}}$ in nuclei. Thus we conclude that the weak density dependence of the four-quark condensate is more realistic. The scalar and vector self-energies of ${\mathrm{\ensuremath{\Lambda}}}_{c}$ for the QM-type dependence are found to be much smaller than those of the light baryons.

Collective effects in the interactions of small systems on the RHIC

It has long been believed that small colliding systems (p+Au, d+Au, 3He + Au) are can only be used to study the collective effects of cold nuclear matter. However, recent studies on the RHIC and LHC accelerators indicate there are flowlike collective effects characterized by the high multiplicity of charged particles produced in these collisions. Whether these effects result from the hydrodynamic expansion of a dense and hot thermalized medium or are caused by the initial state remains an open question. This work reports the results from measuring flow characteristics in d + Au and 3He + Au collisions at an energy of 200 GeV in the PHENIX experiment on the RHIC collider. Attempts to describe the results theoretically are discussed.

Light flavour hadron production in the ALICE experiment at LHC

Unique among the LHC experiments, ALICE has excellent particle identification capabilities for the measurement of light-flavour hadrons. A large number of hadron species from pions to multi-strange baryons and light nuclei have been measured over a large transverse momentum region. The measurement of the production of these particles is a valuable tool to study the properties of the medium formed in heavy-ion collisions. In particular they give information on the collective phenomena of the fireball, on the parton energy loss in the hot QCD medium and on the hadronization mechanisms such as recombination and statistical hadronization. The measurements in pp and in p-nucleus collisions provide the necessary baseline for heavy-ion data and help to investigate the effects of the ordinary nuclear matter. In this paper some of the main ALICE results on identified light-flavour hadron production in Pb–Pb collisions at √s NN = 2.76 TeV and p–Pb collisions at √s NN = 5.02 TeV will be presented.

Measuring nuclear modification factors for φ-mesons in p + Pb and Pb + Pb collisions in the ALICE experiment at the large hadron collider

Investigations of light vector mesons allow us to understand the mechanisms of hadron production in more detail and to investigate the properties of the dense and hot medium formed in central collisions of heavy ultrarelativistic nuclei. One of the most useful objects for investigations are φ-mesons, since their properties are sensitive to such effects as strangeness enhancement, the restoration of chiral symmetry, and the loss of parton energy that accompanies a phase transition. To investigate these phenomena, meson production must be measured not only in collisions of heavy nuclei but in proton-proton and proton-nuclear collisions as well. Measurements in elementary nucleon-nucleon collisions are used as the basis for comparison to heavier interacting systems. The collision of light and heavy nuclei allows us to evaluate the effects of cold nuclear matter and to interpret the effects of a hot nuclear medium more precisely. Invariant differential production spectra of φ-mesons were measured in the ALICE experiment as a function of transverse momenta in p + p, p + Pb, and Pb + Pb interactions. The distributions of the nuclear modification factors were constructed for φ-mesons on the basis of these data.

In-medium effects around the Fermi energy

We study nuclear stopping in central collisions for heavy-ion induced reactions in the Fermi energy domain (15-100 AMeV). Using the large dataset provided by the 4π array INDRA, we determine that stopping can be directly related to the transport properties in the nuclear medium. By looking specifically at protons, we present a comprehensive body of experimental results concerning the mean free path, the nucleon-nucleon cross-section and in-medium effects in nuclear matter.

In-medium effects for nuclear matter in the Fermi-energy domain

We study nuclear stopping in central collisions for heavy-ion induced reactions in the Fermi energy domain, between $15$ and $100$ A\,\textrm{MeV}. Using the large dataset of exclusive measurements provided by the $4\pi$ array \emph{INDRA}, we determine the relative degree of stopping as a function of system mass and bombarding energy. We show that the stopping can be directly related to the transport properties in the nuclear medium. By looking specifically at free nucleons (here protons), we present for the first time a comprehensive body of experimental results concerning the mean free path, the nucleon-nucleon cross-section and in-medium effects in nuclear matter. It is shown that the mean free path exhibits a maximum at $\lambda_{NN}=9.5 \pm 2$ \textrm{fm}, around $E_{inc}=35-40$ A\,\textrm{MeV} incident energy and decreases toward an asymptotic value $\lambda_{NN}= 4.5 \pm 1$ \textrm{fm} at $E_{inc} = 100$ A\,\textrm{MeV}. After accounting for Pauli blocking of elastic nucleon-nucleon collisions, it is shown that the effective in-medium \emph{NN} cross section is further reduced compared to the free value in this energy range. Therefore, in-medium effects cannot be neglected in the Fermi energy range. These results bring new fundamental inputs for microscopic descriptions of nuclear reactions in the Fermi energy domain.

Modeling of a microscopic optical pion-nucleon potential at energies in the (3, 3)-resonance region and nuclear-matter effect on the pion-nucleon amplitude

Differential cross sections for elastic pion scattering on a number of nuclei at energies between 130 and 290 MeV are calculated. For this purpose, use is made of a microscopic optical pion-nucleus potential that was obtained on the basis of the Glauber theory of high-energy scattering and which is determined by the density distribution of pointlike target nucleons and by the pion-nucleon scattering amplitude. The calculation of the cross sections in question is based on solving the respective relativistic wave equation. Three parameters of the pion-nucleon amplitude are fitted in performing a comparison with experimental data. These are the total cross section for pion-nucleon scattering, the ratio of the real to the imaginary part of the forward pion-nucleon scattering amplitude, and the slope parameter. The resulting values of these parameters are then compared with their counterparts for scattering on free nucleons. The character of nuclear-matter-induced changes in these parameters is studied.

Impact of the nuclear modification of the gluon densities onJ/ψproduction inpPb collisions atsNN=5TeV

We update our previous studies of nuclear-matter effects on J/Psi production in proton-nucleus for the recent LHC pPb runs at sqrt(s_NN)=5 TeV. We have analysed the effects of the modification of the gluon PDFs in nucleus, using an exact kinematics for a 2->2 process, namely g+g->J/Psi+g as expected from LO pQCD. This allows to constrain the transverse-momentum while computing the nuclear modification factor for different rapidities, unlike with the usual simplified kinematics. Owing to the absence of measurement in pp collisions at the same sqrt(s_NN) and owing to the expected significant uncertainties in yield interpolations which would hinder definite interpretations of nuclear modification factor --R_pPb--, we have derived forward-to-backward and central-to-peripheral yield ratios in which the unknown proton-proton yield cancel. These have been computed without and with a transverse-momentum cut, e.g. to comply with the ATLAS and CMS constraints in the central-rapidity region.

ϒ production in pp and pA collisions: from RHIC to the LHC

I discuss Upsilon production in pp collisions at RHIC, Tevatron and LHC energies, in particular the behaviour of the differential cross section in rapidity and the impact of QCD corrections on the P_T differential cross section. I also emphasise the very good agreement between the parameter-free predictions of the Colour-Singlet Model (CSM) and the first LHC data, especially in the region of low transverse momenta, which is the most relevant one for heavy-ion studies. I also show that the CSM predicts Upsilon cross-section ratios in agreement with the most recent LHC data. I then briefly discuss the nuclear-matter effects on Upsilon production at RHIC and the LHC in p(d)A collisions and, by extension, in AA collisions. I argue that a) the Upsilon break-up probability can be neglected, at RHIC and the LHC, b) gluon shadowing --although non-negligible-- is not strong enough to describe forward RHIC data, c) backward RHIC data hints at a gluon EMC effect, possibly stronger than the quark one. Outlooks for the LHC pPb run are also presented.

ϒ production in p(d)A collisions at RHIC and the LHC

We study the effect of nuclear matter in Upsilon production in dAu collisions at RHIC and pPb collisions at the LHC. We find that the nuclear modification factor, R^Upsilon_dAu, measured at RHIC is not satisfactorily reproduced by the conventional effects used in the literature, namely the modification of the gluon distribution in bound nucleons and an --effective-- survival probability for a bound state to escape the nucleus. In particular, we argue that this probability should be close to 1 as opposed to the J/psi case. We note that, at backward rapidities, the unexpected suppression of R^Upsilon_dAu observed by PHENIX hints at the presence of a gluon EMC effect, analogous to the quark EMC effect -- but likely stronger. Further nuclear matter effects, such as saturation and fractional energy loss, are discussed, but none of them fit in a more global picture of quarkonium production. Predictions for Upsilon(nS) for the forthcoming pPb run at 5 TeV at the LHC are also presented.