Nucleon Pair Research Articles

Exclusive studies on short range correlations in nuclei

Short Range Correlations (SRC) are brief fluctuations of high relative momentum nucleon pairs. Properties of SRC have important consequences for nuclear physics, high energy physics, atomic physics, and astrophysics. SRC pairs form some of the densest states of cold matter achievable on Earth, making them an ideal system to study the partonic and nucleonic degrees of freedom in nuclear systems. Hard exclusive breakup reactions, where high-energy probes scatter on SRC pairs, are used to study such properties of SRC pairs as isospin decomposition, nuclear mass and asymmetry dependence, c.m. momentum distribution. Thomas Jefferson National Accelerator Facility (JLab) plays a key role in the SRC program. CLAS (CEBAF Large Acceptance Spectrometer), located in Hall B at JLab, has almost 4πcoverage and is capable of measuring exclusive reactions of the type A(e, e’pp). We will discuss the recent experimental results from JLab and future experiments planned at JLab as well as at JINR.

On The Contribution of the P and D Partial-Wave States to the Binding Energy of the Triton in the Bethe-Salpeter-Faddeev Approach

The influence of the partial-wave states with nonzero orbital moment of the nucleon pair on the binding energy of the triton in the relativistic case is considered. The relativistic generalization of the Faddeev equation in the Bethe-Salpeter formalism is applied. Two-nucleon t matrix is obtained from the Bethe-Salpeter equation with separable kernel of nucleon-nucleon interaction of the rank one. The kernel form factors are the relativistic type of the Yamaguchi functions. The following two-nucleon partial-wave states are considered: $^1S_0$, $^3S_1$, $^3D_1$, $^3P_0$, $^1P_1$, $^3P_1$. The system of the integral equations are solved by using the iteration method. The binding energy of the triton and three-nucleon amplitudes are found. The contribution of the P and D states to the binding energy of triton is given.

Probing short-range NN-correlations in the reaction 12C + p → p + pN + 10A

The exclusive reaction 12C(p, ppN)10A of quasi-elastic knockout of the nucleon from the short-rangecorrelated nucleon pair <NN> in the 12C nucleus by proton with an energy of few GeV is considered in the plane-wave approximation.The two-nucleon spectroscopic factors and momentum distributions of the center of mass motion of NN-pairs are considered by analogy with theory of quasi-elastic knockout of fast deuteron clusters from nuclei (p, Nd) using the translationally-invariant shell model.

ϒ suppression at forward rapidity in Pb–Pb collisions at [formula omitted

Inclusive ϒ(1S) and ϒ(2S) production have been measured in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair sNN=5.02 TeV, using the ALICE detector at the CERN LHC. The ϒ mesons are reconstructed in the centre-of-mass rapidity interval 2.5<y<4 and in the transverse-momentum range pT<15 GeV/c, via their decays to muon pairs. In this Letter, we present results on the inclusive ϒ(1S) nuclear modification factor RAA as a function of collision centrality, transverse momentum and rapidity. The ϒ(1S) and ϒ(2S) RAA, integrated over the centrality range 0–90%, are 0.37±0.02(stat)±0.03(syst) and 0.10±0.04(stat)±0.02(syst), respectively, leading to a ratio RAAϒ(2S)/RAAϒ(1S) of 0.28±0.12(stat)±0.06(syst). The observed ϒ(1S) suppression increases with the centrality of the collision and no significant variation is observed as a function of transverse momentum and rapidity.

Centrality and pseudorapidity dependence of the charged-particle multiplicity density in Xe–Xe collisions at [formula omitted

In this Letter, the ALICE Collaboration presents the first measurements of the charged-particle multiplicity density, dNch/dη, and total charged-particle multiplicity, Nchtot, in Xe–Xe collisions at a centre-of-mass energy per nucleon–nucleon pair of sNN=5.44TeV. The measurements are performed as a function of collision centrality over a wide pseudorapidity range of −3.5<η<5. The values of dNch/dη at mid-rapidity and Nchtot for central collisions, normalised to the number of nucleons participating in the collision (Npart) as a function of sNN follow the trends established in previous heavy-ion measurements. The same quantities are also found to increase as a function of Npart, 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 Npart. 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%.

Study of \u03d2 production in pPb collisions at sqrt{s_{mathrm{NN}}}=8.16 TeV

The production of ϒ(nS) mesons (n = 1, 2, 3) in pPb and Pbp collisions at a centre-of-mass energy per nucleon pair sqrt{s_{mathrm{NN}}}=8.16 TeV is measured by the LHCb experiment, using a data sample corresponding to an integrated luminosity of 31.8 nb−1. The ϒ(nS) mesons are reconstructed through their decays into two opposite-sign muons. The measurements comprise the differential production cross-sections of the ϒ(1S) and ϒ(2S) states, their forward-to-backward ratios and nuclear modification factors. The measurements are performed as a function of the transverse momentum pT and rapidity in the nucleon-nucleon centre-of-mass frame y* of the ϒ(nS) states, in the kinematic range pT < 25 GeV/c and 1.5 < y* < 4.0 (−5.0 < y* < −2.5) for pPb (Pbp) collisions. In addition, production cross-sections for ϒ(3S) are measured integrated over phase space and the production ratios between all three ϒ(nS) states are determined. Suppression for bottomonium in proton-lead collisions is observed, which is particularly visible in the ratios. The results are compared to theoretical models.

Constraints on axion-like particles and nucleon pairing in dense matter from the hot neutron star in HESS J1731-347

If the thermal evolution of the hot young neutron star in the supernova remnant HESS J1731-347 is driven by neutrino emission, it provides a stringent constraint on the coupling of light (mass $\ll 10$ keV) axion-like particles to neutrons. Using Markov-Chain Monte Carlo we find that for the values of axion-neutron coupling $g_{ann}^2 > 7.7 \times 10^{-20}$ (90% c.l.) the axion cooling from the bremsstrahlung reaction $n+n\rightarrow n+n +a$ is too rapid to account for the high observed surface temperature. This implies that the Pecci-Quinn scale or axion decay constant $f_a > 6.7 \times 10^7$ GeV for KSVZ axions and $f_a > 1.7 \times 10^9$ GeV for DFSZ axions. The high temperature of this neutron star also allows us to tighten constraints on the size of the nucleon pairing gaps.

Center of Mass Motion of Short-Range Correlated Nucleon Pairs studied via the A(e,e^{'}pp) Reaction.

Short-range correlated (SRC) nucleon pairs are a vital part of the nucleus, accounting for almost all nucleons with momentum greater than the Fermi momentum (k_{F}). A fundamental characteristic of SRC pairs is having large relative momenta as compared to k_{F}, and smaller center of mass (c.m.) which indicates a small separation distance between the nucleons in the pair. Determining the c.m. momentum distribution of SRC pairs is essential for understanding their formation process. We report here on the extraction of the c.m. motion of proton-proton (pp) SRC pairs in carbon and, for the first time in heavier and ansymetric nuclei: aluminum, iron, and lead, from measurements of the A(e,e^{'}pp) reaction. We find that the pair c.m. motion for these nuclei can be described by a three-dimensional Gaussian with a narrow width ranging from 140 to 170 MeV/c, approximately consistent with the sum of two mean-field nucleon momenta. Comparison with calculations appears to show that the SRC pairs are formed from mean-field nucleons in specific quantum states.

Small bits of cold dense matter

The behavior of QCD at high baryon density and low temperature is crucial to understanding the properties of neutron stars and gravitational waves emitted during their mergers. In this paper we study small systems of baryons in periodic boundary conditions to probe the properties of QCD at high baryon density. By comparing calculations based on nucleon degrees of freedom to simple quark models we show that specific features of the nuclear spectrum, including shell structure and nucleon pairing, emerge if nucleons are the primary degrees of freedom. Very small systems should also be amenable to studies in lattice QCD, unlike larger systems where the fermion sign problem is much more severe. Through comparisons of lattice QCD and nuclear calculations it should be possible to gain, at least at a semi-quantitative level, more understanding of the cold dense equation of state as probed in neutron stars.

Inclusive J/ψ production in Xe–Xe collisions at [formula omitted] TeV

Inclusive J/ψ production is studied in Xe–Xe interactions at a centre-of-mass energy per nucleon pair of sNN=5.44 TeV, using the ALICE detector at the CERN LHC. The J/ψ 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 RAA for inclusive J/ψ, measured in the centrality range 0–90% as well as in the centrality intervals 0–20% and 20–90% are presented. The RAA values are compared to previously published results for Pb–Pb collisions at sNN=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.

Cooling of Small and Massive Hyperonic Stars

Abstract We perform cooling simulations for isolated neutron stars using recently developed equations of state for their core. The equations of state are obtained from new parametrizations of the FSU2 relativistic mean-field functional that reproduce the properties of nuclear matter and finite nuclei, while fulfilling the restrictions on high-density matter deduced from heavy-ion collisions, measurements of massive 2 M ⊙ neutron stars, and neutron star radii below 13 km. We find that two of the models studied, FSU2R (with nucleons) and in particular FSU2H (with nucleons and hyperons), show very good agreement with cooling observations, even without including extensive nucleon pairing. This suggests that the cooling observations are more compatible with an equation of state that produces a soft nuclear symmetry energy, hence it generates small neutron star radii. However, both models favor large stellar masses, above 1.8 M ⊙, to explain the colder isolated neutron stars that have been observed, even if nucleon pairing is present.

Self-Consistent Calculations of the Quadrupole Moments of the Lowest 3– States in Sn and Pb Isotopes

self-consistent approach to anharmonic effects based on the quantum many-body theory is for the first time used to calculate the quadrupole moments of the lowest 3– states in Sn and Pb isotopes, including the 100Sn, 132Sn, and 208Pb doubly magic ones. The consistency between the mean nuclear field and the effective interaction is maintained using the energy-density-functional method with known parameters of the Fayans functional. Thereby, the quadrupole moments of the lowest 3– states of isotopes with nucleon pairing are reliably predicted, and the existing data for the 208Pb isotope are reproduced. It has been shown that the new three-quasiparticle correlations are responsible for slightly more than half of the observed effect and the remaining part is due to the quadrupole polarizability of the nucleus.

Constant temperature model for nuclear level density

We study physics related to the nuclear level density calculated either in a realistic shell model or, equivalently, with the use of the statistical moments method. At excitation energy up to 12–15 MeV, the obtained level density grows exponentially being well described by the so-called constant temperature model. We discuss the physical meaning of the effective temperature parameter and its dependence on the interaction Hamiltonian including nucleon pairing and deformation effects. The possible interpretation relates the underlying physics with the gradual chaotization of typical wave functions rather than with the pairing phase transition.

Measurement of prompt and nonprompt charmonium suppression in text {PbPb} collisions at 5.02,text {Te}text {V}

The nuclear modification factors of {mathrm {J}/psi } and psi text {(2S)} mesons are measured in text {PbPb} collisions at a centre-of-mass energy per nucleon pair of sqrt{smash [b]{s_{_{text {NN}}}}} = 5.02,text {Te}text {V} . The analysis is based on text {PbPb} and mathrm {p}mathrm {p} data samples collected by CMS at the LHC in 2015, corresponding to integrated luminosities of 464,mu mathrm {b}^{-1} and 28,text {pb}^text {-1}, respectively. The measurements are performed in the dimuon rapidity range of |y | < 2.4 as a function of centrality, rapidity, and transverse momentum (p_{mathrm {T}} ) from p_{mathrm {T}} =3{,text {Ge}text {V}/}text {c} in the most forward region and up to 50{,text {Ge}text {V}/}text {c}. Both prompt and nonprompt (coming from b hadron decays) {mathrm {J}/psi } mesons are observed to be increasingly suppressed with centrality, with a magnitude similar to the one observed at sqrt{smash [b]{s_{_{text {NN}}}}} = 2.76,text {Te}text {V} for the two {mathrm {J}/psi } meson components. No dependence on rapidity is observed for either prompt or nonprompt {mathrm {J}/psi } mesons. An indication of a lower prompt {mathrm {J}/psi } meson suppression at p_{mathrm {T}} > 25{,text {Ge}text {V}/}text {c} is seen with respect to that observed at intermediate p_{mathrm {T}}. The prompt psi text {(2S)} meson yield is found to be more suppressed than that of the prompt {mathrm {J}/psi } mesons in the entire p_{mathrm {T}} range.

Nuclear modification factor of D0 mesons in PbPb collisions at [formula omitted

The transverse momentum (pT) spectrum of prompt D0 mesons and their antiparticles has been measured via the hadronic decay channels D0→K−π+ and D‾0→K+π− in pp and PbPb collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair with the CMS detector at the LHC. The measurement is performed in the D0 meson pT range of 2–100GeV/c and in the rapidity range of |y|<1. The pp (PbPb) dataset used for this analysis corresponds to an integrated luminosity of 27.4 pb−1 (530 μb−1 ). The measured D0 meson pT spectrum in pp collisions is well described by perturbative QCD calculations. The nuclear modification factor, comparing D0 meson yields in PbPb and pp collisions, was extracted for both minimum-bias and the 10% most central PbPb interactions. For central events, the D0 meson yield in the PbPb collisions is suppressed by a factor of 5–6 compared to the pp reference in the pT range of 6–10GeV/c. For D0 mesons in the high-pT range of 60–100GeV/c, a significantly smaller suppression is observed. The results are also compared to theoretical calculations.

Measurement of Prompt D^{0} Meson Azimuthal Anisotropy in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The prompt D^{0} meson azimuthal anisotropy coefficients, v_{2} and v_{3}, are measured at midrapidity (|y|<1.0) in Pb-Pb collisions at a center-of-mass energy sqrt[s_{NN}]=5.02 TeV per nucleon pair with data collected by the CMS experiment. The measurement is performed in the transverse momentum (p_{T}) range of 1 to 40 GeV/c, for central and midcentral collisions. The v_{2} coefficient is found to be positive throughout the p_{T} range studied. The first measurement of the prompt D^{0} meson v_{3} coefficient is performed, and values up to 0.07 are observed for p_{T} around 4 GeV/c. Compared to measurements of charged particles, a similar p_{T} dependence, but smaller magnitude for p_{T}<6 GeV/c, is found for prompt D^{0} meson v_{2} and v_{3} coefficients. The results are consistent with the presence of collective motion of charm quarks at low p_{T} and a path length dependence of charm quark energy loss at high p_{T}, thereby providing new constraints on the theoretical description of the interactions between charm quarks and the quark-gluon plasma.

Universal scaling of the pion, kaon and proton pT spectra in Pb[sbnd]Pb collisions at 2.76 TeV

With the experimental data collected by the ALICE collaboration in PbPb collisions at a center-of-mass energy per nucleon pair 2.76 TeV for six different centralities (0–5%, 5–10%, 10–20%, 20–40%, 40–60% and 60–80%), we investigate the scaling property of the pion, kaon and proton transverse momentum (pT) spectra at these centralities. We show that in the low pT region with pT≤ 2.75 (3.10 and 2.35) GeV/c the pion (kaon and proton) spectra exhibit a scaling behaviour independent of the centrality of the collisions. This scaling behaviour arises when these spectra are presented in terms of a suitable variable, z=pT/K. The scaling parameter K is determined by the quality factor method and is parameterized by a〈Npart〉b, where 〈Npart〉 is the average value of the number of participating nucleons, a and b are free parameters, b characterizes the rate at which lnK changes with ln〈Npart〉. The values of b for pions and kaons are consistent within uncertainties, while they are smaller than that for protons. In the high pT region, due to the suppression of the spectra, a violation of the proposed scaling is observed going from central to peripheral collisions. The more peripheral the collisions are, the more clearly violated the proposed scaling becomes. In the framework of the colour string percolation model, we argue that the pions, kaons and protons originate from the fragmentation of clusters which are formed by strings overlapping and the cluster's fragmentation functions are different for different hadrons. The scaling behaviour of the pion, kaon and proton spectra in the low pT region can be simultaneously explained by the colour string percolation model in a qualitative way.

Elliptic flow of electrons from beauty-hadron decays extracted from Pb\u2013Pb collision data at sqrt{s_mathrm{NN}} = 2.76 hbox {TeV}

We present a calculation of the elliptic flow of electrons from beauty-hadron decays in semi-central Pb–Pb collisions at centre-of-mass energy per colliding nucleon pair, represented as sqrt{s_mathrm{NN}}, of 2.76 TeV. The result is obtained by the subtraction of the charm-quark contribution in the elliptic flow of electrons from heavy-flavour hadron decays in semi-central Pb–Pb collisions at sqrt{s_mathrm{NN}} = 2.76 hbox {TeV} recently made publicly available by the ALICE collaboration.

0νββ -decay nuclear matrix element for light and heavy neutrino mass mechanisms from deformed quasiparticle random-phase approximation calculations for Ge76, Se82, Te130, Xe136 , and Nd150 with isospin restoration

In this work, with restored isospin symmetry, we evaluated the neutrinoless double beta decay nuclear matrix elements for $^{76}$Ge, $^{82}$Se, $^{130}$Te, $^{136}$Xe and $^{150}$Nd for both the light and heavy neutrino mass mechanisms using the deformed QRPA approach with realistic forces. We give detailed decompositions of the nuclear matrix elements over different intermediate states and nucleon pairs, and discuss how these decompositions are affected by the model space truncations. Compared to the spherical calculations, our results show reductions from $30\%$ to about $60\%$ of the nuclear matrix elements for the calculated isotopes mainly due to the presence of BCS overlap factor between the initial and final ground states. The comparison between different nucleon-nucleon forces with corresponding Short-Range-Correlations (src) shows, that the choice of the NN force gives roughly $20\%$ deviations for light exchange neutrino mechanism and much larger deviations for the heavy neutrino exchange mechanism.

Measurement of the Splitting Function in pp and Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions, but it is not known where in the shower evolution the modifications occur. The momentum ratio of the two leading partons, resolved as subjets, provides information about the parton shower evolution. This substructure observable, known as the splitting function, reflects the process of a parton splitting into two other partons and has been measured for jets with transverse momentum between 140 and 500GeV, in pp and PbPb collisions at a center-of-mass energy of 5.02TeV per nucleon pair. In central PbPb collisions, the splitting function indicates a more unbalanced momentum ratio, compared to peripheral PbPb and pp collisions.. The measurements are compared to various predictions from event generators and analytical calculations.