Nucleon Pair Research Articles

D-Meson Azimuthal Anisotropy in Midcentral Pb-Pb Collisions at sqrt[s]_{NN}=5.02 TeV.

The azimuthal anisotropy coefficient v_{2} of prompt D^{0}, D^{+}, D^{*+}, and D_{s}^{+} mesons was measured in midcentral (30%-50% centrality class) Pb-Pb collisions at a center-of-mass energy per nucleon pair sqrt[s_{NN}]=5.02 TeV, with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays at midrapidity, |y|<0.8, in the transverse momentum interval 1<p_{T}<24 GeV/c. The measured D-meson v_{2} has similar values as that of charged pions. The D_{s}^{+} v_{2}, measured for the first time, is found to be compatible with that of nonstrange D mesons. The measurements are compared with theoretical calculations of charm-quark transport in a hydrodynamically expanding medium and have the potential to constrain medium parameters.

Comparing transverse momentum balance of b jet pairs in pp and PbPb collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV

The transverse momentum balance of pairs of back-to-back b quark jets in PbPb and pp collisions recorded with the CMS detector at the LHC is reported. The center-of-mass energy in both collision systems is 5.02 TeV per nucleon pair. Compared to the pp collision baseline, b quark jets have a larger imbalance in the most central PbPb collisions, as expected from the jet quenching effect. The data are also compared to the corresponding measurement with inclusive dijets. In the most central collisions, the imbalance of b quark dijets is comparable to that of inclusive dijets.

Short-range correlation in high-momentum antisymmetrized molecular dynamics

We propose a new variational method for treating short-range repulsion of bare nuclear force for nuclei in antisymmetrized molecular dynamics (AMD). In AMD, the short-range correlation is described in terms of large imaginary centroids of Gaussian wave packets of nucleon pairs in opposite signs, causing high-momentum components in nucleon pair. We superpose these AMD basis states and name this method "high-momentum AMD" (HM-AMD), which is capable of describing strong tensor correlation (Prog. Theor. Exp. Phys. (2017) 111D01). In this paper, we extend HM-AMD by including up to two kinds of nucleon pairs in each AMD basis state utilizing the cluster expansion, which produces many-body correlations involving high-momentum components. We investigate how much HM-AMD describes the short-range correlation by showing the results for $^3$H using the Argonne V4$^\prime$ central potential. It is found that HM-AMD reproduces the results of few-body calculations and also the tensor-optimized AMD. This means that HM-AMD is a powerful approach to describe the short-range correlation in nuclei. In HM-AMD, momentum directions of nucleon pairs isotropically contribute to the short-range correlation, which is different from the tensor correlation.

Nucleon Isovector Pairing in Nuclei: Microscopic Approach, Boson Representation, and Collective Model

Nucleon pair correlations in atomic nuclei are analyzed within a nuclear microscopic model with residual isovector pairing forces. These are formulated in the boson representation of fermion operators whereby the collective mode of pair excitations can be isolated without restricting the size of the one-particle basis. This method allows one to analyze the fluctuations in the nonsuperfluid phase of nuclear matter, its phase transition to the superfluid phase, and strong pair correlations. The performance of the method is exemplified by numerical results for the nuclei in the vicinity of the doubly magic 56Ni nucleus.

The nuclear contacts and short range correlations in nuclei

Atomic nuclei are complex strongly interacting systems and their exact theoretical description is a long-standing challenge. An approximate description of nuclei can be achieved by separating its short and long range structure. This separation of scales stands at the heart of the nuclear shell model and effective field theories that describe the long-range structure of the nucleus using a mean-field approximation. We present here an effective description of the complementary short-range structure using contact terms and stylized two-body asymptotic wave functions. The possibility to extract the nuclear contacts from experimental data is presented. Regions in the two-body momentum distribution dominated by high-momentum, close-proximity, nucleon pairs are identified and compared to experimental data. The amount of short-range correlated (SRC) nucleon pairs is determined and compared to measurements. Non-combinatorial isospin symmetry for SRC pairs is identified. The obtained one-body momentum distributions indicate dominance of SRC pairs above the nuclear Fermi-momentum.

Measurement of quarkonium production in proton\u2013lead and proton\u2013proton collisions at 5.02~mathrm {TeV} with the ATLAS detector

The modification of the production of J/psi , psi (2mathrm {S}), and varUpsilon (nmathrm {S}) (n = 1, 2, 3) in p+Pb collisions with respect to their production in pp collisions has been studied. The p+Pb and pp datasets used in this paper correspond to integrated luminosities of 28~mathrm {nb}^{-1} and 25~mathrm {pb}^{-1} respectively, collected in 2013 and 2015 by the ATLAS detector at the LHC, both at a centre-of-mass energy per nucleon pair of 5.02 TeV. The quarkonium states are reconstructed in the dimuon decay channel. The yields of J/psi and psi (mathrm {2S}) are separated into prompt and non-prompt sources. The measured quarkonium differential cross sections are presented as a function of rapidity and transverse momentum, as is the nuclear modification factor, R_{pmathrm {Pb}} for J/psi and varUpsilon (nmathrm {S}). No significant modification of the J/psi production is observed while varUpsilon (nmathrm {S}) production is found to be suppressed at low transverse momentum in p+Pb collisions relative to pp collisions. The production of excited charmonium and bottomonium states is found to be suppressed relative to that of the ground states in central p+Pb collisions.

Pair correlations in low-lying T=0 states of odd-odd nuclei with six nucleons

In this paper, we study pair correlations in low-lying $T=0$ states for two typical cases of odd-odd $N=Z$ nuclei. The first case is six nucleons in a single $j=9/2$ shell, for which we study the $S$-broken-pair approximation, the isoscalar spin-1 pair condensation, and the isoscalar spin-aligned pair condensation, with schematic interactions. In the second case, we study pair approximations and correlation energies for $^{22}\mathrm{Na}$, $^{34}\mathrm{Cl}$, $^{46}\mathrm{V}$, $^{62}\mathrm{Ga}$, and $^{94}\mathrm{Ag}$ in multi-$j$ shells with effective interactions. A few $T=0$ states are found to be well represented by isoscalar nucleon pairs. The isoscalar spin-aligned pairs play an important role for the yrast $T=0$ states with $I\ensuremath{\sim}2j$ and $I\ensuremath{\sim}{I}_{\mathrm{max}}$ in $^{22}\mathrm{Na}$, $^{46}\mathrm{V}$, and $^{94}\mathrm{Ag}$. The overlap between the isoscalar $\mathcal{J}=1$ pair wave function and the shell-model wave function is around 0.5 for the $I=1,3$ states of $^{34}\mathrm{Cl}$ and the $I=1$ state of $^{94}\mathrm{Ag}$. The $I=9$ state of $^{62}\mathrm{Ga}$ is very well described by the isoscalar $\mathcal{J}=3$ pair condensation. The broken-pair approximation (which is similar to the 2-quasiparticle excitation of the isovector pair condensation) is appropriate for quite few states, such as the $I=1\ensuremath{-}3$ states of $^{34}\mathrm{Cl}$ and the $I=5$ state of $^{62}\mathrm{Ga}$. The correlation energies are presented in this paper. It is noted that the picture based on nucleon-pair wave functions is not always in agreement with the picture based on correlation energies.

Absence of paired crossing in the positive parity bands of Cs124

High-spin states in Cs124 were populated in the Ni64(Ni64,p3n) reaction and the Gammasphere detector array was used to measure γ-ray coincidences. Both positive- and negative-parity bands, including bands with chiral configurations, have been extended to higher spin, where a shape change has been observed. The configurations of the bands before and after the alignment are discussed within the framework of the cranked Nilsson-Strutinsky model. The calculations suggest that the nucleus undergoes a shape transition from triaxial to prolate around spin I≃22 of the positive-parity states. The alignment gain of 8â.,, observed in the positive-parity bands, is due to partial alignment of several valence nucleons. This indicates the absence of band crossing due to paired nucleons in the bands.

Scaling Properties of the Mean Multiplicity and Pseudorapidity Density in e−+e+, e±+p, p( p ¯ )+p, p+A and A+A(B) Collisions

The charged-particle pseudorapidity density ( d N ch / d η ) for p( p ¯ )+p, p+A and A+A(B) collisions and the mean multiplicity 〈 N ch 〉 for e − +e + , e ± + p , and p( p ¯ )+p collisions are studied for a wide range of beam energies ( s ). Characteristic scaling patterns are observed for both d N ch / d η and 〈 N ch 〉 , consistent with a thermal particle production mechanism for the bulk of the soft particles created in all of these systems. The scaling patterns found also validate an essential role for quark participants in these collisions. The measured values for d N ch / d η and 〈 N ch 〉 are observed to factorize into contributions that depend on log ( s ) and the number of nucleon or quark participant pairs N pp . The quantification of these contributions gives expressions that serve to systematize d N ch / d η and 〈 N ch 〉 measurements spanning nearly 4 orders of magnitude in s and to predict their values as a function of s and N pp .

J/ψ production in proton-lead collisions at 8 TeV with the LHCb detector

The production of J/ψ mesons is studied in proton-lead collisions at a centre-of-mass energy per nucleon pair [see formula in PDF] = 8.16 TeV with the LHCb detector. The double differential cross-sections of prompt and nonprompt J/ψ production are measured as functions of the J/ψ transverse momentum and rapidity. Nuclear modification factors are determined. The results are compared with different theoretical calculations.

Coarse graining the Bethe–Goldstone equation: Nucleon–nucleon high-momentum components

The delta-shell representation of the nuclear force allows a simplified treatment of nuclear correlations. We show how this applies to the Bethe-Goldstone equation as an integral equation in coordinate space with a few mesh points, which is solved by inversion of a 5-dimensional square matrix in the single channel cases and a $10\times10$ matrix for the tensor-coupled channels. This allows us to readily obtain the high momentum distribution, for all partial waves, of a back-to-back correlated nucleon pair in nuclear matter. We find that the probability of finding a high-momentum correlated neutron-proton pair is about 18 times that of a proton-proton one, as a result of the strong tensor force, thus confirming in an independent way previous results and measurements.

High-momentum antisymmetrized molecular dynamics compared with tensor-optimized shell model for strong tensor correlation

We treat the tensor correlation in antisymmetrized molecular dynamics (AMD) including large-relative-momentum components among nucleon pairs for finite nuclei. The tensor correlation is described by using large imaginary centroid vectors of Gaussian wave packets for nucleon pairs with opposite directions, which makes a large relative momentum. We superpose the AMD basis states, in which one nucleon pair has various relative momenta for all directions; this new method is called "high-momentum AMD" (HM-AMD). We show the results for $^4$He using the effective interaction having a strong tensor force. It is found that HM-AMD provides a large tensor matrix element comparable to the case of the tensor-optimized shell model (TOSM), in which the two-particle-two-hole (2p-2h) excitations are fully included to describe the tensor correlation. The results of two methods agree with each other at the level of the Hamiltonian components of $^4$He. This indicates that in HM-AMD the high-momentum components described by the imaginary centroid vectors of the nucleon pair provide the equivalent effect of the 2p-2h excitations for the tensor correlation.

Two-particle multiplets splitting as a guideline in nucleon pairing estimations

The ground state multiplet structure for nuclei over the wide range of mass number $A$ was calculated in $\delta$-approximation and different mass relations for pairing energy was analysed in this work. Correlation between the calculated multiplet structure and experimental data offer a guideline in deciding between mass relations for nucleon pairing.

Charm meson production in Pb–Pb collisions at the LHC in HYDJET++ model

The phenomenological analysis of J/ψ and D meson production in Pb–Pb collisions at a center-of-mass energy of 2.76 TeV per nucleon pair is fulfilled in the framework of the two-component HYDJET++ model, which includes the thermal and nonthermal production mechanisms. A significant fraction of D-mesons is found to be in a kinetic equilibrium with the created matter, while J/ψ-mesons are characterized by earlier (as compared to light hadrons) freeze-out.

Measurement of the B^{±} Meson Nuclear Modification Factor in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The differential production cross sections of B^{±} mesons are measured via the exclusive decay channels B^{±}→J/ψK^{±}→μ^{+}μ^{-}K^{±} as a function of transverse momentum in pp and Pb-Pb collisions at a center-of-mass energy sqrt[s_{NN}]=5.02 TeV per nucleon pair with the CMS detector at the LHC. The pp(Pb-Pb) data set used for this analysis corresponds to an integrated luminosity of 28.0 pb^{-1} (351 μb^{-1}). The measurement is performed in the B^{±} meson transverse momentum range of 7 to 50 GeV/c, in the rapidity interval |y|<2.4. In this kinematic range, a strong suppression of the production cross section by about a factor of 2 is observed in the Pb-Pb system in comparison to the expectation from pp reference data. These results are found to be roughly compatible with theoretical calculations incorporating beauty quark diffusion and energy loss in a quark-gluon plasma.

Jet Substructure through Splitting Functions And Mass in pp and PbPb collisions at 5.02 TeV with CMS

We present recent results on measurements of jet substructures using grooming techniques with pp and PbPb data collected with the CMS detector at a center-of-mass energy of 5.02 TeV per nucleon pair. The grooming technique is used to focus on the hard structure of the jet by extracting the two subjets corresponding to the hardest parton splitting. This allows us to study medium-induced gluon emission properties and the evolution of partons through dense QCD matter. The hard jet structure is sensitive to the virtuality evolution of a parton in the medium, as well as the role of (de)coherent gluon emitters. Results and prospects on the transverse momentum balance, mass and angular difference of the two hard subjets over a wide range of jet transverse momentum and various collision centrality selections are discussed.

Prompt and nonprompt J/ψ production and nuclear modification in pPb collisions at [formula omitted

The production of J/ψ mesons is studied in proton-lead collisions at the centre-of-mass energy per nucleon pair sNN=8.16 TeV with the LHCb detector at the LHC. The double differential cross-sections of prompt and nonprompt J/ψ production are measured as a function of the J/ψ transverse momentum and rapidity in the nucleon–nucleon centre-of-mass frame. Forward-to-backward ratios and nuclear modification factors are determined. The results are compared with theoretical calculations based on collinear factorisation using nuclear parton distribution functions, on the colour glass condensate or on coherent energy loss models.

Power series expansion method in tensor-optimized antisymmetrized molecular dynamics beyond the Jastrow correlation method

We developed a new variational method for tensor-optimized antisymmetrized molecular dynamics (TOAMD) for nuclei. In TOAMD, the correlation functions for the tensor force and the short-range repulsion are introduced and used in the power series form of the wave function, which is different from the Jastrow method. Here, nucleon pairs are correlated in multi-steps with different forms, while they are correlated only once including all pairs in the Jastrow correlation method. Each correlation function in every term is independently optimized in the variation of total energy in TOAMD. For $s$-shell nuclei using the nucleon-nucleon interaction, the energies in TOAMD are better than those in the variational Monte Carlo method with the Jastrow correlation function. This means that the power series expansion using the correlation functions in TOAMD describes the nuclei better than the Jastrow correlation method.

Reconstruction of low-momentum protons with the Fine-Grained Detector of the T2K experiment

Low-momentum protons are important in the searches for neutrino interactions with correlated pairs of nucleons, 2p-2h. An attempt using the T2K 0ff-axis near detector data was made to use vertex activity (energy deposited near the vertex of the neutrino interaction) to search for a signature of such low-momentum particles. The analysis is based on extracting a signal of non-reconstructed particles expected to be associated to the vertex activity, which is later used by a neural network to detect the presence of the low-momentum proton in the interaction.

Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC

Light-by-light scattering (γγ → γγ) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead ions. Using 480 μb−1 of lead–lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, here we report evidence for light-by-light scattering. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events. After background subtraction and analysis corrections, the fiducial cross-section of the process Pb + Pb (γγ) → Pb(∗) + Pb(∗)γγ, for photon transverse energy ET > 3 GeV, photon absolute pseudorapidity |η| < 2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 ± 24 (stat.) ± 17 (syst.) nb, which is in agreement with the standard model predictions.