Nucleon Level Research Articles

Assessing the theory-data tension in neutrino-induced charged pion production: The effect of final-state nucleon distortion

Pion production on nuclei constitutes a significant part of the total cross section in experiments involving few-GeV neutrinos. Combined analyses of data on deuterium and heavier nuclei points to tensions between the bubble-chamber data and the data of the $\mathrm{MINER}\ensuremath{\nu}\mathrm{A}$ experiment, which are often ascribed to unspecified nuclear effects. In experimental analysis use is made of approximate treatments of nuclear dynamics, usually in a Fermi gas approach with classical treatments of the reaction mechanism, and fits are often performed by simply rescaling cross sections. To understand the origin of these tensions, check the validity of approximations, and to further advance the description of neutrino pion production on nuclei, a microscopic quantum mechanical framework is needed to compute nuclear matrix elements. We use the local approximation to the relativistic distorted wave impulse approximation to calculate the nuclear matrix elements. We include the distortion of wave functions of the final-state nucleon in a real energy-dependent potential. We compare results with and without distortion. To perform this comparison under conditions relevant to neutrino experiments, we compute cross sections for the $\mathrm{MINER}\ensuremath{\nu}\mathrm{A}$ and T2K charged-pion production datasets. The inclusion of nucleon distortion leads to a reduction of the cross section up to 10%, but to no significant change in shape of the flux-averaged cross sections. Results with and without distortion compare favorably to experimental data, with the exception of the low-${Q}^{2}$ $\mathrm{MINER}\ensuremath{\nu}\mathrm{A}$ ${\ensuremath{\pi}}^{+}$ data. We point out that hydrogen target data from BEBC is also overpredicted at low ${Q}^{2}$, and the data-model discrepancy is similar in shape and magnitude as what is found in comparison to $\mathrm{MINER}\ensuremath{\nu}\mathrm{A}$ data. Including nucleon distortion alone cannot explain the overprediction of low-${Q}^{2}$ cross sections measured by $\mathrm{MINER}\ensuremath{\nu}\mathrm{A}$. The similar overprediction of BEBC data on hydrogen means that it is impossible to ascribe this discrepancy solely to a nuclear effect. Axial form factors might not be constrained in a satisfactory way by the Argonne and Brookhaven National Laboratories (ANL/BNL) data alone. Axial couplings and their ${Q}^{2}$ dependence should ideally be derived from more precise data on hydrogen and deuterium. Nuclear matrix elements should be tested with e.g., electron scattering data for which nucleon level physics is better constrained.

A new method of gamma level gauge using a position-sensitive sensor with rod plastic scintillator

Radiometric measuring has many applications, and nucleonic level measurement is one of them. The traditional level gauge works for detecting the level in the tank according to the changes of radiation amount due to the changes of the level in liquid or solid materials. In this study, we proposed a new method of level gauging with some significant advantages for the continuous level meter. In our experimental job, we used a plastic scintillation rod detector and tried to detect the time difference between two photomultiplier tubes to define the approximate point of radiation exposure to the detector. Results show that our innovative method has some clear advantages when compared to the traditional level measurement. This position-sensitive detector succeeded in improving the method of calibration. Also, this method is independent of the type and activity of the radioactive source. Furthermore, this new method helps reduce the source activity, so from viewpoint of health and environmental issues, our method has a lot of benefits. This method has another significant result, and we succeeded in improving the accuracy of the level measurement ten times better than traditional level measurement.

Calculated Event Rates for Axion Detection via Atomic and Nuclear Processes

The possibility of detection of 5.5 MeV and 14.4 keV solar axions by observing axion-induced nuclear and atomic transitions is investigated. The presence of nuclear transitions between spin orbit partners can be manifested by the subsequent deexcitation via gamma ray emissions. The transition rates can also be studied in the context of radiative axion absorption by a nucleus. The elementary interaction is obtained in the context of the axion-quark couplings predicted by existing axion models. Then, these couplings will be transformed to the nucleon level utilizing reasonable existing models, which lead to effective transition operators. Using these operators, we calculate the needed nuclear matrix elements employing wave functions obtained in the context of the nuclear shell model. With these ingredients, we discuss possibilities of experimental observation of the axion-induced nuclear gamma rays. In the second part, we will examine the axion-induced production of X-rays (axion-photon conversion) or ionization from deeply bound electron orbits. In this case, the axion electron coupling is predicted by existing axion models; no renormalization is needed. The experimental signal is the observation of directly produced electrons and/or the emission of hard X-rays and Auger electrons, following the deexcitation of the final atom. Critical discussion is made on the experimental feasibility of detecting the solar axions by using multiton scale NaI detectors.

Sensitivity of EDM experiments in paramagnetic atoms and molecules to hadronic CP violation

Experiments searching for the electric dipole moment (EDM) of the electron $d_e$ utilise atomic/molecular states with one or more uncompensated electron spins, and these paramagnetic systems have recently achieved remarkable sensitivity to $d_e$. If the source of $CP$ violation resides entirely in the hadronic sector, the two-photon exchange processes between electrons and the nucleus induce $CP$-odd semileptonic interactions, parametrised by the Wilson coefficient $C_{SP}$, and provide the dominant source of EDMs in paramagnetic systems instead of $d_e$. We evaluate the $C_{SP}$ coefficients induced by the leading hadronic sources of $CP$ violation, namely nucleon EDMs and $CP$-odd pion-nucleon couplings, by calculating the nucleon-number-enhanced $CP$-odd nuclear scalar polarisability, employing chiral perturbation theory at the nucleon level and the Fermi-gas model for the nucleus. This allows us to translate the ACME EDM limits from paramagnetic ThO into novel independent constraints on the QCD theta term $|\bar \theta| < 3 \times 10^{-8}$, proton EDM $|d_p| < 2 \times 10^{-23}\,e\,{\rm cm}$, isoscalar $CP$-odd pion-nucleon coupling $|\bar g^{(1)}_{\pi NN}| < 4 \times 10^{-10}$, and colour EDMs of quarks $|\tilde d_u - \tilde d_d| < 2 \times 10^{-24}\,{\rm cm}$. We note that further experimental progress with EDM experiments in paramagnetic systems may allow them to rival the sensitivity of EDM experiments with neutrons and diamagnetic atoms to these quantities.

Disentangling contributions to small-system collectivity via scans of light nucleus-nucleus collisions

The observation of multi-particle azimuthal correlations in high-energy small-system collisions has led to intense debate on its origin and the possible coexistence from two competing theoretical scenarios: one based on initial-state intrinsic momentum anisotropy (ISM), and the other based on final-state collective response to the collision geometry (FSM). To complement the previous scan of asymmetric collision systems ($p$+Au, $d$+Au and He+Au), we propose a scan of small symmetric collision systems at RHIC, such as C+C, O+O, Al+Al and Ar+Ar $\sqrt{s_{\mathrm{NN}}}=0.2$ TeV, to provide further insights in disentangling contributions from these two scenarios. These symmetric small systems have the advantage of providing a better controlled initial geometry dominated by the average shape of the overlap region, as opposed to fluctuation-driven geometries in asymmetric systems. A transport model is employed to investigate the expected geometry response in the FSM scenario. Different trends of elliptic flow with increasing charge particle multiplicity are observed between symmetric and asymmetric systems, while triangular flow appears to show a similar behavior. Furthermore, a comparison of O+O collisions at $\sqrt{s_{\mathrm{NN}}}=0.2$ TeV and at $\sqrt{s_{\mathrm{NN}}}=2.76-7$ TeV, as proposed at the LHC, provides a unique opportunity to disentangle the collision geometry effects at nucleon level from those arising from subnucleon fluctuations.

Angular distributions in electroweak pion production off nucleons: Odd parity hadron terms, strong relative phases, and model dependence

The study of pion production in nuclei is important for signal and background determinations in current and future neutrino oscillation experiments. The first step, however, is to understand the pion production reactions at the free nucleon level. We present an exhaustive study of the charged-current and neutral-current neutrino and antineutrino pion production off nucleons, paying a special attention to the angular distributions of the outgoing pion. We show, using general arguments, that parity violation and time-reversal odd correlations in the weak differential cross sections are generated from the interference between different contributions to the hadronic current that are not relatively real. Next, we present a detailed comparison of three, state of the art, microscopic models for electroweak pion production off nucleons, and we also confront their predictions with polarized electron data, as a test of the vector content of these models. We also illustrate the importance of carrying out a comprehensive test at the level of outgoing pion angular distributions, going beyond comparisons done for partially integrated cross sections, where model differences cancel to a certain extent. Finally, we observe that all charged and neutral current distributions show sizable anisotropies, and identify channels for which parity violating effects are clearly visible. Based on the above results, we conclude that the use of isotropic distributions for the pions in the center of mass of the final pion-nucleon system, as assumed by some of the Monte Carlo event generators, needs to be improved by incorporating the findings of microscopic calculations.

First-order Coriolis-coupling for the rotational spectrum of a tetrahedrally deformed core plus one-particle system

The possible existence of shape-coexisting nuclear configurations with tetrahedral symmetry is receiving an increasing attention due to unprecedented nuclear structure properties, in particular in terms of the exotic 4-fold nucleonic level degeneracies and the expected long lifetimes which may become a new decisive argument in the exotic nuclei research programs. The present article addresses the rotational structure properties of the tetrahedrally-symmetric even-even core configurations coupled with a single valence nucleon. We focus on the properties of the associated Coriolis-coupling Hamiltonian proposing the solutions based on the explicit construction of the bases of the irreducible representations of the tetrahedral point-group on the one-hand side and the microscopic angular-momentum and parity projection nuclear mean-field approach on the other. It is shown that for one-particle occupying an orbital belonging to the $E_{1/2}$ or $E_{5/2}$ irreducible representation, the rotational spectrum splits into two sequences, the structures analogous to those of the $K=1/2$ rotational bands in the axially symmetric nuclei. Although the spectrum is generally more complicated for one-particle occupying a 4-fold degenerate orbital belonging to the $G_{3/2}$ representation, an appearance of the correlated double-sequence structures persists. The spectra of the doubly-magic tetrahedral core plus one-particle systems can be well interpreted using the analytical solutions of the first order Coriolis-coupling Hamiltonian. We introduce the notion of the generalized decoupling parameters, which determine the size of the energy-splitting between the double-sequence structures.

The charged component of the vacuum field as the source of electric force in the modernized Le Sage’s model

The formula is derived for the electric force inside a uniformly charged spherical body, as well as for the Coulomb force between the charged bodies from the standpoint of the model of the vacuum field with charged particles. The parameters of the fluxes of charged particles are estimated, including the energy density, energy flux and cross section of interaction with the charged matter. The energy density of gravitons in the Le Sage’s gravitation model is expressed in terms of the strong gravitational constant. The charge to mass ratio is determined for the charged particles that make up photons and the charged component of the gravitational field. These particles are identified as praons, while the praon level of matter is considered a lower level relative to the nucleon level of matter. The analysis of the main problems of the Le Sage’s model shows that these problems can be eliminated in the modernized model.

Review of the electric dipole moment of light nuclei

In this paper, we summarize the theoretical development on the electric dipole moment (CEDM) of light nuclei. We first describe the nucleon level CP violation and its parametrization. We then present the results of calculations of the EDM of light nuclei in the ab initio approach and in the cluster model. The analysis of the effect of several models beyond standard model (CBSM) is presented together with the prospects for its discovery. The advantage of the EDM of light nuclei is focused in the point of view of the many-body physics. The evaluations of the nuclear EDM generated by the [Formula: see text]-term and by the CP phase of the Cabibbo–Kobayashi–Maskawa (CKM) matrix are also reviewed.

Scaling of nuclear modification factors for hadrons and light nuclei

The number of constituent quarks (NCQ-) scaling of hadrons and the number of constituent nucleons (NCN-) scaling of light nuclei are proposed for nuclear modification factors ($R_{cp}$) of hadrons and light nuclei, respectively, according to the experimental investigations in relativistic heavy-ion collisions. Based on coalescence mechanism the scalings are performed for pions and protons in quark level, and light nuclei $d (\bar d)$ and $^3$He for nucleonic level, respectively, formed in Au + Au and Pb + Pb collisions and nice scaling behaviour emerges. NCQ or NCN scaling law of $R_{cp}$ can be respectively taken as a probe for quark or nucleon coalescence mechanism for the formation of hadron or light nuclei in relativistic heavy-ion collisions.

Overcoming velocity suppression in dark-matter direct-detection experiments

Pseudoscalar couplings between Standard-Model quarks and dark matter are normally not considered relevant for dark-matter direct-detection experiments because they lead to velocity-suppressed scattering cross-sections in the non-relativistic limit. However, at the nucleon level, such couplings are effectively enhanced by factors of order ${\cal O}(m_N/m_q)\sim 10^3$, where $m_N$ and $m_q$ are appropriate nucleon and quark masses respectively. This enhancement can thus be sufficient to overcome the corresponding velocity suppression, implying --- contrary to common lore --- that direct-detection experiments can indeed be sensitive to pseudoscalar couplings. In this work, we explain how this enhancement arises, and present a model-independent analysis of pseudoscalar interactions at direct-detection experiments. We also identify those portions of the corresponding dark-matter parameter space which can be probed at current and future experiments of this type, and discuss the role of isospin violation in enhancing the corresponding experimental reach.

Transverse energy and charged particle production in heavy-ion collisions: From RHIC to LHC

We study the charged particle and transverse energy production mechanism from AGS, SPS, Relativistic Heavy-Ion Collider (RHIC) to Large Hadron Collider (LHC) energies in the framework of nucleon and quark participants. At RHIC and LHC energies, the number of nucleons-normalized charged particle and transverse energy density in pseudorapidity, which shows a monotonic rise with centrality, turns out to be an almost centrality independent scaling behavior when normalized to the number of participant quarks. A universal function which is a combination of logarithmic and power-law, describes well the charged particle and transverse energy production both at nucleon and quark participant level for the whole range of collision energies. Energy dependent production mechanisms are discussed both for nucleonic and partonic level. Predictions are made for the pseudorapidity densities of transverse energy, charged particle multiplicity and their ratio (the barometric observable, [Formula: see text]) at mid-rapidity for Pb + Pb collisions at [Formula: see text]. A comparison with models based on gluon saturation and statistical hadron gas is made for the energy dependence of [Formula: see text].

Singleπproduction in neutrino-nucleus scattering

We study one pion production in both charged and neutral current neutrino nucleus scattering for neutrino energies below 2 GeV. We use a theoretical model for one pion production at the nucleon level that we correct for medium effects. The results are incorporated into a cascade program that apart from production also includes the pion final state interaction inside the nucleus. Besides, in some specific channels coherent pion production is also possible and we evaluate its contribution as well. Our results for total and differential cross sections are compared with recent data from the MiniBooNE Collaboration. The model provides an overall acceptable description of data, better for NC than for CC channels, although theory is systematically below data. Differential cross sections, folded with the full neutrino flux, show that most of the missing pions lie on the forward direction and at high energies.

Charged kaon production by coherent scattering of neutrinos and antineutrinos on nuclei

With the aim of achieving a better and more complete understanding of neutrino interactions with nuclear targets, the coherent production of charged kaons induced by neutrinos and antineutrinos is investigated in the energy range of some of the current neutrino experiments. We follow a microscopic approach which, at the nucleon level, incorporates the most important mechanisms allowed by the chiral-symmetry-breaking pattern of QCD. The distortion of the outgoing $K$ ($\overline{K}$) is taken into account by solving the Klein-Gordon equation with realistic optical potentials. Angular and momentum distributions, as well as the energy and nuclear dependence of the total cross section, are studied.

Analysis of the (n,f) Reaction in the Plutonium Isotopes

This paper describes the modified Hauser-Feshbach formalism used to com- pute accurately fission cross sections for low-energy neutrons (from a few keV up to 5.5 MeV) in presence of intermediate structure in the second well. Application to the large plutonium isotope family (236 to 244) has been made with in particular reliable predic- tions of the cross sections of the short-lived nuclides. Special attention is paid to the choice of the model parameters entering in the calculations. The decay of a compound nucleus by fission is a complex multi-dimensional problem, whose de- scription still requires many model parameters. The elucidation and physical understanding of these parameters is an ongoing research process, and this paper reports our progress in the study of the fis- sion cross-sections for a long sequence of plutonium isotopes. Our ultimate goal is to replace many fitting parameters ubiquitous in current data evaluations with physically sound theory with fewer pa- rameters. If successful, such an approach would lead to reliable predictions of the fission cross-sections of nuclides for which measurements are unavailable or very poor, while at the same time deepen our understanding of the fission process. Here we present results obtained with a modern version of the AVXSF code (AVerage Cross Sec- tion of Fission), which treats in great detail the intermediate structure due to the presence of a sec- ond well on the fission path. The correct modeling of the class-II states properties and in particular, their coupling with the levels present in the first well as well as to the continuum, is crucial to infer meaningful barrier heights. Experimentally, those barrier heights can be inferred from an analysis of low-energy neutron-induced fission cross sections in the case of fertile nuclei, and transfer reactions leading to fission, e.g., ( t, p f ), in the case of fissile nuclei. The AVXSF code can treat both situations adequately in order to extract fission barrier heights. In the following, the theory and main equations implemented in the AVXSF code are introduced. Approximate formulae valid in specific situations are also discussed. A Monte Carlo option is used when analytic approximations fail and a full sampling of the level characteristics distributions is needed. We then present preliminary results on the neutron-induced fission cross sections obtained for a suite of plutonium isotopes, for which a consistent set of model input parameters was devel- oped. Our analysis encompasses neutron incident energies from a few keV up to 5.5 MeV, just below the onset of second-chance fission. The importance of fission barrier heights, nucleon pairing gaps and level densities is emphasized. This paper concludes on some examples of predicted cross sections with some reliability for the short-lived nuclides of the Pu series.

Nuclear PDFs

The current status of nuclear parton distribution functions (nPDFs) is reviewed with a particular emphasis on the recent nuclear CTEQ global analysis framework. We summarize the results of an analysis of ℓA deep inelastic scattering (DIS) and Drell-Yan (DY) data. Subsequently, we turn to an analysis of νA DIS data. We compute nuclear correction factors needed in global analyses of proton PDFs to correct the νA DIS data to the free nucleon level. These correction factors turn out to be surprisingly different from the expectations based on the ℓA DIS+DY data. We consider the possibility to reconcile the νA with the ℓA data by a suitable compromise fit to the combined νA+ℓA+DY data sets and show that an acceptable compromise does not exist. Furthermore, we discuss the uncertainty of the nuclear gluon distribution and propose prompt photon plus heavy quark production in pA collisions as a suitable process to constrain both, the nuclear gluon and heavy quark distributions.

Understanding nuclear shape phase transitions at the nucleon level with a boson mapping approach

Implementing a shell model Hamiltonian with monopole-pair, quadrupole-pair and quadrupole–quadrupole interactions between nucleons, we study the shape phases of nuclei and their transitions, with the Dyson boson mapping approach. The investigation concentrates on the influence of each interaction on the nuclear shape phases. A correspondence between the strength of each of the interactions and the nuclear shape phases is obtained. The investigation also indicates that increasing the quadrupole-pair interaction strength can induce the vibrational to the axially prolate rotational shape phase transition and enhancing the quadrupole–quadrupole interaction can drive the phase transition from the axially oblate rotational to the axially prolate rotational, with the γ-soft rotational being the critical point.

SOME ASPECTS OF SHAPE PHASE TRANSITION IN EVEN-EVEN NUCLEI

We have reviewed the study on the signatures of shape phase transition in even-even nuclei and investigated the statistical properties of the E (5) and X (5) symmetries and their location in the Casten's triangle and the effect of some modes of nucleon-nucleon interaction on the nuclear shape phases and their transitions. We show that the ratios of the E 2 transition rates and the ratio of the energies of [Formula: see text] state and the [Formula: see text] state can serve as the effective order parameters not only identifying the nucleon number driven phase transition but also distinguishing the first order one form the second order one. Our calculations show that the states in E (5) and X (5) symmetries are almost regular ones and the X (5) symmetry may actually locate at the middle of the "AW arc" inside the Casten's triangle and the E (5) symmetry at the U (5)- O (6) chain. Our study also indicates that strong or weak monopole-pair interaction leads to a vibration or an axially oblate deformation. While quadrupole-pair and QQ interactions induce deformations. Then a complete shape phase structure of nucleus is given in a study at nucleon level.

Effects of orbital occupancies on the neutrinoless ββ matrix element of 76Ge

In this work we use the recently measured neutron occupancies in the 76Ge and 76Se nuclei as a guideline to define the neutron quasiparticle states in the 1p0f0g shell. We define the proton quasiparticles by inspecting the odd-mass nuclei adjacent to 76Ge and 76Se. We insert the resulting quasiparticles in a proton–neutron quasiparticle random-phase approximation (pnQRPA) calculation of the nuclear matrix element of the neutrinoless double beta (0νββ) decay of 76Ge. A realistic model space and effective microscopic two-nucleon interactions are used. We include the nucleon–nucleon short-range correlations and other relevant corrections at the nucleon level. It is found that the resulting 0νββ matrix element is smaller than in the previous pnQRPA calculations, and closer to the recently reported shell-model results.

Two topics in the physics of light nuclei

I review how tensor forces affect the ground-state structure of nuclei, and how isospin-symmetry-breaking corrections at the nucleon and nuclear level contaminate the asymmetry measured in parity-violating electron scattering from nuclei, complicating the extraction of the strange-quark form factors from these measurements.