Neutron-proton Mass Difference Research Articles

The electromagnetic fine-structure constant in primordial nucleosynthesis revisited

We study the dependence of the primordial nuclear abundances as a function of the electromagnetic fine-structure constant alpha , keeping all other fundamental constants fixed. We update the leading nuclear reaction rates, in particular the electromagnetic contribution to the neutron-proton mass difference pertinent to beta -decays, and go beyond certain approximations made in the literature. In particular, we include the temperature-dependence of the leading nuclear reactions rates and assess the systematic uncertainties by using four different publicly available codes for Big Bang nucleosynthesis. Disregarding the unsolved so-called lithium-problem, we find that the current values for the observationally based {}^{2}text {H} and {}^{4}text {He} abundances restrict the fractional change in the fine-structure constant to less than 2% , which is a tighter bound than found in earlier works on the subject.

Charge-symmetry-breaking effects on neutron β decay in nonrelativistic quark models

A formalism for the study of charge-symmetry-breaking (CSB) effects is discussed and used to analyze the effects of charge-symmetry breaking on neutron $\ensuremath{\beta}$ decay. The effect of including CSB reduces the $\ensuremath{\beta}$-decay matrix element by an amount on the order of ${10}^{\ensuremath{-}4}$, a value much larger than the previous estimate. The earlier calculation used the neutron-proton mass difference as the CSB operator instead of the matrix element of the sum of the individual terms between the ground and excited states. The electromagnetic and dynamic effects of the up-down quark mass difference oppose the up-down quark mass difference leading to a small $n\ensuremath{-}p$ mass difference, but add coherently in computing the excitation matrix elements causing large enhancements. The current uncertainty in the value of ${V}_{ud}$ is also on the order of ${10}^{\ensuremath{-}4}$. An improvement of that uncertainty by an order of magnitude would require that charge-symmetry-breaking effects should be included in future analyses.

Sum rule for the Compton amplitude and implications for the proton\u2013neutron mass difference

The Cottingham formula expresses the leading contribution of the electromagnetic interaction to the proton-neutron mass difference as an integral over the forward Compton amplitude. Since quarks and gluons reggeize, the dispersive representation of this amplitude requires a subtraction. We assume that the asymptotic behaviour is dominated by Reggeon exchange. This leads to a sum rule that expresses the subtraction function in terms of measurable quantities. The evaluation of this sum rule leads to m_{mathrm{QED}}^{p-n}=0.58pm 0.16,text {MeV}.

θ -dependence of light nuclei and nucleosynthesis

We investigate the impact of the QCD vacuum at nonzero $\theta$ on the properties of light nuclei, Big Bang nucleosynthesis, and stellar nucleosynthesis. Our analysis starts with a calculation of the $\theta$-dependence of the neutron-proton mass difference and neutron decay using chiral perturbation theory. We then discuss the $\theta$-dependence of the nucleon-nucleon interaction using a one-boson-exchange model and compute the properties of the two-nucleon system. Using the universal properties of four-component fermions at large scattering length, we then deduce the binding energies of the three-nucleon and four-nucleon systems. Based on these results, we discuss the implications for primordial abundances of light nuclei, the production of nuclei in stellar environments, and implications for an anthropic view of the universe.

Calculation of Neutron-Proton Mass Difference by the Monte Carlo Method

Calculation results of the Monte Carlo method of the average energy of the electrostatic interaction between the quarks are presented to the neutron and proton. The proposed model of the distribution of quarks in protons and neutrons is possible to assess the area which included a strong (gluon) interaction. Given the fact that the probability of finding a quark in the field with strong interaction is less than one, there is a good agreement between the experimental and calculated values of the mass difference between the neutron and the proton.

High resolution mass separator dipole design studies for SPES project

The purposes of the SPES (Selective Production of Exotic Species) project at INFN laboratory in Legnaro (Italy) is to study nuclei close to the drip lines. Therefore, a High-Resolution Mass Separator (HRMS) must provide full separation of the ions with a resolution 1/20000, to be sensible to the proton-neutron mass difference in the fission products. SPES HRMS consists of: two 90° magnet dipoles, one electrostatic multipole in between them, six electrostatic quadrupoles, two electrostatic hexapoles and two electrostatic triplets before and after the slits on the object and image point. All these components will be installed on a high voltage platform with a maximum operating voltage of -240 kV. Before entering the HRMS, a 40 keV energy beam go through an RFQ Cooler, designed to have an output energy spread of 1 eV. Mass separation within target resolution is the most critical part: dipoles must provide a magnetic field homogeneity of 4 10−5 throughout beam occupancy (half magnet pole surface), at a field intensity of 0.562 T for the reference ion 132 Sn. Therefore, a very accurate dipole design is mandatory. This contribute will show the studies which lead to a possible dipole design.

Neutron-proton mass difference from gauge/gravity duality

Using gauge/gravity duality as a tool, we compute the strong sector, isospin breaking induced contribution to the neutron-proton mass difference in the Witten-Sakai-Sugimoto model of large $N$ QCD with two non-degenerate light flavors. The mass difference, for which we provide an analytic expression, turns out to be positive and proportional to the down-up quark mass splitting, consistently with expectations and previous estimates based on effective QCD models. Extrapolating the model parameters to fit QCD hadronic observables, we find that the strong sector contribution to the nucleon mass splitting overcomes the electromagnetic contribution and is about $0.25\%$ of the average nucleon mass in the model, a result which approaches recent lattice QCD estimates. Our formula is extended to resonances and $\Delta$ baryons. We thus use it to compute the strong sector contribution to $\Delta$ baryons mass differences. Finally, we also provide details of how isospin breaking affects the holographic instanton solution describing the baryons.

Fine structure of the cross sections of e+e− annihilation near the thresholds of [formula omitted] and [formula omitted] production

The energy dependence of the cross sections of pp¯, nn¯, and meson production in e+e− annihilation in the vicinity of the pp¯ and nn¯ thresholds is studied. The proton–neutron mass difference and the pp¯ Coulomb interaction are taken into account. The values of the cross sections are very sensitive to the parameters of the optical potential. It is shown that the commonly accepted factorization approach for the account of the Coulomb interaction does not work well enough in the vicinity of the threshold due to the finite size of the optical potential well.

$\rho^{0} - \omega$ ρ 0 - ω mixing in the presence of a weak magnetic field

We calculate the momentum dependence of the $\rho^0-\omega$ mixing amplitude in vacuum with vector nucleon-nucleon interaction in presence of a constant homogeneous weak magnetic field background. The mixing amplitude is generated by the nucleon-nucleon ($NN$) interaction and thus driven by the neutron-proton mass difference along with a constant magnetic field. We find a significant effect of magnetic field on the mixing amplitude. We also calculate the Charge symmetry violating (CSV) $NN$ potential induced by the magnetic field dependent mixing amplitude. The presence of the magnetic field influences the $NN$ potential substantially which can have important consequences in highly magnetized astrophysical compact objects, such as magnetars. The most important observation of this work is that the mixing amplitude is non-zero, leading to positive contribute to the CSV potential if the proton and neutron masses are taken to be equal.

A simple mass-splitting mechanism in the Skyrme model

It is shown that the addition of a single chiral symmetry breaking term to the standard ω-meson variant of the nuclear Skyrme model can reproduce the proton–neutron mass difference.

First-Order Symmetry Energy Induced by Neutron—Proton Mass Difference**Supported by the National Natural Science Foundation of China under Grant Nos 11405223, 11175219, 11275271 and 11435014, the National Basic Research Program of China under Grant No 2013CB834405, the Knowledge Innovation Project of Chinese Academy of Sciences under Grant No KJCX2-EW-N01, the Funds for Creative Research Groups of China under Grant No 11321064,

The 1st-order symmetry energy coefficient of nuclear matter induced merely by the neutron—proton (n—p) mass difference is derived analytically, which turns out to be completely model-independent. Based on this result, the 1st-order symmetry energy Esym,1(npDM) (A) of heavy nuclei such as 208Pb induced by the n—p mass difference is investigated with the help of a local density approximation combined with the Skyrme energy density functionals. Although Esym,1(npDM) (A) is small compared with the second-order symmetry energy, it cannot be dropped simply for an accurate estimation of nuclear masses as it is still larger than the rms deviation given by some accurate mass formulas. It is therefore suggested that one perhaps needs to distinguish the neutron mass from the proton one in the construction of nuclear density functionals.

The surprising influence of late charged current weak interactions on Big Bang Nucleosynthesis

The weak interaction charged current processes (νe+n↔p+e−; ν¯e+p↔n+e+; n↔p+e−+ν¯e) interconvert neutrons and protons in the early universe and have significant influence on Big Bang Nucleosynthesis (BBN) light-element abundance yields, particularly that for 4He. We demonstrate that the influence of these processes is still significant even when they operate well below temperatures T∼0.7 MeV usually invoked for “weak freeze-out,” and in fact down nearly into the alpha-particle formation epoch (T≈0.1 MeV). This physics is correctly captured in commonly used BBN codes, though this late-time, low-temperature persistent effect of the isospin-changing weak processes, and the sensitivity of the associated rates to lepton energy distribution functions and blocking factors are not widely appreciated. We quantify this late-time influence by analyzing weak interaction rate dependence on the neutron lifetime, lepton energy distribution functions, entropy, the proton–neutron mass difference, and Hubble expansion rate. The effects we point out here render BBN a keen probe of any beyond-standard-model physics that alters lepton number/energy distributions, even subtly, in epochs of the early universe all the way down to near T=100 keV.

Classic calculations of static properties of nucleons reexamined

Classic calculations of the magnetic moments mu_p and mu_n of the nucleons using the traditional exponential kernel show instability with respect to variations of the Borel mass as well as arbitrariness with respect to the choice of the onset of perturbative QCD. The use of a polynomial kernel, the coefficients of which are determined by the masses of the nucleon resonances stabilizes the calculation and provides much better damping of the unknown contribution of the nucleon continuum. The method is also applied to the evaluation of the coupling gA of proton to the axial current and to the strong part of the neutron-proton mass difference Delta M_np. All these quantities depend sensitively on the value of the 4-quark condensate < 0 | qqqq | 0 > and the value < 0 | qqqq | 0 > ~ 1.5< 0 | qq | 0 >^2 reproduces the experimental results.

The Role of One Single Lambda Hyperon on Binding Energy Difference of Hypernuclear Mirror Pair

Investigation on isospin symmetry in light Lambda hypernuclei is one of the most important issues in hypernuclear physics. In order to know the influences introduced by a single Lambda hyperon, we study the binding energy difference of mirror hypernuclear pair with mass A = 16, 18, 28, 40, and 42 using a time-odd triaxial relativistic mean field theory. Effects as the spin-orbit interaction, the time-odd component of vector fields, the core polarization, the proton-neutron mass difference, and the center-of-mass energy correction are self-consistently considered. Compared to the reported results of ordinary nuclei, the binding energy difference of mirror hypernuclei shows trivial change. With core polarization modified by an impurity hyperon, the isospin nonconserving effect between proton and neutron is hardly reduced for nuclei in study.

A note on the neutron–proton mass difference

The Search and Discovery story “The neutron and proton weigh in, theoretically,” by Sung Chang (Physics Today, June 2015, page 17), reports on very important research determining the neutron and proton masses and mass difference. However, the interpretation in the penultimate paragraph, based on hypothetically varying the neutron–proton mass difference—or the electromagnetic coupling strength or other fundamental parameters—is too narrow.There is strong phenomenological and theoretical motivation for an underlying theory in which the couplings are unified at short distances—a grand unified theory. If they are, then, for example, after Big Bang nucleosynthesis the number of neutrons, and most other relevant quantities, are affected by all the couplings and would change too.11. G. L. Kane, M. J. Perry, A. N. Zytkow, New Astron. 7, 45 (2002). https://doi.org/10.1016/S1384-1076(01)00088-4 Without a calculation of all the combined effects, one cannot draw any reliable conclusions.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. G. L. Kane, M. J. Perry, A. N. Zytkow, New Astron. 7, 45 (2002). https://doi.org/10.1016/S1384-1076(01)00088-4, Google ScholarCrossref© 2015 American Institute of Physics.

The neutron and proton weigh in, theoretically

The mass difference between the neutron and proton—about 0.14%—is known experimentally with an impressive precision of 4 parts in 10 million. But calculating that difference from scratch via quantum chromodynamics (QCD), the theory of the strong force, is another matter altogether.The simplest description of neutrons and protons posits them as bound states of three “valence” quarks (up, up, down for protons and up, down, down for neutrons). Analogous to the way photons mediate the electromagnetic force between charged electrons, gluons mediate the strong force between quarks, which carry color charge. (See the article by Frank Wilczek, Physics Today, August 2000, page 22.) But unlike neutral photons, gluons also carry color charge and therefore interact with each other. One consequence is that perturbation theory, so successful for quantum electrodynamics (QED), fails spectacularly for QCD at the GeV energy characteristic of neutrons and protons.In lattice QCD, spacetime is discretized into a four-dimensional lattice with quarks at each lattice site. Gluon interactions connect only quarks on neighboring sites, which makes the problem tractable for numerical calculations. (See the article by Carleton DeTar and Steven Gottlieb, Physics Today, February 2004, page 45.) But a closer look inside a neutron or proton, whose complexity is indicated by the figure, reveals a roiling sea of virtual quark–antiquark pairs and gluons, which can give even modern supercomputers indigestion. To complicate matters, those sea quarks and antiquarks, like the valence quarks, also interact electromagnetically.The neutron and proton, in addition to having three valence quarks (larger balls), are filled with a virtual sea of gluons (red springs) and quark (purple)–antiquark (green) pairs. The quarks and antiquarks also interact electromagnetically by exchanging photons (gray wavy lines).PPT|High resolutionTheorists have dealt with the resulting computational difficulties by resorting to such approximations as including only two or three of the six types of quarks, assigning equal mass to the up and down quarks, or ignoring electromagnetic effects. Now an international collaboration led by Zoltán Fodor at the University of Wuppertal in Germany has calculated the neutron and proton masses with a precision of 0.03%, and thus has made the most accurate theoretical determination of their difference to date.11. S. Borsanyi et al., Science 347, 1452 (2015). https://doi.org/10.1126/science.1257050In simulations that produced a combined 60 terabytes of data, the researchers used four types of quarks: up, down, strange, and charm, each with a different mass. Crucially, after a series of QCD-only runs, they turned on electromagnetic interactions in the quark–antiquark sea, a step that was easier said than done. For example, the finite-sized QCD lattice requires imposing boundary conditions on the long-range electromagnetic interactions. The team had to figure out how to control the large artifacts that can crop up when applying those boundary conditions. “The theoretical approach, the algorithmic steps, which were developed for the strong interaction, had to be reworked,” says Fodor. In all, the group combined 41 independent simulations to reach the final result.The new calculation wasn’t just a computational feat. The results revealed in detail a competition between electromagnetic effects and the mass difference between the up and down quarks. That the up quark is lighter than the down quark increases the neutron’s mass relative to the proton’s, whereas the QED contribution does the opposite.“For the first time, all effects have been included and controlled to the first nonvanishing order in the fine structure constant,” says Thomas Blum of the University of Connecticut. Important next steps, he says, are to improve the accuracy of the calculated light quark masses and to apply similar electromagnetic corrections to other phenomena such as the decay of kaons, which pair strange quarks or antiquarks with up or down antiquarks or quarks. Observations of kaon decay in 1964 led particle physicists to discover a fundamental symmetry-breaking process called charge–parity violation.Fodor and his collaborators think their findings also have important implications for cosmology. “Imagine that the electromagnetic coupling was twice as large,” he explains. “Hydrogen atoms would collapse through electron capture.” In the other direction, if the neutron–proton mass difference were much larger than it is, faster neutron beta decay would have left the universe with far fewer neutrons at the end of Big Bang nucleosynthesis. Stellar fusion of hydrogen and the production of heavy elements would be more difficult.Robert Jaffe of MIT agrees. “How finely tuned do the parameters of the standard model have to be for complex structures like human observers to form?” he asks. “Only lattice quantum field theory can explore questions like this.”REFERENCESSection:ChooseTop of pageABSTRACTREFERENCES <<1. S. Borsanyi et al., Science 347, 1452 (2015). https://doi.org/10.1126/science.1257050, Google ScholarCrossref© 2015 American Institute of Physics.

BPS Skyrmions as neutron stars

The BPS Skyrme model has been demonstrated already to provide a physically intriguing and quantitatively reliable description of nuclear matter. Indeed, the model has both the symmetries and the energy–momentum tensor of a perfect fluid, and thus represents a field theoretic realization of the “liquid droplet” model of nuclear matter. In addition, the classical soliton solutions together with some obvious corrections (spin–isospin quantization, Coulomb energy, proton–neutron mass difference) provide an accurate modeling of nuclear binding energies for heavier nuclei. These results lead to the rather natural proposal to try to describe also neutron stars by the BPS Skyrme model coupled to gravity. We find that the resulting self-gravitating BPS Skyrmions provide excellent results as well as some new perspectives for the description of bulk properties of neutron stars when the parameter values of the model are extracted from nuclear physics. Specifically, the maximum possible mass of a neutron star before black-hole formation sets in is a few solar masses, the precise value of which depends on the precise values of the model parameters, and the resulting neutron star radius is of the order of 10 km.

Constraining axion dark matter with Big Bang Nucleosynthesis

We show that Big Bang Nucleosynthesis (BBN) significantly constrains axion-like dark matter. The axion acts like an oscillating QCD θ angle that redshifts in the early Universe, increasing the neutron–proton mass difference at neutron freeze-out. An axion-like particle that couples too strongly to QCD results in the underproduction of He4 during BBN and is thus excluded. The BBN bound overlaps with much of the parameter space that would be covered by proposed searches for a time-varying neutron EDM. The QCD axion does not couple strongly enough to affect BBN.

Progress in resolving charge symmetry violation in nucleon structure

Recent work unambiguously resolves the level of charge symmetry violation in moments of parton distributions using (2 + 1)-flavor lattice QCD. We introduce the methods used for that analysis by applying them to determine the strong contribution to the proton–neutron mass difference. We also summarize related work which reveals that the fraction of baryon spin which is carried by the quarks is in fact structure-dependent rather than universal across the baryon octet.

Analytic evaluation of electromagnetic neutron–proton mass difference in Skyrme model

We calculate, here, the electromagnetic (em) neutron–proton mass difference analytically by using arctan ansatz in Skyrme model. The value so obtained is compared with those given in the literature. Our value agrees well with the widely accepted value in the literature.