Nucleon Bremsstrahlung Research Articles

Probing dark gauge boson with observations from neutron stars

We present an investigation on the production of light dark gauge bosons by the nucleon bremsstrahlung processes in the core of neutron stars. The dark vector is assumed to be a $U(1{)}_{B\ensuremath{-}L}$ gauge boson with a mass much below keV. We calculate the emission rate of the dark vector produced by the nucleon bremsstrahlung in the degenerate nuclear matter. In addition, we take into account the photon-dark vector conversion for the photon luminosity observed at infinity. Combining with the observation of J1856 surface luminosity, we find that a recently discovered excess of J1856 hard x-ray emission in the 2--8 keV energy range by X-ray Multi-Mirror Mission-Newton and Chandra x-ray telescopes could be consistently explained by a dark vector with gauge coupling ${e}^{\ensuremath{'}}=5.56\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$, mixing angle $ϵ=1.29\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$, and mass ${m}_{{\ensuremath{\gamma}}^{\ensuremath{'}}}\ensuremath{\lesssim}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{eV}$. We also show that the mixing angle $ϵ>7.97\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ for ${m}_{{\ensuremath{\gamma}}^{\ensuremath{'}}}\ensuremath{\lesssim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{ }\mathrm{eV}$ and the gauge coupling ${e}^{\ensuremath{'}}>4.13\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ for ${m}_{{\ensuremath{\gamma}}^{\ensuremath{'}}}\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{keV}$ have been excluded at 95% confidence level by the J1856 surface luminosity observation. Our best-fit dark vector model satisfies the current limits on hard x-ray intensities from the Swift and INTEGRAL hard x-ray surveys. Future hard x-ray experiments such as the Nuclear Spectroscopic Telescope Array may give a further test on our model.

Upper Limit on the QCD Axion Mass from Isolated Neutron Star Cooling.

The quantum chromodynamics (QCD) axion may modify the cooling rates of neutron stars (NSs). The axions are produced within the NS cores from nucleon bremsstrahlung and, when the nucleons are in superfluid states, Cooper pair breaking and formation processes. We show that four of the nearby isolated magnificent seven NSs along with PSR J0659 are prime candidates for axion cooling studies because they are coeval, with ages of a few hundred thousand years known from kinematic considerations, and they have well-measured surface luminosities. We compare these data to dedicated NS cooling simulations incorporating axions, profiling over uncertainties related to the equation of state, NS masses, surface compositions, and superfluidity. Our calculations of the axion and neutrino emissivities include high-density suppression factors that also affect SN 1987A and previous NS cooling limits on axions. We find no evidence for axions in the isolated NS data, and within the context of the Kim-Shifman-Vainshtein-Zakharov QCD axion model, we constrain m_{a}≲16 meV at 95% confidence level. An improved understanding of NS cooling and nucleon superfluidity could further improve these limits or lead to the discovery of the axion at weaker couplings.

Light scalars in neutron star mergers

Due to their unique set of multimessenger signals, neutron star mergers have emerged as a novel environment for studies of new physics beyond the Standard Model (SM). As a case study, we consider the simplest extension of the SM scalar sector involving a light CP-even scalar singlet S mixing with the SM Higgs boson. These S particles can be produced abundantly in neutron star mergers via the nucleon bremsstrahlung process. We show that the S particles may either be trapped in or stream freely out of the merger remnant, depending on the S mass, its mixing with the SM Higgs boson, and the temperature and baryon density in the merger. In the free-streaming region, the scalar S will provide an extra channel to cool down the merger remnant, with cooling timescales as small as \U0001d4aa(ms). On the other hand, in the trapped region, the Bose gas of S particles could contribute a larger thermal conductivity than the trapped neutrinos in some parts of the parameter space, thus leading to faster thermal equilibration than expected. Therefore, future observations of the early postmerger phase of a neutron star merger could effectively probe a unique range of the S parameter space, largely complementary to the existing and future laboratory and supernova limits. In view of these results, we hope the merger simulation community will be motivated to implement the effects of light CP-even scalars into their simulations in both the free-streaming and trapped regimes.

Revisiting supernova constraints on a light CP-even scalar

A light CP-even Standard Model (SM) gauge-singlet scalar S can be produced abundantly in the supernova core, via the nucleon bremsstrahlung process N N → N N S, due to its mixing with the SM Higgs boson. Including the effective S coupling to both nucleons and the pion mediators, we evaluate the production amplitude for the S particle and point out a key difference with the well-known light CP-odd scalar (axion) and vector boson (dark photon) cases. Taking the subsequent decay and re-absorption of S into account, we present a complete calculation of the energy loss rate for the S particle. We then use the SN1987A luminosity constraints to derive an updated limit on the mixing of the scalar S with the SM Higgs boson. We find that the mixing angle sinθ with the SM Higgs is excluded only in the narrow range of 3.5 × 10−7 to 2.5 × 10−5, depending on the scalar mass up to the two-pion threshold, beyond which the supernova limit disappears. This result has important consequences for the laboratory searches for light scalars.

Chiral Effective Field Theory Description of Neutrino Nucleon–Nucleon Bremsstrahlung in Supernova Matter

Abstract We revisit the rates of neutrino pair emission and absorption from nucleon–nucleon bremsstrahlung in supernova matter using the T-matrix formalism in the long-wavelength limit. Based on two-body potentials of chiral effective field theory (χEFT), we solve the Lippmann–Schwinger equation for the T-matrix including non-diagonal contributions. We consider final-state Pauli blocking and hence our calculations are valid for nucleons with an arbitrary degree of degeneracy. We also explore the in-medium effects on the T-matrix and find that they are relatively small for supernova matter. We compare our results with one-pion exchange rates, commonly used in supernova simulations, and calculations using an effective on-shell diagonal T-matrix from measured phase shifts. We estimate that multiple-scattering effects and correlations due to the random phase approximation introduce small corrections on top of the T-matrix results at subsaturation densities. A numerical table of the structure function is provided that can be used in supernova simulations.

Virginia Ruth Brown

Virginia Ruth Brown died on 8 February 2016 in Chevy Chase, Maryland, after a long battle with cancer. She was a wonderful colleague, friend, and distinguished theoretical nuclear physicist. During her long career, Virginia made important contributions to our understanding of the nucleon–nucleon interaction, nuclear reaction theory, and nuclear structure, among other accomplishments. She loved physics and had a deep commitment to getting to the heart of things.Virginia Ruth BrownPPT|High resolutionBorn on 11 March 1934 in Massachusetts, Virginia received her PhD, under the supervision of Bernard Margolis, from McGill University in 1964. She was a postdoctoral fellow at Yale University and then worked at Lawrence Livermore National Laboratory (LLNL) from 1964 until 1995. She was an NSF program officer for nuclear theory from 1995 to 1998. Following that, she was a visiting professor in the physics department of the University of Maryland in College Park and a visiting scientist in the Laboratory for Nuclear Science at MIT.During Virginia’s early work on nucleon–nucleon bremsstrahlung, she explored the effects of parity nonconservation and demonstrated the importance of meson exchange in neutron–proton bremsstrahlung. Her calculations showed that meson exchange currents contributed a factor of 2 and were not dominated by one-pion exchange. Throughout her career she continued refining and extending her calculations, including for relativistic effects and noncoplanarity. She compared her results with experiment, most notably with the data on neutron–proton bremsstrahlung that Stephen Wender, June Matthews, and their colleagues obtained at the Los Alamos Neutron Science Center. When Virginia was at MIT she interacted frequently with Matthews’s group and directed the research of several of Matthews’s undergraduate and graduate students.Virginia’s collaboration with experimentalists started soon after she joined LLNL; most prominently, she worked with John Anderson on charge-exchange reactions in nuclei.Around 1973 Virginia also began her long partnership with theorist Victor Madsen. They developed the nuclear-structure and reaction theory needed to understand charge-exchange reactions; their breakthroughs included a model for mixing giant resonance states with the low-lying states that had been experimentally observed at the time. Their most important contribution was the systematic inclusion of isospin degrees of freedom that were required to understand the data. It was successfully applied to inelastic scattering processes without any additional modifications.One of us (Bernstein) remembers how stimulating it was to work with Virginia. In the mid 1970s, he received a letter from her about how Madsen’s and her isospin-dependence calculations could explain the puzzles and regularities that he had observed in inelastic alpha-particle scattering. Their collaboration started in their first meeting, and over many years it yielded a productive series of papers. They covered electromagnetic tests of the accuracy of the observed neutron and proton transition matrix elements and their observation using different hadronic probes and electromagnetic methods such as electron scattering and Coulomb excitation. He remembers with great pleasure his annual spring-break trip to the University of California, Berkeley, and LLNL to work with Virginia and to enjoy great dinners with her in Berkeley and San Francisco. Their friendship and collaboration were reinforced while she was working as a visiting scientist at MIT from 1998 until her death.Virginia was a deeply involved member of the American Physical Society (APS), including as secretary/treasurer of the division of nuclear physics (DNP) from 1986 to 1995. Because of her numerous contributions, her colleagues chose her in 2003 to receive the division’s first Distinguished Service Award.Another of us (Gibson) saw firsthand Virginia’s remarkable contributions to APS. He first met Virginia when he was a postdoctoral fellow at LLNL in 1968. Their closest connection came through the DNP, when he succeeded her as secretary/treasurer in 1995, and she mentored him in that time-consuming position.Virginia’s efforts as part of the DNP leadership strengthened the division’s fall meetings, assisted in funding APS’s Hans Bethe and Herman Feshbach Prizes and the division’s Nuclear Physics Dissertation Award, initiated the Department of Energy and NSF’s Nuclear Science Advisory Committee Long Range Plan town meetings, and established the DNP archive and historical record.While at NSF, Virginia suggested exploring a joint meeting with the nuclear physicists of the Physical Society of Japan. She chaired the first of the resulting series of successful international physics meetings in 2001 and remained instrumental in their success through the 2014 joint meeting. After Virginia left NSF, she was a visiting scientist at Los Alamos National Laboratory, where she conducted research and shared meals with Gibson and another of us (Seestrom). During one visit, she and Gibson drafted a history of the DNP. Virginia’s imprint on the division is indelible, and the DNP owes her a great debt of gratitude.With her contagious enthusiasm and laughter, Virginia was a dear friend who will be sorely missed.© 2016 American Institute of Physics.

Constraints on axions and axionlike particles from Fermi Large Area Telescope observations of neutron stars

We present constraints on the nature of axions and axionlike particles (ALPs) by analyzing gamma--ray data from neutron stars using the Fermi Large Area Telescope. In addition to axions solving the strong CP problem of particle physics, axions and ALPs are also possible dark matter candidates. We investigate axions and ALPs produced by nucleon--nucleon bremsstrahlung within neutron stars. We derive a phenomenological model for the gamma--ray spectrum arising from subsequent axion decays. By analyzing 5 years of gamma-ray data (between 60 MeV and 200 MeV) for a sample of 4 nearby neutron stars, we do not find evidence for an axion or ALP signal, thus we obtain a combined 95\% confidence level upper limit on the axion mass of 7.9$\times 10^{-2}$ eV, which corresponds to a lower limit for the Peccei-Quinn scale $f_a$ of 7.6$\times 10^7$ GeV. Our constraints are more stringent than previous results probing the same physical process, and are competitive with results probing axions and ALPs by different mechanisms.

A NEW MULTI-ENERGY NEUTRINO RADIATION-HYDRODYNAMICS CODE IN FULL GENERAL RELATIVITY AND ITS APPLICATION TO THE GRAVITATIONAL COLLAPSE OF MASSIVE STARS

ABSTRACT We present a new multi-dimensional radiation-hydrodynamics code for massive stellar core-collapse in full general relativity (GR). Employing an M1 analytical closure scheme, we solve spectral neutrino transport of the radiation energy and momentum based on a truncated moment formalism. Regarding neutrino opacities, we take into account a baseline set in state-of-the-art simulations, in which inelastic neutrino–electron scattering, thermal neutrino production via pair annihilation, and nucleon–nucleon bremsstrahlung are included. While the Einstein field equations and the spatial advection terms in the radiation-hydrodynamics equations are evolved explicitly, the source terms due to neutrino–matter interactions and energy shift in the radiation moment equations are integrated implicitly by an iteration method. To verify our code, we first perform a series of standard radiation tests with analytical solutions that include the check of gravitational redshift and Doppler shift. A good agreement in these tests supports the reliability of the GR multi-energy neutrino transport scheme. We then conduct several test simulations of core-collapse, bounce, and shock stall of a 15 M ⊙ ?> star in the Cartesian coordinates and make a detailed comparison with published results. Our code performs quite well to reproduce the results of full Boltzmann neutrino transport especially before bounce. In the postbounce phase, our code basically performs well, however, there are several differences that are most likely to come from the insufficient spatial resolution in our current 3D-GR models. For clarifying the resolution dependence and extending the code comparison in the late postbounce phase, we discuss that next-generation Exaflops class supercomputers are needed at least.

PROPAGATION AND NEUTRINO OSCILLATIONS IN THE BASE OF A HIGHLY MAGNETIZED GAMMA-RAY BURST FIREBALL FLOW

Neutrons play an important role in the dynamics of gamma-ray bursts. The presence of neutrons in the baryon-loaded fireball is expected. If the neutrons abundance is comparable to that of protons, important features may be observed such as quasi-thermal multi-GeV neutrinos in coincidence with a subphotospheric $\gamma$-ray emission, nucleosynthesis at later times and rebrightening of the afterglow emission. Additionally, thermal MeV neutrinos are created by electron-positron annihilation, electron (positron) capture on protons (neutrons) and nucleonic bremsstrahlung. Although MeV neutrinos are difficult to detect, quasi-thermal GeV neutrinos are expected in km$^3$ detectors and/or DeepCore+IceCube. In this paper, we show that neutrino oscillations have outstanding implications for the dynamics of the fireball evolution and also that they can be detected through their flavor ratio on Earth. For that we derive the resonance and charged-neutrality conditions as well as the neutrino self-energy and effective potential up to order $m_W^{-4}$ at strong, moderate and weak magnetic field approximation to constrain the dynamics of the fireball. We found important implications: i) resonant oscillations are suppressed for high baryon densities as well as neutrons abundance larger than that of protons and ii) the effect of magnetic field is to decrease the proton-to-neutron ratio aside from the number of multi-GeV neutrinos expected in DeepCore detector. Also we estimate the GeV neutrino flavor ratios along the jet and on Earth.

TSALLIS STATISTICS AND THE ROLE OF A STABILIZED RADION IN THE SUPERNOVAE SN1987A COOLING

The radion in the two-brane Randall–Sundrum model is required to stabilize the size of the fifth (extra) spatial dimension. It can be copiously produced inside the supernova core due to electron–positron annihilation (e+e-→ϕ), plasmon–plasmon annihilation (γP+γP→ϕ) and nucleon–nucleon bremsstrahlung and can take away the energy released in SN1987A explosion. Working within the q-deformed statistics (Tsallis statistics) and using the "Raffelt criterion" on the supernovae cooling rate [Formula: see text], we find that in Case I (cooling due to e+e-→ϕ channel): for q = 1.22, as the radion mass mϕ changes from 20 GeV to 150 GeV, the lower bound 〈ϕ〉 changes from 7 TeV to ~1.5 TeV and in Case II (cooling due to γP+γP→ϕ channel): for q = 1.11, as mϕ ranges from 20 GeV to 150 GeV, the lower bound 〈ϕ〉 changes from 201 TeV to 3.3 TeV. We investigate the dependence of 〈ϕ〉 on q and found that in Case I: mϕ = 50(100) GeV , 〈ϕ〉 changes from 0.5(0.2) TeV (for q = 1.18) to 5.5(4.8) TeV (for q = 1.30) and in Case II: for mϕ = 50(100) GeV , 〈ϕ〉 changes from 0.8(~0.1) TeV (for q = 1.09) to 569(216) TeV (for q = 1.13). We also verified that the normal Fermi–Dirac and Bose–Einstein statistics get recovered from the Tsallis statistics in the q→1 limit.

INTERPLAY OF NEUTRINO OPACITIES IN CORE-COLLAPSE SUPERNOVA SIMULATIONS

We have conducted a series of numerical experiments using spherically symmetric, general relativistic, neutrino radiation hydrodynamics with the code Agile-BOLTZTRAN to examine the effects of modern neutrino opacities on the development of supernova simulations. We test the effects of opacities by removing opacities or by undoing opacity improvements for individual opacities and groups of opacities. We find that improvements to electron capture (EC) on nuclei, namely EC on an ensemble of nuclei using modern nuclear structure models rather than the simpler independent-particle approximation (IPA) for EC on a mean nucleus, plays the most important role during core collapse of all tested neutrino opacities. Low-energy neutrinos emitted by modern nuclear EC preferentially escape during collapse without the energy downscattering on electrons required to enhance neutrino escape and deleptonization for the models with IPA nuclear EC. During shock breakout the primary influence on the emergent neutrinos arises from NIS on electrons. For the accretion phase, non-isoenergetic scattering on free nucleons and pair emission by $e^+e^-$ annihilation have the largest impact on the neutrino emission and shock evolution. Other opacities evaluated, including nucleon--nucleon bremsstrahlung and especially neutrino--positron scattering, have little measurable impact on neutrino emission or shock dynamics. Modern treatments of nuclear electron capture, $e^+e^-$-annihilation pair emission, and non-isoenergetic scattering on electrons and free nucleons are critical elements of core-collapse simulations of all dimensionality.

Short-range nuclear effects on axion emissivities by nucleon–nucleon bremsstrahlung

The rates of axion emission by nucleon–nucleon (NN) bremsstrahlung are reconsidered by taking into account the NN short-range correlations. The analytical formulas for the neutron–neutron, proton–proton and neutron–proton processes with the inclusion of the full momentum dependence of one- and two-pion exchange nuclear potentials, in the non-degenerate limit, are explicitly given. We find that the two-pion exchange (short-range) effects can give a significant contribution to the emission rates, and are temperature dependent. Also, other short-range nuclear effects such as effective nucleon mass, polarization effects and use of correlated wave functions are discussed. The trend of all these nuclear effects is to diminish the corresponding axion emission rates. Furthermore, we estimate that the values of the emission rates calculated with the inclusion of all these effects can differ from the corresponding ones derived with constant nuclear matrix elements by a factor of ∼24. This leads to an uncertainty factor of ∼4.9 when extracting bounds of the axion parameters.

Current conservation and analytic determination of the magnetic moment of the Δ resonance in the πN bremsstrahlung: I. Formulation without quark degrees of freedom

The pion–nucleon bremsstrahlung π + N⟹γ′ + π′ + N′ is studied in a new form of current conservation. According to this condition, the part of the on shell external particle πN radiation amplitude and the corresponding on shell internal particle radiation amplitude cancel each other. In particular, it is shown that the double Δ exchange diagram with the Δ − γ′Δ′ vertex cancels against the appropriate part of the external particle radiation diagrams. This cancellation determines the Δ − γΔ form factors through the Δ − πN vertices. Consequently, a model-independent relation between the magnetic dipole moments of the Δ+ and Δ++ resonances and the full magnetic moment of the proton μp is obtained, where μΔ is expressed by μp as and in agreement with the values extracted from the experimental cross section of the π+p → γ′π+p reaction.

Neutrino oscillation in a magnetized gamma-ray burst fireball

Neutrinos of energies of about 5--20 MeV are produced due to the stellar collapse or merger events that trigger the gamma-ray burst. Also low energy MeV neutrinos are produced within the fireball due to electron-positron annihilation and nucleonic bremsstrahlung. Many of these neutrinos will propagate through the dense and relativistic magnetized plasma of the fireball. We have studied the possibility of resonant oscillation of ${\ensuremath{\nu}}_{e}\ensuremath{\leftrightarrow}{\ensuremath{\nu}}_{\ensuremath{\mu},\ensuremath{\tau}}$ by taking into account the neutrino oscillation parameters from SNO, super Kamiokande, and liquid scintillator detector. Using the resonance condition, we have calculated the resonance length for these neutrinos and also for the fireball observables, like lepton asymmetry and the baryon load that are estimated based on the assumed fireball radius of 100 km.

Pion mass effects on axion emission from neutron stars through NN bremsstrahlung processes

The rates of axion emission by nucleon–nucleon bremsstrahlung are calculated with the inclusion of the full momentum contribution from a nuclear one pion exchange (OPE) potential. The contributions of the neutron–neutron ( nn), proton–proton ( pp) and neutron–proton ( np) processes in both the non-degenerate and degenerate limits are explicitly given. We find that the finite-momentum corrections to the emissivities are quantitatively significant for the non-degenerate regime and temperature-dependent, and should affect the existing axion mass bounds. The trend of these nuclear effects is to diminish the emissivities.

Bounds on large extra dimensions from the photon fusion process in SN1987A

The constraint on the Arkani-Hamed, Dimopoulos and Dvali model of extra dimensionscoming from photon annihilation into Kaluza–Klein gravitons in supernova cores isrevisited. In the two-photon process, for a conservative choice of the core parameters, weobtain the bound on the fundamental Planck scale TeV. The combined rate of energy loss due to nucleon–nucleon bremsstrahlung and photon annihilationprocesses is rederived, and it shows that the combined bounds add only a second decimal place toM*. The present study may strengthen the results that are available in thecurrent literature for the graviton emission from SN1987A, which puts verystrong constraints on models with large extra dimensions for the case ofn = 3.

Magnetohydrodynamics of Neutrino-Cooled Accretion Tori around a Rotating Black Hole in General Relativity

We present our first numerical results of axisymmetric magnetohydrodynamic simula- tions for neutrino-cooled accretion tori around rotating black holes in general relativity. We consider tori of mass ∼ 0.1-0.4Maround a black hole of mass M =4 Mand spin a =0 - 0.9M ; such systems are candidates for the central engines of gamma-ray bursts (GRBs) formed after the collapse of massive rotating stellar cores and the merger of a black hole and a neutron star. In this paper, we consider the short-term evolution of a torus for a duration of ≈ 60 ms, focusing on short-hard GRBs. Simulations were performed with a plausible microphysical equation of state that takes into account neutronization, the nuclear statistical equilibrium of a gas of free nucleons and α-particles, black body radiation, and a relativistic Fermi gas (neutrinos, electrons, and positrons). Neutrino-emission processes, such as e ± capture onto free nucleons, e ± pair annihilation, plasmon decay, and nucleon- nucleon bremsstrahlung are taken into account as cooling processes. Magnetic braking and the magnetorotational instability in the accretion tori play a role in angular momentum redistribution, which causes turbulent motion, resultant shock heating, and mass accretion onto the black hole. The mass accretion rate is found to be ˙ M∗ ∼ 1-10M� /s, and the shock heating increases the temperature to ∼ 10 11 K. This results in a maximum neutrino emission rate of Lν = several ×10 53 ergs/s and a conversion efficiency Lν / ˙ M∗c 2 on the order of a few percent for tori with mass Mt ≈ 0.1-0.4Mand for moderately high black hole spins. These results are similar to previous results in which the phenomenological α-viscosity prescription with the α-parameter of αv =0 .01-0.1 is used. It is also found that the neutrino luminosity can be enhanced by the black hole spin, in particular for large spins, i.e., a & 0.75M ;i f the accretion flow is optically thin with respect to neutrinos, the conversion efficiency may be & 10% for a & 0.9M. Angular momentum transport, and the resulting shock heating caused by magnetic stress induce time-varying neutrino luminosity, which is a favorable property for explaining the variability of the luminosity curve of GRBs.

Proton–proton bremsstrahlung towards the elastic limit at 190 MeV incident beam energy

A series of nucleon–nucleon bremsstrahlung (NNγ) experiments at 190 MeV incident beam energy have been performed at KVI in order to gain more insight into the dynamics governing the bremsstrahlung reaction. After initial measurements wherein the bremsstrahlung process was studied far away from the elastic limit, a new study was used to probe the process nearer to the elastic limit by measuring at lower photon energies. Measured cross sections and analyzing powers are compared with the predictions of a microscopic model and those of two soft-photon models. The theoretical calculations overestimate the data by up to ≈30%, for some kinematics.

Monte Carlo Study of Supernova Neutrino Spectra Formation

The neutrino flux and spectra formation in a supernova core is studied by using a Monte Carlo code. The dominant opacity contribution forl is elastic scattering on nucleonslN ! Nl, wherel always stands for eitherl or � � . In addition, we switch on or off a variety of processes that allow for the exchange of energy or the creation and destruction of neutrino pairs, notably nucleon bremsstrahlung NN ! NNl � � l, the pair annihilation processes e þ e � ! � l � � l ande � � e ! � ll, recoil and weak magnetism in elastic nucleon scatter- ing, elastic scattering on electronsle � ! e � � l, and elastic scattering on electron neutrinos and antineutri- nosle ! � el andl � � e ! � � el. The least important processes are neutrino-neutrino scattering and e + e � annihilation. The formation of the spectra and fluxes ofl is dominated by the nucleonic processes, i.e., bremsstrahlung and elastic scattering with recoil, but alsoee annihilation andlescattering contribute significantly. When all processes are included, the spectral shape of the emitted neutrino flux is always '' pinched,'' i.e., the width of the spectrum is smaller than that of a thermal spectrum with the same average energy. In all of our cases we find that the averagel energy exceeds the average � � e energy by only a small amount, 10% being a typical number. Weak-magnetism effects cause the opacity ofl to differ slightly from that of � � l, translating into differences of the luminosities and average energies of a few percent. Depending on the density, temperature, and composition profile, the flavor-dependent luminosities Le , Le , and Ll can mutually differ from each other by up to a factor of 2 in either direction. Subject headings: diffusion — neutrinos — scattering — supernovae: general

Electron Neutrino Pair Annihilation: A New Source for Muon and Tau Neutrinos in Supernovae

We show that in a supernova core the annihilation process nu_e nu_e-bar -> nu_{mu,tau} nu_{mu,tau}-bar is always more important than the traditional reaction e^+ e^- -> nu_{mu,tau} nu_{mu,tau}-bar as a source for muon and tau neutrino pairs. We study the impact of the new process by means of a Monte Carlo transport code with a static stellar background model and by means of a self-consistent hydrodynamical simulation with Boltzmann neutrino transport. Nucleon bremsstrahlung NN -> NN nu_{mu,tau} nu_{mu,tau}-bar is also included as another important source term. Taking into account nu_e nu_e-bar -> nu_{mu,tau} nu_{mu,tau}-bar increases the nu_mu and nu_tau luminosities by as much as 20% while the spectra remain almost unaffected. In our hydrodynamical simulation the shock was somewhat weakened. Elastic nu_{mu,tau} nu_e and nu_{mu,tau} nu_e scattering is not negligible but less important than nu_{mu,tau} e^+ or e^- scattering. Its influence on the nu_{mu,tau} fluxes and spectra is small after all other processes have been included.