arXiv subjects

C. J. Horowitz

Publications and source records attributed to C. J. Horowitz.

At least 163 records · Page 9Linked to original sources

arXiv2000

Parity Violating Measurements of Neutron Densities

Parity violating electron nucleus scattering is a clean and powerful tool for measuring the spatial distributions of neutrons in nuclei with unprecedented accuracy. Parity violation arises from the interference of electromagnetic and weak neutral amplitudes, and the $Z^0$ of the Standard Model couples primarily to neutrons at low $Q^2$. The data can be interpreted with as much confidence as electromagnetic scattering. After briefly reviewing the present theoretical and experimental knowledge of neutron densities, we discuss possible parity violation measurements, their theoretical interpretation, and applications. The experiments are feasible at existing facilities. We show that theoretical corrections are either small or well understood, which makes the interpretation clean. The quantitative relationship to atomic parity nonconservation observables is examined, and we show that the electron scattering asymmetries can be directly applied to atomic PNC because the observables have approximately the same dependence on nuclear shape.

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arXiv1999

Charge-conjugation violating neutrino interactions in supernovae

The well known charge conjugation violating interactions in the Standard Model increase neutrino- and decrease anti-neutrino- nucleon cross sections. This impacts neutrino transport in core collapse supernovae through "recoil" corrections of order the neutrino energy $k$ over the nucleon mass $M$. All $k/M$ corrections to neutrino transport deep inside a protoneutron star are calculated from angular integrals of the Boltzmann equation. We find these corrections significantly modify neutrino currents at high temperatures. This produces a large mu and tau number for the protoneutron star and can change the ratio of neutrons to protons. In addition, the relative size of neutrino mean free paths changes. At high temperatures, the electron anti-neutrino mean free path becomes {\it longer} than that for mu or tau neutrinos.

astro-ph↗

arXiv1999

High energy solar neutrinos and p-wave contributions to ^3He(p,\nue^+)^4He

High energy solar neutrinos can come from the hep reaction ^3He(p,\nue^+)^4He with a large end point energy of 18.8 MeV. Understanding the hep reaction may be important for interpreting solar neutrino spectra. We calculate the contribution of the axial charge transition $^3P_0\to ^1S_0$ to the hep thermonuclear S factor using a one-body reaction model involving a nucleon moving in optical potentials. Our result is comparable to or larger than previous calculations of the s-wave Gamow Teller contribution. This indicates that the hep reaction may have p-wave strength leading to an enhancement of the S factor.

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arXiv1999

Nucleosynthesis in Supernovae

Core collapse supernovae are dominated by energy transport from neutrinos. Therefore, some supernova properties could depend on symetries and features of the standard model weak interactions. The cross section for neutrino capture is larger than that for antineutrino capture by one term of order the neutrino energy over the nucleon mass. This reduces the ratio of neutrons to protons in the $ν$-driven wind above a protoneutron star by approximately 20 % and may significantly hinder r-process nucleosynthesis.

astro-ph↗

arXiv1998

The Mu and Tau Number of Supernovae

The neutrino-nucleon cross section is slightly larger than that for $\barν-N$. Therefore, $\barν$ will escape more quickly from core collapse supernovae leaving the star $ν$ rich. A diffusion formalism is used to calculate the time evolution of the mu and tau lepton number densities. These quickly reach steady state equilibria. We estimate that a protoneutron star with a maximum temperature near 50 MeV will contain over 50 % more $ν_μ$ and $ν_τ$ than $\barν_μ$ and $\barν_τ$. Supernovae may be the only known systems with large mu and or tau lepton numbers.

hep-ph↗

arXiv1998

Parity Violating Elastic Electron Scattering and Coulomb Distortions

Parity violating elastic electron-nucleus scattering provides an accurate and model independent measurement of neutron densities, because the $Z^0$ couples primarily to neutrons. Coulomb distortion corrections to the parity violating asymmetry $A_l$ are calculated exactly using a relativistic optical model. Distortions significantly reduce $A_l$ in a heavy nucleus. However even with distortions, an experiment to measure the neutron radius is feasible. This will aid the interpretation of future atomic parity violation measurements and provide fundamental nuclear structure information. Coulomb distortions and small differences between neutron and proton radii could be important for a standard model test on $^4$He, $^{12}$C or $^{16}$O.

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arXiv1997

Neutrino Annihilation in Stellar Magnetic Fields

The potential enhancement of the cross section $ν\barν\to e^+e^-$ in the presence of a magnetic field is of critical interest for the study of supernovae and as a possible mechanism for gamma ray bursts. While parity violation(PV) in this reaction in free space is forbidden by CP, the presence of a CP non-invariant background gas of electrons creates an asymmetry in the cross section which may contribute to the asymmetry of a supernova and the natal velocities of neutron stars. We calculate the cross section in the presence of fields as high as $B = 10^{16}$G and find no significant enhancement of the cross section with magnetic field strength. By studying the systematics of our results as a function of environmental variables(field strength, density,temperature), we extrapolate the relative strength of the parity violating terms to the weak field limit, and find the parity violation is insufficient to provide the ``kick'' required to explain the observed velocities of neutron stars.

astro-ph↗

arXiv1997

Cumulative Parity Violation in Supernovae

Supernovae provide a unique opportunity for large scale parity violation because they are dominated by neutrinos. We calculate the parity violating asymmetry A of neutrino emission in a strong magnetic field. We assume the neutrinos elastically scatter many times from slightly polarized neutrons. Because of multple interactions, A grows with the optical thickness of the proto-neutron star and may be much larger than previous estimates. As a result, the neutron star could recoil at a significant velocity.

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arXiv1997

Macroscopic Parity Violation and Supernova Asymmetries

Core collapse supernovae are dominated by weakly interacting neutrinos. This provides a unique opportunity for macroscopic parity violation. We speculate that parity violation in a strong magnetic field can lead to an asymmetry in the explosion and a recoil of the newly formed neutron star. We estimate the asymmetry from neutrino-polarized-neutron elastic scattering, polarized electron capture and neutrino-nucleus elastic scattering in a (partially) polarized electron gas.

hep-ph↗

arXiv1996

Neutrino Trapping in a Supernova and Ion Screening

Neutrino-nucleus elastic scattering is reduced in dense matter because of correlations between ions. The static structure factor for a plasma of electrons and ions is calculated from Monte Carlo simulations and parameterized with a least squares fit. Our results imply a large increase in the neutrino mean free path. This strongly limits the trapping of neutrinos in a supernova by coherent neutral current interactions.

astro-ph↗

arXiv1996

Detection of Atmospheric Neutrinos and Relativistic Nuclear Structure Effects

Neutrino-nucleus cross sections for the detection of atmospheric neutrinos are calculated in relativistic impulse and random phase approximations. Pion production is estimated via inclusive Delta-hole nuclear excitations. The number of pion events can confuse the identification of the neutrino flavor. As a further source of pions we calculate coherent pion production (where the nucleus remains in the ground state). We examine how these nuclear structure effects influence atmospheric neutrino experiments and study possible improvements in the present detector simulations.

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arXiv1995

Isospin-Violating Meson-Nucleon Vertices as an Alternate Mechanism of Charge-Symmetry Breaking

We compute isospin-violating meson-nucleon coupling constants and their consequent charge-symmetry-breaking nucleon-nucleon potentials. The couplings result from evaluating matrix elements of quark currents between nucleon states in a nonrelativistic constituent quark model; the isospin violations arise from the difference in the up and down constituent quark masses. We find, in particular, that isospin violation in the omega-meson--nucleon vertex dominates the class IV CSB potential obtained from these considerations. We evaluate the resulting spin-singlet--triplet mixing angles, the quantities germane to the difference of neutron and proton analyzing powers measured in elastic $\vec{n}-\vec{p}$ scattering, and find them commensurate to those computed originally using the on-shell value of the $ρ$-$ω$ mixing amplitude. The use of the on-shell $ρ$-$ω$ mixing amplitude at $q^2=0$ has been called into question; rather, the amplitude is zero in a wide class of models. Our model possesses no contribution from $ρ$-$ω$ mixing at $q^2=0$, and we find that omega-meson exchange suffices to explain the measured $n-p$ analyzing power difference~at~183 MeV.

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arXiv1995

Delta Excitations in Neutrino-Nucleus Scattering

We derive the contribution of $Δ$-h excitations to quasielastic charged-current neutrino-nucleus scattering in the framework of relativistic mean-field theory. We discuss the effect of $Δ$ production on the determination of the axial mass $M_A$ in neutrino scattering experiments.

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arXiv1995

Charge-Symmetry-Breaking Potentials from Isospin-Violating Meson-Baryon Coupling Constants

We consider charge-symmetry violations in the nucleon-nucleon force which result from isospin-violating meson-baryon coupling constants. The vector mesons are assumed to couple to the nucleon's electromagnetic current, which we decompose into isoscalar and isovector quark components. We compute these currents in the context of a constituent quark model. The isospin violations in the meson-baryon couplings arise from the difference in the up and down constituent quark masses. We show that class IV charge-symmetry-breaking potentials arise in the resulting $ω$ and $ρ$ exchange contributions to the $NN$ force. The magnitude of these contributions is consistent with that phenomenologically required by the measured difference of $n$ and $p$ analyzing powers in elastic $\vec{n}-\vec{p}$ scattering at 183 MeV.

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arXiv1995

Relativistic treatment of the charged-current reaction ${}^{12}{\rm C}(ν_μ,μ^{-})$ near threshold

We compute the cross section for the charged-current reaction ${}^{12}{\rm C}(ν_μ,μ^{-})$ near threshold using a relativistic mean-field formalism. A reduced value of the nucleon mass in the medium --- coupled to the finite muon mass --- leads to a 30\% reduction in the cross section relative to a free Fermi-gas estimate. Isovector RPA correlations, which are strongly repulsive at these momentum transfers, reduce the cross section by an additional 15-30\%. Hence, relativistic nuclear-structure effects can account for the more than a factor-of-two reduction in the cross section recently reported by the LSND collaboration.

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arXiv1994

Relativistic nuclear structure effects in quasielastic neutrino scattering

Charged-current cross sections are calculated for quasielastic neutrino and antineutrino scattering using a relativistic meson-nucleon model. We examine how nuclear-structure effects, such as relativistic random-phase-approximation (RPA) corrections and momentum-dependent nucleon self-energies, influence the extraction of the axial form factor of the nucleon. RPA corrections are important only at low-momentum transfers. In contrast, the momentum dependence of the relativistic self-energies changes appreciably the value of the axial-mass parameter, $M_A$, extracted from dipole fits to the axial form factor. Using Brookhaven's experimental neutrino spectrum we estimate the sensitivity of M$_A$ to various relativistic nuclear-structure effects.

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arXiv1994

Relativistic models for quasielastic $(e,e')$\ at large momentum transfers

Inclusive quasielastic response functions are calculated for electron scattering in a relativistic model including momentum dependent scalar and vector mean fields. The momentum dependence of the mean fields is taken from Dirac optical fits to proton nucleus scattering and is important in describing data at momentum transfers of 1 GeV/c or larger. Our simple model is applicable for quasielastic scattering over a large range of momentum transfers.

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arXiv1994

Relativistic treatment of spin-transfer observables in quasielastic $(\vec{\bf p},\vec{\bf n})$ scattering

We calculate all spin-transfer observables for the quasielastic $(\vec{p},\vec{n})$ reaction in a relativistic plane-wave impulse approximation. The nuclear-structure information is contained in a large set of nuclear-response functions that are computed in nuclear matter using a relativistic random-phase approximation to the Walecka model. A reduced value of the nucleon mass in the medium induces important dynamical changes in the residual isovector interaction relative to its nonrelativistic counterpart. As a result, good agreement is found for all spin observables --- including the spin-longitudinal to spin-transverse ratio --- when compared to the original ($q=1.72$~fm$^{-1}$) NTOF experiment. In contrast, the spin-longitudinal to spin-transverse ratio is underpredicted at $q=1.2$~fm$^{-1}$ and overpredicted at $q=2.5$~fm$^{-1}$. We comment on the role of distortions as a possible solution to this discrepancy.

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