arXiv subjects

C. J. Horowitz

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

At least 19 recordsLinked to original sources

arXiv2026

Precision Neutron Skins of $^{208}$Pb and $^{48}$Ca from Parity-Violating Electron Scattering

We have measured the parity-violating elastic electron scattering asymmetry in the PREX and CREX experiments on ${}^{208}$Pb and ${}^{48}$Ca respectively; these are both doubly-magic nuclei whose excited states can be discriminated from the ground state by the high resolution spectrometers in Hall A at Jefferson Lab. This asymmetry provides a precise determination of the weak charge form factor at one $Q^2$ and pins down the neutron radius in these two nuclei in a relatively clean and model-independent way. This is because the $Z^0$ boson of the weak interaction couples primarily to neutrons. The heavier lead nucleus, with a neutron excess, provides an interpretation of the neutron skin thickness in terms of properties of bulk neutron matter. For the lighter ${}^{48}$Ca nucleus, which is also rich in neutrons, comparisons to microscopic nuclear theory calculations are sensitive to poorly constrained 3-neutron forces. The weak neutral form factors $F_W(Q^2)$ were extracted to be $0.368 \pm 0.013$ at $Q = 0.3977 {\rm\ fm}^{-1}$ for $^{208}$Pb from PREX-2 and $0.1304 \pm 0.0055$ at $Q = 0.8733 {\rm\ fm}^{-1}$ for $^{48}$Ca. The form factor differences $(F_{ch}-F_W)(Q^2)$ were calculated to be $0.041 \pm 0.013$ at $Q = 0.3977 {\rm fm}^{-1}$ for $^{208}$Pb from PREX-2 and $0.0277 \pm 0.0055$ at $Q = 0.8733 {\rm fm}^{-1}$ for $^{48}$Ca. Correcting for Coulomb distortions and using nuclear model information, we find the neutron skin thicknesses to be $R^{208}_{\rm skin} = 0.283 \pm 0.071$ fm combining PREX-1 and PREX-2 and $R^{48}_{\rm skin} = 0.121 \pm 0.035$ fm. This paper provides a full description of the special experimental and data analysis techniques employed for precisely measuring these small asymmetries.

nucl-ex↗

arXiv2026

Electroweak Radiative Corrections to Parity-Violating Electron-Nucleus Scattering

Parity-violating electron-scattering provides a largely model independent way of measuring neutron densities in nuclei that has important implications for the structure of nuclei and neutron stars. In this paper we calculate radiative corrections to the parity-violating asymmetry $A_{\rm pv}$ in electron-nucleus scattering including vertex and vacuum polarization contributions. We find large cancellations between the vertex corrections to the vector and axial-vector vertices. As a result the total radiative correction in 2nd Born approximation is dominated by vacuum polarization and is of order -0.5\%. Coulomb distortions have modest effects on radiative corrections for $^{12}$C and $^{48}$Ca nuclei. For $^{208}$Pb Coulomb distortions reduce the total radiative correction to only about 0.1\%. Therefore, these corrections are not important for the interpretation of the PREX and MREX experiments on $^{208}$Pb and for the CREX experiment on $^{48}$Ca. However, radiative corrections must be carefully included for a precision measurement of the weak charge of $^{12}$C.

nucl-th↗

arXiv2026

Comment on QED Corrections to the Parity Violating Asymmetry in High-Energy Electron-Nucleus Scattering

In a recent letter Roca-Maza and Jakubassa-Amundsen claim a large $\approx 5\%$ radiative correction to the parity-violating asymmetry $A_{pv}$ in electron-nucleus scattering. If true, this would be important for the interpretation of the PREX, CREX, and MREX experiments. However, Roca-Maza and Jakubassa-Amundsen do not explicitly include the axial-vector vertex correction to $Z^0$ exchange (at the momentum transfer of the experiment). We show that including this cancels much of the change in $A_{pv}$.

nucl-th↗

arXiv2025

Bulk viscosity from neutron decays to dark baryons in neutron star matter

The existence of a dark baryon that mixes with the neutron leads to the possibility of neutron decay into a dark sector. Such dark decays have been studied as possibly relevant for the neutron decay anomaly and for their potential impacts on neutron stars. The most popular formulation is a dark sector consisting of a dark baryon $\chi$ and a dark scalar $\phi$, where a neutron in vacuum decays 1% or less of the time via the channel $n\rightarrow \chi+\phi$. In this work, we consider the effect of this additional neutron decay channel on transport in neutrons star mergers. We find that the neutron dark decay rate in medium is quite slow, and thus the dark baryons modify the dense matter equation of state in a way that decreases the Urca bulk viscosity by, at most, a factor of 2-3. However, if the neutron dark decay was to occur more rapidly, then the bulk viscosity at merger temperatures of tens of MeV would be strongly enhanced, potentially rapidly damping oscillations in merger environments and therefore providing a signature of slowly equilibrating matter in the merger.

astro-ph.HE↗

arXiv2024

Limiting Rotation Rate of Neutron Stars from Crust Breaking and Gravitational Waves

Neutron stars are not observed to spin faster than about half their breakup rate. This limiting rotational frequency may be related to the strength of their crusts. As a star spins up from accretion, centrifugal forces stress the crust. We perform finite-element simulations of rotating neutron stars and find that the crust fails at rotation rates about half the breakup rate. Given uncertainties in microphysics, we have not determined the crust configuration after this failure. Instead, we argue that the crust may fail in an asymmetric way and could produce a configuration with a significant ellipticity (fractional difference in moments of inertia). If the ellipticity is large, a rotating star will radiate gravitational waves that may limit further spin up. These stars may be promising sources for LIGO / VIRGO and next generation gravitational wave detectors.

astro-ph.HE↗

arXiv2024

Finite-Element Simulations of Rotating Neutron Stars with Anisotropic Crusts and Continuous Gravitational Waves

``Mountains'', or non-axisymmetrical deformations in the elastic crust of rotating neutron stars are efficient radiators of continuous gravitational waves. Recently, small anisotropies were observed in the solid innermost inner core of the Earth. We implement three-dimensional finite-element simulations to study mountains sourced by modest anisotropies in the solid crust of rotating neutron stars. We find that anisotropic mountains may be detectable by current ground-based gravitational-wave detectors and might explain several observed phenomena, confirming the results of a previous work on less realistic neutron star models. In particular, we find that a slightly anisotropic neutron star crust that changes its rotation rate modestly can support an ellipticity of a few $\times$ 10$^{-9}$, which is equivalent to the upper bounds on the ellipticity of some nearby and rapidlly spinning pulsars.

gr-qc↗

arXiv2024

Density Dependence of the Symmetry Energy in the Post PREX-CREX Era

The recently published CREX results suggest a rather peculiar picture for the density dependence of the symmetry energy. Whereas PREX favors a large neutron skin thickness in $^{208}$Pb, thereby suggesting a stiff equation of state, CREX suggests instead a much softer equation of state. This discrepancy has caused a large spur in the theoretical community since no model has been able to simultaneously reproduce within $1σ$ the PREX and CREX results. Motivated by a novel correlation between a CREX observable and a combination of bulk symmetry energy parameters, we calibrate three new covariant energy density functionals that reproduce binding energies and charge radii of spherical nuclei - and also accommodate the constraints imposed by PREX and CREX. Given that these models suggest a stiff equation of state at high densities, predictions for neutron star properties are also discussed.

nucl-th↗

arXiv2023

Anisotropic neutron star crust, solar system mountains, and gravitational waves

"Mountains" or non-axisymmetric deformations of rotating neutron stars (NS) efficiently radiate gravitational waves (GW). We consider analogies between NS mountains and surface features of solar system bodies. Both NS and moons such as Europa or Enceladus have thin crusts over deep oceans while Mercury has a thin crust over a large metallic core. Thin sheets may wrinkle in universal ways. Europa has linear features, Enceladus has "Tiger" stripes, and Mercury has lobate scarps. NS may have analogous features. The innermost inner core of the Earth is anisotropic with a shear modulus that depends on direction. If NS crust material is also anisotropic this will produce an ellipticity, when the crust is stressed, that grows with spin frequency. This yields a braking index (log derivative of spin down rate assuming only GW spin down) very different from $n=5$ and could explain the maximum spin observed for neutron stars and a possible minimum ellipticity of millisecond pulsars.

astro-ph.HE↗

arXiv2023

Electron Scattering and Neutrino Physics

A thorough understanding of neutrino-nucleus scattering physics is crucial for the successful execution of the entire US neutrino physics program. Neutrino-nucleus interaction constitutes one of the biggest systematic uncertainties in neutrino experiments - both at intermediate energies affecting long-baseline Deep Underground Neutrino Experiment (DUNE), as well as at low energies affecting coherent scattering neutrino program - and could well be the difference between achieving or missing discovery level precision. To this end, electron-nucleus scattering experiments provide vital information to test, assess and validate different nuclear models and event generators intended to be used in neutrino experiments. In this white paper, we highlight connections between electron- and neutrino-nucleus scattering physics at energies ranging from 10s of MeV to a few GeV, review the status of ongoing and planned electron scattering experiments, identify gaps, and layout a path forward that benefits the neutrino community. We also highlight the systemic challenges with respect to the divide between the nuclear and high-energy physics communities and funding that presents additional hurdle in mobilizing these connections to the benefit of neutrino programs.

hep-ex↗

arXiv2023

Detectability of Sub-Solar Mass Neutron Stars Through a Template Bank Search

We study the detectability of gravitational-wave signals from sub-solar mass binary neutron star systems by the current generation of ground-based gravitational-wave detectors. We find that finite size effects from large tidal deformabilities of the neutron stars and lower merger frequencies can significantly impact the sensitivity of the detectors to these sources. By simulating a matched-filter based search using injected binary neutron star signals with tidal deformabilities derived from physically motivated equations of state, we calculate the reduction in sensitivity of the detectors. We conclude that the loss in sensitive volume can be as high as $78.4 \%$ for an equal mass binary system of chirp mass $0.17 \, \textrm{M}_{\odot}$, in a search conducted using binary black hole template banks. We use this loss in sensitive volume, in combination with the results from the search for sub-solar mass binaries conducted on data collected by the LIGO-Virgo observatories during their first three observing runs, to obtain a conservative upper limit on the merger rate of sub-solar mass binary neutron stars. Since the discovery of a low-mass neutron star would provide new insight into formation mechanisms of neutron stars and further constrain the equation of state of dense nuclear matter, our result merits a dedicated search for sub-solar mass binary neutron star signals.

astro-ph.HE↗

arXiv2022

Neutron Star Crust Can Support A Large Ellipticity

Non-axisymmetrical deformations of the crust on rapidly rotating neutron stars are one of the main targets of searches for continuous gravitational waves. The maximum ellipticity, or fractional difference in moments of inertia, that can be supported by deformations of the crust (known as "mountains") provides an important upper limit on the strength of these continuous gravitational wave sources. We use the formalism of Gittins et al 2021, along with a deforming force that acts mainly in the transverse direction, to obtain a maximum ellipticity of 7.4$\times$10$^{-6}$. This is larger than the original results of Gittins et al 2021 but consistent with earlier calculations by Ushomirsky et al 2000. This suggests that rotating neutron stars could be strong sources of continuous gravitational waves.

gr-qc↗

arXiv2022

Nuclear fission reaction simulations in compact stars

Type-Ia supernovae (SN Ia) are powerful stellar explosions that provide important distance indicators in cosmology. Recently, we proposed a new SN Ia mechanism that involves a nuclear fission chain-reaction in an isolated white dwarf [PRL 126, 1311010]. Here we perform novel reaction network simulations of the actinide-rich first solids in a cooling white dwarf. The network includes neutron-capture and fission reactions on a range of U and Th isotopes with various possible values for U-235 enrichment. We find, for modest U-235 enrichments, neutron-capture on U-238 and Th-232 can breed additional fissile nuclei so that a significant fraction of all U and Th nuclei may fission during the chain-reaction. The resulting large energy release could ignite thermonuclear carbon burning and possibly trigger a SN Ia.

astro-ph.SR↗

arXiv2022

New Measurements of the Beam-Normal Single Spin Asymmetry in Elastic Electron Scattering Over a Range of Spin-0 Nuclei

We report precision determinations of the beam normal single spin asymmetries ($A_n$) in the elastic scattering of 0.95 and 2.18~GeV electrons off $^{12}$C, $^{40}$Ca, $^{48}$Ca, and $^{208}$Pb at very forward angles where the most detailed theoretical calculations have been performed. The first measurements of $A_n$ for $^{40}$Ca and $^{48}$Ca are found to be similar to that of $^{12}$C, consistent with expectations thus demonstrating the validity of theoretical calculations for nuclei with Z~$\leq20$. We also report $A_n$ for $^{208}$Pb at two new momentum transfers (Q$^2$) extending the previous measurement. Our new data confirm the surprising result previously reported, with all three data points showing significant disagreement with the results from the $Z\leq 20$ nuclei. These data confirm our basic understanding of the underlying dynamics that govern $A_n$ for nuclei containing $\lesssim 50$ nucleons, but point to the need for further investigation to understand the unusual $A_n$ behaviour discovered for scattering off $^{208}$Pb.

nucl-ex↗

arXiv2022

Ignition of carbon burning from nuclear fission in compact stars

Type-Ia supernovae (SN Ia) are powerful stellar explosions that provide important distance indicators in cosmology. Recently, we proposed a new SN Ia mechanism that involves a nuclear fission chain reaction in an isolated white dwarf (WD) [PRL 126, 1311010]. The first solids that form as a WD starts to freeze are actinide rich and potentially support a fission chain reaction. In this letter we explore thermonuclear ignition from fission heating. We perform thermal diffusion simulations and find at high densities, above about 7x10^8 g/cm^3, that the fission heating can ignite carbon burning. This could produce a SN Ia or another kind of astrophysical transient.

astro-ph.SR↗

arXiv2022

Precision Determination of the Neutral Weak Form Factor of $^{48}$Ca

We report a precise measurement of the parity-violating asymmetry $A_{\rm PV}$ in the elastic scattering of longitudinally polarized electrons from $^{48}{\rm Ca}$. We measure $A_{\rm PV} =2668\pm 106\ {\rm (stat)}\pm 40\ {\rm (syst)}$ parts per billion, leading to an extraction of the neutral weak form factor $F_{\rm W} (q=0.8733$ fm$^{-1}) = 0.1304 \pm 0.0052 \ {\rm (stat)}\pm 0.0020\ {\rm (syst)}$ and the charge minus the weak form factor $F_{\rm ch} - F_{\rm W} = 0.0277\pm 0.0055$. The resulting neutron skin thickness $R_n-R_p=0.121 \pm 0.026\ {\rm (exp)} \pm 0.024\ {\rm (model)}$~fm is relatively thin yet consistent with many model calculations. The combined CREX and PREX results will have implications for future energy density functional calculations and on the density dependence of the symmetry energy of nuclear matter.

nucl-ex↗

arXiv2022

First Determination of the 27Al Neutron Distribution Radius from a Parity-Violating Electron Scattering Measurement

We report the first measurement of the parity-violating elastic electron scattering asymmetry on 27Al. The 27Al elastic asymmetry is A_PV = 2.16 +- 0.11 (stat) +- 0.16 (syst) ppm, and was measured at =0.02357 +- 0.0001 GeV^2, = 7.61 +- 0.02 degrees, and = 1.157 GeV with the Qweak apparatus at Jefferson Lab. Predictions using a simple Born approximation as well as more sophisticated distorted-wave calculations are in good agreement with this result. From this asymmetry the 27Al neutron radius R_n = 2.89 +- 0.12 fm was determined using a many-models correlation technique. The corresponding neutron skin thickness R_n-R_p = -0.04 +- 0.12 fm is small, as expected for a light nucleus with a neutron excess of only 1. This result thus serves as a successful benchmark for electroweak determinations of neutron radii on heavier nuclei. A tree-level approach was used to extract the 27Al weak radius R_w = 3.00 +- 0.15 fm, and the weak skin thickness R_wk - R_ch = -0.04 +- 0.15 fm. The weak form factor at this Q^2 is F_wk = 0.39 +- 0.04.

nucl-ex↗

arXiv2022

Next Generation Observatories -- Report from the Dawn VI Workshop; October 5-7 2021

The workshop Dawn VI: Next Generation Observatories took place online over three days, 5-7 October, 2021. More than 200 physicists and astronomers attended to contribute to, and learn from, a discussion of next-generation ground-based gravitational-wave detectors. The program was centered on the next generation of ground-based gravitational-wave observatories and their synergy with the greater landscape of scientific observatories of the 2030s. Cosmic Explorer (CE), a concept developed with US National Science Foundation support, was a particular focus; Einstein Telescope (ET), the European next generation concept, is an important complement and partner in forming a network. The concluding summary of the meeting expressed the sentiment that the observational science accessible to CE and ET, also in combination with data from other non-GW observatories, will stimulate a very broad community of analysts and yield insights which are exciting given the access to GWs from the entire universe. The need, and desire, for closer collaboration between ET and CE was expressed; a three-detector network is optimal for delivering much of the science. The science opportunities afforded by CE and ET are broad and compelling, impacting a wide range of disciplines in physics and high energy astrophysics. There was a consensus that CE is a concept that can deliver the promised science. A strong endorsement of Cosmic Explorer, as described in the CE Horizon Study, is a primary outcome of DAWN VI.

astro-ph.IM↗

arXiv2022

Gravitational search for near Earth black holes or other compact dark objects

Primordial black holes, with masses comparable to asteroids, are an attractive possibility for dark matter. In addition, other forms of dark matter could form compact dark objects (CDO). We search for small tidal accelerations from low mass black holes or CDOs orbiting near the Earth, and find none. Using about 10 years of data from the superconducting gravimeters in the Black Forest Observatory in South-Western Germany and at Djougou, Northern Benin in Western Africa we set an upper limit on the maximum mass of any dark object orbiting the Earth as a function of orbital radius. For semi-major axis less than two earth radii we exclude all black holes or CDOs with masses larger than 6.7x10^{13} kg. Lower mass primordial black holes may be strongly constrained by Hawking radiation. We conclude that near Earth black holes are extremely unlikely.

gr-qc↗