arXiv subjects

Bijaya Acharya

Publications and source records attributed to Bijaya Acharya.

16 recordsLinked to original sources

arXiv2024

16O electroweak response functions from first principles

We present calculations of various electroweak response functions for the 16O nucleus obtained using coupled-cluster theory in conjunction with the Lorentz integral transform method. We employ nuclear forces derived at next-to-leading order and next-to-next-to-leading order in chiral effective field theory and perform a Bayesian analysis to assess uncertainties. Our results are in good agreement with available electron-scattering data at q~ 326 MeV/c. Additionally, we provide several predictions for the weak response functions in the quasi-elastic peak region at q= 300 and 400 MeV/c, which are critical for long-baseline neutrino experiments.

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arXiv2024

Uncertainty estimation and anomaly detection in chiral effective field theory studies of key nuclear electroweak processes

Chiral effective field theory ($χ$EFT) is a powerful tool for studying electroweak processes in nuclei. I discuss $χ$EFT calculations of three key nuclear electroweak processes: primordial deuterium production, proton-proton fusion, and magnetic dipole excitations of $^{48}\mathrm{Ca}$. This article showcases $χ$EFT's ability to quantify theory uncertainties at the appropriate level of rigor for addressing the different precision demands of these three processes.

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arXiv2023

$^{40}$Ca transverse response function from coupled-cluster theory

We present calculations of the $^{40}$Ca transverse response function obtained from coupled-cluster theory used in conjunction with the Lorentz integral transform method. We employ nuclear forces derived at next-to-next-to leading order in chiral effective field theory with and without $Δ$ degrees of freedom. We first benchmark this approach on the $^4$He nucleus and compare both the transverse sum rule and the response function to earlier calculations based on different methods. As expected from the power counting of the chiral expansion of electromagnetic currents and from previous studies, our results retaining only one-body term underestimate the experimental data for $^4$He by about $20\%$. However, when the method is applied to $^{40}$Ca at the same order of the expansion, response functions do not lack strength and agree well with the world electron scattering data. We discuss various sources of theoretical uncertainties and comment on the comparison of our results with the available experiments.

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arXiv2023

Revisiting proton-proton fusion in chiral effective field theory

We calculate the $S$-factor for proton-proton fusion using chiral effective field theory interactions and currents. By performing order-by-order calculations with a variety of chiral interactions that are regularized and calibrated in different ways, we assess the uncertainty in the $S$-factor from the truncation of the effective field theory expansion and from the sensitivity of the $S$-factor to the short-distance axial current determined from three- and four-nucleon observables. We find that $S(0)=(4.100\pm0.019\mathrm{(syst)}\pm0.013\mathrm{(stat)}\pm0.008(g_A))\times10^{-23}~\mathrm{MeV\,fm}^2\,,$ where the three uncertainties arise, respectively, from the truncation of the effective field theory expansion, use of the two-nucleon axial current fit to few-nucleon observables and variation of the axial coupling constant within the recommended range. The increased value of $S(0)$ compared to previous calculations is mainly driven by an increase in the recommended value for the axial coupling constant and is in agreement with a recent analysis based on pionless effective field theory.

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arXiv2022

Muon capture on the deuteron in chiral effective field theory

We consider the capture of a muon on a deuteron. An uncertainty analysis of the dominant channels is important for a careful analysis of forthcoming experimental data. We quantify the theoretical uncertainties of chiral effective-field-theory predictions of the muon-deuteron capture rate from the relevant neutron-neutron partial wave channels in the final state. We study the dependence on the cutoff used to regularize the interactions, low-energy constants calibrated using different fitting data and strategies, and truncation of the effective-field-theory expansion of the currents. Combining these approaches gives as an estimate of $Γ^{1/2}_{μd} = 399.1 \pm 7.6 \pm 4.4$ s$^{-1}$ for capture from the atomic doublet state, and $Γ^{3/2}_{μd} = 12.31 \pm 0.47 \pm 0.04$ s$^{-1}$ for capture from the quartet state.

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arXiv2022

Uncertainty quantification in electromagnetic observables of nuclei

We present strategies to quantify theoretical uncertainties in modern ab-initio calculations of electromagnetic observables in light and medium-mass nuclei. We discuss how uncertainties build up from various sources, such as the approximations introduced by the few- or many-body solver and the truncation of the chiral effective field theory expansion. We review the recent progress encompassing a broad range of electromagnetic observables in stable and unstable nuclei.

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arXiv2022

Bayesian analysis of nuclear polarizability corrections to the Lamb shift of muonic H-atoms and He-ions

The extraction of nuclear charge radii from spectroscopy experiments in muonic atoms is currently limited by the large uncertainties associated with the theoretical evaluation of the nuclear polarizability effects. To facilitate calculations, these polarizability corrections are conventionally expressed as an expansion in a dimensionless parameter $η$, which has been argued in previous literature to hold an approximate value of 0.33 in light-nuclear systems. In this work, we check this claim by doing a Bayesian analysis of the nuclear-polarizability corrections to the Lamb shift in $μ^2$H and $μ^3$H atoms and in $μ^3$He$^+$ and $μ^4$He$^+$ ions at various orders in the $η$-expansion. Our analysis supports the claim that $η\ll 1$ in these systems and finds truncation uncertainties that are similar to the previous estimate, the only exception being the truncation uncertainties in the $μ^3$He$^+$ ion, which are found to be larger.

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arXiv2022

Final-state interactions and spin structure in $E1$ breakup of $^11$Li in Halo EFT

We calculate the $E1$ breakup of the $2n$ halo nucleus $^{11}$Li in Halo Effective Field Theory (Halo EFT) at leading order. In Halo EFT, $^{11}$Li is treated as a three-body system of a $^{9}$Li core and two neutrons. We present a detailed investigation of final-state interactions (FSI) in the neutron-neutron $(nn)$ and neutron-core $(nc)$ channels. We employ Moller operators to formulate an expansion scheme that satisfies the non-energy-weighted cluster sum rule and successively includes higher-order terms in the multiple-scattering series for the FSI. Computing the $E1$ strength up to third order in this scheme, we observe apparent convergence and good agreement with experiment. The neutron-neutron FSI is by far the most important contribution and largely determines the maximum value of the $E1$ distribution. However, inclusion of $nc$ FSI does shift the peak position to slightly lower energies. Moreover, we investigate the sensitivity of the $E1$ response to the spin structure of the neutron-${}^9$Li interaction. We contrast results for an interaction that is the same in the spin-1 and spin-2 channels with one that is only operative in the spin-2 channel, and find that good agreement with experimental data is only obtained if the interaction is present in both spin channels. The latter case is shown to be equivalent to a calculation in which the spin of $^9$Li is neglected.

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arXiv2022

Gaussian process error modeling for chiral effective-field-theory calculations of $np\leftrightarrow dγ$ at low energies

We calculate the energy-dependent cross section of the $np\leftrightarrow dγ$ process in chiral effective field theory and apply state-of-the-art tools for quantification of theory uncertainty. We focus on the low-energy regime, where the magnetic dipole and the electric dipole transitions cross over, including the range relevant for big-bang nucleosynthesis. Working with the leading one- and two-body electromagnetic currents, we study the order-by-order convergence of this observable in the chiral expansion of the nuclear potential. We find that the Gaussian process error model describes the observed convergence very well, allowing us to present Bayesian credible intervals for the truncation error with correlations between the cross sections at different energies taken into account. We obtain a 1$σ$ estimate of about 0.2\% for the uncertainty from the truncation of the nuclear potential. This is an important step towards calculations with statistically interpretable uncertainties for astrophysical reactions involving light nuclei.

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arXiv2021

Dispersive evaluation of the Lamb shift in muonic deuterium from chiral effective field theory

We merge the dispersive relation approach and the ab initio method to compute nuclear structure corrections to the Lamb shift in muonic deuterium. We calculate the deuteron response functions and corresponding uncertainties up to next-to-next-to-next-to-leading order in chiral effective field theory and compare our results to selected electromagnetic data to test the validity of the theory. We then feed response functions calculated over a wide range of kinematics to the dispersion-theory formalism and show that an improved accuracy is obtained compared to that with the use of available experimental data in the dispersive analysis. This opens up the possibility of applying this hybrid method to other light muonic atoms and supplementing experimental data with ab initio theory for kinematics where data are scarce or difficult to measure with the goal of reducing uncertainties in estimates of nuclear structure effects in atomic spectroscopy.

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arXiv2020

Neutrino-deuteron scattering: Uncertainty quantification and new $L_{1,A}$ constraints

We study neutral- and charged-current (anti)neutrino-induced dissociation of the deuteron at energies from threshold up to 150 MeV by employing potentials, as well as one- and two-body currents, derived in chiral effective field theory ($χ$EFT). We provide uncertainty estimates from $χ$EFT truncations of the electroweak current, dependences on the $χ$EFT cutoff and variations in the pool of fit data used to fix the low-energy constants of $χ$EFT. At 100 MeV of incident (anti)neutrino energy, these uncertainties amount to about 2-3\% and are smaller than the sensitivity of the cross sections to the single-nucleon axial form factor, which amounts to 5\% if one varies the range of the nucleon axial radius within the bands determined by recent lattice quantum chromodynamics evaluations and phenomenological extractions. We conclude that a precise determination of the nucleon axial form factor is required for a high-precision calculation of the neutrino-deuteron cross sections at energies higher than 100 MeV. By matching our low-energy $χ$EFT results to those of pionless effective field theory ($\cancelπ$EFT), we provide new constraints for the counterterm $L_{1,A}$ that parameterizes the strength of the axial two-body current in $\cancelπ$EFT. We obtain a value of $4.9^{+1.9}_{-1.5}\mathrm{fm}^3$ at renormalization scale set to pion mass, which is compatible with, albeit narrower than, previous experimental determinations, and comparable to a recent lattice quantum chromodynamics calculation.

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arXiv2019

Universal behavior of $p$-wave proton-proton fusion near threshold

We calculate the $p$-wave contribution to the proton-proton fusion $S$ factor and its energy derivative in pionless effective field theory (EFT) up to next-to-leading order. The leading contributions are given by a recoil piece from the Gamow-Teller and Fermi operators, and from relativistic $1/m$ suppressed weak interaction operators. We obtain the value of $(2.5\pm0.3 )\times 10^{-28}~\mathrm{MeV\ fm^2}$ for the $S$ factor and $(2.2\pm0.2) \times 10^{-26}~\mathrm{fm^2}$ for its energy derivative at threshold. These are smaller than the results of a prior study that employed chiral EFT by several orders of magnitude. We conclude that, contrary to what has been previously reported, the $p$-wave contribution does not need to be considered in a high-precision determination of the $S$ factor at astrophysical energies. Combined with the chiral EFT calculation of Acharya {\it et al.} [Phys. Lett. B \bf{760}, 584 (2016)] for the $s$-wave channel, this gives a total threshold $S$ factor of $S(0) = (4.047^{+0.024}_{-0.032}) \times 10^{-23}~{\rm MeV~fm}^2$.

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arXiv2019

Effective-field-theory predictions of the muon-deuteron capture rate

We quantify the theoretical uncertainties of chiral effective-field-theory predictions of the muon-deuteron capture rate. Theoretical error estimates of this low-energy process is important for a reliable interpretation of forthcoming experimental results by the MuSun collaboration. Specifically, we estimate the three dominant sources of uncertainties that impact theoretical calculations of this rate: those resulting from uncertainties in the pool of fit data used to constrain the coupling constants in the nuclear interaction, those due to the truncation of the effective field theory, and those due to uncertainties in the axial radius of the nucleon. For the capture rate into the ${}^1S_0$ channel, we find an uncertainty of approximately $4.6~s^{-1}$ due to the truncation in the effective field theory and an uncertainty of $3.9~s^{-1}$ due to the uncertainty in the axial radius of the nucleon, both of which are similar in size to the targeted experimental precision.

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arXiv2017

The Efimov effect for heteronuclear three-body systems at positive scattering length and finite temperature

We study the recombination process of three atoms scattering into an atom and diatomic molecule in heteronuclear mixtures of ultracold atomic gases with large and positive interspecies scattering length at finite temperature. We calculate the temperature dependence of the three-body recombination rates by extracting universal scaling functions that parametrize the energy dependence of the scattering matrix. We compare our results to experimental data for the 40K-87Rb mixture and make a prediction for 6Li-87Rb. We find that contributions from higher partial wave channels significantly impact the total rate and, in systems with particularly large mass imbalance, can even obliterate the recombination minima associated with the Efimov effect.

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arXiv2016

An effective-field-theory analysis of Efimov physics in heteronuclear mixtures of ultracold atomic gases

We use an effective-field-theory framework to analyze the Efimov effect in heteronuclear three-body systems consisting of two species of atoms with a large interspecies scattering length. In the leading-order description of this theory, various three-body observables in heteronuclear mixtures can be universally parameterized by one three-body parameter. We present the next-to-leading corrections, which include the effects of the finite interspecies effective range and the finite intraspecies scattering length, to various three-body observables. We show that only one additional three-body parameter is required to render the theory predictive at this order. By including the effective range and intraspecies scattering length corrections, we derive a set of universal relations that connect the different Efimov features near the interspecies Feshbach resonance. Furthermore, we show that these relations can be interpreted in terms of the running of the three-body counterterms that naturally emerge from proper renormalization. Finally, we make predictions for recombination observables of a number of atomic systems that are of experimental interest.

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arXiv2015

Properties of Lithium-11 and Carbon-22 at leading order in halo effective field theory

We study the $^{11}\mathrm{Li}$ and $^{22}\mathrm{C}$ nuclei at leading order (LO) in halo effective field theory (Halo EFT). Using the value of the $^{22}\mathrm{C}$ rms matter radius deduced in Ref. [1] as an input in a LO calculation, we simultaneously constrained the values of the two-neutron (2$n$) separation energy of $^{22}\mathrm{C}$ and the virtual-state energy of the $^{20}\mathrm{C}-$neutron system (hereafter denoted $^{21}$C). The 1$-σ$ uncertainty of the input rms matter radius datum, along with the theory error estimated from the anticipated size of the higher-order terms in the Halo EFT expansion, gave an upper bound of about 100 keV for the 2$n$ separation energy. We also study the electric dipole excitation of 2$n$ halo nuclei to a continuum state of two neutrons and the core at LO in Halo EFT. We first compare our results with the $^{11}\mathrm{Li}$ data from a Coulomb dissociation experiment and obtain good agreement within the theoretical uncertainty of a LO calculation. We then obtain the low-energy spectrum of $B(E1)$ of this transition at several different values of the 2$n$ separation energy of $^{22}\mathrm{C}$ and the virtual-state energy of $^{21}\mathrm{C}$. Our predictions can be compared to the outcome of an ongoing experiment on the Coulomb dissociation of $^{22}\mathrm{C}$ to obtain tighter constraints on the two- and three-body energies in the $^{22}\mathrm{C}$ system.

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