arXiv subjects

Rajan Gupta

Publications and source records attributed to Rajan Gupta.

At least 19 recordsLinked to original sources

arXiv2026

Lattice QCD calculation of the pion-nucleon coupling $\bar{g}_0$ induced by the QCD $\Theta$-term

We present lattice QCD results for the CP violating pion-nucleon coupling $\bar{g}_0$ induced by the QCD $\overline\Theta$ term from the analysis of three 2+1+1-flavor ensembles generated with highly improved staggered quarks (HISQ) by the MILC collaboration. These ensembles are at lattice spacing $a\approx 0.09~\text{fm}$ and pion masses of 313, 226 and 138 MeV, respectively. The coupling $\bar{g}_0$ is extracted in two ways. First, from the matrix element of the correlation between the pesudoscalar current and the topological charge evaluated between the nucleon ground state. The data for correlation functions with both the pseudoscalar and axial vector exhibit large contamination from the $N\pi$ excited state. We show that these can be controlled at the leading order using chiral perturbation theory ($\chi$PT) and the axial Ward identity (AWI, also called the partially conserved axial current (PCAC) relation). The result after removing the $N\pi$ contamination and extrapolating to the physical pion mass is, however, noisy: $\bar{g}_0/(2F_\pi)=-7(63)\times 10^{-3} \,{\overline{\Theta}}$. The more precise result $\bar{g}_0/(2F_\pi)=17.4(1.9)\times 10^{-3} \,{\overline{\Theta}}$ is obtained using low energy effective field theory methods or equivalently the AWI. Since contamination from the $N\pi$ excited states arises in the calculation of many nucleon matrix elements, we give an extended discussion on them and the use of the AWI for controlling them in the calculation of $\bar g_0$.

hep-lat↗

arXiv2026

The Momentum Fraction, Helicity and Transversity Isovector Moments of Nucleons from \texorpdfstring{$2+1$}{2+1}-flavor Lattice QCD

Results for the isovector momentum fraction, $\langle x \rangle_{u-d}$, helicity moment, $\langle x \rangle_{\Delta u-\Delta d}$, and the transversity moment, $\langle x\rangle_{\delta u-\delta d}$, of the nucleon are presented using high-statistics data on thirteen NME ensembles of gauge configurations generated by the JLab/W\&M/LANL/MIT/Marseille collaborations using $2+1$-flavors of dynamical Wilson-clover quarks. The much higher statistics facilitated better control over all systematics compared to our previous lattice calculation. The least controlled systematic---excited-state contamination---is quantified by studying the variation of the results as a function of three estimates of the mass gap of the first excited state, obtained from two- and three-point correlation functions. The final results are obtained using a simultaneous fit to extrapolate in the lattice spacing, $a$, pion and kaon masses, $M_\pi$ and $M_K$, and the finite volume parameter, $M_\pi L$. The data show no significant finite-volume correction, and some dependence on the lattice spacing and the renormalization factors. The largest systematic uncertainty is due to possible remaining excited states contributions. Our final results, in the $\overline{\rm MS}$ scheme at 2~GeV, are $\langle x \rangle_{u-d} = 0.154(10)(9)$, $\langle x \rangle_{\Delta u-\Delta d} = 0.177(10)(15)$ and $\langle x \rangle_{\delta u-\delta d} = 0.197(12)(18)$, where the first error is the overall statistical uncertainty and the second represents the various systematic uncertainties added in quadrature. Results for the momentum fraction and helicity moment are consistent with phenomenological global fit values, while the transversity moment is a prediction.

hep-lat↗

arXiv2026

The Spectrum and Scale Setting on 2+1-flavor NME Lattices

This paper describes the thirteen ensembles, named NME, generated with 2+1-flavor Wilson-clover fermions by the JLab/W\&M/LANL/MIT/Marseille collaborations, and presents an analysis of the meson and baryon spectrum, decay constants $f_\pi$ and $f_K$, flow scales $t_0$ and $w_0$, and time histories of the $\Theta$ and Weinberg operators under gradient flow. Using these quantities, the physical point values of the two flow scales, ${t_0^{\rm Phy}}$ and ${w_0^{\rm Phy}}$, and the ratio $\mathop{f_K / f_\pi}^{\rm Phy}$ are determined. The masses of the octet and decuplet baryons are analyzed using both the next-to-leading order (NLO) and the next-next-to-leading order (NNLO) ansatz from heavy baryon chiral perturbation theory (HB$\chi$PT). The NNLO fit to the octet baryons, $M_N$, $M_\Sigma$, $M_\Lambda$ and $M_\Xi$, is preferred while the corresponding fits to the decuplet Omega mass, $M_\Omega$, are not distinguished. We also present a study of the autocorrelations in the data and show that there is no evidence, even at large flow time, of the freezing of the topological charge or the Weinberg three-gluon operator.

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arXiv2025

Flavor diagonal nucleon charges using clover fermions on MILC HISQ ensembles

We present lattice results for the flavor diagonal charges of the proton from the analysis of eight ensembles generated using 2+1+1-flavors of highly improved staggered quarks (HISQ) by the MILC collaboration. The calculation includes all the needed connected and disconnected contributions to nucleon three-point function. For extracting matrix elements using fits to the spectral decomposition of these correlation functions, two strategies to remove excited state contributions are employed and compared. To renormalize these charges, the 2+1-flavor mixing matrix is calculated in the RI-sMOM intermediate scheme on the lattice. The final results are presented in the $\overline{\text{MS}}$ scheme at scale 2GeV. The axial charges for the proton are $g_A^u = 0.781(25)$, $g_A^d = -0.440(39)$, and $g_A^s = -0.055(9)$; the tensor charges are $g_T^u = 0.782(28)$, $g_T^d = -0.195(16)$, and $g_T^s = -0.0016(12)$; and the scalar charges are $g_S^u = 9.39(88)$, $g_S^d = 8.84(93)$, and $g_S^s = 0.37(14)$. Results for the neutron are given by the $u \leftrightarrow d$ interchange. Results for the sigma terms are $\sigma_{\pi N}|_{\rm standard} = 42(6)~{\rm MeV}$ from a "standard" analysis and $\sigma_{\pi N}|_{N \pi} = 61(6)~{\rm MeV}$ from a "$N\pi$" analysis that includes the contributions of multihadron $N\pi $ excited states as motivated by chiral perturbation theory. Our preferred value $\sigma_{\pi N}|_{N \pi}$ is consistent with the phenomenological extraction from $\pi- N$ scattering data. The strangeness content of the proton, for which the "standard" analysis is appropriate, is $\sigma_{s}|_{\rm standard} = 35(13)~{\rm MeV}$.

hep-lat↗

arXiv2025

Lattice gauge ensembles and data management

We summarize the status of lattice QCD ensemble generation efforts and their data management characteristics. Namely, these proceedings combine the contributions to a dedicated parallel session during the 41st International Symposium on Lattice Field Theory (Lattice 2024), during which representatives of 16 lattice QCD collaborations provided details on their simulation program, with focus on plans for publication, data management, and storage requirements. The parallel session was organized by the International Lattice Data Grid (ILDG), following an open call to the lattice QCD community for participation in the session.

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arXiv2025

Gradient flow of the Weinberg operator

We present preliminary results on the susceptibilities involving the CP-violating (CPV) Weinberg three-gluon operator and the topological $\Theta$ term using the gradient flow scheme, and study their continuum and chiral extrapolations. These are used to provide an estimate of the $\Theta$ induced by the Weinberg operator in theories with the Peccei-Quinn (PQ) mechanism. Combined with the calculations of the matrix elements (MEs) of quark-bilinears between nucleon states, such calculations will enable estimates of the electric dipole moments (EDMs) and CPV pion-nucleon couplings due to the Weinberg operator, thereby providing robust constraints on beyond the standard model (BSM) physics.

hep-lat↗

arXiv2025

Inference of response functions with the help of machine learning algorithms

Response functions are a key quantity to describe the near-equilibrium dynamics of strongly-interacting many-body systems. Recent techniques that attempt to overcome the challenges of calculating these \emph{ab initio} have employed expansions in terms of orthogonal polynomials. We employ a neural network prediction algorithm to reconstruct a response function $S(\omega)$ defined over a range in frequencies $\omega$. We represent the calculated response function as a truncated Chebyshev series whose coefficients can be optimized to reduce the representation error. We compare the quality of response functions obtained using coefficients calculated using a neural network (NN) algorithm with those computed using the Gaussian Integral Transform (GIT) method. In the regime where only a small number of terms in the Chebyshev series are retained, we find that the NN scheme outperforms the GIT method.

quant-ph↗

arXiv2024

Neutron electric dipole moment from isovector quark chromo-electric dipole moment

We present results from our lattice QCD study of the contribution of the isovector quark cEDM (qcEDM) operator to the neutron EDM. The calculation was carried out on four 2+1+1-flavor highly improved staggered quark ensembles (provided to us by the MILC collaboration) using Wilson-clover quarks to construct correlation functions. We use the nonsinglet axial Ward identity including corrections up to O(a) to show how to control the power-divergent mixing of the isovector qcEDM operator with the lower dimensional pseudoscalar operator. Results for the nEDM are presented after conversion to the MS scheme at the leading-log order.

hep-lat↗

arXiv2024

Isovector Axial Charge and Form Factors of Nucleons from Lattice QCD

I present an overview of the calculations of the isovector axial vector form factor of the nucleon, $G_A(Q^2)$, using lattice QCD. Based on a comparison of results from various collaborations, a case is made that lattice results are now consistent within 10\%. A similar level of uncertainty is found also in the axial charge $g_A^{u-d}$, the mean squared axial charge radius, $\langle r_A^2 \rangle$, the induced pseudoscalar charge $g_P^\ast$, and the pion-nucleon coupling $g_{\pi NN}$. These lattice results for $G_A(Q^2)$ are already compatible with those obtained from the recent MINER$\nu$A experiment but lie 2-3$\sigma$ higher than the phenomenological extraction from the old $\nu$-deuterium bubble chamber scattering data for $Q^2 > 0.3$~GeV${}^2$. Fits to our data show that the dipole ansatz does not have enough parameters to parameterize the form factor over the range $0 \le Q^2 \le 1$~GeV${}^2$, whereas even a $z^2$ truncation of the $z$-expansion or a low order Pad\'e are sufficient. Looking ahead, lattice QCD calculations will provide increasingly precise results over the range $0 \le Q^2 \lesssim 1$~GeV${}^2$, and MINER$\nu$A-like experiments will extend the range to $Q^2 \sim 2$~GeV${}^2$ or higher. To increase precision of lattice data to the percent level, new developments are needed to address two related issues: the exponentially falling signal-to-noise ratio in all nucleon correlation functions and removing excited state contributions. Nevertheless, even with the current methodology, significant reduction in errors is expected over the next few years with higher statistics data on more ensembles closer to the physical point.

hep-lat↗

arXiv2024

Progress report on testing robustness of the Newton method in data analysis on 2-point correlation function using a MILC HISQ ensemble

We report recent progress in data analysis on the two point correlation functions which will be prerequisite to obtain semileptonic form factors for the $B_{(s)} \to D_{(s)}\ell\nu$ decays. We use a MILC HISQ ensemble for the measurement. We use the HISQ action for light quarks, and the Oktay-Kronfeld (OK) action for the heavy quarks ($b$ and $c$). We used a sequential Bayesian method for the data analysis. Here we test the new fitting methodology of Benjamin J.~Choi in a completely independent manner.

hep-lat↗

arXiv2024

Current progress on the semileptonic form factors for $\bar{B} \to D^{\ast} \ell \bar{\nu}$ decay using the Oktay-Kronfeld action

We present recent progress in calculating the semileptonic form factors $h_{A_1}(w)$ for the $\bar{B} \to D^{\ast} \ell \bar{\nu}$ decays. We use the Oktay-Kronfeld (OK) action for the charm and bottom valence quarks and the HISQ action for light quarks. We adopt the Newton method combined with the scanning method to find a good initial guess for the $\chi^2$ minimizer in the fitting of the 2pt correlation functions. The main advantage is that the Newton method lets us to consume all the time slices allowed by the physical positivity. We report the first, reliable, but preliminary results for $h_{A_1}(w)/\rho_{A_1}$ at zero recoil ($w=1$). Here we use a MILC HISQ ensemble ($a = 0.12$ fm, $M_{\pi}$ = 220 MeV, and $N_f = 2 + 1 + 1$ flavors).

hep-lat↗

arXiv2024

Update on flavor diagonal nucleon charges from clover fermions

We present a summary of the full calculation of the axial, scalar and tensor flavor diagonal charges of the nucleon carried out using Wilson-clover fermions on eight ensembles generated using 2+1+1-flavors of highly improved staggered quarks (HISQ) by the MILC collaboration. We also give results for the $3\times 3$ matrix of renormalization factors between the RI-sMOM and $\overline{\rm MS}$ scheme for the 2+1 flavor theory that include flavor mixing. Preliminary results for $g_{A,S,T}^{u,d,s}$ are presented in the $\overline{\rm MS}$ scheme at scale 2 GeV.

hep-lat↗

arXiv2023

Phases of 2d massless QCD with qubit regularization

We investigate the possibility of reproducing the continuum physics of 2d SU(N) gauge theory coupled to a single flavor of massless Dirac fermions using qubit regularization. The continuum theory is described by N free fermions in the ultraviolet (UV) and a coset Wess-Zumino-Witten (WZW) model in the infrared (IR). In this work, we explore how well these features can be reproduced using the Kogut-Susskind Hamiltonian with a finite-dimensional link Hilbert space and a generalized Hubbard coupling. Using strong coupling expansions, we show that our model exhibits a gapped dimer phase and another phase described by a spin-chain. Furthermore, for N=2, using tensor network methods, we show that there is a second-order phase transition between these two phases. The critical theory at the transition can be understood as an SU(2)_1 WZW model, using which we determine the phase diagram of our model quantitatively. Using the confinement properties of the model we argue how the UV physics of free fermions could also emerge, but may require further modifications to our model.

hep-lat↗

arXiv2023

Confronting axial-vector form factor from lattice QCD with MINERvA antineutrino-proton data

We compare recent MINERvA antineutrino-hydrogen charged-current measurements to phenomenological predictions of the axial-vector form factor based on fits to all available electron scattering and deuterium bubble-chamber data and to representative lattice-QCD (LQCD) determination by the PNDME Collaboration. While there is $1$--$2σ$ agreement in the cross section with MINERvA data for each bin in $Q^2$, we identify three regions with different relevance and opportunity for LQCD predictions. For $Q^2 \lesssim 0.2~\mathrm{GeV}^2$, the phenomenological extractions have large number of data points and LQCD is competitive, while MINERvA data have large errors. For $0.2~\mathrm{GeV}^2 \lesssim Q^2 \lesssim 1~\mathrm{GeV}^2$, LQCD is competitive with the MINERvA determination, and both give values larger than from phenomenological extraction. For $Q^2 > 1~\mathrm{GeV}^2$, the MINERvA data are the most precise. Our analysis indicates that with improving precision of MINERvA-like experiments and LQCD data, the uncertainty in the nucleon axial-vector form factor will be steadily reduced.

hep-lat↗

arXiv2023

Topological terms with qubit regularization and relativistic quantum circuits

Qubit regularization provides a rich framework to explore quantum field theories. The freedom to choose how the important symmetries of the theory are embedded in the qubit regularization scheme allows us to construct new lattice models with rich phase diagrams. Some of the phases can contain topological terms which lead to critical phases. In this work we introduce and study the SU(3)-F qubit regularization scheme to embed the SO(3) spin-symmetry. We argue that qubit models in this regularization scheme contain several phases including a critical phase which describes the k = 1 Wess-Zumino-Witten (WZW) conformal field theory (CFT) at long distances, and two massive phases one of which is trvially gapped and the other which breaks the lattice translation symmetry. We construct a simple space-time Euclidean lattice model with a single coupling U and study it using the Monte Carlo method. We show the model has a critical phase at small U and a trivially massive phase at large U with a first order transition separating the two. Another feature of our model is that it is symmetric under space-time rotations, which means the temporal and spatial lattice spacing are connected to each other. The unitary time evolution operator obtained by a Wick rotation of the transfer matrix of our model can help us compute the physics of the k = 1 WZW CFT in real time without the need for tuning the temporal lattice spacing to zero. We use this idea to introduce the concept of a relativistic quantum circuit on a discrete space-time lattice.

hep-lat↗

arXiv2023

The case for an EIC Theory Alliance: Theoretical Challenges of the EIC

We outline the physics opportunities provided by the Electron Ion Collider (EIC). These include the study of the parton structure of the nucleon and nuclei, the onset of gluon saturation, the production of jets and heavy flavor, hadron spectroscopy and tests of fundamental symmetries. We review the present status and future challenges in EIC theory that have to be addressed in order to realize this ambitious and impactful physics program, including how to engage a diverse and inclusive workforce. In order to address these many-fold challenges, we propose a coordinated effort involving theory groups with differing expertise is needed. We discuss the scientific goals and scope of such an EIC Theory Alliance.

hep-ph↗

arXiv2023

Nucleon Isovector Axial Form Factors

We present results for the isovector axial vector form factors obtained using thirteen 2+1+1-flavor highly improved staggered quark (HISQ) ensembles generated by the MILC collaboration. The calculation of nucleon two- and three-point correlation functions has been done using Wilson-clover fermions. In the analysis of these data, we quantify the sensitivity of the results to strategies used for removing excited state contamination and invoke the partially conserved axial current relation between the form factors to choose between them. Our data driven analysis includes removing contributions from multihadron $N \pi$ states that make significant contributions. Our final results are: $g_A = 1.292 (53)_\text{stat}\,(24)_\text{sys}$ for the axial charge; $g_S = 1.085 (50)_\text{stat}\, (103)_\text{sys}$ and $g_T = 0.991 (21)_\text{stat}\, (10)_\text{sys}$ for the scalar and tensor charges; $\langle r_A^2 \rangle = 0.439 (56)_\text{stat} (34)_\text{sys}$ fm${}^2$ for the mean squared axial charge radius, $g_P^\ast = 9.03(47)_\text{stat}(42)_\text{sys} $ for the induced pseudoscalar charge; and $g_{\pi NN} = 14.14(81)_\text{stat}(85)_\text{sys}$ for the pion-nucleon coupling. We also provide a parameterization of the axial form factor $G_A(Q^2)$ over the range $0 \le Q^2 \le 1$ GeV${}^2$ for use in phenomenology and a comparison with other lattice determinations. We find that the various lattice data agree within 10\% but are significantly different from the extraction of $G_A(Q^2)$ from the $\nu$-deuterium scattering data.

hep-lat↗

arXiv2023

Quark Chromo-Electric Dipole Moment Operator on the Lattice

We present a lattice QCD study of the contribution of the isovector quark chromo-electric dipole moment (qcEDM) operator to the nucleon electric dipole moments (nEDM). The calculation was carried out on four 2+1+1-flavor of highly improved staggered quark (HISQ) ensembles using Wilson-clover quarks to construct correlation functions. This clover-on-HISQ formulation is not fully $O(a)$ improved, and gives rise to additional systematics over and above those due to removing excited state contributions to getting ground-state matrix elements, and the final chiral and continuum extrapolations to get the physical result. We use the non-singlet axial Ward identity including corrections up to $O(a)$ to show how to control the power-divergent mixing of the isovector qcEDM operator with the lower dimensional pseudoscalar operator. The residual corrections are observed to give rise to $O(25\%)$ violations in relations arising from the axial Ward identity. We devise three methods attempting to control the resulting uncertainty in the CP violating form factor; each of these, however, can have large $O(a^2)$ corrections. Preliminary results for the nEDM due to qcEDM are presented choosing the method giving the most uniform behavior.

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