arXiv subjects

Amarjit Soni

Publications and source records attributed to Amarjit Soni.

At least 19 recordsLinked to original sources

arXiv2026

Minimal dark $SU(2)$ origin of a massless Dirac neutrino

We propose a gauge-symmetry origin of a rank-two Dirac neutrino mass matrix that enforces one exactly massless neutrino, while being consistent with the oscillation data, as well as cosmological constraints. The mechanism relies on a minimal dark $SU(2)_D$ gauge symmetry under which one right-handed-neutrino-like Weyl fermion is charged, thereby forbidding its Standard Model Yukawa coupling. Quantum consistency then fixes the minimal dark-sector completion: Cancellation of the Witten anomaly requires a second fermionic $SU(2)_D$ doublet, while a discrete $Z_4$ symmetry that forbids Majorana masses allows the two dark doublets to form a vectorlike pair. This anomaly-free completion gives rise to a secluded, confining dark sector with a viable dark matter candidate, linking the protected neutrino texture to dark infrared dynamics.

hep-ph↗

arXiv2025

Updated Constraints from Electric Dipole Moments in the MSSM with R-Parity Violation

We revisit the electric dipole moments (EDMs) of quarks and leptons in the Minimal Supersymmetric Standard Model (MSSM) with trilinear $R$-parity violation (RPV). In this framework, EDMs are induced at the two-loop level via RPV interactions. We perform a comprehensive recalculation of several classes of Barr-Zee type diagrams in a general $R_\xi$ gauge. While we find general agreement with previous analytic results in the literature, our work provides a valuable independent cross-check of the complicated calculations. We also point out some subtleties in the intermediate steps and in the choice of the flavor basis for the numerical evaluation of the expressions. By confronting the theoretical predictions with the latest experimental limits on EDMs, we derive updated constraints on combinations of RPV couplings. We highlight an approximate, testable correlation between the proton and neutron EDM that emerges within the considered class of RPV models, offering a distinctive signature for future EDM experiments.

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arXiv2025

Efficient lattice QCD computation of radiative-leptonic-decay form factors at multiple positive and negative photon virtualities

In previous work Phys. Rev. D 107, 074507 (2023), we showed that form factors for radiative leptonic decays of pseudoscalar mesons can be determined efficiently and with high precision from lattice QCD using the ``3d method,'' in which three-point functions are computed for all values of the current-insertion time and the time integral is performed at the data-analysis stage. Here, we demonstrate another benefit of the 3d method: the form factors can be extracted for any number of nonzero photon virtualites from the same three-point functions at no extra cost. We present results for the $D_s\to\ell\nu\gamma^*$ vector form factor as a function of photon energy and photon virtuality, for both positive and negative virtuality. In our analysis, we separately consider the two different time orderings and the different quark flavors in the electromagnetic current. We discuss in detail the behavior of the unwanted exponentials contributing to the three-point functions, as well as the choice of fit models and fit ranges used to remove them for various values of the virtuality. While positive photon virtuality is relevant for decays to multiple charged leptons, negative photon virtuality suppresses soft contributions and is of interest in QCD-factorization studies of the form factors.

hep-lat↗

arXiv2025

Status of the Proton EDM Experiment (pEDM)

The Proton EDM Experiment (pEDM) is the first direct search for the proton electric dipole moment (EDM) with the aim of being the first experiment to probe the Standard Model (SM) prediction of any particle EDM. Phase-I of pEDM will achieve $10^{-29} e\cdot$cm, improving current indirect limits by four orders of magnitude. This will establish a new standard of precision in nucleon EDM searches and offer a unique sensitivity to better understand the Strong CP problem. The experiment is ideally positioned to explore physics beyond the Standard Model (BSM), with sensitivity to axionic dark matter via the signal of an oscillating proton EDM and across a wide mass range of BSM models from $\mathcal{O}(1\text{GeV})$ to $\mathcal{O}(10^3\text{TeV})$. Utilizing the frozen-spin technique in a highly symmetric storage ring that leverages existing infrastructure at Brookhaven National Laboratory (BNL), pEDM builds upon the technological foundation and experimental expertise of the highly successful Muon $g$$-$$2$ Experiments. With significant R\&D and prototyping already underway, pEDM is preparing a conceptual design report (CDR) to offer a cost-effective, high-impact path to discovering new sources of CP violation and advancing our understanding of fundamental physics. It will play a vital role in complementing the physics goals of the next-generation collider while simultaneously contributing to sustaining particle physics research and training early-career researchers during gaps between major collider operations.

hep-ex↗

arXiv2024

Old neutron stars as a new probe of relic neutrinos and sterile neutrino dark matter

We study the kinetic cooling (heating) of old neutron stars due to coherent scattering with relic neutrinos (sterile neutrino dark matter) via Standard Model neutral-current interactions. We take into account several important physical effects, such as gravitational clustering, coherent enhancement, neutron degeneracy and Pauli blocking. We find that the anomalous cooling of nearby neutron stars due to relic neutrino scattering might actually be observable by current and future telescopes operating in the optical to near-infrared frequency band, such as the James Webb Space Telescope (JWST), provided there is a large local relic overdensity that is still allowed. Similarly, the anomalous heating of neutron stars due to coherent scattering with keV-scale sterile neutrino dark matter, could also be observed by JWST or future telescopes, which would probe hitherto unexplored parameter space in the sterile neutrino mass-mixing plane.

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arXiv2024

Theoretical underpinnings of CP-Violation at the High-energy Frontier

We present a general analysis for the discovery potential of CP-violation (CPV) searches in scattering processes at TeV-scale colliders in an effective field theory framework, using the SMEFT basis for higher dimensional operators. In particular, we systematically examine the CP-violating sector of the SMEFT framework in some well motivated limiting cases, based on flavour symmetries of the underlying heavy theory. We show that, under naturality arguments of the underlying new physics (NP) and in the absence of (or suppressed) flavour-changing interactions, there is only a single operator, $Q_{t\phi} = \phi^\dagger \phi \left(\bar q_3 t \right) \tilde{\phi} $ which alters the top-Yukawa coupling, that can generate a non-vanishing CP-violating effect from tree-level SM$\times$NP interference terms. We find, however, that CPV from $Q_{t\phi} = \phi^\dagger \phi \left(\bar q_3 t \right) \tilde{\phi} $ is expected to be at best of $O(1\%)$ and, therefore, very challenging if at all measurable at the LHC or other future high-energy colliders. We then conclude that a potentially measurable CP-violating effect of $O(10\%)$ can arise in high-energy scattering processes ONLY if flavour-changing interactions are present in the underlying NP; in this case a sizable CPV can be generated at the tree-level by pure NP$\times$NP effects and not from SM$\times$NP interference. We provide several examples of CPV at the LHC and at a future $e^+e^-$ collider to support these statements.

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arXiv2024

Theory of CP angles measurement

In the early 80's Sanda-san and collaborators wrote key papers on the direct and clean determination of the unitarity angle $\phi_1$ ($\beta$). This motivated many of us for analogously coming up with ways for direct and clean determinations of the other two unitarity angles, $\phi_2 (\alpha)$ and $\phi_3 (\gamma)$. Current status of these direct determinations as well as our expectations for when Belle-II has 50 $ab^{-1}$ of luminosity and LHCb with some upgrades, will be given. In particular, it is emphasized that for direct determination of $\phi_3$, Belle-II should be able to handle final states in $D^0$ or $\bar D^0$ Dalitz decays, that contain one $\pi^0$ (which are difficult for LHCb) then they may make further inroads in improving the accuracy of $\phi_3$ determination. Early lattice inputs for constraining the unitarity triangle (UT) are briefly recalled. Its crucial role in supporting the Kobayashi-Maskawa theory of CP violation is emphasized. Over the years lattice methods have made significant progress and latest constraints from these for the UT will be discussed as well as compatibility with current direct determinations and some comments on future outlook will be made.

hep-ph↗

arXiv2024

Reappraisal of SU(3)-flavor breaking in $B\rightarrow DP$

In light of recently found deviations of the experimental data from predictions from QCD factorization for $B_{(s)}\rightarrow D_{(s)}P$ decays, where $P=\{\pi,K\}$, we systematically probe the current status of the SU(3)$_F$ expansion from a fit to experimental branching ratio data without any further theory input. We find that the current data are in agreement with the power counting of the SU(3)$_F$ expansion. While the SU(3)$_F$ limit is excluded at $>5\sigma$, amplitude-level SU(3)$_F$-breaking contributions of $\sim 20\%$ suffice for an excellent description of the data. SU(3)$_F$ breaking is needed in tree ($>5\sigma$) and color-suppressed tree ($2.4\sigma$) diagrams. We are not yet sensitive to SU(3)$_F$ breaking in exchange diagrams. From the underlying SU(3)$_F$ parametrization we predict the unmeasured branching ratios $\mathcal{B}(\overline{B}_s^0\rightarrow \pi^- D^+) = 2 \mathcal{B}(\overline{B}_s^0\rightarrow \pi^0 D^0) = [0.3, 7.2] \times 10^{-6}$ of suppressed decays that can be searched for at the LHCb experiment.

hep-ph↗

arXiv2023

Rare K decays off and on the lattice

The importance of rare $K$ decays especially in the context of a kaon unitarity triangle (KUT) is emphasized. The decay $K_L \to \pi^0 \nu \bar \nu$ is theoretically very clean but experimentally extremely challenging. The Standard Model prediction $\mathcal{B}\sim 3 \times 10^{-11}$ is still about two orders of magnitude away from the current experimental upper bound. One way to continue to make progress towards the construction of a KUT is by improving the accuracy in the calculation of $\varepsilon'$. Another way which is the primary focus here is via studies of $K^0 \to \pi^0 \mu^+ \mu^-$. LHCb, J-PARC, the proposed HIKE project, phenomenology, and in fact precision studies on the lattice can all play a very important role in this context.

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arXiv2023

Generic tests of CP-violation in high-$p_\text{T}$ multi-lepton signals at the LHC and beyond

We introduce a modification to the standard expression for tree-level CP-violation in scattering processes at the LHC, which is important when the initial state in not self-conjugate. Based on that, we propose a generic and model-independent search strategy for probing tree-level CP-violation in inclusive multi-lepton signals. We then use TeV-scale 4-fermion operators of the form $tu\ell\ell$ and $tc \ell \ell$ with complex Wilson coefficients as an illustrative example and show that it may generate ${\cal O}(10\%)$ CP asymmetries that should be accessible at the LHC with an integrated luminosity of ${\cal O}(1000)$ fb$^{-1}$.

hep-ph↗

arXiv2023

Long-distance contribution to $ε_K$ from lattice QCD

A lattice QCD approach to the calculation of the long-distance contributions to $ε_K$ is presented. This parameter describes indirect CP violation in $K\toππ$ decay. While the short-distance contribution to $ε_K$ can be accurately calculated in terms of standard model parameters and a single hadronic matrix element, $B_K$, there is a long-distance part which is estimated to be approximately $5\%$ of the total and is more difficult to determine. A method for determining this small but phenomenologically important contribution to $ε_K$ using lattice QCD is proposed and a complete exploratory calculation of the contribution is presented. This exploratory calculation uses an unphysical light quark mass corresponding to a 339 MeV pion mass and an unphysical charm quark mass of 968 MeV, expressed in the $\overline{\mathrm{MS}}$ scheme at 2 GeV. This calculation demonstrates that future work should be able to determine this long-distance contribution from first principles with a controlled error of 10\% or less.

hep-lat↗

arXiv2023

Probing the muon (g-2) anomaly at the LHC in final states with two muons and two taus

The longstanding muon $(g-2)$ anomaly, as well as the persistent hints of lepton flavor universality violation in $B$-meson decays, could be signaling new physics beyond the Standard Model (SM). A minimal $R$-parity-violating supersymmetric framework with light third-generation sfermions (dubbed as 'RPV3') provides a compelling solution to these flavor anomalies, while simultaneously addressing other pressing issues of the SM. We propose a new RPV3 scenario for the solution of the muon $(g-2)$ anomaly, which leads to an interesting LHC signal of $μ^+μ^-τ^+τ^-$ final state. We analyze the Run-2 LHC multilepton data to derive stringent constraints on the sneutrino mass and the relevant RPV coupling in this scenario. We then propose dedicated selection strategies to improve the bound even with the existing dataset. We also show that the high-luminosity LHC will completely cover the remaining muon $(g-2)$-preferred parameter space, thus providing a robust, independent test of the muon $(g-2)$ anomaly.

hep-ph↗

arXiv2023

Exclusive semileptonic $B_s\to K \ell ν$ decays on the lattice

Semileptonic $B_s \to K \ell ν$ decays provide an alternative $b$-decay channel to determine the CKM matrix element $|V_{ub}|$, and to obtain a $R$-ratio to investigate lepton-flavor-universality violations. Results for the CKM matrix element may also shed light on the discrepancies seen between analyses of inclusive or exclusive decays. We calculate the decay form factors using lattice QCD with domain-wall light quarks and a relativistic $b$-quark. We analyze data at three lattice spacings with unitary pion masses down to $268\,\mathrm{MeV}$. Our numerical results are interpolated/extrapolated to physical quark masses and to the continuum to obtain the vector and scalar form factors $f_+(q^2)$ and $f_0(q^2)$ with full error budgets at $q^2$ values spanning the range accessible in our simulations. We provide a possible explanation of tensions found between results for the form factor from different lattice collaborations. Model- and truncation-independent $z$-parameterization fits following a recently proposed Bayesian-inference approach extend our results to the entire allowed kinematic range. Our results can be combined with experimental measurements of $B_s \to D_s$ and $B_s\to K$ semileptonic decays to determine $|V_{ub}|=3.8(6)\times 10^{-3}$. The error is currently dominated by experiment. We compute differential branching fractions and two types of $R$ ratios, the one commonly used as well as a variant better suited to test lepton-flavor universality.

hep-lat↗

arXiv2023

$\Delta I = 3/2$ and $\Delta I = 1/2$ channels of $K\to\pi\pi$ decay at the physical point with periodic boundary conditions

We present a lattice calculation of the $K\to\pi\pi$ matrix elements and amplitudes with both the $\Delta I = 3/2$ and 1/2 channels and $\varepsilon'$, the measure of direct $CP$ violation. We use periodic boundary conditions (PBC), where the correct kinematics of $K\to\pi\pi$ can be achieved via an excited two-pion final state. To overcome the difficulty associated with the extraction of excited states, our previous work \cite{Bai:2015nea,RBC:2020kdj} successfully employed G-parity boundary conditions, where pions are forced to have non-zero momentum enabling the $I=0$ two-pion ground state to express the on-shell kinematics of the $K\to\pi\pi$ decay. Here instead we overcome the problem using the variational method which allows us to resolve the two-pion spectrum and matrix elements up to the relevant energy where the decay amplitude is on-shell. In this paper we report an exploratory calculation of $K\to\pi\pi$ decay amplitudes and $\varepsilon'$ using PBC on a coarser lattice size of $24^3\times64$ with inverse lattice spacing $a^{-1}=1.023$ GeV and the physical pion and kaon masses. The results are promising enough to motivate us to continue our measurements on finer lattice ensembles in order to improve the precision in the near future.

hep-lat↗

arXiv2023

A New Probe of Relic Neutrino Clustering using Cosmogenic Neutrinos

We propose a new probe of cosmic relic neutrinos (C$ν$B) using their resonant scattering against cosmogenic neutrinos. Depending on the lightest neutrino mass and the energy spectrum of the cosmogenic neutrino flux, a Standard Model vector meson (such as a hadronic $ρ$) resonance can be produced via $ν\barν$ annihilation. This leads to a distinct absorption feature in the cosmogenic neutrino flux at an energy solely determined by the meson mass and the neutrino mass, apart from redshift. By numerical coincidence, the position of the $ρ$-resonance overlaps with the originally predicted peak of the Greisen-Zatsepin-Kuzmin (GZK) neutrino flux, which offers an enhanced absorption effect at higher redshifts. We show that this absorption feature in the GZK neutrino flux may be observable in future radio-based neutrino observatories, such as IceCube-Gen2 radio, provided there exists a large overdensity in the C$ν$B distribution. This therefore provides a new probe of C$ν$B clustering at large redshifts, complementary to the laboratory probes (such as KATRIN) at zero redshift.

hep-ph↗

arXiv2023

Isospin 0 and 2 two-pion scattering at physical pion mass using all-to-all propagators with periodic boundary conditions in lattice QCD

A study of two-pion scattering for the isospin channels, $I=0$ and $I=2$, using lattice QCD is presented. Möbius domain wall fermions on top of the Iwasaki-DSDR gauge action for gluons with periodic boundary conditions are used for the lattice computations which are carried out on two ensembles of gauge field configurations generated by the RBC and UKQCD collaborations with physical masses, inverse lattice spacings of 1.023 and 1.378 GeV, and spatial extents of $L=4.63$ and 4.58 fm, respectively. The all-to-all propagator method is employed to compute a matrix of correlation functions of two-pion operators. The generalized eigenvalue problem (GEVP) is solved for a matrix of correlation functions to extract phase shifts with multiple states, two pions with a non-zero relative momentum as well as two pions at rest. Our results for phase shifts for both $I=0$ and $I=2$ channels are consistent with and the Roy Equation and chiral perturbation theory, though at this preliminary stage our errors for $I=0$ are large. An important outcome of this work is that we are successful in extracting two-pion excited states, which are useful for studying $K\toππ$ decay, on physical-mass ensembles using GEVP.

hep-lat↗

arXiv2023

Methods for high-precision determinations of radiative-leptonic decay form factors using lattice QCD

We present a study of lattice-QCD methods to determine the relevant hadronic form factors for radiative leptonic decays of pseudoscalar mesons. We provide numerical results for $D_s^+ \to \ell^+ νγ$. Our calculation is performed using a domain-wall action for all quark flavors and on a single RBC/UKQCD lattice gauge-field ensemble. The first part of the study is how to best control two sources of systematic error inherent in the calculation, specifically the unwanted excited states created by the meson interpolating field, and unwanted exponentials in the sum over intermediate states. Using a 3d sequential propagator allows for better control over unwanted exponentials from intermediate states, while using a 4d sequential propagator allows for better control over excited states. We perform individual analyses of the 3d and 4d methods as well as a combined analysis using both methods, and find that the 3d sequential propagator offers good control over both sources of systematic uncertainties for the smallest number of propagator solves. From there, we further improve the use of a 3d sequential propagator by employing an infinite-volume approximation method, which allows us to calculate the relevant form factors over the entire allowed range of photon energies. We then study improvements gained by performing the calculation using a different three-point function, using ratios of three-point functions, averaging over positive and negative photon momentum, and using an improved method for extracting the structure-dependent part of the axial form factor. The optimal combination of methods yields results for the $D_s^+ \to \ell^+ νγ$ structure-dependent vector and axial form factors in the entire kinematic range with statistical plus fitting uncertainties of order 5%, using 25 gauge configurations with 64 samples per configuration.

hep-lat↗

arXiv2022

Precision tau physics: Challenge for Theory, on and off the lattice

$τ$ is playing an important role in the current B-physics indications from experiments of lepton flavor universality violations(LFUV). This suggests it be given increasing attention theoretically in the coming years, given also the fact that Belle-II will have much larger data samples to study; similar comments also apply to LHCb as well as ATLAS and CMS. The fact that $τ$ decays provide valuable information on its spin is an added advantage. This gains special significance if the current indications of new physics are upheld since naturalness arguments then strongly suggest that new physics should be accompanied by new CP-odd phase(s). Moreover, the fact that the $τ$ mass is around 1.8 GeV,~{\it i.e} a lot less than the B-meson mass makes it much easier candidate for lattice studies in great detail and consequently very likely with greater precision. This should help us test the SM and or BSMs with greater precision as needed.

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