arXiv subjects

Amarjit Soni

Publications and source records attributed to Amarjit Soni.

At least 37 records · Page 2Linked to original sources

arXiv2022

A lattice QCD perspective on weak decays of b and c quarks Snowmass 2022 White Paper

Lattice quantum chromodynamics has proven to be an indispensable method to determine nonperturbative strong contributions to weak decay processes. In this white paper for the Snowmass community planning process we highlight achievements and future avenues of research for lattice calculations of weak $b$ and $c$ quark decays, and point out how these calculations will help to address the anomalies currently in the spotlight of the particle physics community. With future increases in computational resources and algorithmic improvements, percent level (and below) lattice determinations will play a central role in constraining the standard model or identifying new physics.

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arXiv2022

Hints of Natural Supersymmetry in Flavor Anomalies?

The recent results from the Fermilab muon $g-2$ experiment, as well as the persisting hints of lepton flavor universality violation in $B$-meson decays, present a very strong case for flavor-nonuniversal new physics beyond the Standard Model. We assert that a minimal $R$-parity violating supersymmetric scenario with relatively light third-generation sfermions (dubbed as 'RPV3') provides a natural, well-motivated framework for the simultaneous explanation of all flavor anomalies, while being consistent with a multitude of low-energy flavor constraints, as well as with limits from high-energy collider searches. We further propose complementary tests and distinct signatures of this scenario in the high-$p_T$ searches at current and future colliders. Specifically, we find that an sbottom in the mass range of 2-12 TeV accounts for $R_{D^{(*)}}$ and $R_{K^{(*)}}$ flavor anomalies and it only plays a minor role in the $(g-2)_μ$ anomaly, whereas a sneutrino with mass between 0.7-1 TeV is the dominant player for $(g-2)_μ$. In this context, we propose specific collider signatures of sbottom via its decays to $ \overline{ t}(t) μ^+ μ^-$, and of sneutrino pairs with their decays leading to a highly distinctive and spectacular four-muon final state, which can be used to completely probe the RPV3 parameter space of interest.

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arXiv2022

Multi-lepton probes of new physics and lepton-universality in top-quark interactions

We explore the sensitivity to new physics (NP) in the associated production of top-quarks with leptons $pp \to t \bar t \ell^+ \ell^-$, which leads to the multi-leptons signals $pp \to n \ell + {\tt jets} + \not\!\! E_T$, where $n = 2,3,4$. The NP is parameterized via 4-Fermi effective $t\bar{t} \ell^+ \ell^-$ contact interactions of various types, which are generated by multi-TeV heavy scalar, vector or tensor exchanges in $t \bar t \to \ell^+ \ell^-$; we focus on the case of $\ell=e,μ$. We match the 4-Fermi $t t \ell \ell$ terms to the SMEFT operators and also give examples of specific underlying heavy physics that can generate such terms. Analysis of the SM signals and corresponding backgrounds shows that the di-lepton and tri-lepton channels are much better probes of the effective $t\bar{t} \ell^+ \ell^-$ 4-Fermi terms than the four-lepton one at the 13 TeV LHC. Therefore, the best sensitivity is obtained in the di- and tri-lepton channels, for which the dominant background $pp \to t \bar t$ and $pp \to WZ$, respectively, can be essentially eliminated after applying the $2\ell$ and $ 3 \ell$ selections and a sufficiently high invariant mass selection for the opposite sign same flavor (OSSF) lepton-pair. We explore two cases: lepton flavor universal (LFU) NP where the $t t e e$ and $t t μμ$ contact interactions are of same size and LFU violating (LFUV) NP, where the scale of the $t t μμ$ terms is assumed to be much lower. We show that in both cases it is possible to obtain new 95\% CL bounds on the scale of the $t t \ell \ell$ contact interactions at the level $Λ\gtrsim 2-3$ TeV, which are considerably tighter than the current bounds on these 4-Fermi terms.

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arXiv2022

The storage ring proton EDM experiment

We describe a proposal to search for an intrinsic electric dipole moment (EDM) of the proton with a sensitivity of \targetsens, based on the vertical rotation of the polarization of a stored proton beam. The New Physics reach is of order $10^~3$TeV mass scale. Observation of the proton EDM provides the best probe of CP-violation in the Higgs sector, at a level of sensitivity that may be inaccessible to electron-EDM experiments. The improvement in the sensitivity to $θ_{QCD}$, a parameter crucial in axion and axion dark matter physics, is about three orders of magnitude.

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arXiv2022

Discovering new physics in rare kaon decays

The decays and mixing of $K$ mesons are remarkably sensitive to the weak interactions of quarks and leptons at high energies. They provide important tests of the standard model at both first and second order in the Fermi constant $G_F$ and offer a window into possible new phenomena at energies as high as 1,000 TeV. These possibilities become even more compelling as the growing capabilities of lattice QCD make high-precision standard model predictions possible. Here we discuss and attempt to forecast some of these capabilities.

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arXiv2022

$K\toππ$ decay matrix elements at the physical point with periodic boundary conditions

We calculate $K\toππ$ matrix elements using periodic boundary conditions as an independent calculation from our previous study with G-parity boundary conditions. We present our preliminary results for $K\toππ$ three-point functions and matrix elements on a $24^3, a^{-1} = 1$~GeV, $2+1$-flavor Möbius DWF ensemble at physical pion and kaon masses generated by the RBC and UKQCD collaborations and discuss the prospect for high-precision computation of $\varepsilon'$ with periodic boundary conditions.

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arXiv2021

Form factors for semileptonic $B\toπ$, $B_s\to K$ and $B_s\to D_s$ decays

We report on our determinations of $B\to π\ellν$, $B_s\to K \ell ν$ and $B_s\to D_s \ell ν$ semileptonic form factors. In addition we discuss the determination of $R$-ratios testing lepton-flavor universality and suggest an improved ratio. Our calculations are based on the set of 2+1 flavor domain-wall Iwasaki gauge field configurations generated by the RBC/UKQCD collaboration with three lattice spacings of $1/a = 1.78$, $2.38$, and $2.79\,\text{GeV}$. We use the relativistic heavy quark action for $b$ quarks and charm quarks are simulated with the Möbius domain-wall fermion action.

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arXiv2021

Meeting the Challenges for Relic Neutrino Detection

Inspired by Gounaris-Sakurai and Lee-Zumino, we postulate that the weak vector and axial vector currents are dominated by $J^{PC} = 1^{--}$ and $1^{++}$ resonances respectively in the appropriate channels of $ν+ \bar ν$ annihilation into quark-antiquark pairs when an ultrahigh-energy incoming $ν\ (\bar ν)$ strikes a relic $\bar ν\ (ν)$. Despite this and some other ideas, it appears the detection of relic neutrinos with just the Standard Model interactions seems extremely difficult at existing or future neutrino telescopes. Thus any positive signal would be due to some non-standard interactions of neutrinos.

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arXiv2021

Controlling unwanted exponentials in lattice calculations of radiative leptonic decays

Two important sources of systematic errors in lattice QCD calculations of radiative leptonic decays are unwanted exponentials in the sum over intermediate states and unwanted excited states created by the meson interpolating field. Performing the calculation using a 3d sequential propagator allows for better control over the systematic uncertainties from intermediate states, while using a 4d sequential propagator allows for better control over the systematic uncertainties from excited states. We calculate form factors using both methods and compare how reliably each controls these systematic errors. We also employ a hybrid approach involving global fits to data from both methods.

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arXiv2021

Enhancement of charm CP violation due to nearby resonances

Quantitative understanding of CP violation is extremely important as naturalness reasoning strongly suggests that new physics should be accompanied by beyond the Standard Model CP-odd phases. In 2019 LHCb made the first $5 σ$ discovery of CP violation in the charm system, leading to the new world average $Δa_{CP}^{\mathrm{dir}} = -0.00161 \pm 0.00028$. While some calculations have found this observation as requiring new physics, we suggest that scalar resonances nearby to $m_{D^0}$, in particular, $f_0(1710)$ and/or $f_0(1790)$ cause enhancements for CP violation within the SM. Thereby, our calculations based on the Standard Model suggest compatibility with the LHCb observations for now. However, experimental information especially on $f_0(1790)$ is rather sparse limiting our accuracy and further data on these resonances is strongly advocated.

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arXiv2021

Proton decay matrix elements on the lattice at physical pion mass

Proton decay is a major prediction of Grand-Unified Theories (GUT) and its observation would indicate baryon number violation that is required for baryogenesis. Many decades of searching for proton decay have constrained its rate and ruled out some of the simplest GUT models. Apart from the baryon number-violating interactions, this rate also depends on transition amplitudes between the proton and mesons or leptons produced in the decay, which are matrix elements of three-quark operators. We report nonperturbative calculation of these matrix elements for the most studied two-body decay channels into a meson and antilepton done on a lattice with physical light and strange quark masses and lattice spacings $a\approx0.14$ and 0.20 fm. We perform nonperturbative renormalization and excited state analysis to control associated systematic effects. Our results largely agree with previous lattice calculations done with heavier quark masses and thus remove ambiguity in ruling out some simple GUT theories due to quark mass dependence of hadron structure.

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arXiv2021

New flavor physics in di- and tri-lepton events from single-top at the LHC and beyond

The associated production of a single-top with opposite-sign same-flavor (OSSF) di-leptons, $pp \to t \ell^+ \ell^-$ and $ pp \to t \ell^+ \ell^- + j$ ($j=$light jet), can lead to striking tri-lepton $pp \to \ell^\prime \ell^+ \ell^- + X$ and di-lepton $pp \to \ell^+ \ell^- + j_b + X$ ($j_b=b$-jet) events at the LHC, after the top decays. Although these rather generic multi-lepton signals are flavor-blind, they can be generated by new 4-Fermi flavor changing (FC) $u_i t \ell \ell$ scalar, vector and tensor interactions ($u_i \in u,c$), which we study in this paper; we match the FC $u_i t \ell \ell$ 4-Fermi terms to the SMEFT operators and also to different types of FC underlying heavy physics. The main backgrounds to these di- and tri-lepton signals arise from $t \bar t$, $Z$+jets and $VV$ ($V=W,Z$) production, but they can be essentially eliminated with a sufficiently high invariant mass selection on the OSSF di-leptons, $m_{\ell^+ \ell^-}^{\tt min}(OSSF) > 1$ TeV; the use of $b$-tagging as an additional selection in the di-lepton final state case also proves very useful. We find, for example, that the expected 95\% CL bounds on the scale of a tensor(vector) $u t μμ$ interaction, with the current $\sim 140$ fb$^{-1}$ of LHC data, are $Λ< 5(3.2) $ TeV or $Λ< 4.1(2.7)$ TeV, if analyzed via the di-muon $μ^+ μ^- + j_b$ signal or the $e μ^+ μ^-$ tri-lepton one, respectively. The expected reach at the HL-LHC with 3000 fb$^{-1}$ of data is $Λ< 7.1(4.7)$ TeV and $Λ< 2.4(1.5)$ TeV for the corresponding $u t μμ$ and $c t μμ$ operators. We also study the potential sensitivity at future 27 TeV and 100 TeV high-energy LHC successors and also discuss the possible implications of this class of FC 4-Fermi effective interactions on lepton non-universality tests at the LHC.

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arXiv2020

Nonperturbative calculations of form factors for exclusive semileptonic $B_{(s)}$ decays

Precise theoretical predictions derived from the Standard Model are a key ingredient in searches for new physics in the flavor sector. The large mass and long lifetime of the $b$ quark make processes involving $b$ quarks of particular interest. We use lattice simulations to perform nonperturbative QCD calculations for semileptonic $B_{(s)}$ decays. We present results from our determinations of $B_s\to D_s \ell ν$ and $B_s\to K \ell ν$ semileptonic form factors and provide an outlook for our $B\to π\ellν$ calculation. In addition we discuss the determination of $R$-ratios testing lepton-flavor universality and suggest use of an improved ratio. Our calculations are based on the set of 2+1 flavor domain wall Iwasaki gauge field configurations generated by the RBC-UKQCD collaboration featuring three lattice spacings of $1/a = 1.78$, $2.38$, and $2.79\,\text{GeV}$. Heavy $b$-quarks are simulated using the relativistic heavy quark action.

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arXiv2020

Direct CP violation and the $ΔI=1/2$ rule in $K\toππ$ decay from the Standard Model

We present a lattice QCD calculation of the $ΔI=1/2$, $K\toππ$ decay amplitude $A_0$ and $\varepsilon'$, the measure of direct CP-violation in $K\toππ$ decay, improving our 2015 calculation of these quantities. Both calculations were performed with physical kinematics on a $32^3\times 64$ lattice with an inverse lattice spacing of $a^{-1}=1.3784(68)$ GeV. However, the current calculation includes nearly four times the statistics and numerous technical improvements allowing us to more reliably isolate the $ππ$ ground-state and more accurately relate the lattice operators to those defined in the Standard Model. We find ${\rm Re}(A_0)=2.99(0.32)(0.59)\times 10^{-7}$ GeV and ${\rm Im}(A_0)=-6.98(0.62)(1.44)\times 10^{-11}$ GeV, where the errors are statistical and systematic, respectively. The former agrees well with the experimental result ${\rm Re}(A_0)=3.3201(18)\times 10^{-7}$ GeV. These results for $A_0$ can be combined with our earlier lattice calculation of $A_2$ to obtain ${\rm Re}(\varepsilon'/\varepsilon)=21.7(2.6)(6.2)(5.0) \times 10^{-4}$, where the third error represents omitted isospin breaking effects, and Re$(A_0)$/Re$(A_2) = 19.9(2.3)(4.4)$. The first agrees well with the experimental result of ${\rm Re}(\varepsilon'/\varepsilon)=16.6(2.3)\times 10^{-4}$. A comparison of the second with the observed ratio Re$(A_0)/$Re$(A_2) = 22.45(6)$, demonstrates the Standard Model origin of this "$ΔI = 1/2$ rule" enhancement.

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arXiv2020

High $p_T$ correlated tests of lepton universality in lepton(s) + jet(s) processes; an EFT analysis

We suggest a new class of tests for searching for lepton flavor non-universality (LFNU) using ratio observables and based on correlations among the underlying LFNU new physics (NP) effects in several (seemingly independent) di-lepton and single lepton + jet(s) processes. This is demonstrated by studying the effects generated by LFNU 4-Fermi interactions involving 3rd generation quarks. We find that the sensitivity to the scale ($Λ$) of the LFNU 4-Fermi operators significantly improves when the correlations among the various di-lepton +jets and single-lepton + jets processes are used, reaching $Λ\sim {\cal O}(10)$~TeV at the HL-LHC.

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arXiv2020

Relations between $b\rightarrow cτν$ Decay Modes in Scalar Models

As a consequence of the Ward identity for hadronic matrix elements, we find relations between the differential decay rates of semileptonic decay modes with the underlying quark-level transition $b\rightarrow cτν$, which are valid in scalar models. The decay-mode dependent scalar form factor is the only necessary theoretical ingredient for the relations. Otherwise, they combine measurable decay rates as a function of the invariant mass-squared of the lepton pair $q^2$ in such a way that a universal decay-mode independent function is found for decays to vector and pseudoscalar mesons, respectively. This can be applied to the decays $B\rightarrow D^{*}τν$, $B_s\rightarrow D_s^*τν$, $B_c\rightarrow J/ψτν$ and $B\rightarrow Dτν$, $B_s\rightarrow D_sτν$, $B_c\rightarrow η_cτν$, with implications for $R(D^{(*)})$, $R(D_s^{(*)})$, $R(J/ψ)$, $R(η_c)$, and $\mathcal{B}(B_c\rightarrow τν)$. The slope and curvature of the characteristic $q^2$-dependence is proportional to scalar new physics parameters, facilitating their straight forward extraction, complementary to global fits.

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arXiv2020

Addressing $R_{D^{()}}$, $R_{K^{()}}$, muon $g-2$ and ANITA anomalies in a minimal $R$-parity violating supersymmetric framework

We analyze the recent hints of lepton flavor universality violation in both charged-current and neutral-current rare decays of $B$-mesons in an $R$-parity violating supersymmetric scenario. Motivated by simplicity and minimality, we had earlier postulated the third-generation superpartners to be the lightest (calling the scenario "RPV3") and explicitly showed that it preserves gauge coupling unification and of course has the usual attribute of naturally addressing the Higgs radiative stability. Here we show that both $R_{D^{(*)}}$ and $R_{K^{(*)}}$ flavor anomalies can be addressed in this RPV3 framework. Interestingly, this scenario may also be able to accommodate two other seemingly disparate anomalies, namely, the longstanding discrepancy in the muon $(g-2)$, as well as the recent anomalous upgoing ultra-high energy ANITA events. Based on symmetry arguments, we consider three different benchmark points for the relevant RPV3 couplings and carve out the regions of parameter space where all (or some) of these anomalies can be simultaneously explained. We find it remarkable that such overlap regions exist, given the plethora of precision low-energy and high-energy experimental constraints on the minimal model parameter space. The third-generation superpartners needed in this theoretical construction are all in the 1-10 TeV range, accessible at the LHC and/or next-generation hadron collider. We also discuss some testable predictions for the lepton-flavor-violating decays of the tau-lepton and $B$-mesons for the current and future $B$-physics experiments, such as LHCb and Belle II. Complementary tests of the flavor anomalies in the high-$p_T$ regime in collider experiments such as the LHC are also discussed.

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arXiv2020

SU(3)-breaking ratios for $D_{(s)}$ and $B_{(s)}$ mesons

We present results for the $SU(3)$ breaking ratios of decay constants $f_{D_s}/f_D$ and $f_{B_s}/f_B$ and - for the first time with physical pion masses - the ratio of bag parameters $B_{B_s}/B_{B_d}$, as well as the ratio $ξ$, forming the ratio of the nonpeturbative contributions to neutral $B_{(s)}$ meson mixing. Our results are based on Lattice QCD simulations with chirally symmetric 2+1 dynamical flavors of domain wall fermions. Eight ensembles at three different lattice spacing in the range $a = 0.11 - 0.07\,\mathrm{fm}$ enter the analysis two of which feature physical light quark masses. Multiple heavy quark masses are simulated ranging from below the charm quark mass to half the bottom quark mass. The $SU(3)$ breaking ratios display a very benign heavy mass behaviour allowing for extrapolation to the physical bottom quark mass. The results in the continuum limit including all sources of systematic errors are $f_{D_s}/f_D = 1.1740(51)_\mathrm{stat}(^{+68}_{-68})_\mathrm{sys}$, $f_{B_s}/f_B = 1.1949(60)_\mathrm{stat}(^{+\hphantom{0}95}_{-175})_\mathrm{sys}$, $B_{B_s}/B_{B_d} = 0.9984(45)_\mathrm{stat}(^{+80}_{-63})_\mathrm{sys}$ and $ξ= 1.1939(67)_\mathrm{stat}(^{+\hphantom{0}95}_{-177})_\mathrm{sys}$. Combining these with experimentally measured values we extract the ratios of CKM matrix elements $|V_{cd}/V_{cs}| = 0.2164(57)_\mathrm{exp}(^{+12}_{-12})_\mathrm{lat}$ and $|V_{td}/V_{ts}| = 0.20329(41)_\mathrm{exp}(^{+162}_{-301})_\mathrm{lat}$.

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