arXiv subjects

Amarjit Soni

Publications and source records attributed to Amarjit Soni.

At least 55 records · Page 3Linked to original sources

arXiv2020

Charm CP: $ΔA_{CP}$ and Radiative decays

Motivated by the very important discovery of CP violation in charm-decays for the first time, by the LHCb collaboration, the role of nearby resonances such as the scalar $f_0(1710)$ in accounting for the observed CP is discussed. It is suggested that the influence of such a resonance may also explain the long-standing puzzle of such a large breaking of SU(3) seen in its decays. It is also explained that intervention of such resonance(s) will render first principles calculations of $ΔA_{CP}$ rather difficult. Instead, it is proposed that searches for CP violation in simple radiative final states, such as $D^0 \to γϕ[\to K^+ K^-]$, $D^0 \to l^+ l^- π$ etc. has much better chance of theoretical precision studies and therefore theoretical and experimental investigations therein are strongly urged.

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arXiv2019

Semileptonic $B\toπ\ellν$, $B\to D\ellν$, $B_s\to K\ellν$, and $B_s\to D_s\ellν$ decays

We present updates for our nonperturbative lattice QCD calculations to determine semileptonic form factors for exclusive $B\to π\ellν$, $B\to D \ellν$, $B_s\to K\ell ν$, and $B_s\to D_s\ellν$ decays. Our calculation is based on RBC-UKQCD's set of $2+1$-dynamical-flavor gauge field ensembles. In the valence sector we use domain wall fermions for up/down, strange and charm quarks, whereas bottom quarks are simulated with the relativistic heavy quark action. The continuum limit is based on three lattice spacings. Using kinematical $z$ expansions we aim to obtain form factors over the full $q^2$ range. These form factors are the basis for predicting ratios addressing lepton flavor universality or, when combined with experimental results, to obtain CKM matrix elements $|V_{ub}|$ and $|V_{cb}|$.

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arXiv2019

Phenomenology of TeV-scale scalar Leptoquarks in the EFT

We examine new aspects of leptoquark (LQ) phenomenology using effective field theory (EFT). We construct a complete set of leading effective operators involving SU(2) singlets scalar LQ and the SM fields up to dimension six. We show that, while the renormalizable LQ-lepton-quark interaction Lagrangian can address the persistent hints for physics beyond the Standard Model in the B-decays $\bar B \to D^{(*)} τ\barν$, $\bar B \to \bar K \ell^+ \ell^-$ and in the measured anomalous magnetic moment of the muon, the LQ higher dimensional effective operators may lead to new interesting effects associated with lepton number violation. These include the generation of one-loop sub-eV Majorana neutrino masses, mediation of neutrinoless double-$β$ decay and novel LQ collider signals. For the latter, we focus on 3rd generation LQ ($ϕ_3$) in a framework with an approximate $Z_3$ generation symmetry, and show that one class of the dimension five LQ operators may give rise to a striking asymmetric same-charge $ϕ_3 ϕ_3$ pair-production signal, which leads to low background same-sign leptons signals at the LHC. For example, with $M_{ϕ_3} \sim 1$ TeV and a new physics scale of $Λ\sim 5$ TeV, we expect about $5000$ positively charged $τ^+ τ^+$ events via $pp \to ϕ_3 ϕ_3 \to τ^+ τ^+ + 2 \cdot j_b$ ($j_b$=b-jet) at the 13 TeV LHC with an integrated luminosity of 300 fb$^{-1}$. It is interesting to note that, in the LQ EFT framework, the expected same-sign lepton signals have a rate which is several times larger than the QCD LQ-mediated opposite-sign leptons signals, $gg,q \bar q \to ϕ_3 ϕ_3^* \to \ell^+ \ell^- +X$. We also consider the same-sign charged lepton signals in the LQ EFT framework at higher energy hadron colliders such as a 27 TeV HE-LHC and a 100 TeV FCC-hh.

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arXiv2019

R-Parity Violating Supersymmetry and the 125 GeV Higgs signals

We study the impact of R-parity violating Supersymmetry (RPV SUSY) on the 125 GeV Higgs production and decay modes at the LHC. We assume a heavy SUSY spectrum with multi-TeV squarks and SU(2) scalar singlets as well as the decoupling limit in the SUSY Higgs sector. In this case the lightest CP-even Higgs is SM-like when R-parity is conserved. In contrast, we show that when R-parity violating interactions are added to the SUSY framework, significant deviations may occur in some production and decay channels of the 125 GeV Higgs-like state. Indeed, we assume a single-flavor (mostly third generation) Bilinear RPV (BRPV) interactions, which generate Higgs-sneutrino mixing, lepton-chargino mixing and neutrino-neutralino mixing, and find that notable deviations of ${\cal O}(20-30\%)$ may be expected in the Higgs signal strength observables in some channels, e.g., in $p p \to h \to μ^+ μ^-, τ^+ τ^-$. Moreover, we find that new and detectable signals associated with BRPV Higgs decays to gauginos, $h\rightarrowν_τ\tildeχ_{2}^{0}$ and $h\rightarrowτ^{\pm}χ_{2}^{\mp}$, may also arise in this scenario. These decays yield a typical signature of $h \to τ^\pm \ell^\mp + {\not\!\! E_T}$ ($\ell =e,μ,τ$) that can be much larger than in the SM, and may also be accompanied by an ${\cal O}(20-30\%)$ enhancement in the di-photon signal $pp \to h \to γγ$. We also examine potential interesting signals of Trilinear R-parity violation (TRPV) interactions in the production and decays of the Higgs-sneutrino BRPV mixed state (assuming it is the 125 GeV scalar) and show that, in this case also, large deviations up to ${\cal O}(100\%)$ are expected in e.g., $p p \to h \to μ^+ μ^-, τ^+ τ^-$, which are sensitive to the BRPV$\times$TRPV coupling product.

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arXiv2019

Radiative leptonic decays on the lattice

Adding a hard photon to the final state of a leptonic pseudoscalar-meson decay lifts the helicity suppression and can provide sensitivity to a larger set of operators in the weak effective Hamiltonian. Furthermore, radiative leptonic $B$ decays at high photon energy are well suited to constrain the first inverse moment of the $B$-meson light-cone distribution amplitude, an important parameter in the theory of nonleptonic $B$ decays. We demonstrate that the calculation of radiative leptonic decays is possible using Euclidean lattice QCD, and present preliminary numerical results for $D_s^+ \to \ell^+ νγ$ and $K^- \to \ell^-\barνγ$.

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arXiv2019

Flavor Changing Heavy Higgs Interactions with Leptons at Hadron Colliders

In a general two Higgs doublet model, we study flavor changing neutral Higgs (FCNH) decays into leptons at hadron colliders, $pp \to ϕ^0 \to τ^\mpμ^\pm +X$, where $ϕ^0$ could be a CP-even scalar ($h^0$, $H^0$) or a CP-odd pseudoscalar ($A^0$). The light Higgs boson $h^0$ is found to resemble closely the Standard Model Higgs boson at the Large Hadron Collider. In the alignment limit of $\cos(β-α) \cong 0$ for $h^0$--$H^0$ mixing, FCNH couplings of $h^0$ are naturally suppressed, but such couplings of the heavier $H^0, A^0$ are sustained by $\sin(β-α) \simeq 1$. We evaluate physics backgrounds from dominant processes with realistic acceptance cuts and tagging efficiencies. We find promising results for $\sqrt{s} = 14$ TeV, which we extend further to $\sqrt{s} = 27$ TeV and 100 TeV future pp colliders.

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arXiv2019

Resonance enhancement of Charm CP

It is suggested that a nearby $0^{++}$ resonance, $f_0$(1710) of mass $m_f=1723 MeV$ and width $Γ=139$ MeV is playing a significant role in efficiently providing the strong (CP-conserving) and weak (CP-odd) phase simultaneously in the recently observed direct CP asymmetry $ΔA_{CP}$ by the LHCb collaboration. The direct CP arises by the well known penguin-tree interference wherein the virtual b-quark in the c-u penguin is the source of the Kobayashi-Maskawa CP-odd phase, $γ$, in the SM. Loop (penguin) corrections generate left-right operators enhancing coupling to the $0^{++}$ scalar resonance. The scalar resonance is likely rich in gluonic content perhaps leading to a better understanding of large breaking of flavor SU(3) that has been known for a long-time. Approximate calculations give a rough understanding of the observed size of the CP asymmetry. The mechanism leads to several interesting implications which can be experimentally studied and tested. Moreover, in an analogous fashion to $f_0$, 4-quark operators also generate $P \times P$, P being a pseudo-scalar bilinear, which may be dominated by the nearby $η(1760)$ of width about 250 MeV that can influence final states such as 4 $π$'s, $η^{\prime}(η) + π^+ + π^-$ etc which could exhibit CP violating triple correlation or energy asymmetries. We also briefly discuss CP violation in radiative charm decays and suggest that simple final states $γρ$ and $γϕ$ are best suited for sizeable asymmetries as well as providing precise tests of the SM.

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arXiv2019

Semi-leptonic form factors for $B_s \to K \ell ν$ and $B_s \to D_s \ell ν$

Semi-leptonic $B_s \to K \ell ν$ and $B_s \to D_s \ell ν$ decays provide an alternative $b$-decay channel to determine the CKM matrix elements $|V_{ub}|$ and $|V_{cb}|$ or to obtain $R$-ratios to investigate lepton flavor universality violations. In addition, these decays may shed further light on the discrepancies seen in the analysis of inclusive vs. exclusive decays. Using the nonperturbative methods of lattice QCD, theoretical results are obtained with good precision and full control over systematic uncertainties. This talk will highlight ongoing efforts of the $B$-physics program by the RBC-UKQCD collaboration.

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arXiv2018

Model-Independent Determination of $B_c^+ \to η_c\, \ell^+\, ν$ Form Factors

We derive model-independent bounds on the form factors for the decay $B_c^+ \to η_c\, \ell^+\, ν$ including full mass effects, i.e. $\ell = e,\, μ, \text{ and } τ$. The bounds are obtained by using the BGL parameterization for the form factors, and fitting to the preliminary lattice data of the HPQCD Collaboration. Our main result after bounding the form factors is the Standard Model (SM) prediction for the ratio of branching fractions $R(η_c) = \mathcal{B}(B_c^+ \to η_c\, τ^+\, ν_τ) / \mathcal{B}(B_c^+ \to η_c\, μ^+\, ν_μ)$. We find $R(η_c)|_{\text{SM}} = 0.31^{+0.04}_{-0.02}$, and argue that a measurement of $R(η_c)$ is within the reach of LHCb during the high luminosity run of the LHC. In addition, using the heavy-quark spin symmetry of the $B_c$ meson we relate our results for $B_c^+ \to η_c\, \ell^+\, ν$ to those for $B_c^+ \to J/ψ\, \ell^+\, ν$ yielding the estimate $R(J/ψ)|_{\text{SM}} = 0.26 \pm 0.02$ in good agreement with other determinations.

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arXiv2018

A universally enhanced light-quarks Yukawa couplings paradigm

We propose that natural TeV-scale new physics (NP) with ${\cal O}(1)$ couplings to the standard model (SM) quarks may lead to a universal enhancement of the Yukawa couplings of all the light quarks, perhaps to a size comparable to that of the SM b-quark Yukawa coupling, i.e., $y_q \sim {\cal O}(y_b^{SM})$ for $q=u,d,c,s$. This scenario is described within an effective field theory (EFT) extension of the SM, for which a potential contribution of certain dimension six effective operators to the light quarks Yukawa couplings is $y_q \sim {\cal O} \left( f \frac{v^2}{Λ^2} \right)$, where $v$ is the Higgs vacuum expectation value (VEV), $v=246$ GeV, $Λ$ is the typical scale of the underlying heavy NP and $f$ is the corresponding Wilson coefficient which depends on its properties and details. In particular, we study the case of $y_q \sim 0.025 \sim y_b^{SM}$, which is the typical size of the enhanced light-quark Yukawa couplings if the NP scale is around $Λ\sim 1.5$ TeV and the NP couplings are natural, i.e., $f \sim {\cal O}(1)$. We also explore this enhanced light quarks Yukawa paradigm in extensions of the SM which contain TeV-scale vector-like quarks and we match them to the specific higher dimensional effective operators in the EFT description. We discuss the constraints on this scenario and the flavor structure of the underlying NP dynamics and suggest some resulting "smoking gun" signals that should be searched for at the LHC, such as multi-Higgs production $pp \to hh,hhh$ and single Higgs production in association with a high $p_T$ jet ($j$) or photon $pp \to hj,h γ$ and with a single top-quark $pp \to h t$.

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arXiv2018

Light-quarks Yukawa couplings and new physics in exclusive high-$p_T$ Higgs + jet and Higgs + $b$-jet events

We suggest that the exclusive Higgs + light (or b)-jet production at the LHC, $pp \to h+j(j_b)$, is a rather sensitive probe of the light-quarks Yukawa couplings and of other forms of new physics (NP) in the Higgs-gluon $hgg$ and quark-gluon $qqg$ interactions. We study the Higgs $p_T$-distribution in $pp \to h+j(j_b) \to γγ+ j(j_b)$, i.e., in $h+j(j_b)$ production followed by the Higgs decay $h \to γγ$, employing the ($p_T$-dependent) signal strength formalism to probe various types of NP which are relevant to these processes and which we parameterize either as scaled Standard Model (SM) couplings (the kappa-framework) and/or through new higher dimensional effective operators (the SMEFT framework). We find that the exclusive $h+j(j_b)$ production at the 13 TeV LHC is sensitive to various NP scenarios, with typical scales ranging from a few TeV to ${\cal O}(10)$ TeV, depending on the flavor, chirality and Lorentz structure of the underlying physics.

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arXiv2017

Towards a non-perturbative calculation of Weak Hamiltonian Wilson coefficients

We propose a method to compute the Wilson coefficients of the weak effective Hamiltonian to all orders in the strong coupling constant using Lattice QCD simulations. We perform our calculations adopting an unphysically light weak boson mass of around $2~\mathrm{GeV}$. We demonstrate that systematic errors for the Wilson coefficients $C_1$ and $C_2$, related to the current-current four-quark operators, can be controlled and present a path towards precise determinations in subsequent works.

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arXiv2017

$R_{D^{(*)}}$ anomaly: A possible hint for natural supersymmetry with $R$-parity violation

Recently, several $B$-physics experiments have reported an appreciable deviation from the Standard Model (SM) in the tree-level observables $R_{D^{(*)}}$; the combined weighted average now stands at $\approx 4 σ$. We first show the anomaly necessarily implies model-independent collider signals of the form $pp \to b τν$ that should be expediously searched for at ATLAS/CMS as a complementary test of the anomaly. Next we suggest a possible interconnection of the anomaly with the radiative stability of the Standard Model Higgs boson and point to a minimal effective supersymmetric scenario with $R$-parity violation as the underlying cause. We also comment on the possibility of simultaneously explaining the recently reported $R_{K^{(*)}}$ anomaly in this setup.

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arXiv2017

A Cosmic Selection Rule for Glueball Dark Matter Relic Density

We point out a unique mechanism to produce the relic abundance for glueball dark matter from a gauged $SU(N)_d$ hidden sector which is bridged to the standard model sector through heavy vectorlike quarks colored under gauge interactions from both sides. A necessary ingredient of our assumption is that the vectorlike quarks, produced either thermally or non-thermally, are abundant enough to dominate the universe for some time in the early universe. They later undergo dark color confinement and form unstable vectorlike-quarkonium states which annihilate decay and reheat the visible and dark sectors. The ratio of entropy dumped into two sectors and the final energy budget in the dark glueballs is only determined by low energy parameters, including the intrinsic scale of the dark $SU(N)_d$, $Λ_d$, and number of dark colors, $N_d$, but depend weakly on parameters in the ultraviolet such as the vectorlike quark mass or the initial condition. We call this a cosmic selection rule for the glueball dark matter relic density.

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arXiv2017

Gravitational Waves From SU(N) Glueball Dark Matter

A hidden sector with pure non-abelian gauge symmetry is an elegant and just about the simplest model of dark matter. In this model the dark matter candidate is the lightest bound state made of the confined gauge fields, the dark glueball. In spite of its simplicity, the model has been shown to have several interesting non-standard implications in cosmology. In this work, we explore the gravitational waves from binary boson stars made of self-gravitating dark glueball fields as a natural and important consequence. We derive the dark SU($N$) star mass and radius as functions of the only two fundamental parameters in the model, the glueball mass $m$ and the number of colors $N$, and identify the regions that could be probed by the LIGO and future gravitational wave observatories.

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arXiv2017

Improved lattice computation of proton decay matrix elements

We present an improved result of lattice computation of the proton decay matrix elements in $N_f=2+1$ QCD. In this study, the significant improvement of statistical accuracy by adopting the error reduction technique of All-mode-averaging, is achieved for relevant form factor to proton (and also neutron) decay on the gauge ensemble of $N_f=2+1$ domain-wall fermions in $m_π=0.34$--0.69 GeV on 2.7~fm$^3$ lattice as used in our previous work \cite{Aoki:2013yxa}. We improve total accuracy of matrix elements to 10--15\% from 30--40\% for $p\rightarrowπe^+$ or from 20--40\% for $p\rightarrow K \barν$. The accuracy of the low energy constants $α$ and $β$ in the leading-order baryon chiral perturbation theory (BChPT) of proton decay are also improved. The relevant form factors of $p\rightarrow π$ estimated through the "direct" lattice calculation from three-point function appear to be 1.4 times smaller than those from the "indirect" method using BChPT with $α$ and $β$. It turns out that the utilization of our result will provide a factor 2--3 larger proton partial lifetime than that obtained using BChPT. We also discuss the use of these parameters in a dark matter model.

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arXiv2016

Form factors for semi-leptonic $B$ decays

Semi-leptonic $B$ decays provide promising channels to test the Standard Model, search for signs of new physics, or determine fundamental parameters like CKM matrix elements. We present an update on our calculation of short distance contributions to GIM suppressed rare $B$ decays focusing in particular on $B_s\to ϕ\ell^+ \ell^-$ decays. Furthermore we show first results for our calculation of $B_{(s)}\to D_{(s)}^{(*)}\ellν$ semi-leptonic decays involving $b\to c$ transitions. Our calculations are based on RBC-UKQCD's 2+1 flavor domain-wall fermion and Iwasaki gauge field configurations featuring three lattice spacings in the range $1.73$ GeV $\le a^{-1} \le 2.77$ GeV and pion masses down to the physical value. We calculate the form factors by simulating $b$-quarks using the relativistic heavy quark action, create light $u/d$ and $s$ quarks with standard domain-wall kernel, and use optimised Möbius domain-wall fermions for charm quarks.

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arXiv2016

Correlating new physics signals in $B \to D^{(*)} τν_τ$ with $B \to τν_τ$

Semileptonic and purely leptonic decays of B meson to $τ$, such as $B\to D^{(\ast)}τν_τ$ and $B\toτν_τ$ are studied. Recognizing that there already were some weak hints of possible deviations from the SM in the measurements of $\mathcal{B}(B\toτν_τ)$ by \Babar~and Belle and the fact that detection of the $τ$ also occurs in the measurements of $B\to D^{(\ast)}τν_τ$, we stress the importance of joint studies of these processes, whenever possible. For this purpose, as an illustration, we introduce the observable, $\mathcal{R}(D^{(*)})/\mathcal{B}(B\toτν_τ)$ where, for one thing, the unknown systematics due to $τ$ identification are expected to largely cancel. We show that all measurements of this observable are consistent with the existing data, within somewhat largish experimental errors, with the predictions of the SM. We stress that precise experimental measurement and comparison with theory of the branching ratio for $B \to τν_τ$ is extremely important for a reliable search of new physics. Furthermore, in view of the anticipated improved precision in experiments in the next few years, in addition to $\mathcal{R}(D^{(*)})$, host of other ratios analogous to $\mathcal{R}(D^{(*)})/\mathcal{B}(B \to τν_τ)$ in the SM are suggested for lattice calculations as well, so that for more stringent tests of the SM, correlations in lattice calculations can be properly taken into account to enhance precision.

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