arXiv subjects

Alakabha Datta

Publications and source records attributed to Alakabha Datta.

At least 37 records · Page 2Linked to original sources

arXiv2021

Axion-like particles resolve the $B \to πK$ and g-2 anomalies

We offer a new solution to an old puzzle in the penguin-dominated $B\toπK$ decays. The puzzle is the inconsistency among the measurements of the branching ratios and CP asymmetries of the four $B\toπK$ decays: $B^+ \to π^+ K^0$, $B^+\to π^0 K^+$, $B_d^0\toπ^- K^+$, $B_d^0\toπ^0 K^0$. We solve the $B\toπK$ puzzle by considering the effect of an axion-like particle (ALP) that mixes with the $π^0$ and has mass close to the $π^0$ mass. We show that the ALP can also explain the anomalies in the electron and muon anomalous magnetic moments.

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arXiv2021

Anomalous dimensions from Yukawa couplings in SMNEFT: four-fermion operators

The Standard Model Neutrino Effective Field Theory (SMNEFT) is the Standard Model Effective Field Theory (SMEFT) augmented with right-handed neutrinos. Building on our previous work, arXiv:2010.12109, we calculate the Yukawa coupling contributions to the one-loop anomalous dimension matrix for the 11 dimension-six four-fermion SMNEFT operators. We also present the new contributions to the anomalous dimension matrix for the 14 four-fermion SMEFT operators that mix with the SMNEFT operators through the Yukawa couplings of the right-handed neutrinos.

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arXiv2020

Anomalous dimensions from gauge couplings in SMEFT with right-handed neutrinos

Standard Model Neutrino Effective Field Theory (SMNEFT) is an effective theory with Standard Model (SM) gauge-invariant operators constructed only from SM and right-handed neutrino fields. For the full set of dimension-six SMNEFT operators, we present the gauge coupling terms of the one-loop anomalous dimension matrix for renormalization group evolution (RGE) of the Wilson coefficients between a new physics scale and the electroweak scale. We find that the SMNEFT operators can be divided into five subsets which are closed under RGE. Our results apply for both Dirac and Majorana neutrinos. We also discuss the operator mixing pattern numerically and comment on some interesting phenomenological implications.

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arXiv2020

A Measurable Angular Distribution for ${\bar B} \to D^{*} τ^- {\barν}_τ$ Decays

At present, the measurements of $R_{D^{(*)}}$ and $R_{J/ψ}$ hint at new physics (NP) in $b \to c τ^- {\barν}$ decays. The angular distribution of ${\bar B} \to D^* (\to D π) \, τ^{-} {\barν}_τ$ would be useful for getting information about the NP, but it cannot be measured. The reason is that the three-momentum ${\vec p}_τ$ cannot be determined precisely since the decay products of the $τ^-$ include an undetected $ν_τ$. In this paper, we construct a measurable angular distribution by considering the additional decay $τ^- \to π^- ν_τ$. The full process is ${\bar B} \to D^* (\to D π') \, τ^{-} (\to π^- ν_τ) {\barν}_τ$, which includes three final-state particles whose three-momenta can be measured: $D$, $π'$, $π^-$. The magnitudes and relative phases of all the NP parameters can be extracted from a fit to this angular distribution. One can measure CP-violating angular asymmmetries. If one integrates over some of the five kinematic parameters parametrizing the angular distribution, one obtains (i) familiar observables such as the $q^2$ distribution and the $D^*$ polarization, and (ii) new observables associated with the $π^-$ emitted in the $τ$ decay: the forward-backward asymmetry of the $π^-$ and the CP-violating triple-product asymmetry.

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arXiv2020

Dark sector origin of the KOTO and MiniBooNE anomalies

We present a dark sector model that reproduces the KOTO, MiniBooNE and muon anomalous magnetic moment anomalies. The dark sector is comprised of a light scalar singlet $S$ that has a large coupling to a slightly heavier sterile neutrino that mixes with the active neutrinos. The scalar couples to standard model fermions via Yukawa couplings, and to photons via a higher-dimensional coupling. The KOTO signal is a result of the flavor-changing penguin process $K_L \to π^0 S$ followed by the decay of $S$ to neutrinos. The sterile neutrino produced in neutrino-nucleus scattering at MiniBooNE decays to an active neutrino and $S$, which decays electromagnetically and creates an event excess at low energies.

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arXiv2020

Resolving the $(g-2)_μ$ and $B$ anomalies with leptoquarks and a dark Higgs boson

At present, there are outstanding discrepancies between standard model predictions and measurements of the muon's $g-2$ and several $B$-meson properties. We resolve these anomalies by considering a two-Higgs-doublet model extended to include leptoquarks and a dark Higgs boson $S$. The leptoquarks modify $B$-meson decays and also induce an $S γγ$ coupling, which contributes to the muon's $g-2$ through a Barr-Zee diagram. We show that, for TeV-scale leptoquarks and dark Higgs boson masses $m_{S} \sim 10-200~\text{MeV}$, a consistent resolution to all of the anomalies exists. The model predicts interesting new decays, such as $B \to K^{(*)} e^+ e^-$, $B \to K^{(*)} γγ$, $K \to πγγ$, and $h \to γγγγ$, with branching fractions not far below current bounds.

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arXiv2019

A unified explanation of $b \to s μ^+ μ^-$ anomalies, neutrino masses and $B\rightarrow πK$ puzzle

Anomalies in semi-leptonic $B$ decays could indicate new physics beyond the standard model(SM). There is an older puzzle in non-leptonic $B \to πK$ decays. The new particles, leptoquarks and diquarks, required to solve the semi-leptonic and the non-leptonic puzzles can also generate neutrino masses and mixing at loop level. We show that a consistent framework to explain the $B$ anomalies and the neutrino masses is possible and we make predictions for certain rare nonleptonic $B$ decays.

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arXiv2019

The $B$ Anomalies and New Physics in $b \to s e^+ e^-$

We investigate the implications of the latest LHCb measurement of $R_K$ for NP explanations of the $B$ anomalies. The previous data could be explained if the $b \to s μ^+ μ^-$ NP is in (I) $C_{9,{\rm NP}}^{μμ}$ or (II) $C_{9,{\rm NP}}^{μμ} = -C_{10,{\rm NP}}^{μμ}$, with scenario (I) providing a better explanation than scenario (II). This continues to hold with the new measurement of $R_K$. However, for both scenarios, this measurement leads to a slight tension of $O(1σ)$ between separate fits to the $b \to s μ^+ μ^-$ and $R_{K^{(*)}}$ data. In this paper, we investigate whether this tension can be alleviated with the addition of NP in $b \to s e^+ e^-$. In particular, we examine the effect of adding such NP to scenarios (I) and (II). We find several scenarios in which this leads to improvements in the fits. $Z'$ and LQ models with contributions to both $b \to s μ^+ μ^-$ and $b \to s e^+ e^-$ can reproduce the data, but only within scenarios based on (II). If the tension persists in future measurements, it may be necessary to consider NP models with more than one particle contributing to $b \to s \ell^+ \ell^-$.

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arXiv2019

CP Violation in ${\bar B}^0\to D^{*+}μ^-{\barν}_μ$

In order to explain the observed anomalies in the measurements of $R_{D^{(*)}}$ and $R_{J/ψ}$, a variety of new-physics (NP) models that contribute to $b\to cτ^-{\barν}$ have been proposed. In this paper, we show how CP-violating observables can be used to distinguish these NP models. Because ${\vec p}_τ$ cannot be measured (the decay products of the $τ$ include the undetected $ν_τ$), obtaining the angular distribution of ${\bar B}^0\to D^{*+}τ^{-}{\barν}_τ$ is problematic. Instead, we focus here on ${\bar B}^0\to D^{*+}(\to D^0 π^+)μ^- {\barν}_μ$. This process may also receive contributions from the same NP, and LHCb intends to measure the CP-violating angular asymmetries in this decay. There are two classes of NP models that contribute to $b\to cμ^-{\barν}_μ$. These involve (i) a $W'$ (two types) or (ii) a leptoquark (LQ) (six types). The most popular NP models predict no CP-violating effects, so the measurement of nonzero CP-violating symmetries would rule them out. Furthermore these measurements allow one to distinguish the $W'$ and LQ models, and to differentiate among several LQ models.

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arXiv2019

Neutrino scattering and B anomalies from hidden sector portals

We examine current constraints on and the future sensitivity to the strength of couplings between quarks and neutrinos in the presence of a form factor generated from loop effects of hidden sector particles that interact with quarks via new interactions. We consider models associated with either vector or scalar interactions of quarks and leptons generated by hidden sector dynamics. We study constraints on these models using data from coherent elastic neutrino-nucleus scattering and solar neutrino experiments and demonstrate how these new interactions may be discovered by utilizing the recoil spectra. We show that our framework can be naturally extended to explain the lepton universality violating neutral current B decay anomalies, and that in a model framework the constraints from neutrino scattering can have implications for these anomalies.

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arXiv2018

Diagnosing New Physics with LUV and LFV B Decays

An important prediction of the standard model is the universality of the gauge interactions of the three generation of charged leptons. Violation of this universality would be a clean evidence of new physics (NP) beyond the standard model. In recent times anomalies in measurements of certain $B$ decays indicate violation of lepton universality (LUV). I will discuss how one may probe this LUV new physics via related decays and distributions. I will point out that LUV new physics can often lead to lepton flavor violation (LFV) and I will discuss some promising decays to look for LFV new physics.

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arXiv2018

New physics in inclusive $B \to X_c\ell \barν$ decay in light of $R(D^{(*)})$ measurements

In this work we study the effects of new-physics (NP) operators with different Lorentz structures on the inclusive $B \to X_cτ\barν$ decay and make predictions for the ratio of total decay rates $R(X_c)=\frac{Γ(B \to X_cτ\barν_τ)}{Γ(B \to X_c \ell \barν_\ell)}$ with $\ell=e, μ$, the differential decay rates, $\frac{dΓ}{dq^2}$ and $\frac{dΓ}{dE_τ}$, forward-backward asymmetry $A_{FB}$ and the ratio of the differential decay rates $B(q^2)=\frac{dΓ(B \to X_cτ\barν_τ)/dq^2}{dΓ(B \to X_c \ell \barν_\ell)/dq^2}$. In addition, we introduce some leptoquark models as explicit models of the NP operators and study their effects on the inclusive decay. We consider $\mathcal{O}(α_s)$ radiative and $1/m_b$ nonperturbative corrections to the Standard Model (SM) decay rate and ignore their small effects in the NP contributions.

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arXiv2018

New light mediators for the $R_K$ and $R_{K^*}$ puzzles

The measurements of $R_{K}$ and $R_K^{*}$ provide hints for the violation of lepton universality. However, it is generally difficult to explain the $R_{K^*}$ measurement in the low $q^2$ range, $0.045 \le q^2 \le 1.1$ GeV$^2$. Light mediators offer a solution by making the Wilson coefficients $q^2$ dependent. We check if new lepton nonuniversal interactions mediated by a scalar ($S$) or vector particle ($Z^\prime$) of mass between $10-200$ MeV can reproduce the data. We find that a 25 MeV $Z^\prime$ with a $q^2$-dependent $b-s$ coupling and that couples to the electron but not the muon can explain all three anomalies in conjunction with other measurements. A similar 25 MeV $S$ provides a good fit to all relevant data except $R_{K^*}$ in the low $q^2$ bin. A 25 MeV $Z^\prime$ with a $q^2$-dependent $b-s$ coupling and that couples to the muon but not the electron provides a good fit to the combination of the $R_K$ and $R_{K^*}$ data, but does not fit $R_{K^*}$ in the low $q^2$ bin well.

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arXiv2018

Diagnosing New Physics in $b \to c \, τ\, ν_τ$ decays in the light of the recent BaBar result

The BaBar Collaboration has recently reported the measurement of the ratio of the branching fractions of $\bar{B} \to D(D^{*}) τ^{-} \bar{ν_τ}$ to $\bar{B} \to D(D^{*}) \ell^{-} \bar{ν_\ell}$ which deviates from the Standard Model prediction by 2$σ$(2.7$σ$). This deviation goes up to 3.4$σ$ level when the two measurements in the $D$ and $D^*$ modes are taken together and could indicate new physics. Using an effective Lagrangian for the new physics we study the implication of these results and calculate other observables that can shed light on the nature of the new physics. We show that the measurements of the forward-backward asymmetries and the $τ$ and $D^*$ polarization fractions can distinguish among the various couplings of the new physics operators.

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arXiv2018

The $B \to πK$ Puzzle Revisited

For a number of years, there has been a certain inconsistency among the measurements of the branching ratios and CP asymmetries of the four $B \to πK$ decays ($B^+ \to π^+ K^0$, $B^+ \to π^0 K^+$, $B^0 \to π^- K^+$, $B^0 \to π^0 K^0$). In this paper, we re-examine this $B \to πK$ puzzle. We find that the key unknown parameter is $|C'/T'|$, the ratio of color-suppressed and color-allowed tree amplitudes. If this ratio is large, $|C'/T'| = 0.5$, the SM can explain the data. But if it is small, $|C'/T'| = 0.2$, the SM cannot explain the $B \to πK$ puzzle -- new physics (NP) is needed. The two types of NP that can contribute to $B \to πK$ at tree level are $Z'$ bosons and diquarks. $Z'$ models can explain the puzzle if the $Z'$ couples to right-handed $u{\bar u}$ and/or $d{\bar d}$, with $g_R^{dd} \ne g_R^{uu}$. Interestingly, half of the many $Z'$ models proposed to explain the present anomalies in $b \to s μ^+ μ^-$ decays have the required $Z'$ couplings to $u{\bar u}$ and/or $d{\bar d}$. Such models could potentially explain both the $b \to s μ^+ μ^-$ anomalies and the $B \to πK$ puzzle. The addition of a color sextet diquark that couples to $ud$ can also explain the puzzle.

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arXiv2017

New Physics in $b \to s μ^+ μ^-$ after the Measurement of $R_{K^*}$

The recent measurement of $R_{K^*}$ is yet another hint of new physics (NP), and supports the idea that it is present in $b\to sμ^+μ^-$ decays. We perform a combined model-independent and model-dependent analysis in order to deduce properties of this NP. Like others, we find that the NP must obey one of two scenarios: (I) $C_9^{μμ}({\rm NP}) < 0$ or (II) $C_9^{μμ}({\rm NP}) = - C_{10}^{μμ}({\rm NP}) < 0$. A third scenario, (III) $C_9^{μμ}({\rm NP}) = - C_{9}^{\prime μμ}({\rm NP})$, is rejected largely because it predicts $R_K = 1$, in disagreement with experiment. The simplest NP models involve the tree-level exchange of a leptoquark (LQ) or a $Z'$ boson. We show that scenario (II) can arise in LQ or $Z'$ models, but scenario (I) is only possible with a $Z'$. Fits to $Z'$ models must take into account the additional constraints from $B^0_s$-${\bar B}^0_s$ mixing and neutrino trident production. Although the LQs must be heavy, O(TeV), we find that the $Z'$ can be light, e.g., $M_{Z'} = 10$ GeV or 200 MeV.

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arXiv2017

Phenomenology of $Λ_b \to Λ_c τ\barν_τ$ using lattice QCD calculations

In a recent paper we studied the effect of new-physics operators with different Lorentz structures on the semileptonic $Λ_b \to Λ_c τ\barν_τ$ decay. This decay is of interest in light of the $R({D^{(*)}})$ puzzle in the semileptonic $\bar{B} \to D^{(*)} τ{\barν}_τ$ decays. In this work we add tensor operators to extend our previous results and consider both model-independent new physics (NP) and specific classes of models proposed to address the $R({D^{(*)}})$ puzzle. We show that a measurement of $R(Λ_c) = {\cal B}[Λ_b \to Λ_c τ\barν_τ] / {\cal B}[Λ_b \to Λ_c \ell \barν_{\ell}]$ can strongly constrain the NP parameters of models discussed for the $R({D^{(*)}})$ puzzle. We use form factors from lattice QCD to calculate all $Λ_b \to Λ_c τ\barν_τ$ observables. The $Λ_b \to Λ_c$ tensor form factors had not previously been determined in lattice QCD, and we present new lattice results for these form factors here.

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arXiv2017

A light $Z^\prime$ for the $R_K$ puzzle and nonstandard neutrino interactions

We show that the $R_K$ puzzle in LHCb data and the discrepancy in the anomalous magnetic moment of the muon can be simultaneously explained if a 10 MeV mass $Z^\prime$ boson couples to the muon but not the electron, and that clear evidence of the nonstandard matter interactions of neutrinos induced by this coupling may be found at DUNE.

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