arXiv subjects

Damir Bečirević

Publications and source records attributed to Damir Bečirević.

At least 19 recordsLinked to original sources

arXiv2026

Probing the neutrino mass through semileptonic meson decays

We argue that a detailed analysis of semileptonic decays can test the possibility of a massive neutrino. The key observable, related to the forward-backward asymmetry, is exactly zero for a massless neutrino but becomes non-zero if the neutral lepton is heavy and interacts with Standard Model fields via left-handed operators. For right-handed interactions, this quantity differs significantly from zero even for a massless right-handed neutrino. We demonstrate this explicitly using the example of a pseudoscalar meson decaying into another pseudoscalar meson. A similar discussion applies to decays into a vector meson, with an additional subtlety addressed in this work.

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arXiv2025

Right-handed interactions in puzzling $B$-decays

We explain the difference between the measured decay widths of $b \to c τν$ processes and of $B\to Kν\barν$ and their values predicted in the Standard Model by introducing the right-handed interactions both to quarks and to leptons. At low energy scales, in addition to the Standard Model particles, we assume the presence of an additional neutral lepton (right-handed neutrino). We then show a specific realization of such a scenario in a model with a single scalar leptoquark ($S_1$), and discuss the corresponding phenomenology.

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arXiv2024

$R_{D^{(*)}}$ and survival of the fittest scalar leptoquark

Motivated by the long-standing discrepancy in lepton flavor universality ratios $R_D$ and $R_{D^{\ast}}$ we assess the status of scalar leptoquark states $R_2$, $\widetilde R_2$ and $S_1$ which can in principle provide a desired enhancement of $\mathcal{B}(B\to D^{(\ast )}τν)$ in a minimal setup with two Yukawa couplings only. We consider unavoidable low-energy constraints, $Z$-pole measurements as well as high-$p_T$ constraints. After setting mass of each leptoquark to $1.5$ TeV we find that of all considered states only $S_1$ leptoquark, coupled to both chiralities of leptons and quarks, is still a completely viable solution while the scenario with $R_2$ is in growing tension with $Γ(Z \to ττ)$ and with the LHC constraints on the di-tau tails at high-$p_T$. We comment on the future experimental tests of $S_1$ scenario.

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arXiv2024

Lepton flavor violation in exclusive $b\to d\ell_i\ell_j$ and $b\to s\ell_i\ell_j$ decay modes

We discuss the exclusive lepton flavor violating (LFV) decays modes based on $b\to d\ell_i\ell_j$ and $b\to s\ell_i\ell_j$ by considering the ground state mesons and baryons. After spelling out the expressions for such decay rates in a low energy effective theory which includes generic contributions arising from physics beyond the Standard Model (BSM), we show that the experimental bounds on meson decays can be used to bound the corresponding modes involving baryons. We find, for example, $\mathcal{B}(Λ_b\to Λμτ)\lesssim 4\times 10^{-5}$. We also consider two specific models and constrain the relevant LFV couplings by using the low energy observables. In the first model we assume the Higgs mediated LFV and find the resulting decay rates to be too small to be experimentally detectable. We also emphasize that the regions favored by the bounds $\mathcal{B}(h\toμτ)^\mathrm{Atlas}$ and $\mathcal{B}(h\to eτ)^\mathrm{Atlas}$ are not compatible with $\mathcal{B}(μ\to eγ)^\mathrm{MEG}$ to $1σ$. In the second model we assume LFV mediated by a heavy $Z'$ boson and find that the corresponding $b$-hadron branching fractions can be $\mathcal{O}(10^{-6})$, thus possibly within experimental reach at LHCb and Belle~II.

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arXiv2023

Revisiting $B\to K^{(\ast)} ν\barν$ decays in the Standard Model and beyond

In this letter we revisit the Standard Model predictions for $\mathcal{B}(B\to K^{(\ast)}ν\barν)$ and discuss the opportunities that open up when combining its partial decay rate with that of $B\to K^{(\ast)}\ell\ell$. In the Standard Model a suitable ratio of these two modes can be used to extract $C_9^\mathrm{eff}$, which is essential for a reliable phenomenological analysis of the $B\to K^{(\ast)}\ell\ell$ angular observables. The same ratio also proves to be more sensitive to the presence of New Physics in many plausible extensions of the Standard Model. We also suggest that the separate measurement of $\mathcal{B}(B\to Kν\barν)$ for high and for low $q^2$'s can be helpful for testing the assumed shape of the vector form factor, because the lattice QCD data are obtained at high $q^2$'s, whereas the low $q^2$ region is obtained through an extrapolation.

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arXiv2022

Looking for the effects of New Physics in the $Λ_b \longrightarrow Λ_c(\to Λπ) \ell ν$ decay mode

One of the most pragmatic ways to look for the effects of New Physics at low energy scales is to study a detailed angular distribution of various decay modes, and in particular those based on $b\to c\ell\bar ν_\ell$. In this paper we focus onto $Λ_b \longrightarrow Λ_c(\to Λπ) \ell ν$ in a generic effective theory setup in which, besides the Standard Model, we allow for all the possible covariant dimension-six effective operators capturing the contributions arising at high energy scales, beyond the Standard Model. We list a number of observables that could be used as a diagnostic tool to check for the presence of New Physics and to discriminate among its various scenarios. We also briefly comment on $Λ_c \to Λlν$.

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arXiv2022

On a model with two scalar leptoquarks -- $R_2$ and $S_3$

We discuss a model that can accommodate the $B$-physics anomalies, based on combining two scalar leptoquarks, $R_2$ and $S_3$, of mass $\mathcal{O}(1\,\mathrm{TeV})$, and that we proposed in our previous paper. We update the analysis of its parameter space and show that a model remains viable and consistent with a number of low energy and high energy flavor physics constraints. Since the model predicts a non-zero New Physics phase, we discuss the possibility to test its contribution to the neutron electric dipole moment and to the angular distributions of the exclusive $b\to cτ\bar ν$ decays. We find that the model can provide a significant enhancement to $\mathcal{B}(B\to K^{(\ast)}νν)$ and provides both the upper and lower bounds to $\mathcal{B}(B\to K^{(\ast)}μτ)$.

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arXiv2021

New Physics effects in leptonic and semileptonic decays

We discuss the possibilities of extracting the constraints on New Physics by using the current data on the leptonic and semileptonic decays of pseudoscalar mesons. In doing so we use a general low energy Lagrangian that besides the vector and axial operators also includes the (pseudo-)scalar and tensor ones. In obtaining constraints on New Physics couplings we combine the experimental information concerning several decay modes with the accurate lattice QCD results for the hadronic matrix elements. We propose to study new observables that can be extracted from the angular analysis of the semileptonic decays and discuss their values both in the Standard Model and in some specific scenarios of physics beyond the Standard Model.

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arXiv2021

On the single leptoquark solutions to the $B$-physics anomalies

We revisit the possibilities of accommodating the experimental indications of the lepton flavor universality violation in $b$-hadron decays in the minimal scenarios in which the Standard Model is extended by the presence of a single $\mathcal{O}(1\,\mathrm{TeV})$ leptoquark state. To do so we combine the most recent low energy flavor physics constraints, including $R_{K^{(\ast)}}^\mathrm{exp}$ and $R_{D^{(\ast)}}^\mathrm{exp}$, and combine them with the bounds on the leptoquark masses and their couplings to quarks and leptons as inferred from the direct searches at the LHC and the studies of the large $p_T$ tails of the $pp\to \ell\ell$ differential cross section. We find that none of the scalar leptoquarks of $m_\mathrm{LQ} \simeq 1÷2$ TeV can accommodate the $B$-anomalies alone. Only the vector leptoquark, known as $U_1$, can provide a viable solution which, in the minimal setup, provides an interesting prediction, i.e. a lower bound to the lepton flavor violating $b\to sμ^\pmτ^\mp$ decay modes, such as $\mathcal{B}(B\to Kμτ) \gtrsim 0.7\times 10^{-7}$.

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arXiv2020

Enhanced CP asymmetries in $B \to K μ^+ μ^-$

We show that the current values of $R_K^\mathrm{exp}$ and $R_{K^\ast}^\mathrm{exp}$ can be accommodated by allowing a nonzero New Physics coupling $δC_9^{μμ}$ to be complex, both in the scenario in which only $δC_9^{μμ}$ is affected, and in the scenario with complex $δC_{9,10}^{μμ}$ satisfying $δC_{9}^{μμ}=-δC_{10}^{μμ}$. A presence of the weak CP-violating phase can then be tested by measuring the CP-asymmetry, $\mathcal{A}_\mathrm{CP}$. We show that this asymmetry is enhanced around the peak of each $c\bar c$-resonance, and in fact more pronounced in the close vicinity of $J/ψ$ and $ψ(2S)$. Therefore, measuring $\mathcal{A}_\mathrm{CP}$ before and after the resonances' peak could be revelatory of the CP-violation that originates from beyond the Standard Model, or to be a significant constrain when building a realistic scenario of New Physics.

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arXiv2019

Palatable Leptoquark Scenarios for Lepton Flavor Violation in Exclusive $b\to s\ell_1\ell_2$ modes

We examine various scenarios that involve a light ${\cal O}(1 TeV)$ leptoquark state and select those which are compatible with the current experimental values for $\mathcal{B}(B_s\to μμ)$, $\mathcal{B}(B\to Kμμ)_{\mathrm{large}-q^2}$, $R_K=\mathcal{B^\prime}(B\to Kμμ)/\mathcal{B^\prime}(B\to Kee)$, and which lead to predictions consistent with other experimental data. We show that two such scenarios are phenomenologically plausible, namely the one with a doublet of scalar leptoquarks of hypercharge $1/6$, and the one with a triplet of vector leptoquarks of hypercharge $2/3$. We also argue that a model with a singlet scalar leptoquark of hypercharge $1/3$ is not viable. Using the present experimental data as constraints, it is shown that the exclusive lepton flavor violating decays, $\mathcal{B}(B_s\to μτ)$, $\mathcal{B}(B\to K μτ)$ and $\mathcal{B}(B\to K^\ast μτ)$, can be as large as $\mathcal{O}(10^{-5})$.

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arXiv2018

Closing the window on single leptoquark solutions to the $B$-physics anomalies

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both $B$-physics anomalies, viz. $R_{D^{(\ast)}}^\mathrm{exp}> R_{D^{(\ast)}}^\mathrm{SM}$ or/and $R_{K^{(\ast)}}^\mathrm{exp}< R_{K^{(\ast)}}^\mathrm{SM}$. To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass $m_\mathrm{LQ} \simeq 1$ TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for $m_\mathrm{LQ} \simeq 1÷2$ TeV is a vector leptoquark, known as $U_1$, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating $b\to s\ell_1^\pm\ell_2^\mp$ modes, such as $\mathcal{B}(B\to Kμτ)$. Improving the experimental upper bound on $\mathcal{B}(B\to Kμτ)$ by two orders of magnitude could compromise the viability of the minimal $U_1$ model as well.

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arXiv2018

Scalar leptoquarks from GUT to accommodate the $B$-physics anomalies

We address the $B$-physics anomalies within a two scalar leptoquark model. The low-energy flavor structure of our set-up originates from two $SU(5)$ operators that relate Yukawa couplings of the two leptoquarks. The proposed scenario has a UV completion, can accommodate all measured lepton flavor universality ratios in $B$-meson decays, is consistent with related flavor observables, and is compatible with direct searches at the LHC. We provide prospects for future discoveries of the two light leptoquarks at the LHC and predict several yet-to-be-measured flavor observables.

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arXiv2018

Seeking leptoquarks in IceCube

We investigate the sensitivity of IceCube(-Gen2) to a scalar leptoquark scenario with couplings only to heavy quark flavors which may be connected to solving discrepancies in $B$-meson semileptonic decays. We take into account, for the first time, the non-negligible neutrino-gluon cross section induced by leptoquarks, and we systematically account for indirect and direct constraints which have been overlooked in previous studies. We conclude that IceCube(-Gen2) can only probe the light LQ regime, already disfavored by the combination of flavor physics constraints, electroweak precision data and the direct searches at the LHC.

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arXiv2018

A leptoquark model to accommodate $R_K^\mathrm{exp} <R_K^\mathrm{SM} $ and $R_{K^\ast}^\mathrm{exp}< R_{K^\ast}^\mathrm{SM}$

We show that a modification of the model with a low energy scalar leptoquark state carrying hypercharge $Y=7/6$ allows to accommodate both $R_K<1$ and $R_{K^\ast}<1$, through loop effects, consistent with recent observations made at LHCb. We describe details of the model, compute the relevant Wilson coefficient and, after discussing a number of constraints, we examine the phenomenological consequences of the model. The bounds on the lepton flavor violating decay rates, induced by this model, include $\mathcal{B}(Z\toμτ) \lesssim \mathcal{O}(10^{-7})$, and $\mathcal{B}(B\to Kμτ) \lesssim \mathcal{O}(10^{-9})$. We also comment on the interpretation of the bounds on the leptoquark mass obtained from the direct searches at LHC.

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arXiv2017

Two Higgs Doublet Models and $b\to s$ exclusive decays

We computed the leading order Wilson coefficients relevant to all the exclusive $b\to s\ell^+\ell^-$ decays in the framework of Two Higgs Doublet Model (2HDM) with a softly broken $\mathbb{Z}_2$ symmetry by including the $\mathcal{O}(m_b)$ corrections. We elucidate the issue of appropriate matching between the full and the effective theory when dealing with the (pseudo-)scalar operators for which keeping the external momenta different from zero is necessary. We then make a phenomenological analysis by using the measured $\mathcal{B}(B_s\to μ^+μ^-)$ and $\mathcal{B}(B\to K μ^+μ^-)_{\mathrm{high}-q^2}$, for which the hadronic uncertainties are well controlled, and discussed their impact on various types of 2HDM. A brief discussion of the decays with $τ$-leptons in the final state is also made.

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arXiv2017

Unexpected pattern of transitions to radial excitations by heavy quark current

Motivated by the claimed possibility of a large contribution of the first radial excitation of the $D^{(\ast )}$ to the $B$ semileptonic decay into charmed mesons, also invoked to solve the "$1/2$ vs. $3/2$ semileptonic puzzle", we discuss the transitions to heavy-light radial excitations by a heavy $b \to c$ quark current. We first consider a HQET sum rule, which provides a bound on the slopes of Isgur-Wise functions which we then calculate in the Bakamjian-Thomas framework which both guaranties covariance in the heavy quark limit and satisfies a set of HQET sum rules. We observe a remarkable property that for a large variety of wave functions the transition to the first radial excitation is very small while the transition matrix element to the second radial excitation is large and dominant in saturating the HQET sum rule. This is opposite to what is found in non-relativistic models, where the transition to the first radial excitation dominates the sum rule. The relative magnitude of the transition to the second excitation appears to be weakly dependent on the dynamical scale (radius of the bound states), and the same holds true for the slope of the elastic transition. These features could be tested in the heavy mass limit of lattice QCD. This pattern is shown to be related to the general structure of the Bakamjian-Thomas model, it is independent of the spin structure of the approach and derives mainly from the Lorentz transformation of the spatial wave function, a feature often disregarded in quark models.

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arXiv2017

$B$ decays to radially excited $D$ mesons in Heavy Quark Effective Theory

Semileptonic transitions $\overline{B} \to D^{(n)} \ell \overlineν$, where $D^{(n)} (n \not = 0)$ is a radially excited meson, have recently attracted much attention as a way to understand some puzzles between theory and data. Following closely the formalism of Falk and Neubert for the elastic case, we study the $1/m_Q$ corrections to the heavy quark limit, in which the inelastic Isgur-Wise function vanishes at zero recoil, $ξ^{(n)}(1) = 0\ (n \not = 0)$. We find simple formulas that involve the derivative $ξ^{(n)'}(1)$, and we propose a number of ways of isolating this quantity in practice. We formulate also a generalization to the inelastic case of Luke's theorem. On the other hand, although some $1/m_Q$ HQET results are satisfied in the Bakamjian-Thomas relativistic quark model, we emphasize the problems concerning these corrections in this scheme.

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