arXiv subjects

Damir Becirevic

Publications and source records attributed to Damir Becirevic.

At least 19 recordsLinked to original sources

arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

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arXiv2024

Understanding the first measurement of $\mathcal{B}(B\to K ν\barν)$

Recently, Belle II reported on the first measurement of $\mathcal{B}(B^\pm\to K^\pm ν\barν)$ which appears to be almost $3σ$ larger than predicted in the Standard Model. We point out the important correlation with $\mathcal{B}(B\to K^{\ast} ν\barν)$ so that the measurement of that decay mode could help restraining the possible options for building the model of New Physics. We then try to interpret this new experimental result in terms of physics beyond the Standard Model by using SMEFT and find that a scenario with coupling only to $τ$ can accommodate the current experimental constraints but fails in getting a desired $R_{D^{(\ast )}}^\mathrm{exp}/R_{D^{(\ast )}}^\mathrm{SM}$, unless one turns the other SMEFT operators that are not related to $b\to s\ell\ell$ or/and $b\to sνν$.

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arXiv2022

Charged Current Anomalies with Baryons

Motivated by the first observation of $Λ_b\toΛ_c~τ\barν$ by LHCb, we discuss various observables that can be derived from angular distribution of this decay which, if measured, could be essential for explanation of the $b\to cτ\barν$ anomalies. Furthermore, we show that the measurement of $R_{Λ_c}=\mathcal{B}(Λ_b\toΛ_cτ\barν)/\mathcal{B}(Λ_b\toΛ_cμ\barν)$ does not change the current global picture arising from $R_D^{\rm}$ and $R_{D^*}^{\rm}$ being larger than predicted in the Standard Model. Finally, we show the example of a few angular observables, with and without a secondary decay, that could have a strong discriminating power on New Physics.

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arXiv2019

Simple operator formulation of the Bakamjian-Thomas approach to heavy quark current , with generalisation to HQET, and with applications to transitions of Lambda-b

Starting from the general formulation of current matrix elements in the Bakamjian-Thomas (BT) approach to quark models, certain aspects of their heavy quark limit are exhibited in an intuitive form, allowing a common treatment for any number of quarks and angular momenta j,j'. We use the general Falk interpretation of Isgur-Wise functions as overlaps of light parton states in motion. Then, in BT, for small collinear velocities, the motion of these states is described by a very simple dimensionless differential operator O acting on the internal wave functions of the light quarks, which is a relativistic generalisation of the dipole operator times the quark mass : it appears as the generator of Lorentz transformations for N free light particles.

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arXiv2019

Probing low energy scalar leptoquarks by the leptonic $W$ and $Z$ couplings

We compute the generic one-loop contribution involving scalar leptoquarks (LQ) to the $W$ and $Z$ leptonic decay widths. In our computation we include for the first time the finite terms and the corrections due to the external momenta of the electroweak bosons, which is a step beyond the leading-logarithmic approximation considered in the literature so far. We show that the terms we include can be numerically quite significant. They amount to about $20\%$ for scalar LQ masses below $1.5$~TeV, as currently allowed by the direct searches at the LHC. To further illustrate the relevance of our results we revisit a model with two light scalar LQs, proposed to accommodate the $B$-physics anomalies. We show that the finite terms we computed can reduce the tension with the $Z$-pole data.

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arXiv2019

Lepton Flavor Universality tests through angular observables of $\overline{B}\to D^{(\ast)}\ell\overlineν$ decay modes

We discuss the possibility of using the observables deduced from the angular distribution of the $B\to D^{(\ast)} \ell\barν$ decays to test the effects of lepton flavor universality violation (LFUV). We show that the measurement of even a subset of these observables could be very helpful in distinguishing the Lorentz structure of the New Physics contributions to these decays. To do so we use the low energy effective theory in which besides the Standard Model contribution we add all possible Lorentz structures with the couplings (Wilson coefficients) that are determined by matching theory with the measured ratios $R{(D^{(\ast)})}^\mathrm{exp}$. We argue that even in the situation in which the measured $R{(D^{(\ast)})}^\mathrm{exp}$ becomes fully compatible with the Standard Model, one can still have significant New Physics contributions the size of which could be probed by measuring the observables discussed in this paper and comparing them with their Standard Model predictions.

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arXiv2018

A model to accommodate the $B$-physics anomalies

After briefly reviewing the status of the $B$-physics anomalies, describing the challenges of building a scenario of physics beyond the Standard Model that can accommodate the observed departures from lepton flavor universality, I discuss a new model, based on extending the Standard Model by two light [$\mathcal{O}(1\,\mathrm{TeV})$] scalar leptoquarks. That model, in addition to satisfying a number of flavor physics constraints both at low and high energies, also allows for a $SU(5)$ unification.

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arXiv2017

Sterile neutrinos facing kaon physics experiments

We discuss weak kaon decays in a scenario in which the Standard Model is extended by massive sterile fermions. After revisiting the analytical expressions for leptonic and semileptonic decays we derive the expressions for decay rates with two neutrinos in the final state. By using a simple effective model with only one sterile neutrino, compatible with all current experimental bounds and general theoretical constraints, we conduct a thorough numerical analysis which reveals that the impact of the presence of massive sterile neutrinos on kaon weak decays is very small, less than $1\%$ on decay rates. The only exception is $\mathcal{B} (K_L\to νν)$, which can go up to $\mathcal{O}( 10^{-10})$, thus possibly within the reach of the KOTO experiment. In other words, if all the future measurements of weak kaon decays turn out to be compatible with the Standard Model predictions, this would not rule out the existence of massive light sterile neutrinos with non-negligible active-sterile mixing. Instead, for a sterile neutrino of mass below $m_K$, one might obtain a huge enhancement of $\mathcal{B} (K_L\to νν)$, otherwise negligibly small in the Standard Model.

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arXiv2016

Lepton Flavor Violation in Exclusive $b\to s$ Decays

Starting from the general effective hamiltonian relevant to the $b\to s$ transitions, we derive the expressions for the full angular distributions of the $B \to K^{(\ast )} \ell_1 \ell_2$ decay modes, as well as for ${\cal B}(B_s \to \ell_1\ell_2 )$ ($\ell_1\neq \ell_2$). We point out the differences in the treatment of the lepton flavor violating modes with respect to the lepton flavor conserving ones. Concerning the relevant Wilson coefficients we evaluate them in two different scenarios: (i) The (pseudo-)scalar coefficients are obtained using the (pseudo-)scalar coupling extracted from the experimental non-zero value of ${\cal B}(h\to μτ)$, (ii) Revisiting a $Z^\prime$-model in which the flavor changing neutral couplings are allowed. We provide the numerical estimates of the branching fractions of the above-mentioned modes in both scenarios.

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arXiv2016

Can the new resonance at LHC be a CP-Odd Higgs boson?

A plausible explanation of the recent experimental indication of a resonance in the two-photon spectrum at LHC is that it corresponds to the CP-odd Higgs boson. We explore such a possibility in a generic framework of the two Higgs doublet models (2HDM), and combine $m_A \approx 750$GeV with the known $m_h =125.7(4)$~GeV to show that the charged Higgs boson and the other CP-even scalar masses become bounded from bellow and from above. We show that this possibility is also consistent with the electroweak precision data and the low energy observables, which we test in a few leptonic and semileptonic decay modes.

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arXiv2016

Angular distributions of $\bar B \to D^{(\ast)}\ell\bar ν_\ell$ decays and search of New Physics

We derive the expressions for the full angular distributions of $\bar B\to D\ell\bar ν_\ell$ and $\bar B\to D^{\ast }\ell\bar ν_\ell$ decays and discuss the spectra on each angle separately. The coefficient functions, depending on helicity amplitudes, can then be combined in an ensemble of observables which can then be used to check for the presence of New Physics. We examine the sensitivity of each of these observables on the presence of non-Standard Model interaction terms at low energies. The expressions presented here are general and can be used for studying any other semileptonic pseudoscalar to pseudoscalar/vector meson decay. We also examine the problem of pollution of the $\bar B\to D^{\ast}(\to Dπ)_S\ell\bar ν_\ell$ decay sample by the $\bar B\to D_0^{\ast}(\to Dπ)\ell\bar ν_\ell$ events, and point out that a measurement of two particular quantities could clarify whether or not the $(Dπ)_{S-\rm wave}$ in the vicinity of $D^\ast$-peak is (approximately) described by the Breit-Wigner formula.

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arXiv2015

Lepton flavor violating decays of vector quarkonia and of the $Z$ boson

We address the impact of sterile fermions on the lepton flavor violating decays of quarkonia as well as of the $Z$ boson. We compute the relevant Wilson coefficients and show that the ${\rm B}(V\to\ell_α\ell_β)$, where $V=ϕ,ψ^{(n)}$, $Υ^{(n)},Z$ can be significantly enhanced in the case of large sterile fermion masses and a non-negligible active-sterile mixing. We illustrate that feature in a specific minimal realization of the inverse seesaw mechanism, known as $(2,3)$-ISS, and in an effective model in which the presence of non-standard sterile fermions is parameterized by means of one heavy sterile (Majorana) neutrino.

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arXiv2014

Lattice QCD estimate of the $η_{c}(2S)\to J/ψγ$ decay rate

We compute the hadronic matrix element relevant to the physical radiative decay $η_{c}(2S)\to J/ψγ$ by means of lattice QCD. We use the (maximally) twisted mass QCD action with Nf=2 light dynamical quarks and from the computations made at four lattice spacings we were able to take the continuum limit. The value of the mass ratio $m_{η_c(2S)}/m_{η_c(1S)}$ we obtain is consistent with the experimental value, and our prediction for the form factor is $V^{η_{c}(2S)\to J/ψγ}(0)\equiv V_{12}(0)=0.32(6)(2)$, leading to $Γ(η_c (2S) \to J/ψγ) = (15.7\pm 5.7)$ keV, which is much larger than $Γ(ψ(2S) \to η_cγ)$ and within reach of modern experiments.

hep-lat↗

arXiv2014

Insight into $D/B\to π\ell ν_\ell$ decay using the pole models

After improving the knowledge about residua of the semileptonic form factor at its first two poles we show that $f_+^{Dπ}(q^2)$ is not saturated when compared with the experimental data. To fill the difference we approximate the rest of discontinuity by an effective pole and show that the data can be described very well with the position of the effective pole larger than the next excitation in the spectrum of $D^\ast$ state. The results of fits with experimental data also suggest the validity of superconvergence which in the pole models translates to a vanishing of the sum of residua of the form factor at all poles. A similar discussion in the case of $B\to π\ellν_\ell$ leads to the possibility of extracting $\vert V_{ub}\vert$, the error of which appears to be dominated by $g_{B^\ast Bπ}$, which can be nowadays computed on the lattice. In evaluating the residua of the form factors at their nearest pole we needed the vector meson decay constants $f_{D^\ast}$ and $f_{B^\ast}$, which we computed by using the numerical simulations of QCD on the lattice with $N_{\rm f}=2$ dynamical quarks. We obtain, $f_{D^\ast}/f_D=1.208(27)$ and $f_{B^\ast}/f_B=1.051(17)$.

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arXiv2014

'Bs --> Ds l nu' near zero recoil in and beyond the Standard Model

We compute the normalization of the form factor entering the Bs --> Ds l nu decay amplitude by using numerical simulations of QCD on the lattice. From our study with Nf=2 dynamical light quarks, and by employing the maximally twisted Wilson quark action, we obtain in the continuum limit G(1) = 1.052(46). We also compute the scalar and tensor form factors in the region near zero recoil and find f0(t0)/f+(t0)=0.77(2), fT(t0,mb)/f+(t0)=1.08(7), for t0=11.5 GeV^2. These latter results are useful for searching the effects of physics beyond the Standard Model in Bs --> Ds l nu decays. Our results for the similar form factors relevant to the non-strange case indicate that the method employed here can be used to achieve the precision determination of the B --> D l nu decay amplitude as well.

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arXiv2014

Lattice QCD study of $B_s\to D_s \ell \barν_\ell$ decay near zero recoil

We study the hadronic matrix elements describing the $B_s \to D_s\ell \barν_\ell$ decay in and beyond the Standard Model. By using the twisted mass QCD on the lattice with $N_f=2$ dynamical flavors we compute the normalization $\mathscr G_s(1)$ of the form factor dominating $B_s\to D_s \ell \barν_\ell$ in the SM. We also make the first lattice determination of $F_0(q^2)/F_+(q^2)$ and $F_T(q^2)/F_+(q^2)$ near zero recoil (near $q^2_{\rm max}$). We briefly discuss the non-strange case $B \to D\ell \barν_\ell$ as well.

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arXiv2014

Lattice QCD and QCD Sum Rule determination of the decay constants of eta_c, J/psi and hc states

We compute the decay constants of the lowest ccbar-states with quantum numbers J(PC)=0(-+) [eta_c], 1(--) [J/psi], and 1(+-) [hc] by using lattice QCD and QCD sum rules. We consider the coupling of J/psi to both the vector and tensor currents. Lattice QCD results are obtained from the unquenched (Nf=2) simulations using twisted mass QCD at four lattice spacings, allowing us to take the continuum limit. On the QCD sum rule side we use the moment sum rules. The results are then used to discuss the rate of eta_c --> gamma gamma decay, and to comment on the factorization in B --> X K decays, with X being either eta_c or J/psi.

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arXiv2013

On the significance of B-decays to radially excited D

We discuss the possibilities of testing the recent claims of a relatively large value of the B --> D' transition form factor through B0--> D' pi+ and B- --> D' pi- decay modes. To estimate the width of the latter we need the decay constant of the radially excited D-meson that we computed in lattice QCD with Nf=2 dynamical flavors and after taking the continuum limit we found f(D')=117(25) MeV. We also provide an update for the values of the D-meson decay constants obtained by using maximally twisted mass QCD.

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