arXiv subjects

Damir Becirevic

Publications and source records attributed to Damir Becirevic.

At least 37 records · Page 2Linked to original sources

arXiv2013

Theoretical estimate of the $D^* \to Dπ$ decay rate

We present the results of our lattice QCD study of the $g_{D^* Dπ}i$ coupling, relevant to the $D^\ast \to Dπ$ decay. Our computation is made on the gauge field configurations that include $N_f=2$ dynamical light quarks by using the twisted mass QCD action. From the results obtained at four different lattice spacings we were able to take for the first time the continuum limit of this quantity computed on the lattice. Our final value, $g_c=0.53(3)(3)$, leads to $Γ(D^{\ast +}\to \bar{D}^0 π^+)= (50\pm 5\pm 6)$ keV, and is in good agreement with the experimental results for the width of the charged $D^\ast$-meson.

hep-lat↗

arXiv2013

Testing the Standard Model in $B \to D τ\bar ν$ decay with minimal theory input

Recent experimental results for the ratio of the branching fractions of $\bar B \to D^{(*)}τν_τ$ and $B\to D^{(*)} μν_μ$ decays came as a surprise and lead to a discussion of possibility to constraining new physics through these modes. Here we focus on $Br(B \to D τ\barν_τ)/Br(B \to D μ\barν_μ)$ and argue that the result is consistent with the Standard Model within $2\,σ$ and that the test of compatibility of this ratio with the Standard Model can be done experimentally with a minimal theory input. We also show that these two decay channels can provide us with quite good constraints of the new physics couplings.

hep-ph↗

arXiv2013

Radiative decays of charmonia on the lattice

We present the results of our lattice QCD study of the hadronic matrix elements relevant to the physical radiative J/psi -> eta_c gamma and hc -> eta_c gamma decays. From computations with Nf=2 dynamical quark in twisted mass QCD at four lattice spacings, we were able to take the continuum limit and obtained Gamma(J/psi -> eta_c gamma) = 2.64(11) keV and Gamma(hc -> eta_c gamma) = 0.72(5) MeV. We also computed the the hyperfine splitting and found that it does not depend from the sea quark mass and we obtain Delta = m_J/psi - m_eta_c = 112(4) MeV.

hep-lat↗

arXiv2012

Lattice QCD study of the radiative decays $J/ψ\to η_cγ$ and $h_c\to η_cγ$

We present the results of our lattice QCD study of the hadronic matrix elements relevant to the physical radiative $J/ψ\to η_cγ$ and $h_c\to η_cγ$ decays. We used the twisted mass QCD action with $N_{\rm f}=2$ light dynamical quarks and from the computations made at four lattice spacings we were able to take the continuum limit. Besides the form factors parameterizing the above decays we also computed: (i) the hyperfine splitting and obtained $Δ= 112\pm 4$ MeV, (ii) the annihilation constant $f_{J/ψ}$ which agrees with the one inferred from the measured $Γ(J/ψ\to e^+e^-)$.

hep-lat↗

arXiv2012

Impact of $B\to K^\ast_0 \ell^+\ell^-$ on the New Physics search in $B\to K^\ast \ell^+\ell^-$ decay

We discuss the uncertainty related to the amount of unwanted $B\to K_0^\ast (Kπ)\ell^+\ell^-$ events in the sample of $B\to K^\ast (Kπ)\ell^+\ell^-$. Those events can increase the measured differential decay rate by up to 10% in the low $q^2$ region, and can be a source of non-negligible uncertainty in the full angular distribution of the $B\to K^\ast (Kπ)\ell^+\ell^-$ decay. Although the transverse asymmetries should be unaffected by the presence of the $S$-wave $Kπ$ pairs, coming from the scalar $K_0^\ast$ meson, we show that in practice their normalization might be sensitive to those events and could entail a sizable uncertainty in transverse asymmetries around $q^2=2 GeV^2$. For other $q^2$'s that error is under about 10%.

hep-ph↗

arXiv2012

$\bar B\to Dτ\bar ν_τ$ vs. $\bar B\to Dμ\bar ν_μ$

Recent experimental results for the ratio of the branching fractions of $\bar B\to D^{(*)}τ\bar ν_τ$ and $\bar B\to D^{(*)}μ\bar ν_μ$ decays came as a surprise and lead to a discussion of possibility to constraining New Physics through these modes. Here we focus on ${\cal B}(\bar B\to Dτ\bar ν_τ)/{\cal B}(\bar B\to Dμ\bar ν_μ)$ and argue that the result is consistent with the Standard Model within $2σ$, and that the test of compatibility of this ratio with the Standard Model can be done experimentally with a minimal theory input. We also show that these two decay channels can provide us with quite good constraints of the New Physics couplings.

hep-ph↗

arXiv2012

Proposal to study $B_s \to \bar D_{sJ}$ transitions

It is proposed to clear some of the puzzles of B decay to the broad $\bar{D}^{\ast\ast}$ states by studying the corresponding decay with strange states $B_s^0 \to D_{s0}^{\ast -} π^+$ at LHCb. Interpretation of the results should be easier due to the narrowness of the $D_{s0}^{\ast -}$ state.

hep-ph↗

arXiv2012

Complementarity of Bs->mu+mu- and B-> K l+l- in New Physics searches

This year, LHC provided a very stringent bound on Br(Bs->mu+mu-), bringing it closer to the value predicted by the Standard Model (SM). Bs->mu+mu- was believed to be the golden mode at LHCb to find SUSY because a large enhancement was expected in the regime of moderate and large values of tanb. Other scenarios are still possible and a correlation with other decay channels is needed. We show that a complementary information on New Physics (NP) can be obtained model-indepedently from the B-> K l+l- decay mode. We provide a prediction Br(B-> K l+l-) based on the first lattice QCD results for all three relevant form factors, f0(q^2), f+(q^2) and fT(q^2). We were then able to provide the model-independent bounds on the complex couplings to scalar and pseudoscalr operators in the b->s sector.

hep-ph↗

arXiv2012

Complementarity of the constraints on New Physics from B_s -> mu+ mu- and from B -> K l+l- decays

We discuss the advantages of combining the experimental bound on Br(B_s -> mu+ mu-) and the measured Br(B -> K l+l-) to get the model independent constraints on physics beyond the Standard Model. Since the two decays give complementary information, one can study not only the absolute values of the Wilson coefficients that are zero in the Standard Model, but also their phases. To identify the sector in which the new physics might appear, information about the shapes of the transverse asymmetries in B -> K* l+l- at low q^2's can be particularly useful. We also emphasize the importance of measuring the forward-backward asymmetry in B -> K l+l- decay at large q^2's.

hep-ph↗

arXiv2012

Future prospects for the determination of the Wilson coefficient $C_{7γ}^\prime$

We discuss the possibilities of assessing a non-zero $C_{7γ}^\prime$ from the direct and the indirect measurements of the photon polarization in the exclusive $b \to sγ^{(*)}$ decays. We focus on three methods and explore the following three decay modes: $B \to K^*(\to K_Sπ^0)γ$, $B \to K_1(\to Kππ)γ$, and $B \to K^*(\to Kπ)\ell^+\ell^-$. By studying different New Physics scenarios we show that the future measurement of conveniently defined observables in these decays could provide us with the full determination of $C_{7γ}$ and $C_{7γ}^\prime$.

hep-ph↗

arXiv2012

Pionic couplings to the lowest heavy-light mesons of positive and negative parity

We present the method and compute the strong couplings of the lowest and first orbitally excited heavy-light mesons to a soft pion in the static heavy quark limit on the lattice. Besides the usual g^ and g couplings, we were able to make the first computation of the coupling h using the relevant radial distributions. Our results are obtained from the simulations of QCD with Nf=2 light Wilson-Clover quarks, combined with the improved static quark actions. The hierarchy among couplings that emerges from our study is g < g^ < h.

hep-lat↗

arXiv2012

On transverse asymmetries in B --> K* l+l-

We discuss the three independent asymmetries, AT(2)(q2), AT(im)(q2) and AT(re)(q2), that one can build from the amplitudes A_perp(q2) and A_par(q2). These quantities are expected to be accessible from the new B-physics experiments, they are sensitive to the presence of new physics, and they are not significantly sensitive to hadronic uncertainties. Studying their low q2-dependence can be helpful in discerning among various possible new physics scenarios. All three asymmetries can be extracted from the full angular analysis of B-->K* l+l-. Our formulas apply to both the massless and the massive lepton case.

hep-ph↗

arXiv2012

D-meson decay constants and a check of factorization in non-leptonic B-decays

We compute the vector meson decay constants fD*, fDs* from the simulation of twisted mass QCD on the lattice with Nf = 2 dynamical quarks. When combining their values with the pseudoscalar D(s)-meson decay constants, we were able (i) to show that the heavy quark spin symmetry breaking effects with the charm quark are large, fDs*/fDs = 1.26(3), and (ii) to check the factorization approximation in a few specific B-meson non-leptonic decay modes. Besides our main results, fD* = 278 \pm 13 \pm 10 MeV, and fDs* = 311 \pm 9 MeV, other phenomenologically interesting results of this paper are: fDs*/fD* = 1.16 \pm 0.02 \pm 0.06, fDs*/fD = 1.46 \pm 0.05 \pm 0.06, and fDs/fD* = 0.89 \pm 0.02 \pm 0.03. Finally, we correct the value for B(B0 \rightarrow D+ pi-) quoted by PDG, and find B(B0 \rightarrow D+ pi-) = (7.8 \pm 1.4) \times 10-7. Alternatively, by using the ratios discussed in this paper, we obtain B(B0 \rightarrow D+ pi-) = (8.3 \pm 1.0 \pm 0.8)\times10-7.

hep-lat↗

arXiv2011

Spatial distributions in static heavy-light mesons: a comparison of quark models with lattice QCD

Lattice measurements of spatial distributions of the light quark bilinear densities in static mesons allow to test directly and in detail the wave functions of quark models. These distributions are gauge invariant quantities directly related to the spatial distribution of wave functions. We make a detailed comparison of the recent lattice QCD results with our own quark models, formulated previously for quite different purposes. We find a striking agreement not only between our two quark models, but also with the lattice QCD data for the ground state in an important range of distances up to about 4/GeV. Moreover the agreement extends to the L=1 states [j^P=(1/2)^+]. An explanation of several particular features completely at odds with the non-relativistic approximation is provided. A rather direct, somewhat unexpected and of course approximate relation between wave functions of certain quark models and QCD has been established.

hep-ph↗

arXiv2009

Soft photons in semileptonic B \to D decays

Determination of V_cb in exclusive semileptonic decays is crucial consistency check against the V_cb determined inclusively. Anticipated precision of V_cb at the Super Flavor factory is ~1%, with most of the theoretical error due to hadronic form factor uncertainties. However, at this level of precision treating electromagnetic effects becomes inevitable. In addition to virtual photon corrections there are also emissions of real photons which are soft enough to avoid detection. The bremsstrahlung part is completely universal and is accounted for in the experimental analyses. However, the so-called structure dependent contribution, which probes the hadronic content of the process and is infrared finite, has been neglected so far. To this end, we estimated fraction of radiative events which are identified as semileptonic by experiment.

hep-ph↗

arXiv2009

On internal structure of the heavy-light mesons

We compute the radial distributions of the vector and axial charge density as well as of the matter density in the heavy-light mesons on the lattice. The results for the lowest lying static heavy-light mesons and their first excited counterparts are obtained with Nf=2 dynamical quarks of Wilson (Clover) type and with the various improved static heavy quark actions. From these distributions we were able to compute the corresponding charges and slopes of the form factors (i.e. ). Those results are also presented, and briefly discussed too.

hep-lat↗

arXiv2009

g(B*Bpi)-coupling in the static heavy quark limit

By means of QCD simulations on the lattice, we compute the coupling of the heavy-light mesons to a soft pion in the static heavy quark limit. The gauge field configurations used in this calculations include the effect of N_f=2 dynamical Wilson quarks, while for the static quark propagator we use its improved form (so called HYP). On the basis of our results we obtain that the coupling g=0.44 +/- 0.03 (+0.07/-0.00), where the second error is flat (not gaussian).

hep-ph↗

arXiv2009

Soft Photon Problem in Leptonic B-decays

We point out at the peculiarity of B --> mu nu decay, namely the enhancement of the soft photon events which originate from the structure dependent part of the B --> mu nu gamma amplitude. This may be a dominant source of systematic uncertainty and compromise the projected experimental uncertainty on Gamma(B --> mu nu). We show that the effect of these soft photons can be controlled if the experimental cut on identification of soft photons is lowered and especially if the better resolution in identifying the momentum of muon emerging from B --> mu nu, is made. A lattice QCD computation of the relevant form factors would be highly helpful for a better numerical control over the structure dependent soft photon emission.

hep-ph↗