Nonzero Recoil Research Articles

Semileptonic form factors for Brightarrow D^*ell nu at nonzero recoil from 2+1-flavor lattice QCD

We present the first unquenched lattice-QCD calculation of the form factors for the decay Brightarrow D^*ell nu at nonzero recoil. Our analysis includes 15 MILC ensembles with N_f=2+1 flavors of asqtad sea quarks, with a strange quark mass close to its physical mass. The lattice spacings range from aapprox 0.15 fm down to 0.045 fm, while the ratio between the light- and the strange-quark masses ranges from 0.05 to 0.4. The valence b and c quarks are treated using the Wilson-clover action with the Fermilab interpretation, whereas the light sector employs asqtad staggered fermions. We extrapolate our results to the physical point in the continuum limit using rooted staggered heavy-light meson chiral perturbation theory. Then we apply a model-independent parametrization to extend the form factors to the full kinematic range. With this parametrization we perform a joint lattice-QCD/experiment fit using several experimental datasets to determine the CKM matrix element |V_{cb}|. We obtain left| V_{cb}right| = (38.40 pm 0.68_{text {th}} pm 0.34_{text {exp}} pm 0.18_{text {EM}})times 10^{-3}. The first error is theoretical, the second comes from experiment and the last one includes electromagnetic and electroweak uncertainties, with an overall chi ^2text {/dof} = 126/84, which illustrates the tensions between the experimental data sets, and between theory and experiment. This result is in agreement with previous exclusive determinations, but the tension with the inclusive determination remains. Finally, we integrate the differential decay rate obtained solely from lattice data to predict R(D^*) = 0.265 pm 0.013, which confirms the current tension between theory and experiment.

Exclusive determinations of vert V_{cb} vert and R(D^{*}) through unitarity

In this work we apply the Dispersive Matrix (DM) method of Di Carlo et al. (Phys Rev D 104:054502, 2021) and Martinelli et al. (Phys Rev D 105:034503, 2022) to the lattice computations of the Form Factors (FFs) entering the semileptonic B rightarrow D^* ell nu _ell decays, recently produced by the FNAL/MILC Collaborations (Fermilab Lattice, MILC collaboration, Semileptonic form factors for B rightarrow D^*ell nu at nonzero recoil from 2 + 1-flavor lattice QCD. arXiv:2105.14019) at small, but non-vanishing values of the recoil variable (w-1). Thanks to the DM method we obtain the FFs in the whole kinematical range accessible to the decay in a completely model-independent and non-perturbative way, implementing exactly both unitarity and kinematical constraints. Using our theoretical bands of the FFs we extract vert V_{cb} vert from the experimental data and compute the theoretical value of R(D^*). Our final result for vert V_{cb} vert reads vert V_{cb} vert = (41.3 pm 1.7) cdot 10^{-3}, compatible with the most recent inclusive estimate at the 0.5sigma level. Moreover, we obtain the pure theoretical value R(D^*) = 0.275 pm 0.008, which is compatible with the experimental world average at the sim 1.3 sigma level.

Revisiting fits to B0→D*−ℓ+νℓ to measure |Vcb| with novel methods and preliminary LQCD data at nonzero recoil

We present a study of fits to exclusive ${B}^{0}\ensuremath{\rightarrow}{D}^{*\ensuremath{-}}{\ensuremath{\ell}}^{+}{\ensuremath{\nu}}_{\ensuremath{\ell}}$ measurements for the determination of the Cabibbo-Kobayashi-Maskawa matrix element magnitude $|{V}_{cb}|$, based on the most recent Belle untagged measurement. Results are obtained with the Caprini-Lellouch-Neubert (CLN) and Boyd-Grinstein-Lebed (BGL) form factor parametrizations, with and without the inclusion of preliminary lattice QCD (LQCD) measurements of form factors at nonzero hadronic recoil from the JLQCD Collaboration. The CLN and BGL fits are also studied in different scenarios with reduced theoretical assumptions and at higher-order expansions, respectively. To avoid bias from high systematic error correlations, we employ a novel technique in the field of $B$-physics phenomenology with a toy Monte Carlo method using a Cholesky decomposition of the covariance matrix. Using additional input from lattice QCD calculations of form factors at nonzero recoil, in collaboration with JLQCD, allows for well-defined fit results with reduced model dependence in CLN and BGL. The results obtained are consistent between different configurations, ultimately providing a method for a more model-independent exclusive measurement of $|{V}_{cb}|$. Using preliminary LQCD inputs and assuming a value of $\mathcal{F}(1)=0.904\ifmmode\pm\else\textpm\fi{}0.012$, we find $|{V}_{cb}|=(38.48\ifmmode\pm\else\textpm\fi{}0.32\ifmmode\pm\else\textpm\fi{}0.97\ifmmode\pm\else\textpm\fi{}0.46)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ in BGL(2,2,2) and $(38.42\ifmmode\pm\else\textpm\fi{}0.35\ifmmode\pm\else\textpm\fi{}1.05\ifmmode\pm\else\textpm\fi{}0.46)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ in CLNnoHQS, where the errors are statistical, systematic, and related to $\mathcal{F}(1)$, respectively.

Solar neutrino scattering with electron into massive sterile neutrino

The recent Xenon1T excess can be explained by solar neutrino scattering with electron via a light mediator, either scalar or vector, in addition to many other explanations from the dark sector. Since only the recoil electron is observable, a keV sterile neutrino instead of an active neutrino can appear in the final state. The sterile neutrino allows pseudoscalar mediator to explain the Xenon1T excess which was thought impossible. In addition, nonzero recoil energy lower bound arises from the sterile neutrino mass, which can be used to testify if the sterile neutrino is massive or not. We also briefly discuss the case of sterile neutrino final state with light Z′ mediator.

Updates on extraction of |Vcb| and SM prediction of R(D*) in B \u2192 D*\u2113\u03bd\u2113 decays

We update the standard model (SM) predictions of R(D*) using the latest results on the decay distributions in B → D∗ℓνℓ (ℓ = μ, e) by Belle collaboration, while extracting |Vcb| at the same time. Depending on the inputs used in the analysis, we define various fit scenarios. Although the central values of the predicted R(D*) in all the scenarios have reduced from its earlier predictions in 2017, the results are consistent with each other within the uncertainties. In this analysis, our prediction of R(D*) is consistent with the respective world average at ∼ 3σ. We have also predicted several angular observables associated with B → D*τντ decays. We note that the predicted FL(D*) is consistent with the corresponding measurement at 2σ. Utilizing these new results, we fit the Wilson coefficients appearing beyond the standard model of particle physics (BSM). To see the trend of SM predictions, we have used the recently published preliminary results on the form-factors at non-zero recoil by the lattice groups like Fermilab-MILC and JLQCD and predicted the observables in B → D∗ℓνℓ, and B → D*τντ decays.

B → D*lv at non-zero recoil

We present preliminary results from our analysis of the form factors for the B → D*lv decay at non-zero recoil. Our analysis includes 15 MILC asqtad ensembles with Nf = 2 + 1 flavors of sea quarks and lattice spacings ranging from a ≈ 0.15 fm down to 0.045 fm. The valence light quarks employ the asqtad action, whereas the heavy quarks are treated using the Fermilab action. We conclude with a discussion of future plans and phenomenological implications. When combined with experimental measurements of the decay rate, our calculation will enable a determination of the CKM matrix element |Vcb|.

Bs→Dsℓν form factors and the fragmentation fraction ratio fs/fd

We present a lattice quantum chromodynamics determination of the scalar and vector form factors for the $B_s \rightarrow D_s \ell \nu$ decay over the full physical range of momentum transfer. In conjunction with future experimental data, our results will provide a new method to extract $|V_{cb}|$, which may elucidate the current tension between exclusive and inclusive determinations of this parameter. Combining the form factor results at non-zero recoil with recent HPQCD results for the $B \rightarrow D \ell \nu$ form factors, we determine the ratios $f^{B_s \rightarrow D_s}_0(M_\pi^2) / f^{B \rightarrow D}_0(M_K^2) = 1.000(62)$ and $f^{B_s \rightarrow D_s}_0(M_\pi^2) / f^{B \rightarrow D}_0(M_\pi^2) = 1.006(62)$. These results give the fragmentation fraction ratios $f_s/f_d = 0.310(30)_{\mathrm{stat.}}(21)_{\mathrm{syst.}}(6)_{\mathrm{theor.}}(38)_{\mathrm{latt.}} $ and $f_s/f_d = 0.307(16)_{\mathrm{stat.}}(21)_{\mathrm{syst.}}(23)_{\mathrm{theor.}}(44)_{\mathrm{latt.}}$, respectively. The fragmentation fraction ratio is an important ingredient in experimental determinations of $B_s$ meson branching fractions at hadron colliders, in particular for the rare decay ${\cal B}(B_s \rightarrow \mu^+ \mu^-)$. In addition to the form factor results, we make the first prediction of the branching fraction ratio $R(D_s) = {\cal B}(B_s\to D_s\tau\nu)/{\cal B}(B_s\to D_s\ell\nu) = 0.301(6)$, where $\ell$ is an electron or muon. Current experimental measurements of the corresponding ratio for the semileptonic decays of $B$ mesons disagree with Standard Model expectations at the level of nearly four standard deviations. Future experimental measurements of $R(D_s)$ may help understand this discrepancy.

Erratum:B→Dlνform factors at nonzero recoil and extraction of|Vcb|[Phys. Rev. D92, 054510 (2015)

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B→Dℓνform factors at nonzero recoil and|Vcb|from2+1-flavor lattice QCD

We present the first unquenched lattice-QCD calculation of the hadronic form factors for the exclusive decay $\overline{B} \rightarrow D \ell \overline{\nu}$ at nonzero recoil. We carry out numerical simulations on fourteen ensembles of gauge-field configurations generated with 2+1 flavors of asqtad-improved staggered sea quarks. The ensembles encompass a wide range of lattice spacings (approximately 0.045 to 0.12 fm) and ratios of light (up and down) to strange sea-quark masses ranging from 0.05 to 0.4. For the $b$ and $c$ valence quarks we use improved Wilson fermions with the Fermilab interpretation, while for the light valence quarks we use asqtad-improved staggered fermions. We extrapolate our results to the physical point using rooted staggered heavy-light meson chiral perturbation theory. We then parameterize the form factors and extend them to the full kinematic range using model-independent functions based on analyticity and unitarity. We present our final results for $f_+(q^2)$ and $f_0(q^2)$, including statistical and systematic errors, as coefficients of a series in the variable $z$ and the covariance matrix between these coefficients. We then fit the lattice form-factor data jointly with the experimentally measured differential decay rate from BaBar to determine the CKM matrix element, $|V_{cb}|=(39.6 \pm 1.7_{\rm QCD+exp} \pm 0.2_{\rm QED})\times 10^{-3}$. As a byproduct of the joint fit we obtain the form factors with improved precision at large recoil. Finally, we use them to update our calculation of the ratio $R(D)$ in the Standard Model, which yields $R(D) = 0.299(11)$.

Motion of an atom due to anisotropic filtering of its resonance fluorescence

The effect of the specific anisotropy of the environment on the dynamics of a resonantly fluorescing atom is analyzed in a one-dimensional model. The environmental anisotropy, which is manifested as different spectral selectivities of the absorption of spontaneous photons emitted by the atom in different directions, results in the anisotropy of the photon emission rate giving rise to a nonzero recoil force on the atom. The effect under optimal conditions can reach one-quarter of the recoil momentum per single photon emission. This force is directed toward the weaker spectral selectivity.

Light quark dependence of the Isgur-Wise function from QCD sum rules.

We study light quark dependence of the Isgur-Wise function for $B_a\rightarrow D_a$ and $B_a\rightarrow D^{*}_a$ in the framework of QCD sum rules. At zero recoil , all the Isgur-Wise functions equal as required by heavy quark symmetry and at non-zero recoil, the Isgur-Wise function for $B_s$ decay falls faster than that for $B_{u,d}$ decay , which is just contrary to the recent prediction of the heavy meson chiral perturbation theory. As by-products, we also estimate SU(3) breaking effects in the mass and the decay constant.

Heavy-baryon weak form factors

It has recently been shown that hadrons containing a single heavy quark exhibit a new flavor-spin symmetry of QCD. We exploit this symmetry to obtain model-independent absolutely normalized predictions for some heavy-baryon weak form factors at zero recoil as well as relations between such form factors at nonzero recoil.