Meson Semileptonic Decays Research Articles

Tomography of the QGP by heavy quarks

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities, the electric conductivity or the charm diffusion coefficient of the hot QCD medium. In this contribution we report on the recent results on the charm dynamics and elliptic flow in Au+Au collisions at RHIC and Pb+Pb reactions at the LHC as well as on the single electron spectra from D— and B—meson semileptonic decays in Au+Au collisions at √sNN =200 and 62.4 GeV. We find that the PHSD approach well describes the Raa and elliptic flow v2 of open charm mesons in Au+Au collisions at √sNN = 200 GeV (from STAR) and 2.76 TeV (from ALICE) as well as the elliptic flow of single electrons at √sNN = 200 and 62.4 GeV (from PHENIX), however, the large Raa at √sNN = 62.4 GeV is not reproduced at all which might indicate a new ’PHENIX puzzle’.

Simulated Measurement of the Ds Meson Semileptonic Decay Form Factor with the P̄ANDA Detector

We simulate the anti-proton and proton interaction to evaluate the P̄ANDA detector performance in the measurement of the semileptonic decay form factor of Ds+→e+νeη. The kinematics of the neutrino have been reconstructed with a complete simulation model of the detector and reconstruction tools. With theoretical predictions of the cross section, we obtain a preliminary estimate of the expected count rate for the future data taking.

Towards the low energy frontier

Unanswered and interesting questions arise in the leptonic sector of the standard model of particle physics: Are there new sources of CP violation? Are the neutrinos Dirac or Majorana particles? Are there more than one Higgs doublet that modify the leptonic or semileptonic decays of mesons? All these questions may be answered by a detailed study of low energy experiments. In this talk we present a short review of some examples where using present or future data on D meson decays, neutrino scattering on electron or coherent scattering on nucleus and neutrinos produced in accelerators may help us to answer those questions.

Leptoquarks, dark matter, and anomalous LHC events

Leptoquarks with mass in the region of 550--650 GeV are a possible candidate for the recent excess seen by CMS in the $eejj$ and $e\ensuremath{\nu}jj$ channels. We discuss models where leptoquarks decay primarily to dark matter and jets, thereby giving a branching to charged lepton and jet final states that can match data. The confluence of proton decay constraints, dark matter indirect and direct detection data, and Higgs invisible decay bounds results in a handful of predictive models that will be conclusively verified or excluded in upcoming direct detection experiments. Along the way, we present robust limits on such leptoquark models stemming from the muon magnetic moment using current and projected experiment sensitivities, as well as from $K$ and $B$ meson mixing, and leptonic and semileptonic meson decays.

Semileptonic meson decays in point-form relativistic quantum mechanics: Unambiguous extraction of weak form factors

The point-form version of the Bakamjian-Thomas construction is applied to the description of several semileptonic decays of mesons. Weak form factors are extracted without ambiguity for pseudoscalar-to-pseudoscalar as well as for pseudoscalar-to-vector transitions of mesons from the most general covariant decomposition of the weak current. No manifestation of cluster-separability violation appears in the form of non-physical contributions to the structure of such a current, in contrast to what happens in the electromagnetic case. Moreover, no frame dependence is observed when we extract the form factors from the most general covariant decomposition of the current, which contrasts with analogous front-form calculations that involve vector mesons in the transition. We present our results for heavy-light meson decays, i.e. $B\to D$, as well as for $B$ and $D$ mesons decaying into $\pi$, $\rho$ and $K^{(*)}$ and perform a numerical comparison with the analogous front-form approach. Differences between point and front forms that are not seen in the heavy-quark limit of $q\bar Q$-systems appear. These differences are attributed to the different role that the non-valence contributions play in the description of hadronic reactions in each form. It is argued how contributions from missing $Z$-graphs can be estimated.

QCD corrections to theBcto charmonia semileptonic decays

We present a detailed analysis on the $B_c$ meson semi-leptonic decays, $B_c \to \eta_c (J/\psi) \ell \nu$, up to next-to-leading order (NLO) QCD correction. We adopt the principle of maximum conformality (PMC) to set the renormalization scales for those decays. After applying the PMC scale setting, we determine the optimal renormalization scale for the $B_c\to\eta_c(J/\psi)$ transition form factors (TFFs). Because of the same $\{\beta_0\}$-terms, the optimal PMC scales at the NLO level are the same for all those TFFs, i.e. $\mu_r^{\rm PMC} \approx 0.8{\rm GeV}$. We adopt a strong coupling model from the massive perturbation theory (MPT) to achieve a reliable pQCD estimation in this low energy region. Furthermore, we adopt a monopole form as an extrapolation for the $B_c\to\eta_c(J/\psi)$ TFFs to all their allowable $q^2$ region. Then, we predict $\Gamma_{B_c \to \eta_c \ell \nu}(\ell=e,\mu) =(71.53^{+11.27}_{-8.90})\times 10^{-15} {\rm GeV}$, $\Gamma_{B_c \to \eta_c \tau \nu}=(27.14^{+5.93}_{-4.33})\times 10^{-15} {\rm GeV}$, $\Gamma_{B_c \to J/\psi \ell \nu}(\ell=e,\mu) =(106.31^{+18.59}_{-14.01}) \times 10^{-15} {\rm GeV}$, $\Gamma_{B_c \to J/\psi \tau \nu} =(28.25^{+6.02}_{-4.35})\times 10^{-15} {\rm GeV}$, where the uncertainties are squared averages of all the mentioned error sources. We show that the present prediction of the production cross section times branching ratio for $B^+_c\to J/\psi \ell^+ v$ relative to that for $B^+ \to J/\psi K^+$, i.e. $\Re(J/\psi \ell^+ \nu)$, is in a better agreement with CDF measurements than the previous predictions.

NLO twist-3 contributions toB→πform factors inkTfactorization

In this paper, we calculate the next-to-leading-order (NLO) twist-3 contribution to the form factors of $B \to \pi$ transitions by employing the $k_{T}$ factorization theorem. All the infrared divergences regulated by the logarithms $\ln(k_{iT}^{2})$ cancel between those from the quark diagrams and from the effective diagrams for the initial $B$ meson wave function and the final pion meson wave function. An infrared finite NLO hard kernel is therefore obtained, which confirms the application of the $k_{T}$ factorization theorem to $B$ meson semileptonic decays at twist-3 level. From our analytical and numerical evaluations, we find that the NLO twist-3 contributions to the form factors $f^{+,0}(q^2)$ of $B \to \pi$ transition are similar in size, but have an opposite sign with the NLO twist-2 contribution, which leads to a large cancelation between these two NLO parts. For the case of $f^+(0)$, for example, the $24\%$ NLO twist-2 enhancement to the full LO prediction is largely canceled by the negative ( about $-17\%$ ) NLO twist-3 contribution, leaving a small and stable $7\%$ enhancement to the full LO prediction in the whole range of $0\leq q^2\leq 12$ GeV$^2$. At the full NLO level, the perturbative QCD prediction is $F^{B \to \pi}(0)=0.269^{+0.054}_{-0.050}$. We also studied the possible effects on the pQCD predictions when different sets of the B meson and pion distribution amplitudes are used in the numerical evaluation.

Remarks on the lifetime of sterile neutrinos and the effect on detection of rare meson decaysM+→M′−ℓ+ℓ+

We review the detection of the $\mathrm{\ensuremath{\Delta}}L=2$ semileptonic meson decays of the form ${M}^{+}\ensuremath{\rightarrow}{M}^{\ensuremath{'}\ensuremath{-}}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{+}$ that we studied in a previous work, mediated by a Majorana neutrino with a mass in the range between the masses of the initial and final mesons. For such a range of masses, the Majorana neutrino will go on its mass shell, and the two charged leptons will appear at displaced vertices or, if the lifetime is long enough, most secondary vertices will fall outside the detector. We study the consequences of this effect on the experimental searches and limits that can be extracted.

Sterile neutrinos in leptonic and semileptonic decays

We address the impact of a modified $W \ell \nu$ coupling on a wide range of observables, such as $\tau$ leptonic and mesonic decays, leptonic decays of pseudoscalar mesons, as well as semileptonic meson decays. In particular, we concentrate on deviations from lepton flavour universality, focusing on the ratios $R_{P} = \Gamma (P \to \ell \nu) / \Gamma (P \to \ell' \nu)$, with $P=K, \pi, D, D_s$, $R(D)={\Gamma (B^+ \to D \tau^+ \nu)}/{\Gamma (B^+ \to D\ell^+ \nu)}$, $R \tau={\Gamma (\tau\to \mu\nu\nu)}/{\Gamma (\tau\to e\nu\nu)}$, $R^{\ell \tau}_P=\Gamma(\tau\to P\nu)/\Gamma(P\to \ell \nu)$, and $\text{BR}(B \to \tau \nu)$. We further consider leptonic gauge boson decays, such as $W\to \ell \nu $ and $Z \to \nu \nu$. For all the above observables, we provide the corresponding complete analytical expressions, derived for the case of massive neutrinos. Working in the framework of the Standard Model extended by additional sterile fermions, which mix with the active (left-handed) neutrinos, we numerically study the impact of active-sterile mixings on the above mentioned observables.

Open heavy flavor measurements ind+Au collisions at PHENIX experiment

The heavy quarks produced in the early stage of heavy-ion collisions are very effective probes of the dense partonic medium produced at RHIC. PHENIX has the ability to measure heavy quark production through single electrons in the central arm spectrometers (|η| < 0.35) and single muons in the forward (backward) muon spectrometers (1.2 < |η| < 2.2). As these single leptons are from open heavy-flavor meson semi-leptonic decays, initial state cold nuclear matter effects on heavy quark production can be probed by measuring the single leptons in d+Au collisions. PHENIX have observed a large enhancement of heavy-flavor electrons in d+Au collisions at mid-rapidity, which indicates strong CNM effects on heavy quark production, in contrast to the suppression observed in Au+Au collisions. Measurement of single muons from open heavy flavor in d+Au collisions at forward (backward) rapidity provide detailed look into rapidity dependent CNM effects as well as the low (high) x parton distribution function within Au nucleus. We discuss recent PHENIX heavy flavor measurements and how they expand our understanding of CNM effects and contribute to the interpretation of other results in heavy-ion collisions.

Kaon semileptonic vector form factor and determination of|Vus|using staggered fermions

Using staggered fermions and twisted boundary conditions, we calculate the K meson semileptonic decay vector form factor at zero momentum transfer. The HISQ formulation is used for the valence quarks, while the sea quarks are simulated with the asqtad action (MILC N_f=2+1 configurations). For the chiral and continuum extrapolation we use two-loop continuum CHPT, supplemented by partially quenched staggered CHPT at one loop. Our result is f_+^{K\pi}(0) = 0.9667+-0.0023+-0.0033, where the first error is statistical and the second is the sum in quadrature of the systematic uncertainties. This result is the first N_f=2+1 calculation with two lattice spacings and a controlled continuum extrapolation. It is also the most precise result to date for the vector form factor and, although the central value is larger than previous unquenched lattice calculations, it is compatible with them within errors. Combining our value for f_+^{K\pi}(0) with the latest experimental measurements of K semileptonic decays, we obtain |V_{us}| = 0.2238+-0.0009+-0.0005, where the first error is from f_+^{K\pi}(0) and the second one is experimental. As a byproduct of our calculation, we obtain the combination of low-energy constants [C_{12}^r+C_{34}^r-(L_5^r)^2](M_\rho) = (3.62+-1.00)x10^{-6}.

Beta decays and non-standard interactions in the LHC era

We consider the role of precision measurements of beta decays and light meson semi-leptonic decays in probing physics beyond the Standard Model in the LHC era. We describe all low-energy charged-current processes within and beyond the Standard Model using an effective field theory framework. We first discuss the theoretical hadronic input which in these precision tests plays a crucial role in setting the baseline for new physics searches. We then review the current and upcoming constraints on the various non-standard operators from the study of decay rates, spectra, and correlations in a broad array of light-quark systems. We finally discuss the interplay with LHC searches, both within models and in an effective theory approach. Our discussion illustrates the independent yet complementary nature of precision beta decay measurements as probes of new physics, showing them to be of continuing importance throughout the LHC era.

Next-to-leading-order corrections toB→πform factors inkTfactorization

We calculate next-to-leading-order (NLO) corrections to the $B\to\pi$ transition form factors at leading twist in the $k_T$ factorization theorem. Light partons off-shell by $k_T^2$ are considered in the quark diagrams, in the effective diagrams for the $B$ meson wave function defined with the effective heavy-quark field, and in the effective diagrams for the pion wave function. It is explicitly demonstrated that the infrared logarithms $\ln k_T^2$ cancel between the above sets of diagrams, as deriving the $k_T$-dependent NLO hard kernel from their difference. The infrared finiteness of the hard kernel confirms the application of the $k_T$ factorization theorem to $B$ meson semileptonic decays. The NLO pion wave function is identical to those constructed from the pion transition and electromagnetic form factors, consistent with its universality. Choosing the renormalization and factorization scales lower than the $B$ meson mass, the NLO corrections are under control: they amount only up to 30% of the form factors at large recoil of the pion, when varying models for the meson wave functions.

SemileptonicBandBsdecays into orbitally excited charmed mesons

The BaBar Collaboration has recently reported products of branching fractions that include $B$ meson semileptonic decays into final states with charged and neutral $D_{1}(2420)$ and $D_{2}^{\ast}(2460)$, two narrow orbitally excited charmed mesons. We evaluate these branching fractions, together with those concerning $D_{0}^{\ast}(2400)$ and $D_{1}'(2430)$ mesons, within the framework of a constituent quark model. The calculation is performed in two steps, one of which involves a semileptonic decay and the other is mediated by a strong process. Our results are in agreement with the experimental data. We also extend the study to semileptonic decays of $B_{s}$ into orbitally excited charmed-strange mesons, providing predictions to the possible measurements to be carried out at LHC.

B semileptonic moments at NNLO

The calculation of the moments in inclusive B meson semileptonic decays is upgraded to O(alphas^2). The first three moments of the lepton energy and invariant hadronic mass distributions are computed for arbitrary cuts on the lepton energy and in various renormalization schemes, finding in general small deviations from the O(alphas^2 beta0) calculation. I also review the relation between MSbar and kinetic heavy quark masses.

Constraint fitting of experimental data with a jet quenching model embedded in a hydrodynamical bulk medium

We present a global fit to single- and double-inclusive suppression data of high-pT particles in central Au+Au collisions at top RHIC energy. We also include in this analysis data on heavy quarks via their D and B meson semi-leptonic decays (i.e. non-photonic electrons). The analysis is based on the parton quenching weights for medium-induced gluon radiation computed in the BDMPS approximation then embedded in a hydrodynamical description of the bulk medium. Our results indicate that values of the transport coefficient more than four times larger than perturbative estimates are preferred by experimental data. This confirms previous calculations based on simpler implementations of the medium geometry or only the single-inclusive suppression. We also comment on the statistical compatibility of the heavy quark data within a radiative-only energy loss scenario, and on the sensitivity of the results to nuclear modification of the parton distribution functions (PDFs) and to assumptions on the energy loss during times in the collision prior to the hydrodynamical behavior.

B→πlνsemileptonic form factor from three-flavor lattice QCD: A model-independent determination of|Vub|

We calculate the form factor ${f}_{+}({q}^{2})$ for $B$-meson semileptonic decay in unquenched lattice QCD with $2+1$ flavors of light sea quarks. We use Asqtad-improved staggered light quarks and a Fermilab bottom quark on gauge configurations generated by the MILC Collaboration. We simulate with several light-quark masses and at two lattice spacings, and extrapolate to the physical quark mass and continuum limit using heavy-light meson staggered chiral perturbation theory. We then fit the lattice result for ${f}_{+}({q}^{2})$ simultaneously with that measured by the BABAR experiment using a parameterization of the form-factor shape in ${q}^{2}$, which relies only on analyticity and unitarity in order to determine the Cabibbo-Kobayashi-Maskawa matrix element $|{V}_{\mathrm{ub}}|$. This approach reduces the total uncertainty in $|{V}_{\mathrm{ub}}|$ by combining the lattice and experimental information in an optimal, model-independent manner. We find a value of $|{V}_{\mathrm{ub}}|\ifmmode\times\else\texttimes\fi{}{10}^{3}=3.38\ifmmode\pm\else\textpm\fi{}0.36$.

Non-photonic electron–hadron correlations at STAR

We present STAR's measurements of azimuthal correlations between non-photonic electrons and charged hadrons in p+p, Cu+Cu and Au+Au collisions at GeV. In p+p collisions, from the non-photonic e–h correlation we have extracted the relative B meson semi-leptonic decay contributions to non-photonic electrons up to electron pt ∼ 9 GeV/c. In central Cu+Cu and Au+Au collisions where a dense medium is created, we find that the e–h correlation on the away side of the trigger non-photonic electron has been modified in comparison with the expectation from PYTHIA simulations.

Heavy meson semileptonic differential decay rate in two dimensions in the large [formula omitted

We study QCD in 1 + 1 dimensions in the large N c limit using light-front Hamiltonian perturbation theory in the 1 / N c expansion. We use this formalism to exactly compute hadronic transition matrix elements for arbitrary currents at leading order in 1 / N c , which we use to write the semileptonic differential decay rate of a heavy meson and its moments. We then compare with the results obtained using an effective field theory approach based on perturbative factorization, with the intention of better understanding quark-hadron duality. A very good numerical agreement is obtained between the exact result and the result using effective theories.

MEASUREMENT OF NON-PHOTONIC ELECTRON ANGULAR CORRELATIONS WITH CHARGED HADRONS

Correlations of non-photonic electrons with charged hadrons are studied using the PYTHIA Monte Carlo event generator in p + p collisions at [Formula: see text]. We propose experimental methods to estimate the relative contributions to non-photonic electrons from charm and bottom decays. We present the measurement of azimuthal correlations between non-photonic electrons and charged hadrons in p + p collisions at [Formula: see text] from STAR. The results are compared to PYTHIA simulations to estimate the relative contributions of D and B meson semi-leptonic decays to non-photonic electrons. Our preliminary result indicates that at pT ∼ 4.0 - 6.0 GeV / c the measured B contribution to non-photonic electrons is comparable to D contribution, and is consistent with the FONLL calculation.