Search

We present new calculations of the leading-order one-loop amplitude and the decay rate for $H\to \nu \bar{\nu} \gamma$ within the standard model, employing reduction to a set of basic Passarino-Veltman functions resulting in more concise expressions compared to prior works. Our result for the total decay rate is $\Gamma(H\to \nu\bar{\nu} \gamma)=1.33\,\text{keV}$, in close agreement with the narrow width approximation (NWA) for the intermediate Z boson. A comparison with previous results available in the literature is also provided. We evaluate the differential decay rate with respect to the energy of the photon and find that the non-resonant contributions from box diagrams beyond the NWA notably affect the kinematic region associated with high-energy photons, where the intermediate Z boson is offshell. This suggests potential implications for searches of dark sector mediators, particularly in future lepton collider experiments., Comment: 12 pages, 3 figures; v2: A few clarifications added, acknowledgments extended

We present updated predictions for lifetimes of doubly charmed baryons, within the heavy quark expansion, including available NLO $\alpha_s$ contributions and newly-computed terms in the $1/m_c$ series. Our improved results confirm the expected hierarchy $$\tau(\Xi_{cc}^{+}) < \tau(\Omega_{cc}^{+}) < \tau(\Xi_{cc}^{++}) \,, $$ while the predicted lifetime $\tau(\Xi_{cc}^{++}) = 0.32 \pm 0.5 ^{+0.8}_{-0.7} \,\textrm{ps} $ is consistent with the recent LHCb determination. We provide predictions for the lifetime ratios of the $\Xi_{cc}^{+}$ and $\Omega_{cc}^+$ baryons relative to the $\Xi_{cc}^{++}$ baryon, namely $\tau(\Xi_{cc}^{+})/\tau(\Xi_{cc}^{++})=0.22\pm 0.05\pm 0.04$ and $\tau(\Omega_{cc}^{+})/\tau(\Xi_{cc}^{++})=0.52\pm 0.13^{+0.03}_{-0.02}$., Comment: 25 pages, 2 figures; v2: few clarifications added, one reference added, matches version accepted for publication

We provide updated predictions of the lifetimes of singly charmed baryons and mesons within the heavy quark expansion, with all known corrections included. A special attention is devoted to the choice of the charm mass and wavefunctions of heavy baryons. Our results accommodate the experimentally-favoured hierarchy of singly charmed baryon lifetimes \begin{eqnarray*} \tau(\Xi_c^0) < \tau(\Lambda_c^+)< \tau(\Omega_c^0) < \tau(\Xi_c^+)\, \end{eqnarray*} in contrast to earlier theoretical findings. Predictions for charmed meson lifetimes and semileptonic branching ratios are also in agreement, within uncertainties, with a recent comprehensive study and with experimental results., Comment: 11 pages, 4 figures. Prepared for Proceedings of the Corfu Summer Institute 2022 "School and Workshops on Elementary Particle Physics and Gravity", 28 August - 1 October, 2022 (CORFU2022)

In the 1990s, very low experimental values for the lifetime ratio $\tau (\Lambda_b)/\tau(B_d)$ triggered a considerable amount of doubt in the applicability of the heavy quark expansion (HQE), which is based on the assumption of quark-hadron duality (QHD) for inclusive total decay rates. However, these low values turned out to be the result of purely experimental problems, and the current HFLAV average reads $\tau (\Lambda_b)/\tau(B_d) = 0.969(6)$. In this work, we present the Standard Model predictions for the $b$-baryon lifetimes within the framework of the HQE. In particular, we include for the first time the contribution of the Darwin term and we update the estimates for the matrix elements of the dimension-six four-quark operators. Within experimental and theoretical uncertainties, we find excellent agreement between the data and the HQE predictions, and thus no indication for any visible violation of QHD. Our numerical results can be summarised by the ratios $\tau (\Lambda_b)/\tau(B_d) = 0.955(14)$, $\tau (\Omega_b^-)/\tau(B_d) = 1.081(42)$, and $\tau (\Xi_b^0)/\tau (\Xi_b^-) = 0.929(28)$., Comment: 36 pages, 3 figures, 8 tables

We provide an extensive study of the lifetimes of singly charmed baryons and mesons, within the heavy quark expansion with all known corrections included. A special attention is devoted to the choice of the charm mass and wavefunctions of heavy baryons. We give our predictions for lifetimes, lifetime ratios, and semileptonic branching ratios of singly charmed baryons. Our results accommodate the experimentally-favoured hierarchy of singly charmed baryon lifetimes \begin{eqnarray*} \tau\left(\Xi_c^{0}\right) < \tau\left(\Lambda_c^{+}\right)< \tau\left(\Omega_c^{0}\right) < \tau\left(\Xi_c^{+}\right)\, \end{eqnarray*} in contrast to earlier theoretical findings. Predictions for charmed meson lifetimes and semileptonic decay rates are in agreement with a recent comprehensive study and experimental results within uncertainties., Comment: 68 pages, 6 figures. v2: two references added; cosmetic changes to all figures; some typos corrected; numerics and conclusions unchanged. Text updated to correspond with journal version

The decay $H\to \ell^+\ell^- \gamma$, $\ell=e,\mu$, receives contributions from $H\to Z[\to \ell^+\ell^-] \gamma$ and a non-resonant contribution, both of which are loop-induced. We describe how one can separate these sub-processes in a gauge-independent way, define the decay rate $\Gamma(H\to Z\gamma)$, and extract the latter from differential $H\to \ell^+\ell^- \gamma$ branching ratios. For $\ell=\mu$ also the tree decay rate, which is driven by the muon Yukawa coupling, is important. We propose kinematic cuts optimized to separate the three contributions, paving the way to the milestones (i) discovery of $H\to Z \gamma$, (ii) discovery of $H\to \mu^+\mu^- \left. \gamma\right|_{\rm tree}$, and (iii) quantification of new physics in both the effective $H$-$Z$-$\gamma$ and non-resonant $H$-$\ell^+$-$\ell^-$-$\gamma$ couplings., Comment: 15 pages, 6 figures; v2: Few typos corrected, preprint number added, acknowledgements extended

Evidence for electron-muon universality violation that has been revealed in $b\to s \ell\ell$ transitions in the observables $R_{K,K^*}$ by the LHCb Collaboration can be explained with spin-1 leptoquarks in $SU(2)_L$ singlet $V_1$ or triplet $V_3$ representations in the ${\cal{O}}(1-10)$ TeV range. We explore the sensitivity of the high luminosity LHC (HL-LHC) and future proton-proton colliders to $V_1$ and $V_3$ in the parameter space connected to $R_{K,K^*}$-data. We consider pair production and single production in association with muons in different flavor benchmarks. Reinterpreting a recent ATLAS search for scalar leptoquarks decaying to $b \mu$ and $j \mu$, we extract improved limits for the leptoquark masses: For gauge boson-type leptoquarks ($\kappa=1$) we obtain $M_{V_1}> 1.9$ TeV, $M_{V_1}> 1.9$ TeV, and $M_{V_1}> 1.7$ TeV for leptoquarks decaying predominantly according to hierarchical, flipped and democratic quark flavor structure, respectively. Future sensitivity projections based on extrapolations of existing ATLAS and CMS searches are worked out. We find that for $\kappa=1$ the mass reach for pair (single) production of $V_1$ can be up to 3 TeV (2.1 TeV) at the HL-LHC and up to 15 TeV (19.9 TeV) at the FCC-hh with $\sqrt{s}=100$ TeV and $20 \, \mbox{ab}^{-1}$. The mass limits and reach for the triplet $V_3$ are similar or higher, depending on flavor. While there is the exciting possibility that leptoquarks addressing the $R_{K,K^*}$-anomalies are observed at the LHC, to fully cover the parameter space $pp$-collisions beyond the LHC-energies are needed., Comment: 16 pages, 8 figures; v2: Clarifying comments, four plots and few references added. Few typos corrected. Conclusions unchanged

We consider a Higgs portal model in which the 125-GeV Higgs boson mixes with a light singlet mediator $h_2$ coupling to particles of a Dark Sector and study potential $b\to s h_2$ decays in the Belle II experiment. Multiplying the gauge-dependent off-shell Standard-Model $b$-$s$-Higgs vertex with the sine of the Higgs mixing angle does not give the correct $b$-$s$-$h_2$ vertex. We clarify this issue by calculating the $b$-$s$-$h_2$ vertex in an arbitrary $R_\xi$ gauge and demonstrate how the $\xi$ dependence cancels from physical decay rates involving an on-shell or off-shell $h_2$. Then we revisit the $b\to s h_2$ phenomenology and point out that a simultaneous study of $B\to K^* h_2$ and $B\to K h_2$ helps to discriminate between the Higgs portal and alternative models of the Dark Sector. We further advocate for the use of the $h_2$ lifetime information contained in displaced-vertex data with $h_2$ decaying back to Standard-Model particles to better constrain the $h_2$ mass or to reveal additional $h_2$ decay modes into long-lived particles., Comment: 9 pages, 4 figures. v.2: minor clarifications, added Table 1, added references, acknowledgements extended

We present new calculations of the differential decay rates for $H\to \ell^+\ell^- \gamma$ with $\ell=e$ or $\mu$ in the Standard Model. The branching fractions and forward-backward asymmetries, defined in terms of the flight direction of the photon relative to the lepton momenta, depend on the cuts on energies and invariant masses of the final state particles. For typical choices of these cuts we find the branching ratios $B(H\to e \bar e \gamma)=5.8\cdot 10^{-5}$ and $B(H\to \mu \bar \mu \gamma)=6.4\cdot 10^{-5}$ and the forward-backward asymmetries $\mathcal{A}^{(e)}_{\text{FB}}=0.343$ and $\mathcal{A}^{(\mu)}_{\text{FB}}=0.255$. We provide compact analytic expressions for the differential decay rates for the use in experimental analyses., Comment: 17 pages, 8 figures, 2 ancillary files; v2: missed color factor added, plots and results changed accordingly, matches the corrected version to appear in PRD

We discuss the possibility of using the observables deduced from the angular distribution of the $B\to D^{(\ast)} \ell\bar{\nu}$ 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., Comment: 29 pages, 5 figures, 5 tables. Some references and minor changes added

In this addendum to arXiv:1811.09603 we update our results including the recent measurement of ${\cal R}(D)$ and ${\cal R}(D^*)$ by the Belle collaboration: ${\cal R}(D)_{\rm Belle} = 0.307\pm0.037\pm0.016$ and ${\cal R}(D^*)_{\rm Belle}=0.283\pm0.018\pm0.014$, resulting in the new HFLAV fit result ${\cal R}(D) = {0.340\pm0.027 \pm 0.013}$, ${\cal R}(D^*) = {0.295\pm0.011 \pm 0.008 }$, exhibiting a $3.1\,\sigma$ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for ${\cal R}(\Lambda_c)$ from our sum rule reads ${\cal R}(\Lambda_c)= \mathcal{R}_{\rm SM}(\Lambda_c) \left( 1.15 \pm 0.04 \right) = 0.38 \pm 0.01 \pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $b\to B_c$ and their implication on the constraint from $B_{u/c}\to\tau\nu$ data., Comment: 9 pages, 7 figures. v2: references added, matches PRD version

The combined analysis of the BaBar, Belle, and LHCb data on $B\to D\tau\nu$, $B\to D^*\tau\nu$ and $B_c\to J/\Psi\tau\nu$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $\tau$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\to\tau\nu)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\to\tau\nu)<10\%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${\rm BR}(B_c\to\tau\nu)$ constraint has a significant impact: depending on whether one uses a limit of $60\%$, $30\%$ or $10\%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60\%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10\%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $B\to D\tau\nu$, $B\to D^*\tau\nu$ and $\Lambda_b\to \Lambda_c\tau\nu$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${\rm BR}(\Lambda_b\to \Lambda_c\tau\nu)$ over its SM prediction by $(24\pm 6)\%$ for the current $\mathcal{R}(D^{(*)})$ data., Comment: 16 pages, 6 figures; v2: clarifying comments and few references added, matches version published in PRD; v3: typo corrected in eq. (28). Updated results based on the HFLAV average for spring 2019 are available in arXiv:1905.08253

We evaluate long-distance electromagnetic (QED) contributions to $\bar{B}{}^0 \to D^+ \tau^{-} \bar{\nu}_{\tau}$ and $B^- \to D^0 \tau^{-} \bar{\nu}_{\tau}$ relative to $\bar{B}{}^0 \to D^+ \mu^{-} \bar{\nu}_{\mu}$ and $B^- \to D^0 \mu^{-} \bar{\nu}_{\mu}$, respectively, in the standard model. We point out that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ are not negligible, contrary to the expectation that radiative corrections are almost canceled out in the ratio of the two branching fractions. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for $R(D^{+})^{\tau/\mu}$ and $R(D^{0})^{\tau/\mu}$ can be amplified by $\sim4\%$ and $\sim3\%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV., Comment: 5 pages, 3 figures; v3: several minor modifications, references added, published version

$B$-physics data and flavor symmetries suggest that leptoquarks can have masses as low as few ${\cal{O}}(\mbox{TeV})$, predominantly decay to third generation quarks, and highlight $pp \to b \mu \mu$ signatures from single production and $pp \to b b \mu \mu$ from pair production. Abandoning flavor symmetries could allow for inverted quark hierarchies, and cause sizable $pp \to j \mu \mu$ and $jj \mu \mu $ cross sections, induced by second generation couplings. Final states with leptons other than muons including lepton flavor violation (LFV) ones can also arise. The corresponding couplings can also be probed by precision studies of the $B \to (X_s, K^*,\phi) ee $ distribution and LFV searches in $B$-decays. We demonstrate sensitivity in single leptoquark production for the LHC and extrapolate to the high luminosity HL-LHC. Exploration of the bulk of the phase space requires a hadron collider beyond the reach of the LHC, with $b$-identification capabilities., Comment: 17 pages

Measurements of the ratio of $B \to K^* \mu \mu $ to $B \to K^* e e $ branching fractions, $R_{K^*}$, by the LHCb collaboration strengthen the hints from previous studies with pseudoscalar kaons, $R_K$, for the breakdown of lepton universality, and therefore the Standard Model (SM), to $\sim 3.5 \sigma$. Complementarity between $R_K$ and $R_{K^*}$ allows to pin down the Dirac structure of the new contributions to be predominantly SM-like chiral, with possible admixture of chirality-flipped contributions of up to ${\cal{O}}( \mbox{few} 10 \%)$. Scalar and vector leptoquark representations $(S_3,V_1,V_3)$ plus possible ($\tilde S_2,V_2$) admixture can explain $R_{K,K^*}$ via tree level exchange. Flavor models naturally predict leptoquark masses not exceeding a few TeV, with couplings to third generation quarks at $O(0.1)$, implying that this scenario can be directly tested at the LHC., Comment: v1: 8 pages, v2: references added, typos corrected, matches published version

The rare decay $B\to K^\ast( \to K\pi) \nu\bar{\nu}$ is expected to play an important role in searches for physics beyond the Standard Model at the near future $B$-physics experiments. We investigate resonant and non-resonant backgrounds that arise beyond the narrow-width approximation for the $K^\ast$. Non-resonant $B\to K\pi \nu\bar{\nu}$ decays are analyzed in the region of low hadronic recoil, where $B \to K \pi$ form factors from Heavy-Hadron-Chiral-Perturbation Theory are available. In a Breit-Wigner-type model interference-induced effects in the $K^*$ signal region are found to be sizable, as large as $20\%$ in the branching ratio. Corresponding effects in the longitudinal polarization fraction $F_L$ are smaller, at most around few \%. Effects of the broad scalar states $K_0^\ast$ and $\kappa$ are at the level of percent in the branching fraction in the $K^\ast$ signal region and negligible in $F_L$. Since the backgrounds to $F_L$ are small this observable constitutes a useful probe of form factors calculations, or alternatively, of right-handed currents in the entire $q^2$-region. The forward-backward asymmetry in the $K \pi$-system, $A_{\rm FB \, L}^K$, with normalization to the longitudinal decay rate probes predominantly S,P-wave interference free of short-distance coefficients and can therefore be used to control the resonant and non-resonant backgrounds., Comment: 14 pages

We analyse $B\to K^\ast \ell\ell$ decays in the region of low hadronic recoil, where an operator product expansion (OPE) in $1/m_b$ applies. Using a local model for charm contributions based on $e^+ e^- \to hadrons$ against the OPE provides a data-driven method to access the limitations to the OPE's accuracy related to binnings in the dilepton mass. Model-independent fits to $B\to K^\ast \mu \mu $ low recoil angular observables exhibit presently only small sensitivity to different charm models. They give similar results as the fits based on the OPE, and are in agreement with the standard model, but leave also room for new physics. Measurements with resolution small enough to probe charm resonances would be desirable., Comment: 18 pages, v2: references and clarifications added, conclusions unchanged, matches published version

We derive the expressions for the full angular distributions of $\bar B\to D\ell\bar \nu_\ell$ and $\bar B\to D^{\ast }\ell\bar \nu_\ell$ 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 $\bar B\to D^{\ast}(\to D\pi)_S\ell\bar \nu_\ell$ decay sample by the $\bar B\to D_0^{\ast}(\to D\pi)\ell\bar \nu_\ell$ events, and point out that a measurement of two particular quantities could clarify whether or not the $(D\pi)_{S-\rm wave}$ in the vicinity of $D^\ast$-peak is (approximately) described by the Breit-Wigner formula., Comment: 29 pages

The LHC collaborations ATLAS and CMS recently reported on the excess of the events in the diphoton final states at the invariant mass of about $750~ \text{GeV}$. In this article we speculate on the possibility that the excess arises from the neutral CP-even component $\phi$ of the scalar triplet $ \Phi $ of the $SU(3)_{c}\times SU(3)_{L}\times U(1)_{X}$ $(3\text{-}3\text{-} 1)$ model that has a $U(1)_{X}$ charge equal to $X=-1/3$ and acquires a vacuum expectation value larger than the electroweak symmetry breaking scale. The interactions of the scalar field $\phi$ to the photon- and gluon-pairs are mediated by the virtual vector-like fermions which appear as components of the anomaly-free chiral fermion representations of the $3\text{ -}3\text{-}1$ gauge group., Comment: 13 pages, 1 figure. Substantially improved version

Current experimental information on the charm meson decay observables in which the $c\to s\ell\nu_\ell$ transitions occur is well compatible with the Standard Model predictions. Recent precise lattice calculations of the $D_s$ meson decay constant and form factors in $D\to K\ell\nu$ decays offer a possibility to search for the small deviations from the Standard Model predictions in the next generation of the high intensity flavour experiments. We revisit constraints from these processes on the new physics contributions in the effective theory approach. We investigate new physics effects which might appear in the differential distributions for the longitudinally and transversely polarised $K^\ast$ in $D\to K^\ast\ell\nu_\ell$ decays. Present constraints from these observables are rather weak, but could be used to constrain new physics effects in the future. In the case of $D\to K\ell\nu$ we identify observables sensitive on new physics contribution coming from the scalar Wilson coefficient, namely the forward-backward and the transversal muon asymmetries. By allowing that new physics modifies only the second lepton generation but not the first one, we identify allowed region for the differential decay rate for the process $D\to K\mu\nu_\mu$ and find that it is allowed to deviate from the Standard Model prediction by only few percent. The lepton flavour universality violation can be tested in the ratio $R_{\mu/e}(q^2)\equiv \frac{d\Gamma^{(\mu)}}{dq^2}/\frac{d\Gamma^{(e)}}{dq^2}$. If the first lepton generation behaves as in the Standard Model, we find, using current constraint on the scalar Wilson coefficient, that the ratio $R_{\mu/e}(q^2)$ is currently allowed to be within the range $(0.9, 1.2)$, depending on the value of $q^2$.

We speculate about the possible interpretations of the recently observed excess in the $h \to \tau \mu$ decay. We derive a robust lower bound on the Higgs boson coupling strength to a tau and a muon, even in presence of the most general new physics affecting other Higgs properties. Then we reevaluate complementary indirect constraints coming from low energy observables as well as from theoretical considerations. In particular, the tentative signal should lead to $\tau \to \mu \gamma$ at rates which could be observed at Belle II. In turn we show that, barring fine-tuned cancellations, the effect can only be accommodated within models with an extended scalar sector. These general conclusions are demonstrated using a number of explicit new physics models. Finally we show how, given the $h \to \tau \mu$ signal, the current and future searches for $\mu \to e \gamma$ and $\mu \to e$ nuclear conversions unambiguously constrain the allowed rates for $h \to \tau e$., Comment: 34 pages, 9 figures, expanded Section V.D with the fine-tuning solution; conclusions unchanged

The presence of a colored scalar that is a weak doublet with fractional electric charges of $|Q|=2/3$ and $|Q|=5/3$ with mass below 1,TeV can provide an explanation of the observed branching ratios in $B \to D^{(*)} \tau \bar \nu$ decays. The required combination of scalar and tensor operators in the effective Hamiltonian for $b \to c \tau \bar \nu$ is generated through the $t$-channel exchange. We focus on a scenario with a minimal set of Yukawa couplings that can address a semitauonic puzzle and show that its resolution puts a nontrivial bound on the product of the scalar couplings to $\bar \tau b$ and $\bar c \nu$. We also derive additional constraints posed by $Z \to b\bar b$, muon magnetic moment, lepton flavor violating decays $\mu \to e \gamma$, $\tau \to \mu \gamma$, $\tau \to e \gamma$, and $\tau$ electric dipole moment. The minimal set of Yukawa couplings is not only compatible with the mass generation in an SU(5) unification framework, a natural environment for colored scalars, but specifies all matter mixing parameters except for one angle in the up-type quark sector. We accordingly spell out predictions for the proton decay signatures through gauge boson exchange and show that $p \rightarrow \pi^0 e^+$ is suppressed with respect to $p \rightarrow K^+ \bar{\nu}$ and even $p \rightarrow K^0 e^+$ in some parts of available parameter space. Impact of the colored scalar embedding in 45-dimensional representation of SU(5) on low-energy phenomenology is also presented. Finally, we make predictions for rare top and charm decays where presence of this scalar can be tested independently., Comment: 30 pp, 6 figures; v2: includes additional constraints

We review possible impact of beyond Standard Model physics in the third generation leptonic and semileptonic B decays., Comment: Proceedings of CKM 2012, the 7th International Workshop on the CKM Unitarity Triangle, University of Cincinnati, USA, 28 September - 2 October 2012

Present measurements of b->c tau nu and b->u tau nu transitions differ from the standard model predictions of lepton flavor universality by almost 4sigma. We examine new physics interpretations of this anomaly. An effective field theory analysis shows that minimal flavor violating models are not preferred as an explanation, but are also not yet excluded. Allowing for general flavor violation, right-right vector and right-left scalar quark currents are identified as viable candidates. We discuss explicit examples of two Higgs doublet models, leptoquarks as well as quark and lepton compositeness. Finally, implications for LHC searches and future measurements at the (super)B- factories are presented., Comment: 5 pages, 2 figures, version as published

B physics has played a prominent role in investigations of new physics effects at low-energies. Presently, the largest discrepancy between a standard model prediction and experimental measurements appears in the branching ratio of the charged current mediated B to tau nu decay, where the large tau mass lifts the helicity suppression arising in leptonic B decays. Less significant systematic deviations are also observed in the semileptonic B to D(*) tau nu rates. Due to the rich spin structure of the final state, the decay mode B to D* tau nu offers a number of tests of such possible standard model deviations. We investigate the most general set of lowest dimensional effective operators leading to helicity suppressed modifications of b to c (semi)leptonic transitions. We explore such contributions to the B to D* tau nu decay amplitudes by determining the differential decay rate, longitudinal D* polarization fraction, D* - tau opening angle asymmetry and the tau helicity asymmetry. We identify the size of possible new physics contributions constrained by the present B to D(*) tau nu rate measurements and find significant modifications are still possible in all of them. In particular, the opening angle asymmetry can be shifted by almost 30%, relative to the standard model prediction, while the tau helicity asymmetry can still deviate by as much as 80%., Comment: 10 pages, 4 figures; corrected several typos, added references, conclusions unchanged