Lepton Flavor Universality Ratios Research Articles

To (b)e or not to (b)e: no electrons at LHCb

We discuss the impact of the recent LHCb update on the two lepton-flavour universality ratios R_K and R_{K^*}, and the CMS update of B({B_s rightarrow mu ^+mu ^-}) regarding the possibility of New Physics in brightarrow sell ^+ell ^- decays. We perform global fits of the New Physics Wilson coefficients defined in the model-independent approach of the Weak Effective Theory at the b-quark mass. We discuss three different frameworks for this analysis: (i) an update limited to the experimental data but using the same theoretical framework as in earlier works, (ii) a full update concerning both the experimental inputs and the theoretical framework, (iii) an analysis without the LHCb results on electron modes. The comparison between these various sets of results allows us to identify the differences stemming from the various components of the analysis: new experimental results, new inputs for the hadronic form factors, the role played by LHCb data on electron modes. As expected, the significance of all New Physics hypotheses gets reduced after the LHCb announcements on R_{K^{(*)}} while the hypothesis of a lepton-flavour-universal contribution to the Wilson coefficient of the semileptonic O_{9ell } operators (possibly with a very small lepton-flavour-universality violating component) is clearly reinforced. We also discuss the possibility of a long-distance charm-loop contribution through a mode-by-mode analysis and we find that the preferred values for the {mathcal C}_{9mu } Wilson coefficient are consistent throughout the different brightarrow smu ^+mu ^- modes and that there is no significant evidence of non-constant q^2 dependencies, which would indicate the presence of a long-distance charm-loop contribution beyond those already included.

D -meson semileptonic decays to pseudoscalars from four-flavor lattice QCD

We present lattice-QCD calculations of the hadronic form factors for the semileptonic decays $D\to\pi\ell\nu$, $D\to K\ell\nu$, and $D_s\to K\ell\nu$. Our calculation uses the highly improved staggered quark (HISQ) action for all valence and sea quarks and includes $N_f=2+1+1$ MILC ensembles with lattice spacings ranging from $a\approx0.12$ fm down to $0.042$ fm. At most lattice spacings, an ensemble with physical-mass light quarks is included. The HISQ action allows all the quarks to be treated with the same relativistic light-quark action, allowing for nonperturbative renormalization using partial conservation of the vector current. We combine our results with experimental measurements of the differential decay rates to determine $|V_{cd}|^{D\to\pi}=0.2238(11)^{\rm Expt}(15)^{\rm QCD}(04)^{\rm EW}(02)^{\rm SIB}[22]^{\rm QED}$ and $|V_{cs}|^{D\to K}=0.9589(23)^{\rm Expt}(40)^{\rm QCD}(15)^{\rm EW}(05)^{\rm SIB}[95]^{\rm QED}$ This result for $|V_{cd}|$ is the most precise to date, with a lattice-QCD error that is, for the first time for the semileptonic extraction, at the same level as the experimental error. Using recent measurements from BES III, we also give the first-ever determination of $|V_{cd}|^{D_s\to K}=0.258(15)^{\rm Expt}(01)^{\rm QCD}[03]^{\rm QED}$ from $D_s\to K \ell\nu$. Our results also furnish new Standard Model calculations of the lepton flavor universality ratios $R^{D\to\pi}=0.98671(17)^{\rm QCD}[500]^{\rm QED}$, $R^{D\to K}=0.97606(16)^{\rm QCD}[500]^{\rm QED}$, and $R^{D_s\to K}=0.98099(10)^{\rm QCD}[500]^{\rm QED}$, which are consistent within $2\sigma$ with experimental measurements. Our extractions of $|V_{cd}|$ and $|V_{cs}|$, when combined with a value for $|V_{cb}|$, provide the most precise test of second-row CKM unitarity, finding agreement with unitarity at the level of one standard deviation.

Confronting the vector leptoquark hypothesis with new low- and high-energy data

In light of new data we present an updated phenomenological analysis of the simplified U_1-leptoquark model addressing charged-current B-meson anomalies. The analysis shows a good compatibility of low-energy data (dominated by the lepton flavor universality ratios R_D and R_{D^*}) with the high-energy constraints posed by pprightarrow tau {bar{tau }} Drell-Yan data. We also show that present data are well compatible with a framework where the leptoquark couples with similar strength to both left- and right-handed third-generation fermions, a scenario that is well-motivated from a model building perspective. We find that the high-energy implications of this setup will be probed at the 95% confidence level in the high-luminosity phase of the LHC.

Standard Model predictions for lepton flavour universality ratios of inclusive semileptonic B decays

We present Standard Model predictions for lepton flavour universality ratios of inclusive B → X(c) ell {overline{nu}}_{ell } . For the ℓ = μ, e, these ratios are very close to unity as expected. For the τ mode, we update the SM prediction for the branching ratio including power-corrections in the heavy-quark expansion up to 1/ {m}_b^3 . These inclusive ratios serve as an important cross-check of the exclusive B → D(*) ell {overline{nu}}_{ell } modes, in which tensions exists between the predictions and measurements in those modes.

Flavor non-universal vector leptoquark imprints in [formula omitted] and ΔF = 2 transitions

We analyze K→πνν¯ rates in a model with a TeV-scale leptoquark addressing B-meson anomalies, based on the flavor non-universal 4321 gauge group featuring third-generation quark-lepton unification. We show that, together with the tight bounds imposed by ΔF=2 amplitudes, the present measurement of B(K+→π+νν¯) already provides a non-trivial constraint on the model parameter space. In the minimal version of the model, the deviations from the Standard Model in B(K+→π+νν¯) are predicted to be in close correlation with non-standard effects in the Lepton Flavor Universality ratios RD and RD⁎. With the help of future data, these correlations can provide a decisive test of the model.

Towards ruling out the charged Higgs interpretation of the {R}_{D^{left(ast right)}} anomaly

Motivated by the notorious anomaly in the lepton flavor universality ratios {R}_{D^{left(ast right)}} , we study the sensitivity of the Large Hadron Collider (LHC) to a low-mass charged Higgs boson H− lighter than 400 GeV in a generic two Higgs doublet model. A combination of current constraints from the Bc→ τν decay, Bs meson mixing data, tau sleptons and di-jet searches at the LHC allows to explain the {R}_{D^{left(ast right)}} anomaly at the 1σ level by a low-mass charged Higgs. In this context, we estimate the reach of an LHC search for resonant H− production, where the final state contains an energetic τ lepton decaying hadronically, a neutrino with large transverse momentum, and an additional b-jet (pp → b + τh + ν). Requiring the additional b-tagged jet in the τν resonance search profits from the suppression of the Standard Model background, and therefore it allows us to judge the low-mass H− interpretation of the {R}_{D^{left(ast right)}} anomaly. To demonstrate this, we perform a fast collider simulation for the τν resonance search with an additional b-tagged jet, and find that most of the interesting parameter region of the whole mass range can already be probed with the current integrated luminosity of 139 fb−1.

Exclusive semileptonic B-meson decays using lattice QCD and unitarity

We present the results of the application of the Dispersion Matrix approach to exclusive semileptonic B-meson decays. This method allows to determine the hadronic form factors in a non-perturbative and completely modelindependent way. Starting from lattice results available at large values of the momentum transfer, the behaviour of the form factors in their whole kinematical range is obtained without introducing any parameterization of their momentum dependence. We will focus on the determination of the Cabibbo-Kobayashi- Maskawa matrix elements |Vcb| and |Vub| through the analysis of B(s) → D(s)(*)lv and B(s) → π(K)ℓν decays. New theoretical determinations of the Lepton Flavour Universality ratios relevant for these transitions will be also presented.

Bottom-flavored mono-tau tails at the LHC

We study the effective field theory sensitivity of an LHC analysis for the τν final state with an associated b-jet. To illustrate the improvement due to the b-tagging, we first recast the recent CMS analysis in the τν channel, using an integrated luminosity of 35.9 fb−1 at sqrt{s} = 13 TeV, and provide limits on all the dimension-six effective operators which contribute to the process. The expected limits from the b-tagged analysis are then derived and compared. We find an improvement of approximately ∼ 30% in the bounds for operators with a b quark. We also discuss in detail possible angular observables to be used as a discriminator between dimension-six operators with different Lorentz structure. Finally, we study the impact of these limits on some simplified scenarios aimed at addressing the observed deviations from the Standard Model in lepton flavor universality ratios of semileptonic B-meson decays. In particular, we compare the collider limits on those scenarios set by our analysis either with or without the b-tagging, assuming an integrated luminosity of 300 fb−1, with relevant low-energy flavor measurements.

Optimized lepton universality tests in Brightarrow V ell {bar{nu }} decays

We propose improved lepton flavor universality (LFU) ratios in semileptonic Prightarrow V ell {bar{nu }} decays, when comparing mu and tau modes, that minimize the theoretical form-factor uncertainties. These optimized ratios are obtained with simple cuts or reweighting of the dilepton mass distributions, which imply a minimum loss of signal on the rare tauonic modes while maximizing the cancellation of theoretical uncertainties among the two modes. We illustrate the usefulness of these observables in B_crightarrow J/psi , B_crightarrow psi (2S), Brightarrow D^* and B_srightarrow D_s^* transitions, showing that in all cases we can reach {mathcal {O}}(1%) uncertainties on the SM predictions of the improved LFU ratios employing conservative form-factor uncertainties.

New physics in brightarrow sell ell transitions at one loop

We investigate new-physics contributions to brightarrow s ell ell transitions in the context of an effective field theory extension of the Standard Model, including operator mixing at one loop. We identify the few scenarios where a single Wilson coefficient, C/Lambda ^2 sim 1/mathrm{TeV}^2, induces a substantial shift in the lepton flavour universality ratios R_K and R_{K^*} at one loop, while evading Z-pole precision tests, collider bounds, and other flavour constraints. Novel fits to the present data are achieved by a left-handed current operator with quark-flavour indices (2, 2) or (3, 3). Interestingly, the running of the Standard Model Yukawa matrices gives the dominant effect for these scenarios. We match the favoured effective-theory scenarios to minimal, single-mediator models, which are subject to additional stringent constraints. Notably, we recognise three viable instances of a leptoquark with one coupling to fermions only. If the anomalies were confirmed, it appears that one-loop explanations have good prospects of being directly tested at the LHC.

Phenomenology of decays in a scalar leptoquark model * * Supported by the National Natural Science Foundation of China (11775092, 11521064, 11435003, 11805077). XY is also supported in part by the startup research funding from CCNU

During the past few years, signs of lepton flavor universality (LFU) violation have been observed in and transitions. Recently, the and polarization fractions and in decay were likewise measured by the Belle collaboration. Motivated by these intriguing results, we revisit the and anomalies in a scalar leptoquark (LQ) model, where two scalar LQs, one of which is a singlet and the other a triplet, are introduced simultaneously. We consider five mediated decays, , , , and , and focus on the LQ effects on the distributions of the branching fractions, LFU ratios, and various angular observables in these decays. Under the combined constraints of the available data on , , , and , we perform scans for the LQ couplings and make predictions for a number of observables. Numerically it is found that both the differential branching fractions and LFU ratios are largely enhanced by the LQ effects, with the latter expected to provide testable signatures at the SuperKEKB and High-Luminosity LHC experiments.

Decay in supersymmetry with R-parity violation * * Supported by the National Natural Science Foundation of China (11775092, 11521064, 11435003, 11805077), XY is also supported in part by the startup research funding from CCNU

In past years, several hints of lepton flavor universality (LFU) violation have emerged from the and data. More recently, the Belle Collaboration has reported the first measurement of the longitudinal polarization fraction in the decay. Motivated by this intriguing result, along with the recent measurements of and polarization, we present the study of decays in supersymmetry (SUSY) with -parity violation (RPV). We consider , , and modes and focus on the branching ratios, LFU ratios, forward-backward asymmetries, polarizations of daughter hadrons, and the lepton. The RPV SUSY was capable of explaining the anomalies at the level, after taking into account various flavor constraints. In the allowed parameter space, the differential branching fractions and LFU ratios are largely enhanced by the SUSY effects, especially in the large dilepton invariant mass region. Moreover, a lower bound 10−6 is obtained. These observables could provide testable signatures at the high-luminosity LHC and SuperKEKB, and correlate with direct searches for SUSY.

Lambda _b decays into Lambda _c^*ell bar{nu }_ell and Lambda _c^*pi ^-[Lambda _c^*=Lambda _c(2595) and Lambda _c(2625)] and heavy quark spin symmetry

We study the implications for Lambda _b rightarrow Lambda _c^*ell bar{nu }_ell and Lambda _b rightarrow Lambda _c^*pi ^-[Lambda _c^*=Lambda _c(2595) and Lambda _c(2625)] decays that can be deduced from heavy quark spin symmetry (HQSS). Identifying the odd parity Lambda _c(2595) and Lambda _c(2625) resonances as HQSS partners, with total angular momentum–parity j_q^P=1^- for the light degrees of freedom, we find that the ratios Gamma (Lambda _brightarrow Lambda _c(2595)pi ^-)/Gamma (Lambda _brightarrow Lambda _c(2625)pi ^-) and Gamma (Lambda _brightarrow Lambda _c(2595) ell bar{nu }_ell )/ Gamma (Lambda _brightarrow Lambda _c(2625) ell bar{nu }_ell ) agree, within errors, with the experimental values given in the Review of Particle Physics. We discuss how future, and more precise, measurements of the above branching fractions could be used to shed light into the inner HQSS structure of the narrow Lambda _c(2595) odd-parity resonance. Namely, we show that such studies would constrain the existence of a sizable j^P_q=0^- component in its wave-function, and/or of a two-pole pattern, in analogy to the case of the similar Lambda (1405) resonance in the strange sector, as suggested by most of the approaches that describe the Lambda _c(2595) as a hadron molecule. We also investigate the lepton flavor universality ratios R[Lambda _c^*] = mathcal{B}(Lambda _b rightarrow Lambda _c^* tau ,bar{nu }_tau )/mathcal{B}(Lambda _b rightarrow Lambda _c^* mu ,bar{nu }_mu ), and discuss how R[Lambda _c(2595)] may be affected by a new source of potentially large systematic errors if there are two Lambda _c(2595) poles.

A light dimuon resonance in B decays?

The observed deviations from the Standard Model in several b→sμμ processes can be explained in terms of a new vector boson produced on-shell in B meson decays. A mass of 2.5–3 GeV and a total width of 10–20% allow to hide the associated dimuon bump in the poorly known charmonium region, and the large invisible decay width can be interpreted in terms of Dark Matter. This proposal predicts a contribution to the muon anomalous magnetic moment, that could explain the long-standing tension with the Standard Model. It also predicts sizeable invisible B decays and a peculiar q2-dependence of the lepton flavor universality ratios RK and RK⁎, that could be tested at the LHCb and Belle-II. This proceeding is based on [F. Sala, D. M. Straub, A New Light Particle in B Decays?, Phys. Lett. B774 (2017) 205–209. arXiv:1704.06188, doi:10.1016/j.physletb.2017.09.072], and slightly extends it with comments about Dark Matter.

Scalar leptoquarks from grand unified theories to accommodate the B -physics anomalies

We address the $B$-physics anomalies within a two scalar leptoquark model. The low-energy flavor structure of our set-up originates from two $SU(5)$ operators that relate Yukawa couplings of the two leptoquarks. The proposed scenario has a UV completion, can accommodate all measured lepton flavor universality ratios in $B$-meson decays, is consistent with related flavor observables, and is compatible with direct searches at the LHC. We provide prospects for future discoveries of the two light leptoquarks at the LHC and predict several yet-to-be-measured flavor observables.

A new light particle in B decays?

We investigate the possibility whether the tensions with SM expectations observed in several b→sℓℓ transitions, including hints for lepton flavour non-universality, could be due to the decay of B into a new light resonance. We find that qualitative agreement with the data can be obtained with a light vector resonance dominantly decaying invisibly. This scenario predicts a shift in the muon anomalous magnetic moment that could explain the long-standing discrepancy. The most stringent constraint comes from searches for B decays with missing energy. A striking prediction is a strong q2 dependence of the lepton flavour universality ratios RK and RK⁎ that should allow to clearly confirm or rule out this possibility experimentally. We also comment on the possible connection of the invisible decay product with Dark Matter.

Vector leptoquark resolution of R and RD(⁎) puzzles

We propose that three recent anomalies in $B$ meson decays, $R_{D^{(*)}}$, $R_K$, and $P_5'$, might be explained by a single vector leptoquark weak triplet state. The constraints on the parameter space are obtained by considering $t \to b \tau^+ \nu$ data, lepton flavor universality tests in the kaon sector, bounds on $B \to K^{(*)} \bar \nu \nu$, bound on the lepton flavor violating decay $B \to K \mu \tau$, and measurements of $b \to c \mu^- \bar\nu$ decays. The presence of such vector leptoquark could be exposed in precise measurements of $t \to b \tau \nu$ and $B \to K^{(*)} \bar \nu \nu$ decays. The model also predicts approximate equality of lepton flavor universality ratios $R_{K^*}$, $R_{K}$, and suppressed branching fraction of $B_s \to \mu^+ \mu^-$.