Physics of leptoquarks in precision experiments and at particle colliders

An angular analysis of the $B^{0}\rightarrow K^{*0}(\rightarrow K^{+}\pi^{-})\mu^{+}\mu^{-}$ decay is presented. The dataset corresponds to an integrated luminosity of $3.0\,{\mbox{fb}^{-1}}$ of $pp$ collision data collected at the LHCb experiment. The complete angular information from the decay is used to determine $C\!P$-averaged observables and $C\!P$ asymmetries, taking account of possible contamination from decays with the $K^{+}\pi^{-}$ system in an S-wave configuration. The angular observables and their correlations are reported in bins of $q^2$, the invariant mass squared of the dimuon system. The observables are determined both from an unbinned maximum likelihood fit and by using the principal moments of the angular distribution. In addition, by fitting for $q^2$-dependent decay amplitudes in the region $1.1<q^{2}<6.0\mathrm{\,Ge\kern -0.1em V}^{2}/c^{4}$, the zero-crossing points of several angular observables are computed. A global fit is performed to the complete set of $C\!P$-averaged observables obtained from the maximum likelihood fit. This fit indicates differences with predictions based on the Standard Model at the level of 3.4 standard deviations. These differences could be explained by contributions from physics beyond the Standard Model, or by an unexpectedly large hadronic effect that is not accounted for in the Standard Model predictions.

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Angular analysis of the $B^{0}\rightarrow K^{*0}\mu^{+}\mu^{-}$ decay using $3\,\mbox{fb}^{-1}$ of integrated luminosity

An angular analysis of the B0 → K*0(→ K+π−)μ+μ− decay is presented. The dataset corresponds to an integrated luminosity of 3.0 fb−1 of pp collision data collected at the LHCb experiment. The complete angular information from the decay is used to determine CP-averaged observables and CP asymmetries, taking account of possible contamination from decays with the K+π− system in an S-wave configuration. The angular observables and their correlations are reported in bins of q2, the invariant mass squared of the dimuon system. The observables are determined both from an unbinned maximum likelihood fit and by using the principal moments of the angular distribution. In addition, by fitting for q2-dependent decay amplitudes in the region 1.1 < q2 < 6.0 GeV2/c4, the zero-crossing points of several angular observables are computed. A global fit is performed to the complete set of CP-averaged observables obtained from the maximum likelihood fit. This fit indicates differences with predictions based on the Standard Model at the level of 3.4 standard deviations. These differences could be explained by contributions from physics beyond the Standard Model, or by an unexpectedly large hadronic effect that is not accounted for in the Standard Model predictions.[Figure not available: see fulltext.]

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Angular analysis of the B 0 → K *0 μ + μ − decay using 3 fb −1 of integrated luminosity

Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a U(2)q ×U(2)l flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an SU(2)L-singlet vector leptoquark emerges as a particularly simple and successful framework.

/pdf/b-physics-anomalies-a-guide-to-combined-explanations-1qj28zwkus.pdf

B -physics anomalies: a guide to combined explanations

Over the past years, experiments accumulated intriguing hints for new physics (NP) in flavor observables, namely in the anomalous magnetic moment of the muon (a
 μ
 ), in R(D
 (∗)) = Br(B → D
 (∗)
 τ ν)/Br(B → D
 (∗)
 lν) and in b → sμ
 +
 μ
 − transitions, which are all at the 3 − 4 σ level In this article we point out that one can explain the R(D
 (∗)) anomaly using two scalar leptoquarks (LQs) with the same mass and coupling to fermions related via a discrete symmetry: an SU(2)
 L
 singlet and an SU(2)
 L
 triplet, both with hypercharge Y = −2/3 In this way, potentially dangerous contributions to b → sνν are avoided and non-CKM suppressed effects in R(D
 (∗)) can be generated This allows for smaller overall couplings to fermions weakening the direct LHC bounds In our model, R(D
 (∗)) is directly correlated to b → sτ +
 τ − transitions where an enhancement by orders of magnitude compared to the standard model (SM) is predicted, such that these decay modes are in the reach of LHCb and BELLE II Furthermore, one can also naturally explain the b → sμ
 +
 μ
 − anomalies (including R(K)) by a C
 9 = −C
 10 like contribution without spoiling μ − e universality in charged current decays In this case sizable effects in b → sτ μ transitions are predicted which are again well within the experimental reach One can even address the longstanding anomaly in a
 μ
 , generating a sizable decay rate for τ → μγ However, we find that out of the three anomalies R(D
 (∗)), b → sμ
 +
 μ
 − and a
 μ
 only two (but any two) can be explained simultaneously We point out that a very similar phenomenology can be achieved using a vector leptoquark SU(2)
 L
 singlet with hypercharge 2/3 In this case, no tuning between couplings is necessary, but the model is non-renormalizable

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Simultaneous explanation of R(D (∗)) and b→sμ + μ −: the last scalar leptoquarks standing

We present a unified explanation for the $B$-decay anomalies in ${R}_{{D}^{(*)}}$ and ${R}_{K}$ together with the anomalous muon magnetic moment, consistent with the constraints from the current measurements of leptonic decay rates and ${D}^{0}\ensuremath{-}{\overline{D}}^{0}$, ${B}_{s}^{0}\ensuremath{-}{\overline{B}}_{s}^{0}$ mixings, within the framework of a minimal left-right symmetric gauge theory motivated by one of the low-energy subgroups of ${E}_{6}$ naturally accommodating leptoquarks.

Towards a unified explanation of R D (*) , R K and (g -2 ) μ anomalies in a left-right model with leptoquarks

Motivated by an explanation of the $R_{K^{(*)}}$ anomalies, we propose a Standard Model extension via two scalar SU(2)$_L$ triplet leptoquarks and three generations of triplet Majorana fermions. The gauge group is reinforced by a $Z_2$ symmetry, ensuring the stability of the lightest $Z_2$-odd particle, which is a potentially viable dark matter candidate. Neutrino mass generation occurs radiatively (at the three-loop level), and leads to important constraints on the leptoquark couplings to leptons. We consider very generic textures for the flavour structure of the $h_1$ leptoquark Yukawa couplings, identifying classes of textures which succeed in saturating the $R_{K^{(*)}}$ anomalies. We subsequently carry a comprehensive analysis of the model's contributions to numerous high-intensity observables such as meson oscillations and decays, as well as charged lepton flavour violating processes, which put severe constraints on the flavour structure of these leptoquark extensions. Our findings suggest that the most constraining observables are $K^+ \to \pi^+ \nu \bar \nu$ decays, and charged lepton flavour violating $\mu -e$ conversion in nuclei (among others). Nevertheless, for several classes of flavour textures and for wide mass regimes of the new mediators (within collider reach), this Standard Model extension successfully addresses neutrino mass generation, explains the current $R_{K^{(*)}}$ tensions, and offers a viable dark matter candidate.

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Reconciling $B$-decay anomalies with neutrino masses, dark matter and constraints from flavour violation.

Recent experimental results from the LHCb, BaBar and Belle collaborations on the semitauonic decays of B meson, $$ \overline{B}\to {D}^{\left(\ast \right)}\tau \overline{\nu} $$
 , showing a significant deviation from the Standard Model (SM), hint towards a new physics scenario beyond the SM. In this work, we show that these enhanced decay rates can be explained within the framework of E
 6 motivated Alternative Left-Right Symmetric Model (ALRSM), which has been successful in explaining the recent CMS excesses and has the feature of accommodating high scale leptogenesis. The R-parity conserving couplings in ALRSM can contribute universally to both $$ \overline{B}\to D\tau \overline{\nu} $$
 and $$ \overline{B}\to {D}^{\ast}\tau \overline{\nu} $$
 via the exchange of scalar leptoquarks. We study the leptonic decays $$ {D}_s^{+}\to {\tau}^{+}\overline{\nu} $$
 , $$ {B}^{+}\to {\tau}^{+}\overline{\nu} $$
 , $$ {D}^{+}\to {\tau}^{+}\overline{\nu} $$
 and $$ {D}^0-{\overline{D}}^0 $$
 mixing to constrain the couplings involved in explaining the enhanced B decay rates and we find that ALRSM can explain the current experimental data on $$ \mathrm{\mathcal{R}}\left({D}^{\left(\ast \right)}\right) $$
 quite well while satisfying these constraints.

/pdf/bar-b-rightarrow-d-ast-tau-bar-nu-excesses-in-alrsm-j3rz5o58ql.pdf

$\bar{B}\rightarrow D^{(\ast)}\tau \bar{\nu}$ excesses in ALRSM constrained from $B$, $D$ decays and $D^{0}-\bar{D}^{0}$ mixing

Recently, several anomalies in flavor physics have been observed, and it was noticed that leptoquarks might account for the deviations from the Standard Model. In this work, we examine the effects of new physics originating from a scalar leptoquark model on the kaon sector. The leptoquark we consider is a TeV-scale particle and within the reach of the LHC. We use the existing experimental data on the several kaon processes including ${K}^{0}\ensuremath{-}{\overline{K}}^{0}$ mixing; rare decays ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$, ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}}$; the short-distance part of ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$; and lepton-flavor-violating decay ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ifmmode\pm\else\textpm\fi{}}{e}^{\ensuremath{\mp}}$ to obtain useful constraints on the model.

Constraints on a scalar leptoquark from the kaon sector

We explore the interplay between $h(125) \to \tau\mu$ search at the LHC and $\tau \to \mu\gamma$ at the up and coming Belle~II experiment, in context of the general two Higgs doublet model with extra Yukawa couplings such as $\rho_{\tau\mu}$. The search for $h \to \tau\mu$ constrains $\rho_{\tau\mu} \cos\gamma$, where $\cos\gamma$ is the $h$--$H$ mixing angle of $h$ with the exotic $CP$-even scalar $H$. For $\tau \to \mu\gamma$, we define the "BSM-benchmark" by setting the extra top Yukawa coupling $\rho_{tt} = 1\ (\cong \lambda_t)$ in two-loop diagrams, and $\cos\gamma = 0$ to decouple $h$. We show that this leading effect due to $H$ and $CP$-odd scalar $A$ can be readily probed by Belle~II, even for the conservative value of $\rho_{\tau\mu} = 0.7\lambda_\tau$. We define the subleading "$h$-benchmark" by setting $\rho_{tt} = 0$ in two-loop diagrams, and take the conservative maximal value of $\cos\gamma = 0.2$. We show that it falls beyond Belle~II reach, but can interfere with the BSM-benchmark effect, which in principle probes the phase of $\rho_{tt}$. We further show that the one-loop $H$, $A$ effect, proportional to $\rho_{\tau\mu}\rho_{\tau\tau}$ in amplitude, is beyond the sensitivity of Belle~II to probe, even for $\rho_{\tau\tau}$ as large as $3\lambda_\tau$. With the working assumption that $\rho_{32}^f, \rho_{33}^f = {\cal O}(\lambda_3^f)$ for all charged fermions $f$, we find good discovery potential for both $\tau$ lepton flavor violation searches in the coming decade.

Girish Kumar

Papers

Towards a unified explanation of R D (*) , R K and (g -2 ) μ anomalies in a left-right model with leptoquarks

Reconciling $B$-decay anomalies with neutrino masses, dark matter and constraints from flavour violation.

$\bar{B}\rightarrow D^{(\ast)}\tau \bar{\nu}$ excesses in ALRSM constrained from $B$, $D$ decays and $D^{0}-\bar{D}^{0}$ mixing

Constraints on a scalar leptoquark from the kaon sector

Coming decade of h→τμ and τ→μγ interplay in τ flavor violation search