Flavor Observables Research Articles

Flavor physics in the multi-Higgs doublet models induced by the left-right symmetry

In this paper, we discuss the multi-Higgs doublet models, that could be effectively induced by the extended Standard Model (SM). In particular, we focus on the phenomenology in the supersymmetric model with left-right (LR) symmetry, where the down-type and the up-type Yukawa couplings are unified and the Yukawa coupling matrices are expected to be hermitian. In this model, several Higgs doublets are introduced to realize the realistic fermion mass matrices, and the heavy Higgs doublets have flavor changing couplings with quarks and leptons. The LR symmetry is assumed to break down at high energy to realize the Type-I seesaw mechanism. The supersymmetry breaking scale is expected to be around 100 TeV to achieve the 125 GeV Higgs. In such a setup, the flavor-dependent interaction of the Higgs fields becomes sizable, so that we especially discuss the flavor physics induced by the heavy Higgs fields in our work. Our prediction depends on the structure of neutrinos, e.g., the neutrino mass ordering. We demonstrate how the flavor structure of the SM affects the flavor violating couplings. In our analysis, we mainly focus on the four-fermi interaction induced by the scalar exchanging, and we propose a simple parameterization for the coefficients. Then, we find the correlations among the flavor observables and, for instance, see that our prediction for the μ → 3e process could be covered by the future experiment, in one case where the neutrino mass hierarchy is normal.

Combined explanations of B-physics anomalies: the sterile neutrino solution

In this paper we provide a combined explanation of charged- and neutral-current B-physics anomalies assuming the presence of a light sterile neutrino NR which contributes to the B → D(*)τν processes. We focus in particular on two simplified models, where the mediator of the flavour anomalies is either a vector leptoquark U1μ ∼ (3, 1, 2/3) or a scalar leptoquark S1 ∼ ( overline{mathbf{3}} , 1, 1/3). We find that U1μ can successfully reproduce the required deviations from the Standard Model while being at the same time compatible with all other flavour and precision observables. The scalar leptoquark instead induces a tension between Bs mixing and the neutral-current anomalies. For both states we present the limits and future projections from direct searches at the LHC finding that, while at present both models are perfectly allowed, all the parameter space will be tested with more luminosity. Finally, we study in detail the cosmological constraints on the sterile neutrino NR and the conditions under which it can be a candidate for 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.

The decay in the model

We study the four-body decay in the Randall–Sundrum model with custodial protection (RSc). By considering the constraints coming from the direct searches of the lightest Kaluza–Klein (KK) excitation of the gluon, electroweak precision tests, the measurements of the Higgs signal strengths at the LHC and from ΔF = 2 flavor observables, we perform a scan of the parameter space of the RSc model and obtain the maximum allowed deviations of the Wilson coefficients for different values of the lightest KK gluon mass . Later, their implications on the observables such as differential branching fraction, longitudinal polarization of the daughter baryon Λ, forward–backward asymmetry with respect to leptonic, hadronic and combined lepton–hadron angles are discussed where we present the analysis of these observables in different bins of di-muon invariant mass squared s ( = q2). It is observed that with the current constraints the Wilson coefficients in the RSc model show slight deviations from their Standard Model values and hence cannot accommodate the discrepancies between the Standard Model calculations of various observables and the LHCb measurements in Λb decays.

A realistic U(2) model of flavor

We propose a simple U(2) model of flavor compatible with an SU(5) GUT structure. All hierarchies in fermion masses and mixings arise from powers of two small parameters that control the U(2) breaking. In contrast to previous U(2) models this setup can be realized without supersymmetry and provides an excellent fit to all SM flavor observables including neutrinos. We also consider a variant of this model based on a D6 × U(1)F flavor symmetry, which closely resembles the U(2) structure, but allows for Majorana neutrino masses from the Weinberg operator. Remarkably, in this case one naturally obtains large mixing angles in the lepton sector from small mixing angles in the quark sector. The model also offers a natural option for addressing the Strong CP Problem and Dark Matter by identifying the Goldstone boson of the U(1)F factor as the QCD axion.

Investigating the 331-Z′ mediated FCNC effects on b → sμ+μ− transitions

The rare semileptonic [Formula: see text] and [Formula: see text] decay modes mediated by the [Formula: see text] transition are studied in this paper. We analyze observables such as the differential branching ratio, lepton polarization asymmetry and forward–backward asymmetry within the standard model in the low [Formula: see text] region. We then perform an analysis of the same modes within the 331 or [Formula: see text] model in which a new heavy [Formula: see text] boson contributes. The effects of the [Formula: see text]-mediated flavor-changing neutral current (FCNC) transitions on these modes are investigated by varying the mass [Formula: see text] and the model parameter [Formula: see text]. The new [Formula: see text] couplings are constrained by flavor observables of [Formula: see text] mixing and branching ratio of [Formula: see text]. We find that for some values of these couplings, the observables are quite sensitive to [Formula: see text] effects and deviations from Standard Model can be large.

Status of rare exclusiveBmeson decays in 2018

This review discusses the present experimental and theoretical status of rare flavor-changing neutral current b-quark decays at the beginning of 2018. It includes a discussion of the experimental situation and details of the currently observed anomalies in measurements of flavor observables, including lepton flavor universality. Progress on the theory side within and beyond the Standard Model theory is also discussed, together with potential New Physics interpretations of the present measurements.

Addressing the B-physics anomalies in a fundamental Composite Higgs model

I present a model addressing coherently the naturalness problem of the electroweak scale and the observed pattern of deviations from the Standard Model in semileptonic decays of B mesons. The Higgs and the two scalar leptoquarks responsible for the B-physics anomalies, {S}_1=left(overline{mathbf{3}},mathbf{1},1/3right) and {S}_3=left(overline{mathbf{3}},mathbf{3},1/3right) , arise as pseudo Nambu-Goldstone bosons of a new strongly coupled sector at the multi-TeV scale. I focus on an explicit realization of such a dynamics in terms of a new strongly coupled gauge interaction and extra vectorlike fermions charged under it. The model presents a very rich phenomenology, ranging from flavour observables, Higgs and electroweak precision measurements, and direct searches of new states at the LHC.

Implications of hidden gauged U(1) model for B anomalies

We propose a hidden gauged $U(1)_H$ $Z'$ model to explain deviations from the Standard Model (SM) values in lepton flavor universality known as $R_K$ and $R_D$ anomalies. The $Z'$ only interacts with the SM fermions via their mixing with vector-like doublet fermions after the $U(1)_H$ symmetry breaking, which leads to $b \to s \mu\mu$ transition through the $Z^{\prime}$ at tree level. Moreover, introducing an additional mediator, inert-Higgs doublet, yields $b\to c \tau \nu$ process via charged scalar contribution at tree level. Using flavio package, we scrutinize adequate sizes of the relevant Wilson coefficients to these two processes by taking various flavor observables into account. It is found that significant mixing between the vector-like and the second generation leptons is needed for the $R_K$ anomaly. A possible explanation of the $R_D$ anomaly can also be simultaneously addressed in a motivated situation, where a single scalar operator plays a dominant role, by the successful model parameters for the $R_K$ anomaly.

R(K(⁎)) from dark matter exchange

Hints of lepton flavor violation have been observed by LHCb in the rate of the decay B→Kμ+μ− relative to that of B→Ke+e−. This can be explained by new scalars and fermions which couple to standard model particles and contribute to these processes at loop level. We explore a simple model of this kind, in which one of the new fermions is a dark matter candidate, while the other is a heavy vector-like quark and the scalar is an inert Higgs doublet. We explore the constraints on this model from flavor observables, dark matter direct detection, and LHC run II searches, and find that, while currently viable, this scenario will be directly tested by future experiments.

The Higgs properties in the MSSM after the LHC Run-2

We scrutinize the parameter space of the SM-like Higgs boson in the minimal supersymmetric standard model (MSSM) under current experimental constraints. The constraints are from (i) the precision electroweak data and various flavor observables; (ii) the direct 22 separate ATLAS searches in Run-1; (iii) the latest LHC Run-2 Higgs data and tri-lepton search of electroweakinos. We perform a scan over the parameter space and find that the Run-2 data can further exclude a part of parameter space. For the property of the SM-like Higgs boson, its gauge couplings further approach to the SM values with a deviation below 0.1%, while its Yukawa couplings [Formula: see text] and [Formula: see text] can still sizably differ from the SM predictions by several tens percent.

30 parameter version of the MSSM in light of the Bs→μ+μ− observation

The R-parity conserving MSSM in light of the decay $B_s\rightarrow\mu^+\mu^-$ with near-SM branching ratio is an interesting platform for studying the complementarity between direct and indirect searches for beyond the SM physics. Based on this, we have analysed the possible impact of the $B_s\rightarrow\mu^+\mu^-$ observation on the posterior sample from the global fit of a 30-parameter MSSM (MSSM-30), and the related Wilson Coefficients. The MSSM-30 is a systematically constructed, symmetry-guided, MSSM parametrization, as opposed to the traditional frames (e.g. pMSSM) with crude treatment of flavor violation parameters. This paper illustrates why phenomenological frames like the MSSM-30 should be preferred to study flavor physics. For the current and future B-physics experimental precision, such a consideration is crucial for suitably assessing supersymmetric contributions to flavor observables.

Can an unbroken flavour symmetry provide an approximate description of lepton masses and mixing?

We provide a complete answer to the following question: what are the flavour groups and representations providing, in the symmetric limit, an approximate description of lepton masses and mixings? We assume that neutrino masses are described by the Weinberg operator. We show that the pattern of lepton masses and mixings only depends on the dimension, type (real, pseudoreal, complex), and equivalence of the irreducible components of the flavour representation, and we find only six viable cases. In all cases the neutrinos are either anarchical or have an inverted hierarchical spectrum. In the context of SU(5) unification, only the anarchical option is allowed. Therefore, if the hint of a normal hierarchical spectrum were confirmed, we would conclude (under the above assumption) that symmetry breaking effects must play a leading order role in the understanding of neutrino flavour observables. In order to obtain the above results, we develop a simple algorithm to determine the form of the lepton masses and mixings directly from the structure of the decomposition of the flavour representation in irreducible components, without the need to specify the form of the lepton mass matrices.

Baryon production from cluster hadronisation

We present an extension to the colour reconnection model in the Monte Carlo event generator Herwig to account for the production of baryons and compare it to a series of observables for soft physics. The new model is able to improve the description of charged-particle multiplicities and hadron flavour observables in pp collisions.

B decay anomalies at LHCb

The LHCb collaboration has provided a plethora of precise measurements of flavour observables in the last years. In the B meson sector some of these results show consistent deviations from Standard Model predictions with a clear tendency to specific New Physics scenarios. B decay anomalies are found in particular related to lepton flavour universality tests and angular observables in Flavour-Changing-Neutral-Current transitions. I review here the LHCb measurements and their experimental caveats. Results from Lattice are crucial in the coming years to verify if these anomalies are explained by QCD effects or if they are unambiguous hints of physics beyond the Standard Model.

Global fits of GUT-scale SUSY models with GAMBIT

We present the most comprehensive global fits to date of three supersymmetric models motivated by grand unification: the constrained minimal supersymmetric standard model (CMSSM), and its Non-Universal Higgs Mass generalisations NUHM1 and NUHM2. We include likelihoods from a number of direct and indirect dark matter searches, a large collection of electroweak precision and flavour observables, direct searches for supersymmetry at LEP and Runs I and II of the LHC, and constraints from Higgs observables. Our analysis improves on existing results not only in terms of the number of included observables, but also in the level of detail with which we treat them, our sampling techniques for scanning the parameter space, and our treatment of nuisance parameters. We show that stau co-annihilation is now ruled out in the CMSSM at more than 95% confidence. Stop co-annihilation turns out to be one of the most promising mechanisms for achieving an appropriate relic density of dark matter in all three models, whilst avoiding all other constraints. We find high-likelihood regions of parameter space featuring light stops and charginos, making them potentially detectable in the near future at the LHC. We also show that tonne-scale direct detection will play a largely complementary role, probing large parts of the remaining viable parameter space, including essentially all models with multi-TeV neutralinos.

FlavBit: a GAMBIT module for computing flavour observables and likelihoods

Flavour physics observables are excellent probes of new physics up to very high energy scales. Here we present FlavBit, the dedicated flavour physics module of the global-fitting package GAMBIT. FlavBit includes custom implementations of various likelihood routines for a wide range of flavour observables, including detailed uncertainties and correlations associated with LHCb measurements of rare, leptonic and semileptonic decays of B and D mesons, kaons and pions. It provides a generalised interface to external theory codes such as SuperIso, allowing users to calculate flavour observables in and beyond the Standard Model, and then test them in detail against all relevant experimental data. We describe FlavBit and its constituent physics in some detail, then give examples from supersymmetry and effective field theory illustrating how it can be used both as a standalone library for flavour physics, and within GAMBIT.

Slepton non-universality in the flavor-effective MSSM

Supersymmetric theories supplemented by an underlying flavor-symmetry {mathcal{G}}_f provide a rich playground for model building aimed at explaining the flavor structure of the Standard Model. In the case where supersymmetry breaking is mediated by gravity, the soft-breaking Lagrangian typically exhibits large tree-level flavor violating effects, even if it stems from an ultraviolet flavor-conserving origin. Building on previous work, we continue our phenomenological analysis of these models with a particular emphasis on leptonic flavor observables. We consider three representative models which aim to explain the flavor structure of the lepton sector, with symmetry groups {mathcal{G}}_f=Delta (27) , A4, and S3.

Lepton-flavor universality limits in warped space

We explore the limits on lepton-flavor universality (LFU) violation in theories where the hierarchy problem is solved by means of a warped extra dimension. In those theories LFU violation, in fermion interaction with Kaluza-Klein modes of gauge bosons, is provided ab initio when different flavor of fermions are differently localized along the extra dimension. As this fact arises from the mass pattern of quarks and leptons, LFU violation is natural in this class of theories. We analyze the experimental data pointing towards LFU violation, as well as the most relevant electroweak and flavor observables, and the LFU tests in the $\mu/e$ and $\tau/\mu$ sectors. We find agreement with $R_{K^{(\ast)}}$ and $R_{D^{(\ast)}}$ data at 95% CL, provided the third generation left-handed fermions are composite ($0.14 < c_{b_L} < 0.28$ and $0.27 < c_{\tau_L} < 0.33$), and find the absolute limits $R_{K^{(\ast)}}\gtrsim 0.79$ and $R_{D^{(\ast)}}/R_{D^{(\ast)}}^{\rm SM}\lesssim 1.13$. Moreover we predict $\mathcal B( B\to K\nu\bar\nu)\gtrsim 1.14\times 10^{-5}$ at 95% CL, smaller than the present experimental upper bound but a few times larger than the Standard Model prediction.

The role of the S3 GUT leptoquark in flavor universality and collider searches

We investigate the ability of the S3 scalar leptoquark to address the recent hints of lepton universality violation in B meson decays. The S3 leptoquark with quantum numbers left(overline{mathbf{3}},mathbf{3},1/3right) naturally emerges in the context of an SU(5) GUT model without any conflict with the stringent limits from observed nucleon stability. Scalar leptoquark S3 with left-handed couplings to 2nd and 3rd generations of charged leptons and down-type quarks seems well-suited to address both RK(∗) and RD(∗) . We quantify this suitability with numerical fits to a plethora of relevant flavor observables. The proposed SU(5) model calls for a second leptoquark state, i.e., {tilde{R}}_2 with quantum numbers (3,2, 1/6), if one is to generate gauge coupling unification and neutrino mass. We accordingly include it in our study to investigate {tilde{R}}_2 ’s ability to offset adverse effects of S3 and thus improve a quality of numerical fits. A global fit of the leptoquark Yukawa couplings shows that large couplings of light S3 to τ leptons are preferred. We furthermore identify Bto {K}^{left(ast right)}overline{nu}nu as the most sensitive channel to probe the preferred region of parameter space. Large couplings of S3 to τ leptons are finally confronted with the experimental searches for τ final states at the Large Hadron Collider. These searches comprise a study of decay products of the leptoquark pair production, as well as, and more importantly, an analysis of the high-mass ττ finalstates.