Physics Anomalies Research Articles

Gauge coupling unification in a minimal non-supersymmetric E6 GUT

We consider a minimal renormalizable non-supersymmetric [Formula: see text] Grand Unified Theory using fundamental representation 27 for fermions and scalars. The scalar with adjoint representation 78 is also taken for direct breaking of [Formula: see text] to SM. The proposed model, guided by TeV-scale vector-like fermions and scalar leptoquark, offers successful gauge unification even in the absence of any intermediate symmetry. Embedded with threshold corrections, it is shown to be compatible with the present experimental limit on proton decay lifetime. The unique feature of the model shows that the GUT threshold corrections to the unification mass is controlled by superheavy gauge bosons only, thereby minimizing the uncertainty of the GUT predictions. The scalar leptoquark and vector-like fermions residing in 27 representation can explain flavor physics anomalies like [Formula: see text] as reported by the LHCb collaboration and the muon anomalous magnetic moment reported by the recent muon [Formula: see text] experiment at Fermilab. The model can also predict a sub-eV scale neutrino at one-loop level via exchange of W and Z gauge bosons through MRIS mechanism.

Explanations of the tentative new physics anomalies and dark matter in the Simple Extension of the Standard Model (SESM)

We revisit the Simple Extension of the Standard Model (SESM), which can account for various tentative new physics anomalies and dark matter. The B physics anomalies, muon anomalous magnetic moment, and dark matter can be explained already in SESM. In this work, we study the unitarity constraint on this model and comment the parameter space on the edge of unitarity bounds. Besides, the complete scalar potential is considered which is needed to address the W boson anomaly and more dark matter (co-)annihilation processes are discussed. We perform a systematic study, and find the viable parameter space which can explain these anomalies and evade all the current experimental constraints.

The Z resonance, inelastic dark matter, and new physics anomalies in the Simple Extension of the Standard Model (SESM) with general scalar potential

We consider the generic scalar potential with CP-violation, and study the Z resonance and inelastic dark matter in the Simple Extension of the Standard Model (SESM), which can explain the dark matter as well as new physics anomalies such as the B physics anomalies and muon anomalous magnetic moment, etc. With the new scalar potential terms, we obtain the mass splittings for the real and imaginary parts of scalar fields. And thus we can have the DM co-annihilation process mediated by Z boson, which couples exclusively to the CP-even and CP-odd parts of scalar fields. This is a brand new feature compared to the previous study. For the CP conserving case, we present the viable parameter space for the Higgs and Z resonances, which can explain the B physics anomalies, muon anomalous magnetic moment, and dark matter relic density, as well as evade the constraint from the XENON1T direct detection simultaneously. For the CP-violating case, we consider the inelastic dark matter, and study four concrete scenarios for the inelastic DM-nucleon scatterings mediated by the Higgs and Z bosons in details. Also, we present the benchmark points which satisfy the aforementioned constraints. Furthermore, we investigate the constraints from the dark matter-electron inelastic scattering processes mediated by the Higgs and Z bosons in light of the XENONnT data. We show that the constraint on the Z mediated process is weak, while the Higgs mediated process excludes the dark matter with mass around several MeV.

Vector-like lepton searches at a muon collider in the context of the 4321 model* *Supported in part by the National Natural Science Foundation of China (12150005, 12075004, . 12061141002) and MOST (2018YFA0403900)

A feasibility study is performed on the search for vector-like leptons (VLLs) at a muon collider in the context of the "4321 model", an ultraviolet-complete model with rich collider phenomenology and the potential to explain several recent existing B physics measurements or anomalies. Pair production and decays of VLLs lead to an interesting final state topology with multi-jets and multi-tau leptons. In this study, we perform a Monte Carlo investigation with various machine learning techniques and examine the projected sensitivity on VLLs over a wide mass range at a TeV-scale muon collider. We find that a 3 TeV muon collider with only of data can be sensitive over the mass range of a VLL up to 1450 GeV in the "4321 model."

The flavourful present and future of 2HDMs at the collider energy frontier

We study the intersection of flavour and collider physics for Two-Higgs-Doublet models of Type I and II. Drawing from the flavour precision-LHC exotics search complementarity, we also provide a projection of the future sensitivity that can be achieved in light of currently available analyses. On the one hand, we find that the parameter space of the 2HDM can be explored significantly further with more data from the LHC with some complementarity with flavour physics. On the other hand, flavour physics results alongside their projections remain powerful tools to constrain the model space in regions where direct sensitivity to new states via exotics searches is lost. Our results further high-light the recently observed flavour physics anomalies as important drivers of new physics searches in the future; we also touch on implications for a strong first order electroweak phase transition.

Flavour anomalies and dark matter assisted unification in SO(10) GUT

With the recent experimental hint of new physics from flavor physics anomalies, combined with the evidence from neutrino masses and dark matter, we consider a minimal extension of SM with a scalar leptoquark and a fermion triplet. The scalar leptoquark with couplings to leptons and quarks can explain lepton flavor non-universality observables RK, {R}_{K^{left(ast right)}} , {R}_{D^{left(ast right)}} and RJ/ψ. Neutral component of fermion triplet provides current abundance of dark matter in the Universe. The interesting feature of the proposal is that the minimal addition of these phenomenologically rich particles (scalar leptoquark and fermion triplet) assist in realizing the unification of the gauge couplings associated with the strong and electroweak forces of standard model when embedded in the non-supersymmetric SO(10) grand unified theory. We discuss on unification mass scale and the corresponding proton decay constraints while taking into account the GUT threshold corrections.

Model with two scalar leptoquarks: R2 and S3

We discuss a model that can accommodate the $B$-physics anomalies, based on combining two scalar leptoquarks, ${R}_{2}$ and ${S}_{3}$, of mass $\mathcal{O}(1\text{ }\text{ }\mathrm{TeV})$, and that we proposed in our previous paper. We update the analysis of its parameter space and show that a model remains viable and consistent with a number of low energy and high energy flavor physics constraints. Since the model predicts a nonzero new physics phase, we discuss the possibility to test its contribution to the neutron electric dipole moment and to the angular distributions of the exclusive $b\ensuremath{\rightarrow}c\ensuremath{\tau}\overline{\ensuremath{\nu}}$ decays. We find that the model can provide a significant enhancement to $\mathcal{B}(B\ensuremath{\rightarrow}{K}^{(*)}\ensuremath{\nu}\ensuremath{\nu})$ and provides both the upper and lower bounds to $\mathcal{B}(B\ensuremath{\rightarrow}{K}^{(*)}\ensuremath{\mu}\ensuremath{\tau})$.

Muonic force behind flavor anomalies

We develop an economical theoretical framework for combined explanations of the flavor physics anomalies involving muons: (g − 2)μ, {R}_{K^{left(ast right)}} , and b → sμ+μ− angular distributions and branching ratios, that was first initiated by some of us in ref. [1]. The Standard Model (SM) is supplemented with a lepton-flavored U(1)X gauge group. The U(1)X gauge boson with the mass of mathcal{O} (0.1) GeV resolves the (g − 2)μ tension. A TeV-scale leptoquark, charged under the U(1)X, carries a muon number and mediates B-decays without prompting charged lepton flavor violation or inducing proton decay. We explore the theory space of the chiral, anomaly-free U(1)X gauge extensions featuring the above scenario, and identify many suitable charge assignments for the SM+3νR fermion content with the integer charges in the range {X}_{F_i} ∈ [−10, 10]. We then carry out a comprehensive phenomenological study of the muonic force in representative benchmark models. Interestingly, we found models which can resolve the tension without conflicting the complementary constraints, and all of the viable parameter space will be tested in future muonic resonance searches. Finally, the catalog of the anomaly-free lepton-non-universal charge assignments motivated us to explore different directions in model building. We present a model in which the muon mass and the (g − 2)μ are generated radiatively from a common short-distance dynamics after the U(1)X breaking. We also show how to charge a vector leptoquark under U(1)μ−τ in a complete gauge model.

Looking beyond the Standard Model with Third Generation Quarks at the LHC

The Large Hadron Collider (LHC) is at the frontier of collider physics today, probing new physics at unprecedented energy scales. Many theories of physics beyond the Standard Model seek to elucidate the underlying mechanism of electroweak symmetry breaking. Given their large Yukawa couplings to the Higgs boson, third generations quarks of the Standard Model, and especially the top quark, play a key role in such theories. Therefore, new particles predicted by these theories often couple preferentially to top and bottom quarks. The favoured coupling to third generation can also be used to explain recently observed flavour physics anomalies in the LHCb, Babar or Belle experiments. This article will review recent searches for new physics performed by the ATLAS and CMS experiments at the LHC, in final states containing top and bottom quarks. In particular, searches for vector-like quarks, leptoquarks, and heavy scalar and gauge bosons will be discussed.

The neutrino magnetic moment portal: cosmology, astrophysics, and direct detection

We revisit the physics of neutrino magnetic moments, focusing in particular on the case where the right-handed, or sterile, neutrinos are heavier (up to several MeV) than the left-handed Standard Model neutrinos. The discussion is centered around the idea of detecting an upscattering event mediated by a transition magnetic moment in a neutrino or dark matter experiment. Considering neutrinos from all known sources, as well as including all available data from XENON1T and Borexino, we derive the strongest up-to-date exclusion limits on the active-to-sterile neutrino transition magnetic moment. We then study complementary constraints from astrophysics and cosmology, performing, in particular, a thorough analysis of BBN . We find that these data sets scrutinize most of the relevant parameter space. Explaining the XENON1T excess with transition magnetic moments is marginally possible if very conservative assumptions are adopted regarding the supernova 1987 A and CMB constraints. Finally, we discuss model-building challenges that arise in scenarios that feature large magnetic moments while keeping neutrino masses well below 1 eV. We present a successful ultraviolet-complete model of this type based on TeV-scale leptoquarks, establishing links with muon magnetic moment, B physics anomalies, and collider searches at the LHC.

Vector leptoquarks beyond tree level. III. Vectorlike fermions and flavor-changing transitions

Extending previous work on this subject, we evaluate the impact of vector-like fermions at next-to-leading order accuracy in models with a massive vector leptoquark embedded in the $SU(4)\times SU(3)^\prime\times SU(2)_L\times U(1)_X$ gauge group. Vector-like fermions induce new sources of flavor symmetry breaking, resulting in tree-level flavor-changing couplings for the leptoquark not present in the minimal version of the model. These, in turn, lead to a series of non-vanishing flavor-changing neutral-current amplitudes at the loop level. We systematically analyze these effects in semileptonic, dipole and $\Delta F=2$ operators. The impact of these corrections in $b\to s\nu\nu$ and $b\to c\tau\nu$ observables are discussed in detail. In particular, we show that, in the parameter region providing a good fit to the $B$-physics anomalies, the model predicts a $10\%$ to $50\%$ enhancement of $\mathcal{B}(B\to K^{(*)}\nu\nu)$.

General neutrino interactions from an effective field theory perspective

General Neutrino Interactions (GNI) are scalar, pseudoscalar, vector, axial vector or tensor interactions of neutrinos with fermions, and generalise the often studied neutrino Non-Standard Interactions (NSI). If GNI arise from heavy new physics, they should be embeddable into effective field theory operators that respect the Standard Model (SM) gauge symmetry. Therefore we consider a full basis of gauge-invariant dimension-six operators involving SM fermions and right-handed singlet neutrinos and map their Wilson coefficients onto GNI parameters. In this embedding we discuss correlations of and bounds on different GNI in the context of charged lepton flavour violation processes and neutrino-fermion scattering, as well as beta decay and coherent neutrino-nucleus scattering. We also study possible UV completions of the relevant dimension-six operators for GNI via leptoquarks that can be related to radiative neutrino masses and B physics anomalies. Details on the numbers of free GNI parameters for Dirac or Majorana neutrinos and for CP violation or conservation are also provided.

Muon anomalies and the SU(5) Yukawa relations

We show that, within the framework of $SU(5)$ Grand Unified Theories (GUTs), multiple vector-like families at the GUT scale which transform under a gauged $U(1)'$ (under which the three chiral families are neutral) can result in a single vector-like family at low energies which can induce non-universal and flavourful $Z'$ couplings, which can account for the B physics anomalies in $R_{K^{(*)}}$. In such theories, we show that the same muon couplings which explain $R_{K^{(*)}}$ also correct the Yukawa relation $Y_e=Y_d^T$ in the muon sector without the need for higher Higgs representations. To illustrate the mechanism, we construct a concrete a model based on $SU(5)\times A_4 \times Z_3\times Z_7$ with two vector-like families at the GUT scale, and two right-handed neutrinos, leading to a successful fit to quark and lepton (including neutrino) masses, mixing angles and CP phases, where the constraints from lepton flavour violation require $Y_e$ to be diagonal.

The leptoquark Hunter\u2019s guide: large coupling

Leptoquarks have recently received much attention especially because they may provide an explanation to the {R}_{D^{left(*right)}}kern0.5em mathrm{and}kern0.5em {R}_{K^{left(*right)}} anomalies in rare B meson decays. In a previous paper we proposed a systematic search strategy for all possible leptoquark flavors by focusing on leptoquark pair production. In this paper, we extend this strategy to large (order unity) leptoquark couplings which offer new search opportunities: single leptoquark production and t-channel leptoquark exchange with dilepton final states. We discuss the unique features of the different search channels and show that they cover complementary regions of parameter space. We collect and update all currently available bounds for the different flavor final states from LHC searches and from atomic parity violation measurements. As an application of our analysis, we find that current limits do not exclude a leptoquark explanation of the B physics anomalies but that the high luminosity run of the LHC will reach the most interesting parameter space.

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.

Searching for leptoquarks at IceCube and the LHC

In the light of recent experimental results from IceCube, LHC searches for scalar leptoquark, and the flavor anomalies $R_K$ and $R_{K^*}$, we analyze two scalar leptoquark models with hypercharge $Y=1/6$ and $Y=7/6$. We consider the 53 high-energy starting events from IceCube and perform a statistical analysis, taking into account both the Standard Model and leptoquark contribution together. The lighter leptoquark states that are in agreement with IceCube are strongly constrained from LHC di-lepton+dijet search. Heavier leptoquarks in the TeV mass range are in agreement both with IceCube and LHC. We furthermore show that leptoquark which explains the $B$-physics anomalies and does not have any coupling with the third generation of quarks and leptons, can be strongly constrained.

Fuzzy dark matter and nonstandard neutrino interactions

We discuss novel ways in which neutrino oscillation experiments can probe dark matter. In particular, we focus on interactions between neutrinos and ultra-light ("fuzzy") dark matter particles with masses of order $10^{-22}$ eV. It has been shown previously that such dark matter candidates are phenomenologically successful and might help ameliorate the tension between predicted and observed small scale structures in the Universe. We argue that coherent forward scattering of neutrinos on fuzzy dark matter particles can significantly alter neutrino oscillation probabilities. These effects could be observable in current and future experiments. We set new limits on fuzzy dark matter interacting with neutrinos using T2K and solar neutrino data, and we estimate the sensitivity of reactor neutrino experiments and of future long-baseline accelerator experiments. These results are based on detailed simulations in GLoBES. We allow the dark matter particle to be either a scalar or a vector boson. In the latter case, we find potentially interesting connections to models addressing various $B$ physics anomalies.

Combined Interpretation of $B$-physics Anomalies and Model Building

Combined Interpretation of $B$-physics Anomalies and Model Building

Explaining B→K(*)ℓ+ℓ− anomaly by radiatively induced coupling in U(1)μ−τ gauge symmetry

We propose an extension of the standard model (SM) with $U(1)_{\mu-\tau}$ gauge symmetry, extra vectorlike quark doublets $Q'_a$ and singlet scalar $\chi$, both of which are charged under $U(1)_{\mu-\tau}$ and carry {odd} dark $Z_2$ parity. Then assuming that $\chi$ is the dark matter (DM) of the universe and imposing various constraints from dark matter search, flavor physics and collider search for $Q'_a$, one can show that radiative corrections to $b\rightarrow s Z^{'*} \rightarrow s l^+ l^- $ involving $Q'_a$ and $\chi$ can induce $\Delta C_9 \sim -1$ which can resolve the LHCb anomalies related with $B\to K^{(*)} \ell^+ \ell^-$. Therefore both DM and $B$ physics anomalies could be accommodated in the model.

Leptoquark model to explain the B -physics anomalies, RK and RD

We show that a model with a scalar leptoquark of hypercharge $Y=1/6$ which includes the light right-handed neutrinos, can successfully describe both of the $B$-physics anomalies, $R_K^{\rm exp} < R_K^{\rm SM}$ and $R_D^{\rm exp} > R_D^{\rm SM}$. We discuss the corresponding low energy effective theory and, after using the known experimental data as constraints, we show that the model is viable and that it offers several predictions which can be tested experimentally.