Minimal Flavor Violation Framework Research Articles

Flavoured Majorana Dark Matter then and now: from freeze-out scenarios to LHC signatures

We study a simplified Dark Matter model in the Dark Minimal Flavour Violation framework. Our model complements the Standard Model with a flavoured Dark Matter Majorana triplet and a coloured scalar mediator that share a Yukawa coupling with the right-handed up-type quarks with the coupling matrix λ. We extend previous work on this topic by exploring a large range of cosmologically viable parameter space, including the coannihilation region and, in particular, the region of conversion-driven freeze-out, while considering constraints from D0–D¯0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {D}^0\\hbox{--} {\\overline{D}}^0 $$\\end{document} mixing as well as constraints from direct and indirect Dark Matter searches. We find various realisations of conversion-driven freeze-out within the model, that open up allowed windows of parameter space towards small mass splittings and very weak Dark Matter couplings. Finally, we probe the model by reinterpreting current LHC searches for missing energy and long-lived particles. We point out gaps in the coverage of current constraints as well as new opportunities to search for the model at the LHC, in particular, the charge asymmetry in single-top production associated with jets and missing energy.

Opening the Higgs portal to lepton-flavoured dark matter

We study a simplified model of lepton-flavoured complex scalar dark matter coupling to right-handed leptons and the Higgs boson. The model is set up in the Dark Minimal Flavour Violation framework. In contrast to previous studies of similar models we consider the most general case and do not a priori constrain the hierarchy of dark matter masses and couplings in any way aside from the chosen parameter ranges. In the first part of the analysis we discuss the impact of Higgs portal interactions and the generalised mass hierarchy on the model’s phenomenology. We find that they render new physics masses around the electroweak scale viable, thus qualifying this model to address the (g − 2)μ puzzle. After reviewing the current situation of the latter, we perform two combined analyses — one in which (g − 2)μ allows for significant new physics effects and one in which it does not. We find that while the latter scenario allows for a larger range of new physics scales, both scenarios are equally viable.

Lepton-flavoured scalar dark matter in Dark Minimal Flavour Violation

We study a simplified model of lepton-flavoured complex scalar dark matter set up in the Dark Minimal Flavour Violation framework. In this model the Standard Model is extended by a scalar dark matter flavour triplet and a charged fermionic mediator, through which dark matter couples to the right-handed charged leptons of the Standard Model. This interaction is parameterized by a new 3 × 3 coupling matrix λ. Consistent with the field content of the model, also the Standard Model’s approximate flavour symmetry is extended to include an additional global U(3) associated with the dark matter flavour triplet. In addition to the Standard Model Yukawa couplings, the new coupling matrix λ is assumed to constitute the only source that violates this extended symmetry. We analyse the parameter space of this model by investigating constraints from collider searches, lepton flavour violating decays, the observed dark matter relic density, and direct as well as indirect dark matter detection experiments. By performing a combined analysis of all constraints we find that restrictions from lepton flavour violating decays, the observed relic density and dark matter nucleon scattering are dominant. The combination of the latter two renders limits from collider searches irrelevant while indirect detection constraints are weak due to a p-wave suppression of the annihilation rate. We conclude that lepton-flavoured scalar dark matter has a rich phenomenology and is a viable dark matter candidate.

Resolving the flavor structure in the MFV-SMEFT

We constrain the flavor structure of Wilson coefficients in the Standard Model Effective Field Theory (SMEFT) from data. In the SMEFT, new physics effects in couplings of up-type and down-type quarks are related through the Cabibbo-Kobayashi-Maskawa mixing matrix. We exploit this relation to pin down potential new sources of flavor symmetry breaking in a global analysis of high- and low-energy data from the LHC, LEP, and b factory experiments. We demonstrate the power of such an analysis by performing a combined fit of effective four-quark and two-quark couplings contributing to a large set of flavor, top-quark, electroweak, and dijet observables. All four sectors are needed to fully resolve the flavor structure of left-chiral four-quark couplings without leaving blind directions in the parameter space. Although we work in the framework of minimal flavor violation, our strategy applies as well to other flavor patterns, like U(2) flavor symmetry or leptoquark scenarios.

Tasting flavoured Majorana dark matter

We study a simplified model of flavoured Majorana dark matter in the Dark Minimal Flavour Violation framework. The model extends the Standard Model by a dark matter flavour triplet and a scalar mediator, through which the new dark fermions couple to right-handed up-type quarks. This interaction is governed by a new coupling matrix λ which is assumed to constitute the only new source of flavour and CP violation. We analyse the parameter space of this model by using constraints from collider searches, D0− overline{D} 0 mixing, cosmology and direct dark matter searches. Throughout our study, we point out crucial differences between the Majorana and Dirac dark matter cases. After performing a combined analysis within the context of all the experimental constraints mentioned above, we analyse which flavour for the dark matter particle is preferred by experimental data. We further investigate if this model is capable of explaining the large measured value of the direct CP asymmetry Delta {A}_{mathrm{CP}}^{mathrm{dir}} in charm decays. We find that significant enhancements with respect to the Standard Model expectation are compatible with all constraints, and even the central value of the measurement can be reached. We also advertise the flavour-violating final state with two same-sign top quarks produced in association with missing transverse energy as a smoking-gun signature for flavoured Majorana dark matter at the LHC.

Flavor violating Higgs couplings in minimal flavor violation

Motivated by the recent LHC data on the lepton-flavor violating (LFV) decays h → ℓ1ℓ2 and Bs,d → ℓ1ℓ2, we study the Higgs-mediated flavor-changing neutral current (FCNC) interactions in the effective field theory (EFT) approach without and with the minimal flavor violation (MFV) hypothesis, and concentrate on the later. After considering the B and K physics data, the various LFV processes, and the LHC Higgs data, severe constraints on the Higgs FCNC couplings are derived, which are dominated by the LHC Higgs data, the {B}_s-{overline{B}}_s mixing, and the μ → eγ decay. In the general case and the MFV framework, allowed ranges of various observables are obtained, such as mathrm{mathcal{B}} (Bs → ℓ1ℓ2), mathrm{mathcal{B}} (h → ℓ1ℓ2), mathrm{mathcal{B}} (h → q1q2), and the branching ratio of μ → e conversion in Al. Future prospects of searching for the Higgs FCNC interactions at the low-energy experiments and the LHC are discussed.

Probing lepton flavour (universality) violation at NA62 and future kaon experiments

Recent results from the LHC’s first run have revealed intriguing departures from lepton flavour universality in the semi-leptonic decays of B-mesons. We discuss the complementary role that rare kaon decays can provide in testing new physics explanations of these flavour anomalies. In the framework of minimal flavour violation, we relate the chiral low-energy constants involved in and with the new physics Wilson coefficients of the b → s effective Hamiltonian. We comment on the determination of these low-energy constants at NA62 and future kaon experiments, as well as the required improvements in sensitivity necessary to test the B-physics anomalies in the kaon sector.

Constraints on aZ′boson within minimal flavor violation

We explore a $Z^\prime$ boson coupled only with the Standard Model (SM) fermions ($Z^\prime f\bar{f}$) in the framework of Minimal Flavor Violation. We study its effects on the processes with lepton flavor violation $\ell_j\to \ell_i\ell_k\bar{\ell}_l$, $\ell_j\to\ell_i\gamma$, $\mu^{-}N\to e^- N$, quark flavor changing neutral currents $b\to s \ell^+\ell^-$, neutral $B$ and $K$ meson mixing, and $e^+e^-\to f\bar{f}$ at the LEP experiment to constrain the parameter space of $Z^\prime$ mass and couplings. We find that among those relevant processes, $\mu\to e\gamma$, $\mu\to 3e$, $\mu$-$e$ conversion and $e^+e^-\to f\bar{f}$ can put more stringent bounds on $Z^\prime f\bar{f}$ couplings normalized by $Z^\prime$ mass. Its implications on various processes are obtained, such as $B$ and $K$ mixing and $B/K\to M \ell_1 \bar\ell_2$ decays. In addition, we also make analysis on $Z^\prime$ signatures at the LHC with $\sqrt{s}=8$ and 13 TeV.

Beyond minimal lepton-flavored Dark Matter

We consider a class of flavored dark matter (DM) theories where dark matter interacts with the Standard Model lepton fields at the renormalizable level. We allow for a general coupling matrix between the dark matter and leptons whose structure is beyond the one permitted by the minimal flavor violation (MFV) assumption. It is assumed that this is the only new source of flavor violation in addition to the Standard Model (SM) Yukawa interactions. The setup can be described by augmenting the SM flavor symmetry by an additional $\mathrm{SU}(3)_{\chi}$, under which the dark matter $\chi$ transforms. This framework is especially phenomenologically rich, due to possible novel flavor-changing interactions which are not present within the more restrictive MFV framework. As a representative case study of this setting, which we call "beyond MFV" (BMFV), we consider Dirac fermion dark matter which transforms as a singlet under the SM gauge group and a triplet under $\mathrm{SU}(3)_{\chi}$. The DM fermion couples to the SM lepton sector through a scalar mediator $\phi$. Unlike the case of quark-flavored DM, we show that there is no $\mathbb{Z}_3$ symmetry within either the MFV or BMFV settings which automatically stabilizes the lepton-flavored DM. We discuss constraints on this setup from flavor-changing processes, DM relic abundance as well as direct and indirect detections. We find that relatively large flavor-changing couplings are possible, while the dark matter mass is still within the phenomenologically interesting region below the TeV scale. Collider signatures which can be potentially searched for at the lepton and hadron colliders are discussed. Finally, we discuss the implications for decaying dark matter, which can appear if an additional stabilizing symmetry is not imposed.

Higgs decay h → μτ with minimal flavor violation

We consider the tentative indication of flavor-violating Higgs boson decay $h\to\mu\tau$ recently reported in the CMS experiment within the framework of minimal flavor violation. Specifically, we adopt the standard model extended with the seesaw mechanism involving right-handed neutrinos plus effective dimension-six operators satisfying the minimal flavor violation principle in the lepton sector. We find that it is possible to accommodate the CMS $h\to\mu\tau$ signal interpretation provided that the right-handed neutrinos couple to the Higgs boson in some nontrivial way. We take into account empirical constraints from other lepton-flavor-violating processes and discuss how future searches for the $\mu\to e\gamma$ decay and $\mu\to e$ conversion in nuclei may further probe the lepton-flavor-violating Higgs couplings.

Minimal lepton flavor violation implications of the b → s anomalies

The latest measurements of rare b → s decays in the LHCb experiment have led to results in tension with the predictions of the standard model (SM), including a tentative indication of the violation of lepton flavor universality. Assuming that this situation will persist because of new physics, we explore some of the potential consequences in the context of the SM extended with the seesaw mechanism involving right-handed neutrinos plus effective dimension-six lepton-quark operators under the framework of minimal flavor violation. We focus on a couple of such operators which can accommodate the LHCb anomalies and conform to the minimal flavor violation hypothesis in both their lepton and quark parts. We examine specifically the lepton-flavor-violating decays B → K(∗)ℓℓ′, B s → ϕℓℓ′, B → (π, ρ)ℓℓ′, and B d,s → ℓℓ′, as well as K L → eμ and K → πeμ, induced by such operators. The estimated branching fractions of some of these decay modes with μτ in the final states are allowed by the pertinent experimental constraints to reach a few times 10−7 if other operators do not yield competitive effects. We also look at the implications for B → K(∗)νν and K → πνν, finding that their rates can be a few times larger than their SM values. These results are testable in future experiments.

Non-minimally flavour violating dark matter

Flavour symmetries provide an appealing mechanism to stabilize the dark matter particle. I present a simple model of quark flavoured dark matter that goes beyond the framework of minimal flavour violation. I discuss the phenomenological implications for direct and indirect dark matter detection experiments, high energy collider searches as well as flavour violating precision data.

Enhanced Electron EDM with Minimal Flavor Violation

The latest data from the ACME experiment have led to the most stringent limit to date on the electric dipole moment de of the electron. Nevertheless, the standard model (SM) prediction for de is many orders of magnitude below the new result, making this observable a powerful probe for physics beyond the SM. We perform a model-independent study of de in the SM plus right handed neutrinos and its extension with the seesaw mechanism under the framework of minimal flavor violation (MFV). We find that de crucially depends on whether neutrinos are Dirac or Majorana fermions. In the Majorana case, de can reach its measured bound, which therefore constrains the scale of MFV to be above a few hundred GeV. We also consider extra CP-violating sources in the Yukawa couplings of the right-handed neutrinos. Such new sources can have important effects on de.

Model building for flavor changing Higgs couplings

If $t\rightarrow hq$ ($q=c,u$) or $h\rightarrow\tau\ell$ ($\ell=\mu,e$) decays are observed, it will be a clear signal of new physics. We investigate whether natural and viable flavor models can saturate the present direct upper bounds without violating the indirect constraints from low energy loop processes. We carry out our analysis in two theoretical frameworks: minimal flavor violation (MFV) and Froggatt-Nielsen symmetry (FN). The simplest models in either framework predict flavor changing couplings that are too small to be directly observed. Yet, in the MFV framework, it is possible to have lepton flavor changing Higgs couplings close to the bound if spurions related to heavy singlet neutrinos play a role. In the FN framework, it is possible to have large flavor changing couplings in both the up and the charged lepton sectors if supersymmetry plays a role.

Fermion EDMs with minimal flavor violation

We study the electric dipole moments (EDMs) of fermions in the standard model supplemented with right-handed neutrinos and its extension including neutrino seesaw mechanism under the framework of minimal flavor violation (MFV). In the quark sector, we find that the current experimental bound on the neutron EDM does not yield a significant restriction on the scale of MFV. In addition, we consider how MFV may affect the contribution of the strong theta-term to the neutron EDM. For the leptons, the existing EDM data also do not lead to strict limits if neutrinos are Dirac particles. On the other hand, if neutrinos are Majorana in nature, we find that the constraints become substantially stronger. Moreover, the results of the latest search for the electron EDM by the ACME Collaboration are sensitive to the MFV scale of order a few hundred GeV or higher. We also look at constraints from $CP$-violating electron-nucleon interactions that have been probed in atomic and molecular EDM searches.

Large electron electric dipole moment in minimal flavor violation framework with Majorana neutrinos

The latest data from the ACME Collaboration have put a stringent constraint on the electric dipole moment d_e of the electron. Nevertheless, the standard model (SM) prediction for d_e is many orders of magnitude below the new result, making this observable a powerful probe for physics beyond the SM. We carry out a model-independent study of d_e in the SM with right handed neutrinos and its extension with the neutrino seesaw mechanism under the framework of minimal flavor violation. We find that d_e crucially depends on whether neutrinos are Dirac or Majorana particles. In the Majorana case, d_e can reach its experimental bound, and it constrains the scale of minimal flavor violation to be above a few hundred GeV or more. We also explore extra CP-violating sources in the Yukawa couplings of the right-handed neutrinos. Such new sources can have important effects on d_e.

Top-flavored dark matter and the forward-backward asymmetry

We propose a simple model where dark matter (DM) carries top flavor and couples to the Standard Model through the top quark within a framework of minimal flavor violation. Top-flavored DM can explain the anomalous top forward-backward asymmetry observed at the Tevatron, while remaining consistent with other top observables at colliders. By virtue of its large coupling to the top, DM acquires a sizable loop coupling to the $Z$ boson, and the relic density is set by annihilation through the $Z$. We also discuss constraints from current direct detection searches, emphasizing the role of spin-dependent searches to probe this scenario.

Indirect probes of the MSSM after the Higgs discovery

AbstractWe study the minimal supersymmetric standard model (MSSM) with minimal flavor violation (MFV), imposing constraints from flavor physics observables and MSSM Higgs searches, in light of the recent discovery of a 125 GeV Higgs boson by ATLAS and CMS. We analyze the electroweak vacuum stability conditions to further restrict the MSSM parameter space. In addition, a connection to ultraviolet physics is shown via an implementation of renormalization group running, which determines the TeV-scale spectrum from a small set of minimal supergravity parameters. Finally, we investigate the impact from dark matter direct detection searches. Our work highlights the complementarity of collider, flavor and dark matter probes in exploring the MSSM, and shows that even in a MFV framework, flavor observables constrain the MSSM parameter space well beyond the current reach of direct SUSY particle searches.

Ultravisible warped model from flavor triviality and improved naturalness

A warped extra-dimensional model, where the Standard Model Yukawa hierarchy is set by UV physics, is shown to have a sweet spot of parameters with improved experimental visibility and possibly naturalness. Upon marginalizing over all the model parameters, a Kaluza-Klein scale of 2.1 TeV can be obtained at 2 sigma (95.4 CL) without conflicting with electroweak precision measurements. Fitting all relevant parameters simultaneously can relax this bound to 1.7 TeV. In this bulk version of the Rattazzi-Zaffaroni shining model, flavor violation is also highly suppressed, yielding a bound of 2.4 TeV. Non-trivial flavor physics at the LHC in the form of flavor gauge bosons is predicted. The model is also characterized by a depletion of the third generation couplings -- as predicted by the general minimal flavor violation framework -- which can be tested via flavor precision measurements. In particular, sizable CP violation in Delta B=2 transitions can be obtained, and there is a natural region where Bs mixing is predicted to be larger than Bd mixing, as favored by recent Tevatron data. Unlike other proposals, the new contributions are not linked to Higgs or any scalar exchange processes.

Light stop decay in the MSSM with minimal flavour violation

In supersymmetric scenarios with a light stop particle $\tilde{t}_1$ and a small mass difference to the lightest supersymmetric particle (LSP) assumed to be the lightest neutralino, the flavour changing neutral current decay $\tilde{t}_1 \to c \tilde{\chi}_1^0$ can be the dominant decay channel and can exceed the four-body stop decay for certain parameter values. In the framework of Minimal Flavour Violation (MFV) this decay is CKM-suppressed, thus inducing long stop lifetimes. Stop decay length measurements at the LHC can then be exploited to test models with minimal flavour breaking through Standard Model Yukawa couplings. The decay width has been given some time ago by an approximate formula, which takes into account the leading logarithms of the MFV scale. In this paper we calculate the exact one-loop decay width in the framework of MFV. The comparison with the approximate result exhibits deviations of the order of 10% for large MFV scales due to the neglected non-logarithmic terms in the approximate decay formula. The difference in the branching ratios is negligible. The large logarithms have to be resummed. The resummation is performed by the solution of the renormalization group equations. The comparison of the exact one-loop result and the tree level flavour changing neutral current decay, which incorporates the resummed logarithms, demonstrates that the resummation effects are important and should be taken into account.