New Sources Of Flavour Violation Research Articles

Single-top final states as a probe of top-flavoured dark matter models at the LHC

Models incorporating flavoured dark matter provide an elegant solution to the dark matter problem, evading the tight LHC and direct direction constraints on simple WIMP models. In Dark Minimal Flavour Violation, a simple framework of flavoured dark matter with new sources of flavour violation, the constraints from thermal freeze-out, direct detection experiments, and flavour physics create well-defined benchmark scenarios for these models. We study the LHC phenomenology of four such scenarios, focusing on final states where a single top quark is produced accompanied by no jets, one jet from the fragmentation of light quarks or a b-tagged jet. For each of these signatures we develop a realistic LHC analysis, and we show that the proposed analyses would increase the parameter space coverage for the four benchmarks, compared to existing flavour-conserving LHC analyses. Finally we show the projected discovery potential of the considered signatures for the full LHC statistics at 14 TeV, and for the High Luminosity LHC.

B anomalies under the lens of electroweak precision

The measurements carried out at LEP and SLC projected us into the precision era of electroweak physics. This has also been relevant in the theoretical interpretation of LHCb and Belle measurements of rare B semileptonic decays, paving the road for new physics with the inference of lepton universality violation in {R}_{K^{left(ast right)}} ratios. The simplest explanation of these flavour anomalies — sizeable one-loop contributions respecting Minimal Flavour Violation — is currently disfavoured by electroweak precision data. In this work, we discuss how to completely relieve the present tension between electroweak constraints and one-loop minimal flavour violating solutions to {R}_{K^{left(ast right)}} . We determine the correlations in the Standard Model Effective Field Theory that highlight the existence of such a possibility. Then, we consider minimal extensions of the Standard Model where our effective-field-theory picture can be realized. We discuss how these solutions to b → sℓℓ anomalies, respecting electroweak precision and without any new source of flavour violation, may point to the existence of a Z′ boson at around the TeV scale, within the discovery potential of LHC, or to leptoquark scenarios.

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.

Combining Pati-Salam and flavour symmetries

We construct an extension of the Standard Model (SM) which is based on grand unification with Pati-Salam symmetry. The setup is supplemented with the idea of spontaneous flavour symmetry breaking which is mediated through flavon fields with renormalizable couplings to new heavy fermions. While we argue that the new gauge bosons in this approach can be sufficiently heavy to be irrelevant at low energies, the fermionic partners of the SM quarks, in particular those for the third generation, can be relatively light and provide new sources of flavour violation. The size of the effects is constrained by the observed values of the SM Yukawa matrices, but in a way that is different from the standard minimal-flavour violation approach. We determine characteristic deviations from the SM that could eventually be observed in future precision measurements.

Enhancement ofBr(Bd→μ+μ−)/Br(Bs→μ+μ−)in supersymmetric unified models

We explain the 2.3 sigma deviation in the recent measurements of the neutral B mesons decay into muon pairs from the standard model prediction in the framework of supersymmetric grand unified models using anti-symmetric coupling as a new source of flavor violation. We show a correlation between the B_d -> mu^+mu^- decay and the CP phase in the B_d -> J/psi K decay and that their deviations from the standard model predictions can be explained after satisfying constraints arising from various hadronic and leptonic rare decay processes, B-bar{B}, K-bar{K} oscillations data and electric dipole moments of electron and neutron. The allowed parameter space is typically represented by pseudoscalar Higgs mass m_A < 1 TeV and tan beta_H (= {v_u}/{v_d}) < 20 for squark and gluino masses around 2 TeV.

Rare flavor processes in Maximally Natural Supersymmetry

We study CP-conserving rare flavor violating processes in the recently proposed theory of Maximally Natural Supersymmetry (MNSUSY). MNSUSY is an unusual supersymmetric (SUSY) extension of the Standard Model (SM) which, remarkably, is un-tuned at present LHC limits. It employs Scherk-Schwarz breaking of SUSY by boundary conditions upon compactifying an underlying 5-dimensional (5D) theory down to 4D, and is not well-described by softly-broken $\mathcal{N}=1$ SUSY, with much different phenomenology than the Minimal Supersymmetric Standard Model (MSSM) and its variants. The usual CP-conserving SUSY-flavor problem is automatically solved in MNSUSY due to a residual almost exact $U(1)_R$ symmetry, naturally heavy and highly degenerate 1st- and 2nd-generation sfermions, and heavy gauginos and Higgsinos. Depending on the exact implementation of MNSUSY there exist important new sources of flavor violation involving gauge boson Kaluza-Klein (KK) excitations. The spatial localization properties of the matter multiplets, in particular the brane localization of the 3rd generation states, imply KK-parity is broken and {\it tree-level} contributions to flavor changing neutral currents are present in general. Nevertheless, we show that simple variants of the basic MNSUSY model are safe from present flavor constraints arising from kaon and $B$-meson oscillations, the rare decays $B_{s,d} \to \mu^+ \mu^-$, $\mu \to {\bar e}ee$ and $\mu$-$e$ conversion in nuclei. We also briefly discuss some special features of the radiative decays $\mu \to e \gamma$ and ${\bar B}\to X_s \gamma$. Future experiments, especially those concerned with lepton flavor violation, should see deviations from SM predictions unless one of the MNSUSY variants with enhanced flavor symmetries is realized.

Stringent tests of constrained Minimal Flavor Violation through ΔF=2 transitions

New Physics contributions to ΔF=2 transitions in the simplest extensions of the Standard Model (SM), the models with constrained Minimal Flavor Violation (CMFV), are parametrized by a single variable S(v), the value of the real box diagram function that in CMFV is bounded from below by its SM value S 0(x t ). With already very precise experimental values of ε K , ΔM d , ΔM s and precise values of the CP-asymmetry $S_{\psi K_{S}}$ and of $\hat{B}_{K}$ entering the evaluation of ε K , the future of CMFV in the ΔF=2 sector depends crucially on the values of |V cb |, |V ub |, γ, $F_{B_{s}} \sqrt{\hat{B}_{B_{s}}}$ and $F_{B_{d}} \sqrt{\hat{B}_{B_{d}}}$ . The ratio ξ of the latter two non-perturbative parameters, already rather precisely determined from lattice calculations, allows then together with ΔM s /ΔM d and $S_{\psi K_{S}}$ to determine the range of the angle γ in the unitarity triangle independently of the value of S(v). Imposing in addition the constraints from |ε K | and ΔM d allows to determine the favorite CMFV values of |V cb |, |V ub |, $F_{B_{s}}\sqrt{\hat{B}_{B_{s}}}$ and $F_{B_{d}} \sqrt{\hat{B}_{B_{d}}}$ as functions of S(v) and γ. The |V cb |4 dependence of ε K allows to determine |V cb | for a given S(v) and γ with a higher precision than it is presently possible using tree-level decays. The same applies to |V ub |, |V td | and |V ts | that are automatically determined as functions of S(v) and γ. We derive correlations between $F_{B_{s}}\sqrt{\hat{B}_{B_{s}}}$ and $F_{B_{d}} \sqrt{\hat{B}_{B_{d}}}$ , |V cb |, |V ub | and γ that should be tested in the coming years. Typically $F_{B_{s}}\sqrt{\hat{B}_{B_{s}}}$ and $F_{B_{d}} \sqrt{\hat{B}_{B_{d}}}$ have to be lower than their present lattice values, while |V cb | has to be higher than its tree-level determination, with a significance depending on the evolution of their errors. Within the SM this would imply values for $\mathcal{B}(K^{+}\rightarrow\pi^{+}\nu\bar{\nu})$ and $\mathcal{B}(K_{L}\rightarrow\pi^{0}\nu\bar{\nu})$ that are typically larger by (15–20) % than those presently quoted in the literature. The region in the space of these three parameters allowed by CMFV indicates tensions in this class of models and hints for the presence of new sources of flavor violation and/or new local operators in ΔF=2 data that are strongly suppressed in these models. As a byproduct we propose to reduce the present uncertainty in the charm contribution to ε K by using the experimental value of ΔM K .

The anatomy of neutral scalars with FCNCs in the flavour precision era

In many extensions of the Standard Model flavour changing neutral current processes can be mediated by tree-level heavy neutral scalars and/or pseudo-scalars H^0(A^0). This generally introduces new sources of flavour violation and CP violation as well as left-handed and right-handed scalar currents. These new physics contributions imply a pattern of deviations from SM expectations for FCNC processes that depends only on the couplings of H^0(A^0) to fermions and on their masses. In situations in which a single H^0 or A^0 dominates NP contributions stringent correlations between Delta F=2 and Delta F=1 observables exist. Anticipating the Flavour Precision Era ahead of us we illustrate this by searching for allowed oases in the landscape of a given model assuming significantly smaller uncertainties in CKM and hadronic parameters than presently available. To this end we analyze Delta F=2 observables in B^0_{s,d}-bar B^0_{s,d} and K^0-bar K^0 systems and rare B and K decays with charged leptons in the final state including both left-handed and right-handed scalar couplings of H^0 and A^0 to quarks in various combinations. We identify a number of correlations between various flavour observables that could test and distinguish these different scenarios. The prominent role of the decays B_{s,d}-> mu^+ mu^- in these studies is emphasized. Imposing the existing flavour constraints, a rich pattern of deviations from the SM expectations in rare B_{s,d} decays emerges provided M_H< 1 TeV. NP effects in rare K decays are very small. Neutral SM Higgs contributions to rare B and K decays turn out to be negligible once the constraints from Delta F=2 processes are taken into account. Finally, we point out striking differences between the correlations found here and in scenarios in which tree-level FCNC are mediated by a new neutral gauge boson Z'.

Impact of squark flavor violation on neutralino dark matter

We discuss the possibility of new sources of flavour violation in the squark sector of supersymmetric models in the context of the dark matter relic density. We show that the corresponding non-minimal flavour violation terms in the squark mass matrices can have an important impact on the thermally averaged (co)annihilation cross section of the neutralino, and in consequence can modify its predicted relic density. We discuss in detail the relevant effects and present a numerical study of neutralino annihilation and coannihilation in this context. We also comment on the LHC phenomenology of the corresponding scenarios.

Constraining flavour symmetries at the EW scale I: the A 4 Higgs potential

We consider an extension of the Standard Model in which the symmetry is enlarged by a global flavour factor A4 and the scalar sector accounts for three copies of the Standard Model Higgs, transforming as a triplet of A4. In this context, we study the most general scalar potential and its minima, performing for each of them a model independent analysis on the related phenomenology. We study the scalar spectrum, the new contributions to the oblique corrections, the decays of the Z and W, the new sources of flavour violation, which all are affected by the introduction of multiple Higgses transforming under A4. We find that this model independent approach discriminates the different minima allowed by the scalar potential.

Flavor mixing in gauge-Higgs unification

We discuss flavor mixing and resultant flavor changing neutral current processes in the SU(3) ⊗ SU(3)color gauge-Higgs unification scenario. To achieve flavor violation is a challenging issue in the scenario, since the Yukawa couplings are originally higher dimensional gauge interactions. We argue that the presence of Z 2-odd bulk masses of fermions plays a crucial role as the new source of flavor violation. Although introducing brane-localized mass terms in addition to the bulk masses is necessary to realize flavor mixing, if the bulk masses were universal among generations, the flavor mixing and flavor changing neutral current processes are known to disappear. We also discuss whether natural flavor conservation is realized in the scenario. It is shown that the new source of flavor violation leads to flavor changing neutral current processes at the tree level due to the exchange of non-zero Kaluza-Klein gauge bosons. As a typical example we calculate the rate of \( {K^0} - {\overline K^0} \) mixing due to the non-zero Kaluza-Klein gluon exchange at the tree level. The obtained result for the mass difference of neutral kaon is suppressed by the inverse powers of the compactification scale. By comparing our prediction with the data we obtain the lower bound of the compactification scale as a function of one unfixed parameter of the theory, which is of \( \mathcal{O}\left( {10} \right) \) TeV, except for some extreme cases. We argue that the reason to get such rather mild lower bound is the presence of “GIM-like” mechanism, which is a genuine feature of GHU scenario.

Radiatively induced flavour violation in the general two-Higgs doublet model with Yukawa alignment

The most general two-Higgs doublet model contains new sources of flavour violation that are usually in conflict with the experimental constraints. One possibility to suppress the exotic contribution to the flavour changing neutral currents consists on imposing the alignment of the Yukawa couplings. This condition presumably holds at a high-energy scale and is spoiled by the radiative corrections. We compute in this Letter the size of the radiatively induced flavour violating Higgs couplings at the electroweak scale. These also yield the absolute lower bound on the size of the exotic contributions to the flavour changing neutral currents in any two-Higgs doublet model, barring cancellations and the existence of discrete symmetries. We show that these contributions are well below the experimental bounds in large regions of the parameter space.

First evidence of new physics in [formula omitted] mixing and its implications

We present an update of the analysis of B s mixing by the UTfit Collaboration, including the very recent tagged analyses of B s → J / Ψ ϕ by the CDF and DO collaborations. We find that the phase of the B s mixing amplitude deviates more than 2.9σ from the Standard Model prediction. This results points to extensions of the Standard Model with new sources of CP violation. We compare this result with the analysis of B d mixing and show that present data favour the presence of a new source of flavour violation. Finally, we consider the impact of this analysis for SUSY models and the possible implications for new physics effects in b → s hadronic penguin decays.

Lepton flavor violation in the little Higgs model withTparity

Little Higgs models with T-parity may provide us with a new source of lepton flavor violation, as such, in this paper we consider the anomalous magnetic moment of the muon (g-2){sub {mu}} and the lepton flavor violating decays {mu}{yields}e{gamma} and {tau}{yields}{mu}{gamma} in the little Higgs model with T-parity [A. Goyal, hep-ph/0609095.]. Our results show that the present experimental constraints of {mu}{yields}e{gamma} are a much more useful tool for constraining the new sources of flavor violation, than from the other processes considered here, which are present in T-parity models.

Low-energy variety of asymmetric SUSY flavor structure

In this Letter we study the low-energy phenomenology and cosmological implications of supersymmetric grand unified theory with asymmetric flavor structure, which is suggested by the recent observation of fermion masses and mixing angles. The predictions of the scenario are rather dependent on a Yukawa parameter fixed by group-theoretical argument. A reduced value of the parameter gives a resolution to the sign problem of supersymmetric Higgs mass μ, with which the theory becomes simultaneously consistent with the experimental data of bottom/tau mass ratio, flavor-changing rare decay of bottom quark, and the muon anomalous magnetic moment. The relic abundance of the lightest superparticle as cold dark matter of the universe is also investigated in light of the three-years WMAP result. A new source of flavor violation is found in the D-term induced scalar masses, that is a distinctive signature of the generation asymmetry.

Flavor violation and extra dimensions

We analyze new sources of flavor violation in models with extra dimensions. We focus on three major classes of five-dimensional models: models with universal extra dimension, models with split fermions, and models with warped extra dimension. We study the implications of these new sources on the associate CP violating asymmetries to the rare B-decays. We show that among these models only the split fermions scenario may accommodate the recent experimental deviation between the CP asymmetry of B d → ϕ K S and sin 2 β .

B physics and supersymmetry

In this talk, I briefly review a selection of SUSY effects in B physics. First, I consider models with Minimal Flavour Violation. Then I discuss SUSY models with new sources of flavour violation in squark mass matrices, analyzing present constraints and possible developments with forthcoming data on b → s and b → d transitions.

Bs→ μ+μ− and B→ Xsμ+μ− in MSSM

Pure leptonic and semileptonic rare B decays, B s → μ + μ − and B→ X s μ + μ − in the minimal supersymmetric Standard Model (MSSM), in particular, gluino and neutralino contributions to the decays, are discussed under Br( B→ X s γ) and other experimental constraints. The general scalar quark mass matrices as the new sources of flavor violation are considered. We present Wilson coefficients of bs transitions from γ, Z, and neutral Higgs boson penguin diagrams by using vertex mixing method to deal with scalar down-type quark flavor changing and also give their expressions in MIA to show different sources of enhancements. We find that under the experimental constraints, with large mixing of left- and right-handed sbottom, C (′) 10 can be enhanced by 10% compared with SM, in two cases, heavy gluino and fine-tuning between δ dLL 23( δ dRR 23) and δ dLR 23( δ dRL 23) terms in C (′) 7. Particularly, C 10 and C 10′ can reach a 20% enhancement in some regions of parameters under experimental constraints. When CP-odd Higgs A 0 is not too heavy (∼250 GeV), and tan β is large (∼40), neutral Higgs boson penguins with gluino and down-type squark in the loop can significantly contribute to the bs transition and the contributions can compete with those due to the chargino and up-type squark loop.

Correlation between ΔMs and B0s,d→μ+μ− in supersymmetry at large tanβ

Considering the MSSM with the CKM matrix as the only source of flavour violation and heavy supersymmetric particles at large tanβ, we analyze the correlation between the increase of the rates of the decays B0s,d→μ+μ− and the suppression of ΔMs, that are caused by the enhanced flavour changing neutral Higgs couplings to down-type quarks. We give analytic formulae for the neutral and charged Higgs couplings to quarks including large tanβ resumed corrections in the SU(2)×U(1) limit and comment briefly on the accuracy of this approximation. For 0.8⩽(ΔMs)exp/(ΔMs)SM⩽0.95 we find 6×10−7⩾BR(B0s→μ+μ−)max⩾4×10−8 and 1.4×10−8⩾BR(B0d→μ+μ−)max⩾1×10−9. For (ΔMs)exp⩾(ΔMs)SM substantial enhancements of B0s,d→μ+μ− relative to the expectations based on the Standard Model are excluded. With (ΔMs)exp>15.0/ps a conservative analysis of (ΔMs)SM gives BR(B0s→μ+μ−)≲1.2×10−6 and BR(B0d→μ+μ−)≲3×10−8. However, we point out that in the less likely scenario in which the squark mixing is so large that the neutral Higgs contributions dominate ΔMs, the rates for B0s,d→μ+μ− increase with increasing ΔMs and the bounds in question are weaker. Violation of all these correlations and bounds would indicate new sources of flavour violation.

U(1)textures and lepton flavor violation

U(1) family symmetries have led to successful predictions of the fermion mass spectrum and the mixing angles of the hadronic sector. In the context of the supersymmetric unified theories, they further imply a non-trivial mass structure for the scalar partners, giving rise to new sources of flavor violation. In the present work, lepton flavor non-conserving processes are examined in the context of the minimal supersymmetric standard model augmented by a U(1)-family symmetry. We calculate the mixing effects on the \mu-> e\gamma and \tau -> \mu\gamma rare decays. All supersymmetric scalar masses involved in the processes are determined at low energies using two loop renormalization group analysis and threshold corrections. Further, various novel effects are considered and found to have important impact on the branching ratios. Thus, a rather interesting result is that when the see-saw mechanism is applied in the (12 X 12)-sneutrino mass matrix, the mixing effects of the Dirac matrix in the effective light sneutrino sector are canceled at first order. In this class of models and for the case that soft term mixing is already present at the GUT scale, tau -> \mu \gamma decays are mostly expected to arise at rates significantly smaller than the current experimental limits. On the other hand, the $\mu \ra e \gamma$ rare decays impose important bounds on the model parameters, particularly on the supersymmetric scalar mass spectrum. In the absence of soft term mixing at high energies, the predicted branching ratios for rare decays are, as expected, well below the experimental bounds.