Tau Yukawa Coupling Research Articles

Constraining the {mathcal {C}}{mathcal {P}} structure of Higgs-fermion couplings with a global LHC fit, the electron EDM and baryogenesis

{mathcal {C}}{mathcal {P}} violation in the Higgs couplings to fermions is an intriguing, but not yet extensively explored possibility. We use inclusive and differential LHC Higgs boson measurements to fit the {mathcal {C}}{mathcal {P}} structure of the Higgs Yukawa couplings. Starting with simple effective models featuring {mathcal {C}}{mathcal {P}} violation in a single Higgs–fermion coupling, we probe well-motivated models with up to nine free parameters. We also investigate the complementarity of LHC constraints with the electron electric dipole moment bound, taking into account the possibility of a modified electron Yukawa coupling, and assess to which extent {mathcal {C}}{mathcal {P}} violation in the Higgs–fermion couplings can contribute to the observed baryon asymmetry of the universe. Even after including the recent analysis of angular correlations in Hrightarrow tau ^+tau ^- decays, we find that a complex tau Yukawa coupling alone may be able to account for the observed baryon asymmetry, but with large uncertainties in the baryogenesis calculation. A combination of complex top and bottom quark Yukawa couplings yields a result four times larger than the sum of their separate contributions, but remains insufficient to account for the observed baryon asymmetry.

Charged lepton flavor violation in light of muon g-2

The recent confirmation of the muon g-2 anomaly by the Fermilab g-2 experiment may harbinger a new era in mu and tau physics. In the context of general two Higgs doublet model, the discrepancy can be explained via one-loop exchange of sub-TeV exotic scalar and pseudoscalars, namely H and A, that have flavor changing neutral couplings rho _{tau mu } and rho _{mu tau } at sim 20 times the usual tau Yukawa coupling, lambda _tau . Taking rho _{ell ell ^prime }sim lambda _{ mathrm min(ell , ell ^prime )}, we show that the above solution to muon g-2 then predicts enhanced rates of various charged lepton flavor violating processes, which should be accessible at upcoming experiments. We cover muon related processes such as mu rightarrow e gamma , mu rightarrow eee and mu N rightarrow e N, and tau decays tau rightarrow mu gamma and tau rightarrow mu mu mu . A similar one-loop diagram with rho _{etau }= rho _{tau e} = mathcal{O}(lambda _e) induces mu rightarrow egamma , bringing the rate right into the sensitivity of the MEG II experiment. The mu egamma dipole can be probed further by mu rightarrow 3e and mu N rightarrow eN. With its promised sensitivity range and ability to use different nuclei, the mu N rightarrow eN conversion experiments can not only make discovery, but access the extra diagonal quark Yukawa couplings rho _{qq}. For the tau lepton, we find that tau rightarrow mu gamma would probe rho _{tau tau } down to lambda _tau or lower, while tau rightarrow 3mu would probe rho _{mu mu } to mathcal{O}(lambda _{mu }).

Probing unification with precision Higgs physics

We propose a novel approach of probing grand unification through precise measurements on the Higgs Yukawa couplings at the LHC. This idea is well motivated by the appearance of effective operators not suppressed by the mass scale of unification ${M}_{\mathrm{U}}$ in realistic models of unification with the minimal structure of Yukawa sector. Such operators modify the Higgs Yukawa couplings in correlated patterns at scale ${M}_{\mathrm{U}}$ that hold up to higher-order corrections. The coherences reveal a feature that, the deviation of tau Yukawa coupling relative to its standard model value at the weak scale is the largest one among the third-generation Yukawa couplings. This feature, if verified by the future LHC, can serve as a hint of unification.

The role of leptons in electroweak baryogenesis

We investigate the role of leptons in electroweak baryogenesis by studying a relatively simple framework inspired by effective field theory that satisfies all Sakharov conditions. In particular, we study the effectiveness of CP-violating source terms induced by dimension-six Yukawa interactions for quarks and charged leptons. Despite the relatively small Yukawa coupling, CP-violating source terms involving taus are quite effective and can account for the observed matter-antimatter asymmetry. We obtain analytical and numerical expressions for the total baryon asymmetry, the former providing important insight into what makes lepton CP violation relatively effective compared to quark CP violation. Leptons also play an important role if the CP-violating source involves top quarks. While the tau Yukawa coupling in the Standard Model is small, it significantly enhances the baryon asymmetry by transferring the chiral asymmetry in quarks, which is washed out by strong sphalerons, to a chiral asymmetry in leptons. We conclude that leptons should not be ignored even if CP violation is limited to the quark sector. The role of leptons can be further increased in scenarios of new physics with additional chiral-symmetry-breaking interactions between quarks and leptons, as can happen in models with additional Higgs bosons or leptoquarks. Finally, we study CP-violating dimension-six Yukawa interactions for lighter quarks and leptons but conclude that these lead to too small baryon asymmetries.

Top-bottom-tau Yukawa coupling unification in the MSSM plus one vectorlike family and fermion masses as IR fixed points

In the MSSM extended by a complete vectorlike family, precise top, bottom and tau Yukawa coupling unification can be achieved assuming SUSY threshold corrections which are typical for comparable superpartner masses. Furthermore, the unification is possible with a large unified coupling, implying that all three fermion masses can be simultaneously close to their IR fixed points. Assuming unified Yukawa couplings of order one or larger, the preferred common scale of new physics (superpartners and vectorlike matter) is in the 3 TeV to 30 TeV range, with larger couplings favoring smaller scales. Splitting superpartner masses from masses of vectorlike fields, the preferred scales extend in both directions. The multi-TeV scale for superpartners is compatible with and independently suggested by the Higgs boson mass.

Looking for the left sneutrino LSP with displaced-vertex searches

We analyze a displaced dilepton signal expected at the LHC for a tau left sneutrino as the lightest supersymmetric particle with a mass in the range $45$-$100$ GeV. The sneutrinos are pair produced via a virtual $W$, $Z$ or $\gamma$ in the $s$ channel and, given the large value of the tau Yukawa coupling, their decays into two dileptons or a dilepton plus missing transverse energy from neutrinos can be significant. The discussion is carried out in the $\mu \nu$SSM, where the presence of $R$-parity violating couplings involving right-handed neutrinos solves the $\mu$ problem and can reproduce the neutrino data. To probe the tau left sneutrinos we compare the predictions of the $\mu \nu$SSM with the ATLAS search for long-lived particles using displaced lepton pairs in $pp$ collisions at $\sqrt s= 8$ TeV, allowing us to constrain the parameter space of the model. We also consider an optimization of the trigger requirements used in existing displaced-vertex searches by means of a High Level Trigger that exploits tracker information. This optimization is generically useful for a light metastable particle decaying into soft charged leptons. The constraints on the sneutrino turn out to be more stringent. We finally discuss the prospects for the $13$ TeV LHC searches as well as further potential optimizations.

Searching for left sneutrino LSP at the LHC

We analyze relevant signals expected at the LHC for a left sneutrino as the lightest supersymmetric particle (LSP). The discussion is carried out in the “[Formula: see text] from [Formula: see text]” supersymmetric standard model [Formula: see text], where the presence of [Formula: see text]-parity breaking couplings involving right-handed neutrinos solves the [Formula: see text] problem and reproduces neutrino data. The sneutrinos are pair produced via a virtual [Formula: see text], [Formula: see text] or [Formula: see text] in the [Formula: see text] channel. From the prompt decay of a pair of left sneutrinos LSPs of any family, a significant diphoton signal plus missing transverse energy (MET) from neutrinos can be present in the mass range 118–132 GeV, with 13 TeV center-of-mass energy and an integrated luminosity of 100 fb[Formula: see text]. In addition, in the case of a pair of tau left sneutrinos LSPs, given the large value of the tau Yukawa coupling diphoton plus leptons and/or multileptons can appear. We find that the number of expected events for the multilepton signal, together with properly adopted search strategies, is sufficient to give a significant evidence for a sneutrino of mass in the range 130–310 GeV, even with the integrated luminosity of 20 fb[Formula: see text]. In the case of the signal producing diphoton plus leptons, an integrated luminosity of 100 fb[Formula: see text] is needed to give a significant evidence in the mass range 95–145 GeV. Finally, we discuss briefly the presence of displaced vertices and the associated range of masses.

Effective field theory approach to trans-TeV supersymmetry: covariant matching, Yukawa unification and Higgs couplings

Dismissing traditional naturalness concerns while embracing the Higgs boson mass measurement and unification motivates careful analysis of trans-TeV supersymmetric theories. We take an effective field theory (EFT) approach, matching the Minimal Supersymmetric Standard Model (MSSM) onto the Standard Model (SM) EFT by integrating out heavy superpartners, and evolving MSSM and SMEFT parameters according to renormalization group equations in each regime. Our matching calculation is facilitated by the recent covariant diagrams formulation of functional matching techniques, with the full one-loop SUSY threshold corrections encoded in just 30 diagrams. Requiring consistent matching onto the SMEFT with its parameters (those in the Higgs potential in particular) measured at low energies, and in addition requiring unification of bottom and tau Yukawa couplings at the scale of gauge coupling unification, we detail the solution space of superpartner masses from the TeV scale to well above. We also provide detailed views of parameter space where Higgs coupling measurements have probing capability at future colliders beyond the reach of direct superpartner searches at the LHC.

Tau flavored dark matter and its impact on tau Yukawa coupling

In this paper we perform a systematic study of the tau flavored dark matter (DM) model by introducing two kinds of mediators (a scalar doublet and a charged scalar singlet). The electromagnetic properties of the DM, as well as their implications in DM direct detections, are analyzed in detail. The model turns out contributing a significant radiative correction to the tau lepton mass, in addition to loosing the tension between the measured DM relic density and constraints of DM direct detections. The loop corrections can be \U0001d4aa(10%) of the total tau mass. Signal rates of the Higgs measurements from the LHC in the h→τ τ and h→ γ γ channels, relative to the Standard Model expectations, can be explained in this model.

Non-custodial warped extra dimensions at the LHC?

With the prospect of improved Higgs measurements at the LHC and at proposed future colliders such as ILC, CLIC and TLEP we study the non-custodial Randall-Sundrum model with bulk SM fields and compare brane and bulk Higgs scenarios. The latter bear resemblance to the well studied type III two-Higgs-doublet models. We compute the electroweak precision observables and argue that incalculable contributions to these, in the form of higher dimensional operators, could have an impact on the T-parameter. This could potentially reduce the bound on the lowest Kaluza-Klein gauge boson masses to the 5 TeV range, making them detectable at the LHC. In a second part, we compute the misalignment between fermion masses and Yukawa couplings caused by vector-like Kaluza-Klein fermions in this setup. The misalignment of the top Yukawa can easily reach 10%, making it observable at the high-luminosity LHC. Corrections to the bottom and tau Yukawa couplings can be at the percent level and detectable at ILC, CLIC or TLEP.

Towards a general analysis of LHC data within two-Higgs-doublet models

The data accumulated so far confirm the Higgs-like nature of the new boson discovered at the LHC. The Standard Model Higgs hypothesis is compatible with the collider results and no significant deviations from the Standard Model have been observed neither in the flavour sector nor in electroweak precision observables. We update the LHC and Tevatron constraints on CP-conserving two-Higgs-doublet models without tree-level flavour-changing neutral currents. While the relative sign between the top Yukawa and the gauge coupling of the 126 GeV Higgs is found be the same as in the SM, at 90% CL, there is a sign degeneracy in the determination of its bottom and tau Yukawa couplings. This results in several disjoint allowed regions in the parameter space. We show how generic sum rules governing the scalar couplings determine the properties of the additional Higgs bosons in the different allowed regions. The role of electroweak precision observables, low-energy flavour constraints and LHC searches for additional scalars to further restrict the available parameter space is also discussed.

Vacuum stability and Higgs diphoton decays in the MSSM

Abstract Current Higgs data at the Large Hadron Collider is compatible with a SM signal at the 2σ level, but the central value of the signal strength in the diphoton channel is enhanced with respect to the SM expectation. If the enhancement resides in the diphoton partial decay width, the data could be accommodated in the Minimally Super-symmetric Standard Model (MSSM) with highly mixed light staus. We revisit the issue of vacuum instability induced by large mixing in the stau sector, including effects of a radiatively-corrected tau Yukawa coupling. Further, we emphasize the importance of taking into account the tan β dependence in the stability bound. While the metastability of the Universe constrains the possible enhancement in the Higgs to diphoton decay width in the light stau scenario, an increase of the order of 50% can be achieved in the region of large tan β. Larger enhancements may be obtained, but would require values of tan β associated with non-perturbative values of the tau Yukawa coupling at scales below the GUT scale, thereby implying the presence of new physics beyond the MSSM.

Non-linear single Higgs MSSM

We present a non-linear MSSM with non-standard Higgs sector and goldstino field. Non-linear supersymmetry for the goldstino couplings is described by the constrained chiral superfield and, as usual, the Standard Model sector is encompassed in suitable chiral and vector supermultiplets. Two models are presented. In the first model (non-linear MSSM312), the second Higgs is replaced by a new supermultiplet of half-family with only a new generation of leptons (or quarks). In the second model, for anomaly cancellation purposes, the second Higgs is retained as a spectator superfield by imposing a discrete symmetry. Both models do not have a μ-problem as a μ-term is forbidden by the discrete symmetry in the case of a spectator second Higgs or not existing at all in the case of a single Higgs. Moreover, the tree level relation between the Higgs mass and the hidden sector SUSY breaking scale f is derived. Finally, we point out a relative suppression by msoft/Λ of the bottom and tau Yukawa couplings with respect to those of the top quark.

YUKAWA UNIFICATION IN SUSY SO(10) IN LIGHT OF THE LHC HIGGS DATA

The status of top–bottom–tau Yukawa coupling unification in supersymmetric SO(10) models is reviewed with a particular emphasis on the implications of the Higgs boson mass in the vicinity of 125 GeV, as suggested by the Large Hadron Collider (LHC) Higgs data. In addition, the recently proposed model with negative μ, D-term splitting of the soft scalar masses and nonuniversal gaugino masses generated by a nonzero F-term in a 24-dimensional representation of SU(5) ⊂ SO(10) is reanalyzed in the context of the 125 GeV Higgs. The condition of top–bottom–tau Yukawa unification together with the Higgs mass of about 125 GeV impose strong lower mass limits on supersymmetry (SUSY) particles. Nevertheless, some of the Minimal Supersymmetric Standard Model (MSSM) particles may be within the reach of LHC. In the case of models with positive μ this is the gluino. While in the case of negative μ these are the pseudoscalar Higgs, the lighter sbottom (sometimes strongly degenerate with LSP leading to sbottom coannihilations), the right-handed down squark and the gluino.

Exploring neutralino dark matter resonance annihilation viabA,bH→bμ+μ−at the LHC

One of the main channels which allows for a large rate of neutralino dark matter annihilation in the early Universe is via the pseudoscalar Higgs A-resonance. In this case, the measured dark matter abundance can be obtained in the minimal supergravity (mSUGRA) model when tan(beta)\sim 50 and 2m_{\tz_1}\sim m_A. We investigate the reaction pp\to b\phi\to b\mu^+\mu^- +X (where \phi =A or H) at the CERN LHC where requiring the tag of a single b-jet allows for amplification of the signal-to-background ratio. The rare but observable Higgs decay to muon pairs allows for a precise measurement of the Higgs boson mass and decay width. We evaluate signal and background using CalcHEP, with muon energy smearing according to the CMS detector. We find that the Higgs width (\Gamma_A) can typically be determined with the accuracy up to \sim 8% (\sim 17%) for m_A\sim 400 (600) GeV assuming 10^3 fb^{-1} of integrated luminosity. Therefore, the pp\to b\phi\to b\mu^+\mu^- +X process provides a unique possibility for direct \Gamma_A measurement at the LHC. While the Higgs width is correlated with the parameter \tan\beta for a given value of m_A, extracting \tan\beta is complicated by an overlap of the A and H peaks, radiative corrections to the b and \tau Yukawa couplings, and the possibility that SUSY decay modes of the Higgs may be open. In the case where a dilepton mass edge from \tz_2\to\ell^+\ell^-\tz_1 is visible, it should be possible to test the relation that 2m_{\tz_1}\sim m_A.

Landscape predictions for the Higgs boson and top quark masses

If the Standard Model is valid up to scales near the Planck mass, and if the cosmological constant and Higgs mass parameters scan on a landscape of vacua, it is well known that the observed orders of magnitude of these quantities can be understood from environmental selection for large-scale structure and atoms. If in addition the Higgs quartic coupling scans, with a probability distribution peaked at low values, environmental selection for a phase having a scale of electroweak symmetry breaking much less than the Planck scale leads to a most probable Higgs mass of 106 GeV. While fluctuations below this are negligible, the upward fluctuation is 25/p GeV, where p measures the strength of the peaking of the a priori distribution of the quartic coupling. If the top Yukawa coupling also scans, the most probable top quark mass is predicted to lie in the range (174--178) GeV, providing the standard model is valid to at least 10^{17} GeV. The downward fluctuation is 35 GeV/ \sqrt{p}, suggesting that p is sufficiently large to give a very precise Higgs mass prediction. While a high reheat temperature after inflation could raise the most probable value of the Higgs mass to 118 GeV, maintaining the successful top prediction suggests that reheating is limited to about 10^8 GeV, and that the most probable value of the Higgs mass remains at 106 GeV. If all Yukawa couplings scan, then the e,u,d and t masses are understood to be outliers having extreme values induced by the pressures of strong environmental selection, while the s, \mu, c, b, \tau Yukawa couplings span only two orders of magnitude, reflecting an a priori distribution peaked around 10^{-3}. Extensions of these ideas allow order of magnitude predictions for neutrino masses, the baryon asymmetry and important parameters of cosmological inflation.

CPviolation in a minimal renormalizable supersymmetricSO(10)model and beyond

We investigate the role of CP phases within the renormalizable SUSY SO(10) GUT with one 10_H, one 126bar_H one 126_H and one 210_H Higgs representations and type II seesaw dominating the neutrino mass matrix. This framework is non trivially predictive in the fermionic sector and connects in a natural way the GUT unification of b and tau Yukawa couplings with the bi-large mixing scenario for neutrinos. On the other hand, existing numerical analysis claim that consistency with quark and charged lepton data prevents the minimal setup from reproducing the observed CP violation via the Cabibbo-Kobayashi-Maskawa (CKM) matrix. We re-examine the issue and find by inspection of the fermion mass sum rules and a detailed numerical scan that, even though the CKM phase takes preferentially values in the second quadrant, the agreement of the minimal model with the data is actually obtained in a non negligible fraction of the parameter space. We then consider a recently proposed renormalizable extension of the minimal model, obtained by adding one chiral 120-dimensional Higgs supermultiplet. We show that within such a setup the CKM phase falls naturally in the observed range. We emphazise the robust predictivity of both models here considered for neutrino parameters that are in the reach of ongoing and future experiments.

Precise determination of the neutral Higgs boson masses in the MSSM

We present the implementation of the radiative corrections of the Higgs sector in three public computer codes for the evaluation of the particle spectrum in the Minimal Supersymmetric Standard Model, Softsusy, Spheno and SuSpect. We incorporate the full one-loop corrections to the Higgs boson masses and the electroweak symmetry breaking conditions, as well as the two-loop corrections controlled by the strong gauge coupling and the Yukawa couplings of the third generation fermions. We include also the corrections controlled by the tau Yukawa coupling that we derived for completeness. The computation is consistently performed in the DRbar renormalisation scheme. In a selected number of MSSM scenarios, we study the effect of these corrections and analyse the impact of some higher order effects. By considering the renormalisation scheme and scale dependence, and the effect of the approximation of zero external momentum in the two-loop corrections, we estimate the theoretical uncertainty on the lighter Higgs boson mass to be 3 to 5 GeV. The uncertainty on Mh due to the experimental error in the measurement of the SM input parameters is approximately of the same size. Finally, we discuss the phenomenological consequences, using the latest value of the top quark mass. We find, in particular, that the most conservative upper bound on the lighter Higgs boson mass in the general MSSM is Mh < 152 GeV and that there is no lower bound on the parameter tan(beta) from non-observation of the MSSM Higgs bosons at LEP2.

Fermion masses and coupling unification inE6:Life in the desert

We present an ${E}_{6}$ grand unified model with a realistic pattern of fermion masses. All standard model fermions are unified in three fundamental 27-plets (i.e., supersymmetry is not invoked), which involve in addition right-handed neutrinos and three families of vectorlike heavy quarks and leptons. The lightest of those can lie in the low-TeV range, being accessible to future collider experiments. As a result of the high symmetry, the masses and mixings of all fermions are closely related. The new heavy fermions play a crucial role for the quark and lepton mass matrices and the bilarge neutrino oscillations. In all channels generation mixing and $\mathcal{C}P$ violation arise from a single antisymmetric matrix. The ${E}_{6}$ breaking proceeds via an intermediate-energy region with ${\mathrm{SU}(3)}_{L}\ifmmode\times\else\texttimes\fi{}{\mathrm{SU}(3)}_{R}\ifmmode\times\else\texttimes\fi{}{\mathrm{SU}(3)}_{C}$ gauge symmetry and a discrete left-right symmetry. This breaking pattern leads in a straightforward way to the unification of the three gauge coupling constants at high scales, providing for a long proton lifetime. The model also provides for the unification of the top, bottom, and tau Yukawa couplings and for new interesting relations in flavor and generation space.

Neutralino relic density in supersymmetric GUTs with no-scale boundary conditions above the unification scale

We investigate SU(5) and SO(10) GUTs with vanishing scalar masses and trilinear scalar couplings at a scale higher than the unification scale. The parameter space of the models, further constrained by b-\tau Yukawa coupling unification, consists of a common gaugino mass and of \tan\beta. We analyze the low energy phenomenology, finding that A-pole annihilations of neutralinos and/or coannihilations with the lightest stau drive the relic density within the cosmologically preferred range in a significant region of the allowed parameter space. Implications for neutralino direct detection and for CERN LHC experiments are also discussed.