Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userB-anomalies related to leptons and lepton flavour universality violation
67
0
0.0
(
0
)
Authors
Andreas Crivellin
Ask ChatGPT about the research
No Arabic abstract
Several experiments observed deviations from the Standard Model (SM) in the flavour sector: LHCb found a $4-5,sigma$ discrepancy compared to the SM in $bto smu^+mu^-$ transitions (recently supported by an Belle analysis) and CMS reported a non-zero measurement of $htomutau$ with a significance of $2.4,sigma$. Furthermore, BELLE, BABAR and LHCb founds hints for the violation of flavour universality in $Bto D^{(*)}tau u$. In addition, there is the long-standing discrepancy in the anomalous magnetic moment of the muon. Interestingly, all these anomalies are related to muons and taus, while the corresponding electron channels seem to be SM like. This suggests that these deviations from the SM might be correlated and we briefly review some selected models providing simultaneous explanations.
rate research
Read More
Rare semileptonic $b to s ell^+ ell^-$ transitions provide some of the most promising frameworks to search for new physics effects. Recent analyses of these decays have indicated an anomalous behaviour in measurements of angular distributions of the decay $B^0to K^*mu^+mu^-$ and lepton-flavour-universality observables. Unambiguously establishing if these deviations have a common nature is of paramount importance in order to understand the observed pattern. We propose a novel approach to independently and complementary probe this hypothesis by performing a simultaneous amplitude analysis of $bar{B}^0 to bar{K}^{*0} mu^+mu^-$ and $bar{B}^0 to bar{K}^{*0} e^+e^-$ decays. This method enables the direct determination of observables that encode potential non-equal couplings of muons and electrons, and are found to be insensitive to nonperturbative QCD effects. If current hints of new physics are confirmed, our approach could allow an early discovery of physics beyond the standard model with LHCb run II data sets.
We consider the recent LHCb result for $B_cto J/psi tau u$ in conjunction with the existing anomalies in $R(D)$ and $R(D^star)$ within the framework of a right-handed current with enhanced couplings to the third generation. The model predicts a linear relation between the observables and their SM values in terms of two combinations of parameters. The strong constraints from $bto s gamma$ on $W-W^prime$ mixing effectively remove one of the combinations of parameters resulting in an approximate proportionality between all three observables and their SM values. To accommodate the current averages for $R(D)$ and $R(D^star)$, the $W^prime$ mass should be near 1 TeV, and possibly accessible to direct searches at the LHC. In this scenario we find that $R(J/psi)$ is enhanced by about 20% with respect to its SM value and about 1.5$sigma$ below the central value of the LHCb measurement. The predicted $dGamma/dq^2$ distribution for $Bto D(D^star) tau u$ is in agreement with the measurement and the model satisfies the constraint from the $B_c$ lifetime.
We propose renormalizable models of new physics that can explain various anomalies observed in decays of B-mesons to electron and muon pairs. The new physics states couple to linear combinations of Standard Model fermions, yielding a pattern of flavour violation that gives a consistent fit to the gamut of flavour data. Accidental symmetries prevent contributions to baryon- and lepton-number-violating processes, as well as enforcing a loop suppression of new physics contributions to flavour violating processes. Data require that the new flavour-breaking couplings are largely aligned with the Yukawa couplings of the SM and so we also explore patterns of flavour symmetry breaking giving rise to this structure.
In recent years, evidence for lepton flavour universality violation beyond the Standard Model has been accumulated. In this context, a singly charged $SU(2)_L$ singlet scalar ($phi^pm$) is very interesting, as it can only have flavour off-diagonal couplings to neutrinos and charged leptons, therefore necessarily violating lepton flavour (universality). In fact, it gives a (necessarily constructive) tree-level effect in $elltoell^prime u u$ processes, while contributing to charged lepton flavour violating only at the loop-level. Therefore, it can provide a common explanation of the hints for new physics in $tautomu u u/tau(mu)to e u u$ and of the Cabibbo Angle Anomaly. Such an explanation predicts ${rm Br }[tauto egamma]$ to be of the order of a few times $10^{-11}$ while ${ rm Br}[tauto emumu]$ can be of the order of $10^{-9}$ for order one couplings and therefore in the reach of forthcoming experiments. Furthermore, we derive a {novel} coupling-independent lower limit on the scalar mass of $approx 200,$GeV by recasting LHC slepton searches. In the scenario preferred by low energy precision data, the lower limit is even strengthened to $approx300,$GeV, showing the complementary between LHC searches and flavour observables. Furthermore, we point out that this model can be tested by reinterpreting DM mono-photon searches at future $e^+e^-$ colliders.
In addition to the existing strong indications for lepton flavour university violation (LFUV) in low energy precision experiments, CMS recently released an analysis of non-resonant di-lepton pairs which could constitute the first sign of LFUV in high-energy LHC searches. In this article we show that the Cabibbo angle anomaly, an (apparent) violation of first row and column CKM unitarity with $approx3,sigma$ significance, and the CMS result can be correlated and commonly explained in a model independent way by the operator $[Q_{ell q}^{(3)}]_{1111} = (bar{ell}_1gamma^{mu}sigma^Iell_1)(bar{q}_1gamma_{mu}sigma^Iq_1)$. This is possible without violating the bounds from the non-resonant di-lepton search of ATLAS (which interestingly also observed slightly more events than expected in the electron channel) nor from $R(pi)=pi tomu u/pi to e u$. We find a combined preference for the new physics hypothesis of $4.5,sigma$ and predict $1.0004<R(pi)<1.0009$ (95%~CL) which can be tested in the near future with the forthcoming results of the PEN experiment.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
