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Register a new userSimultaneous Explanation of the $R_K$ and $R_{D^{(*)}}$ Puzzles: a Model Analysis
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$R_K$ and $R_{D^{(*)}}$ are two $B$-decay measurements that presently exhibit discrepancies with the SM. Recently, using an effective field theory approach, it was demonstrated that a new-physics model can simultaneously explain both the $R_K$ and $R_{D^{(*)}}$ puzzles. There are two UV completions that can give rise to the effective Lagrangian: (i) $VB$: a vector boson that transforms as an $SU(2)_L$ triplet, as in the SM, (ii) $U_1$: an $SU(2)_L$-singlet vector leptoquark. In this paper, we examine these models individually. A key point is that $VB$ contributes to $B^0_s$-${bar B}^0_s$ mixing and $tau to 3mu$, while $U_1$ does not. We show that, when constraints from these processes are taken into account, the $VB$ model is just barely viable. It predicts ${cal B} (tau^-tomu^-mu^+mu^-) simeq 2.1 times 10^{-8}$. This is measurable at Belle II and LHCb, and therefore constitutes a smoking-gun signal of $VB$. For $U_1$, there are several observables that may point to this model. Perhaps the most interesting is the lepton-flavor-violating decay $Upsilon(3S) to mu tau$, which has previously been overlooked in the literature. $U_1$ predicts ${cal B}(Upsilon(3S) to mu tau)|_{rm max} = 8.0 times 10^{-7}$. Thus, if a large value of ${cal B}(Upsilon(3S) to mu tau)$ is observed -- and this should be measurable at Belle II -- the $U_1$ model would be indicated.
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There has been persistent disagreement between the Standard Model (SM) prediction and experimental measurements of $R_{D^{(*)}}=mathcal{B}(bar B rightarrow D^{(*)} tau bar u_tau)/mathcal{B}(bar B rightarrow D^{(*)} l bar u_l)$ $(l=e,mu)$. This anomaly may be addressed by introducing interactions beyond the Standard Model involving new states, such as leptoquarks. Since the processes involved are quark flavor changing, any new states would need to couple to at least two different generations of quarks, requiring a non-trivial flavor structure in the quark sector while avoiding stringent constraints from flavor-changing neutral current processes. In this work, we look at scalar leptoquarks as a possible solution for the $R_{D^{(*)}}$ anomaly under the assumption of $it{minimal~flavor~violation}$ (MFV). We investigate all possible representations for the leptoquarks under the SM quark flavor symmetry group, consistent with asymptotic freedom. We consider constraints on their parameter space from self-consistency of the MFV scenario, perturbativity, the FCNC decay $bto sbar u u$ and precision electroweak observables. We find that none of the scalar leptoquarks can explain the $R_{D^{(*)}}$ anomaly while simultaneously avoiding all constraints within this scenario. Thus scalar leptoquarks with MFV-generated quark couplings do not work as a solution to the $R_{D^{(*)}}$ anomaly.
We address the presently reported significant flavor anomalies in the $K$ and $B$ meson systems such as the CP violating Kaon decay ($epsilon/epsilon$) and lepton-flavor universality violation in $B$ meson decays ($R_{K^{(*)}},$ and also commenting ${R_{D^{(*)}}}$), by proposing flavorful and chiral vector bosons as the new physics constitution at $sim 1,mathrm{TeV}$. Interestingly, if the new (composite) vector bosons are quite heavier than $sim 1,mathrm{TeV}$, we face a difficulty in addressing the anomaly in $epsilon/epsilon$ consistently with the constraint from the $K^0$-$overline{K^0}$ mixing. Both of the anomalies can be addressed within $1sigma$ confidence levels individually, where the relevant parameter space will be investigated by the NA62 and KOTO experiments, in addition to direct searches at the large hadron collider.
Recently, several $B$-physics experiments have reported an appreciable deviation from the Standard Model (SM) in the tree-level observables $R_{D^{(*)}}$; the combined weighted average now stands at $approx 4 sigma$. We first show the anomaly necessarily implies model-independent collider signals of the form $pp to b tau u$ that should be expediously searched for at ATLAS/CMS as a complementary test of the anomaly. Next we suggest a possible interconnection of the anomaly with the radiative stability of the Standard Model Higgs boson and point to a minimal effective supersymmetric scenario with $R$-parity violation as the underlying cause. We also comment on the possibility of simultaneously explaining the recently reported $R_{K^{(*)}}$ anomaly in this setup.
Measurements of the $R_{D^*}$ parameter remain in tension with the standard model prediction, despite recent results helping to close the gap. In this work, we revisit the standard model considerations for the prediction. We pay particular attention to the theoretical prediction considering the full 4-body decay $(Brightarrow l u D^* to l u Dpi)$, which introduces the longitudinal degree of freedom of the $D^*$. We show that this does not introduce sizeable effects at the current precision. This modifies our previous finding (Phys. Rev. D 98 056014 (2018)) where a numerical bug led us to a different conclusion. Thus, the results on $R_{Dpi}$ are consistent with $R_{D^*}$, and the difference between the several values can be traced back to the form factor used and the restrictions incorporated to determine their parameters. There is still tension between the experimental world average and the most accurate theoretical estimate, leaving the possibility of presence of new physics scenarios open.
Recent theoretical developments on $R_D$ and $R_{D^*}$ -- discrepancies between experimental data and the Standard Model predictions have been reported (B anomaly) -- are reviewed. New Physics explanations for the B anomaly and other relevant observables to obtain additional bounds on New Physics are also summarized. This is the proceedings for the talk at CIPANP2018 which was held on May 29 2018.
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