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Register a new userCollider signature of $U_1$ Leptoquark and constraints from $bto c$ observables
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One of the most popular models that is known to be able to solve the lepton flavour universality violating charged ($bto c$) and neutral current ($bto s$) anomalies is the Leptoquark Model. In this work we examine the {it multijet} + $mET$ collider signature of a vector leptoquark ($U_1$) which has the potential to mediate both the charged and neutral current processes at tree level. From our collider analysis we derive the exclusion limits on mass for the $U_1$ leptoquark at 95% C.L. at the current and future experiment of the Large Hadron Collider. We also calculate the effect of such a leptoquark in $bto c$ observables.
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The Leptoquark model has been instrumental in explaining the observed lepton flavour universality violating charged ($bto c$) and neutral ($bto s$) current anomalies that have been the cause for substantial excitement in particle physics recently. In this article we have studied the role of one (designated as $V_2^{frac 43}$) of the components of {boldmath${V}_2$} Vector Leptoquark doublet with electromagnetic charge $frac 43$ in explaining the neutral current ($bto s$) anomalies $R_{K^{(*)}}$ and $B_stomu^+mu^-$. Moreover, we have performed a thorough collider search for this $V_2^{frac 43}$ Leptoquark using $bbar{b} ell^+ ell^-$ ($ellequiv e, mu$) final state at the Large Hadron Collider. From our collider analysis we maximally exclude the mass of the $V_2^{frac 43}$ Leptoquark up to 2340 GeV at 95% confidence level for the 13 TeV Large Hadron Collider for an integrated luminosity of 3000 ${rm fb}^{-1}$. Furthermore, a significant portion of the allowed parameter space that is consistent with the neutral current ($bto s$) observables is excluded by collider analysis.
In a couple of recent publications ( arXiv:1706.08437 and arXiv:1712.01593 ), the authors attempted to achieve simultaneous explanation of the persistent flavor anomalies in $bto s$ and $bto c$ semileptonic decays with a minimal scheme by using only three unknown new parameters. The analysis was obtained with a handful of precise observables. Motivated by their proposal, in this paper we reanalyze the models proposed in the aforementioned papers with a total of 170 observables from those channels including newly available measurements, correlated theoretical results, and constraints. We validate our results by searching for the most influential points and outliers. By analyzing the parameter spaces and their relationship with the constraints, we gain new insight and statistical significance in those models. We also provide a new and precise calculation of $R(J/Psi)$, obtained during the analysis.
The combined analysis of the BaBar, Belle, and LHCb data on $Bto Dtau u$, $Bto D^*tau u$ and $B_cto J/Psitau u$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $tau$ polarizations, and especially on the constraint from the branching fraction ${rm BR}(B_ctotau u)$. We critically review this constraint and do not support the aggressive limit of ${rm BR}(B_ctotau u)<10%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${rm BR}(B_ctotau u)$ constraint has a significant impact: depending on whether one uses a limit of $60%$, $30%$ or $10%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $Bto Dtau u$, $Bto D^*tau u$ and $Lambda_bto Lambda_ctau u$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${rm BR}(Lambda_bto Lambda_ctau u)$ over its SM prediction by $(24pm 6)%$ for the current $mathcal{R}(D^{(*)})$ data.
We show that the current bounds on the leptoquark couplings imply that if leptoquarks are produced in e p collisions, a significant fraction of them could form a leptoquark-quark bound state. The decay of the bound state has a distinct event shape with rapidity gap. A possible application of this observation in the leptoquark search at HERA is discussed.
In this addendum to arXiv:1811.09603 we update our results including the recent measurement of ${cal R}(D)$ and ${cal R}(D^*)$ by the Belle collaboration: ${cal R}(D)_{rm Belle} = 0.307pm0.037pm0.016$ and ${cal R}(D^*)_{rm Belle}=0.283pm0.018pm0.014$, resulting in the new HFLAV fit result ${cal R}(D) = {0.340pm0.027 pm 0.013}$, ${cal R}(D^*) = {0.295pm0.011 pm 0.008 }$, exhibiting a $3.1,sigma$ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for ${cal R}(Lambda_c)$ from our sum rule reads ${cal R}(Lambda_c)= mathcal{R}_{rm SM}(Lambda_c) left( 1.15 pm 0.04 right) = 0.38 pm 0.01 pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $bto B_c$ and their implication on the constraint from $B_{u/c}totau u$ data.
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