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Register a new userNew Physics in B-Bbar mixing in the light of recent LHCb data
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A. Lenz
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We perform model-independent statistical analyses of three scenarios accommodating New Physics (NP) in Delta F=2 flavour-changing neutral current amplitudes. In a scenario in which NP in B_d-B_d-bar and B_s-B_s-bar is uncorrelated, we find the parameter point representing the Standard-Model disfavoured by 2.4 standard deviations. However, recent LHCb data on B_s neutral-meson mixing forbid a good accommodation of the D0 data on the semileptonic CP asymmetry A_SL. We introduce a fourth scenario with NP in both M_12^d,s and Gamma_12^d,s, which can accommodate all data. We discuss the viability of this possibility and emphasise the importance of separate measurements of the CP asymmetries in semileptonic B_d and B_s decays. All results have been obtained with the CKMfitter analysis package, featuring the frequentist statistical approach and using Rfit to handle theoretical uncertainties.
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We analyse three different New Physics scenarios for Delta F=2 flavour-changing neutral currents in the quark sector in the light of recent data on neutral-meson mixing. We parametrise generic New Physics contributions to B_q-Bbar_q mixing (q=d,s), in terms of one complex quantity Delta_q, while three parameters Delta_K^tt, Delta_K^ct and Delta_K^cc are needed to describe K-Kbar mixing. In Scenario I, we consider uncorrelated New Physics contributions in the B_d, B_s, and K sectors. In this scenario, it is only possible to constrain the parameters Delta_d and Delta_s whereas there are no non-trivial constraints on the kaon parameters. In Scenario II, we study the case of Minimal Flavour Violation (MFV) and small bottom Yukawa coupling and Scenario III is the generic MFV case with large bottom Yukawa couplings. Our quantitative analyses consist of global CKM fits within the Rfit frequentist statistical approach, determining the Standard Model parameters and the new physics parameters of the studied scenarios simultaneously. We find that the recent measurements indicating discrepancies with the Standard Model are well accomodated in Scenarios I and III with new mixing phases, with a slight preference for Scenario I that permits different new CP phases in the B_d and B_s systems. Within our statistical framework, we find evidence of New Physics in both B_d and B_s systems. The Standard-Model hypothesis Delta_d=Delta_s=1 is disfavoured with p-values of 3.6 sigma and 3.3 sigma in Scenarios I and III, respectively. We also present an exhaustive list of numerical predictions in each scenario. In particular, we predict the CP phase in B_s -> J psi phi and the difference between the B_s and B_d semileptonic asymmetries, which will be both measured by the LHCb experiment.
In the Standard Model (SM), the rare transitions where a bottom quark decays into a strange quark and a pair of light leptons exhibit a potential sensitivity to physics beyond the SM. In addition, the SM embeds Lepton Flavour Universality (LFU), leading to almost identical probabilities for muon and electron modes. The LHCb collaboration discovered a set of deviations from the SM expectations in decays to muons and also in ratios assessing LFU. Other experiments (Belle, ATLAS, CMS) found consistent measurements, albeit with large error bars. We perform a global fit to all available $bto sell^+ell^-$ data ($ell=e,mu$) in a model-independent way allowing for different patterns of New Physics. For the first time, the NP hypothesis is preferred over the SM by $5,sigma$ in a general case when NP can enter SM-like operators and their chirally-flipped partners. LFU violation is favoured with respect to LFU at the 3-4$,sigma$ level. We discuss the impact of LFU-violating New Physics on the observable $P_5^prime$ from $B to K^*mu^+mu^-$ and we compare our estimate for long-distance charm contributions with an empirical model recently proposed by a group of LHCb experimentalists. Finally, we discuss NP models able to describe this consistent pattern of deviations.
This article is a short and non-exhaustive summary of the prospects to find New Physics with LHCb as was presented at the HCP conference at Toronto on August 26th 2010.
While the LHC did not observe direct evidence for physics beyond the standard model, indirect hints for new physics were uncovered in the flavour sector in the decays $Bto K^*mu^+mu^-$, $Bto Kmu^+mu^-/Bto Ke^+e^-$, $B_stophimu^+mu^-$, $Bto D^{(*)}tau u$ and $htotau^pmmu^mp$. Each observable deviates from the SM predictions at the $2-3,sigma$ level only, but combining all $bto smu^+mu^-$ data via a global fit, one finds $4-5,sigma$ difference for NP compared to the SM and combining $Bto D^{*}tau u$ with $Bto Dtau u$ one obtains $3.9,sigma$. While $Bto D^{(*)}tau u$ and $htotaumu$ can be naturally explained by an extended Higgs sector, the $bto smu^+mu^-$ anomalies point at a $Z$ gauge boson. However, it is also possible to explain $Bto D^{(*)}tau u$ and $bto smu^+mu^-$ simultaneously with leptoquarks while their effect in $htotau^pmmu^mp$ is far too small to account for current data. Combining a 2HDM with a gauged $L_mu-L_tau$ symmetry allows for explaining the $bto smu^+mu^-$ anomalies in combination with $htotau^pmmu^mp$, predicting interesting correlations with $tauto3mu$. In the light of these deviations from the SM we also discuss the possibilities of observing lepton flavour violating $B$ decays (e.g. $Bto K^{(*)}tau^pmmu^mp$ and $B_stotau^pmmu^mp$).
It is interesting to search for new physics beyond the standard model at LHCb. We suggest that weak decays of doubly charmed baryon such as $Xi_{cc}(3520)^+, Xi_{cc}^{++}$ to charmless final states would be a possible signal for new physics. In this work, we consider two models, i.e. the unparticle and $Z$ as examples to study such possibilities. We also discuss the cases for $Xi^0_{bb}, Xi_{bb}^-$ which have not been observed yet, but one can expect to find them when LHCb begins running. Our numerical results show that these two models cannot result in sufficiently large decay widths, therefore if such modes are observed at LHCb, there must be a new physics other than the unparticle or $Z$ models.
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