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Register a new userTree-level FCNC in the B system: from CP asymmetries to rare decays
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G. Barenboim
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Tree-level Flavor-Changing Neutral Currents (FCNC) are characteristic of models with extra vector-like quarks. These new couplings can strongly modify the B^0 CP asymmetries without conflicting with low--energy constraints. In the light of a low CP asymmetry in B --> J/psi K_{S}, we discuss the implications of these contributions. We find that even these low values can be easily accommodated in these models. Furthermore, we show that the new data from B factories tend to favor an O(20) enhancement of the b --> d l bar{l} transition over the SM expectation.
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We show that the study of CP asymmetries in neutral B-meson decays provides a very sensitive probe of flavour-changing neutral currents (FCNC). We introduce two new angles, $alpha_{SM}$ and $beta_{SM}$, whose main feature is that they can be readily obtained from the measurement of the CP asymmetries $a_{J/psi K_s}$, $a_{pi^+ pi^-}$ and the ratio $R_u equiv|V_{ud}V_{ub}^*|/|V_{cd}V_{cb}^*|$, providing a quantitative test of the presence of new physics in a model-independent way. Assuming that new physics is due to the presence of an isosinglet down-type quark, we indicate how to reconstruct the unitarity quadrangles and point out that the measurements of the above asymmetries, within the expected experimental errors, may detect FCNC effects, even for values of $|sum_{i=1}^3 V_{id} V_{ib}^* / (V_{td} V_{tb}^*)| $ at the level of a few times $10^{-2}$.
U-spin symmetry predicts equal CP rate asymmetries with opposite signs in pairs of $Delta S=0$ and $Delta S=1$ $B$ meson decays in which initial and final states are related by U-spin reflection. Of particular interest are six decay modes to final states with pairs of charged pions or kaons, including $B_s to pi^+K^-$ and $B_sto K^+K^-$ for which asymmetries have been reported recently by the LHCb collaboration. After reviewing the current status of these predictions, highlighted by the precision of a relation between asymmetries in $B_s to pi^+K^-$ and $B^0to K^+pi^-$, we perform a perturbative study of U-spin breaking corrections, searching for relations for combined asymmetries which hold to first order. No such relation is found in these six decays, in two-body decays involving a neutral kaon, and in three-body $B^+$ decays to charged pions and kaons.
CP asymmetries have been measured recently by the LHCb collaboration in three-body $B^+$ decays to final states involving charged pions and kaons. Large asymmetries with opposite signs at a level of about 60% have been observed in $B^pmto pi^pm({rm or} K^pm)pi^+pi^-$ and $B^pm to pi^pm K^+K^-$ for restricted regions in the Dalitz plots involving $pi^+pi^-$ and $K^+K^-$ with low invariant mass. U-spin is shown to predict corresponding $Delta S=0$ and $Delta S=1$ asymmetries with opposite signs and inversely proportional to their branching ratios, in analogy with a successful relation predicted thirteen years ago between asymmetries in $B_sto K^-pi^+$ and $B^0 to K^+ pi^-$. We compare these predictions with the measured integrated asymmetries. Effects of specific resonant or non-resonant partial waves on enhanced asymmetries for low-pair-mass regions of the Dalitz plot are studied in $B^pm to pi^pm pi^+pi^-$. The closure of low-mass $pi^+pi^-$ and $K^+K^-$ channels involving only $pipi leftrightarrow Kbar K$ rescattering may explain by CPT approximately equal magnitudes and opposite signs measured in $B^pmto pi^pmpi^+pi^-$ and $B^pm to pi^pm K^+K^-$.
I describe the theoretical progress in the study of semileptonic tree-level B decays, and its interplay with recent experimental results. In particular, I focus on two anomalies: the ratios $R(D^{(*)})=displaystylefrac{{cal B}(B to D^{(*)} tau bar u_tau)}{{cal B}(B to D^{(*)} ell bar u_ell)}$ and the inclusive versus exclusive determination of $|V_{cb}|$. I review a few explanations proposed for such anomalies, and discuss tests to shed light on their origin.
We give an updated summary of the topics covered in Working Group I of the 2nd Workshop on the CKM Unitarity Triangle, with emphasis on the results obtained since the 1st CKM Workshop. The topics covered include the measurement of |V_{ub}|, of |V_{cb}| and of non-perturbative Heavy Quark Expansion parameters, and the determination of b-hadron lifetimes and lifetime differences.
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