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Register a new userLong-Distance Dominance of the CP Asymmetry in B->X_{s,d}+gamma Decays
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Michael Benzke
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We show that in the Standard Model the parametrically leading (by a factor 1/alpha_s) contribution to the inclusive CP asymmetry in B->X_{s,d}+gamma decays arises from a long-distance effect in the interference of the electromagnetic dipole amplitude with the amplitude for an up-quark penguin transition accompanied by soft gluon emission. This contribution is governed by a single hadronic parameter Lambda_{17}^u related to a matrix elements of a non-local operator. In view of current experimental data, a future precision measurement of the flavor-averaged CP asymmetry in B->X_s+gamma will signal the presence of new physics only if a value below -2% is found. A cleaner probe of new physics is offered by the difference of the CP asymmetries in charged versus neutral B-meson decays.
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We report the measurement of the direct $CP$ asymmetry in the radiative $bar{B}rightarrow X_{s+d}gamma$ decay using a data sample of $(772 pm 11)times 10^6$ $Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The $CP$ asymmetry is measured as a function of the photon energy threshold. For $E^{rm{*}}_{gamma} geq 2.1~{rm{GeV}}$, where $E^{rm{*}}_{gamma}$ is the photon energy in the center-of-mass frame, we obtain $mathcal{A}_{CP}(bar{B}rightarrow X_{s+d}gamma)= (2.2 pm 3.9 pm 0.9) %$, consistent with the Standard Model prediction.
We describe a measurement of the direct CP asymmetry between inclusive b -> s gamma and bbar -> sbar gamma decays. This asymmetry is expected to be less than 0.01 in the Standard Model, but could be enhanced up to about 0.10 by new physics contributions. We use a sample of 89 million BB pairs recorded with the BaBar detector at PEP-II, from which we reconstruct a set of 12 exclusive b -> s gamma final states containing one charged or neutral kaon and one to three pions. We measure an asymmetry of A_CP(b -> s gamma) = 0.025 +/- 0.050(stat.) +/- 0.015(syst.), corresponding to an allowed range of -0.06 < A_CP(b -> s gamma) < +0.11 at 90% confidence level.
The $CP$ asymmetry in $B^-to D_s^-D^0$ and $B^-to D^-D^0$ decays is measured using LHCb data corresponding to an integrated luminosity of 3.0 fb$^{-1}$, collected in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV. The results are $A^{CP}(B^-to D_s^-D^0)=(-0.4pm 0.5pm 0.5)%$ and $A^{CP}(B^-to D^-D^0)=( 2.3pm 2.7pm 0.4)%$, where the first uncertainties are statistical and the second systematic. This is the first measurement of $A^{CP}(B^-to D_s^-D^0)$ and the most precise determination of $A^{CP}(B^-to D^-D^0)$. Neither result shows evidence of $CP$ violation.
Radiative penguin decays provide an indirect probe for physics beyond the Standard Model and contribute to the determination of the CKM matrix elements. Copious quantities of B mesons produced at the B-Factories permit precision measurements of radiative penguin decays. We review the experimental status of the radiative penguin processes b --> s(d) gamma.
We calculate tree-level contributions to the inclusive rare $bar B to X_{s(d)} , ell^+ell^-$ decays. At the partonic level they stem from the five-particle process $b to s(d) , q bar q , ell^+ell^-$, with $q in {u,d,s}$. While for $b to d$ transitions such five-body final states contribute at the same order in the Wolfenstein expansion compared to the three-body partonic decay, they are CKM suppressed in $b to s$ decays. In the perturbative expansion, we include all leading-order contributions, as well as partial next-to-leading order QCD and QED effects. In the case of the differential branching ratio, we present all results completely analytically in terms of polylogarithmic functions of at most weight three. We also consider the differential forward-backward asymmetry, where all except one interference could be obtained analytically. From a phenomenological point of view the newly calculated contributions are at the percent level or below.
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