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Register a new userMeasurement of alpha / phi_2 from B to pi pi decays
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A. J. Bevan
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The current results on B to pipi decays and SU2 constraints on the Unitarity Triangle angle alpha or phi_2 from the B-factories are summarised. Based on these measurements, predictions of the isospin analysis constraints at the end of the lifetime of both B-factories are given.
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CP-violating asymmetries in $B to pi pi$ and $B to rho rho$ decays can help specify the weak phase $phi_2 = alpha$ of the Cabibbo-Kobayashi-% Maskawa (CKM) matrix. We discuss the impact of improved measurements of these processes such as will be available in the near future, finding special value in better measurement of the time-dependent CP violation parameter $S_{00}$ in $B^0 to pi^0 pi^0$ and $B^0 to rho^0 rho^0$. Reducing the errors on $B to rho rho$ measurements by a factor of two can potentially lead to an error in $phi_2 = alpha$ just above $2^circ$, at which level isospin-breaking corrections must be considered.
The Dalitz plot analysis technique is used to study the resonant substructures of $B^{-} to D^{+} pi^{-} pi^{-}$ decays in a data sample corresponding to 3.0 ${rm fb}^{-1}$ of $pp$ collision data recorded by the LHCb experiment during 2011 and 2012. A model-independent analysis of the angular moments demonstrates the presence of resonances with spins 1, 2 and 3 at high $D^{+}pi^{-}$ mass. The data are fitted with an amplitude model composed of a quasi-model-independent function to describe the $D^{+}pi^{-}$ S-wave together with virtual contributions from the $D^{*}(2007)^{0}$ and $B^{*0}$ states, and components corresponding to the $D^{*}_{2}(2460)^{0}$, $D^{*}_{1}(2680)^{0}$, $D^{*}_{3}(2760)^{0}$ and $D^{*}_{2}(3000)^{0}$ resonances. The masses and widths of these resonances are determined together with the branching fractions for their production in $B^{-} to D^{+} pi^{-} pi^{-}$ decays. The $D^{+}pi^{-}$ S-wave has phase motion consistent with that expected due to the presence of the $D^{*}_{0}(2400)^{0}$ state. These results constitute the first observations of the $D^{*}_{3}(2760)^{0}$ and $D^{*}_{2}(3000)^{0}$ resonances.
Proton-proton collision data recorded in 2011 and 2012 by the lhcb experiment, co-rres-pon-ding to an integrated luminosity of 3.0invfb, are a-na-lysed to search for the charmless ${B^0 to rho^0 rho^0}$ decay. More than 600 ${B^0 to (pi^+pi^-)(pi^+pi^-)}$ signal decays are selected and used to perform an amplitude analysis from which the ${B^0 to rho^0 rho^0}$ decay is observed for the first time with 7.1 standard deviations significance. The fraction of ${B^0 to rho^0 rho^0}$ decays yielding a longitudinally polarised final state is measured to be $fL = 0.745^{+0.048}_{-0.058} ({rm stat}) pm 0.034 ({rm syst})$. The ${B^0 to rho^0 rho^0}$ branching fraction, using the ${B^0 to phi K^*(892)^{0}}$ decay as reference, is also reported as ${BF(B^0 to rho^0 rho^0) = (0.94 pm 0.17 ({rm stat}) pm 0.09 ({rm syst}) pm 0.06 ({rm BF})) times 10^{-6}}$.
We report a study of the suppressed $B^-to DK^-pi^+pi^-$ and favored $B^-to Dpi^-pi^+pi^-$ decays, where the neutral $D$ meson is detected through its decays to the $K^{mp}pi^{pm}$ and CP-even $K^+K^-$ and $pi^+pi^-$ final states. The measurement is carried out using a proton-proton collision data sample collected by the LHCb experiment, corresponding to an integrated luminosity of 3.0~fb$^{-1}$. We observe the first significant signals in the CP-even final states of the $D$ meson for both the suppressed $B^-to DK^-pi^+pi^-$ and favored $B^-to Dpi^-pi^+pi^-$ modes, as well as in the doubly Cabibbo-suppressed $Dto K^+pi^-$ final state of the $B^-to Dpi^-pi^+pi^-$ decay. Evidence for the ADS suppressed decay $B^{-}to DK^-pi^+pi^-$, with $Dto K^+pi^-$, is also presented. From the observed yields in the $B^-to DK^-pi^+pi^-$, $B^-to Dpi^-pi^+pi^-$ and their charge conjugate decay modes, we measure the value of the weak phase to be $gamma=(74^{+20}_{-19})^{rm o}$. This is one of the most precise single-measurement determinations of $gamma$ to date.
Observations are reported of different sources of $CP$ violation from an amplitude analysis of $B^+ to pi^+ pi^+ pi^-$ decays, based on a data sample corresponding to an integrated luminosity of $3 ; {rm fb}^{-1}$ of $pp$ collisions recorded with the LHCb detector. A large $CP$ asymmetry is observed in the decay amplitude involving the tensor $f_2(1270)$ resonance, and in addition significant $CP$ violation is found in the $pi^+ pi^-$ S-wave at low invariant mass. The presence of $CP$ violation related to interference between the $pi^+ pi^-$ S-wave and the P-wave $B^+ to rho(770)^0 pi^+$ amplitude is also established; this causes large local asymmetries but cancels when integrated over the phase space of the decay. The results provide both qualitative and quantitative new insights into $CP$-violation effects in hadronic $B$ decays.
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