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Register a new userImproving the measurement of the CKM phase $phi_2 = alpha$ in $B to pi pi$ and $B to rho rho$ decays
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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.
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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.
Theoretical errors in the extraction of alpha from B -> pi^+ pi^-, rho^+ rho^-, rho pi decays are usually given in terms of upper bounds on alpha_eff-alpha obtained from isospin or from SU(3) relations, where alpha_eff is measured through CP asymmetries. We show that mild assumptions about magnitudes and strong phases of penguin and tree amplitudes (|P/T| < 1 and |delta| < pi/2) in B -> pi pi and B -> rho rho, imply alpha_eff > alpha, thus reducing by a factor two the error in alpha. Similarly, the assumptions |p_+-/t_+-| < 1, |delta_-| < pi/2 <|delta_+| in B -> rho pi lead to a cancellation between two terms in alpha_eff-alpha. Current data support these conditions.
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 ${BF(B^0 to rho^0 rho^0) = (0.94 pm 0.17 ({rm stat}) pm 0.09 ({rm syst}) pm 0.06 ({rm BF})) times 10^{-6}}$.
Exploring hints of New Physics in the decay modes B -> pi K^* and B -> rho K can shed light on the B -> K pi puzzle. In this talk we discuss supersymmetric contributions to the direct CP asymmetries of the decays B -> pi K^* and B -> rho K within Soft Collinear Effective Theory. We consider non-minimal flavor SUSY contributions mediated by gluino exchange and apply the Mass Insertion Approximation in the analysis. We show that gluino contributions can enhance the CP asymmetries and accommodate the experimental results.
It has been pointed out that the currently most precise determination of the weak phase $phi_2 = alpha$ of the Cabibbo-Kobayashi-Maskawa (CKM) matrix achieved in $B to rhorho$ decays is susceptible to a small correction at a level of $(Gamma_rho/m_rho)^2$ due to an $I=1$ amplitude caused by the $rho$ width. Using Breit-Wigner distributions for the two pairs of pions forming $rho$ mesons, we study the $I=1$ contribution to $Bto rhorho$ decay rates as function of the width and location of the $rho$ band. We find that in the absence of a particular enhancement of the $I=1$ amplitude reducing a single band to a width $Gamma_rho$ at SuperKEKB leads to results which are completely insensitive to the $rho$ width. If the $I=1$ amplitude is dynamically enhanced relative to the $I=0,2$ amplitude one could subject its contribution to a magnifying glass measurement using two separated $rho$ bands of width $Gamma_rho$. Subtraction of the $I=1$ contribution from the measured decay rate would lead to a very precise determination of the $I=0,2$ amplitude needed for performing the isospin analysis.
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