# First measurement of the $CP$-violating phase in $B_s^0 \to \phi \phi$ decays

[to restricted-access page]

A first flavour-tagged measurement of the time-dependent CP-violating asymmetry in $B_s^0 \to \phi\phi$ decays is presented. In this decay channel, the CP-violating weak phase arises due to CP violation in the interference between $B_s^0$-$\bar{B}_s^0$ mixing and the $b \to s \bar{s} s$ gluonic penguin decay amplitude. Using a sample of $pp$ collision data corresponding to an integrated luminosity of $1.0\; fb^{-1}$ and collected at a centre-of-mass energy of $7 \rm TeV$ with the LHCb detector, $880\ \B_s^0 \to \phi\phi$ signal decays are obtained. The CP-violating phase is measured to be in the interval [-2.46, -0.76] \rm rad$at 68% confidence level. The p-value of the Standard Model prediction is 16%. ## Figures and captions  Invariant$K^+K^-K^+K^-$mass distribution for selected$ B ^0_ s \rightarrow \phi \phi$candidates. The total fit (solid line) consists of a double Gaussian signal component together with an exponential background (dotted line). phiphi[..].pdf [24 KiB] HiDef png [203 KiB] Thumbnail [58 KiB] *.C file One-dimensional projections of the$ B ^0_ s \rightarrow \phi\phi$fit for (a) decay time, (b) helicity angle$\Phi$and the cosine of the helicity angles (c)$\theta_1$and (d)$\theta_2$. The data are marked as points, while the solid lines represent the projections of the best fit. The$ C\!P$-even$P$-wave, the$ C\!P$-odd$P$-wave and$S$-wave components are shown by the long dashed, short dashed and dotted lines, respectively. time_p[..].pdf [30 KiB] HiDef png [212 KiB] Thumbnail [60 KiB] *.C file phi_pu[..].pdf [19 KiB] HiDef png [175 KiB] Thumbnail [59 KiB] *.C file ctheta[..].pdf [19 KiB] HiDef png [160 KiB] Thumbnail [56 KiB] *.C file ctheta[..].pdf [19 KiB] HiDef png [159 KiB] Thumbnail [56 KiB] *.C file Negative$\Delta$ln likelihood scan of$\phi_s$. Only the statistical uncertainty is included. RapidL[..].pdf [13 KiB] HiDef png [131 KiB] Thumbnail [32 KiB] *.C file Animated gif made out of all figures. PAPER-2013-007.gif Thumbnail ## Tables and captions  Coefficients of the time-dependent terms and angular functions defined in Eqs. 1 and 2. Amplitudes are defined at$t=0$. Table_1.pdf [54 KiB] HiDef png [125 KiB] Thumbnail [19 KiB] tex code Fit results with statistical and systematic uncertainties. A 68% statistical confidence interval is quoted for$\phi_{ s }$. Amplitudes are defined at$t = 0\$. Table_2.pdf [53 KiB] HiDef png [50 KiB] Thumbnail [8 KiB] tex code

Created on 20 April 2019.Citation count from INSPIRE on 24 April 2019.