Measurement of the CP-violating phase $\phi_s$ in the decay $B^{0}_s \to J/\psi \phi$

[to restricted-access page]


We present a measurement of the time-dependent CP-violating asymmetry in B_s -> J/psi phi decays, using data collected with the LHCb detector at the LHC. The decay time distribution of B_s -> J/psi phi is characterized by the decay widths Gamma_H and Gamma_L of the heavy and light mass eigenstates of the B_s-B_s-bar system and by a CP-violating phase phi_s. In a sample of about 8500 B_s -> J/psi phi events isolated from 0.37 fb^-1 of pp collisions at sqrt(s)=7 TeV we measure phi_s = 0.15 +/- 0.18 (stat) +/- 0.06 (syst) rad. We also find an average B_s decay width Gamma_s == (Gamma_L + Gamma_H)/2 = 0.657 +/- 0.009 (stat) +/- 0.008 (syst) ps^-1 and a decay width difference Delta Gamma_s == Gamma_L - Gamma_H} = 0.123 +/- 0.029 (stat) +/- 0.011 (syst) ps^-1. Our measurement is insensitive to the transformation (phi_s,DeltaGamma_s --> pi - phi_s, - Delta Gamma_s.

Figures and captions

Invariant mass distribution for $ B ^0_s \to\mu^+\mu^- K^+K^-$ candidates with the mass of the $\mu^+\mu^-$ pair constrained to the nominal $J/\psi$ mass. Curves for fitted contributions from signal (dashed), background (dotted) and their sum (solid) are overlaid.

fig1.pdf [11 KiB]
HiDef png [166 KiB]
Thumbnail [133 KiB]
*.C file

Projections for the decay time and transversity angle distributions for events with $m_B$ in a $\pm 20$ $\mathrm{ Me V}$ {} range around the $ B ^0_s$ {} mass. The points are the data. The dashed, dotted and solid lines represent the fitted contributions from signal, background and their sum. The remaining curves correspond to different contributions to the signal, namely the $ C P$ -even P-wave (dashed with single dot), the $ C P$ -odd P-wave (dashed with double dot) and the S-wave (dashed with triple dot).

fig2a.pdf [7 KiB]
HiDef png [204 KiB]
Thumbnail [173 KiB]
*.C file
fig2b.pdf [5 KiB]
HiDef png [181 KiB]
Thumbnail [154 KiB]
*.C file
fig2c.pdf [5 KiB]
HiDef png [189 KiB]
Thumbnail [169 KiB]
*.C file
fig2d.pdf [5 KiB]
HiDef png [189 KiB]
Thumbnail [162 KiB]
*.C file

Likelihood confidence regions in the $\Delta\Gamma_{s}$-$\phi_s$ plane. The black square and error bar corresponds to the Standard Model prediction [10,7].

fig3.pdf [6 KiB]
HiDef png [198 KiB]
Thumbnail [155 KiB]
*.C file

Animated gif made out of all figures.


Tables and captions

Fit results for the solution with $\Delta\Gamma_{s}>0$ with statistical and systematic uncertainties. \\

Table_1.pdf [52 KiB]
HiDef png [49 KiB]
Thumbnail [23 KiB]
tex code

Created on 27 February 2021.