CMS-BPH-13-012

CMS-BPH-13-012 ; CERN-PH-EP-2015-192
Measurement of the CP-violating weak phase $\phi_\mathrm{s}$ and the decay width difference $\Delta\Gamma_\mathrm{s}$ using the ${\mathrm{B^0_s} \to \mathrm{ J } / \psi \phi(1020)}$ decay channel in pp collisions at $\sqrt{s} =$ 8 TeV
CMS Collaboration
27 July 2015
Phys. Lett. B 757 (2016) 97
Abstract: The CP-violating weak phase $\phi_\mathrm{s}$ of the ${\mathrm{B}^0_s}$ meson and the decay width difference $\Delta\Gamma_\mathrm{s}$ of the ${\mathrm{B}^0_s}$ light and heavy mass eigenstates are measured with the CMS detector at the LHC using a data sample of ${\mathrm{B^0_s} \to \mathrm{ J } / \psi \phi(1020)} \to \mu^+\mu^-\mathrm{ K }^+\mathrm{ K }^-$ decays. The analysed data set corresponds to an integrated luminosity of 19.7 fb $^{-1}$ collected in pp collisions at a centre-of-mass energy of 8 TeV. A total of 49 200 reconstructed ${\mathrm{B}^0_s}$ decays are used to extract the values of $\phi_\mathrm{s}$ and $\Delta\Gamma_\mathrm{s}$ by performing a time-dependent and flavour-tagged angular analysis of the $\mu^+ \mu^- \mathrm{ K }^+\mathrm{ K }^-$ final state. The weak phase is measured to be $\phi_\mathrm{s} = -0.075 \pm 0.097 \mathrm{(stat)} \pm 0.031\mathrm{(syst)}$ rad, and the decay width difference is $\Delta\Gamma_\mathrm{s} = 0.095 \pm 0.013 \mathrm{(stat)} \pm 0.007 \mathrm{(syst)}$ ps $^{-1}$ .
Links: e-print arXiv:1507.07527 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; Public twiki page ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Definition of the three angles $\theta _\mathrm {T}$ , $\psi _\mathrm {T}$ , and $\varphi _\mathrm {T}$ describing the decay topology of ${\mathrm {B^0_s} \to \mathrm{ J } / \psi \phi (1020)}$ . See text for details.
png pdf	Figure 2: The $\mathrm{ J } / \psi \mathrm{ K }^+ \mathrm{ K }^-$ invariant mass distribution of the ${\mathrm {B}^0_s}$ candidates. The solid line is a fit to the data (solid markers), the dashed line is the signal component and the dot-dashed line is the background component.
png pdf	Figure 3-a: The $ct$ distribution (a) and its uncertainty $\sigma _{ct}$ (b) of the ${\mathrm {B}^0_s}$ candidates. The solid line is a fit to the data (solid markers), the dashed line is the signal component and the dot-dashed line is the background component. For the $ct$ distribution the pull, defined as the difference between the observed events and the fit function applied to the sum of the signal and background, divided by the statistical uncertainty in the observed events, is displayed in the histogram in the lower panel.
png pdf	Figure 3-b: The $ct$ distribution (a) and its uncertainty $\sigma _{ct}$ (b) of the ${\mathrm {B}^0_s}$ candidates. The solid line is a fit to the data (solid markers), the dashed line is the signal component and the dot-dashed line is the background component. For the $ct$ distribution the pull, defined as the difference between the observed events and the fit function applied to the sum of the signal and background, divided by the statistical uncertainty in the observed events, is displayed in the histogram in the lower panel.
png pdf	Figure 4-a: The mistag probabilities $\omega$ , defined as the ratio of the number of wrongly tagged events divided by the total number of tagged events, as a function of the MLP-NN discriminators for muons (a) and electrons (b). The data points (solid markers) are placed at the average weighted value of the events in each bin. The vertical bars show the statistical uncertainties and the horizontal bars the bin width. The solid line represents the parametrisation curve extracted from the background-subtracted ${\mathrm {B}^\pm }$ data; the dashed and dot-dashed lines refer to the parametrisations extracted from the simulated ${\mathrm {B}^0_s}$ and ${\mathrm {B}^\pm }$ samples, respectively.
png pdf	Figure 4-b: The mistag probabilities $\omega$ , defined as the ratio of the number of wrongly tagged events divided by the total number of tagged events, as a function of the MLP-NN discriminators for muons (a) and electrons (b). The data points (solid markers) are placed at the average weighted value of the events in each bin. The vertical bars show the statistical uncertainties and the horizontal bars the bin width. The solid line represents the parametrisation curve extracted from the background-subtracted ${\mathrm {B}^\pm }$ data; the dashed and dot-dashed lines refer to the parametrisations extracted from the simulated ${\mathrm {B}^0_s}$ and ${\mathrm {B}^\pm }$ samples, respectively.
png pdf	Figure 5-a: The angular distributions ( $\cos\theta _\mathrm {T}$ , $\cos\psi _\mathrm {T}$ , $\varphi _\mathrm {T}$ ) of the ${\mathrm {B}^0_s}$ candidates from data (solid markers). The solid line is the result of the fit, the dashed line is the signal component, and the dot-dashed line is the background component.
png pdf	Figure 5-b: The angular distributions ( $\cos\theta _\mathrm {T}$ , $\cos\psi _\mathrm {T}$ , $\varphi _\mathrm {T}$ ) of the ${\mathrm {B}^0_s}$ candidates from data (solid markers). The solid line is the result of the fit, the dashed line is the signal component, and the dot-dashed line is the background component.
png pdf	Figure 5-c: The angular distributions ( $\cos\theta _\mathrm {T}$ , $\cos\psi _\mathrm {T}$ , $\varphi _\mathrm {T}$ ) of the ${\mathrm {B}^0_s}$ candidates from data (solid markers). The solid line is the result of the fit, the dashed line is the signal component, and the dot-dashed line is the background component.
png pdf	Figure 6: The CMS measured central value and the 68%, 90%, and 95% CL contours in the ${\Delta \Gamma _\mathrm {s}}$ versus $\phi _\mathrm {s}$ plane, together with the SM prediction [3,4]. Uncertainties are statistical only.

Tables
png pdf	Table 1: Angular and time-dependent terms of the signal model.
png pdf	Table 2: Results of the fit to the data. Uncertainties are statistical only.
png pdf	Table 3: Correlation matrix for the statistical uncertainties in the physics fit parameters.
png pdf	Table 4: Summary of the uncertainties in the measurements of the various ${\mathrm {B}^0_s}$ parameters. If no value is reported, then the systematic uncertainty is negligible with respect to the statistical and other systematic uncertainties. The total systematic uncertainty is the quadratic sum of the listed systematic uncertainties.

Summary

Using pp collision data collected by the CMS experiment at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 19.7 fb

$^{-1}$ , 49 200

${\mathrm{B}^0_s} \mathrm{ J } / \psi \phi$ signal candidates were used to measure the weak phase

$\phi_\mathrm{s}$ and the decay width difference

${\Delta \Gamma_\mathrm{s}}$ . The analysis was performed by using opposite-side lepton tagging of the

${\mathrm{B}^0_s}$ flavour at the production time. Both muon and electron tags were used.

The measured values for the weak phase and the decay width difference between the

${\mathrm{B}^0_s}$ mass eigenstates are

$\phi_\mathrm{s} = -0.075 \pm 0.097\mathrm{(stat)}\pm 0.031\mathrm{(syst)}$ rad and

$\Delta\Gamma_\mathrm{s}= 0.095 \pm 0.013\mathrm{(stat)}\pm 0.007\mathrm{(syst)}$ ps

$^{-1}$ , respectively. The measured values are consistent with those obtained by the LHCb Collaboration using

${\mathrm{B}^0_s} \to \mathrm{ J } / \psi\mathrm{ K }^+\mathrm{ K }^-$ decays [34].

Our measured value of

$\phi_\mathrm{s}$ agrees with the SM prediction. Our result confirms

${\Delta \Gamma_\mathrm{s}}$ to be nonzero, with a value consistent with theoretical predictions. The uncertainties in our

$\phi_\mathrm{s}$ and

${\Delta \Gamma_\mathrm{s}}$ measurements are dominated by statistical uncertainties. Our results provide independent reference measurements of

$\phi_\mathrm{s}$ and

${\Delta \Gamma_\mathrm{s}}$ , and contribute to improving the overall precision of these quantities and thereby probing the SM further. Since our measurement precision is still limited by statistical uncertainty, substantial improvement is expected from LHC

$\sqrt{s} =$ 13 TeV high-luminosity running that will be available over the next few years.

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