ATLAS [10] collaborations using J/ψ φ(1020) final states.
3
The average of all of the above
mentioned measurements is φ
s
= −30 ± 33 mrad [13].
4
Previously, using a data sample corresponding to 1 fb
−1
integrated luminosity, the
LHCb collaboration studied the resonant structures in the B
0
s
→ J/ψ K
+
K
−
decay [14]
revealing a rich resonance spectrum in the K
+
K
−
mass distribution. In addition to the
φ(1020) meson, there are significant contributions from the f
0
2
(1525) resonance [15] and
nonresonant S-wave, which are large enough to allow further studies of CP violation. This
paper presents the first measurement of φ
s
using B
0
s
→ J/ψ K
+
K
−
decays, where J/ψ →
µ
+
µ
−
with m
KK
above the φ(1020) region, using data corresponding to an integrated
luminosity of 3 fb
−1
, obtained from pp collisions at the LHC. One third of the data was
collected at a centre-of-mass energy of 7 TeV, and the remainder at 8 TeV. An amplitude
analysis as a function of the B
0
s
proper decay time [16] is performed to determine the CP -
violating phase φ
s
, by measuring simultaneously the CP -even and CP -odd decay amplitudes
for each contributing resonance (and nonresonant S-wave), allowing the improvement of
the φ
s
accuracy and, in addition, further studies of the resonance composition in the decay.
These B
0
s
→ J/ψ K
+
K
−
decays are separated into two K
+
K
−
mass intervals. Those
with m
KK
< 1.05 GeV are called low-mass and correspond to the region of the φ(1020)
resonance, while those with m
KK
> 1.05 GeV are called high-mass. The high-mass region
has not been analyzed for CP violation before, allowing the measurement of CP violation
in several decay modes, including a vector-tensor final state, J/ψ f
0
2
(1525). In the SM the
phase φ
s
is expected to be the same in all such modes. One important difference from
the previous low-mass analysis [7] is that modelling of the m
KK
distribution is included to
distinguish different resonance and nonresonance contributions. In the previous low mass
CP-violation analysis only the φ(1020) resonance and an S-wave amplitude were considered.
This analysis follows very closely the analyses of CP violation in B
0
s
→ J/ψ π
+
π
−
decays [8]
and in B
0
→ J/ψ π
+
π
−
decays [3], and only significant changes with respect to those
measurements are described in this paper. The analysis strategy is to fit the CP -even
and CP -odd components in the decay width probability density functions that describe
the interfering amplitudes in the particle and antiparticle decays. These fits are done as
functions of the B
0
s
proper decay time and in a four-dimensional phase space including the
three helicity angles characterizing the decay and m
KK
. Flavour tagging, described below,
allows us to distinguish between initial B
0
s
and
B
0
s
states.
This paper is organized as follows. Section 2 describes the B
0
s
proper-time dependent
decay widths. Section 3 gives a description of the detector and the associated simulations.
Section 4 contains the event selection procedure and the extracted signal yields. Section 5
shows the measurement of the proper-time resolution and efficiencies for the final state in
the four-dimensional phase space. Section 6 summarizes the identification of the initial
flavour of the state, a process called flavour tagging. Section 7 gives the masses and widths
of resonant states that decay into K
+
K
−
, and the description of a model-independent
S-wave parameterization. Section 8 describes the unbinned likelihood fit procedure used
3
The final states D
+
s
D
−
s
[11] and ψ(2S)φ(1020) [12] are also used by LHCb, but the precisions are not
comparable due to lower statistics.
4
See also updated results and plots available at http://www.slac.stanford.edu/xorg/hfag/.
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