first performs a partial event reconstruction and requires events to have two well-identified
oppositely charged muons with an invariant mass larger than 2.7 GeV/c
2
. The second stage
performs a full event reconstruction. Events are retained for further processing if they
contain a J/ψ → µ
+
µ
−
candidate. The distance between the decay vertex of the J/ψ and
each PV, divided by its uncertainty, is required to be larger than three.
To study the properties of the signal and the most important backgrounds, simulated
pp collisions are generated using Pythia [15, 16] with a specific LHCb configuration [17].
Decays of hadronic particles are described by EvtGen [18], in which final-state radiation
is generated using Photos [19]. The interaction of the generated particles with the detec-
tor, and its response, are implemented using the Geant4 toolkit [20, 21] as described in
ref. [22
]. Other sources of background, such as those from b → ψ(2S) transitions, where the
ψ(2S) decays radiatively to a χ
cJ
meson, are studied using the RapidSim fast simulation
package [23].
3 Selection
A two-step procedure is used to optimize the selection of
B
0
s
→ χ
c1,c2
K
+
K
−
candidates.
These studies use simulation samples together with the high-mass sideband of the data,
5550 < m(χ
c2
K
+
K
−
) < 6150 MeV/c
2
, which is not used for subsequent analysis. In a
first step, loose selection criteria are applied to reduce the background significantly whilst
retaining high signal efficiency. Subsequently, a multivariate selection is used to reduce
further the combinatorial background.
The selection starts from a pair of oppositely charged particles, identified as muons,
that form a common decay vertex. Combinatorial background is suppressed by requiring
that the χ
2
IP
of the muon candidates, defined as the difference between the χ
2
of the
PV reconstructed with and without the considered particle, be larger than four for all
reconstructed PVs. The invariant mass of the dimuon candidate must be within 50 MeV/c
2
of the known J/ψ mass [24].
Photons are selected from well-identified neutral clusters, reconstructed in the electro-
magnetic calorimeter [8
], that have a transverse energy in excess of 700 MeV/c. Selected
J/ψ and photon candidates are combined to form χ
c1,c2
candidates. The invariant mass
of the combination, obtained from a kinematic fit [25
] with a J/ψ mass constraint [24], is
required to be within the range 3400–3700 MeV/c
2
.
Pairs of oppositely charged kaons with p
T
> 200 MeV/c and displaced from all PVs
(χ
2
IP
> 4) are selected. Good kaon identification is achieved by using information from
the RICH detectors. This is combined with kinematic and track quality information using
neural networks which provide a response that varies between 0 and 1 for each of the
different mass hypotheses: kaon (P
K
), pion (P
π
), and proton (P
p
). The closer to one
this value is, the higher the likelihood that the particular mass hypothesis is correct. The
chosen requirements on these variables have an efficiency of (86.8±0.2)% and (86.4±0.2)%
for the
B
0
s
→ χ
c1
K
+
K
−
and B
0
s
→ χ
c2
K
+
K
−
modes, respectively, where the uncertainty
is due the size of the available simulation samples. The invariant mass of the selected kaon
pair is required to be within 15 MeV/c
2
of the known value of the φ mass [24]. These criteria
– 3 –