decay appears to be systematically lower than current SM predictions. Global analyses
of b → s processes favour a modification of the SM at the level of 4 to 5 standard devia-
tions [20–24]. Similar studies of b → d processes are important to understand the flavour
structure of the underlying theory.
This paper presents a search for the decay B
0
s
→ K
∗0
µ
+
µ
−
, where the inclusion of
charge-conjugate processes is implied throughout, using data collected with the LHCb
experiment in pp collisions during Runs 1 and 2 of the LHC. The data set used in this
paper is as follows: 1.0 fb
−1
of integrated luminosity collected at a centre-of-mass energy of
7 TeV during Run 1; 2.0 fb
−1
of integrated luminosity collected at a centre-of-mass energy
of 8 TeV during Run 1; and 1.6 fb
−1
of integrated luminosity collected at a centre-of-mass
energy of 13 TeV during Run 2. Section 2 of this paper describes the LHCb detector and
the experimental setup used for the analysis. Section 3 outlines the selection processes
used to identify signal candidates. Section 4 describes the method used to estimate the
number of B
0
s
→ K
∗0
µ
+
µ
−
decays in the data set. Section 5 describes the determination of
the B
0
s
→ K
∗0
µ
+
µ
−
branching fraction, normalising the number of observed signal decays
to the number of B
0
→ J/ψ K
∗0
decays present in the data set. Section 6 discusses sources
of systematic uncertainty on the B
0
s
→ K
∗0
µ
+
µ
−
branching fraction. Finally, conclusions
are presented in section 7.
2 Detector and simulation
The LHCb detector [25, 26] is a single-arm forward spectrometer covering the
pseudorapidity range 2 < η < 5, designed for the study of particles containing b or c
quarks. The detector includes a high-precision tracking system consisting of a silicon-
strip vertex detector surrounding the pp interaction region [27], a large-area silicon-strip
detector located upstream of a dipole magnet with a bending power of about 4 Tm, and
three stations of silicon-strip detectors and straw drift tubes [28] placed downstream of
the magnet. The tracking system provides a measurement of momentum, p, of charged
particles with a relative uncertainty that varies from 0.5% at low momentum to 1.0% at
200 GeV/c. The minimum distance of a track to a primary vertex (PV), the impact param-
eter (IP), is measured with a resolution of (15 + 29/p
T
) µm, where p
T
is the component of
the momentum transverse to the beam, in GeV/c. Different types of charged hadrons are
distinguished using information from two ring-imaging Cherenkov detectors [29]. Photons,
electrons and hadrons are identified by a calorimeter system consisting of scintillating-
pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter.
Muons are identified by a system composed of alternating layers of iron and multiwire
proportional chambers [30].
The online event selection is performed by a trigger [31]. The trigger consists of a
hardware stage, based on information from the calorimeter and muon systems, followed
by a software stage, which applies a full event reconstruction. The signal candidates are
required to pass through a hardware trigger that selects events containing at least one
muon with p
T
greater than 1 to 2 GeV/c, depending on the data-taking conditions. The
software trigger requires a two-, three- or four-track secondary vertex with a significant
– 2 –
