interactions [7, 8]. Cross section measurements of quarkonium pair production are essential
in understanding SPS and DPS contributions and the parton structure of the proton.
The first quarkonium pair production measurement dates back to 1982 when the NA3
experiment at CERN observed direct production of two J/ψ mesons [9] in π
−
-platinum
interactions with beam energy of 150 GeV and 280 GeV, where the main contribution is
quark-antiquark annihilation. Early theoretical calculations of the pair production cross
section assumed only color-singlet states [10–12]. However, because of the high parton flux
and high center-of-mass energy at the LHC, DPS is expected to play a significant role in
quarkonium pair production [13].
Quarkonium pair production in pp collisions via DPS is assumed to result from two
independent SPS occurrences [7, 13]. Several DPS production processes, including final
states with associated jets, are commonly described by an effective cross section (σ
eff
)
that characterizes the transverse area of the hard partonic interactions [14, 15]. It is
estimated as the product of single quarkonium production cross sections divided by the
corresponding DPS quarkonium pair cross section [16]. Assuming the parton distribution
functions are not correlated, σ
eff
is expected to be independent of the final state and the
center-of-mass energy. Recent measurements of σ
eff
in final states with jets are in the
range 12–20 mb [17–20]. However, measurements of J/ψ pair [21] and J/ψ +Υ(1S) [22]
production in pp collisions yield values of 4.8 and 2.2 mb, respectively. From a fit to
differential cross section measurements of J/ψ pair production with CMS [23], Lansberg
and Shao [24] estimate an effective cross section of 8.2 mb. In this paper, we report the
first observation of Υ(1S) pair production and estimate σ
eff
using this result, along with
theoretical predictions.
2 The CMS detector and muon reconstruction
The central feature of the CMS apparatus is a 13 m long superconducting solenoid of 6 m
internal diameter, providing a magnetic field of 3.8 T. Within the solenoid volume are a
silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter, and
a brass and scintillator hadron calorimeter, each composed of a barrel and two endcap
sections. Extensive forward calorimetry complements the coverage provided by the barrel
and endcap detectors. Muons are measured in gas-ionization detectors embedded in the
steel flux-return yoke outside the solenoid. The main subdetectors used for the present
analysis are the muon detection system and the silicon tracker.
Charged particle trajectories are reconstructed by the silicon tracker in the pseudora-
pidity range |η| < 2.5. The innermost component of the tracker consists of three cylindrical
layers of pixel detectors in the barrel region and two endcap disks at each end of the barrel.
The strip tracker has 10 barrel layers and 12 endcap disks at each end of the barrel. There
are 66 million 100 × 150 µm
2
silicon pixels and more than 9 million silicon strips. Strip
pitches vary between 80 µm and 120 µm in the inner barrel, while the inner disk sensors
have a pitch between 100 µm and 141 µm. For charged particles of transverse momentum
1 < p
T
< 10 GeV and |η| < 1.4, the relative track resolution is typically 1.5% in p
T
and
25–90 (45–50) µm in the transverse (longitudinal) impact parameter [25].
– 2 –
