2011 QCD and High Energy Interactions - Rencontres de Moriond ...

ATLAS - consists of four parts (monitored drift tubes, cathode strip chambers, thin-gap chambers
and resistive plate chambers). It covers |η| < 2.7 and due to its toroidal magnetic field it
can measure transverse momenta with σpT /pT = 10% at pT = 1 TeV. This system also provides
important muon triggers.
Two levels of muon reconstruction are used. Muons that have an ID track matched to an MS
track are fully reconstructed (combined muons) and their momentum is measured by refitting
their track through the detector to give the best precision. Some muons have a clear ID track,
but only track segments in the MS so they can be reconstructed only partially (tagged muons).
The ATLAS trigger consists of three levels, Level 1 (L1), Level 2 (L2) and Event Filter
(EF). L1 trigger is a hardware trigger based on MS, Calorimeters and Minimum Bias Trigger
Scintillator (MBTS). L2 and EF triggers are software triggers and are together called High
Level Trigger (HLT). Due to a constant increase of data rate throughout 2010, varying trigger
conditions and thresholds were used to take data for measurements presented in this paper.
3 Quarkonia
Reconstructed invariant mass distributions of c¯c and b ¯ b mesons decaying into muon pairs are
used to study Inner Detector performance and alignment algorithms. The resulting spectra
obtained from the analysis of 41 pb −1 (full 2010 ATLAS dataset) are shown at Fig. 1. The
fitted mass values are in a good agreement with the world averages.
Di-muon Candidates / ( 0.017 GeV )
100
80
60
40
20
3
× 10
ATLAS Preliminary
∫
s = 7 TeV
-1
L dt = 41 pb
Data: 2010
Fit projection
Fit projection of bkg.
NJ/
ψ = 846000 ± 1000
µ = 3.095 ± 0.003 GeV
J/ ψ
σ = 65 ± 1 MeV
J/ ψ
0
2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2
mµ
µ [GeV]
Events / ( 0.1 GeV )
3
× 10
ATLAS Preliminary
10
-1
s = 7 TeV ∫ L dt ~ 41.0 pb
5
Data 2010 : Opposite Sign
Fit Projection
Fit Projection of Background
Barrel + Barrel
N( Υ 1S)
= 16300 ± 200 (stat.)
N( Υ 2S)
= 4800 ± 200 (stat.)
N( Υ 3S)
= 2300 ± 100 (stat.)
0
8 9 10 11 12
Inv. M( µ µ ) [GeV]
Figure 1: Di-muon invariant mass spectrum at the J/ψ and ψ(2S) mass range (right) and at the Υ(1S,2S,3S)
mass range
4 J/ψ → µ + µ − Differential Cross-sections
The study of charmonium production is not interesting for detector performance evaluation
only. Processes leading to this production are not well understood. Therefore attention has
been paid in ATLAS to measure the J/ψ production cross-section. The analysis was based on
an integrated luminosity of 2.3 pb −1 and results of inclusive differential cross-section, its prompt
and non-prompt fraction in four rapidity bins covering |y| < 2.4 and J/ψ pT range between 1
and 70 GeV have been obtained. A detailed description can be found here 2 .
4.1 Inclusive J/ψ → µ + µ − Differential Cross-section
In order to recover true number of J/ψ decays several corrections for detector efficiency, bin
migration and acceptance effects have to be taken into account. Therefore a weight w is applied
to each observed J/ψ candidate defined as
1/w = P = A·M·ε 2 trk ·ε+ µ (p+ T ,η+ )·ε − µ (p− T ,η− )·εtrig
(1)