Studies of Muon Reconstruction
Efficiency in ATLAS using 2012 Data
Feng Bi
From: University of Science and
Technology of China
Advisors: Jianbei Liu, Junjie Zhu
Email: bifnhlp@mail.ustc.edu.cn
The ATLAS Detector
• ATLAS is generalpurpose large-scale
detector system
designed to study
many categories of
physics in high
energy hadron
collisions.
• These include
searches for Higgs,
SUSY, antimatter
and even the
signature of
microscopic black
hole.
45 meters long, 25 meters in diameter
Muon Detection at ATLAS
• Muons appear as the final detectable particles in many interesting and important
physics processes. (e.g., H->μ+ μ− μ+ μ− ) • Muons interact with matter mainly
through ionization and excitation
H->μ+ μ− μ+ μ− candidate
processes, thus can easily penetrate
through a lot of detector elements.
• So muons are usually identified
using ionization detectors located
Inner Detector
outmost in an experiment. (e.g. the
muon spectrometer in the case of
ATLAS)
• ATLAS muon spectrometer is
composed of : Monitored Drift
Tube (MDT), Cathode Strip
Muon spectrometer
Chambers (CSC), Resistive Plate
Chambers (RPC), and Thin Gap
Chambers (TGC) etc.
• The major detectors involved in
muon detection at ATLAS:
• Inner tracking detector (ID)
• Muon sepectrometer (MS)
Muon Reconstruction and Muon Categories
• Two muon reconstruction algorithms at ATLAS:
Staco (chain 1): statistically combines the measurements of a muon in both
MS and ID.
Muid (chain 2): performs a track fit using all muon hits in MS and ID.
• Four types of reconstructed muons:
Combined Muon (CB): reconstructed by matching the ID and MS muon track.
Stand-Alone Muon: MS track extrapolated to the interaction point.
Segment Tagged Muon (ST):
ID track matched to MS
segments.
Calorimter Tagged Muon: ID
track matched to clusters in the
Calorimeter.
Muon Reconstruction Efficiency and Scale Factor (SF)
• As ID efficiency is very high ~100%, we measure muon
efficiency using ID as a reference.
– 𝝐𝑹𝒆 = 𝝐𝑰𝑫 × 𝝐𝑴𝑺 × 𝝐𝑴𝒂𝒕𝒄𝒉
– Assuming 𝝐𝑰𝑫 ~ 1, 𝝐𝑹𝒆 ≈ 𝝐𝑴𝑺 × 𝝐𝑴𝒂𝒕𝒄𝒉
• We measure muon efficiency both in data and in MC
simulation, and derive a scale factor defined as the ratio of
𝝐𝒅𝒂𝒕𝒂
data efficiency to the MC one, 𝑺𝑭 =
.
𝝐𝒎𝒄
• The scale factor represents the modeling of muon
reconstruction efficiency in our simulation, and is applied to
the simulation to correct for the mis-modeling of muon
reconstruction efficiency.
Tag and Probe Method
Z boson decays to μ+ μ− provide us a excellent
muon source with low background.
We use muons from Z-> μ+ μ− to perform the
muon efficiency measurement with Tag-Probe
method
•
•
Select Tag muon: selected from the combined
muons with high quality (including pt cut, eta cut,
z0 cut, isolation requirement and trigger match).
2. Select Probe Muon: selected from Inner Detector
tracks after pt, eta, z0, charge, angular distance
cuts. Also use Z mass window constraint to pick a
probe track that gives a M(tag, probe) within a
mass window.
3. Use the probe track to calculate muon
reconstruction efficiency:
𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒑𝒓𝒐𝒃𝒆𝒔 𝒎𝒂𝒕𝒄𝒉𝒆𝒅 𝒘𝒊𝒕𝒉 𝑪𝑩(𝑺𝑻)
𝝐𝑹𝒆 =
𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒕𝒐𝒕𝒂𝒍 𝒑𝒓𝒐𝒃𝒆𝒔
Selected Z events
using Tag-Probe
pairs
1.
Residual background in the Z mass window is subtracted
using MC simulations. Background considered: Z->tautau,
Z->tautaubb, W->taunu, W->munu, W->cc, W->bb, and ttbar.
Zmumu MC used in the analysis:
mc12_8TeV.107660.AlpgenJimmy_AUET2CTEQ6L
1_ZmumuNp0.merge.NTUP_SMWZ.e1218_s146
9_s1470_r3542_r3549_p1067_tid00855469_00/
NTUP_SMWZ.00855469._000271.root.1
Efficiencis vs. Pt
The ST muons’
efficiency is
higher than the
CB ones’.
Chain 1 CB
Chain 1 CBST
And the CB
efficiency of
Chain 2 is
higher than
Chain 1’s.
Chain 2 CB
Chain 2 CBST
Efficiencis vs. eta
Chain 1 CB
Chain 2 CB
Chain 1 CBST
Chain 2 CBST
• Inefficiency at
Eta=0 due to no
muon
chambers
instrumented
in the region.
• Inefficiency in
transition
region at
Eta=1.2 due to
missing EE
chambers (to
be installed).
• Segment
muons can
recover the
inefficiency in
the transition
region.
Efficiencies vs. phi
Chain 1 CB
Chain 1 CBST
Chain 2 CB
Chain 2 CBST
• Obvious
efficiency drops
in the two “feet”
(phi~-1, ~-2) of
the Muon
spectrometer.
• Clear sector
patter in muon
efficiency (large
sectors vs. small
sectors)
Summary
• Muon reconstruction performance is crucial to the LHC
physics program.
• High efficient muon reconstruction is demanded in a lot of
physics studies at LHC.
• Muon reconstruction efficiency measured using Tag-Probe
method on Z->mumu events for 2012 8TeV data.
• The pt, eta and phi dependence of muon efficiency studied for
both Staco and Muid muon reconstruction algorithms, and for
both “Combined” and “Combined+Segment” muons.
• Muon reonstruction efficiency SF (eff_data/eff_mc) also
derived and can be applied to simulation to correct for mismodelling of muon reconstruction effiency.
• What can be further improved or studied
– Background estimation particularly in low pT region
– Estimation of systematic uncertainties
Thanks for your attention!