ATLAS Performance in Run 1

performs precise pT measurements and requires isolation criteria for leptons, reducing the event rate to below 3.5 kHz.

The last triggering step is done via EF, which uses offline analysis procedures such as track and vertex reconstruction, on fully-built events. It uses all available information to further decrease the event rate to about 200 Hz with an average event processing time of∼4s [71]. Events passing the EF are stored permanently on local storage disks to be used for offline analyses.

3.3. ATLAS Performance in Run 1

The first physics data delivered to the ATLAS experiment in 2010 was at the centre-of-mass energy of √

s = 7 TeV. ATLAS recorded 5.08 fb⁻¹ out of 5.46 fb⁻¹ collision data delivered by the LHC by end of 2011, with an instantaneous luminosity of 3.7× 10³³ cm⁻²s⁻¹, where 4.57 fb⁻¹ (∼ 90%) passed the ATLAS quality criteria for physics analyses. In 2012 the centre-of-mass energy of collisions increased to√

s= 8 TeV, where 22.8 fb⁻¹ of data delivered by theLHC, 21.3 fb⁻¹ of which was recorded by ATLAS and 20.3 fb⁻¹ (∼95%) fulfilled the ATLAS quality criteria for physics analyses⁸. Figure3.9a

8A newer calibration study in [81] has re-evaluated the delivered data as 21.7 fb⁻¹, of which 20.2 fb⁻¹ pass the good quality conditions for physics analysis.

Figure 3.8.: Block diagram of the ATLAS trigger and data acquisition systems [80].

shows the data taking efficiency by comparing the delivered data by theLHC, recorded by ATLAS and certified as good quality data for physics analysis as a function of the running time during the Run 1 of the LHC [82].

The instantaneous luminosity of 2012 data increased by about factor of two (3.7× 10³³ cm⁻²s⁻¹) with respect to 2011. This is followed by an increase in the number of interactions per bunch crossing in addition to the collision of interest, causing pile-up background. The pile-pile-up background affects physics object reconstruction, which has a direct impact on the performance. Figure 3.9b shows the luminosity-weighted distribution of the mean number of interactions per bunch crossing for the 2011 and 2012 data taking periods. The mean number of interactions per bunch crossing is calculated from the instantaneous luminosity per bunch as:

µ= Lbunch×σinel

fr

. (3.4)

In Equation 3.4, Lbunch refers to the instantaneous luminosity per bunch, σ_inel is the inelastic cross section which is taken to be 71.5 mb (73 mb) for 7 TeV (8 TeV), andfr is theLHCrevolution frequency equal to 11.2455 kHz [83].

In next chapter, the ATLAS optimisation efforts to reduce the dependency of the reconstruction performance to pile-up effects are discussed in more details.

Month in Year Jan Apr Jul Oct Jan Apr Jul Oct

-1fbTotal Integrated Luminosity

0

Mean Number of Interactions per Crossing

0 5 10 15 20 25 30 35 40 45

/0.1]-1Recorded Luminosity [pb

0

180 ATLASOnline Luminosity

> = 20.7

Figure 3.9.: The comparison of cumulative luminosity delivered, recorded and certified as good quality for physics analysis (a) and the luminosity-weighted distribu-tion of the mean number of interacdistribu-tions per crossing (b) for 2011 and 2012 data [82].

3.3. ATLAS Performance in Run 1

Subdetector Number of Channels Operational Fraction

Pixels 80 M 95.0%

SCT Silicon Strips 6.3 M 99.3%

TRT Transition Radiation Tracker 350 k 97.5%

LAr EM Calorimeter 170 k 99.9%

Tile calorimeter 9800 98.3%

Hadronic endcap LAr calorimeter 5600 99.6%

Forward LAr calorimeter 3500 99.8%

LVL1 Calo trigger 7160 100%

LVL1 Muon RPC trigger 370 k 100%

LVL1 Muon TGC trigger 320 k 100%

MDT Muon Drift Tubes 350 k 99.7%

CSC Cathode Strip Chambers 31 k 96.0%

RPC Barrel Muon Chambers 370 k 97.1%

TGC Endcap Muon Chambers 320 k 98.2%

Table 3.1.: The operational fraction of each of the ATLAS sub-detectors [84].

An overview of the ATLAS detector performance for the 2012 data taking period is presented in Table 3.1, which gives the operational fraction of each of the ATLAS sub-detectors. During Run I of the LHC, the ATLAS detector achieved the fraction of operational channels of >95%.

4

Object Definition

Carrying out a complete physics analysis requires several processes to be performed to convert the electrical signals, i.e. electrical currents and voltages, measured in different sub-detectors to sensible physics information. This information is used in the particle’s tracks and energy deposition reconstruction, which is in turn used to reconstruct various physics objects. These physics objects can be described at different levels which are sketched in Figure 4.1.

As mentioned in Section 2.2.1, the high energy proton-proton collision is effectively a parton-parton collision. The first level of this interaction which is indeed the hard interaction process is called parton level. The final state of the hard process contains quarks and gluons which undergo the so-called hadronization process due to the colour confinement phenomenon (see Section 2.1.2). The electrons and photons in addition to the secondary particles produced via the hadronization process produce the particle shower, which is detectable in both tracking and calorimetry systems of the detector. This level is called theparticle level. The interaction between these particles and the different sub-detector components forms the detector level which is also known as reconstruction level.

The reconstruction level is visualised by the experiments performed at the LHC via using the collision event display to trace the paths of particles produced in a collision.

The event display is very helpful in visualising specific physical processes and for checking that the detector and software function properly. Figure4.2presents att¯candidate event in the ATLAS event display. The physics objects of interests are shown in colours.

The measurement of theW boson polarisation discussed in this thesis is performed by selecting events containing one electron or muon, jets and missing transverse momentum,

Figure 4.1.: Illustration of a particle detection process and the different levels of object descriptions.

due to the presence of a neutrino from the leptonic decay mode of one of theW bosons in the final state. In this chapter the identification and reconstruction of those objects are discussed.