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of the two detectors also results into a higher rejection of background muons, mainly coming from decays in flight of pions and kaons.

Analysis of 2008 MC simulation

In this analysis a reasonable quality of combined muons was guaranteed with a loose require- ment that the tracks should match with χ2 < 100 and the inner detector track used for the combination be the best match to the track in the muon spectrometer, if multiple inner detec- tor tracks fit. Just as for electrons the muons are required to be isolated, by demanding that the total calorimeter energy deposited in a cone of ∆R < 0.3 around the muon be less than 10 GeV. Muons found to overlap with a jet within ∆R < 0.4 are removed from the collection. Analysis of 2010 data

The selection cuts for good muons have changed for the analysis of the first ATLAS data. The new cuts are listed in Table 3.1. The goodness-of-fit of the combined track of matched ID and MS hits is required to be better than χ2

match< 150. The distance to the primary vertex is

measured in the z direction, along the beamline. The cut on pT imbalance prevents ID and MS tracks with very different pT being combined to form a muon, and the isolation cut prevents muons from e.g. b-decay entering the analysis.

In the bottom half of Table 3.1 the requirements of the inner detector track part of the combined muon are listed, demanding a minimum number of Pixel, SCT and TRT hits. There is also a limit on the number of outliers (hits associated with the track, but not used in deter- mining the track parameters since their contribution to the fit χ2 is too large) relative to the total number of TRT tracks. This requirement is split over different regions in pseudorapidity: in the central part of the detector the number of TRT hits is required to be larger than five, and the relative number of outliers is limited. In the forward region, there is no requirement on the number of TRT hits, but if it is larger than five, the relative number of outliers is limited.

3.4

Missing Transverse Energy

The missing transverse energy (Emiss

T ) is a very important object in SUSY/mSUGRA analyses, as the presence of a massive weakly interacting particle in SUSY events is the cause of large Emiss

T . This can be used to distinguish these events from the SM background.

The measurement of missing transverse energy requires very detailed calibration and mea- surement of all the sub-detector components of ATLAS. The non-interacting particles, such as neutrinos in the SM and the Lightest Supersymmetric Particle (LSP) for SUSY, carry away energy. To measure this missing energy, all energy deposits in the detector must be measured and calibrated correctly. Since at the LHC the boost of the produced particles along the beam- axis is unknown, one can only speak of missing energy measurement in the transverse plane, or Emiss

T .

Analysis of 2008 MC simulation The Emiss

T algorithm first calculates the sum in the two transverse directions, x and y as follows:

Description Cut χ2

match (combined muons only) χ2match< 150

Distance to primary vertex _|dz| < 10 mm

Impact parameter significance d/σd< 5

Pseudorapidity _{|η| < 2.4}

pT imbalance if pM ST < 50 GeV (pTM S− pIDT )/pIDT >−0.4

Isolation, cone size 0.2 ptcone20< 1.8 GeV

Distance to closest jet ∆Rjet> 0.4

ID track requirements for muons

Nr. of SCT hits NSCT ≥ 6

Nr. of Pixel hist NP ix≥ 1

TRT outliers |η| ≤ 1.9 Nh+o> 5 AND No/Nh+o< 0.9

TRT outliers |η| > 1.9 Nh+o< 6 OR No/Nh+o< 0.9

Table 3.1: Muon object definition. The variable d denotes the track impact parameter, σdthe uncer-

tainty on the impact parameter. The isolation in a cone size of 0.2 is called ptcone20. No is the number of outliers, Nh+othe number of hits plus the number of outliers.

where Ex,yCalo = − X TopoCells Ex,y, (3.6) EMuonBoyx,y = − X muons Ex,y (3.7) ECryox,y = − X jets wCryoqEEM3

x,y × Ex,yHAD. (3.8)

To suppress noise, only calorimeter cells that are associated to a TopoCluster contribute to ECalo

x,y . For the muon term, the momenta as measured in the muon spectrometer by the combined algorithm are used to prevent double counting, since the calorimeter term already accounts for the muon energy deposits in the calorimeter. All the combined muons in the |η| < 2.5 are summed, while for the region uncovered by the inner detector 2.5 < |η| < 2.7 the standalone muon spectrometer momenta are used. The last term of Equation 3.5 accounts for the energy lost in the dead material of the cryostat between the electromagnetic and hadronic calorimeters. The last layer of the electromagnetic LAr calorimeter, EEM3

x,y , is compared to the first layer of the hadronic calorimeter, EHAD

x,y , and for each jet a calibration weight, wCryo, is applied.

The ECalo

x,y calculation is further refined by calibrating each contribution according to the reconstructed object it is assigned to. The assignment adheres to the following order: electrons, photons, muons, hadronicaly decaying taus, b-jets and finally light jets. Thus ECalo

x,y (Ex,yFinal) is replaced by the refined ERefCalo

x,y (Ex,yRefFinal) defined as: ERefCalo

x,y = Ex,yRefEle+ Ex,yRefGamma+ Ex,yRefTau+ ERefJet

x,y + ERefMuonx,y + Ex,yCellOut.

(3.9) As before each term in equation 3.9 is calculated as the negative sum of calibrated cells inside the specific object. All TopoClusters calorimeter cells without an object assignment are

3.5. TRANSVERSE MASS 63