Límites específicos

7.2.1

Triggering

Online selection of interesting events was done by requiring a muon trigger in each event. The specific trigger used changed over the period of data taking, as trigger requirements shifted with

Period Trigger ´ Ldt (pb−1)

D L1 MU6 0.288

E EF mu10 MSonly 0.937

F-H EF mu13 14.4

I EF mu13 tight 19.0

Table 7.1: The triggers used for the different data periods. The number in the trigger name denotes the muon pT threshold.

increasing instantaneous luminosity delivered by the LHC. Starting with only the L1 trigger, later the L2 and EF triggers were added to further reduce the amount of data recorded by ATLAS. To keep the trigger rate under control, the trigger thresholds were gradually increased. The addition of the high level triggers, and the increase of the trigger threshold does not have a significant effect on the number of events that passed the selection. The offline event selection asks for one muon with a pT > 20 GeV. The L1 efficiency of each trigger for finding such a muon is the same for a wide range of trigger thresholds. This can be seen from the shape of the muon trigger turn on curve, a steep rise around the threshold followed by a plateau at higher pT. See Figure 4.11(a) in section 4.5, which shows that the efficiency of the muon trigger is about 75% for the L1 muon trigger with a pT threshold of 6 GeV. Furthermore for an offline muon with pT > 20 GeV the L2 and EF triggers are close to 100% efficient with respect to the L1 trigger, see Figures 4.11(b) and 4.12.

Data taking was divided in different periods, labeled A-I, when changes to the LHC or the detector were made. The data used in this chapter was taken during period D-I. Periods A-C contained only 16 nb−1_{. Table 7.1 shows the triggers for the different periods of data taking,} and the amount of integrated luminosity recorded with that trigger. For MC an EF trigger requirement with a pT threshold of 10 GeV was used. Again, as the offline lepton selection asks for muons with a pT of at least 20 GeV, well into the plateau region of both the data and MC triggers, the efficiencies of both triggers is compatible.

7.2.2

Run selection, Integrated luminosity

After passing the trigger, events are recorded and reconstructed. Not all recorded events con- tain data of sufficient quality to be suitable for physics analysis. A subdetector malfunction, or issues with beam stability could have effected the recorded data. Therefore recorded events went through a first selection requiring stable beams from the LHC, and all detector subsys- tems in operation. Besides an overall flag set by the ATLAS run coordinator each subsystem and trigger system records a flag for each run indicating smooth running of the system, and absence of abnormalities in the recorded data., e.g. symmetric φ distribution within acceptance and expected occupancy in different regions of the subdetector. These selection requirements resulted in a total integrated luminosity of 35 pb−1_{± 3.4% [66].}

During period D, with a total integrated luminosity of 288 nb−1_{the RPC reported ongoing} issues, and data was accepted with less than 90% of the RPC functioning. The problematic RPC data was then reprocessed offline in order to be usable for physics analysis.

7.2. DATA SELECTION 129 Process Cross Section (pb)

W +jets + l + 0 partons 8.5 · 103 W +jets + l + 1 partons 1.6 · 103 W +jets + l + 2 partons 4.5 · 102 W +jets + l + 3 partons 1.2 · 102 W +jets + l + 4 partons 31 W +jets + l + 5 partons 8.5

Table 7.2: The cross section for the W + jets + 1 lepton ALPGEN samples, with NNLO k-factor applied. These samples were generated for all three lepton flavors.

7.2.3

MC samples

In the previous chapter the generators and simulation packages used to create MC samples for signal and background were described in some detail. The MC samples used in this analysis were mostly created using the same tools. This means t¯t events were generated using MC@NLO and W +jets events using ALPGEN. ALPGEN was also used to create a second sample of t¯t events, used to estimate the systematic effect of using a specific generator. The different center of mass energy used in these samples resulted in different cross sections compared to the ones used in the previous chapter. For t¯t generated with MC@NLO, a NNLO cross section of 159 pb was used, with the generator level filter efficiency (selecting only leptonic t¯t decays) of 0.56. For ALPGEN t¯t, the cross sections are listed in Table 7.3. For W +jets, the cross sections are listed in Table 7.2.

Process Cross Section (pb)

t¯t_{→ lνlν + 0 partons} 5.8 t¯t_{→ lνlν + 1 partons} 5.7 t¯t_{→ lνlν + 2 partons} 3.6 t¯t_{→ lνlν + 3 partons} 2.3 t¯t→ lνqq + 0 partons 24 t¯t→ lνqq + 1 partons 24 t¯t→ lνqq + 2 partons 15 t¯t_{→ lνqq + 3 partons} 9.4

Table 7.3: The cross section for the t¯t ALPGEN samples, with an NLO k-factor applied. Samples were

generated for all three lepton flavors.

For QCD specific samples were generated using PYTHIA, where one muon was already requested at generator level, thus reducing the number of events that need to be generated and processed for this analysis. The cross sections for these QCD samples are listed in Table 7.4.

A new grid was defined in the (m0,m1/2) plane of mSUGRA phase space, with A0= 0 GeV, tan β = 3 and sgn(µ) = +, and with (80 < m0< 850 GeV) and (100 < m1/2< 340 GeV). The spacing of the grid was twice as close for lower values (m0< 440 GeV and m1/2< 220 GeV), the region where exclusion/discovery was to be expected. For each point of the grid a full simulation was made of the mSUGRA model with the corresponding set of parameters.

Process Cross Section (pb) Di-jet + µ + 0 partons 8.4 · 105 Di-jet + µ + 1 partons 8.2 · 105 Di-jet + µ + 2 partons 2.2 · 105 Di-jet + µ + 3 partons 2.9 · 104 Di-jet + µ + 4 partons 2.0 · 103

Table 7.4: The LO cross section for the QCD+µ PYTHIA di-jet samples.

The new background samples were generated at the collision energy of 7 TeV. The CTEQ66 [11] set was used for the proton parton density function. A new detector description model was used with updated detector geometry based on the early 2010 collision data. The GEANT4 [52] program was used to simulate the detector response.

When proton beams are collided at high intensity, as the LHC is designed to do, the probability of more than one pair of protons colliding during one bunch crossing becomes non-negligible. When designing an analysis this effect (called pileup) must be taken into consideration, in order to be applicable to the high intensity environment at the LHC. The MC samples used in this analysis take pileup into account, by adding a number of minimum bias collisions to each simulated event. The extra vertices are reweighted to match the average number of vertices found in data.

7.2.4

Event selection

The definitions of the physics objects were previously described in section 3. They differ from the definitions used in the 10 TeV MC analysis, incorporating improved knowledge of the detector, and dealing with experimental challenges of analyzing actual data.

To get a good signal over background ratio, a set of cuts was applied to the events passing the trigger. A summary of these cuts is listed in Table 7.5. Events were selected with exactly one muon with pT > 20 GeV, reducing the QCD background considerably. Events with a second lepton (electron or muon) with pT > 10 GeV were discarded, reducing the Z+jets and dileptonic t¯t background. This second lepton veto also eliminates the overlap with other SUSY analyses in the ATLAS collaboration requiring two or more leptons. Four jets with |η| < 2.5 and pT > 30 GeV were required, with a requirement for the leading jet to have pT > 60 GeV. Requiring four jets means the mass of the hadronicaly decaying top can be reconstructed, thus separating these events from other backgrounds and signal. An important feature of the method described in this chapter is that it not only distinguishes background from signal, but also the different background contributions, giving a valuable cross check on the validity of the result. The cut on the ETmiss/Meff ratio greatly reduces the number of QCD events in the data sample.

The signal and control region are defined in the (Emiss

T , MT) plane. The full region in the (Emiss

T , MT) plane where the background model is to be applied is defined by 30 < ETmiss < 900 GeV and 40 < MT < 900GeV. This region (denoted by the full region throughout this chapter) is split in a control region (CR) and a signal region (SR). The upper edge of the control region is defined by Emiss

T < 125 GeV or MT < 100 GeV, thus defining an L-shaped region, shown in Figure 7.1. The signal region is defined by Emiss

T > 125 GeV and MT > 100 GeV. Finally events in the signal region were required to have Meff > 500 GeV.

7.2. DATA SELECTION 131 Description Selection Nr. of muons pT > 20 GeV Nµ20= 1 Nr. of muons pT > 10 GeV Nµ10= 1 Nr. of electrons pT > 10 GeV Ne10= 0 Nr. of Jets pT > 60 GeV Nj60≥ 1 Nr. of Jets pT > 30 GeV Nj30≥ 4 Emiss T ETmiss> 30 GeV MT MT> 40 GeV

Meff ETmiss/Meff > 0.25

Control region MT< 100 GeV∨ ETmiss< 125 GeV

Signal region MT> 100 GeV∧ ETmiss> 125 GeV, Meff > 500 GeV

Table 7.5: Event selection criteria

100 GeV 125 GeV MT ET miss Signal Region Control Region

Figure 7.1: Definition of the control and signal regions in the MT/ETmissplane

The cuts on MT and ETmisswere designed to reduce the W +jets and t¯t background. The Meff cut is designed to enhance the signal over background ratio for the gluino and squark masses that are expected to be probed by this method. These cuts were adopted from the ATLAS SUSY analysis [67].

The cut on Meff poses an issue for the analysis presented here. Applying the same cut on the CR would reduce the background statistics to such a degree that it would be impossible to find the shape of the distributions. Thus an extra dimension has been added to the problem of extrapolation, requiring a fourth dimension to be added to the model. This dimension would add extra shape parameters to the model, and be strongly correlated to the Emiss

T dimension. To keep the model simple, the efficiency of the Meff cut is not a parameter of the fit, but is taken directly from MC.

A study of the expected significance attainable by this method averaged over all SUSY points for different values of the Emiss

T and MT boundary of the signal region shows that the effect of the Meff cut could possibly be reproduced by posing a more stringent cut on EmissT , namely Emiss

T & 200 GeV. This would restore the data driven policy of the method. The cuts of the ATLAS SUSY group were adopted here in order to retain some measure of comparability

Component Event count Corrected count Full Range T 1 25.3 W 11 .2 T 2 2.8 T P 7.2 5.4 CW 29 .2 30.5 total MC 40 data 46 Control region T 1 25.0 W 10.9 T 2 2.5 T P 7.1 5.3 CW 28.7 30.5 total MC 39 data 43 Signal region T 1 0.1 W 0.2 T 2 0.3 T P 0.06 0.05 CW 0.3 0.3 total MC 0.6 data 1

Table 7.6: The event counts for both data and MC simulated samples of the SM backgrounds in the full, control and signal region. The MC counts are normalized to an integrated luminosity of 35 pb−1. The t¯t sample is split in T 1 and T 2. The T 1 sample is split in T P and T C. The T C sample is added to the W sample to form CW . The yields of the CW (T P ) sample in the third column is used to compare with the fitted yields. These are corrected for contamination by T P (T C) events.

to other results of the group.

Table 7.6 shows the expected number of events for the background MC samples normalized to an integrated luminosity of 35 pb−1. The same table shows the measured number of events in data. The numbers are quoted for the full (EmissT , MT) range, the signal region and the control region.