Track Reconstruction

Detailed Monte Carlo studies were needed to prepare for the data analysis, with particular attention being given to a reliable Monte Carlo prediction of the p f variable. Appropriate techniques to validate the analysis method using a data-driven approach had to be developed, which also illustrates the challenges one faces at the LHC to perform this analysis. 

Lea succeeded in measuring for the first time the inclusive fo-quark production cross section at a centre-of-mass-energy of 7 TeV using 8.1 nb of data collected by CMS during the first month of data-taking in April 2010. This result represents a remarkable achievement at this early stage of data-taking and analysis. 

In addition to their impact on the pixel operation, the results of the work described in this thesis also set the stage for new analysis techniques using the CMS detector. The results also provide an excellent example of the close link between the development of forefront technologies and major progress in fundamental science, by exploiting the new energy frontier offered by the LHC. 

It was a great pleasure and a unique experience to work in a large international collaboration like CMS. This work would not have been possible without the help of many people and I am grateful to everybody who contributed to its successful completion. However, there are some people I would like to especially mention. 

I wish to express my sincere thank to Felicitas Pauss for giving me the opportunity to do my thesis under her supervision. I very much appreciated her competent advise, her enthusiasm and her continues support. 

---

Fig. 4.8 Distribution of the number of tracks reconstructed within a TrackJet...

For collision data the track reconstruction was performed using the CTF algorithm (see Sect. 2.2.4) and the tracking detectors had been aligned based on the same procedure as described in [5]. 

Tracks with a minimum transverse momentum of pj > 0.3 GeV and a pseudorapidity of -2.5 < r] < 2.5 were selected and the quality cuts listed in Table4.4 were applied. The performance of the track reconstruction for the data is compared to the MC simulation. In Fig. 5.2 the distribution of the track multiplicity and the track transverse momentum, pseudorapidity and azimuthal angle are shown. While the shape of the transverse momentum spectrum in data is well described by MC, the disagreement between data and simulation in track multiplicity and track pseudorapidity is due to an imperfect D6T tuning as discussed in [6]. The asymmetry in the... 

In Fig. 5.4 the number of tracks reconstructed within a TrackJet and the transverse momentum of the highest transverse momentum track are compared to the MC simulation and a good agreement is found. These results are relevant in view of the measurement of the -quark production cross-section as the analysis presented here is based on a precise determination of the muon momentum with respect to the TrackJet direction. A good understanding of the TrackJet reconstruction and a reliable simulation of the TrackJet distributions are thus of utmost importance. 

The CMS pixel detector allows for high precision tracking in the region closest to the interaction point in a particularly harsh environment characterized by a high track multiplicity and heavy irradiation. The main purpose of the pixel detector is the reconstruction of secondary vertices from heavy flavor and tau decays and the generation of track seeds for track reconstruction. 

Furthermore, the cosmic data were used to estimate the hit efficiency of the BPIX modules from data. This is done by extrapolating tracks reconstructed in the silicon tracker to the pixel detector and checking the presence of a compatible hit. The efficiency is then defined as the number of tracks with a compatible pixel hit divided by the total number of tracks. For the efficiency measurement only tracks to which one additional hit was associated in both the top and bottom half of the pixel detector were selected. A layer efficiency averaged over the operable modules of (97.1 1.4)%, (97.1 1.9)% and (96.4 2.6)% was measured for the first, second, and third barrel layers, respectively. 

The determination of the cross section relies on the performance of the muon and track reconstruction. The muon and TrackJet kinematical disuibutions were compared to the MC prediction and the simulation was found to be in good agreement with data. Furthermore, A detailed study of the dominant sources of systematic uncertainty has been performed in the scope of this work. 

Fig. 2.4 Track reconstruction efficiency for muons (left) and pions (right) with transverse momenta of 1,10 and lOOGeV [4]...

The expected performance of the track reconstruction is shown in Fig. 2.4 for muons, pions and hadrons. The track reconstruction efficiency for high energy muons is about 99% and drops at 7 > 2.1 due to the reduced coverage of the forward pixel detector. For pions and hadrons the efficiency is in general lower because of interactions with the material in the tracker. The material budget of the CMS tracker in units of radiation length is presented in Fig. 2.5. 

Figure 29 (Qin and Liu, 1982) shows the behavior of individual particles above the distributor recorded by video camera of small clusters of particles, coated with a fluorescent material and spot-illuminated by a pulse of ultra violet light from an optical fiber. The sequential images, of which Fig. 29 just represents exposures after stated time intervals, were reconstructed to form the track of motion of the particle cluster shown in Fig. 30. Neither this track nor visual observation of the shallow bed while fluidized, reveal any vestige of bubbles. Instead, the particles are thrown up by the high velocity jets issuing from the distributor orifices to several times their static bed height.

Audit trails refer to a journal that records modifications to electronic records. The persons or automated processes operating on the user s behalf may perform these modifications. The audit-trail mechanism provides the capability to reconstruct the data prior to a modification, and therefore must not obscure the previously recorded data. The tracking mechanism must include a computer-generated time stamp that indicates the time of the entry. Audit trails are computer generated and can be part of the record that has been modified or a standalone record linked to the modified record. 

An audit trail is a journal recording any modifications carried out by the users, or by the processes operating on behalf of the user, to the electronic records. This mechanism provides the capability to reconstruct modified data and must not obscure the previously entered data. The tracking mechanism includes adding computer-generated date and time stamps to the record, to indicate when the record was modified, the types of modifications performed, and the identity of the person performing the modification to the record. 

The benefits of the CLP are numerous the fact that it is a turnkey program with an independent QA oversight provided by the EPA being the most important one. The observance of the CLP analytical protocols as a contract requirement ensures that the data obtained at different CLP laboratories are comparable. Extensive documentation makes data tracking and reconstruction possible negotiated prices for laboratory contracts and the economy of scale due to large numbers of samples make the CLP cost-effective and efficient. 

---