REFERENTE NORMATIVO

One year of data taking at the LHC results in 15 Petabytes (15 million Gigabytes) of data, produced by the four experiments, which has to be carefully analyzed by physicists worldwide

4.4. ATLAS COMPUTING 37

Figure 4.19: Two magnetic coils and their support structures during the building phase of ATLAS (left) and theirGeoModel_{description (right).}

to discover new physics processes. Moreover, billions of complex theoretical simulations of the proton-proton collision must be calculated.

In the former LEP experiments, the computer processing was done at a computer farm, near to the experiment itself. For LHC, storage and processing requirements exceed by far the capacities available at a single site and hence a new approach was chosen, which is commonly known under LHC Computing Grid (LCG) project [57]. This computing grid provides to infrastructure for the storage of data and the necessary computing power for the physics analyses and simulations.

The data distribution follows a so-called Tier-

USA Italy UK GridKa@FZK Germany Munich LRZ France Tier 0 CERN Computer center LMU Munich Tier 3 Tier 2 Tier 1

Figure 4.20: Illustration of Tier-structure of the LCG.

structure (Figure 4.20). The LHC data is recorded in a first step on tape at so called Tier-0 center at CERN. From there, it is fur- ther distributed to worldwide Tier-1 centers (e.g. GridKa@FZK), which store also a large part of data and provide a twenty-four hour support. The Tier-2 centers like the computer cluster at the LMU Munich (LRZ) store only a small part of data since they are designated for user specific physics analysis and simula- tion. The Grid is accessed via the lowest hier- archy level (Tier-3), which are small computer clusters or individual PCS of physicists. The Tier-structure has several advantages. First of, several copies of data exists, which ensures

that data safety. The single Tier centers are independent from each other which minimizes the critical points in the infrastructure. A further advantage is the cost sharing for maintenance and support through the several national computer centers.

The LCG project involves dedicated hardware and software developments. Obviously, an adequate bandwidth is needed for the data distribution within the grid. The grid-software must be compatible with heterogeneous hardware and must also ensure coherent software at all connected computers. Distributed data must be identifiable by the user and stored redundantly. Moreover a fair access to all resources for all users (load balancing) must be guaranteed and a secure access to more than 100 sites without local accounts must be

38 CHAPTER 4. THE ATLAS EXPERIMENT provided.

Several tests of the grid infrastructure called data challenges have been performed during the last years to ensure the full functionality of the grid with the start of the data taking.

Part II

Muon Spectrometer Performance

“We haven’t got the money, so we’ve got to think!”

Ernest Rutherford1

Chapter 5

Expected Performance of an Ideal

Muon Spectrometer Setup

The Muon Spectrometer, one of the biggest and most complex detectors ever designed, re- quires a detailed and flexible simulation to deal with questions related to design optimization and detailed physics studies which will lay the basis for the first discoveries of new physics. Hence it is crucial also for this thesis to understand the Muon Spectrometer and its simulation in detail in order to give meaningful predictions of physics analysis on first data.

The simulation of the Muon Spectrometer includes a detailed description of several thousand detection chambers, a detailed material description of support structures and a precise model of the expected torodial magnetic field. A short survey of the simulation side of the ATLAS Muon Spectrometer can be found in [8]. The validation and the development of the Muon Spectrometer software was crucial to prepare the physics analysis of this thesis. A more detailed discussion can be found in appendix A.1.

The results of the expected performance of an ideal Muon Spectrometer, gained from the developed validation algorithms, are discussed in this chapter. The label ’ideal’ means that all parts of the Muon Spectrometer are fully functional, calibrated, placed at their nominal positions and operate with nominal resolutions. The following study is mainly based on a single muon sample, containing 10,000 events, with a transverse momentum of 50GeV, fully simulated and reconstructed within Athena _{software release 12.0.6. The transverse} momentum of 50GeV was chosen, because the Muon Spectrometer is expected to have its best performance at these energies. Moreover, standard physics processes like the decay of theW orZ boson, which play an important role already in the first phase of LHC, have final state muons in this energy-regime.

1

Sir Ernest Rutherford, born 1871 in New Zealand, was the first physicists who distin- guished betweenα,β andγradiation and introduced the term ”half life”. After receiv- ing the Nobel prize in chemistry in 1908, he postulated the so-called Rutherford atomic model, which he derived from his famous experiment of the deflection ofα particles from a thin gold film.

42

CHAPTER 5. EXPECTED PERFORMANCE OF AN IDEAL MUON SPECTROMETER SETUP ) True )/(1/p Rec -1/p True (1/p -0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4 Number of Entries 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Gaussian g0 4 Gaussian g

Figure 5.1: Definition of the transverse momen- tum (pT) resolution. g0 is the fitted Gaussian of

iteration step 0, g₄ is the fitted Gaussian of iter- ation step 4. φ η d 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Number of Entries 10 2 10 3 10

Figure 5.2: Distribution of the distancedη,φ for

tracks reconstructed by the ATLAS Muon Spec- trometer to Monte Carlo generated muons in a

50GeV single muon Monte Carlo Sample.