Study of the forward photons productions in pp collisions at sqrt{s} = 7 TeV with the ZDC detector of the ATLAS experiment

The Zero Degree Calorimeters (ZDC) of the ATLAS experiment at the Large Hadron Collider (LHC) are compact hadronic calorimeters that are located exactly on the incident beam axis on either side of the interaction point (IP) in the ATLAS Detector, 140 m downstream from the IP. They thus observe forward going neutral particles that are produced in heavy ion (HI), pA or pp collisions. The ZDC uses Tungsten plates as absorber material and quartz rods interspersed in the absorber as active media. The energetic charged particles crossing the quartz rods produce Cherenkov light, which is then detected by photomultipliers and sent to the front end electronics for processing. The ZDC are longitudinally seg- mented in 4 modules and some modules are also equipped with transverse position sensitivity. The rst section (the one closer to the interaction point) acts as an electromagnetic calorimeter, while the remaining 3 act as a hadronic calorimeter. Both ZDC arms have the rst hadronic module transversely segmented and, in one of the arms, the electromagnetic module is transversely segmented for position in- formation about the incident particles. Each ZDC module individually provides energy and trigger information. In heavy ion collisions the ZDC measures "spectator" neutrons, providing an im- portant handle on Pb-Pb collisions centrality and allowing ATLAS to trigger on ultra-peripheral collisions. The ZDC is a versatile device in that it serve to study heavy ion physics, pp physics and provide a tool to tune both the HI and pp beams. It is designed to be as radiation hard as practicable, since the radiation levels in the position of the ZDC are extremely high. While the ZDC is a physically small device, it occupies a large and critical region of phase space. The ZDC lls the transverse aperture of the neutral particle ab- sorber (TAN) and captures all neutral particles leaving the interaction point with pseudorapidity >8.5. Up to now the ZDC has been used for the analysis of HI interactions. This work presents the rst results obtained with the ATLAS ZDC in the analysis of pp collisions at p s = 7 TeV, starting with the study of the detector calibration and performance in pp interactions, with the main goal to study the high energy pho- ton production in p-p collisions, at the large energy that is available at the LHC. This measurement will be compared with the predictions of dierent MonteCarlo generators (Pythia and EPOS) that are usually adopted in the simulation of mini- mum bias events at the LHC. These generators are tuned and validated only in the central part of ATLAS, while the ZDC measurement will help in understanding the MC generators behavior in the forward region. This measurement has many drawbacks in the simulation of the impact of the high energy cosmic rays in the atmosphere. In the following we will illustrate the physics potential of the device and will describe the ZDC detector in detail. The data analysis will start from the de- tector calibration, with particular emphasis on the measurement of electromag- netic shower. This is achieved through the reconstruction of the 0 invariant mass peak, adopting approximations that are needed because of the specic ZDC de- sign. A MC simulation of the detector will be used in order to test and validate data. Three generators has been tested and compared with experimental results: Pythia6, Pythia8 and EPOS, with dierent tunings. After a description of the main features of the selected generators, they will be used to obtain predictions on the 0 reconstruction and the energy and position distributions of single photon candidates in the ZDC. Finally, the last chapter we will present the comparison between MC and data regarding the single photon energy distribution.