• ATLAS
• calibrazione dei jet
• calorimetri
• fotoni
• jet
• LHC
• Monte Carlo
• scala di energia dei jet
• test beam
• validazione del Monte Carlo

The Large Hadron Collider (LHC) is a 27 km long circular particle accelerator located at CERN near Geneva, Switzerland. After a troubled start in September 2008 the LHC began its first period of data taking in November 2009 by colliding protons with an energy in the center of mass of sqrt(s)=900 GeV and later, in December, with a collision energy of 2.36 TeV.

After the winter pause, LHC has restarted operations in February 2010 accelerating the two proton beams to an energy of 3.5 TeV to achieve a center-of-mass energy of 7 TeV with a peak luminosity of 2.1E32 cm^-2s^-1.

The aim of this run is to deliver at least 1 fb^-1 of integrated luminosity by the end of 2011 and several fb^-1 by the end of the year 2012. A year-long upgrade shut-down will be necessary to let LHC reach the design energy of sqrt(s)=14 TeV and the design luminosity of 10E34 m^-2s^-1.

The current theoretical framework, which describe the physics at subatomic length scales is called Standard Model. The SM has shown a remarkable agreement with the huge amount of experimental results gathered in the last 50 years. Nonetheless important questions remain unanswered: why are there three generations of quarks and three flavors of leptons, and what is the explanation of their weak charged current mixings and CP violation? What generate particle masses? Can all particle interactions be unified in a simple gauge group? How can we fit gravity into this picture?

Several Standard Model extensions like the Higgs boson, super symmetry or extra-dimensions have been proposed to answer these questions. Most of these theories, including the Standard Model predict some new phenomena at the TeV mass scale.

LHC will explore an energy range never accessed before in experimentally controlled conditions (the Tevatron proton-anti-proton collider, the LHC predecessor, has a collision energy of 1.96 TeV). The high energy physics community is hopeful that it will play a fundamental role in exploring possible extensions of the Standard Model but also in investigating its precise predictions. The results obtained at LHC could also shed light in some fundamental cosmological questions such as, for example, the dark matter origin.

Four detector sit on the LHC interaction points. Atlas and CMS are two general purpose experiments, Alice is designed to observe lead ions collisions and LHCb is an asymmetric detector especially designed to study b meson production in proton-proton collisions.

The Atlas experiment was designed to accommodate the wide spectrum of possible physics signatures in the TeV mass scale. The measurements planned by the Atlas collaboration range from the already explored areas of the SM (jet production, b and t quark physics, the tau lepton, the W and Z bosons and direct photon production) to its possible extensions (the Higgs boson) and from that to even more exciting possibilities (for instance: new vector bosons, new leptons or quark generations, quark internal structure and super symmetric particles).

Regardless of the complexity of the underlying physics scenario the basic objects that compose the observable final state are just a few: photons, electrons, muons, light hadrons generally grouped in jets, secondary vertices that provide heavy flavor tagging and missing transverse energy that hints at an invisible particle that did not interact in the detector (or at a malfunctioning calorimeter).

Many of these studies, from the top mass and the jet cross section to the Higgs and super symmetry searches require precise measurement of the jet characteristics. The experimental requirements, especially for a precise jet energy scale determination, are often very demanding. Typically, an absolute systematic uncertainty of better than 1% is desirable for precision physics like the measurement of the top quark mass, and the reconstruction of some super symmetric final states.

The Atlas calorimeters are intrinsically non-compensating (they have a higher response for electrons and photons than for hadrons) and non-uniform in eta due to the use of different technology and the presence of dead material and non instrumented regions.

The Atlas calorimeters have been calibrated using electrons during the test beam periods. These periods also served as a test bench to tune the Monte Carlo simulation to closely reproduce the detector response to known particles: electrons, pions, protons and muons. These simulations were then used to compute the corrections necessary to calibrate the response to jets from the electromagnetic scale to the jet energy scale in order to correct the linearity response and improve the energy resolution.

After the start of the data taking several physics channels will be exploited to assess the quality of the calibrations and, if necessary, to obtain correction factors. Different physics channels allow us to study different jet properties in various kinematic and geometrical regions.

Di-jet and multi-jet events will be used to cross-check the relative response across different pseudo-rapidity and pT regions. Z+jet, photon+jet and the hadronic decay of the W will provide a reference for the absolute scale.

The photon+jet channel (being the one with largest cross section) is the first candidate to check the absolute scale of the jet calibration. In events with photon and a recoiling jet, the transverse momentum balance can be exploited to estimate the jet energy using the measurement on the photon energy, whose scale is much better under control. The main background to this channel is given by QCD events where one jet is misidentified as a photon.

In this work we show the analysis of the first of 38 pb^-1 of integrated luminosity collected in 2010. An appropriate sample of events is selected, with the photon in the central region, |eta|<1.4 to cross check the Monte Carlo based calibration up to pT~250 GeV with a combined statistic and systematic uncertainty of 2%. The jet-jet background contamination will be studied by selecting a sample of events with enriched background to estimate and subtract its effect on the photon+jet sample.

The reliability of the jet calibration is an essential ingredient for most physics, both known and unknown, in the Atlas experiment. A method to validate the absolute scale of the Monte Carlo derived jet calibration is very important to provide the Atlas community with one of the fundamental tools to obtain outstanding physics results.

The work on the determination of the signal efficiencies and background contribution to the photon+jet channel is also at the base of the a measurement of the cross section for the associated production.

Following a similar measurement by the D0 collaboration the photon+jet cross section will be measured in four different conditions depending on the rapidity of the jet (central vs. forward jets) and the relative position of the jet and the photon (same side vs. opposite side of the detector). Besides testing of the theoretical prediction this measurement can prove important in probing the gluon content of the proton.

The first chapter of this work introduces the theoretical framework of the Standard Model and some of its possible extensions. In the second chapter the details of the prompt photon theory are reviewed together with a short summary of the previous experimental measurements.

Chapters 3 and 4 review the Atlas experimental apparatus with a particular focus on the calorimeters system. Following the introduction of the Atlas calorimeter, chapter 5 presents a contribute of the candidate to validate two Monte Carlo simulations (Geant4 and Fluka) using the experimental data acquired during the test beam periods of the TileCal hadronic calorimeter.

Chapter 6 and 7 describe the reconstruction of photon and jet in the Atlas detector highlighting the performance and problematics relative to the energy resolution and linearity. The chapter on jet reconstruction builds upon the notions introduced in chapter 1 to introduce the concept of jet algorithms together with a discussion of their characteristics.

The last two chapters report the candidate's work on the associated production of a photon and a jet. The measurement of the differential cross section ds/dpT_photon dy_photon dy_jet (split up in four different regions) is shown in detail in chapter 8 while chapter 9 presents the use of the photon-jet balancing in the transverse plane to measure the uncertainty over the jet energy calibration.