Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

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14 CHAPTER 1: The Standard Model of Particle Physicstheory beyond the Standard Model from which the Standard Model Lagrangian canbe derived.Another question that is brought to mind is the following. The mass of the top quark,which is the heaviest observed elementary particle of the model, is so close to the scaleof the electroweak symmetry breaking v ∼ 246 GeV . This opens the question if thetop quark becomes massive during the spontaneous symmetry breaking mechanism orinteracting with another gauge field results in giving a mass to the top quark.Also, due to the so-called hierarchy problem, there is a huge gap between M Z ∼ 90 GeV ,the energy scale of the weak interaction of the model, and the Plank mass scaleM Pl ∼ 10 19 GeV , which is the fundamental scale of gravity. Then, there have been proposalsthat the current Standard Model is only an effective version of a unified theorywith an energy scale of the order of TeV. Another question which remains unansweredwithin the Standard Model is the lack of a candidate for dark matter. Therefore, withso many open questions in the model, some new extention to the Standard Modelhave been proposed, such as supersymmetry. The Minimal Supersymmetric StandardModel (MSSM), predicts a candidate for dark matter and also provides an answer tothe hierarchy problem. In order to confirm the validity of such extentions to the model,colliders with higher energies are required.Besides the above mentioned challenges of the model, there is another important oneto be noted. Since there has not yet been any experimental evidence for the existenceof the Higgs particle, the procedure of giving mass to the weak bosons is not experimentallyconfirmed. Given that a Higgs boson is not discovered, the Standard Modelof particle physics is still correct but needs some mechanism to explain the origin ofmass in nature. That is why the next generation of the accelerator machines, such asthe LHC, is constructed to run at higher center of mass energies. One of the importantgoals of the LHC project, in addition to provide information on the presence of any newphysics beyond the Standard Model, is to search for the only missing particle predictedby the Standard Model. If a Higgs particle exists with a mass less than 1 TeV, theLHC would be able to discover it.
Chapter 2The CMS experiment at the LHCThe Standard Model of particle physics is the most precise theory describing the matterand the forces, which has been widely tested at high energy physics colliders. Despiteof the good agreement presented between the theory and the experiment, there isstill some part for which there has not yet been any experimental evidence, beingthe mechanism of the electroweak symmetry breaking. The discovery of the Higgsboson would confirm the way of giving mass to the particles described by the theory.Therefore, the idea of constructing the Large Hadron Collider (LHC) was introducedto mainly search for the missing Higgs particle at higher energy scales of physics whichhad not yet been reached.The giant LHC machine is described in more detail in Section 2.1 while the descriptionof one of the general-purpose detectors of the LHC, namely the CMS experiment, isgiven in Section 2.2.2.1 The Large Hadron ColliderThe Large Hadron Collider [29] (LHC) is a proton-proton accelerator installed in atunnel of 26.7 km circumference which was originally built for the Large Electron-Positron collider [30] (LEP). The tunnel is constructed underground, on average about100 m below the surface and is located on the border between Switzerland and France,near Geneva. The LHC, employed by the European Organisation of Nuclear Research(CERN), is currently running at the center of mass energy of √ s = 7 TeV which is afactor of two below the design value of √ s = 14 TeV . Accelerating beams of particlesup to several TeV, the LHC has become the most powerful collider machine. The firstproton-proton collisions at a center of mass energy of 7 TeV happened in March 2010.In addition to colliding beams of protons, the LHC also provides ion-ion collisions [31].Lead nuclei are used as the heavy ion in these collisions.The idea of constructing a hadron collider was approved at 1995 by CERN. It wasdecided to use the same pre-accelerating and underground facilities which had alreadybeen built to serve the LEP experiment [32]. The choice of colliding protons insteadof electrons is due to some physics reasons. An important source of energy loss of a15
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Chapter 5Estimating of the Jet Ener
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CHAPTER 5: Estimating of the Jet En
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2χ / ndf18.76 / 7p0 9.492e-08 ±9.
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Chapter 6ConclusionThe Standard Mod
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CHAPTER 6: Conclusion 129The method
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Bibliography[1] M. E. Peskin and D.
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BIBLIOGRAPHY 133[40] The ALICE Coll
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BIBLIOGRAPHY 135[76] The CMS Collab
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SummaryJets appear in the final sta
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