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

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16 CHAPTER 2: The CMS experiment at the LHCrotating particle when moving around a circle of radius R, is synchrotron radiationwhich is expressed as∆E ∝ 1 ( E) 4,R mwhere E is the particle energy and m is the particle mass. Therefore, it is obvious thatheavier particles loose a smaller amount of energy compared to less heavier particles.As the bending radius was already fixed by choosing the same tunnel as LEP, usingprotons instead of electrons would yield to obtain a more stable beam of rotatingparticles and consume less power.Despite of providing beams of particles with less energy loss per turn, proton-protoncolliders have their own problems as well. Since protons are composite particles, thento achieve physics goals up to an energy scale of s 1 , the collider needs to provide acenter of mass energy of s 2 which must be higher, hence s 2 ≫ s 1 . This is due to thatthe proton constituents carry a fraction of the energy of the proton and the interactiontakes place between these constituents. The decision of operating the LHC at a centerof mass energy of a few TeV is to make it possible to scan the energy scale of physicsinteractions up to 1 TeV scale. Since one of the main goals of the construction of theLHC is to search for the Standard Model Higgs boson, the theory implies an upperlimit on the mass of the Higgs boson to be less than one TeV. Hence a hadron colliderwith a center of mass energy of a few TeV would allow to discover that missing piece,if it exists.Prior to their path in the LHC, the proton beams go through the CERN pre-acceleratormachines which are explained in Section 2.1.1. The LHC has various detectors, eachof which is specialized for particular physics purposes that are discussed briefly inSection 2.1.2.2.1.1 The Proton Accelerator ChainBefore being injected into the LHC, protons are pre-accelerated through the CERNaccelerator complex [33] depicted in Figure 2.1.The current acceleration facilities have served for decades when providing beams ofelectrons and positrons to the LEP collider. For the LHC operation, they have beenupgraded to provide beams of protons for collisions at unprecedent energies.A duoplasmatron, where emitted electrons from a cathode filament ionize the hydrogengas, provides the source of protons for the LHC. The produced protons are thentransfered to the first stage of the accelerator chain, the linear accelerator called Linac,which is based on radio frequency technology. In a Linac machine, the accelerationoccurs by oscillating an electric field with an appropriate frequency. Due to this reason,particles traverse through the accelerator discontinuously while making so-calledbunches, each of which consists of thousands of particles. When leaving the Linac, theprotons reach an energy of 50 MeV.Energetic protons are fed to a booster prior to be injected into the Proton Synchrotron(PS) for further acceleration. The booster, which is the first circular accelerator in theacceleration chain, enhances the energy of the protons to 1.4 GeV. Subsequently, theprotons are more accelerated while circulating in the 630 m circumference of the PS ring
CHAPTER 2: The CMS experiment at the LHC 17Figure 2.1: The CERN accelerator complex comprising the full particle acceleratorchain. Starting on the left hand side, protons are accelerated while moving through thelinear accelerator (Linac), the booster, the Proton Synchrotron (PS), and the SuperProton Synchrotron (SPS). Finally, the protons are injected into the Large HadronCollider to reach the desired energy.to reach an energy of 26 GeV, and are then fed to the Super Proton Synchrotron (SPS).The SPS, which is the final stage in the succession of the pre-accelerator complex, providesprotons with an energy of 450 GeV and inject them into the LHC machine forfinal acceleration and subsequently collisions.The proton beams finally enter the LHC tunnel which contains two beam pipes. Theinjected beams of particles move inside the pipes in opposite directions and cross eachother at four points of the ring, where the detectors are located. The beams are guidedto travel on a circular path with the use of a strong magnetic field, which is obtained usingsuperconducting electromagnets. Each superconducting electromagnet, made fromcoils of superconducting wire, has a length of about 15 m and a weight of around 27tonnes. To be operational, the temperature must be lowered to values of the order ofa few Kelvin. The dipole magnets could provide a magnetic field of about 8.3 T whichkeeps the beams to move around the ring with a designed energy of about 7 TeV perbeam.While more than 1200 dipole magnets are used to make the proton beams circulated,an additional 400 quadrupole magnets are used in order to make the proton beamsfocused. It is important to have very narrow beams when they are crossing, as it increasesan important parameter of the machine, which is called the luminosity L anddefined as [34]L = f N AN B4πσ x σ y,where σ x and σ y represent the beam profiles in horizontal and vertical directions atthe interaction points. Also, N A and N B are the number of particles per bunch in thebeams of type A and B, respectively. The f parameter is the frequency of crossing ofthe bunches of the two beams. It is obvious that an increased number of particles perbunch or more focused beams would result into a larger luminosity.
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Page 5 and 6: IntroductionThe Standard Model of p
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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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Measurement of the Jet Energy Scale in the CMS experiment ... - IIHE

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