The Compact Muon Solenoid (CMS) experiment is one of two large general-purpose particle physics detectors built at the proton-proton Large Hadron Collider (LHC) at CERN in Switzerland. Approximately 3600 people from 183 scientific institutes, representing 38 countries, form the CMS collaboration who built and now operates the detector. First proton beams have already circulated at LHC, while regular physics runs are expected by spring 2009.
The main goals of CMS are:
• to explore physics at the TeV scale
• to discover the Higgs boson
• to look for evidence of physics beyond the standard model, such as supersymmetry, or extra dimensions
The innermost detector system of CMS is a silicon-based tracker. Surrounding it is a scintillating crystal electromagnetic calorimeter, which is itself surrounded with a sampling calorimeter for hadrons. The tracker and the calorimetrers are compact enough to fit inside the CMS solenoid which generates a powerful magnetic field of 4 Tesla. Outside the magnet are the large muon detectors, which are inside the return yoke of the magnet.
The Department of Physics of Catania together with the Catania Section of INFN has contributed to CMS through the efforts of a group of physicists and technicians who participated both to the Tracker design and construction and also to the investigation of some crucial SuSy discovery channels. In particular several hundred silicon tracker modules were micro-bonded and tested in Catania during the last few years. After that the group has been involved in the detector installation and commissioning at CERN and in data taking with cosmic rays and beam halo. A non-negligible contribution to the campaign of power-supply qualification tests has also been given. Furthermore, several studies on SuSy discovery potential of CMS have been performed in Catania, especially based on kinematical end-point analysis of a few suitable invariant mass spectra. A study of an upgraded layout of the CMS tracker detector, in view of the future factor-ten increase of luminosity (Super Large Hadron Collider scenario), has been also performed. More details on these contributions are given below.
The CMS Tracker
The innermost section of the CMS tracker consists of a pixel detector, divided in a barrel and two End-Caps.
The remainder of the tracking volume is equipped with micro-strip detectors. The Silicon Strip Tracker consists of four subsystems. The Tracker Inner Barrel (TIB) and Tracker Inner Disks (TID) provide coverage at radii of less than 50 cm. The coverage extends to distances from the interaction point along the direction of the beam line (identified with the z-axis) of less than 110cm. The Tracker Outer Barrel extends the coverage in the central tracker up to a radius of 108 cm, while the Tracker End Caps (TEC) complete the forward coverage.
The TIB provides four measurements in a cylindrical geometry. The two innermost layers are equipped with double sided modules - consisting of two single sided sensors glued back-to-back at a stereo angle of 100 mrad. Three TID disks at each side provide coverage for tracks at large pseudo-rapidity. Each disk is equipped with modules arranged in three rings: the modules on the two innermost rings are double-sided, the outermost ring is single-sided.
In Catania 566 modules were micro-bonded and tested (411 TIB modules and 155 TEC modules ). About 500 of them were processed during the years 2005-2007.
Susy Seachers at CMS
The Standard Model is the theory describing the fundamental interactions of the known elementary particles . Despite its tremendous phenomenological success, it is not believed to be theoretically satisfactory, and is regarded only as a low-energy effective theory of a more fundamental theory.
Several extensions of the Standard Model have been proposed by the theorists. Supersymmetry is considered by the physics community as one of the most elegant and attractive extensions. If Supersymmetry exists, it is predicted to manifest itself at the TeV scale, and to be therefore observable at the LHC.
The Catania CMS group has given an important contribution to the studies on Supersymmetry performed with simulated events. The aim of such a kind of studies is to estimate the detector capability to observe a significant statistical excess of Supersymmetric events over the expected Standard Model background, and to evaluate the possibility to measure physical quantities characterizing the supersymmetric particles once they are discovered.
Catania has focussed on the final states containing two same-flavour opposite-sign leptons with jets and transverse missing energy. The study performed by the Catania group has shown how CMS will be capable to discover Supersymmetry in this final state within the first few weeks of data taking, in a favourable scenario, or in a few years in more difficult scenarios. In addition, the work performed in Catania has shown how the CMS experiment will be able to observe the typical dilepton-endpoint, which is one of the most important smoking guns for Supersymmetry. The work of the Catania group produced an official CMS NOTE and was included in the CMS Physics Technical Design Report (CERN-LHCC 2006-021). After the Physics Technical Design Report, the work of Catania on Supersymmetry continued, producing a study demonstrating the possibility to measure the masses of the Supersymmetric particles after a few years of data taking at LHC.
The contribution of the Catania group to the CMS SUSY working group is now of fundamental importance. Catania holds responsibility for the analysis with leptons in the final states as well as for the study of the trigger aspects of SuSy studies.
The LHC will provide unprecedented sensitivity to Standard Model and beyond the Standard Model Physics. However, some important Standard Model measurements as well as a wide part of the spectrum of particles predicted by many promising theoretical models of New Physics are likely beyond the LHC reach. For such observations, a factor-ten increase in LHC statistics will have a major impact. A luminosity upgrade is therefore planned for the LHC. The SLHC, besides offering the possibility to increase the physics potential, will create an extreme operating environment for the detectors, particularly the tracking devices. An increase in the number of minimum-bias events by at least an order of magnitude beyond the levels envisioned for design luminosity creates the need to handle much higher occupancies and, for the innermost layers, unprecedented levels of radiation. This will require a fully upgraded tracking system featuring a higher granularity, while trying not to exceed the material budget and power levels of the current tracker. The much higher rate of interactions may also push the limits of the Level-1 trigger system. Efforts have already begun to address these issues. Several working groups have been created inside the CMS Collaboration to address all the relevant issues for SLHC. Catania, in particular, is involved in the study and layout design of the new tracking system which must be able to cope with the very hostile SLHC environment. A detailed simulation program has been developed and Catania is playing a fundamental role in coordinating these studies.
GRUPPO DI RICERCA
S. Albergo, M. Chiorboli, S. Costa, M. Galanti, N. Giudice, N. Guardone, A. Tricomi, C. Tuvè, R. Potenza, M. A. Saizu [IFIN-HH, Bucharest (Romania)], V. Sparti
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