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In the Standard Model of particle physics quarks and leptons, which are the so-called fermions, are described as fundamental, elementary particles, with their interactions described as mediated by means of the exchange of another set of elementary particles, more precisely the bosons. In the case of the electromagnetic interaction the force carrying particles are photons, in the weak interaction case they are W and Z bosons, and finally it is the gluons in the case of the strong interaction. Ever since the experimental discovery of the W and Z bosons, uncovering the mechanism by which they and the fermions acquire mass became one of the primary goals for particle physics. The Standard Model dictates that the W and Z bosons acquire their masses through the Brout-Englert-Higgs symmetry breaking mechanism, giving rise to a massive scalar particle, the Standard Model Higgs boson.
2 The LHC
Constructed at CERN in a 27 km long tunnel, The Large Hadron Collider aims to probe the TeV energy scale as the worlds largest particle collider. One of the main scientific goals of the LHC was demystifying the electroweak symmetry breaking mechanism by means of searching for in the Standard Model postulated Higgs boson.
The collider has been constructed to accelerate and collide protons at centre-of-mass energies of approximately 14 TeV, and to achieve an instantaneous luminosity of more than
. The counter-rotating proton bunches are separated by a mere 25 ns, giving rise to a bunch crossing rate equalling 40 MHz.
The LHC began operating as of 2010 at
. Following this, in 2011 the number of bunches making up the beams was raised to 1380, changing the separation between the bunches to 50 ns, and giving rise to a significant increase in the luminosity. The centre-of-mass energy was then increased to 8 TeV in 2012, and during that time period the luminosity was also further raised, reaching maximum luminosities of near
7 ? 1
In 2011 and during the first months of data taking in 2012, the data accumulated by the CMS experiment, corresponding to slightly more than
per year, was analyzed, which resulted in the first observation of the Higgs boson.
3 The CMS experiment
Having been designed as a general-purpose detector, CMS can identify and reconstruct photons, muons, electrons, hadronic jets, and the missing of transverse momentum, carried away by weakly interacting particles, very precisely. CMS consists of multiple sub-detectors, each making use of different technologies, calibration and reconstruction methods.
The backbone of CMS is a superconducting solenoid, giving rise to an axial magnetic field of
. Both the central tracker and the calorimeters are positioned inside the the bore of the solenoid. The steel flux return yoke outside the solenoid is filled with ionized gas detectors which is used to detect and reconstruct muons. Trajectories of electrically charged particles are measured by a silicon pixel and strip tracker. This instrument has full coverage within a pseudo-rapidity range of
<2.5 . A hadronic calorimeter, HCAL, and an electromagnetic calorimeter, ECAL, surround this tracking region and azimuthally covers a range inside of | ? | <3 . ECAL's barrel itself is located inside | ? | <1.5 with additional coverage by the ECAL endcaps within 1.5<| ? | <3.0 . Next to that there is also a pre-shower detector covering the in the area in between 1.65<| ? | <2.6 . The pre-shower detector is capable of recording the x, y position of incoming particles through two planes of silicon sensors. Finally, the coverage of the calorimeter is enlarged up to | ? | <5.0 by the Cherenkov forward calorimeter.