Trayectorias laborales y trayectorias de formación

The inner tracking system is the detector that is the closest to the interaction point, and its functions include the estimation of the charged particles trajectories, used to provide a measurement of their momenta as described in Section2.2.2, as well as allowing the positional determination of interaction or decay vertices by extrapolat- ing the trajectories inside the interaction region. The detection of charged particle trajectories, or tracks for short, is carried out by several silicon detector layers placed non-uniformly around the collision volume, as shown in Figure2.6. The placement of layers is symmetric inϕ, the outermost layers contained within a supporting cyl- indrical structure of 2.5 m of diameter and 5.8 m of length.

r (cm) 0 10 20 30 40 50 60 70 80 90 100 110 z (cm) -300 -200 -100 0 100 200 300 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0_{→ η→}0.2 0.4 0.6 0.8 1.0 1.2 1.4 − TEC TEC+ TOB TIB − TID TID+ PIXEL

Figure 2.6: Cross sectional view of the CMS detector inner tracker detector in the r_{− z} plane, detailing the position of detecting layers as well as the main detector sub- components. The tracker is approximately symmetric aroundr = 0, so only the top half is shown. Figure has been adapted from [65].

The detector is composed of two main parts: a silicon pixel detector system situ- ated very close to the interaction point and a much larger strip detector arrangement placed outside the former. The disposition on the detecting layers allows to detect tracks within a pseudo-rapidity range defined by _{|η| < 2.5. Both systems have to} deal with the efficient tracking of hundred of charged particles, at a rate of 40 MHz, produced from each bunch crossing. A successful apparatus in such a environment requires a short response time, as well as to be composed of many small detecting elements. The latter property is commonly referred as high granularity, and allows to keep the number of detected track points (i.e. hits) per detector unit at acceptable levels.

Being so close to the collision region, the set-up has also to sustain very high particle fluxes during long periods of time, up to1MHz/mm2 _{at the first pixel layer.}

Therefore, resistance to radiation damage of the detecting elements and the accom- panying electronics, dubbed as radiation-hardness, is an essential specification. Addi- tionally, the amount of material present in the particle trajectories has to be kept to a minimum, to avoid stochastic secondary interactions that would degrade the preci- sion and efficiency of track determination. The use of silicon semiconductor detector technologies [66] in the CMS tracking system is thus motivated by a combination of all previously mentioned reasons. In total, the CMS tracking system is composed of 1440 pixel detector modules and 15148 strip detector modules, accounting for an active area over200m2_.

The pixel detector, the innermost detecting system of the CMS experiment, is comprised by a total of 66 million silicon cells placed in 1440 modules around the collision region. Each pixel cell has an area of100×150µm2_{and a thickness of285µm,}

providing two-dimensional local track hit coordinates with a resolution around in the cell surface plane about 20 µm, that can in turn be used to compute the global three-dimensional hit location with high accuracy after accounting for the precise location of the detecting module. As depicted in Figure 2.6, the pixel detector is composed by three barrel layers (i.e. placed around the collision region in an cylindrical arrangement), located at radii of 4.4 cm, 7.3 cm and 10.2 cm respectively, and two forward disks at each side at distance of 34.5 cm and 46.6 cm from the nominal interaction point.

The rest of the tracking system, placed outside the pixel detector, is constituted of several silicon strip detector modules organised in four different sub-detectors, referred as TIB, TID, TOB and TEC in Figure 2.6. The inner part of the strip tracker, adjacent to the pixel detector, is composed of four barrel layers of strip

modules constituting the tracker inner barrel (TIB) section, and three module layers arranged in disks at at each side forming the tracker inner disk (TID). Further away from the interaction region, the outer strip tracker, comprising of six barrel layers in the tracker outer barrel (TOB) and nine disks at each side forming the tracker endcaps (TEC). The strip specifications varies depending on the sub-detector, with thicknesses ranging from320µm to 500µm, and pitches (i.e. distances between strips) from80 µm to 184 µm.

The strips are placed longitudinally parallel to the beam line in the barrel modules and radially in the perpendicular plane in the endcap disks, with silicon strip lengths ranging from 10 cm to 20 cm, and in an overlapping tiled setting (see Figure 2.6) Each strip layer provides a single local coordinate for a particle track hit, aligned with ϕ both the barrel and the endcap disk. A second coordinate can be easily obtained taking into account the placement on the module, thus obtaining the r coordinate in the barrel andz in the endcap disks. In order to provide information on the unknown coordinate in each case, some layers of the tracker (in blue colour in Figure2.6) are composed of two modules instead on one, with a small tilt of 0.1 rad that allows to obtain a precise 3D coordinate for a track hit by combining the two local coordinates and their module positions.