FOTOPERIODISMO Y DOCUMENTALISMO SOBRE INMIGRACIÓN

The tracking system in LHCb consists of the VELO, described in Section 2.2.1, and four planar tracking stations: the Tracker Turicensis (TT) located upstream of the dipole magnet and the other three stations T1, T2 and T3 located downstream. The T-stations include two type of detectors: the Inner Tracker (IT) and Outer Tracker (OT). VELO and TT use silicon microstrip sensors. In T1-T3, silicon microstrips are used in IT which covers the region close to the beam pipe, whereas straw-tubes are employed in the outer region of the stations.

The Tracker Turicensis

The Tracker Turicensis (TT) [45], formerly known as the Trigger Tracker, is located between the RICH1 (see Section 2.2.4) and the magnet. Besides providing additional information on the tracks recorded in the VELO that traverse the tracking stations, the TT is also used in the following two cases. First, it participates in the Level-1 trigger to assign transverse momentum information to large impact parameter tracks. Second, it is used in the offline analysis to reconstruct the trajectories of low momentum particles that are bent out of the acceptance by the magnetic field and not reaching the tracking stations T1-T3; long-lived neutral particles decaying outside of the VELO, such as K0

s,

The TT consists of four detection layers, grouped in two stations TTa, TTb. Each station contains two layer and is separated by 27 cm. The first and the fourth layer have vertical detection strips, while the second and the third layer have detection strips rotated by a stereo angle of +5◦ _{and −5}◦_{, respectively as shown in Figure2.6. The silicon}

sensors in the TT are single sided p+_{-on-n 500 µm thick sensors. The sensors have size}

of (9.64 × 9.44) cm in width and length and carry 512 silicon microstrips with a pitch of 183 µm. ~30 cm

TTb

TTa

Figure 2.6: Layout of four TT layers.

The Inner Tracker

The Inner Tracker (IT) [46] covers the region close to the beam pipe where the occupancy is high. Similar to the TT, the IT use silicon microstrip sensors with a strip pitch of 198 µm. The thickness of the IT sensors are of 320 µm when they are not ganged together and 410 µm when the two sensors are assembled to form a “long” module: having a larger thickness allows to maintain the S/N ratio above 15. As illustrated in Figure2.7, each station of the IT is composed of four boxes in a cross-shaped layout. Each box

595 4 5 0 (a) x x v v x x uu 15.5 20.0 1.0 x x v v x xuu x x v v x xuu x x v v x xuu INNER TRACKER OUTER TRACKER (b)

Figure 2.7: Front view (a) and top view (b) of a tracking station. The IT is shown in orange and the OT in blue. Dimensions are given in cm.

2 1 .8 4 1 .4 52.9 125.6 36.35 36.35 19.8 (a) 2 1 .0 6 4 1 .3 3 52.09 125.91 36.91 36.91 19.8 23.6 (b)

Figure 2.8: Layout of x-layer (a) and stereo layer (b) in a IT station. Dimensions are given in cm. The single sensors (at the top and bottom) are 320 µm thick, the two sensor modules are 410 µm thick.

consists of four layers of silicon sensors which arrange in x-u-v-x configuration where the x-layers have the microstrips vertical whereas the u- and v-layer are rotated by ±5◦_.

The layout of an x-layer and of a stereo layer (u- or v-layer) in a IT station are shown in Figure2.8.

The IT covers only 1.3% of the total acceptance around the beam pipe, but approximately 20% of all charged particles produced at the interaction point do pass through its area.

(a) (b)

Figure 2.9: (a) Perspective view of the three OT stations (blue) surrounding the IT stations (purple); (b) The OT layout of a vertical layer.

The Outer Tracker

The Outer Tracker (OT) [47] covers the outer region of the three T-stations with an active area of 6 m × 5 m, surrounding the Inner Tracker. As for the IT, the layout of the OT consists of four layers in a x-u-v-x arrangement (see Figure2.7): the modules in the x-layers are oriented vertically, whereas those in the u- and v- layers are tilted by ±5◦

with respect to the vertical.

An OT detector is designed as an array of individual straw-tube modules. Each module contains two staggered layers (monolayers) of drift-tubes with inner diameters of 4.9 mm. A combination of Argon (70%) and CO2 (30%) is used in order to have the drift time is

shorter than 50 ns and a sufficient drift-coordinate resolution of about 190 µm.

Each detector plane is divided into two types of modules: full (F) and short (S) modules (see Figure 2.9b). The F modules have an active length of 4850 mm and contain a total of 256 straws. The S modules, located above and below the beam pipe, have about half the length of the F modules and contain 128 drift tubes. Each detector plane consists of 14 long and 8 short modules. In total, the complete OT is composed of 168 long and 96 short modules corresponding to about 55000 channels.

Track reconstruction

To find the particle trajectories from the VELO to the calorimeters, the correct hits in the VELO, the TT, the IT, and the OT are combined by the track reconstruction software.

The software aims to find all tracks in the event which leave sufficient detector hits. Depending on their trajectories in the LHCb tracking system, the tracks are classified in different types as depicted in Figure 2.10 and described in the following

• Long tracks traverse all the tracking system from the VELO up to the T-stations. These have the most precise momentum measurement, therefore are the most useful for physics. These long tracks are reconstructed in 95 % of the cases using the “forward tracking” algorithm when the inputs are the VELO seeds to which a cluster in the T-stations is added to define a trajectory in the T-stations. Additional clusters are searched for in the T-stations. The track candidate is then kept if it satisfies some quality criteria. The “track matching” algorithm matches the T-seeds with the VELO seeds which have not been used in the “forward tracking” algorithm. The algorithm estimates the momentum of the T-seed using the “pT

kick” method and a “good” match is chosen according to a χ2 _{criterion. This}

“track matching” algorithm allows to reconstruct about 5 % of the long tracks. • Upstream tracks traverse only the VELO and the TT stations. They are mostly

low momentum tracks that are bent out of the acceptance by the magnetic field before reaching the T-stations.

• Downstream tracks traverse only the TT and the T-stations, and have no hits in the VELO. They allow reconstruction of decay products that decay outside the VELO acceptance, such as K0

s, Λ.

• VELO tracks traverse only the VELO. They have a large polar angle and are very useful for the primary vertex reconstruction.

• T tracks traverse only the T-stations. They are typically produced in secondary interactions, and are used in the RICH2 reconstruction.

Once tracks have been found, their trajectories are refitted with a Kalman filter [48] which accounts for multiple scattering and corrects for dE/dx energy loss. The quality of the reconstructed tracks is estimated by the χ2 _{of the fit.}