More details on the ALICE ITS

The Inner Tracking System (ITS) during LHC Run1 and Run 2

The description below refers to the ITS used during the first ten years of operation of ALICE. During LS2 a new ITS has been constructed see below, Upgrade of the ITS.

The Inner Tracking System (ITS) of the ALICE experiment is made out of six layers of silicon detectors exploiting three different technologies (pixel, drift and strip). It covers the central pseudorapidity range of |η| < 0.9 and its distance from the beam line ranges from r = 3.9 cm for the innermost pixel layer up to r = 43 cm for the outermost strip layer. The main tasks of the ITS are to reconstruct the primary and secondary vertices, to track and identify charged particles with a low pt cutoff and to improve the momentum resolution at high pt.

Technical Details

Each layer has hermetic structure and it is coaxial with the beam pipe. The ITS covers the pseudorapidity range |η|≤ 0.9 and the distance from the nominal beam line ranges from 3.9 cm for the innermost layer up to 43 cm for the outermost. The two innermost layers are made of Silicon Pixel Detectors (SPD), the two central layers of Silicon Drift Detectors (SDD) and the two outermost layers of double sided Silicon Strip Detectors (SSD). The ITS has the main purposes of providing both primary and secondary vertices reconstruction and of improving the ALICE barrel tracking capabilities in the vicinity of the interaction point. Furthermore, as a standalone tracker, the ITS recovers particles which do not reach or are missed by the external barrel detector, due to acceptance limitations and momentum cutoff. 

Performance

The performance is crucial to measure the vertices of secondary tracks originatins from the weak decays of strange, charm and beauty particles, which are located close to the collision vertex. Secondary vertices separated by at least 100 μm from the primary vertex can be distinguished. 

The SPD trackls selected within a window are combined to extract the vertex position. This method is used to monitor the interaction position quasi-online at the early stage of the tracking procedure and to measure important first-physics observables, as for example the charged particle multiplicity density at midrapidity. A second method - that reaches a better resolution - is applied at the end of the tracking procedure; it is based on the straigh line approximation of the reconstructed tracks in the vicinity of the vertex. 

Collaboration

The countries that were involved in the construction and operation of the detector are: Czech Republic, Finland, France, Italy, Russia,  Mexico, Poland, Russia, Slovakia, The Netherlands, USA, Ukraine, Switzerland. 

Upgrade of the ITS

During the second LHC long shutdown (LS2) a new, ultra-light, high-resolution Inner Tracking System (ITS) was constructed.  This is a key element of the ALICE upgrade. With respect to the one previously installed in ALICE, the new ITS will significantly improve the tracking precision, the tracking efficiency at low-transverse momenta and the readout rate capabilities. The new ITS consists of seven concentric detector layers based on a 50-μm thick CMOS pixel sensor, with a pixel pitch of about 30 x 30 μmfor a total of about 12.5 Giga pixels covering an active surface of about 10 m2. A key feature of the new ITS is the very low mass of the three innermost layers, which have a material thickness of 0.3% X0 per layer. 

The requirements for the design of the new ITS were : to improve the impact parameter resolution by a factor of ~3 in r-φ and a factor of ~5 in z. To achieve this the first layer is closer to the interaction point, 21 mm instead of 39 mm.

The beam pipe radius has been reduced from 29 mm to 18.2 mm

The material budget has been reduced from 1.14 %X0 to 0.3 % X0

The pixel size has been reduced (50 μm x 425 μm) -> O(30 μm x 30 μm)

The countries that participate in the construction of the new ITS are: China, Czech Republic, France, Italy, Netherlands, Pakistan, Republic of Korea, Russia, Slovakia, Switzerland, Thailand, Ukraine, United Kingdom and the United States.