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Silicon Tracking System and Micro-Vertex Detector

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Overview of the detector system

The Silicon Tracking System (STS) is the central component of the CBM experiment. It serves for track and momentum measurement of all charged particles produced at the target. Between the target and the STS, a Micro-Vertex Detector (MVD) enables to distinguish particle decay vertices from the event vertex. A benchmark observable is the D meson (“open charm”), a rare probe that has to be identified via its hadronic decays $ D^0 \rightarrow K^-\pi^+ $ and $ D^\pm \rightarrow K^\mp\pi^\pm\pi^\pm $. This challenging task requires a detector with high position resolution, very low material budget, high radiation tolerance and a fast self-triggered readout.

The current STS-MVD layout consists of 8 stations that are placed inside a magnetic dipole field which provides the bending power required for momentum determination with an accuracy of about Δp/p = 1%.

The STS comprises 8 detector stations:
  • Stations are placed in 30, 35, 40, 50, 60, 75, 95, 100 cm distance from the target fully based on low-mass Silicon micro-strip detectors for the track point measurement. The stations have ladder structure and are build of 300 μm thick double-sided silicon micro strip sensors. To achieve low-mass detector with 60 μm strip pitch read-out electronics is placed at the perimeter of the STS. Signals from sectors are sent through thin capton micro-cables to the front-end boards.

The MVD comprises two detector stations:
  • They are located at 5 and 10 cm (alternatively 10 and 20 cm) downstream of the target and will be installed in a vacuum vessel. We plan to use very thin pixel detectors with very high spatial resulution there. Monolithic Active Pixel Sensors (MAPS) with a pixel size of 40x40 μm2, yielding a spatial resolution of 3 μm, and a thickness of 100 μm would perfectly fulfill our requirements concerning vertex resolution which is needed to measure the displaced vertices of D mesons. The R&D on MAPS concentrates on the improvement of radiation hardness and readout speed.

Simulations are being performed to optimize the number of STS and MVD stations, their layout and the material budget of the detectors, for efficient track finding, high-resolution momentum measurement and vertex selection. The simulations are closely linked to the detector R&D activities.

STS/MVD challenge

Au+Au collisions, 25 GeV/nucleon:
  • high track densities:
    600 charged particles in polar acceptance 2.5 - 25 deg
  • high r/o speed, radiation hardness:
    10 MHz interaction rate (109 ions/s on 1% λint target), only high-level triggers.

(Left) Simulation of a central Au+Au collision at 25 GeV/nucleon in the STS.
(Right) Illustration of rare "open charm" decays to be identified as detached vertices in about 100 μm distance from the event vertex.

STS/MVD detector layout

The standard STS/MVD detector in the CBM detector simulation.

(Left) Schematical cross section of the detector concept. The target is on the left hand side. The first two (or three) stations from the target are the Micro Vertex Detector (MVD). They will be built from thin MAPS pixel detectors that may be installed in vacuum. The remaining stations form the Silicon Tracking System (STS) for the track and momentum reconstruction of all charged particles. They will be built from micro-strip detectors.
(Right) View of the STS/MVD in the dipole magnet with the beam pipe and the vacuum section of the MVD shown.

STS Tracking station layout

The stations are built of sensors arranged in vertical modules with fixed horizontal size of 6cm. The strip length was matched to a maximum occupancy of less then 5% and results in a vertical size of one sensor from 2 to 6 cm. Stereo angle between front and back strips is 15 deg, wchich can be achieve in two setups: one side of the detector has its strips oriented vetricaly, while the other has strips under 15 deg, or strips are rotated by ±7.5 deg on the front and back planes. The arrangement of sensors and modules in a single station is presented in figure below.


STS Performance

Tracking performed with two stations of hybrid pixel detectors (discs, no detailed internal structure yet) and four micro-strip stations of the "vertical" layout.

  • Track reconstruction efficiency
    • as function of the track momentum
    • Track finder: Cellular Automaton and Kalman Filter
    • vertex position included
    • tracks with hits in >3 consecutive stations
    • central URQMD events, Au+Au 25~GeV/nucleon

Track reconstruction efficiency.

  • Momentum resolution
    • as function of a simplified effective silicon detector thickness, taking into account sensors + support/cables etc
    • averaged over all momenta

Momentum resolution

MVD station layout

The MAPS based detector stations have no detailed layout yet. In the simulation, they are treated as silicon discs segmented into a pixel grid.

MVD Performance together with the STS

  • Spatial resolution of the reconstructed primary and $ D^0 $ decay vertices:

MVD primary and D0 decay vertex resolution.

  • Mass resolution of $ D^0 $ decays with the STS-MVD:

Mass resolution of Do decays with the STS-MVD

  • $ D^0 $ reconstruction in the presence of background.:

D0 reconstruction in the presence of background.

Published articles on the STS/MVD

Follow this link: PublicStsPublishedArticles.

STS/MVD workgroup pages (login required)

Follow these links:

Topic revision: r10 - 16 Feb 2009, AnnaKotynia
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