C O M P A S S

Common Muon and Proton Apparatus for Structure and Spectroscopy

Purpose:

The COMPASS experiment is a new fixed-target experiment planned to start at the CERN SPS by the year 2000. The apparatus is a new two-stage magnetic spectrometer with particle identification and calorimetry.
The physics goals are to investigate hadron structure and hadron spectroscopy by using muon or hadron beams.

Physics with the Muon beam:

The main goal of COMPASS is to measure the parton spin distribution functions, in particular the contribution of gluons to the nucleon spin.
This can be achieved by means of the photon-gluon-fusion. A rather clean signature for photon-gluon-fusion is the production of a charm-anticharm-pair. The experiment is therefore designed to efficiently reconstruct D⁰ mesons, the most abundantly produced charm mesons.

In addition, strange sea-quarks may contribute to the nucleon spin. This is investigated by reconstructing the polarisation of Lamdas produced in deep inelastic scattering.
The transverse and longitudinal spin distribution function will be measuered as well.

Physics with the Hadron beam:

The Primakoff reaction, h A -> h gamma A, where h is Pi or K, will be used to measure electric and magnetic polarisabilities of the incoming hadrons predicted by chiral perturbation theory. The production of vector mesons and possibly of exotic non-qqbar states by a similar process (exchange of a virtual photon between and h and A) will be investigated, as well.
In central production and diffractive dissociation, using proton, pi or kaon beams on a proton target, glueballs and other non-qqbar states are expected to be abundantly produced. In particular, the production and decay of f₀ mesons in the mass range of 1 - 2 GeV will be studied, where the lightest scalar glueball is located according to QCD predictions, and of tensor mesons around 2 GeV (QCD prediction for the tensor glueball).
Goals of the extensive charm program are to study the influence of strong interaction in semileptonic decays and to look for doubly charmed baryons never observed so far.

The Experimental Setup:

The COMPASS apparatus is a two stage magnetic spectrometer to obtain a good momentum resolution from 1 GeV up to 150 GeV at high rates.
Both stages will be equipped with RICH detectors for identification of charged particles, electromagnetic and hadronic calorimeters and muon filters. Tracking detectors will be multistaged using silicon microstrips, gaseous strips and straw tubes. Scintillating fibre detectors will be used as fast beam detectors for timing and triggering.

Munich University (LMU) is coordinating the construction of the Large Angle Tracking Station LAT-TS2 (marked in yellow) between SM1 and RICH1 .