T-438 Objectives

The prototype electromagnetic calorimeter and shower-maximum detector were constructed to test the conceptual design for the endcap electromagnetic calorimeter (EEMC) for the STAR detector at Brookhaven National Laboratory (BNL).

The STAR detector will primarily be used to search for evidence of the formation of a quark-gluon plasma in the collisions of very high energy heavy ions at the Relativistic Heavy Ion Collider (RHIC) at BNL. The RHIC rings will also be capable of accelerating and then colliding polarized proton beams, up to a maximum CM energy of 500 GeV. Polarized proton collisions will be studied at RHIC at high luminosities (0.2 nb^-1 s^-1).

The STAR EEMC has been primarily designed for use in polarized proton collisions. It will detect direct photons, gaining exquisite sensitivity to the degree to which the gluons within the proton are polarized. The EEMC will also be used to detect positrons and electrons at forward angles, arising from the decay of W^± bosons produced in polarized proton collisions. Parity-violating spin asymmetries in W^± production provides important new information about the polarization of the quark-antiquark sea within the proton. For both measurements, the addition of the EEMC to STAR allows the study of asymmetric parton collisions.

To successfully carry out this physics program, the EEMC must be capable of:

• distinguishing single direct photons from pairs of photons produced from the decay of neutral mesons, prolifically produced in pp collisions. The requirement is that at least 80% of the neutral meson background is suppressed by a ‘shower shape analysis’ of the information from the fine granularity shower maximum detector (SMD).

• distinguishing electrons and positrons arising from W^± decay from energetic charged hadrons. Information from the preshower layers of the EEMC, the SMD and the difference between the transverse energy measured by the EEMC and the transverse momentum measured by the STAR time projection chamber must be capable of providing more than an order of magnitude suppression of the charged hadron background.

• The EEMC response must be linear over a broad dynamic range, including ~0.3 GeV photons, up to ~150 GeV electrons.

The prototype EEMC and SMD were constructed to test the applicability of the ‘shower shape analysis’ for discrimination between single photons and photon pairs arising from the decay of energetic neutral mesons. A second objective was to study the linearity of the calorimeter.

Send comments to:

L.C. Bland (IUCF), for the T-438 collaboration

Last revised: 31 October 1999