TREK detector and method of experiment



Overview

The TREK detector is constructed by upgrading the E246 detector which has been used at the KEK-PS. It consists of a Superconducting Toroidal Mgnenet, an active kaon stopping target, a CsI(Tl) electromagnetic calorimeter, a charged particle tracking system, which identify the Kμ3 decays, and an active muon polarimeter which measures prcisely the transverse compomnent of muon polarization. The Toroidal Magnet has 12 identical gaps. Muons from the Kμ3 decays are momentum-analysed by the magnet and the tracking chambers and stopped in the active polarimeter. Two photons from π0 decay in the Kμ3 decay are detected by the CsI(Tl) calorimeter. The kinematics of the Kμ3 is uniquely determined by the measurements of μ+ and π0. The transverse polarization of muon is determined by observing the azimuthal asymmetry of μ+ decay positrons. The principle of the experiment is shown below.

TREK detector


Principle of the experiment

In order to observe a very small value of PT in the presence of large in-plane polarization components, a double ratio measurement is performed. The T-violating positron asymmetry AT is extracted as a difference of the azimuthal asymmetry between the cases where the π0 is emitted in the forward direction along the beam axis, namely the toroid axis, and the π0 in the backward direction, because the diffrential PT has the opposite signs there. The positon azimuthal asymmetry is determined as the emission ratio difference between clock-wise direction and counter-clock-wise direction. PT can be further extracted from AT by dividing it by the analyzing power α and the kinematical attenuation factor of the measurement. This π0 forward/backward analysis scheme can eliminate almost all the systematic errors when combined the 12-fold rotational symmetry of the spectrometer.

Principle of the experiment


Active polarimeter

Transverse muon polarization is measured in the active muon polarimeter which consists of muon stopper plates and gap drift chambers. Incoming muons are stopped in the stopper plates with a probability more than 85%. Decay positrons are detected with gap drift chambers and the emission direction is determined. Enegy is also roughly determined by the number of penetrating stopper plates. In this way, a large solid angle for positron detection and a high analyzing power of polarization are assured. Since the decay vertex is localozed for each event the backgraound is drastically suppressed. In order to hold the transverse compoment of the polarization and ensure the polarization alignment to the detector system, the active polarimeter is put in a high precision magnetic field.

Active polarimeter


Active target

Incident positive kaons are stopped in the active target which is a bundle of rectangular scintillating fibers. A decay vertex is determined from the energy deposit distribution of a stopping kaon and the tracking of decay muons. The target fibers provide also spatial information. Each fiber will be read by a Geiger-mode avalanche photo-diode with high internal gain (Hamamatsu MPPC) though optical light guide fibers of several 10 cm. The MPPC's locate off beam axis and stand against the beam irradiation. The bundle of the fibers is surrounded by trigger counters with relatively high light outputs.

Active target


Tracking system

The E246 tracking system will be upgraded substantially in order to strengthen the suppression capability of the Kπ2 background. In each spectrometer gap, two new chambers are added. They are made of gas electron multipliers (GEM) to meet the requirements from higher event rates at J-PARC and to obtain stable operation. The innermost C1 chamber will be a cylindrical one. The C2,C3 and C4 chambers of E246 will be used with a modification of the C3-C4 ditance. In order to improve the tracking resolution, spectrometer gaps are filled with He gas.

Tracking system


CsI(Tl) calorimeter

The electromagnetic calorimeter comprizes 768 CsI(Tl) modules forming a tower structure with a large solid angle coverage of about 3/4 of 4π. For chaged particles entering the spectrometer gaps there 12 holes in addition to the beam entrance and exit. The PIN photodiode readout employed in E246 will be replaced by avalanche photodiode (APD) readout to increase the rate performance. The output wave form from the preamplifiers is read by an FADC, which resolves also pile-up signal, if any.

CsI(Tl) calorimeter


Superconducting Toroidal Magnet

The Supercnducting Toroidal Magnet provides a high magnetic field up to 1.8 T at the magnet center. For the lower excitation used in the TREK experiment the field is nearly dipolar. The magnet was assembled with a very high precision to ensure the 12-fold roratinal symmetry. The coils are cooled indirectly with a two-phase flow of He supplied by an online refrigerator.

Superconducting Toroidal Magnet

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