The document summarizes a proposed experiment called TREK at the J-PARC facility in Japan. The experiment aims to search for violations of time reversal symmetry by measuring the transverse muon polarization (PT) in stopped kaon (K+) decays with high precision (10-4). An active polarimeter will be used to stop muons from K+ decays and detect the asymmetry in positron emissions to extract the T-violating parameter PT, which is predicted to be very small in the Standard Model but could indicate new physics if measured to be non-zero. The upgraded TREK detector design, including an active scintillating fiber target, electromagnetic calorimeter, tracking chambers, and active muon polarimeter, is described

1. Search for non-standard model physics using the transverse muon
polarization in stopped K+ decay
S. Bianchin and TREK Collaboration
Citation: AIP Conf. Proc. 1441, 567 (2012); doi: 10.1063/1.3700619
View online: http://dx.doi.org/10.1063/1.3700619
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2. Search for Non-standard Model Physics Using the Transverse
Muon Polarization in Stopped K+ Decay
S. Bianchin for the TREK Collaboration
University of British Columbia, Vancouver, BC V6T 1Z1, Canada
Abstract. The J-PARC TREK Collaboration proposes an experiment dedicated to the search for violation of time reversal
invariance (T ) in the K + → π 0 µ + ν decay. The transverse muon polarization (PT ) in stopped kaon decay will be measured
with high precision (10−4 ). An active polarimeter will be used to stop muons and to detect decay positrons asymmetry at the
same time.
Keywords: kaon, transverse muon polarization, symmetry
PACS: 11.30.Er, 12.60.Fr, 13.20.Eb
THE TREK EXPERIMENT AT J-PARC
+
The transverse muon polarization (PT ) in the K + → π 0 µ + ν (Kµ3 ) decay is the polarization component normal to the
decay plane and a T-odd observable defined by the correlation of the π 0 and µ + momentum vectors pπ 0 and pµ +
and the µ + spin σµ , as PT = σµ · (pπ 0 × pµ + )/|pπ 0 × pµ + |. A non-zero value of PT would be a clear evidence for
violation of time reversal invariance (T ) [1], since the spurious effects from final state interactions are very small [2].
Moreover, PT is almost vanishing (∼ 10−7 ) in the Standard Model (SM) with the Kobayashi-Maskawa scheme [3] and
is, therefore, a very sensitive probe of CP violation mechanisms and new physics beyond the SM. Different models
[4] such as those with multi-Higgs doublets or leptoquarks, and also some SUSY models allow a value of PT as large
as 10−3 ∼ 10−5 .
The most recent research on PT has been performed at KEK. The E246 experiment results were consistent with no
T violation but provided the world best limit of PT = −0.0017 ± 0.0023(stat) ± 0.0011(syst) [5] and constrained
the parameter spaces of several contender models. These results were, however, statistics limited, mainly due to
insufficient accelerator beam intensities despite the smaller systematics errors.
The TREK experiment intends to continue the PT experiment further at the J-PARC facility where higher accelerator
beam intensity will be available and a higher experimental sensitivity is expected. The J-PARC TREK experiment
should improve the sensitivity by at least a factor 20 and reach δ PT ∼ 10−4 by using a K + beam with an intensity of
2.1 × 106 kaons per second and analyzing ∼ 2.4 × 109 Kµ3 decays [6].
In order to observe a very small value of PT in the presence of large in-plane polarization components, a double ratio
measurement will be performed (as was the case in the E246 experiment) by using the toroidal spectometer setup and
by stopping the K + beam in the newly developed active scintillating fiber target. The T-violating positron asymmetry
AT will then be extracted as the difference of the azimutal asymmetry between the cases where a π 0 is emitted in
the forward direction along the beam axis and those where the π 0 is emitted in the backward direction, because the
differential PT has the opposite sign. AT can be expressed as:
AT = (A f wd − Abwd )/2 (1)
where the f wd and bwd asymmetries are calculated using the "clockwise" (cw) and "counter-clockwise" (ccw) positron
emission rates N cw and N ccw (Fig. 1)
N cw ccw
f wd(bwd) − N f wd(bwd)
A f wd(bwd) = (2)
N cw
f wd(bwd)
+ N ccw
f wd(bwd)
19th Particles and Nuclei International Conference (PANIC11)
AIP Conf. Proc. 1441, 567-569 (2012); doi: 10.1063/1.3700619
© 2012 American Institute of Physics 978-0-7354-1035-0/$30.00
567
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3. FIGURE 1. Cross section side view (left) and end view (right) of the TREK experimental setup at J-PARC. The momentum
vectors of charged particles and photons are determined by the toroidal spectrometer and the CsI(Tl) calorimeter, respectively.
The transverse muon polarization PT can finally be extracted by dividing AT by the analyzing power α and the
kinematical attenuation factor of the measurement and expressed as:
PT = AT /α < cosθT > (3)
THE TREK DETECTOR
The proposed TREK detector is an upgraded version of the experimental setup used in the KEK E246 experiment and
consists of a superconducting toroidal magnet, an active kaon stopping target, a CsI(Tl) electromagnetic calorimeter,
a charge particle tracking system which identifies the Kµ3 decays and an active muon polarimeter which measures
precisely the transverse component of the muon polarization.
The superconducting toroidal magnet has 12 identical sections and provides a magnetic field up to 1.8 T in the center
of each gap (Fig. 1). For the lower excitation energies used in the TREK experiment, the magnetic field is nearly
dipolar. The magnet was assembled with high precision to ensure a 12-fold rotational symmetry.
Incident positive kaons will be stopped in the newly developed active target which consists of a bundle of
3 × 3 × 200 mm3 rectangular scintillating fibers. A decay vertex will be determined from the energy deposit distribu-
tion of a stopping kaon and by the tracking of decay muons. Spatial information will be provided as well.
Each fiber will be read by a Geiger mode avalanche photodiode with high internal gain (Hamamatsu MPPC) through a
WLS optical light guide fiber of about 140 cm in length. Moreover, the bundle of fibers will be surrounded by trigger
counters with relatively high light output.
The already existing E246 electromagnetic calorimeter comprized 768 CsI(Tl) modules forming a tower structure
with a large solid angle coverage of about 75% of 4π. 12 so-called "muon holes" have been implemented in addition
to the beam entrance and exit holes to allow the charged particles to enter the spectrometer gaps.
The PIN photodiode readout employed in the E246 experiment will be replaced by an avalance photodiode (APD)
readout system in order to increase the rate performance. The output wave form from the preamplifiers will be read
568
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4. FIGURE 2. Schematics of the tracking system in the E246 setup (left) and the proposed setup for the J-PARC experiment (right).
by Flash-ADCs resolving pile-up signals, if any.
The E246 tracking system will be upgraded substantially in order to strengthen the suppression capability of the Kπ2
background. Two new chambers (C0 and C1) will be added in each spectrometer gap. These chambers will be made
of Gas Electron Multipliers (GEM) counters to meet the requirements for the higher event rate at the J-PARC facility
and ensure stable operation. The innermost chamber (C0) will be cylindrical in shape. The C2, C3 and C4 E246
chambers will be used with an increase of the distance between C3 and C4. Moreover, in order to improve the tracking
resolution, the spectrometer gaps will be filled with He gas (Fig. 2).
The transverse muon polarization will be measured in an active muon polarimeter. Two differents types of polarime-
ters are currently being investigated. The first one consists of muon stopper plates and gap drift chambers, whereas the
second one is made of muon tubes. Incoming muons will be stopped with a probability of about 85%. Decay positrons
will then be detected and the emission direction will be determined. The muon energy will be roughly determined by
the number of stopper plates or muon tubes which have been traversed. In this way, a large solid angle for positron
detection and a high analyzing power will be assured. Moreover, since the decay vertex is localized for each event,
the background will be drastically reduced [6].
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4. G. Bélanger, C.Q. Geng, Phys. Rev. D 44 (1991) 2789;
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6. J-PARC E06 (TREK) proposal;
http://j-parc.jp/NuclPart/Proposal_e.html
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