1. Introduction
The Standard Model (SM) has successfully explained the results of most particle and nuclear physics experiments [1]. However, it is believed to be just an effective low-energy description of fundamental interactions. The existence of dark matter and dark energy is completely beyond the scope of the SM, even though they make up most of the Universe. The fact that neutrinos have mass also cannot be explained by the SM. Therefore, the search for physics beyond the SM plays an important role in exploring the unknown areas of physics. The E36 experiment aimed to search for new physics beyond the SM mainly in terms of lepton universality violation. Furthermore, it also searches for dark photons possibly related to dark matter.
Recently the LHCb collaboration has made a measurement of the ratio of the branching ratio of the
Materials, methods and results
1.1. Kl2 decay and lepton Universality
Lepton universality is an assumption of the SM. Any violation of lepton universality is clearly a sign of new physics beyond the SM. Lepton universality states that the weak couplings of leptons are identical. A group of useful decays is the leptonic decays of psudoscalar mesons. In the kaon sector, the decay width of
In E36, the decay width ratio of
is measured.
Here
Any deviation of
The Minimal Supersymmetric Standard Model (MSSM), which is the minimal extension to the SM that realizes supersymmetry with R parity is one candidate for new physics to be tested by
The proton radius seems a completely different topic from lepton universality. However, the measurements of the proton radius used two leptons: muon and electron. The discrepancy between muonic and electronic measurements is 7 standard deviations [10, 11, 12, 13, 14]. Possible explanations are: violation of
1.2. Search for dark photons
A dark photon is a U(1) extension of the SM in the ``dark'' sector. The ``dark'' sector means that the particles do not interact with charges. However, the dark photon can mix with the normal photon with a parameter
1.3. Experimental status of
Recently, the KLOE and NA62 groups have published their results of
This result quantitively agrees with the SM prediction. The NA62 experiment at CERN collected a large sample of decays during a dedicated run in 2007-8 and their final goal was a measurement of
2. Experiment setup and status
A time-reversal experiment with kaons (TREK) was the original motivation for forming the collaboration. E06 [18] will be carried out at the extended Hadron Facility at the Japan Proton Accelerator Research Complex (J-PARC) in Japan. The E36 [19] experiment used many of the detector components proposed for E06, which are basically the E246 detectors [20, 21, 22]. The distinctive feature of the E36 experiment is utilizing a stopped kaon beam, which has quite different systematics from the previous in-flight experiments and is complementary to them.
The E36 experiment was carried out in the Hadron Hall at J-PARC in 2015. The area became available for installation in mid-November 2014 and the installation was completed in April 2015. The commission and test runs were performed in June. The commissioning data quality and the detector condition were checked during the summer. Several improvements were implemented including a trigger adjustment, and a new TTC (see Figure 2) to improve the trigger efficiency. The production run was performed during October to December. The data acquisition (DAQ) was very stable during the whole experiment data-taking period with 10% dead-time at a trigger rate of 250 Hz [23].
2.1. J-PARC
J-PARC is an experimental facility with a set of high-intensity proton accelerators and several experimental sites utilizing the proton beam. It is open to users from around the world and located 150 km northeast of Tokyo. There are three proton accelerators: a 400 MeV linear accelerator, a 3 GeV rapid-cycling synchrotron (RCS) and a main ring synchrotron (MR) with 50 GeV (currently 30 GeV). There are three experimental facilities using the proton beam: the materials and life science experimental facility (MLF), the hadron experimental facility, and the T2K experimental facility. Secondary beams are produced in the hadron facility and used for different experiments including E36.
2.2. Beam line
A secondary
2.3. Overview of detector
A schematic side view of the detector is shown in Figure 2. A Fitch Cherenkov counter is used to identify kaons and pions in the beam. The beam is slowed down in a BeO degrader. An active target stops the beam and measures the decay vertex with the help of the spiral fiber tracker (SFT). An aerogel Cherenkov counter (AC), a Pb-glass counter (PGC) and a time-of-flight system (TOF1 and TOF2) identify electrons and muons. The spectrometer is composed of a superconducting toroidal magnet with 12 gaps, one multi-wire proportional chamber (MWPC) before each gap (C2) and two MWPCs rotated at 90 degrees after the gap (C3, C4). Radiative photons are detected by the CsI(Tl) calorimeter. A thin 3 mm trigger counter (TTC) was added after summer 2015. This increased the trigger purity with no hit position dependence.
2.4. Central detector
The active target downstream of the Fitch Cherenkov counter was a scintillator bundle made of 256 pieces of
2.5. Spectrometer and particle identification
The central component of the spectrometer is a superconducting toroidal magnet with 12 iron sectors separated by 12 gaps. The target was placed in the center of the sectors. The superconducting magnet can generate a magnetic field up to 1.8 T. A field of 1.5 T was used for the production runs.
To distinguish
In the two-independent analysis scheme, the method of Kalman Filter is also being tried and tuned. Preliminary results showed the successful reconstruction of
2.6. The CsI(Tl) calorimeter
The photon calorimeter was used to detect the radiative photons, for determining the disturbing structure dependent term, for the
3. Conclusion
The E36 collaboration has successfully completed data taking. Enough production, calibration and normalization data have been collected. The data quality has been checked with semi-online analysis. Off-line analysis is now underway. Offline calibration methods are being developed and implemented. Independent and collaborative efforts are distributed onto different tasks.