T2K
The Warwick T2K Group
Academics: Prof Gary Barker, Dr Steve Boyd, Dr Xianguo Lu
Postdoctoral Fellows: Dr David Hadley
Students: Ms Katharina Lachner, Mr Nicholas Latham, Mr Weijun Li, Mr Komninos-John Plows
The Project
T2K measures the oscillation parameters of muon neutrinos by using a near and far detector positioned in the J-PARC muon neutrino beam. The near detector also has an independent physics programme e.g. measuring neutrino interaction cross sections.
The Beam
The J-PARC accelerator chain begins with two Linacs feeding protons into a 3GeV synchrotron. These are sent either to a neutron and muon facility, for materials and life sciences, or passed on to be accelerated further in the 30GeV main ring. In addition to a variety of hadronic experiments, it is this ring which is used for generating the T2K neutrino beam.
With a design power of 0.75 MW, the proton beam is directed onto a graphite target. Collisions in the target generate pions and kaons which are then selected and focused by three magnetic horns with fields up to 2.1T and directed down a 98m decay pipe in which they decay to produce a beam of predominantly muon neutrinos (there is a small background of electron neutrinos from kaon decays and secondary decays of muons ~4%). At the end of the decay pipe is the muon monitor: a 7x7 grid of ionisation chambers, which give the experiment a handle on the beam production rate.
T2K is the world's first "off-axis" neutrino experiment, with the near and far detectors both placed 2.5° off the beam direction. Placing the detectors off-axis means that they only see neutrinos with a much reduced spread in energy which reduces potential backgrounds in the far detector.
The Near Detectors
Downstream from the target (280m) lie T2K's two near detectors: INGRID and ND280.
ND280
ND280 is a hybrid detector, responsible both for measuring the initial neutrino flux from the beam and for making neutrino-nucleon cross-section measurements (essential for predicting the expected flux at the far detector).
The central part of the detector, the "basket", contains multiple tracking modules. The most upstream of these is the Pi-Zero Detector (P0D): X-Y layers of triangular scintillator bars are separated by either lead sheets or target volumes. These target volumes can be filled/emptied with water to provide a target comparable to that at the far detector enabling cross-sections to determine the interaction rate at the far detector. The cross-section for neutral pion production in water is of particular importance to the experiment since this represents the largest background for νe appearance at Super-K. With this measurement in mind, the X and Y scintillator planes are separated by a lead foil to encourage photon conversion.
Downstream of the P0D is the "tracker": three Time Projection Chambers (TPCs) separated by two Fine Grained Detectors (FGDs). The TPC makes high-resolution momentum measurements from curvature in the magnetic field of particles produced in the FGDs which are scintillator-only detectors for good position resolution. The first FGD uses the carbon scintillator as its target mass while the second FGD contains six water targets. Comparison of the interaction rates between the two FGDs enables the (predominantly scintillator) ND280 measurements to predict the (water) interaction rate at Super-K.
The most downstream module in the basket is the Downstream Electromagnetic Calorimeter (ECal). Its primary responsibility is to catch high-momentum tracks escaping the tracker to reconstruct their energy/momentum.
The basket is entirely surrounded by the Barrel (around the tracker and downstream ECal) and P0D ECals, each of which is composed of six (two side, two top and two bottom) modules. Though the ECal detectors are similar in many respects there are some differences in construction. The Barrel ECal will assist in the reconstruction of tracks escaping the tracker at high angles, while the P0D ECal, which is too small to reconstruct tracks/showers, will catch escaping pi-zeros which have not converted in the P0D and veto incoming particles. Construction of all six P0D ECal modules took place here, at the University of Warwick.
Finally, gaps in the magnet are instrumented with large scintillator "paddles".The Side Muon Ranging Detector (SMRD) tags outgoing muons as well as vetoing incoming cosmic rays and particles from rock-neutrino interactions.
INGRID
INGRID is a beam monitoring and profiling detector consisting of seven vertical and seven horizontal modules which combined form a cross centred on the beam axis. Each module is constructed of layers of iron and scintillators, which are read out using the same fibres, photosensors and electronics as the ND280 ECals. It is placed upstream of the ND280.
The Far Detector
Super-Kamiokande (Super-K) is probably the most famous experiment in the history of neutrino physics. Originally built to search for proton decay, it played a significant role in the discovery and study of both solar and atmospheric neutrino oscillations. It has also served as the far-detector for the K2K oscillation experiment prior to being part of T2K.
Super-K is sited near Kamioka, around 300km west of the T2K beamline at J-PARC, in a disused mine under Mount Kamiokako which acts to shield it from cosmic-ray muons. The experiment itself is a 50,000ton water-Cherenkov detector surrounded by over 10,000 photo-multiplier tubes (PMTs). By profiling the Cherenkov rings it can differentiate muon-neutrino and electron-neutrino-induced interactions and provides the vital directional information required to select neutrinos originating from T2K.
Physics Results
- In 2011 T2K was the first experiment to see an indication of electron neutrino appearance from a muon neutrino beam. This award-winning publication has now accrued well over 1000 citations. With the accumulation of more data, the statistical significance of this result increased first to the 3 σ and then past the 7 σ level, firmly establishing the non-zero value of θ13.
- By measuring the disappearance of muon neutrinos at the far detector T2K has set a World-best measurement of the mixing angle θ23 (the first time an accelerator experiment has achieved this):
- In 2020, T2K published the strongest constraint yet on the parameter that governs CP violation in the neutrino sector; a parameter called δcp (delta-CP). Using beams of muon neutrinos and muon antineutrinos, T2K studied how these particles and antiparticles transitioned into electron neutrinos and electron antineutrinos, respectively. The T2K data was shown to disfavour almost half of the possible values δcp could take at the 3σ confident level. This was reported in the April 2020 edition of Nature (Nature 580, 339-344 (2020)), and was selected by Nature as one of the 10 most remarkable discoveries of 2020.
Near Detector Upgrade: SuperFGD
The near detector upgrade is designed to bring down the systematic uncertainty level of the δcp measurement from about 6% to 4% by increasing angular acceptance and providing better reconstruction. As the current Fine-Grained Detectors (FGDs) have a bar structure of plastic scintillators aligned in either x or y directions, essentially only forward-going tracks could be reconstructed well and accepted. An example of a neutrino interaction in the current ND280 is shown below.
In the upgrade, the current Pi0 detector is replaced by a new granular plastic scintillator, the Super Fine-Grained Detector (SuperFGD), and two high-angle time projection chambers (HA-TPCs) as shown below. Additional scintillator planes are placed surrounding the SuperFGD and HA-TPCs for timing measurements at sub-nanosecond precision. This design significantly improves angular acceptance for large-angle and backwards-going tracks.
SuperFGD has a granular structure, with a dimension of 192×184×56 cm3 and it consists of around 2 million optically independent plastic scintillator cubes, each has a size of 1×1×1 cm3. Each cube has 3 holes in the x, y and z directions for the insertion of wavelength shifting (WLS) fibres. Hence, SuperFGD could provide projection information onto 3 orthogonal planes and thus could offer much better reconstruction to study neutrino interactions compared to the existing FGDs. Moreover, the small size of the cube brings excellent track resolution, thereby lowering the reconstruction threshold and offering a probe into previously inaccessible kinematic phase space of the neutrino interaction products.
