The T2K Experiment - From Tokai To Kamioka - Where is the anti-matter?

From the beginning of 2010, the T2K experiment will fire a beam of muon-neutrinos from Tokai on Japan's east coast, 300km accross the country to a detector at Kamioka. It hopes to investigate the phenomenon of "neutrino oscillations" by looking for "muon neutrinos" oscillating into "electron neutrinos".  A million pound detector has been built at the University of Warwick as part of a vital experiment to investigate fundamental particles - neutrinos.

The experiment aims to measure neutrinos at the start of their journey and then again at the end 300 kilometres away to see how they've changed. Understanding neutrinos will tell us more about the physics of the universe and help explain why the universe is made of matter rather than anti-matter.  Associate Professor, Dr Gary Barker said, "It's thought that in the Big Bang that created the universe, matter and anti-matter were created in equal amounts, but it's clear that everything we observe today is only consisting of matter, so the question is where has the anti-matter gone?" 50 trillion neutrinos from the sun pass through us every single second, but as we do not notice these they are hard for scientists to detect. The T2K experiment generates its own beam of neutrinos rather than relying on studying those from the sun.

The detector built at Warwick as been installed in J-PARC on Japan's east coast. Here, scientists accelerate protons into a target and use them to produce a beam of neutrinos. They fire a beam of muon-neutrinos through the Warwick detector aiming at a second detector 300km away at another laboratory called Super-KAMIOKANDE at Kamioka and measure its behaviour. The experiment hopes to investigate the phenomenon of "neutrino oscillations" by looking for "muon neutrinos" oscillating into "electron neutrinos".

Work at Warwick

There are 62 institutes across 12 different countries contributing to the T2K experiment. Within this broad collaboration Warwick has made some important and significant contributions towards construction, quality assurance, calibration hardware and software analysis.

Warwick's construction responsibilities include the testing of photosensors and optical fibres for the entire ND280 detector and construction of all 6 modules of the P0D ECal. Warwick are also important contributors throughout the software framework including the calibration, tracking and particle ID packages.

On the Eastern coast of Japan, the national accelerator complex J-PARC accelerates a 30GeV proton beam onto a graphite target, generating mesons which decay to produce a beam of muon neutrinos. 280m downstream lies the near-detector ND280. Its task is to measure the initial beam flux and to make critical measurements of neutrino interaction cross-sections.

The T2K far detector is located 295km west of J-PARC near Kamioka: Super-Kamiokande is a 50kton water-Cherenkov detector which will measure the muon and electron neutrino fluxes after oscillation. Both detectors are situated 2.5° off the beam-axis to give a sharper energy spectrum than conventional on-axis beams.
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The Near Detectors

Downstream from the target (280m) lie T2K's two near detectors: INGRID and ND280.

ND280

ND280 is a hybrid detector, situated off the beam axis, which will be responsible both for measuring the initial neutrino flux from the beam and for making neutrino-nucleon cross-section measurments (essential for predicting the expected flux at the far detector). It is built within the magent originally used in the famous *UA1 experiment.

*The UA1 high energy physics experiment  ran at CERN  from 1981 until 1993 on the SPS collider. The discovery of the W and Z bosons by this experiment and UA2  in 1982 led to the Nobel Prize for physics being awarded to Carlo Rubbia and Simon van der Meer in 1984. It was named as the first experiment in the CERN "Underground Area", i.e. located underground outside of the two main CERN sites at an interaction point on the underground SPS accelerator which was modified at the same time to convert it into a collider.

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INGRID

INGRID is beam monitoring and profiling detector which consists 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 scintillator, which are read out using the same fibres, photosensors and electronics as the ND280 ECals. It is placed upstream of the ND280.

Neutrinos

The current theory of particle physics, known as the Standard Model, includes 12 particles which make up matter (the physical contents of the Universe). Of these 6 are quarks: they combine to form particles such as protons and neutrons, which in turn make up the atomic nuclei found in the periodic table. Then there are the 3 leptons: a family containing the electron, which amongst other things is responsible for electricity, and two identical but heavier particles known as the muon (μ) and tau (τ). Together, leptons and quarks form all the matter which we see around us.

The last 3 particles in the Standard Model are not often encountered in every day life: the neutrinos.

Although they are the least understood particles in the Standard Model, neutrinos are the most numerous particles in the Universe: around 50 trillion neutrinos from the Sun pass through your body every second, and many more come from elsewhere. Why then do we not notice them?

Neutrinos can only interact with other matter via the "Weak nuclear" force, the most feeble of forces in the Standard Model. Such weak interactions mean that the probability that they will interact with nearby particles is extremely low. So rare are their interactions, that the average neutrino could pass through over 10 lightyears of lead without ever noticing.

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Detection

You may be wondering how, if neutrinos hardly interact with matter, we can see them in particle physics experiments.

The three neutrinos are the electron neutrino (νe), muon neutrino (νμ) and tau neutrino (ντ). You may notice that they are named after the three leptons mentioned earlier. This is because, in the rare event that a neutrino does interact with matter it will produce the lepton with which it is associated. For example, if an electron neutrino were to interact with a proton, an electron would be produced.



This is how we tell that neutrinos are travelling through our detector: we look not for the neutrinos but for the particles resulting from their interactions, and we can identify which type of neutrino interacted by identifying the particle produced. Of course, we need a lot of neutrinos if we hope to see even a few interactions!

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Neutrino Oscillations

Neutrinos were first postulated by Wolfgang Pauli in 1930 and first detected in 1956 by Reines and Cowan (a feat for which they received the 1995 Nobel Prize). However it was in the late 1960's that they became the focus of serious research. Theorists modelling how nuclear fusion worked in the Sun had made very specific predictions about the number of electron neutrinos being produced, but experiments designed to measure those neutrinos consistently found around a third of the number they were expecting. Likewise, measurements of muon neutrinos, from cosmic ray collisions in the upper atmosphere, also showed a reduction in the number compared with that predicted.

Eventually, experiments were able to explain these mysterious disappearances in terms of "neutrino oscillations" - the oscillation from one neutrino type to another. The electron neutrinos from the Sun were oscillating into muon and tau neutrinos (which the solar experiments could not measure), and atmospheric muon neutrinos were oscillating into tau neutrinos. Further experiments measured this phenomenon more accurately using better detectors and by generating neutrinos artificially in neutrino beams.

Solar neutrinos:                 νe → νμ , ντ
Atmospheric neutrinos:     νμ → ντ
T2K:                                   νμ → νe

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T2K

T2K is the first of the "next generation" neutrino oscillation experiments designed to look for this transition: it features a high powered muon neutrino beam with detectors placed off the beam axis to give a cleaner energy spectrum. It will for the first time measure muon to electron neutrino oscillations while also improving the precision of previously measured paramaters.

The experiment begins with the generation a beam of muon neutrinos at the J-PARC accelerator complex on the Eastern coast of Japan. Immediately downstream is the near-detector (ND280) which will enable a measurement of the initial beam. Construction of ND280 and analysis of its data is the primary contribution of the UK in T2K. The beam then travels almost 300km through Japan during which time it is hoped that some of the muon neutrinos will oscillate into electron neutrinos, the number of which will be measured by Super-Kamiokande (Super-K).

Super-K is a cylindrical tank, over 40m tall, which contains 50kT of ultra-pure water. It is located 1000m underground in a disused mine and can identify both electron and muon neutrino interactions within the water. The experiment will then compare the data from SuperK with that in ND280 to measure, at high precision, neutrino oscillation paramaters which have never been observed before!