Soudan2 Result on Proton Decay
References
Soudan2 Results on Atmospheric Neutrinos
High energy cosmic rays hitting air molecules at the top of the atmosphere produce large numbers pi-mesons. The conservation of lepton number requires that decay products of each meson include two muon-type neutrinos and one electron type neutrino. After correction for the detector sensitivity and other minor effects the result of measurements on these neutrinos can be compared with expectation in the form of a ratio. Two earlier experiments, IMB and Kamioka, reported a ratio of 0.6 instead of 1.0. The errors were small but the experiments used the same method, a water cherenkov detector. Recent data from SuperKamioka supports this result. In Jan 1997 Soudan2 published a preliminary result [1] based on half their existing data of 0.72+-0.19(+0.05-0.07) using a completely different detector, an iron calorimeter [3,4]. A more recent result [2] announced at the June 97 Capri Meeting is 0.60+-0.14(+0.05- 0.07) based on 3.2 kton years. A new and completely independent analysis of the data from 26/8/91 to 22/12/96 yields R=0.60+-0.14(stat)+-0.05 (see [5] Stassinakis thesis text on this website). In this analysis the zenith angle distribution is consistent with flat and delta m-squared > 5×10**-3 eV**2 at 90% confidence. Further data is being collected.
References
1. "Measurement of the atmospheric neutrino flavour composition in Soudan 2," W.W.M. Allison et al. Physics Letters B391(1997)491.
2. " " H R Gallagher, Proc Capri...
3. "The Soudan 2 detector. The operation and performance of the tracking calorimeter modules," W.W.M. Allison et al. Nuclear Instruments and Methods in Physics Research A381(1996)385-397.
4. "The Soudan 2 detector. The design and construction of the tracking calorimeter modules" W.W.M. Allison et al., Nuclear Instruments and Methods in Physics Research A376(1996)36-48.
5. "A Study of the Atmospheric Neutrino Flavour Content Using the Soudan2 Detector" A. Stassinakis, D Phil thesis (Univ of Oxford) 1998
Background Information on Neutrino Oscillations
From Professor Wade Allison, University of Oxford
Introduction
The Question is "What is the Universe made of, and how does it work?". In the last twenty years a much fuller answer in the form of a picture of fundamental constituents known as the Standard Model has been established. The plot has unfolded step-by-step as in an Agatha Christie ‘who-dunnit’ and the cast of characters (the quarks, leptons and bosons) are increasingly well known to us.
However we are not yet at the end of the story. The Hercule Poirots of physics have serious questions on which to exercise their ‘little grey cells’. Some experiments have been finding unexpected results – and some of these anomalies are being confirmed by new experiments and further analysis. This report is about new data from an experiment, Soudan2, which provides such confirmation about the behaviour of neutrinos. An intensive programme is being mounted in this area including an ingenious experiment (SNO) whose construction is nearing completion and a breath-taking third experiment (MINOS) taking shape on the drawing board.
What are we trying to do? How should we go about doing it?
The crowning glory of 19th Century Physics was the marriage of Electricity and Magnetism – with the resulting applications from motors and electric lights, to radio and telephone. However, by the late 1890’s discrepancies were starting to appear – the discovery of radioactivity, of X-rays and of the electron were quite unexpected. These lead the way to the Physics of the 20th Century, that is Quantum Theory and Relativity, and thence to our understanding of the Atomic Nucleus and the Standard Model. This knowledge has spawned a multitude of developments from modern electronics and telecommunications, to new materials and approaches to the biological sciences.
One hundred years later in the closing years of the 1990’s how may we find the new discrepancies that will trigger the Physics of the 21st Century? One method is based on the idea that the next layer of reality is more tightly bound within matter than we have previously probed. This approach leads to the construction of more powerful accelerators such as the Large Hadron Collider, now approved for construction at CERN. A second method, complementary to the first, is based on the proposition that the New Physics may be discovered by doing very sensitive experiments, looking for very rare phenomena or very slow processes. This is the approach whose success is reported here. Such small effects may be very important. Gravity, which at a particle-by-particle level is the ‘weakest’ known force, shows us that ‘weak’ effects can be of outstanding importance in the Universe as a whole.
What is a sensitive experiment? As an example consider communication by radio with a space probe in a remote part of the solar system. In such an experiment we would record the rectified ‘beats’ between a carefully tuned oscillator in the radio receiver and the radio waves picked up from the distant spacecraft – that is what ‘tuning in’ to any radio station involves. And then listen! By analogy, can we use a similar fine-tuning technique in an experiment with quantum waves to ‘listen’ for breakdown of the Standard Model? The answer is ‘Yes’. Such an experiment was already been done as early as 1963 looking at the quantum beats between two particles, the two K-zero mesons which differ in mass by 7 parts in 1015. The results of this fine-tune experiment show anomalies which are still not fully understood today. We may ask whether there are any other pairs of particles whose masses are so finely tuned and whose interference might be observed.
Neutrinos
There is a triplet of particles, the three neutrinos, which, as far as we know, have equal masses and therefore are prime candidates for a ‘tuned’ experiment. In the Standard Model they are quite distinct and may not interfere. However, if they could and if they had very slightly different masses, then, as our earlier discussion suggests, studying their propagation would be a sensitive way to look for New Physics. The electron neutrino is the electrically neutral brother of the electron – and the muon neutrino is brother to the mu lepton, and the tau neutrino to the tau lepton. While the relationship within each pair of brothers is very well understood, the reason for the existence of three pairs is not understood at all. Neutrinos are not rare. A large fraction of the Universe consists of neutrinos. There are at least 500 million in every cubic metre (left over from the Big Bang). Being electrically neutral they are highly elusive and, for instance, can pass through the whole earth with only a small chance of being absorbed. We are quite oblivious of the flux of 6 1014 electron neutrinos per square metre from the Sun which pass through us each second – or which should do.
Trouble with neutrinos from the Sun
We know enough about the nuclear physics of the working of the Sun to be able to calculate with some confidence the flux of electron neutrinos radiating out from its core. Do these arrive at the Earth? The answer is ‘Apparently not’. Between a third and two thirds of these electron neutrinos ‘get lost’ on their way from the centre of the Sun to the Earth. This is now a well established experimental result known as the Solar Neutrino Anomaly. Perhaps the neutrino type (or ‘flavour’) oscillates due to the ‘tuning’, so that by the time they reach the Earth some of them are no longer neutrinos of the electron type. The SNO experiment, whose construction is planned for completion this year, should provide the definitive answer to this question by detecting the flux of ‘missing’ neutrinos as well as the electron ones. The Soudan experiment is not directly concerned with the Solar Neutrino Anomaly but with a second 'outbreak' of the unexpected. How closely the two are related is an important question to which we have no answers at present.
Trouble with neutrinos produced in the earth’s atmosphere
Neutrinos are also produced by the collision of cosmic radiation from outer space with air molecules in the upper atmosphere. In this case too we can calculate the flux (to within 20%) and the mix of neutrino species (to within 5%). Two experiments (Kamioka and IMB) in Japan and the USA already suggested some years ago that these neutrinos too change their type on their way between their production and their detection in deep underground experiments. These two experiments are very similar; they consist of large volumes of water in which the small flashes of light emitted by the charged particles released by the interacting neutrinos are detected. These experiments are not easy. Would the observations be confirmed by other quite different experiments? Would the conclusion that these neutrinos change their nature on propagation through the earth be substantiated? Only now, with the report of new results from the Soudan2 experiment, has this become clearer. Soudan2 employs a completely different technique, the Iron Calorimeter, yet the Soudan2 experiment confirms the result of the three water experiments. The new MINOS experiment is designed to take the next step; it will be the first able to detect and distinguish all three different types of neutrino in a long baseline neutrino experiment under controlled ‘laboratory’ conditions.
…and the consequences?
Just a minute! Do we know that the anomalous result indicates that neutrinos oscillate? The answer to this is an important 'NO'. There is however no other hypothesis at this time. Further data is needed to show beyond reasonable doubt that neutrinos travelling short distances differ from those travelling long distances. More data is needed.
If neutrinos do oscillate, they have to have mass, or at least one of them does. The precise value is uncertain but around 1/10 eV or less. The consequences of these discoveries are not yet clear. Will they power a train? Or nourish an economy? Certainly not – although that has been said before! What is certain is that neutrinos play an important role in Cosmology, and mankind would be wise to understand more about these particles which we have been all around us (but unseen) since the dawn of time. The next experiments, such as SNO and MINOS, are designed to take the investigation further.
Return to Oxford Neutrino Home Page
References to a summary of other relevant experiments around the World
An applet to demostrate oscillations between two types of neutrinos.