Amazing Quantum Physics News: Now It Can Be Told!
Originally posted by anarchist_nomad at Now It Can Be Told!
In recent days, gentle readers, you may recall me mentioning within these pages that there were significant new results on the way from the Tokai-to-Kamioka long-baseline neutrino oscillation experiment (or T2K for short). After weeks of checking and validation[*], those results were released to the world today and can now be shared.
This morning, my boss was on the BBC Radio 4 programme Today to discuss the new announcement. Press releases can also be seen by clicking on one of the following links:
To summarize, there are three known types of neutrinos but, thanks to the wonders of quantum mechanics, they match up differently depending on how you look at them. In one way of looking, you get three distinct "flavours": the electron neutrino (which pairs with an electron), the muon neutrino (which pairs with the electron's heavier cousin, the muon), and the tau neutrino (which pairs with the beast of the lepton family, the tau). If you examine the neutrinos by mass, you also see three types -- or three distinct masses. So far, not terribly surprising, right? Well hold on, because here comes the good part:
The three mass states and the three flavour states do not correspond to each other. If you take a garden variety neutron and wait about fifteen minutes, it will decay into a proton, an electron, and an electron neutrino[**]. That electron neutrino has a definite flavour... but not a definite mass. Its mass is actually a mixture of the three mass states. Since objects of different mass with the same energy travel at different speeds -- consider, for instance, pitching a baseball versus a bowling ball -- the three mass states of that electron neutrino travel in a manner that is slightly different for each. When you later go to look at that neutrino again, the mixture may have changed enough to make that electron neutrino look like a muon neutrino. Or a tau neutrino. Wait a bit longer and look again, though, because it may well have changed back into an electron neutrino once more!
This is a process called "neutrino oscillation" and it is not a part of the Standard Model of Particle Physics! It was first discovered by my thesis experiment, Super-Kamiokande, in 1998, when they measured a lack of muon neutrinos (caused by some of them turning into the much more difficult to detect tau neutrinos). Since then, other experiments have replicated these results: The K2K and MINOS experiments have produced beams of muon neutrinos to confirm this disappearance, and both Super-Kamiokande and SNO have seen electron neutrinos from the Sun disappear as they turn into the two other types. Similarly, the KamLAND experiment noticed electron anti-neutrinos from nuclear reactors vanishing as they turned into the corresponding other types of anti-neutrinos.
Since there are three types of neutrinos (call them ν1, ν2, and ν3), there are three types of mixing possible: mixing between 1&2, mixing between 2&3, and mixing between 1&3.
The initial discovery by Super-Kamiokande in 1998, described above, is mixing between types 2&3. So is the mixing studied by K2K and MINOS. The Solar neutrino studies by Super-Kamiokande and SNO measured mixing between types 1&2. However, there has been no strong indication of mixing between types 1&3. Until now. This is the big news from T2K. Although we have only collected about ~3% of our expected total data[***], we already see compelling signs of this mixing! Exciting, no?
For those who may still be unimpressed, I should point out that measurements of the three types of mixing is an essential pre-requisite for measurements of another property of neutrinos, which is called "CP violation". The "C" stands for "charge conjugation", which is basically means flipping a particle to its anti-particle: An electron to a positron, a proton to an anti-proton, et cetera. The "P" stands for parity; if you look at your hands, you will see an example of a parity flip -- the two are the same except for a mirror image transformation. It was once believed that matter and anti-matter were the same if you flipped both "C" and "P". We now know that this is not quite true. That is a good thing, as were they the same in every way, equal amounts of matter and anti-matter would have been created in the Big Bang... then subsequently annihilated together and left nothing behind to make us! So CP symmetry is not exactly; however, the small amount by which this symmetry is violated amongst the fundamental particles known as quarks is not nearly sufficient to explain why we live in a universe that is filled with matter. Measuring the CP violation amongst neutrinos may give us the answer.
For those who are interested in learning more, feel free to download a pre-print of our publication, which was submitted to Physical Review Letters on Monday and will be available on the arXiv server tomorrow. There is no need to wait, as you can get a copy of the pre-print here.
Also, if any of you have any questions about this result, dear friends, do feel free to ask. I anticipate the next few days being particularly busy, but I will try to answer any questions as quickly as possible!
[*] Which was the reason that I postponed my trip to Seoul until next year.
[**] Technically, this case gives an electron anti-neutrino... but don't worry about that difference right now. For the sake of this explanation, the two can be treated as the same.
[***] And will be getting no more until about early next year, thanks to the East Japan Big Earthquake and Disaster.
In recent days, gentle readers, you may recall me mentioning within these pages that there were significant new results on the way from the Tokai-to-Kamioka long-baseline neutrino oscillation experiment (or T2K for short). After weeks of checking and validation[*], those results were released to the world today and can now be shared.
This morning, my boss was on the BBC Radio 4 programme Today to discuss the new announcement. Press releases can also be seen by clicking on one of the following links:
- The UK's Science & Technology Facilities Council [STFC] (Click here)
- The Japan Proton Accelerator Research Complex [J-PARC] (Click here)
To summarize, there are three known types of neutrinos but, thanks to the wonders of quantum mechanics, they match up differently depending on how you look at them. In one way of looking, you get three distinct "flavours": the electron neutrino (which pairs with an electron), the muon neutrino (which pairs with the electron's heavier cousin, the muon), and the tau neutrino (which pairs with the beast of the lepton family, the tau). If you examine the neutrinos by mass, you also see three types -- or three distinct masses. So far, not terribly surprising, right? Well hold on, because here comes the good part:
The three mass states and the three flavour states do not correspond to each other. If you take a garden variety neutron and wait about fifteen minutes, it will decay into a proton, an electron, and an electron neutrino[**]. That electron neutrino has a definite flavour... but not a definite mass. Its mass is actually a mixture of the three mass states. Since objects of different mass with the same energy travel at different speeds -- consider, for instance, pitching a baseball versus a bowling ball -- the three mass states of that electron neutrino travel in a manner that is slightly different for each. When you later go to look at that neutrino again, the mixture may have changed enough to make that electron neutrino look like a muon neutrino. Or a tau neutrino. Wait a bit longer and look again, though, because it may well have changed back into an electron neutrino once more!
This is a process called "neutrino oscillation" and it is not a part of the Standard Model of Particle Physics! It was first discovered by my thesis experiment, Super-Kamiokande, in 1998, when they measured a lack of muon neutrinos (caused by some of them turning into the much more difficult to detect tau neutrinos). Since then, other experiments have replicated these results: The K2K and MINOS experiments have produced beams of muon neutrinos to confirm this disappearance, and both Super-Kamiokande and SNO have seen electron neutrinos from the Sun disappear as they turn into the two other types. Similarly, the KamLAND experiment noticed electron anti-neutrinos from nuclear reactors vanishing as they turned into the corresponding other types of anti-neutrinos.
Since there are three types of neutrinos (call them ν1, ν2, and ν3), there are three types of mixing possible: mixing between 1&2, mixing between 2&3, and mixing between 1&3.
The initial discovery by Super-Kamiokande in 1998, described above, is mixing between types 2&3. So is the mixing studied by K2K and MINOS. The Solar neutrino studies by Super-Kamiokande and SNO measured mixing between types 1&2. However, there has been no strong indication of mixing between types 1&3. Until now. This is the big news from T2K. Although we have only collected about ~3% of our expected total data[***], we already see compelling signs of this mixing! Exciting, no?
For those who may still be unimpressed, I should point out that measurements of the three types of mixing is an essential pre-requisite for measurements of another property of neutrinos, which is called "CP violation". The "C" stands for "charge conjugation", which is basically means flipping a particle to its anti-particle: An electron to a positron, a proton to an anti-proton, et cetera. The "P" stands for parity; if you look at your hands, you will see an example of a parity flip -- the two are the same except for a mirror image transformation. It was once believed that matter and anti-matter were the same if you flipped both "C" and "P". We now know that this is not quite true. That is a good thing, as were they the same in every way, equal amounts of matter and anti-matter would have been created in the Big Bang... then subsequently annihilated together and left nothing behind to make us! So CP symmetry is not exactly; however, the small amount by which this symmetry is violated amongst the fundamental particles known as quarks is not nearly sufficient to explain why we live in a universe that is filled with matter. Measuring the CP violation amongst neutrinos may give us the answer.
For those who are interested in learning more, feel free to download a pre-print of our publication, which was submitted to Physical Review Letters on Monday and will be available on the arXiv server tomorrow. There is no need to wait, as you can get a copy of the pre-print here.
Also, if any of you have any questions about this result, dear friends, do feel free to ask. I anticipate the next few days being particularly busy, but I will try to answer any questions as quickly as possible!
[*] Which was the reason that I postponed my trip to Seoul until next year.
[**] Technically, this case gives an electron anti-neutrino... but don't worry about that difference right now. For the sake of this explanation, the two can be treated as the same.
[***] And will be getting no more until about early next year, thanks to the East Japan Big Earthquake and Disaster.