4 ms·

A question for those who are up to date with their particle physics: how does one know that the neutrinos detected were the ones that they actually sent? Is it

by redcap 15y ago

A question for those who are up to date with their particle physics: how does one know that the neutrinos detected were the ones that they actually sent? Is it based on the direction they were sent from and the amount of neutrinos received? I asked because the one thing that the media repeats is that there's a lot of neutrinos hitting us right now, so how do we know they're the ones we're looking for? Also, is there a normal speed that neutrinos travel at? Can you catch one at rest?

lutorm 15y ago
There may be a lot of neutrinos around, but they are extremely hard to detect. The fact that you see a bunch of them at a time and a place where you expect based on how they were created makes it very unlikely that they are not associated. There are craploads of neutrinos left over from the Big Bang (see http://en.wikipedia.org/wiki/Cosmic_neutrino_background http://en.wikipedia.org/wiki/Cosmic_neutrino_background) but they are very low energy and are impossible to detect. They are likely the closest you can find neutrinos at rest. How fast they go depends on their rest mass, which we only know to be small but nonzero.
pbhjpbhj 15y ago
It's a statistical issue to some extent. How do you find your marble amongst a billion other marbles? Well if you know the colour and how hot it is then it's going to be relatively easy (use IR camera, select for temperature then pick out by visible colour spectrum). Same here, AFAICT. They're using pion and kaon decay that produces muon neutrinos of known energies (in a range) travelling in a known direction. Solar neutrinos (http://en.wikipedia.org/wiki/Solar_neutrino http://en.wikipedia.org/wiki/Solar_neutrino) are created as electron neutrinos, though they flavour shift, they are mainly coming the direction of the sun and have differing energies to those created in the OPERA experiment via the CERN SPS. I'd guess that you can cut out a lot of the background neutrino flux by running the proton capture (to generate the neutrinos) at around noon; I don't know if they time things to put the detector orthogonal to the local galactic plane but that would (to this layman) seem sensible too. One thing that did strike me (lol?) was that the kaon/pion decay happens along a 1095m decay tube. As a sibling comment says they're not tracking individual neutrinos (you can't detect them twice) they're tracking a range of energy events (via the muon's created) at a range of positions along a 1km tube (via the muon trajectories presumably) and pairing them with a range of events at the other end such that they need to know the length of the transit over 730km to an accuracy of 20cm. It's amazing stuff. Edit: I should probably have said that of course they allow for the variances and the statistical nature and other systematic uncertainties, the graph showing the calculated uncertainties can be seen on the Wikipedia page, http://en.wikipedia.org/wiki/CNGS http://en.wikipedia.org/wiki/CNGS.

by redcap 15y ago

lutorm 15y ago

There may be a lot of neutrinos around, but they are extremely hard to detect. The fact that you see a bunch of them at a time and a place where you expect based on how they were created makes it very unlikely that they are not associated. There are craploads of neutrinos left over from the Big Bang (see http://en.wikipedia.org/wiki/Cosmic_neutrino_background http://en.wikipedia.org/wiki/Cosmic_neutrino_background) but they are very low energy and are impossible to detect. They are likely the closest you can find neutrinos at rest. How fast they go depends on their rest mass, which we only know to be small but nonzero.

pbhjpbhj 15y ago

It's a statistical issue to some extent. How do you find your marble amongst a billion other marbles? Well if you know the colour and how hot it is then it's going to be relatively easy (use IR camera, select for temperature then pick out by visible colour spectrum). Same here, AFAICT. They're using pion and kaon decay that produces muon neutrinos of known energies (in a range) travelling in a known direction. Solar neutrinos (http://en.wikipedia.org/wiki/Solar_neutrino http://en.wikipedia.org/wiki/Solar_neutrino) are created as electron neutrinos, though they flavour shift, they are mainly coming the direction of the sun and have differing energies to those created in the OPERA experiment via the CERN SPS. I'd guess that you can cut out a lot of the background neutrino flux by running the proton capture (to generate the neutrinos) at around noon; I don't know if they time things to put the detector orthogonal to the local galactic plane but that would (to this layman) seem sensible too. One thing that did strike me (lol?) was that the kaon/pion decay happens along a 1095m decay tube. As a sibling comment says they're not tracking individual neutrinos (you can't detect them twice) they're tracking a range of energy events (via the muon's created) at a range of positions along a 1km tube (via the muon trajectories presumably) and pairing them with a range of events at the other end such that they need to know the length of the transit over 730km to an accuracy of 20cm. It's amazing stuff. Edit: I should probably have said that of course they allow for the variances and the statistical nature and other systematic uncertainties, the graph showing the calculated uncertainties can be seen on the Wikipedia page, http://en.wikipedia.org/wiki/CNGS http://en.wikipedia.org/wiki/CNGS.