## The Nobel Prize for Neutrino Oscillations

Posted in The Universe and Stuff with tags , , , , , , , , on October 6, 2015 by telescoper

Well the Nobel Prize for Physics in 2015 has been announced. It has been awarded jointly to Takaaki Kajita and Arthur B. McDonald for..

the discovery of neutrino oscillations, which prove that neutrinos have mass.

You can read the full citation here. Congratulations to them both. Some physicists around here were caught by surprise because the 2002 Nobel Prize was also awarded for neutrino physics, but it is fair because this award goes for a direct measurement of neutrino oscillations, which is an important breakthrough in its own right; the earlier award was for measurements of solar neutrinos. For a nice description of the background you could do worse than the Grauniad blog post by Jon Butterworth about neutrino physics.

In brief the a process in which neutrinos (which have three distinct flavour states, associated with the electron, mu and tau leptons) can change flavour as they propagate. It’s quite a weird thing to spring on students who previously thought that lepton number (which denotes the flavour) was always conserved. I remember years ago having to explain this phenomenon to third-year students taking my particle physics course.  I decided to start with an analogy based on more familiar physics, but it didn’t go to plan.

A charged fermion such as an electron (or in fact anything that has a magnetic moment, which would include, e.g. the neutron)  has spin and, according to standard quantum mechanics, the component of this in any direction can  can be described in terms of two basis states, say $|\uparrow>$ and $|\downarrow>$ for spin in the $z$ direction. In general, however, the spin state will be a superposition of these, e.g.

$\frac{1}{\sqrt{2}} \left( |\uparrow> + |\downarrow>\right)$

In this example, as long as the particle is travelling through empty space, the probability of finding it with spin “up” is  50%, as is the probability of finding it in the spin “down” state. Once a measurement is made, the state collapses into a definite “up” or “down” wherein it remains until something else is done to it.

If, on the other hand, the particle  is travelling through a region where there is a  magnetic field the “spin-up” and “spin-down” states can acquire different energies owing to the interaction between the spin and the magnetic field. This is important because it means the bits of the wave function describing the up and down states evolve at different rates, and this  has measurable consequences: measurements made at different positions yield different probabilities of finding the spin pointing in different directions. In effect, the spin vector of the  particle performs  a sort of oscillation, similar to the classical phenomenon called  precession.

The mathematical description of neutrino oscillations is very similar to this, except it’s not the spin part of the wavefunction being affected by an external field that breaks the symmetry between “up” and “down”. Instead the flavour part of the wavefunction is “precessing” because the flavour states don’t coincide with the eigenstates of the Hamiltonian that describes the neutrinos’ evolution. However, it does require that different neutrino types have intrinsically different energies  in quite  a similar way similar to the spin-precession example. In the context of neutrinos however the difference in energy means a difference in mass, and if there’s a difference in mass then not all flavours of neutrino can be massless.

Although the analogy I used isn’t a perfect, I thought  it was a good way of getting across the basic idea. Unfortunately, however, when I subsequently asked an examination question about neutrino oscillations I got a significant number of answers that said “neutrino oscillations happen when a neutrino travels through a magnetic field….”. Sigh. Neutrinos don’t interact with  magnetic fields, you see…

Anyway, today’s announcment also prompts me to mention that neutrino physics is one of the main research interests in our Experimental Particle Physics group here at Sussex. You can read a recent post here about an important milestone in the development of the NOvA Experiment which involves several members of the Department of Physics and Astronomy in the School of Mathematical and Physical Sciences here at the University of Sussex. Here’s the University of Sussex’s press release on the subject. In fact Art McDonald is a current collaborator of our neutrino physicists, who have been celebrating his award today!

Neutrino physics is a fascinating subject even to someone like me, who isn’t really a particle physicist. My impression of the field is that was fairly moribund until about the turn of the millennium  when the first measurement of atmospheric neutrino oscillations was announced. All of a sudden there was evidence that neutrinos can’t all be massless (as many of us had long assumed, at least as far as lecturing was concerned).  Now the humble neutrino is the subject of intense experimental activity, not only in the USA and UK but all around the world in a way that would have been difficult to predict twenty years ago.

But then, as the physicist Niels Bohr famously observed, “Prediction is very difficult. Especially about the future.”

## NOvA and Neutrinos

Posted in The Universe and Stuff with tags , , , , on March 11, 2014 by telescoper

Yesterday’s Grauniad blog post by Jon Butterworth about neutrino physics reminded me that I forgot to post about an important milestone in the development of the NOvA Experiment which involves several members of the Department of Physics and Astronomy in the School of Mathematical and Physical Sciences here at the University of Sussex. Here’s the University of Sussex’s press release on the subject, which came out a couple of weeks ago.

The NOvA experiment consists of two enormous  particle detectors, one at the Fermi National Accelerator Laboratory “Fermilab” near Chicago and the other in Minnesota. The neutrinos are actually generated  at Fermilab; the particle beam is then aimed  at the detectors the, one near the source at Fermilab, and the other in Ash River, Minnesota, near the Canadian border. The particles, sent in their billions every couple of seconds, complete the 500-mile trip in less than three milliseconds.

The point is that the experiment has managed for the first time to actually detect neutrinos through the 500 miles of rock separating the two ends of the experiment. This is obviously just a first step, but it’s equally obviously a crucial one.

Colleagues from Sussex University are strongly involved in  calibrating and fine-tuning the detector, which produces light when particles pass through it. Dr Abbey Waldron and PhD student Luke Vinton have developed a calibration procedure that uses known properties of  muons to calibrate precise measurements of the neutrinos, which are less well understood.  The detector sees 200,000 particle interactions a second, produced by cosmic rays bombarding the atmosphere, and scientists can’t record every single one. Sussex’s Dr Matthew Tamsett has developed a trigger algorithm that searches for events that look like neutrinos among the billions of other particle interactions.

Neutrino physics is an interesting subject to someone like me, who isn’t really a particle physicist. My impression of the field is that was fairly moribund until 1998 when the first measurement of atmospheric neutrino oscillations was announced. All of a sudden there was evidence that neutrinos can’t all be massless (as many of us had long assumed, at least as far as lecturing was concerned).  Now the humble neutrino is the subject of intense experimental activity, not only in the USA and UK but all around the world in a way that would have been difficult to predict twenty years ago.

But then, as the physicist Niels Bohr famously observed, “Prediction is very difficult. Especially about the future.”

## Hot News! Supernova in M82

Posted in The Universe and Stuff with tags , , , , , on January 22, 2014 by telescoper

Very exciting news today – a supernova has gone off in Messier 82. In fact, according to this sequence of images from Japan it actually started to brighten about a week ago:

Being arranged in Japanese fashion, you have to read these from top to bottom but starting at the right, so the supernova can be seen to be steadily brightening, i.e. decreasing in magnitude from 17.0 to 11.9. That means it’s now visible with binoculars and will have been seen already by many amateur astronomers. The exciting question this time is whether we’ll get any neutrinos from it!

UPDATE: I’m told that, close as it is, M82 is probably too far to detect neutrinos. Boo.

This is the nearest supernova since 1987a which was observed in, er, 1987. This is the nearest Type Ia supernova for a very long time (possibly 1937), so it’s of considerable interest for the use of such objects in cosmology. There have been other close ones since the nearest one I can remember, 1987a, which was observed in, er, 1987 but all have been Type II.

UPDATE: Thanks for the people who pointed out my error which I’ve left in to show that I don’t know much about supernovae so you shouldn’t phone me up to ask.

## Neutrino Physics in a Small Universe

Posted in Biographical, The Universe and Stuff with tags , , , , , , , on April 23, 2013 by telescoper

I’ve only just got time for a quick lunchtime post before I head off to attend an afternoon of Mathematics presentations, but it’s a one of those nice bits of news that I like to mention on here from time to time.

It is my pleasure to pass on the wonderful news that one of my colleagues in the School of Mathematical and Physical Sciences here at the University of Sussex,  Dr Jeff Hartnell,. has been awarded  the High Energy Particle Physics prize of the Institute of Physics, which means that his name has now been added to the illustrious list of previous winners. The prize is awarded annually by the HEPP Group, a subject group in the Nuclear and Particle Physics Division of the IOP, to a researcher in the UK who has made an outstanding contribution to their field of study early in their career (within 12 years of being awarded their first degree).

There’s a very nice piece about this award here which reveals, amongst other things, that many moons ago at Nottingham I was Jeff’s undergraduate tutor! In fact Jeff also attended a third-year course on Theoretical Elementary Particle Physics I taught in those days. That he survived those experience and went on to be a world-leading physicist speaks volumes! Not only that, it’s also evidence that the world of physics is smaller than we sometimes suppose. I’ve crossed paths with a number of my new colleagues at various times in the past, but it’s particularly rewarding to see someone you taught as an undergraduate go on to a highly successful career as a scientist. Jeff was awarded a prestigious ERC grant this year too!

Jeff is currently in the USA helping to set up the largest-ever experiment in neutrinos to be built there, called NOvA. You can click on the preceding links for more technical details, and I also found this interesting video showing the NOvA detector being assembled. Particle physics experiments are never small, are they?

p.s. Why do they insist on writing “metric ton” instead of “tonne”?

## Neutrino Timing Glitch?

Posted in The Universe and Stuff with tags , , , , on February 23, 2012 by telescoper

You may recall the kerfuffle last September when physicists connected with the OPERA experiment at the Gran Sasso National Laboratory in Italy produced a paper suggesting that neutrinos might travel at speeds greater than that of light. I posted on that story myself and even composed a poem specially for the occasion at no extra charge:

Do neutrinos go faster than light?
Some physicists think that they might.
In the cold light of day,
I am sorry to say,
The story is probably shite

Well news began to break last night that OPERA scientists had identified an error. The first story I read was a bit shaky on the question of attribution, so I decided to sleep on it and see whether anything emerged that seemed sounder before posting on here. Later on last night an item in Nature News appeared which looks a bit better grounded:

But according to a statement OPERA began circulating today, two possible problems have now been found with its set-up. As many physicists had speculated might be the case, both are related to the experiment’s pioneering use of Global Positioning System (GPS) signals to synchronize atomic clocks at each end of its neutrino beam. First, the passage of time on the clocks between the arrival of the synchronizing signal has to be interpolated and OPERA now says this may not have been done correctly. Second, there was a possible faulty connection between the GPS signal and the OPERA master clock.

We should wait for a more definitive announcement from OPERA about these possible errors, but if it does turn out that technical glitches are responsible for the neutrino speed result then it won’t be entirely unexpected. A faulty cable connection does sound a bit lame, however. I hope they weren’t relying on a USB connection….

Anyway, as I mentioned in a comment elsewhere the arXiv paper from OPERA has now received about 230 citations, although it has not appeared in a refereed journal.  If it turns out to have been a completely wrong result, what does that tell you about the use of citations to measure “quality”?

UPDATE: There is now an official press release from CERN, confirming the unofficial reports mentioned above:

The OPERA collaboration has informed its funding agencies and host laboratories that it has identified two possible effects that could have an influence on its neutrino timing measurement. These both require further tests with a short pulsed beam. If confirmed, one would increase the size of the measured effect, the other would diminish it. The first possible effect concerns an oscillator used to provide the time stamps for GPS synchronizations. It could have led to an overestimate of the neutrino’s time of flight. The second concerns the optical fibre connector that brings the external GPS signal to the OPERA master clock, which may not have been functioning correctly when the measurements were taken. If this is the case, it could have led to an underestimate of the time of flight of the neutrinos. The potential extent of these two effects is being studied by the OPERA collaboration. New measurements with short pulsed beams are scheduled for May.