Profile
Susan Cartwright
My CV
-
Education:
Larbert High School, 1970-76; Glasgow University, 1976-80 (BSc) and 1980-83 (PhD).
-
Qualifications:
Scottish O-grades (predecessors of Standard grades) in Arithmetic, English, Chemistry, French, Geography, Latin, Mathematics and Physics. Scottish Highers in Biology, Chemistry, English, French, Geography, Mathematics and Physics. BSc (1st class) in Astronomy and Natural Philosophy. PhD in particle physics.
-
Work History:
1983-86: postdoctoral researcher working for the Rutherford Appleton Laboratory (but based in DESY, Hamburg). 1987-88, postdoctoral researcher for MIT (based in SLAC, California). 1989 onwards, current post.
-
Current Job:
Senior Lecturer in Physics and Astrophysics, University of Sheffield
-
About Me:
I am a neutrino physicist based at Sheffield University. I work on the T2K experiment in Japan. I enjoy walking in the country, reading, and listening to music.
-
Read more
I live in Sheffield with my two budgies. I like books and hate social media – the only things tweeting in my house are the budgies. I speak decent German and can read and write French, but my attempt to learn Mandarin to talk to our Chinese students was a bit of a disaster (I can say “Hello” and “Thank you” and “My name is Susan”). I am an enthusiastic reader of science fiction and once won a signed book and a mug in one of Charlie Stross’s blog competitions.
-
Read more
Neutrinos are probably the second most common particle in the whole universe, after photons, but they interact so weakly that they are incredibly difficult to detect. There are three distinct types, associated with their three charged partner, the electron, the muon, and the tau, but one of the great discoveries of late 20th century particle physics was that neutrinos produced as one type sometimes later interact as a different type, a phenomenon known as neutrino oscillations. The mathematics of this personality change include a factor which distinguishes between matter and antimatter, and it is possible that in the first fraction of a second after the Big Bang this distinction might have led to the situation we see today, where the Universe contains matter but almost no antimatter. (This never happens in our particle accelerators like the LHC, where every particle created is matched by an antiparticle.)
I work on an experiment called T2K, in Japan. We use an intense beam of protons to create a nearly pure beam of muon-type neutrinos. These neutrinos travel 295 km across Japan, where a tiny fraction of them interact in our far detector, Super-Kamiokande, a massive water tank 40 m in diameter by 40 m high (imagine a dozen Olympic swimming pools stacked on top of each other…). We are interested in the composition of the beam as sampled here: what fraction of our initial muon neutrinos have become tau neutrinos (answer – most of them), and what fraction have become electron neutrinos (not many – but, crucially, not zero)? Now we are making the measurements again with an anti-neutrino beam, to see if there is a difference.
This experiment is an international collaboration, with colleagues from Japan, the UK, the USA, Canada and Europe – like most of my British colleagues, I work mostly on the near detector, ND280, which is situated only 280 m from where the beam starts and measures the neutrinos before they have had a chance to change type. This is necessary because our beam is not quite 100% muon neutrinos: a few electron neutrinos are produced as well, so we have to check how many there are initially, and subtract this from the number we find at Super-K, in order to measure the probability that a neutrino that was originally muon-type interacts at Super-K as electron-type.
I’m also working on the proposed successor to T2K, called Hyper-Kamiokande. While Super-K contains 50000 tons of water, Hyper-K is designed to be 20 times larger, containing A MILLION TONS. Both Super-K and Hyper-K are located in mine workings under mountains, to shield us from background from cosmic rays, which would otherwise drown out our neutrino signal (we measure the muon or electron produced when a neutrino interacts – cosmic ray particles are already muons, and would look too much like interacting muon neutrinos in Super-K): we’re going to need a very large cavern for Hyper-K!
More about the T2K experiment can be found on our website, t2k-experiment.org. I can be heard talking about neutrinos on Radio 4’s In Our Time with Melvyn Bragg in the In Our Time Archive (we’ve made the Science Top 10!).
Apart from being a neutrino physicist, I graduated with a degree in Physics and Astronomy (or, as they called it then, Astronomy and Natural Philosophy – if it was good enough for Newton, it was good enough for us) at the University of Glasgow, did my PhD at the DESY lab in Hamburg, again with Glasgow, and then worked for the Rutherford Appleton Laboratory and MIT before settling in Sheffield. I teach neutrino physics, problem solving, introductory cosmology, the history of astronomy and particle astrophysics to undergraduate students, give talks to astronomical societies and schools, like walking and bird-watching, own a lineolated parakeet and a budgie, and am trying to learn Chinese calligraphy.
-
My Typical Day:
My typical day includes lecturing, talking to students, participating in meetings, thinking about neutrinos, and trying to write code that does what I want it to do instead of what I told it to do.
-
Read more
I am a senior lecturer in a busy and expanding physics department, so quite a lot of my typical day is taken up by teaching-related activities like giving lectures, supervising project students, helping students who are having problems, and marking. I am head of the Study Abroad programme, so sometimes I have an email from a Sheffield student who’s currently on placement in Australia, Canada or the USA, asking for advice about choosing courses. I also try to spend some time each day talking to my PhD students, Sam and Jost – though Jost is writing up his thesis and will soon go off to find a real job!
As a member of Hyper-Kamiokande, I am currently working on the prospects for detecting neutrinos from supernovae – exploding massive stars. In 1987, Super-Kamiokande’s much smaller predecessor, Kamiokande-II, detected a dizen neutrinos from a supernova in the Large Magellanic Cloud, 160000 light-years away. Hyper-K is about 300 times bigger than Kamiokande-II, and a supernova in our Galaxy would be about 5 times closer than SN 1987A, so we could expect to see something like 100000 neutrinos if a supernova exploded near the centre of our Galaxy. This would give us an unprecedented window into the actual stellar explosion – unlike the visible light, the neutrinos come from close to the exploding core of the star. It might also tell us more about the neutrinos themselves. Although we can’t predict when the next Galactic supernova will happen, it’s fair to say that one is overdue: the last two seen on Earth occurred only 32 years apart, in 1572 and 1604, so it seems a bit unfair that 400 years have passed without another one! I am corresponding with supernova experts in Germany, who have produced a simulation that tells me how many neutrinos to expect from an exploding star of a given mass, and my colleague Matt and I are working out from that how many to expect in Hyper-K, allowing for their varying energies and for the fact that no experiment detects 100% of the interactions that occur (we miss the ones that have too low an energy, and may accidentally reject ones that happen to look like background).
This is not Hyper-K’s main focus – like T2K, it will be supplied with a beam of neutrinos from the J-PARC accelerator, 295 km away on Japan’s east coast – but because of its huge size it will be an excellent detector for neutrinos from other sources such as the Sun, cosmic rays hitting the Earth’s atmosphere and – yes – supernovae. It’s only fair to remember that the first evidence for changing neutrino types came from solar and atmospheric neutrinos, not from neutrino beams.
-
What I'd do with the prize money:
I think I would hide the solar system in the university campus.
-
The most exciting thing that's happened this year in my research area:
One exciting thing that has happened fairly recently is the detection of very high energy neutrinos from an active galaxy. This should help us to answer a question that has been puzzling particle physicists and astrophysicists for nearly a century: what is the origin of cosmic rays?
Cosmic rays are charged particles that impact the Earth from outer space. Some of them have enormously high energies: far, far higher than anything we can manage at CERN. The problem is that because cosmic rays are charged, their trajectories are bent around by the Galaxy’s magnetic field, which means that they arrive from fairly random directions and do not point back to their source. However, interactions with random gas atoms in the source – whatever it is – should lead to the production of unstable particles called pions, which decay to produce neutrinos. Neutrinos are not charged and therefore not deflected by magnetic fields, so they should allow us to identify possible sources of cosmic rays.
IceCube, a giant neutrino detector buried in the ice of the South Pole, has been detecting high-energy neutrinos from outer space for some years now, but has never been able to relate them to any specific object. However, last year, one very well-measured neutrino was found to point back to an active galactic nucleus (a galaxy with a certal supermassive black hole that is generating vast amounts of energy by capturing surrounding gas). This is the first clear indication that active galaxies are one of the sources of ultra-high-energy cosmic rays.
-
My latest work:
I am currently working on neutrinos from supernovae. Just over 30 years ago, a couple of dozen neutrinos were detected from a massive star that exploded in the Large Magellanic Cloud, 160 thousand light years away. Someday – preferably someday quite soon! – a similar massive star will be seen to explode in our own Galaxy (we expect that this happens about twice per century, on average), and with much larger neutrino detectors and a closer source, we would expect to see thousands of neutrinos. This has the potential to lead to a greater understanding of the process by which massive stars explode, which is not only interesting in its own right but important in understanding how we came to be here: the oxygen you are breathing was produced in just such a massive-star explosion. We are trying to understand the extent to which observations of neutrinos from a Galactic supernova could help us to distinguish between different models of the explosion (and perhaps also tell us more about neutrinos themselves).
-
My favourite misconception about my area of science:
People who refer to particle accelerators as “atom smashers”. Aaarrgghh, grrrr, grumble grumble. Most particle accelerators accelerate large bunches of individual particles (in currently operating accelerators, generally protons), in a pipe which has been very carefully pumped down to a good vacuum. If particles from the beam encounter stray atoms, the result (a so-called beam-gas interaction) is actually annoying background: the fewer atoms that get smashed, the better!
What’s wrong with calling them “particle accelerators”? It’s what they do: they accelerate particles. It’s not a complicated piece of technical jargon!