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Sally Shaw | University College London | UK

Read Bio

New LUX Results on WIMP-Nucleon Scattering

Monday, December 14th, 2015

It’s amazing that so much hard work (and such high levels of stress) can be condensed down so much… 5 pages, 3 plots and a table – and the new world leading limit on the WIMP-nucleon spin-independent elastic scattering cross section, of course.
Yes, the LUX Run 3 reanalysis results are finally out. It’s been in the works for over a year, and it has been a genuinely wonderful experience to watch this paper grow – and seeing my own plot in there has felt like sending forth a child into the world!
As much as I worked to improve our signal efficiency at low energies, the real star of the LUX reanalysis show was the “D-D” calibration – D-D standing for deuterium-deuterium. We calibrated the detector’s response to nuclear recoils (which we expect WIMP dark matter to cause) with something that sounds like it is out of science fiction, a D-D generator. This generator uses the fusion of deuterium (think heavy hydrogen – one proton, one neutron) to generate neutrons that are focussed into a beam and sent into the detector.

Quick LUX 101 – LUX is a dark matter search experiment. Dark matter is that mysterious dark, massive substance that makes up 27% of our universe. How does LUX look for dark matter? Well, it is a ‘dual phase xenon TPC’ detector, and it lives 4850 feet underground at the Sanford Underground Research Facility.  It must be underground to shield it from as much cosmic radiation as possible – as it is looking for a very rare, weakly interacting dark matter particle called a WIMP. LUX is basically a big tank of liquid xenon, with a gas layer on top. It is sensitive to particles that enter this xenon – photons and electrons cause what we call an electron recoil (think of them bouncing off an atomic electron) whilst neutrons cause a nuclear recoil (bounce off a xenon nucleus). We expect that WIMPs will interact with the atomic nuclei too, just incredibly rarely – so understanding the detector response to these nuclear recoils is of utmost importance.  Both these electron recoils and nuclear recoils, inside the liquid xenon cause a flash of light, a signal we call “S1”, the scintillation signal. Any light in LUX is picked up by two arrays of photomultiplier tubes, 122 in total. Recoils can cause ionisation of electrons; electrons are ‘knocked off’ their atoms by the collision. If you place an electric field over the xenon volume, you can actually push these electrons along, instead of letting them recombine with their atoms. In LUX, the electrons are pushed all the way to the top, and into the gaseous xenon later. They then cause a second flash of light via scintillation in the gas, “S2”, the ionisation signal (as its source is the ionised electrons). Two signals mean two things – discrimination between electron recoils (background) and nuclear recoils (possible dark matter signal!) due to the differing distribution of energy between S1 and S2 for each recoil, and secondly, 3D position reconstruction. XY coordinates can be determined from looking at which photomultiplier tubes light up, whilst the time between the S1 and S2 tells us the depth of the interaction. This XYZ position is very important; we use the xenon to shield itself from radiation from the detector materials itself, or from the surrounding rock. If we have the 3D position of all our events, we can only look in the very inner region of the detector, where it is very quiet, for those rare dark matter interactions.

Schematic of the LUX detector

Schematic of the LUX detector. On the left, it is demonstrated how the S1 and S2 signals can provide 3D position reconstruction. The right shows the inside of the detector, and the position of the photomultiplier tubes that collect light emitted by the scintillation of xenon.


Back to the deuterium-deuterium fusion neutron gun – it’s actually a wonderfully simple but extremely clever idea. We fire a beam of neutrons into our detector, all at the same energy (monoenergetic or monochromatic), at a set position. We then select events in our data along that beam, and look for those neutrons that scattered a second time in the detector. Because of that XYZ position reconstruction, if we have signals from two different scatters, we can actually determine the angle of scattering. As the initial energy is known, allows the energy of the recoil to then be calculated, via simple kinematics. Matching the recoil energy with the size of the two signals allows us to calibrate the nuclear recoil response of the detector extremely well.  The light yield (in S1), tougher to measure than the charge yield (in S2), as we are talking about individual photons, was measured as low as 1.1keV. (keV are kiloelectronvolts, or 1000x the energy of a single electron moved across a potential difference of 1V. In other words, a tiny quantity. 1keV is only 1.6×10-16 joules!)  The charge yield was measured below 1keV. In the previous LUX results, we had assumed a conservative hard cut off – ie we would measure no light for recoils below 3 keV. Now we know that isn’t the case, and can extend our sensitivity to lower energies – which corresponds to lighter WIMPs.

Screenshot 2015-12-14 20.28.48

Upper limits on the spin-independent elastic WIMP-nucleon cross section at 90% CL. Observed limit in black, with the 1- and 2-σ ranges of background-only trials shaded green and yellow.

This improvement in low energy calibration, as well as a more streamlined and improved analysis framework, has led to a huge improvement in LUX’s low WIMP mass limit. In the plot above, which shows the WIMP mass against the probability of interaction with a nucleus, everything above the black line is now ruled out. If you’ve been following the WIMP dark matter saga, you will know that a few experiments were claiming hints of signals in that low mass region, but this new result definitely lays those signals to rest.

Getting a paper ready for publication has turned out to be far harder work than I expected. It requires a lot of teamwork, perseverance, brain power and very importantly, the ability to take on criticism and use it to improve. I must have remade the LUX efficiency plot over 100 times, and a fair few of those times it was because someone didn’t quite like the way I’d formatted it. In a collaboration, you have to be willing to learn from others and compromise. In the last few days before we finished I did not benefit at all from my UK time zone, as I stayed up later and later to finish things off. But – it was worth it! Now, if I search for my name on arXiv, I come up 4 times (3 LUX papers and the LZ Conceptual Design Report). As pathetic as it sounds, this is actually quite exciting for me, and is what I hope to be the foundations of a long career in physics.

The new LUX results are obviously nowhere near as exciting as an actual WIMP discovery, but it’s another step on the way there. LUX Run 4 is in full swing, where we will obtain over 3 times more data, increasing our sensitivity even further , and who knows – those WIMPs might just finally show their faces.


About to start a Physics degree? Hold on tight…

Tuesday, September 8th, 2015

It’s the beginning of September, which means two things. 1- I haven’t managed to write a blog in 6 months (turns out second year is busy!), and 2- it’s nearly the start of the academic year. These two facts have inspired me to write a post aimed for all those fresh-faced 18 year olds about to embark on the adventure of university. Or college for the Americans– a weird concept to us Brits, as college is in fact where 16 year olds go to do mainly non-academic courses like “Travel and Tourism” and “Hair and Beauty”. Right now in the UK, thousands and thousands of teenagers are probably getting increasingly nervous as their start dates in September and October loom nearer and nearer. I thought I would write something for anyone who is about to attempt a physics degree at university, with none of that prospectus fluff.

I think the most succinct way to sum up my undergraduate degree is “play hard, work harder”! It wasn’t easy, some of the time it felt downright impossible but at the end of the day I had fun, I made friends, I got a masters degree and finally a PhD place. What more can you ask for from university?

Other Physicists
So, first things first, before you do any physics, you are going to see and meet some of your coursemates. I’m going to be brutally honest here: physicists are weird. They just are. To dedicate yourself to a subject like physics you just need to have a little bit of weirdness in you, that seems to be a fundamental law. This is not always a bad thing. A disclaimer here, some of my best friends are physicists. My colleagues at UCL are a brilliant, funny and sociable bunch. My boyfriend, although now having sold his soul to the actual law, I met studying the laws of physics at uni. There is nothing wrong with physicists as a whole, but a lot of them are a little strange. You may encounter people who are painfully socially awkward, wear fedoras or suits to lectures, LARPers, guys with LOTS of hair (both on their head and faces), posh kids buying Grey Goose, poor kids living on supernoodles and beans, international kids, gamers, heavy drinkers, those that are politically driven, or lazy with questionable hygiene, and on rare occasions, women.
I joke, I think my course was ~10% girls by the masters year, so not too rare. It’s getting better, and I think we are better off than computer sciences, but we are still in the minority. If you are so lucky to be female amongst the physicists, be warned, they may stare, and they will probably know who you are when you can’t possibly be expected to remember all those generic male faces! I can’t count the times I was approached by strangers, usually in clubs but once on a train, with the line ‘you’re that girl who does physics!’.
I had the good luck of having another physicist in my halls of residence – a rather normal one, who played guitar and drunk a lot. We were flatmates for many years, and we stuck together in lectures and labs as much as we could. In my first year, I remember having a very strong aversion to making any other friends on my course. They’re all awkward and weird and nerdy, I said. I don’t want to hang out with them, they wont be fun, I said. The important thing here is I WAS WRONG. After I realised my flatmate was leaving after third year with a bachelors, I made a bit of effort to meet people, and made some extremely good friends in physics, who I still see often. And guess what? They aren’t weird and no fun. They are really great guys, and I wish I had made friends with them earlier.
So don’t write anyone off immediately. Be sociable, chat to people. People will be shy (I was, and still am) and awkward, but give everyone a chance. You wont get on with everyone, but you may be surprised at who you do end up friends with. Physicists are usually a little bit odd – but they are also often a lot of fun.

The Actual Physics
A very important thing to understand when you start a course like physics (or maths, or any science really) is that unless you are some supreme genius, there will be people cleverer than you. Lots of them. If you are going to a top uni with very high entry grades (Warwick at the time was AAB, A*s weren’t available yet) then chances are you are going to feel a little bit inferior. I went from being the top of my physics A-level class to somewhere in the upper quartile, and at first it was a little disconcerting. But don’t worry – there are more qualities to a person than their grades!

Doing stuff like this in the library will make you feel clever.

Doing stuff like this in the library will make you feel clever.

For my first few weeks at Warwick, I was in a bit of a panic. We had a course called “Physics Foundations” which you may think sounds like some nice gentle introductory course. Wrong. We were thrown in the deep end. It bared almost no resemblance to A-level physics (there was, thank God, a mechanics course that did, but it came after Special Relativity, which also had me panicking a fair bit) and involved all sorts of notation and nomenclature I’d never even heard of (like ’tilde’?!). I also had not done further maths, and whilst they brought us up to speed in maths quite quickly I did feel a little disadvantaged by my lack of knowledge on imaginary numbers. I genuinely spent the first few weeks thinking I chose the wrong course. What had felt so right at A-level, so naturally the thing I was best at, was now giving me an identity crisis. I began wondering how I would explain to my friends that I had failed.

And then, a miracle happened. I talked to other people. I talked to the students in my tutor group and my seminar groups. And guess what – they were all just as confused as me. This was a wonderful realisation. I also noticed my problem sheet marks were actually not so bad. I didn’t always understand what I was doing but I seemed to be doing it the right way. This is another important thing to note – do not expect to understand your lectures. I didn’t understand much at all until I did problems, past papers and proper revision – often in the third term! Do not panic early on if things aren’t going in. Do not think problem sheets aren’t important. They help, seriously.

You’re going to need to do some work. You’re going to notice your hallmates doing humanities having only 8 hours a week of contact time, whereas you’re closer to 25. You will be swamped every week with problems and lab reports and will have problems classes on top of your lectures. You WILL hate labs – I am yet to speak to anyone who really enjoyed them. But it is all essential to your development as a competent physicist (honestly..) and you will be glad of it in the long run.

Start of degree vs end of degree. Proof that blondes don’t have more fun? 

Living Conditions
Now this is an interesting one. You are probably going to be in student halls. Brace yourself. Here are some things that WILL happen:
– If you drink, you will vomit (probably multiple times).
– Again, if you drink, you will be forced to down a dirty pint (the worst I ever saw contained whisky, milk, garlic and and beer). And then you will probably vomit.
– You will really hate 9am lectures. Especially if you’re hungover/still drunk
– Everything will be a mess. All the time. No one will wash up
– You will encounter the panicked rush far too early to sort out a house for your second year, and you may end up not even liking the people you are gong to live with by the end of term
– The toilet will be covered in all manners of disgusting bodily fluids on multiple occasions.
– You will get freshers flu and feel ill for weeks and your lecture halls will be filled with the sound of coughing.
– Someone will not understand how to use a washing machine (and there may even be someone who takes their laundry home to their mum)
– You will inevitably fall out with someone who was initially your best friend
– There will be some romance and some drama. Some couples will last, others will not. Inevitably, people will start breaking up with home boyfriends/girlfriends.
– You wont change your sheets for an unholy amount of time.
– There will be people in your halls you didn’t even know existed until you awkwardly encounter them in the corridor at the end of term or cooking in the middle of the night.
– You will feel sad and miss your parents, your pets and your home friends, no matter how much fun you’re having.
Sound fun? Unfortunately, this seems to be what it takes to get yourself a physics degree. Things might improve when you move off campus into a house, but this is heavily dependent on your choice of housemates. Really, you have to work out what works best for you in order to survive student living. Maybe you wont mind the mess and the mould. What I will say though is please, please wash your sheets at least once a term. It’s gross.

There’s going to be blood, sweat and tears. Literally. There’s going to be fights and drama and emotional and intellectual struggles. There’s going to be regret and awful hangovers. There will be late nights writing lab reports or finishing problems. You will want to tear your hair out over the electromagnetic field of an infinite charged plane, or a pulley with mass, or second order differential equations, or whether or not γμ is a four vector (spoiler: it isn’t). You will hate some lecturers – worst are the ones that pick people out to answer questions, some will send you to sleep and others you will love and respect. You are going to hate physics, you’re going to love physics, and you’re going to question yourself why the hell you chose it. But in the end, if you make it out with your degree, you’ve done something incredible, and a lot of doors will be open to you. I always knew I wanted to stay with physics and my four years at Warwick left me still enjoying physics and well prepared for a PhD.

If you’re about to start your degree – it’s going to be a wild ride, but it may just be some of the best years of your life. Good luck!


Vote LUX, and give an underdog a chance

Wednesday, March 25th, 2015

I’ve had a busy few weeks after getting back from America, so apologies for the lack of blogging! Some things I’ve been up to:
– Presenting my work on LUX to MPs at the Houses of Parliament for the SET for Britain competition. No prizes, but lots of interesting questions from MPs, for example: “and what can you do with dark matter once you find it?”. I think he was looking for monetary gain, so perhaps I should have claimed dark matter will be the zero-carbon fuel of the future!
– Supplementing my lowly salary by marking an enormous pile of undergraduate problem sheets and by participating in paid eye-tracking studies for both the UCL psychology department and a marketing company
– The usual work on analysing LUX data and trying to improve our sensitivity to low mass dark matter.
And on Saturday, I will be on a panel of “experts” (how this has happened I don’t know) giving a talk as part of the UCL Your Universe festival. The discussion is aptly titled “Light into the Dark: Mystery of the Invisible Universe”, and if you’re in London and interested in this sort of thing, you should come along. Free tickets are available here.

I will hopefully be back to posting more regularly now, but first, a bit of promotion!

Symmetry Magazine are running a competition to find “which physics machine will reign supreme” and you can vote right here.

Symmetry Magazine's

Physics Madness: Symmetry Magazine’s tournament to find the champion physics experiment

The first round matches LUX with the LHC, and considering we are a collaboration of just over 100 (compared to CERN’s thousands of scientists) with nothing like the media coverage the LHC gets, we’re feeling like a bit of an underdog.
But you can’t just vote for us because we’re an underdog, so here are some reasons you should #voteLUX:

-For spin-dependent WIMP-nucleon scattering for WIMPs above ~8GeV, LUX is 10,000x more sensitive than the LHC (see figure below).
-LUX cost millions of dollars, the LHC cost billions.
-It’s possible to have an understanding of how LUX works in its entirety. The LHC is too big and has too many detectors for that!
-The LHC is 175m underground. LUX is 1,478m underground, over 8x deeper, and so is much better shielded from cosmic rays.
-The LHC has encountered problems both times it has tried to start up. LUX is running smoothly right now!
-I actually feel kind of bad now, because I like the LHC, so I will stop.

Dark matter sensitivity limits

Dark matter sensitivity limits, comparing LHC results to LUX in red. The x axis is the mass of the dark matter particle, and the y axis is its interaction probability. The smaller this number, the greater the sensitivity.

Anyway, if you fancy giving the world’s most sensitive dark matter detector a hint of a chance in it’s battle against the behemoth LHC, vote LUX. Let’s beat the system!


A day in the life of a Black Hills WIMP hunter

Tuesday, February 3rd, 2015

The Black Hills of South Dakota may seem an unlikely location to hunt for dark matter, even if the name does seem fitting. If they are known for one thing, it’s gold – and gold requires a mine. Gold mining means deep underground caverns, which just happen to be the perfect home for low background experiments such as dark matter searches thanks to the cosmic ray shielding properties of thousands of feet of rock.

I am currently in Lead, S.D. working underground on LUX, the Large Underground Xenon detector. LUX sits in the Davis campus of SURF – the Sanford Underground Research Facility, an underground lab built in the Homestake gold mine. The Davis campus is named after Ray Davis, whose famous Homestake neutrino experiment was the first to detect neutrinos from the sun. LUX now sits in the same cavern that once housed his ground-breaking experiment.

To cut a long story short, LUX is a big tank of xenon that produces light when particles pass through it. We collect that light with sensors called photomultiplier tubes and search through the data for possible dark matter signals. In particular, we look for WIMPs – Weakly Interacting Massive Particles, the most promising dark matter candidate. Placing LUX deep underground in a mine cuts away lots of background from particles streaming down from space and the atmosphere, as those particles are absorbed by the rock. (For a bit of a more technical insight, I recommend this article, which was written during my shifts last year.)

But what do we, the physicists, actually do out here? Our detector is currently in WIMP search mode, waiting patiently for any sign of dark matter, but it needs a bit of a (human) hand. To give you an insight, here is a typical day in Lead, SD:

5.15am – I wake up. Getting up this early is a little unnecessary, but I like to have some time to wake up in the morning! I have a chat with my boyfriend back in London and spend some time reading my emails as I am 7 hours behind the UK over here.

7.00am – We leave to drive up to the mine. Lead is a tiny town and it only takes a few minutes. The streets look like they are straight out of a cowboy film!

Beautiful morning view from outside SURF. There are two shafts, we use the Yates. The Ross shaft is visible centre-right.

Beautiful morning view from outside SURF. There are two shafts, we use the Yates. The Ross shaft is visible centre-right.

7.15am – We arrive at SURF. We’re always a bit rushed; we grab our head torches and head to the changing area. Overalls, steel capped dirty boots, helmets, safety glasses, self-rescuers and the head torch all have to be donned. It’s unpleasantly hot, as the water running down the lift shaft must not be allowed to freeze. We take two golden tags with our name on and place one on a board to show we have gone underground, the other stays on your person (to identify your body…? Doesn’t bear thinking about!).

Me in my fashionable mining gear, 4850 feet underground at SURF

Me in my fashionable mining gear, 4850 feet underground at SURF

7.30am – The cage (literally a big metal cage that acts as our ride downwards) departs from ground level. The cage operators have impeccable timing and take care of opening and closing the door and contacting the hoist operator, who will lower us down from the surface. Usually the morning cage isn’t too busy (there is an earlier one at 7am) but it’s still not the most pleasant experience. Sometimes we are all a little too close for comfort – miners don’t tend to be small guys! Also, if you stand in the wrong place you get cold water dripping on you for the whole journey.

7.45am – The cage arrives at the 4850 level – 4850 feet underground. We leave the cage and head to the bootwash. LUX and the other main experiment at SURF, Majorana, both need clean conditions as they are low background experiments. Any dirt treaded in to the lab could contain radioactive elements that would be very bad for our detectors. For Majorana, the need for cleanliness is much higher than LUX, and so they have a cleanroom that requires them to wear special body suits, face masks and hair nets. I am extremely glad that isn’t necessary for LUX! After cleaning our boots, we remove all of our gear except for the glasses, change into clean steel-capped boots (I obviously have pink ones!) and new hard hats.

8.00am – Morning meeting. We all gather in the LUX control room and discuss the plan for the day. This can vary wildly depending on the decisions made in the weekly planning meeting. We may have taken data or seen conditions that suggest something needs investigating or fixing, or it might just be boring old WIMP search mode where nothing special needs to be done. The control room is the only place where we can remove our hard hats and safety glasses – stopping people leaving the room without them has to be done regularly!

12.00pm – We tend to take a break for lunch. Throughout the morning, everyone will have been going about their various shifting duties – monitoring all aspects of the detector, sampling xenon to check its purity, injecting krypton for calibration, refilling the liquid nitrogen store etc. We may have received some training from off-site system experts or attended a meeting, depending on what day it is. Conditions underground are pretty good, you start to forget where you are – only the lack of windows and bumpy walls remind you! During my previous visit 10 months ago we still had incinerator toilets – the less said about these, the better! They often broke down after the lunchtime rush and if you needed the toilet you had to put all your dirty mining gear back on and go to use the chemical toilets out in the mine. Now, thank god, we actually have running water!

View from the lower Davis. The water tank containing LUX is visible in the centre.

View from the lower Davis. The water tank containing LUX is visible in the centre.

4.00pm –If we are lucky, we get the cage up at 4. If something has gone wrong or there is enough to be done we may have to stay till the next cage at 4.45pm or even the latest at 5.30pm. In an emergency we may be able to come up later but we prefer to not have to do that! So it’s back to being squished in a damp dark cage full of South Dakotan miners!

6.00pm – dinnertime! Often someone will cook a group meal or we will head out to get food either in Lead, Deadwood or Spearfish. If it’s Friday, we go to Lewie’s for greasy burgers to do the “pub quiz” (I have been attempting to teach my American colleagues some proper English) . We tend to do very well in the trivia sections of the quiz, but the music round is our weakness. There’s too much country for us outsiders. The quiz host pulls several names out of a hat for prizes each week; so far on this trip I’ve won an extra large bud-light t-shirt (bear in mind I wear XS…) and some hot wings, which as a vegetarian I couldn’t eat!

8.00pm – depending on the person / how much work they have to do, some of us may continue to work. I mainly do analysis work and sometimes find I don’t have much time to get it done during the underground day so sometimes I try to get a bit done at night. I am also currently the shift manager so I have to fill in a shift report detailing what we have done each day.

10.00pm – bedtime. I’m wiped out by this stage and fall straight asleep, usually dreaming about LUX.

Devil's Tower

Devil’s Tower, Wyoming

But it’s not all hard work. Every other weekend we get 4 non-underground days, which gives us a little time to see the sights of South Dakota (or, if gambling is your thing, there are plenty of casinos in Deadwood)! On my last visit, I visited Mount Rushmore (smaller than you’d think) and Crazy Horse (much bigger than you’d think!), the latter being an enormous mountain carving that has been in progress for over 50 years – still, only his face is complete. If we go somewhere far, someone always has to stay close to SURF in case of an emergency – our detector might need us! This weekend we headed over to the neighbouring state of Wyoming to see Devil’s Tower; an ethereal protrusion of volcanic rock 1,267 feet above the surrounding ground! It was an incredible sight, although temperatures had dropped and I spent most of the time there jumping around trying to restore circulation to my hands. I have Raynaud’s syndrome, which means the moment I get slightly cold my fingers turn white and become extremely painful! We actually had -19 C (-2F) here in Lead a couple of days ago – not fun! Luckily, it’s warm underground!

Speaking of the weather, this time of year there’s snow, so much snow! It’s crazy to think how Britain comes to a standstill with schools and businesses closing when we get a tiny smearing of snow, whilst here several feet overnight is not uncommon. But life goes on in Lead, and we usually still make it up the hill to the mine!

The LUX collaboration. This is standard Lead weather!

The LUX collaboration, demonstrating a standard Lead winter! There seems to be a hairy impostor in this photo…

Lead is a place very different to London –  everyone is so friendly and pleasant! There’s no avoiding all human interaction like on the tube in London – everyone says hello! It is difficult eating as a vegetarian here, but most places have been accommodating and have allowed me to order special things (e.g. a salmon salad with no salmon, a Reuben with no beef!). The locals are always interested to hear how things are going at SURF. Amusingly, one resident excitedly asked “You’re from Pizza Lab?” after they heard us discussing the lab! One big shock to me, however, was finding out about the gun laws here in S.D. – concealed carry permits can be issued and apparently most people you see will be carrying a gun (maybe an exaggeration? But maybe not, you just can’t tell!). But then again all the gun laws in America seem alien to us Brits!

The staff at SURF are also extremely accommodating, helping us get underground in emergencies, and their health and safety policies are commendable. Right now, most shifters are getting trained as “guides” – each research team has a guide who is responsible for getting you to safety, whether that is above ground or in the refuge chamber.

The refuge chamber is something we all hope to never have to use. In the event that we cannot reach the ground from either of the two mine shafts, and that the rest of the mine is dangerous to inhabit (for example a fire causing a lack of oxygen), this is where we would go. It has carbon dioxide scrubbers, oxygen, water and a huge supply of “nutrition bars’ – rock hard bars containing a whopping 500 calories each so that someone could easily survive on two to four a day. There is enough oxygen for the entire underground population to survive for many days, awaiting rescue – but it’s not something we like to think about happening – especially since the toilets are just buckets!

As much as I prefer to be safely in front of my laptop, with no million dollar detectors in my hands and not facing the risks of working underground (note to self, do NOT go back on the Wikipedia list of mining accidents!), I do enjoy being on-site. It makes me feel like I’m actually part of something. We are the “underground crew”, a team of physicists travelling 4850 feet below Earth’s surface every day to take care of our precious detector. We keep things running smoothly, allowing LUX and our colleagues off-site to keep on searching for dark matter! Who knows, the Black Hills may yet bear some dark matter fruit!


The Theory of Everything

Thursday, January 15th, 2015

Last night I went to see The Theory of Everything, the biographical film about Stephen Hawking, adapted from the memoir of his ex-wife, Jane Wilde Hawking. News literally just in – it has been nominated for the Best Picture and Adapted Screenplay Oscars, and there are Best Actor and Best Actress nominations for Eddie Redmayne (Stephen) and Felicity Jones (Jane). Arguably today’s most famous scientist, Stephen Hawking is a theoretical physicist and cosmologist, now holding the position of Director of Research at Cambridge’s Centre for Theoretical Cosmology. He suffers from motor neurone disease; a degenerative disease that has left him unable to move most of the muscles in his body. He now communicates by selecting letters and words on a computer screen using one muscle in his cheek. His computerised voice is world famous and instantly recognisable. He is responsible for ground-breaking work on black holes and general relativity.


I thought the film was fantastically made and the acting incredible; Redmayne’s portrayal of Hawking’s physical condition was uncanny. I shed a few tears at the plight of this man surviving against all the odds whilst doing incredible theoretical physics, and his wife, ever patient and loving, taking care of him and bearing his children despite his health getting only worse.
There have been some complaints about the lack of focus in the film on Hawking’s scientific work; the film instead focuses mainly on his relationship with Jane and their struggle as his condition deteriorates. This should not be a surprise when the film was adapted from Jane’s own writing. If you want to know more about Hawking’s work in physics, then I strongly recommend his physics books. I first attempted to read A Brief History of Time, his most famous publication, age 11. This was obviously optimistic of me, and I gave up after the first couple of chapters. I tried again during my A-levels but never got round to finishing it, but having now studied cosmology and general relativity in much more detail I fully intend to give it another try! I have however read The Universe in a Nutshell, a more accessible book on the history of modern physics and cosmology, as well as discussions on that holy grail of physics, and the title of the film, a ‘theory of everything’.

But what is a theory of everything? Also known as a ‘final theory’, an ‘ultimate theory’, and a ‘master theory’, it sounds rather grand. A ToE would elegantly explain our universe, maybe even in just one equation, linking all the aspects that we can not currently reconcile with each other. It would allow a deep understanding of the universe we live in, as Hawking himself professed despite being an atheist:

If we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason — for then we would know the mind of God.

Sounds good, right? The ultimate triumph. Unfortunately, so far, attempts at developing a ToE have not delivered. Why not? First we need to understand a little more about the physics we know and understand.
Our universe has four forces governing everything that happens within it:

  • Electromagnetism – the interaction of photons and charged particles that we are familiar with in electricity, magnets, etc.
  • Weak force – the interaction responsible for radioactive decay.
  • Strong force – the interaction that binds together the protons and neutrons in a nucleus
  • Gravity – the attraction of bodies with mass to each other, the reason we don’t fly away from the Earth and why the Earth orbits the sun (and also why we know about dark matter!)

Why four? No one knows. It has been shown that at electromagnetism and the weak force can be combined into an ‘electroweak’ force at high energies. This means that in our everyday low energy universe (as opposed to the hot dense universe shortly after the big bang) that electromagnetism and the weak force are just two faces of the same force.

If electromagnetism and the weak force can be combined, can we do the same with the strong force and gravity? Combining the electroweak and the strong force results in a “GUT” – a Grand Unified Theory, (NB despite being grand, this does not yet include gravity). The energy required to see the joining of the strong and the electroweak would be beyond the levels we could reach with particle colliders. We do not currently have a generally accepted GUT, but there are lots of complicated theories in the works.
The final step to a ToE would be the joining of gravity with a GUT theory. This is the real sticking point. As Jane illustrates with a pea and a potato over dinner in the film, the unification of quantum field theory (the pea) on the tiny scales with general relativity (the potato) on large scales has so far proven undoable.
Quantum field theory is what we particle physicists deal with, the standard model of particle physics, tiny things like photons and quarks and electrons, all interacting via electromagnetism, the weak force and the strong force. General relativity is far in the other direction; stars, galaxies, galaxy clusters. Big things with lots of mass, causing curvatures in space-time that manifests as gravity. Both quantum field theory and general relativity have been tested to extreme precision – they both work perfectly on their relative scales. So where does the problem in joining them lie?

Hawking’s greatest work is on black holes; the infinitely small and dense aftermath of the collapse of an enormous star. Once a star greater than about 23 solar masses runs out of fuel to produce energy, its core collapses under its own weight, expelling its outer layers in an explosion called a supernova that outshines its own galaxy. If the core is big enough, it will continue collapsing until it becomes a ‘space-time singularity’ – a point in space infinitely small and dense, where not even light can escape.
When we try to understand the physics inside that point, we start encountering problems. We need both quantum field theory and general relativity – we have a tiny tiny space but a huge mass, and infinities start popping up all over the place. The maths just doesn’t work.

The evolution of stars, showing how a sufficiently large star can end its life as a black hole

The evolution of stars, showing how a sufficiently large star can end its life as a black hole

Stephen Hawking, with the computerised speech system that has allowed him to communicate after losing his ability to speak

Stephen Hawking, with the computerised speech system that has allowed him to communicate and continue his physics work after losing his ability to speak

Hawking has dedicated much of his life to trying to unify these two pillars of modern physics, so far with no luck. This begs the question, if his incredible mind cannot do it, what hope do we have? Currently, a popular approach is string theory – the theory that everything is made of tiny strings, vibrating in many (up to 26!) dimensions. This may sound silly, but it’s actually quite elegant – each different particle is made of a string vibrating in a different mode. An issue with string theory currently is it offers no easily testable predictions. Some of the best minds of today are working on this, so there is still hope!

Stephen Hawking is clearly an incredible man. He has a level of intelligence and a talent in mathematics and physics most of us physicists can only dream of. However, I believe Jane also deserves a huge amount of credit. The diagnosis of motor neurone disease came only shortly after they began dating, but she embarked on a life with him, marrying him and having his children, taking on the mammoth task of caring for him mostly alone, despite his prognosis of only 2 years to live.

Of course, Hawking has far exceeded those two years. He is now 73, reaching what is basically a normal life expectancy despite having a disease that has an average survival from onset of only 3-4 years. He was diagnosed aged only 21. Diseases such as his are tragic, leaving a person’s mind totally intact but trapped inside a failing body. Many would just give up, but Hawking’s love for both Jane and physics drove him to persevere and become the esteemed professor he is today.

I strongly recommend watching The Theory of Everything, even to those uninterested in cosmology. It’s a beautiful, romantic drama set in picturesque Cambridge, emotionally powerful and moving, and certainly does not require you to understand the physics!


Two Royal Visits: Physics at the Royal Opera House and the Royal Observatory, Greenwich

Monday, December 1st, 2014

I’ve had an extremely busy few weeks! We are preparing the next LUX paper, and it’s been a hectic learning curve for me. Most importantly, I now know to never expect anything to be ready on time! It’s really exciting to know that soon I will actually have my name on a published paper – if amongst 100+ others, but that’s how it goes in experimental particle physics. It will still be a proud moment for me; I might start to feel like I’m actually useful. I managed to squeeze in a couple of trips since my last post, one of which was to a talk entitled “Insights: what makes the perfect song?” at London’s Royal Opera House. This was an Institute of Physics event, and the speaker was none other than Professor Brian Cox.

I can almost guarantee if you are British you will have heard of Brian Cox. He is a particle physicist at the University of Manchester, and a member of ATLAS, one of the general purpose LHC detectors. His floppy hair and Mancunian accent are a familiar sight and sound on the BBC; he is always expressing his love for physics and the universe in documentaries such as “Wonders of the Universe”, “Wonders of the Solar System” and most recently “Human Universe”.
I actually first saw Brian on an episode of Horizon (a British science documentary) back in 2008 entitled “What on Earth is wrong with Gravity?”. I was only a few months into AS levels in Physics, Maths, Chemistry and Biology and actually had not yet chosen to go on to do my degree in physics; I was keen at this point to go down either a medical or biochemistry route. The Horizon episode focussed on the uncomfortable discord between quantum mechanics and general relativity, and fascinated me. I was reminded how much I loved physics (my spirit had been somewhat broken by early physics AS-level classes on materials where we learnt about stress, strain and brittle fractures). I believe Horizon and Brian had at least a small influence in my decision to go on and study physics at university. I have found Cox to be like marmite amongst physicists, either loved or hated. Hate is dominant among men (jealous perhaps?) who believe he is not deserving of his professorship status, and is only a face for the media.
I managed to convince my Brian Cox-hating boyfriend to attend the Institute of Physics talk with me. He also studied physics but holds no love for it (he is selling his soul to law!); it was the music aspect that convinced him to come. Although the evening was enjoyable, we were both fairly disappointed in the lack of actual science content. The panel comprised of Christine Rice, an opera singer with a physics degree, Philip Ball, writer of “The Music Instinct”, and Maria Witek, a researcher in neuroscience.

L-R:  Christine Rice, Philip Ball, Maria Wiltek and Brian Cox at Insights: what makes the perfect song?

Despite their commendable qualifications, there were no convincing arguments as to why certain notes sound good together for example, and why our brain reacts the way it does to different things in music. We did see some statistics on the frequency of different musical intervals, saw how syncopated beats make you want to dance more (see Don’t Stop Till You Get Enough – Michael Jackson) and learnt about the Gestalt principles of harmonic progression. We really enjoyed Joe Stilgoe, a jazz pianist’s whose jaunty performances were definitely the highlight. The discussion was thought-provoking but I had been hoping for something on of the waveforms of musical harmonies and melodies, of the modes of vibration on a string, what happens in the brain when we hear a song we love, etc.

I believe the night had been more intended for Royal Opera house frequenters, of which we definitely aren’t (call me uncultured but opera singing hurts my ears), rather than physicists. As the woman sitting beside us excitedly told us, “I’m retired, I spend all the time I can learning. I just love opera and I’m here all the time, so I come to anything like this.”, perhaps this event was aimed more at the likes of her, whose attitude to being retired I found admirable (although the first thing she said to us was “I’m only here for him!” meaning Brian Cox…).

Pictured: evidence of Brian Cox doing real physics work, alongside UCL head of physics and Guardian blogger Jon Butterworth

Pictured: evidence of Brian Cox doing real physics work, alongside UCL head of physics and Guardian blogger Jon Butterworth

Sometimes, after spending weeks on end staring at lifeless code, I forget how much I love physics. The enthusiasm of a great science communicator can remind me, and Brian Cox is one of the best. He has done great things for public attitudes towards physics, whether or not he does his fair share as a professor of experimental particle physics! (I have, by the way, heard from an ATLAS member that in fact he has published some excellent papers with respectable people, such as UCL’s very own Jon Butterworth…).

I could see Lady Gaga in this... planet dress at Royal Observatory Greenwich

I could see Lady Gaga in this… a steampunk planet dress at Royal Observatory Greenwich

My second excursion was to the Royal Observatory in Greenwich. The main purpose of my visit (again, dragging along my tolerant boyfriend) was to see a show called “The Dark Universe”. Admittedly, we got a bit confused, and spent most of our trip looking at old parts of telescopes and clocks, as well as a bizarre steampunk exhibition, before we found where the actual astronomy stuff was. I would say the observatory is well worth a visit – but be warned, it’s up an incredibly steep hill. It always amazes me what the astronomers of the past managed without computers. The 30-minute show was in the Planetarium, so we were seated reclining under a dome (a set-up my boyfriend decided should be implemented in all cinemas!). Neil deGrasse Tyson narrated – another great science communicator, and I was thoroughly impressed. The visuals were stunning and the physics accurate, but still understandable to non-scientists. LUX didn’t feature in the section on dark matter (AMS – the Alpha Magnetic Spectrometer, mounted on the International Space Station, provides much better visuals!) but there was a mention of detectors deep underground, which satisfied me. The film focused on the astounding fact that we do not understand a whopping 95.1% of our universe, and I feel really lucky to be one of those people attempting to help reduce that by 26.8% by uncovering the nature of dark matter.

To add to my busy schedule this week is an offsite shift for LUX. This involves keeping an eye on the detector and the data we are taking whilst those in the USA sleep to make sure all is well. We have a vast array of sensors monitoring our detector’s condition, meaning I can see with the click of a button if something has gone wrong. This job feels like rather a lot of responsibility; I’m hoping in my next post I can say it passed without incident!


Interstellar: Bring tissues, and maybe a general relativity textbook

Monday, November 10th, 2014

On Saturday, I went to see Interstellar at the London BFI IMAX. I wouldn’t usually be so extravagant; my usual cinema trips are on 2-for-1 Orange Wednesdays. But I felt Interstellar was worth seeing in all its IMAX high resolution glory – and it definitely was.  The film, directed by Christopher Nolan, is an epic masterpiece describing the journey of Cooper (Matthew McConaughey), a pilot-turned farmer-turned pilot again out of our galaxy in search of a new home for humans. The Earth is blighted by, well, blight, and the human race is starving. It paints a grim picture of our potential future here on Earth, and it seems entirely plausible.

The black hole featured in Interstellar. Equations from physicist Kip Thorne were used to model the lensing of light around the black hole.

The black hole featured in Interstellar. Equations from physicist Kip Thorne were used to model the lensing of light around the black hole.

Interstellar did an excellent job of using physics. This should be expected – the filmmakers worked with prominent theorist Kip Thorne. Thorne has worked on all those delicious-sounding areas of cosmology that I wish I had the brains for – black holes, wormholes, quantum gravity, gravitational waves, relativistic stars, etc. Genuine equations were used to visualise the black hole and wormhole featured in Interstellar – a fact that the majority of the audience would not know or particularly care about, but satisfies physicists. There is something to be said for not straying too far from real science in films when we live in a world plagued by quack-science that has tarred the word quantum (for examples, just search for quantum healing, or note that searching for quantum crystals brings up a site for buying “quantum balance crystals” before it brings up anything related to quantum mechanics and solid-state physics).

I was surprised to find posts on the internet “explaining” Interstellar. The film wrapped everything up nicely in my opinion. But most people haven’t done a course in General Relativity! I don’t pretend to be any sort of expert, it was 2 years ago and I can barely remember the maths but I do know the basic concept of gravitational time dilation. It is this, it seems, that was confusing people – how time was passing at a different rate for those on Earth and those in space.

I don’t want to give away any spoilers, but Cooper ends up close to a black hole (aptly named “Gargantua”), where time runs much more slowly for him than anyone further away. This is a strange and frightening thought for us humans who spend our lives moving consistently forwards in time at the same rate (at least within our perception). In reality, you are ageing slightly faster at the top of a skyscraper than you are at the Earth’s surface – but the effect is too small to notice. It is, however, definitely there. General and special relativity are used to correct the time given by GPS satellites – they are in a weaker gravitational field than we are down on Earth, and so their clocks run slightly faster. Without this correction, GPS would not work.

To understand relativity, you need to remember that time is just another dimension, a fact that becomes important later in the film. Like our 3D space is warped around a massive object, so is time. The foundations of general relativity lie in something called the “equivalence principle”. Einstein himself wrote this as:

“A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body.


A geodesic is a path minimising line connecting two points on a sphere. Everything falling under gravity is following one of these lines in curved space-time.

What this means is that under gravity, all things will accelerate at the same rate, independent of their mass. We see this on Earth, where that rate of acceleration is 9.81m/s. A feather and a rock dropped together reach the ground at the same moment (ignoring air resistance!).

Next, Einstein deduced that an object in “free-fall”, i.e. an object with only gravity acting upon it, is not actually accelerating – there is no force of gravity. This was actually one of those rare “my mind is blown” moments I had during my degree.  An object in free-fall is not accelerating – it is simply following a geodesic in curved space-time. A geodesic is the analogy to a straight line within curved space – think if it as the shortest path between two points on a sphere.

I’m going a bit off tangent here, but general relativity is a fascinating subject! What I wanted to get to is the time dilation part. Why does time run slower for someone in a strong gravitational field? It actually comes back to special relativity, general relativity’s less scary little brother. I was taught special relativity in the first year of my degree, and it was the first time I felt like I was learning real exciting physics. The postulates of SR are:

  • The laws of physics are the same in all inertial frames of reference.
  • The speed of light in free space has the same value c in all inertial frames of reference.
A beam of light in an accelerating rocket appears curved to an outside observer. This would be the same for a free-falling laboratory.

A beam of light in an accelerating rocket appears curved to an outside observer. This would be the same for a free-falling laboratory.

Combining the principle of a freely-falling (i.e. travelling on a geodesic in a gravitational field) laboratory and applying special relativity introduces time dilation. The Pound and Rebka experiment is helpful to read up on for understanding this. By the laws of SR, both an observer inside the laboratory and one outside should measure the speed of light as c. Imagine a beam of light in the laboratory, the observer outside sees the path of light bend as the laboratory falls, whilst the observer inside sees a straight line as they are in an inertial frame. This means that for the outside observer, the light has travelled a longer (curved) path. As light always travels at c, the observer will deduce that more time has passed inside the laboratory than the person inside will measure. The stronger the gravitational field, the faster the free-fall, and the more the light will appear curved to the outside observer – so the time dilation factor increases with the field strength.

The strength of the black hole’s field in Interstellar means that minutes for Cooper become years for those outside. He is “free-falling” at an incredible rate, so his clock is running thousands of times slower than the ones on Earth, but it feels totally normal to him. He sees his own clock running at a normal speed, but he knows that the ones on Earth are running much faster. Emotions run high as every second he spends on his mission could be years he is missing of his children’s’ lives.  I came out of the film an emotional wreck – I’d shed many tears and my chest felt tight, it was genuinely traumatic. Don’t get me wrong, I cry at a lot of films (I even cried when Gandalf died in Lord of the Rings, even though I’d read the books and so knew he was fine.) but I’ve never left one still feeling so upset. But that helps make it a brilliant film – not just the special effects, the beautiful images of space and stars and black holes, but the human reality; at the end of the day, it is just a father fighting to save his children.

I strongly recommend you go see this film, whether you are a physicist or not. I could go on for a lot longer and discuss the paradoxes some of the wormhole travel introduces as well as some other puzzles, but that would reveal spoilers, so instead I’ll just stop here!  Interstellar is heartbreaking, but also breathtaking, and also warns us to take care of our planet. It’s not so easy to find another, and for god’s sake don’t stop investing in science! Make sure you bring tissues.



Have we detected Dark Matter Axions?

Wednesday, October 22nd, 2014

An interesting headline piqued my interest when browsing the social networking and news website Reddit the other day. It simply said:

“The first direct detection of dark matter particles may have been achieved.”

Well, that was news to me! 
Obviously, the key word here is “may”. Nonetheless, I was intrigued, not being aware of any direct detection experiments publishing such results around this time. As a member of LUX, there are usually collaboration-wide emails sent out when a big paper is published by a rival group, most recently the DarkSide-50 results . Often an email like this is followed by a chain of comments, both good and bad, from the senior members of our group. I can’t imagine there being a day where I think I could read a paper and instantly have intelligent criticisms to share like those guys – but maybe when I’ve been in the dark matter business for 20+ years I will!

It is useful to look at other work similar to our own. We can learn from the mistakes and successes of the other groups within our community, and most of the time rivalry is friendly and professional. 
So obviously I took a look at this claimed direct detection. Note that there are three methods to dark matter detection, see figure. To summarise quickly,

The three routes to dark matter detection

  • Direct detection is the observation of an interaction of a dark matter particle with a standard model one
  • Indirect detection is the observation of annihilation products that have no apparent standard model source and so are assumed to be the products of dark matter annihilation.
  • Production is the measurement of missing energy and momentum in a particle interaction (generally a collider experiment) that could signify the creation of dark matter (this method must be very careful, as this is how the neutrinos are measured in collider experiments).

So I was rather surprised to find the article linked was about a space telescope – the XMM-Newton observatory. These sort of experiments are usually for indirect detection. The replies on the Reddit link reflected my own doubt – aside from the personification of x-rays, this comment was also my first thought:

“If they detected x-rays who are produced by dark matter axions then it’s not direct detection.”

These x-rays supposedly come from a particle called an axion – a dark matter candidate. But to address the comment, I considered LUX, a direct dark matter detector, where what we are actually detecting is photons. These are produced by the recoil of a xenon nuclei that interacted with a dark matter particle, and yet we call it direct – because the dark matter has interacted with a standard model particle, the xenon. 
So to determine whether this possible axion detection is direct, we need to understand the effect producing the x-rays. And for that, we need to know about axions.

I haven’t personally studied axions much at all. At the beginning of my PhD, I read a paper called “Expected Sensitivity to Galactic/Solar Axions and Bosonic Super-WIMPs based on the Axio-electric Effect in Liquid Xenon Dark Matter Detectors” – but I couldn’t tell you a single thing from that paper now, without re-reading it. After some research I have a bit more understanding under my belt, and for those of you that are physicists, I can summarise the idea:

  • The axion is a light boson, proposed by Roberto Peccei and Helen Quinn in 1977 to solve the strong CP problem (why does QCD not break CP-symmetry when there is no theoretical reason it shouldn’t?).
  • The introduction of the particle causes the strong CP violation to go to zero (by some fancy maths that I can’t pretend to understand!).
It has been considered as a cold dark matter candidate because it is neutral and very weakly interacting, and could have been produced with the right abundance.
Conversion of an axion to  a photon within a magnetic field (Yamanaka, Masato et al)

Conversion of an axion to a photon within a magnetic field (Yamanaka, Masato et al)

For non-physicists, the key thing to understand is that the axion is a particle predicted by a separate theory (nothing to do with dark matter) that solves another problem in physics. It just so happens that its properties make it a suitable candidate for dark matter. Sounds good so far – the axion kills two birds with one stone. We could detect a dark matter axion via an effect that converts an axion to an x-ray photon within a magnetic field. The XMM-Newton observatory orbits the Earth and looks for x-rays produced by the conversion of an axion within the Earth’s magnetic field. Although there is no particular interaction with a standard model particle (one is produced), the axion is not annihilating to produce the photons, so I think it is fair to call this direct detection.

What about the actual results? What has actually been detected is a seasonal variation in the cosmic x-ray background. The conversion signal is expected to be greater in summer due to the changing visibility of the magnetic field region facing the sun, and that’s exactly what was observed. In the paper’s conclusion the authors state:

“On the basis of our results from XMM-Newton, it appears plausible that axions – dark matter particle candidates – are indeed produced in the core of the Sun and do indeed convert to soft X-rays in the magnetic field of the Earth, giving rise to a significant, seasonally-variable component of the 2-6 keV CXB”



Conversion of solar axions into photons within the Earth’s magnetic field (University of Leicester)

Note the language used – “it appears plausible”. This attitude of physicists to always be cautious and hold back from bold claims is a wise one – look what happened to BICEP2. It is something I am personally becoming familiar with, last week having come across a lovely LUX event that passed my initial cuts and looked very much like it could have been a WIMP. My project partner from my masters degree at the University of Warwick is now a new PhD student at UCL – and he takes great joy in embarrassing me in whatever way he can. So after I shared my findings with him, he told everyone we came across that I had found WIMPs. Even upon running into my supervisor, he asked “Have you seen Sally’s WIMP?”. I was not pleased – that is not a claim I want to make as a mere second year PhD student. Sadly, but not unexpectedly, my “WIMP” has now been cut away. But not for one second did I truly believe it could have been one – surely there’s no way I‘m going to be the one that discovers dark matter! (Universe, feel free to prove me wrong.)

These XMM-Newton results are nice, but tentative – they need confirming by more experiments. I can’t help but wonder how many big discoveries end up delayed or even discarded due to the cautiousness of physicists, who can scarcely believe they have found something so great. I look forward to the time when someone actually comes out and says ‘We did it – we found it.” with certainty. It would be extra nice if it were LUX. But realistically, to really convince anyone that dark matter has been found, detection via several different methods and in several different places is needed. There is a lot of work to do yet.

It’s an exciting time to be in this field, and papers like the XMM-Newton one keep us on our toes! LUX will be starting up again soon for what we hope will be a 300 day run, and an increase in sensitivity to WIMPs of around 5x. Maybe it’s time for me to re-read that paper on the axio-electric effect in liquid xenon detectors!


Physics Laboratory: Back to Basics

Friday, October 10th, 2014

Dark matter –  it’s essential to our universe, it’s mysterious and it brings to mind cool things like space, stars, and galaxies. I have been fascinated by it since I was a child, and I feel very lucky to be a part for the search for it. But that’s not actually what I’m going to be talking about today.

I am a graduate student just starting my second year in the High Energy Physics group at UCL, London. Ironically, as a dark matter physicist working in the LUX (Large Underground Xenon detector) and LZ (LUX-ZEPLIN) collaborations, I’m actually dealing with very low energy physics.
When people ask what I do, I find myself saying different things, to differing responses:

  1. “I’m doing a PhD in physics” – reaction: person slowly backs away
  2. “I’m doing a PhD in particle physics” – reaction: some interest, mention of the LHC, person mildly impressed
  3. “I’m doing a PhD in astro-particle physics” – reaction: mild confusion but still interested, probably still mention the Large Hadron Collider
  4. “I’m looking for dark matter!” – reaction: awe, excitement, lots of questions

This obviously isn’t true in all cases, but has been the general pattern assumed. Admittedly, I enjoy that people are impressed, but sometimes I struggle to find a way to explain to people not in physics what I actually do day to day. Often I just say, “it’s a lot of computer programming; I analyse data from a detector to help towards finding a dark matter signal”, but that still induces a panicked look in a lot of people.

Nevertheless, I actually came across a group of people who didn’t ask anything about what I actually do last week, and I found myself going right back to basics in terms of the physics I think about daily. Term has just started, and that means one thing: undergraduates. The frequent noise they make as they stampede past my office going the wrong way to labs makes me wonder if the main reason for sending them away for so long is to give the researchers the chance to do their work in peace.

Nonetheless, somehow I found myself in the undergraduate lab on Friday. I had to ask myself why on earth I had chosen to demonstrate – I am, almost by definition, terrible in a lab. I am clumsy and awkward, and even the most simple equipment feels unwieldy in my hands. During my own undergrad, my overall practical mark always brought my average mark down for the year. My masters project was, thank god, entirely computational. But thanks to a moment of madness (and the prospect of earning a little cash, as London living on a PhD stipend is hard), I have signed up to be a lab demonstrator for the new first year physicists.

Things started off awkwardly as I was told to brief them on the experiment and realised I had not a great deal to say.  I got more into the swing of things as time went by, but I still felt like I’d been thrown in the deep end. I told the students I was a second year PhD student; one of them got the wrong end of the stick and asked if I knew a student who was a second year undergrad here. I told him I was postgraduate and he looked quite embarrassed, whilst I couldn’t help but laugh at the thought of the chaos that would ensue if a second year demonstrated the first year labs.


The oscilloscope: the nemesis of physics undergrads in labs everywhere

None of them asked what my PhD was in. They weren’t interested – somehow I had become a faceless authority who told them what to do and had no other purpose. I am not surprised – they are brand new to university, and more importantly, they were pretty distracted by the new experience of the laboratory. That’s not to say they particularly enjoyed it, they seemed to have very little enthusiasm for the experiment. It was a very simple task: measuring the speed of sound in air using a frequency generator, an oscillator and a ruler. For someone now accustomed to dealing with data from a high tech dark matter detector, it was bizarre! I do find the more advanced physics I learn, the worse I become at the basics, and I had to go aside for a moment with a pen and paper to reconcile the theory in my head – it was embarrassing, to say the least!

Their frustration at the task was evident – there were frequent complaints over the length of time they were writing for, over the experimental ‘aims’ and ‘objectives’, of the fact they needed to introduce their diagrams before drawing them, etc. Eyes were rolling at me. I was going to have to really try to drill it in that this was indeed an important exercise. The panic I could sense from them was a horrible reminder of how I used to feel in my own labs. It’s hard to understand at that point that this isn’t just some form of torture, you are actually learning some very valuable and transferrable skills about how to conduct a real experiment. Some examples:

  1. Learn to write EVERYTHING down, you might end up in court over something and some tiny detail might save you.
  2. Get your errors right. You cannot claim a discovery without an uncertainty, that’s just physics. Its difficult to grasp, but you can never fully prove a hypothesis, only provide solid evidence towards it.
  3. Understand the health and safety risks – they seem pointless and stupid when the only real risk seems to be tripping over your bags, but speaking as someone who has worked down a mine with pressurised gases, high voltages and radioactive sources, they are extremely important and may be the difference between life and death.

In the end, I think my group did well. They got the right number for the speed of sound and their lab books weren’t a complete disaster. A few actually thanked me on their way out. 

It was a bit of a relief to get back to my laptop where I actually feel like I know what I am doing, but the experience was a stark reminder of where I was 5 years ago and how much I have learned. Choosing physics for university means you will have to struggle to understand things, work hard and exhaust yourself, but in all honestly it was completely worth it, at least for me. Measuring the speed of sound in air is just the beginning. One day, some of those students might be measuring the quarks inside a proton, or a distant black hole, or the quantum mechanical properties of a semiconductor. 

I’m back in the labs this afternoon, and I am actually quite looking forward to seeing how they cope this week, when we study that essential pillar of physics, conservation of momentum. I just hope they don’t start throwing steel ball-bearings at each other. Wish me luck.