Particle Dreams

(Originally published Jul 30, 2012 on
(This is an old piece, but few people ever actually read it, so I thought I’d take it out and give it an airing for the benefit of anybody who never got round to reading about the Higgs boson discovery at the time. And because Sarah deserves the exposure. Enjoy.)

What is it like to be a particle physicist, probing the deep questions of existence, at a time when the most significant discovery for a generation (the announcement by CERN on July 4th of what looks like the discovery of the Higgs boson, in case you missed it) has splashed the discipline across the pages of every news outlet in the land?

Particle physics PhD candidate, Sarah Baker.

Sarah Baker is a 27 year-old PhD student just months away from completing the thesis she has worked on for the last four years. She’s been working with Jonathan Butterworth at UCL investigating a particular “search channel” for the Higgs in data coming out of the ATLAS detector at CERN.

Asked the question above, she said: “Amazing – I didn’t really imagine we’d ever have a day when I was working in physics like we had the other day.” She added even though everyone involved in her project knew a discovery would be announced: “Everyone just felt: ‘this is the best day to be a physicist.’

“I’m so lucky I’ve come in at the beginning of my career and the last four years of what has basically been 20 years of work. A lot of people I know flew to CERN just so they could sit in the auditorium. They camped out to make sure they got a seat, and we saw them on the big screen, turning round and waving at us, because we were texting them. I think everyone shared that feeling… apart from the people who lost bets.”

Talking about how she ended up helping hunt for the Higgs, Sarah said she was very interested in the big questions when she was younger – space, time, what makes up the universe. She went to Reading University to study physics, but they didn’t specialise in particle physics and it was only during a summer job at an oceanography company that she would pass the Rutherford Appleton laboratory every day on the train between Reading and Didcot. Looking out at the mini-accelerator lab through the window she realised that was where she wanted to be and so she emailed them asking if they wanted a work experience student. They said yes and after a couple of weeks work shadowing someone she was hooked and “it all went from there”.

From molecules to atoms to protons, neutrons and electrons, physics has gradually revealed the structure of matter, finally showing that protons and neutrons are made of quarks. These appear to be the fundamental building blocks of matter, but subatomic particles can be further divided into fermions and bosons. Fermions are the “matter” particles including both quarks and leptons (electrons and neutrinos are types of lepton). Bosons are the “force” particles, which give rise to the fundamental forces – electromagnetic, strong nuclear and weak nuclear. These particles and the mathematics describing their relationships, are what physicists refer to as the “Standard Model” of particle physics.

Photons are the particle of the electromagnetic force, responsible for light, magnetism, etc., whereas the weak nuclear force, responsible for radioactive decay, is carried by W and Z bosons. Particles that feel the strong nuclear force are “bound” together by the particle of the strong force – the gluon – so for example, a proton is made of quarks held together by gluons.

So when protons are smashed together in the LHC at approaching light speed, quarks and gluons collide at high enough energies to recreate conditions just after the big bang. This creates other particles, such as the Higgs, which, because it isn’t stable (unlike, for instance, a proton), immediately decays into other things, which in turn may decay again. A Higgs has distinct probabilities for decaying into various particle pairs – including pairs of leptons, W or Z bosons, quarks, or photons (but since photons are massless this only happens indirectly via a “quantum loop”).

It is in reconstructing these paths from collision to debris that physicists search for evidence of the Higgs and they call the paths in which it should (theoretically) fleetingly exist “search channels”. The channel Sarah has been investigating involves two quarks colliding to produce a W/H boson that radiates a Higgs, which then decays into a pair of bottom quarks (quarks exist in top, bottom, strange, charm, up and down varieties), which then decay again, into a spray-like “jet” of hadrons (particles made of quarks).

Because bottom quarks are the most likely particle a low mass Higgs can decay into, this should be an important channel. The problem is a huge number of jets are produced in the LHC that have nothing to do with the Higgs and this swamps the channel with background noise. By only looking at specific decays (ones with very high “transverse momentum”) a lot of this background is removed, but the jets produced are what Sarah called “collimated”, meaning they fly apart at very narrow angles. So, rather than looking for two separate jets, physicists reconstruct the decay process from the internal structure of a larger jet. Sarah said: “Substructure methods are very in vogue in ATLAS right now.”

Some pictures were necessary to help explain.

The main reason the Higgs was dreamed up, almost half a century ago, was to explain something called “electroweak symmetry breaking”. The electromagnetic and weak nuclear forces behave very similarly at high energies and mathematics revealed that they are actually aspects of a larger whole – leaving the problem of explaining why they behave differently under everyday conditions. The mechanism Peter Higgs (and others) came up with to solve this involved an omnipresent field which gives particles that interact with it their mass, but leaves others unaffected. For instance, the bosons of the weak nuclear force move through this “Higgs field” like sponge through treacle, acquiring mass in the process, but the electromagnetic force photons zip through untouched and so are massless. Not only does this “break the symmetry” between the electromagnetic and weak forces, it also builds a mechanism for explaining why (some) particles have mass into the Standard Model. But an implication of this theory was that this field must have a particle associated with it – something that had never been seen before. So began the hunt for the “Higgs boson” and it now looks as though it, or something very like it, has finally been found.

Is there a sense in which discovery of the Higgs is actually less interesting than the alternative? Complete compatibility with prediction will leave us with no clues about how to attack problems that the Standard Model can’t address, and after all, many of the most exciting scientific advances have come from unexpected results. Sarah said: “I suppose to find nothing would have been more interesting to me than finding the Higgs, but I didn’t think: ‘I wish we hadn’t found it’ – it was very exciting.

“I’m still holding out hope because it’s five-sigma consistent [meaning the probability of this result being a chance occurence is less than 1 in 2 million] with the standard model but all of its properties haven’t been measured, so it could still not be the Standard Model Higgs. It’s completely consistent with what we expected to be there, so it’s very unlikely, but maybe they’ll find it doesn’t couple to something it should. Then I think we’re in the ideal situation where we’ve found something, it’s new, and it’s unexplained.”

Sarah’s channel wasn’t one in which the Higgs was declared to have been discovered in the announcement on July 4th. She said: “It hasn’t been seen everywhere it should exist yet. Not in the channel we were working on – but I don’t think anyone really minded that.” But if it is the Higgs predicted by the Standard Model, it should eventually show up in all valid channels.

So part of CERN’s job now will be to search the other channels in an effort to pin down all the properties of this fabled particle. Departures from the precise predictions of the Standard Model could point the way to genuinely new physics. Sarah said: “If they don’t find it in this channel – if it doesn’t couple to the quarks, it’s not a Standard Model Higgs. It could still be a Higgs; because there are lots of ‘beyond the standard model’ theories like supersymmetry that require different variations on the same idea.”

Supersymmetry is a theory which attempts to address some issues the Standard Model can’t account for, such as dark matter, which is thought to make up 80% of the universe. It is currently a purely theoretical extension to the Standard Model, but also a candidate “theory of everything” which attempts to unite all four of the fundamental forces (i.e. including gravity, which is left out of the Standard Model altogether) in a similar fashion to the way the Higgs mechanism unites the electromagnetic and weak forces.

So why is all this so important? For Sarah there’s a fundamental aspect: “Of understanding how we work and what the universe is made of – getting another piece of the jigsaw.” But also: “For physics it’s a validation of the scientific method – predict, search, discover, predict again.”

Sarah added: “It’s not the end of the story. In some sense it’s just the beginning of the next step of particle physics. The standard model doesn’t explain everything – it’s just a theory, which is working so far, but it doesn’t contain everything. Unless this turns out to be something a bit more exotic than the Standard Model Higgs, we’ve found the answer to why things have mass, but there’s a lot more to do.”

So this discovery seems to mark the conclusion of a search for confirmation, which potentially moves physics forward into greater unknowns. Sarah agrees: “Given we have this understanding now, what next? How do we incorporate gravity? How do we unite relativity and quantum theory? It’s interesting because you don’t know what the practical applications in the future might be. When they discovered the electron, it was just an experiment they did and they had no idea of the kind of world we’d live in today because of that discovery. I can’t think of a practical use for a Higgs particle, but I think that’s a really exciting point in physics, where we really don’t know what the future is.” She added: “I’m glad I’m here to see it really.”

After four years of working hard and being poor, Sarah had started to have doubts about her career choice. She said: “I was starting to be realistic about how many jobs there are. I always thought I’ll go as far as I can, and at the point where I can’t anymore, I’ll do something else. I’ve fulfilled a goal in life, which was to be a research physicist. “ But she added: “Then something like this happening sparks up the enthusiasm again. So I think I want to stay, see what’s next – be part of whatever’s coming up.”

So what would she say to someone contemplating a career in physics? “For me, it’s one of the most interesting jobs you can do. There’s a day to day grind – you’re there, your code’s not working, deadline coming up… but you wake up each day and think: ‘I’m going to look at the meaning of the universe today’.

“I wouldn’t consider myself a natural born physicist. My dad’s an artist, my mum’s a social worker and my talent in school was art not physics, but I think if you find something really interesting, you can make yourself learn the maths. It’s like any job – you train for several years. For me it’s more an interest, which I’m lucky enough to have a place in. I remember my hands pressed against the train window, looking at Rutherford Appleton Laboratory, thinking: ‘I really want to do this – and I see no reason why I can’t.’”

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