Could the Higgs Nobel be the end of particle physics?

This article was originally published at The Conversation on the 8th October 2013 and was subsequently republished on a number of other platforms including Scientific American. As of 19th October 2013 it has been read by over 23,000 people worldwide. Read the original article here.

At around 11am this morning, Peter Higgs’s phone rang. A man from Stockholm was on the line, with the news that every particle physicist had been waiting for: he had won the Nobel Prize.

Higgs’s win will be rightly celebrated in the physics community, not only because of his crucial contribution to our understanding of particles and forces, but also because the unfailingly-modest Peter Higgs is the kind of man people like to cheer for.

But today’s celebrations mask a growing anxiety among physicists. Though the discovery of the Higgs boson was an undoubted triumph, it was seen by many as a foregone conclusion and hasn’t got us any closer to answering some of the most troubling problems in fundamental science.

One senior CERN physicist I spoke to recently even went so far as to say that he was “totally unexcited by the discovery of the Higgs boson”. Though perhaps not the typical reaction it is true that for many, its discovery threatens to close the chapter of 20^th century physics without giving us a hint of how to start writing the next page.

Until the 4^th July last year, when physicists at the Large Hadron Collider (LHC) announced its discovery, the Higgs boson remained the last missing piece of the Standard Model of particle physics, a theory that describes all the particles that make up the world we live in with stunning accuracy. The Standard Model has passed every experimental test thrown at it with flying colours, and yet has some huge (and rather embarrassing) holes.

Thanks to astronomical measurements, we now believe that the stuff described by the Standard Model, the matter that makes up the stars, planets and ultimately us, only accounts for a tiny fraction of the total content of the universe. We appear to be a thin layer of froth, floating on top of an invisible ocean of dark matter and dark energy, about which we know almost nothing.

Worse still, according to the Standard Model, we shouldn’t exist at all. The theory predicts that after the Big Bang equal quantities of matter and antimatter should have obliterated each other, leaving an empty universe, devoid of anything, including theoretical physicists. Any theory that predicts the non-existence of its authors is unquestionably in some trouble.

Both of these are good scientific reasons to doubt that the Standard Model is the end of the story when it comes to the laws of physics. But there is another, aesthetic principle that has lead many physicists to doubt its completeness – the principle of “naturalness”.

The Standard Model is regarded as a highly unnatural theory. Aside from having a large number of different particles and forces, many of which seem surplus to requirement, it is also very precariously balanced. If you change any of the 20+ numbers that have to be put into the theory even a little, you rapidly find yourself living in a universe without atoms, and therefore lethal to wobbly flesh-coloured things made of atoms, like us. This spooky “fine-tuning” worries many physicists, leaving the universe looking as though it has been set up in just the right way for life to exist.

Peter Higgs’s Nobel-Prize-winning boson provides us with one of the worst cases of unnatural fine-tuning. One of the many surprising discoveries of the 20^th century was the realisation that empty space is very far from empty. The vacuum is, in fact, a broiling soup of invisible “virtual” particles, constantly popping in and out of existence.

The conventional wisdom states that as the Higgs boson passes through the vacuum it interacts with this soup of virtual particles and this interaction drives its mass to an absolutely enormous value – potentially up to 100,000,000,000,000,000 times larger than the one measured at the LHC. To restore the Higgs mass to a sensible number requires unbelievable level of fiddly fine-tuning, to an accuracy of one part in a hundred thousand trillion.

To give you a sense of just how improbable this is, imagine a gigantic seesaw with half the population of the Earth on one end and half on the other. If the seesaw had to be balanced to the same precision as the fine-tuning in the Higgs mass, you would have to make sure that both ends of the seesaw had the same weight to a precision equal to the weight of a single strand of hair.

Theorists have attempted to tame the unruly Higgs mass by proposing extensions of the Standard Model, the most popular of which is “supersymmetry”, which introduces a heavier super-particle or “sparticle” for every particle in the Standard Model. These sparticles cancel out the effect of the virtual particles in the vacuum, reducing the Higgs mass to a reasonable value and eliminating the need for any unpleasant fine-tuning.

But aside from being nice and natural, supersymmetry has a number of other pleasing features that have made it extremely popular with theorists. Perhaps its best selling point is that one of these sparticles provides a neat explanation for the mysterious dark matter that makes up about a quarter of the universe.

So although discovering the Higgs boson was vaunted in the media as the main reason for building the 27 kilometre, nine billion dollar Large Hadron Collider, what most physicists have really been waiting for is the sign of something new. As Higgs himself said shortly after the discovery last year “[the Higgs boson] is not the most interesting thing that the LHC is looking for”.

So far however, the LHC has turned up a big fat nothing.

If supersymmetry is really responsible for keeping the Higgs mass low, then sparticles should show up at energies not much higher than where the LHC found the Higgs. The fact that nothing has been found has already ruled out many of the most popular forms of supersymmetry. Those theories that survive now require a certain amount of fine-tuning themselves, albeit only to about one part in ten.

This has led some theorists to challenge the long-cherished principle of naturalness, some going so far as to say it should be abandoned altogether.

James Wells, Nima Arkani-Hamed and Savas Dimopoulos proposed a new idea known as “split-supersymmetry” which accepts fine-tuning in the Higgs mass but keeps the other nice features of supersymmetry, like a dark matter particle.

This may sound like a rather technical difference, but the implications for the nature of our universe could not be more profound. Arkani-Hamed and Dimopoulos argue that the reason we live in an apparently fine-tuned universe is that ours is one among an effectively infinite number of different universes, all with different laws of physics. The constants of nature are what they are because if they were different atoms could not form, and hence we wouldn’t be around to wonder about them.

This anthropic argument is in part motivated by developments in string theory, a potential “theory of everything”, for which there are a vast number (10⁵⁰⁰) different possible universes with different laws of physics. This huge number of universes is often used as a criticism of string theory, sometimes derided as a “theory of everything else” as no one has so far found a solution that corresponds to the universe we live in. However, if Arkani-Hamed and Dimopoulos are right, the lack of new physics at the LHC could be indirect evidence for the existence of the very multiverse anticipated by string theory.

All of this could be rather bad news for the LHC. If the battle for naturalness is lost, then there is no reason why new particles must appear in the next few years. Arkani-Hamed has recently been campaigning for an even larger collider, four times longer and seven times more powerful than the LHC. This monster collider could be used to settle the question once and for all, but though Arkani-Hamed’s ambition is admirable, its hard to imagine such a machine getting the approval of politicians in the near future, especially if the LHC fails to find anything beyond the Higgs.

We are at a critical juncture in particle physics. Perhaps after it restarts the LHC will uncover new particles, naturalness will survive and particle physicists will stay in business. There are reasons to be optimistic after all – we know that there must be something new that explains dark matter, and there remains a good chance that the LHC will find it.

But perhaps, just perhaps, the LHC will find nothing. Peter’s boson could be particle physics’ swansong, the last particle of the accelerator age. Though a worrying possibility for experimentalists, such a result could lead to a profound shift in our understanding of the universe, and our place in it.

Countdown to Collider

On Monday 9 January 2012 I arrived at the Science Museum for my first day in gainful employment. My student days were over - I'd handed in my PhD dissertation three months earlier, moved to a flat in south London and was now the first "Science Museum Fellow of Modern Science" - clearly an important post, if only because the title contains "science" not once, but twice.

The job is a strange mixture of research physicist and curator. For the first half of the week I formed half the content team scoping a project known as "the LHC exhibition"; a potential show all about CERN's Large Hadron Collider. For the second half of the week I was back at the Cavendish Laboratory in Cambridge, starting my new life as a fully fledged PostDoc on the LHCb experiment.

Twenty one months, and a lot of train journeys later, "LHC" has been reborn as Collider: step inside the world'sgreatest experiment. Meanwhile the real LHC has switched off for its long shutdown, after a year of running that saw the Higgs boson cornered and a huge quantity of precious data recorded - so much, in fact, that we are still busily analysing it now.

My job has evolved too. I arrived at the Science Museum as something of an unknown quantity and I suspect my colleagues were unsure what could usefully be done with a particle physicist with no museum experience.  After a few months, I guess I must have shown that I could be of some use, as I was made "Head of Content", equivalent to exhibition curator.

Being involved in the development of an exhibition from the beginning has been a hugely rewarding experience. I've met and worked with an incredible range of talented people, from exhibition designers, curators, historians, graphic artists, animators, film makers, sound artists and writers to the engineers in charge of the LHC, world-class physicists who planned and built the gigantic CERN experiments and even (briefly) Peter Higgs.

As for research, though it certainly isn't easy, I've found it is possible to get stuff done on three (and even two) days a week. Though I admit to a lingering feeling of guilt at not being able to engage to the same degree as my full-time colleagues, I've still managed to produce published research along with a very patient PhD student. I've also been blessed with a very supportive boss, who has shown great patience when I've disappeared for exhibition meetings at design studios in east London.

Now there are just two months to go until Collider opens its doors to the public. Until last week it was still an exhibition on paper only, but two things happened on Thursday to make it suddenly real. First a lorry-load of objects from CERN arrived at the Science Museum. The conservation lab is now packed with superconducting magnets, accelerating cavities, particle detectors and even a block of crystal. Opening a box to see the massive two-tonne LHC magnet that will be the star of the exhibition, up until that point just a line on a spreadsheet, was pretty exciting.

The other thing was standing in the empty gallery, looking at the blank canvas that will become Collider. I can't wait to see what people make of it.