Editor's note: The following essay is reprinted with permission from The Conversation, an online publication covering the latest research.
By Harry Cliff, University of Cambridge
The 2013 Nobel Prize in Physics has been awarded to François Englert and Peter Higgs for their work that explains why subatomic particles have mass. They predicted the existence of the Higgs boson, a fundamental particle, which was confirmed last year by experiments conducted at CERN’s Large Hadron Collider.
But today’s celebrations mask a growing anxiety among physicists. The discovery of the Higgs boson is an undoubted triumph, but many note that it hasn’t brought us any closer to answering some of the most troubling problems in fundamental science.
A senior physicist went so far as to tell me that he was “totally unexcited by the discovery of the Higgs boson”. Though not the typical reaction, this discovery threatens to close a chapter of 20th century physics without a hint of how to start writing the next page.
Until 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 colors, and yet has some rather embarrassing holes.
According to astronomical measurements, the matter described by the Standard Model that makes up the stars, planets and ultimately us, only accounts for a tiny fraction 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.
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 led 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. 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.
The Higgs’s boson provides us with one of the worst cases of unnatural fine-tuning. A surprising discovery of the 20th century was the realization that empty space is 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 a hundred million billion times larger than the one measured at the LHC.
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.
Supersymmetry has other features that have made it popular with physicists. 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.
Although discovering the Higgs boson may have been put forward as the main reason for building the 27km Large Hadron Collider (LHC), what most physicists have really been waiting for is a 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 nothing.
If supersymmetry is really responsible for keeping the Higgs boson’s 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 popular forms of supersymmetry.
This has led some theorists to abandon naturalness altogether. One relatively new idea known as “split-supersymmetry” 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 technical difference, but the implications for the nature of our universe are profound. The argument is that we live in a fine-tuned universe because it happens to be one among an effectively infinite number of different universes, each 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 (roughly 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 split-supersymmetry is 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. Some physicists are 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 it’s hard to imagine that such a machine will get the go ahead, 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 in 2015, it 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. The Higgs 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.
Harry Cliff is affiliated with the University of Cambridge, CERN, the LHCb experiment.