If gossip on various physics blogs pans out, the biggest moment for physics in nearly two decades is just days away. The possible announcement on July 4 of the long-sought Higgs boson would put the last critical piece of the Standard Model of Physics in place, a crowning achievement built on a half-century of work by thousands of scientists. A moment worthy of fireworks.
But there’s a problem: The Higgs boson is starting to look just a little too ordinary.
As physicists at Europe’s Large Hadron Collider prepare to present their latest update in the hunt for the Higgs boson — the strange particle that exists everywhere in space and interacts with all other elementary particles, giving them their mass — other physicists are preparing for disappointment.
That’s because scientists have been secretly hoping all along that, when they finally found the Higgs, it would be an interesting particle with unexpected behaviors — even somewhat unruly. A perfectly well-behaved Higgs leaves less room for new, exciting physics — the kind that theorists have been wishing would show up at the LHC.
The current situation has some physicists starting to worry and, if coming years fail to turn up interesting results, the field could be headed for a crisis.
Since the mid-20th century, particle physicists have been developing a theory known as the Standard Model, which accounts for all the known forces and subatomic particles in the universe. While this model has proven time and time again to be extremely good at predicting particles and forces that were later discovered experimentally, it is not the final theory of everything. The Standard Model still has various problems that stubbornly refuse to cooperate.
Simulated particle collision events in the CMS experiment that would indicate the presence of supersymmetry. CMS collaboration/CERN
Many contenders have stepped up to account for the discrepancies of the Standard Model but none has been more adored than a theory known as supersymmetry. In order to fix the Standard Model, supersymmetry posits that all known particles have a much more massive superpartner lurking in the subatomic world.
“For particle physicists, the more symmetry there is, the nicer a theory is,” said theoretical physicist Csaba Csaki of Cornell University. “So upon first seeing it, most particle physicists fell in love with [supersymmetry].”
The tricky part is that the LHC, in addition to searching for the Higgs, has also been looking for these heavy supersymmetric superpartners. But thus far, nothing is showing up. Furthermore, all indications are that scientists will find that the Higgs weighs 125 gigaelectronvolts (GeV) – or about 125 times more than a proton – which means that it sits exactly where the Standard Model expected it to be.
Great news for the troublesome Standard Model, not so much for its savior, supersymmetry.
Supersymmetry was first proposed in the 1960s and developed seriously during the heyday of particle physics in the 1970s and ‘80s. Back then, large particle accelerators were smashing subatomic particles together and discovering a slew of new bits and pieces, including quarks and the W and Z bosons. Supersymmetry was put forth as an extension of the Standard Model, but the predicted particles were out of reach for atom smashers of that era.
Before the LHC was up and running in 2010, many physicists were hopeful that it would uncover some evidence for supersymmetry. Despite a few promising results, experimental confirmation of the idea keeps failing to show up.
This has a few in the community beginning to seriously doubt their darling supersymmetry will ever be a viable theory.
“It’s a beautiful theory, and I would love it if it were true,” said particle physicist Tommaso Dorigo, who works on one of the LHC’s two main experiments. “But there is not any compelling evidence.”
For two decades, people have been claiming the supersymmetry results were just a few years away, Dorigo added. So as those few years kept coming and going with no results, physicists have tried explaining the non-appearance of these particles by making additions and elaborations to supersymmetry.
Already, the simplest versions of supersymmetry have been ruled out and a Higgs boson at 125 GeV could require even more changes, making many physicists nervous, Csaki said. Tweaking the theory to explain why even the lightest of the predicted superpartners haven’t shown up destroys some of supersymmetry’s beauty, he said.
For instance, one of the best aspects of supersymmetry is that many of its extra subatomic particles make excellent dark matter candidates. Altering supersymmetry could get rid of these potential dark matter particles, and further changes might make the theory even less useful.
“One day we may just look at it and ask if this is still the theory that we’re in love with,” Csaki said.
Of course, all is not yet lost. The LHC is still smashing particles together and, over the next few years, it will do so at higher and higher energies, perhaps finally bringing supersymmetry to light. While the accelerator will be shutdown in 2013 for repairs, 2014 and 2015 will have the machine running at its top capacity.
Many physicists are eager to see if the lightest predicted superpartner – the supersymmetric top quark, or stop quark – will show up. The stop quark is at the heart of supersymmetry and is needed to explain many properties of the Higgs. Without it, many physicists could give up on supersymmetry entirely.
“If after two years of running at high luminosity at the LHC they don’t see anything, we will be out of ideas of the conventional sort,” said Csaki. “We will be in some kind of crisis.”
While troubling, this situation doesn’t bring physics to a grinding halt. The Standard Model still has holes in it, and something needs to account for the dark matter and energy in the universe. Alternative theories to supersymmetry exist. Some require additional forces in nature, new interactions among particles, or for the Higgs boson itself to be composed of simpler pieces.
“However those models have their own problems to be a consistent models of nature,” wrote particle physicist Rahmat Rahmat from the University of Mississippi, who also works on the CMS experiment, in an email to Wired.
As yet, supersymmetry is still the front-runner for theories beyond the Standard Model and most physicists remain optimistic for its prospects.
“I’m really hopeful that besides the discovery of the Higgs, we will also soon see something else,” said Csaki.
Image: The giant detector for the CMS experiment, one of the main Higgs-searching experiments at the LHC. CMS collaboration/CERN
