With the Large Hadron Collider unable to find the particles that the theory says must exist, the field of particle physics is back to its "nightmare scenario"
LHC TUNNEL: No hints of ?new physics? beyond the predictions of the Standard Model have turned up in experiments at the Large Hadron Collider, a 17-mile circular tunnel at CERN Laboratory in Switzerland that slams protons together at high energies. Image: CERN
From Simons Science News
As a young theorist in Moscow in 1982, Mikhail Shifman became enthralled with an elegant new theory called supersymmetry that attempted to incorporate the known elementary particles into a more complete inventory of the universe.
?My papers from that time really radiate enthusiasm,? said Shifman, now a 63-year-old professor at the University of Minnesota. Over the decades, he and thousands of other physicists developed the supersymmetry hypothesis, confident that experiments would confirm it. ?But nature apparently doesn?t want it,? he said. ?At least not in its original simple form.?
With the world?s largest supercollider unable to find any of the particles the theory says must exist, Shifman is joining a growing chorus of researchers urging their peers to change course.
In an essay posted last month on the physics website arXiv.org, Shifman called on his colleagues to abandon the path of ?developing contrived baroque-like aesthetically unappealing modifications? of supersymmetry to get around the fact that more straightforward versions of the theory have failed experimental tests. The time has come, he wrote, to ?start thinking and developing new ideas.?
But there is little to build on. So far, no hints of "new physics" beyond the Standard Model ? the accepted set of equations describing the known elementary particles ? have shown up in experiments at the Large Hadron Collider, operated by the European research laboratory CERN outside Geneva, or anywhere else. (The recently discovered Higgs boson was predicted by the Standard Model.) The latest round of proton-smashing experiments, presented earlier this month at the Hadron Collider Physics conference in Kyoto, Japan, ruled out another broad class of supersymmetry models, as well as other theories of ?new physics,? by finding nothing unexpected in the rates of several particle decays.
?Of course, it is disappointing,? Shifman said. ?We?re not gods. We?re not prophets. In the absence of some guidance from experimental data, how do you guess something about nature??
Younger particle physicists now face a tough choice: follow the decades-long trail their mentors blazed, adopting ever more contrived versions of supersymmetry, or strike out on their own, without guidance from any intriguing new data.
"It's a difficult question that most of us are trying not to answer yet," said Adam Falkowski, a theoretical particle physicist from the University of Paris-South in Orsay, France, who is currently working at CERN. In a blog post about the recent experimental results, Falkowski joked that it was time to start applying for jobs in neuroscience.
?There?s no way you can really call it encouraging,? said Stephen Martin, a high-energy particle physicist at Northern Illinois University who works on supersymmetry, or SUSY for short. ?I?m certainly not someone who believes SUSY has to be right; I just can?t think of anything better.?
Supersymmetry has dominated the particle physics landscape for decades, to the exclusion of all but a few alternative theories of physics beyond the Standard Model.
?It's hard to overstate just how much particle physicists of the past 20 to 30 years have invested in SUSY as a hypothesis, so the failure of the idea is going to have major implications for the field,? said Peter Woit, a particle theorist and mathematician at Columbia University.
The theory is alluring for three primary reasons: It predicts the existence of particles that could constitute "dark matter," an invisible substance that permeates the outskirts of galaxies. It unifies three of the fundamental forces at high energies. And ? by far the biggest motivation for studying supersymmetry ? it solves a conundrum in physics known as the hierarchy problem.