Saturday, June 18, 2022

Anything beyond the Higgs? Is collider physics over?

Adrian Cho, Ten years after the Higgs, physicists face the nightmare of finding nothing else, Science, 13 June 2022.

Before the 27-kilometer-long ring-shaped LHC started to take data in 2010, physicists fretted that it might produce the Higgs and nothing else, leaving no clue to what lies beyond the standard model. So far, that nightmare scenario is coming true. “It’s a bit disappointing,” allows Barry Barish, a physicist at the California Institute of Technology. “I thought we would discover supersymmetry,” the leading extension of the standard model.

It’s too early to despair, many physicists say. After 3 years of upgrades, the LHC is now powering up for the third of five planned runs, and some new particle could emerge in the billions of proton-proton collisions it will produce every second. In fact, the LHC should run for another 16 years, and with further upgrades should collect 16 times as much data as it already has. All those data could reveal subtle signs of novel particles and phenomena.

Still, some researchers say the writing is on the wall for collider physics. “If they don’t find anything, this field is dead,” says Juan Collar, a physicist at the University of Chicago who hunts dark matter in smaller experiments. John Ellis, a theorist at King’s College London, says hopes of a sudden breakthrough have given way to the prospect of a long, uncertain grind toward discovery. “It’s going to be like pulling teeth, not like teeth falling out.”

Since the 1970s, physicists have been locked in a wrestling match with the standard model. It holds that ordinary matter consists of lightweight particles called up quarks and down quarks—which bond in trios to make protons and neutrons—along with electrons and featherweight particles called electron neutrinos. Two sets of heavier particles lurk in the vacuum and can be blasted into fleeting existence in particle collisions. All interact by exchanging other particles: The photon conveys the electromagnetic force, the gluon carries the strong force that binds quarks, and the massive W and Z bosons carry the weak force.

The standard model describes everything scientists have seen at particle colliders so far. Yet it cannot be the ultimate theory of nature. It leaves out the force of gravity, and it doesn’t include mysterious, invisible dark matter, which appears to outweigh ordinary matter in the universe six to one.

And what of naturalness and supersymmetry? The lack of further discoveries puts them in jeopardy:

A notion called naturalness suggested the low mass of the Higgs more or less guaranteed the existence of new particles within the LHC’s grasp. According to quantum mechanics, any particles lurking “virtually” in the vacuum will interact with real ones and affect their properties. That’s exactly how virtual Higgs bosons give other particles their mass.

That physics cuts both ways, however. The Higgs boson’s mass ought to be pulled dramatically upward by other standard model particles in the vacuum—especially the top quark, a heavier version of the up quark that weighs 184 times as much as the proton. That doesn’t happen, so theorists have reasoned that at least one other new particle with a similar mass and just the right properties—in particular, a different spin—must exist in the vacuum to “naturally” counter the effects of the top quark.

The theoretical concept known as supersymmetry would supply such particles. For every known standard model particle, it posits a heavier partner with a different spin. Lurking in the vacuum, those partners would not only keep the Higgs’s mass from running away, but would also help explain how the Higgs field, which pervades the vacuum like an unextinguishable electric field, came into being. Supersymmetric particles might even account for dark matter.

But instead of those hoped-for particles, what have emerged in the past decade are tantalizing anomalies—small discrepancies between observations and standard model predictions—that physicists will explore in the LHC’s next 3-year run.

There's more at the link.

See this post, Lost in Math: Sabine Hossenfelder at Stevens Institute.

H/t 3QD.

No comments:

Post a Comment