Physicists should be ecstatic right-now. Taken at face value, the surprisingly strong magnetism of the elementary particles called muons, revealed by an experiment this month, suggests that the established theory of fundamental particles is incomplete. If the discrepancy pans out, it might be the 1st time that the idea has failled for observations since its inception five decades ago — and there’s nothing physicists love quite proving a theory wrong.
But instead of pointing to a latest and revolutionary theory, the result — announced on 7 April by the Muon g – 2 experiment near Chicago, Illinois — poses a riddle. It seems maddeningly hard to define it in a way that’s compatible with everything else physicists realize elementary particles. and extra anomalies in muon’s behaviour, reported in March by a collider experiment, only make that task harder. The result’s that researchers need to perform the theoretical-physics equivalent of a triple somersault to make an evidence work.
Take supersymmetry, or SUSY, a theory that a lot of physicists once thought was the foremost promising for extending the present paradigm,standard model of particle physics . Supersymmetry comes in many variants, but generally , it posits that each particle within the standard model features a yet-to-be-discovered heavier counterpart, called a superpartner. Superpartners might be among the ‘virtual particles’ that constantly enter and out of the empty space surrounding the muon, a quantum effect that might help to define why this particle’s magnetic flux is stronger than expected.
If so, these particles could solve two mysteries at once: muon magnetism & dark matter, the unseen stuff that, through its gravitational pull, seems to stay galaxies from flying apart.
Until 10 years ago, various lines of evidence had suggested that a superpartner weighing the maximum amount as a couple of hundred protons could constitute dark matter. Many expected that the collisions at the Large Hadron Collider (LHC) outside Geneva, Switzerland, would produce a plethora of those new particles, but thus far none has materialized. the info that the LHC has produced thus far suggest that typical superpartners, if they exist, cannot weigh less-than 1,000 protons (the bounds are often higher depending on the sort of superparticle and therefore the flavour of supersymmetry theory).
“Many people would say supersymmetry is nearly dead,” says Dominik Stöckinger, a theoretical physicist at the Dresden University of Technology in Germany, who may be a member of the Muon g – 2 collaboration. But he still sees it as a plausible thanks to explain his experiment’s findings. “If you check out it as compared to the other ideas, it’s not worse than the others,” he says.
There is one method in which Muon g – 2 could resurrect supersymmetry and also provide evidence for dark matter, Stöckinger says. There might be not one superpartner, but two appearing in LHC collisions, both of roughly similar masses — say, around 550 and 500 protons. Collisions would create the more massive one, which might then rapidly decay into two particles: the lighter superpartner plus a run-of-the-mill, standard-model particle carrying away the 50 protons’ worth of mass difference.
The LHC detectors are well-equipped to reveal this type of decay as long because the ordinary particle — the one that carries away the mass difference between the 2 superpartners — is large enough. But a really light particle could escape unobserved. “This is well-known to be a blind spot for LHC,” says Michael Peskin, a theoretician at the SLAC National Accelerator Laboratory in Menlo Park, California.
The trouble is that models that include two superpartners with similar masses also tend to predict that the Universe should contain a way larger amount of dark matter than astronomers observe. So a further mechanism would be needed — one which will reduce the quantity of predicted dark matter, Peskin explains. This adds complexity to the idea. For it to suit the observations, all its parts would need to work “just so”.
Meanwhile, physicists have uncovered more hints that muons behave oddly. An experiment at the LHC, called LHCb, has found tentative evidence that muons occur significantly less often than electrons because the breakdown products of certain heavier particles called B mesons. consistent with the standard model, muons are alleged to be just like electrons in every way except from their mass, which is 207 times larger. As a consequence, B mesons should produce electrons and muons at rates that are nearly equal.
The LHCb muon anomalies suffer from same problem because the new muon-magnetism finding: various possible explanations exist, but they’re all “ad hoc”, says physicist Adam Falkowski, at the University of Paris-Saclay. “I’m quite appalled by this procession of zombie SUSY models dragged out of their graves,” says Falkowski.
The task of explaining Muon g – 2’s results becomes even harder when researchers try concoct a theory that matches both those findings and therefore the LHCb results, physicists say. “Extremely few models could explain both simultaneously,” says Stöckinger. especially , the supersymmetry model that explains Muon g – 2 and dark matter would do nothing for LHCb.
Some solutions nevertheless exist that would miraculously fit both. One is that the leptoquark — a hypothetical particle that would have the ability to-transform a quark into either a muon or an electron (which are both samples of a lepton). Leptoquarks could resurrect an effort made by physicists within the 1970s to realize a ‘grand unification’ of particle physics , showing that its three fundamental forces — strong, weak and electromagnetic — are all aspects of same force.
Most of the grand-unification schemes of that era failed experimental tests, and therefore the surviving leptoquark models became more complicated — but they still have their fans. “Leptoquarks could solve another big mystery: why different families of particles have such different masses,” says Gino Isidori, a theoretician at the University of Zurich in Switzerland. One family is formed of the lighter quarks — the constituents of protons and neutrons — and therefore the electron. Another has heavier quarks and therefore the muon, and a 3rd family has even heavier counterparts.
Apart from the leptoquark, there’s one other major contender which may reconcile both the LHCb and Muon g – 2 discrepancies. it’s a particle called the Z′ boson due to its similarity with the Z boson, which carries the ‘weak force’ liable for nuclear decay. It, too, could help to unravel the mystery of the three families, says Ben Allanach, a theorist at the University of Cambridge, UK. “We’re building models where some features begin very naturally, you’ll understand these hierarchies,” he says. He adds that both leptoquarks and therefore the Z′ boson have an advantage: they still haven’t been completely ruled out by the LHC, but the machine should ultimately see them if they exist.
The LHC is currently undergoing an upgrade, and it’ll start to smash protons together again in April 2022. The approaching deluge of info could strengthen the muon anomalies and maybe provide hints of the long-sought new particles (although a proposed electron–positron collider, primarily designed to review the Higgs boson, could be needed to deal with a number of the LHC’s blind spots, Peskin says). Meanwhile, beginning next year, Muon g – 2 will release further measurements. Once it’s known more precisely, the dimensions of the discrepancy between muon magnetism and theory could itself rule out some explanations and point to others.
Unless, that is, the discrepancies disappear and therefore the standard model wins again. A new calculation, reported this month, of standard model’s prediction for muon magnetism gave a worth much closer to the experimental result. So far, those that have bet against standard model have always lost, which makes physicists cautious. “We are — maybe — at the start of a new era,” Stöckinger says.
The findings are reported on Physical Review Letters & arxiv