The so-called muon anomaly, first seen in an experiment at Brookhaven Nationwide Laboratory in 2001, hasn’t budged. For 20 years, this slight discrepancy between the calculated worth of the muon’s magnetic second and its experimentally decided one has lingered at a significance of about 3.7 sigma. That may be a confidence degree of 99.98 p.c, or a few one-in-4,500 likelihood the discrepancy is a random fluctuation. With the simply introduced outcomes from the Muon g-2 experiment at Fermi Nationwide Laboratory in Batavia, Unwell., the importance has elevated to 4.2 sigma. That may be a confidence degree of about 99.997 p.c, or a few one-in-40,000 likelihood for the noticed deviation to be a coincidence. By itself, the brand new Fermilab measurement has solely 3.3 sigma significance, however as a result of it reproduces the sooner discovering from Brookhaven, the mixed significance has risen to 4.2 sigma. Nonetheless, the latter falls in need of particle physicists’ five-sigma discovery threshold.
The end result has been long-awaited due to its potential to lastly break the Normal Mannequin of particle physics, a set of the to this point identified basic constituents of matter that has been in place for about 50 years. This mannequin presently comprises a pair dozen particles, however most of them are unstable and due to this fact can’t be discovered simply by trying on the matter that usually surrounds us. The unstable particles are, nonetheless, naturally produced in extremely energetic occasions, resembling when cosmic rays hit the higher ambiance. They’re additionally made in lab-created particle collisions, resembling these used within the Fermilab experiments to measure the muon’s magnetic second.
The muon was one of many first unstable particles identified, with its discovery relationship again to 1936. It’s a heavier model of the electron, and just like the latter particle, it’s electrically charged. The muon has a lifetime of about two microseconds. For particle physicists, that’s a very long time, which is why the particle lends itself to precision measurements. The muon’s magnetic second determines how briskly the particle’s spin axis orbits round magnetic discipline traces. To measure it at Fermilab, physicists create muons and preserve them getting in a circle of about 15 meters in diameter with highly effective magnets. The muons ultimately decay, and from the distribution of the decay merchandise, one can infer the their magnetic second.
The result’s normally quoted because the “g-2” the place “g” is the magnetic second. The “2” is included as a result of the worth is shut to 2—and within the deviations from two are the quantum contributions that physicists are concerned about. These contributions come from vacuum fluctuations that include all particles, albeit in digital kind: they solely seem briefly earlier than disappearing once more. Which means that if there are extra particles than these within the Normal Mannequin, they need to contribute to the muon g-2—therefore its relevance. A deviation from the Normal Mannequin prediction may due to this fact imply that there are extra particles than these which are at the moment identified—or that there’s another new physics, resembling further dimensions of area.
So how are we to gauge the 4.2-sigma discrepancy between the Normal Mannequin’s prediction and the brand new measurement? To begin with, it’s useful to recollect the explanation that particle physicists use the five-sigma commonplace to start with. The reason being not a lot that particle physics is one way or the other intrinsically extra exact than different areas of science or that particle physicists are so significantly better at doing experiments. It’s primarily that particle physicists have a number of information. And the extra information you could have, the extra seemingly you might be to search out random fluctuations that coincidentally appear like a sign. Particle physicists started to generally use the five-sigma criterion within the mid-Nineties to avoid wasting themselves from the embarrassment of getting too many “discoveries” that later develop into mere statistical fluctuations.
However after all 5 sigma is a completely arbitrary lower, and particle physicists additionally talk about anomalies effectively under that restrict. Certainly, fairly a couple of three- and four-sigma anomalies have come and gone through the years. The Higgs boson, for instance, was already “found” in 1996, when a sign of about 4 sigma appeared on the Giant Electron-Positron Collider (LEP) at CERN close to Geneva—after which disappeared once more. (The Higgs was conclusively detected in 2012 by LEP’s successor, the Giant Hadron Collider, or LHC.) Additionally in 1996 quark substructures have been discovered at round three sigma. They, too, disappeared.
In 2003 indicators of supersymmetry—a conjectured extension of the Normal Mannequin that introduces new particles—have been seen at LEP, additionally at round three sigma. However quickly they have been gone. On the LHC in 2015, we noticed the diphoton anomaly, which lingered round 4 sigma earlier than it vanished. There have additionally been some some beautiful six-sigma discoveries that weren’t confirmed, such because the 1998 “superjets” at Fermilab’s Tevatron (even now nobody actually is aware of what they have been) or the 2004 pentaquark sighting on the Hydrogen Epoch of Reionization Array, or HERA (pentaquarks weren’t really detected till 2015).
This historical past ought to enable you gauge how critically to take any particle physics declare with a statistical significance of 4.2 sigma. However after all the g-2 anomaly has in its favor the truth that its significance has gotten stronger quite than weaker.
What does the persistence of the anomaly imply? Excessive-precision experiments at low vitality, resembling this one, complement high-energy experiments. They will present comparable data as a result of, in precept, all of the contributions from excessive energies are additionally current at low energies. It’s simply that they’re very small—we’re speaking a few discrepancy between experiment and concept on the eleventh digit after the decimal level.
In apply, because of this the calculations for the predictions have to precisely account for lots of tiny contributions to succeed in the required precision. In particle physics, these calculations are executed utilizing Feynman diagrams—little graphs with nodes and hyperlinks that denote particles and their interactions. They’re a mathematical instrument to maintain monitor of which integrals have to be calculated.
These calculations change into extra concerned with greater precision as a result of there are extra and larger diagrams. For the muon g-2, physicists needed to calculate greater than 15,000 diagrams. Though computer systems assist drastically within the process, these calculations stay fairly difficult. A specific headache is the hadronic contribution. Hadrons are composite particles product of a number of quarks held collectively by gluons. Calculating these hadronic contributions to the g-2 worth is notoriously tough, and it’s presently the biggest supply of error on the theoretical facet. There are after all additionally numerous cross-measurements that play a task, such because the predictions that rely upon the values of different constants, together with the plenty of leptons and coupling constants.
Thus, the discrepancy may quite mundanely imply that there’s one thing fallacious with the Normal Mannequin calculation, with the hadronic contributions as the first suspect. However there may be additionally the chance that the shortcoming lies throughout the Normal Mannequin itself and never our calculation. Perhaps the discrepancy comes from new particles—supersymmetric particles are the most well-liked candidates. The issue with this rationalization is that supersymmetry isn’t a mannequin— as an alternative it’s a property of a lot of fashions, with totally different fashions from that better entire every yielding totally different predictions. Amongst different issues, the g-2 contributions rely upon the plenty of the hypothetical supersymmetric particles, that are unknown. So for now it’s unimaginable to attribute the discrepancy to supersymmetry specifically.
Fermilab’s new high-precision measurement of the magnetic second is a outstanding experimental achievement. Nevertheless it’s too quickly to declare the Normal Mannequin damaged.
That is an opinion and evaluation article.