The Fermilab Muon Measurement Might or Might Not Point to New Physics, But…

by akoloy

On April 7, particle physicists all around the world had been excited and energized by the announcement of a measurement of the conduct of muons—the heavier, unstable subatomic cousins of electrons—that differed considerably from the anticipated worth.

A century from now, wanting again on this second, will historians perceive this pleasure? They definitely received’t see a significant turning level within the historical past of science. No puzzle was solved, no new particle or area was found, no paradigm shifted in our image of nature. What occurred on April 7 was simply an announcement that the muon’s wobble—its worth is named g-2—had been measured a bit of extra exactly than earlier than, and that the worldwide high-energy physics group was due to this fact a bit of extra assured that different particles and fields are on the market but to be found.

Nevertheless, historians of science will see this as a particular second, not due to the measurement however due to the measuring. The first outcomes of the experiment at Fermilab was the result of a exceptional and maybe even unprecedented set of interactions between a very numerous set of scientific cultures that, over 60 years, advanced independently but required one another.

Early theoretical calculations of g-2 in accordance with quantum electrodynamics acquired a jolt in 1966 when Cornell theorist Toichiro Kinoshita realized that his earlier research had well-prepared him to work out its worth. His first calculations had been by hand, however quickly his calculations grew to become too unwieldy to be carried out that manner and he grew to become depending on computer systems and particular software program. To make the prediction ever extra exact, he needed to incorporate work by totally different teams of theorists who specialised within the huge and numerous panoply of interacting particles and forces that subtly affect the g-2 worth. ((Kinoshita is retired, and at the moment the theoretical worth is labored on by greater than 100 physicists.) The consequence was a selected prediction, counting on the contributions of many theorists, with a minuscule error bar that made a transparent experimental goal.

The preliminary experimental work on a g-2 measurement, which started at CERN in 1959, concerned a multistep course of. The experimenters used a particle accelerator to make unstable particles referred to as pions, then channeled these right into a flat magnet the place the pions decayed into muons. The muons had been compelled to show in circles, and the whirling muons had been made to “walk” in steps down the magnet. The muons emerged from the opposite finish of the magnet right into a field-free area the place their orientation may very well be measured, permitting the experimenters to deduce their g-2.

The subsequent experiment, which began at CERN in 1966, used a extra highly effective accelerator to provide and inject bigger numbers of pions right into a five-meter-diameter storage ring with a magnetic gradient to include the ensuing hordes of muons. The third CERN experiment, which started operations in 1969, was a significant leap ahead. It used a a lot bigger 14-meter-diameter storage ring and ran at a sure “magic” power the place the electrical area wouldn’t have an effect on the muon spin. This made it doable to have a uniform magnetic area, dramatically sharpening the sensitivity of the measurement. But with that enhanced sensitivity got here new sources of precision-sabotaging instrumental noise; one other set of strategies needed to be utilized to scale back uncertainties within the magnets and to measure the magnetic area.

The fourth technology of g-2 experiments—begun at Brookhaven National Laboratory in 1999—required much more years of laborious wrestle to beat again sources of error and management numerous disruptive elements. Like the third CERN experiment, it used a storage ring, 3.1 giga-electron-volt muons, the magic power, and a uniform area; however in contrast to the CERN experiment it had a better flux, muon injection moderately than pion injection, superconducting magnets, a trolley outfitted with NMR probes that may very well be run round contained in the vacuum chamber to verify the magnetic area, and a kicker contained in the storage ring.

These and different options added to the experiment’s complexity and expense. The experiment concerned 60 physicists from 11 institutes; it issued its g-2 worth in 2004. In 2013, the Brookhaven g-2 storage ring was transported to Fermilab and given new life, rebuilt and operated with a bunch of ever-more refined and complex new tips wanted to additional push the outer limits of precision. Ultimately, all these overlapping a long time of labor collectively produced the measurement introduced this month, one with a tiny error bar that made it significant to check with the theoretical prediction, which by then additionally had a slim error bar.

The late Francis Farley, the spokesperson for the very first g-2 experiment at CERN, as soon as informed me, “What the theorists do and what we experimenters do is completely different. They talk about Feynman diagrams, amplitudes, integrals, expansions and a whole lot of complex mathematics. We hook up an accelerator to beam lines and steering magnets to the device itself, which is stuffed with wires, thousands of cables, timing devices, sensors and such things. It’s two totally different worlds! But they come out with a number, we come out with a number, and these numbers agree to parts per million! It’s unbelievable! That is the most astonishing thing for me!”

At the April 7 announcement, the taking part physicists displayed a graph with two error bars, one for the theoretical prediction and the opposite for the experimental measurement. All the thrill sprang from the tiny however indeniable hole—2.5 elements per billion—between the 2. If both bar had been wider, it could have blended into the opposite, and the measurement wouldn’t have indicated physics awaiting discovery. To make the experiment occur, the scientific group, and the federal government companies offering the funding, had positioned monumental belief within the worldwide group of collaborators.

What will amaze historians of science sooner or later, I believe, shall be that at the moment’s scientists may produce that puny however revealing hole in any respect.

This is an opinion and evaluation article.

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