When physicists strip neutrons from atomic nuclei, put them in a bottle, then rely what number of stay there after a while, they infer that neutrons radioactively decay in 14 minutes and 39 seconds, on common. However when different physicists generate beams of neutrons and tally the rising protons—the particles that free neutrons decay into—they peg the common neutron lifetime at round 14 minutes and 48 seconds.
The discrepancy between the “bottle” and “beam” measurements has continued since each strategies of gauging the neutron’s longevity started yielding leads to the 1990s. At first, all of the measurements have been so imprecise that no person nervous. Progressively, although, each strategies have improved, and nonetheless they disagree. Now, researchers at Los Alamos Nationwide Laboratory in New Mexico have made the most precise bottle measurement of the neutron lifetime but, utilizing a brand new sort of bottle that eliminates doable sources of error in earlier designs. The end result, which is able to quickly seem within the journal Science, reinforces the discrepancy with beam experiments and will increase the prospect that it displays new physics somewhat than mere experimental error.
However what new physics? In January, two theoretical physicists put forward an exciting speculation about the reason for the discrepancy. Bartosz Fornal and Benjamin Grinstein of the College of California, San Diego, argued that neutrons would possibly typically decay into dark matter—the invisible particles that appear to make up six-sevenths of the matter within the universe primarily based on their gravitational affect, whereas evading a long time of experimental searches. If neutrons typically transmogrify into darkish matter particles as a substitute of protons, then they’d disappear from bottles at a sooner charge than protons seem in beams, precisely as noticed.
Fornal and Grinstein decided that, within the easiest state of affairs, the hypothetical darkish matter particle’s mass should fall between 937.9 and 938.eight mega-electron volts, and neutron decaying into such a particle would emit a gamma ray of a selected vitality. “This can be a very concrete sign that experimentalists can search for,” Fornal stated in an interview.
The UCNtau experimental staff in Los Alamos—named for ultracold neutrons and tau, the Greek image for the neutron lifetime—heard about Fornal and Grinstein’s paper final month, simply as they have been gearing up for one more experimental run. Nearly instantly, Zhaowen Tang and Chris Morris, members of the collaboration, realized they might mount a germanium detector onto their bottle equipment to measure gamma-ray emissions whereas neutrons decayed inside. “Zhaowen went off and constructed a stand, and we received collectively the elements for our detector and put them up subsequent to the tank and began taking information,” Morris stated.
Information evaluation was equally fast. On Feb. 7, only one month after Fornal and Grinstein’s speculation appeared, the UCNtau staff reported the results of their experimental test on the physics preprint web site arxiv.org: They declare to have dominated out the presence of the telltale gamma rays with 99 p.c certainty. Commenting on the result, Fornal famous that the darkish matter speculation just isn’t completely excluded: A second state of affairs exists during which the neutron decays into two darkish matter particles, somewhat than one in all them and a gamma ray. With no clear experimental signature, this state of affairs shall be far tougher to check. (Fornal and Grinstein’s paper, and the UCNtau staff’s, are actually concurrently below evaluate for publication in Bodily Overview Letters.)
So there’s no proof of darkish matter. But the neutron lifetime discrepancy is stronger than ever. And whether or not free neutrons reside 14 minutes and 39 or 48 seconds, on common, really issues.
Physicists must know the neutron’s lifetime to be able to calculate the relative abundances of hydrogen and helium that might have been produced throughout the universe’s first jiffy. The sooner neutrons decayed to protons in that interval, the less would have existed later to be integrated into helium nuclei. “That steadiness of hydrogen and helium is to begin with a really delicate take a look at of the dynamics of the Big Bang,” stated Geoffrey Greene, a nuclear physicist on the College of Tennessee and Oak Ridge Nationwide Laboratory, “however it additionally tells us how stars are going to type over the subsequent billions of years,” since galaxies with extra hydrogen type extra huge, and ultimately extra explosive, stars. Thus, the neutron lifetime impacts predictions of the universe’s far future.
Moreover, each neutrons and protons are literally composites of elementary particles referred to as quarks which can be held collectively by gluons. Outdoors of steady atomic nuclei, neutrons decay when one in all their down quarks undergoes weak nuclear decay into an up quark, remodeling the neutron right into a positively charged proton and spitting out a detrimental electron and an antineutrino in compensation. Quarks and gluons can’t themselves be studied in isolation, which makes neutron decays, in Greene’s phrases, “our greatest surrogate for the elementary quark interactions.”
The lingering nine-second uncertainty within the neutron lifetime wants resolving for these causes. However nobody has a clue what’s fallacious. Greene, who’s a veteran of beam experiments, stated, “All of us have gone over very rigorously everyone’s experiment, and if we knew the place the issue was we might establish it.”
The discrepancy first turned a severe matter in 2005, when a bunch led by Anatoli Serebrov of the Petersburg Nuclear Physics Institute in Russia and physicists on the Nationwide Institute of Requirements and Know-how (NIST) in Gaithersburg, Maryland, reported bottle and beam measurements, respectively, that have been individually very exact—the bottle measurement was estimated to be at most one second off, and the beam one at most three seconds—however which differed from one another by eight seconds.
Many design enhancements, unbiased checks and head scratches later, the hole between the world-average bottle and beam measurements has solely grown barely—to 9 seconds—whereas each error margins have shrunk. This leaves two potentialities, stated Peter Geltenbort, a nuclear physicist on the Institut Laue-Langevin in France who was on Serebrov’s staff in 2005 and is now a part of UCNtau: “Both there may be actually some unique new physics,” or “everybody was overestimating their precision.”
Beam practitioners at NIST and elsewhere have labored to know and decrease the various sources of uncertainty of their experiments, together with within the depth of their neutron beam, the amount of the detector that the beam passes by, and the effectivity of the detector, which picks up protons produced by decaying neutrons alongside the beam’s size. For years, Greene significantly mistrusted the beam-intensity measurement, however unbiased checks have exonerated it. “At this level I don’t have a greatest candidate of a scientific impact that’s been ignored,” he stated.
On the bottle facet of the story, consultants suspected that neutrons may be getting absorbed into their bottles’ partitions regardless of the surfaces being coated with a clean and reflective materials, and even after correcting for wall losses by various the bottle measurement. Alternatively, the usual means of counting surviving neutrons within the bottles may need been lossy.
However the brand new UCNtau experiment has eradicated each explanations. As an alternative of storing neutrons in a fabric bottle, the Los Alamos scientists trapped them utilizing magnetic fields. And somewhat than transporting surviving neutrons to an exterior detector, they employed an in situ detector that dips into the magnetic bottle and rapidly absorbs all of the neutrons inside. (Every absorption produces a flash of sunshine that will get picked up by phototubes.) But their ultimate reply corroborates that of earlier bottle experiments.
The one possibility is to press on. “Everyone is transferring ahead,” Morris stated. He and the UCNtau staff are nonetheless gathering information and ending up an evaluation that features twice as a lot information as within the forthcoming Science paper. They goal to ultimately measure tau with an uncertainty of simply zero.2 second. On the beam facet, a bunch at NIST led by Jeffrey Nico is taking information now and expects to have leads to two years, aiming for one-second uncertainty, whereas an experiment in Japan referred to as J-PARC can also be getting below means.
NIST and J-PARC will both corroborate UCNtau’s end result, deciding the neutron lifetime as soon as and for all, or the saga will proceed.
“The strain that these two unbiased strategies disagree is what drives the development within the experiments,” Greene stated. If solely the bottle or the beam method had been developed, physicists may need gone ahead with the fallacious worth for tau plugged into their calculations. “The advantage of getting two unbiased strategies is it retains you sincere. I used to work on the Nationwide Bureau of Requirements, they usually’d say, ‘A person with one watch is aware of what time it’s; a person with two is rarely certain.’”
Original story reprinted with permission from Quanta Magazine, an editorially unbiased publication of the Simons Foundation whose mission is to reinforce public understanding of science by overlaying analysis developments and traits in arithmetic and the bodily and life sciences.