A pricey and controversial space-based cosmic ray detector has discovered potential indicators of darkish matter, the invisible stuff thought to produce a lot of the universe’s mass. Or so says Samuel Ting, a particle physicist on the Massachusetts Institute of Expertise in Cambridge and chief of the Alpha Magnetic Spectrometer (AMS), which is perched on the Worldwide Area Station (ISS).
Nonetheless, time is operating out for the growing older detector, and lots of researchers are skeptical concerning the darkish matter interpretation, which Ting dances round with typical coyness. “For those who take heed to the storyline, it does sound like that is the place we’re headed, however we by no means fairly get there,” says Angela Olinto, a cosmic ray physicist on the College of Chicago in Illinois.
The co-winner of the 1976 Nobel Prize in Physics, Ting, 83, jetted world wide to drum up $1.5 billion for the AMS, and wooed NASA and the Division of Vitality (DOE) into backing it. After astronauts bolted the 8500-kilogram, doughnut-shaped detector to the ISS in Might 2011, it started to measure the mass, cost, and vitality of the billions of cosmic rays—charged particles from house—that cross down its maw. Nearly all of them are protons, electrons, and light-weight nuclei similar to helium, however a valuable few encompass antimatter particles similar to positrons. They stand out as a result of, within the magnetic area of the AMS, their paths bend in the wrong way from these of their matter counterparts.
In 2014, AMS researchers reported an sudden flux of positrons that kicked in at energies above 10 giga-electron volts (GeV) and appeared to fade by about 300 GeV. The surplus might come from darkish matter particles colliding and annihilating each other to provide electron-positron pairs, and the vitality of the falloff may level to the mass of the darkish matter particles. Now, with 3 times as many knowledge, AMS researchers have clearly resolved that vitality cutoff. The positron extra begins at 25 GeV and falls sharply at 284 GeV, the 227-member AMS crew reported final week in Bodily Evaluate Letters. “It is necessary since you do begin to see a turnaround” within the vitality spectrum, Olinto says. The cutoff is per heavy darkish matter particles with a mass of about 800 GeV, the researchers report.
The AMS paper acknowledges that darkish matter annihilation is only one potential rationalization for the positrons. They may additionally come from an earthly astrophysical object, similar to a pulsar—a spinning neutron star. However Ting emphasizes the steepness of the cutoff. “The cutoff additionally goes in a short time, similar to [the signal from] darkish matter collisions,” he says.
In a 3rd risk, the positrons might come from the interactions of cosmic rays themselves. Cosmic ray protons rising from remnants of supernova explosions commonly slam into atomic nuclei in interstellar house to create “secondary” cosmic rays, together with positrons. AMS researchers say they’ve dominated out that rationalization for the sign, as a result of the proton collisions ought to produce a protracted tail within the positron spectrum as an alternative of a pointy falloff. However Greg Tarlé, a cosmic ray physicist on the College of Michigan in Ann Arbor, says the AMS knowledge reveal a telltale similarity between the vitality spectrum of the positrons and that of the protons, supporting the concept that the protons are the supply. “It is the AMS knowledge itself that give the perfect proof for the positrons being secondaries,” Tarlé says.
Each rationalization for the positron extra has important issues, cosmic ray consultants say, however Ting insists the AMS should still kind all of it out. The detector might run for the remaining life span of the ISS, maybe till 2024. The AMS crew will then have twice as many knowledge, sufficient to inform whether or not the positron spectrum dives as steeply as darkish matter situations predict, Ting says. Stephane Coutu, a physicist at Pennsylvania State College in College Park, disagrees. Doubling the info will shrink the error bars simply 30%, he says, too little to resolve the difficulty. “They’re mainly finished,” Coutu says. “The remaining is gilding a lily.”
In Might 2018, a federal advisory panel reached an analogous conclusion. In 2017, the White Home proposed slashing DOE’s analysis price range by 17%. In response, officers in DOE’s excessive vitality physics program, which funds the AMS’s $4.5 million working price range, held a assessment to rank 13 ongoing initiatives. The AMS tied for final. The issue lay not with the experiment, however with the theories to interpret its knowledge, says Paul Grannis, a physicist on the State College of New York in Stony Brook who led the assessment. The theoretical uncertainties are “so massive that something you would do to enhance the info can have little or no influence,” Grannis says. In the long run, Congress boosted the 2018 excessive vitality physics price range by 10%, and DOE officers say they don’t have any plans to chop the AMS.
Ting can be holding out for a unique jaw-dropping discovery: heavy antimatter nuclei. It might be enormous as a result of antinuclei heavier than a deuteron—a proton and a neutron—can’t be made in cosmic ray interactions and must originate in some area of the universe dominated by antimatter. Ting claims the AMS has captured a couple of antihelium nuclei. Coutu says a mountain of proof already proves no antimatter areas exist, so the unpublished alerts have to be spurious, maybe produced by misidentified helium nuclei.
The antimatter declare, too, might stay untested. Regardless of final yr’s reprieve, the AMS faces an unsure future. Pumps that cool key detector parts want changing, and the repair would require a spacewalk, scheduled for October. “It is no massive deal,” Ting says, though he will not assure success.
If the AMS stops working, it’ll go away behind an impressive legacy, even when it is not the one Ting envisions. The detector has collected beautiful knowledge on cosmic rays similar to nuclei of helium, boron, beryllium, and carbon. The info are serving to scientists perceive what produces these abnormal cosmic rays, and the way they journey by house. “The cosmic ray knowledge that they are producing is unbelievable,” says Tarlé, usually a vocal critic of Ting. “It would not have been finished if Sam hadn’t satisfied DOE and NASA to do it.”