The AMS results are based on some 25 billion recorded events, including 400,000 positrons, recorded over the course of one and a half years. Positrons are the antimatter counterpart of the electron. The AMS detector measured the ratio of positrons to the total number of electrons and positrons. This “positron fraction”, should be small, and should fall as energy increases, according to standard astrophysics. Instead, AMS results show that the positron fraction increases from roughly 5% at an energy of 10 GeV to more than 15% at 250 GeV. The data showed no variation over time, nor any preferred incoming direction.
The excess of antimatter in the cosmic ray flux was first observed nearly two decades ago, but could not be measured with enough certainty. The AMS data provides the most accurate measurements to date. According to AMS spokesperson Samuel Ting of the Massachusetts Institute of Technology, “AMS is the first experiment to measure to 1% accuracy in space.” The Nobel Prize winning particle physicist is the force behind AMS.
What produced the excess in the positron fraction? A tantalizing possibility is that the positrons arise from dark matter. A theory known as supersymmetry proposes that dark matter consists of weakly interacting massive particles, or WIMPs. Positrons could be produced when two WIMPs collide and annihilate each other, producing an electron-positron pair, with the energy of those particles limited by the mass of the WIMPs. If that is the case, theorists expect that the fraction should increase with energy levels until it reaches a “cutoff” energy where the fraction would begin to fall again. AMS researchers see signs that the positron fraction levels off at 250 GeV, which could mean that the cutoff might be just beyond that energy level.
There may be other possibilities for the cause or causes of the excess positrons. One of the other leading candidates is that pulsars are creating positrons. But Ting remains hopeful, even while acknowledging other possibilities, saying “Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin.”
Image credit: artistic visualization of dark matter/NPACI Visualization Services