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article imageHas dark matter finally been detected? AMS scientists drop a hint

By JohnThomas Didymus     Feb 20, 2013 in Science
Researchers have hinted "big news" for the world in the search for dark matter. The physicist in charge of the Alpha Magnetic Spectrometer (AMS) experiment dropped a hint at the annual meeting of the American Association for the Advancement of Science.
The Alpha Magnetic Spectrometer (AMS) is a special state-of-the-art particle detector which "is performing [in space] very accurate measurements of cosmic ray with unprecedented sensitivity."
The equipment was installed on the International Space Station (ISS) to detect positrons and electrons generated during dark matter particle-antiparticle interactions.
MIT physicist and Nobel Laureate Samuel Ting, is the AMS principal investigator. He said that the latest AMS experiment results will begin to be released when the first paper detailing the results is submitted for publication.
Space.com reports that although Ting did not say precisely what new scientific information or data the AMS experiment paper will reveal, he said enough to indicate that the results are relevant to the mystery of dark matter, an invisible mysterious stuff though to constitute most of the mass of the universe but which is not detectable by ordinary means.
According to physicists, the gravitational force of the mysterious dark matter supposedly determines the large scale structure of spacetime. Its existence was first suspected when calculations forced scientists to postulate "missing mass" in explanation of gravitation-related structure and motion of galaxies, and the large-scale structure of the physical universe.
Physicists point out, for instance, that based solely on the gravitational force contribution of visible mass in the universe, the galaxies should not hold together. There must be an infusion of "invisible mass" to explain the stability of large scale galactic structures. Scientists postulate dark matter as constituting 23% of cosmic mass-energy, dark energy 73%, and observable matter only 4%.
Space.com reports that Ting, to whet the appetite of his audience, said the anticipated paper "will not be a minor paper."
According to Ting, the findings of the study were so significant in the context of the current status of knowledge that the researchers rewrote the paper 30 times to get it just right. But he said that the findings represents only a "small step" toward elucidation of the mystery of dark matter. He also cautioned that they may not yet have the final answer.
The BBC reports he said: "We've waited 18 years to write this paper, and we're now making the final check. I would imagine in two or three weeks, we should be able to make an announcement. We have six analysis groups to analyse the same results. With physicists, as you know, everybody has their own interpretations, and we're now making sure everyone agrees with each other. And this is pretty much done now."
According to Space.com, the $2 billion AMS equipment installed on the ISS in May 2011 is able to detect positrons and electrons produced by dark matter particle-antiparticle interactions in our galaxy. Ting says the AMS has so far detected 25 billion particle events, including 8 billion electrons and positrons.
The Alpha Magnetic Spectrometer (AMS) works on the theoretical notion that dark matter is made of WIMPS (weakly interacting massive particles), consisting of matter particle-antiparticle pairs that annihilate each other to release an electron and its positron antiparticle.
Scientists hope that by measuring the ratio of electrons to positrons and the behavior of any excess of positrons across the energy spectrum, they may edge towards a better understanding of dark matter.
Space.com reports Ting said that the first paper will give details of how many of the particles were found and what their energies were.
According to physicists, a scenario in which the AMS experiment detects an excess of positrons peaking at a certain energy could indicate a detection of dark matter because while electrons are all around us, only very few processes are known to generate positrons.
Michael Turner, director of the Kavli Institute for Cosmological Physics at the University of Chicago, told AFP: "On the cosmology side we now understand that this mysterious dark matter holds together our galaxy and the rest of the Universe.
"And the tantalizing thing on the cosmology side is that we have an airtight case that the dark matter is made of something new... there is no particle in the Standard Model that can account for dark matter."
Space.com reports he said: "'The smoking gun signature is a rise and then a dramatic fall' in the number of positrons with respect to energy, because the positrons produced by dark matter annihilation would have a very specific energy, depending on the mass of the WIMPs making up dark matter." He said: "That's the key signature that would arise."
Physicists explain that another important signal will be whether the positrons originated from one or from all directions in space. If the positrons were generated by dark matter annihilations they should come from all directions in space. However, if they were generated from some other more familiar process, such as a supernova explosion, they would come from only one direction.
Professor Ting told the BBC: "Dark matter is supposed to be everywhere. So if we see the positrons coming from a particular direction, it means astrophysics like a pulsar (a type of neutron star) is responsible for the signal, not dark matter."
Lisa Randall of the Harvard University pointed out that, "There is a lot of stuff that can mimic dark matter." She said the scientific community is eagerly awaiting the AMS results to assess whether the experiment has detected dark matter.
But even if it turns out after rigorous examination of the results that AMS has not detected dark matter, scientists are full of optimism that the problem of cosmic dark matter will be resolved very soon because there are other ongoing experiments such as the Large Hadron Collider in Switzerland.
Turner said: "We are so excited because we believe we are on the threshold of a major discovery. We believe this will be the decade of the wimp." BBC reports he added: "Theory says that this particle might weigh somewhere between 30, 40 and 300 times what the proton does, so somewhere between 30 and maybe 1,000 GeV. The LHC can produce particles of that mass, Sam Ting's AMS detector can see particles of that mass annihilating, and then the detectors deep underground are also sensitive to particles of this mass. If we get very lucky, if Santa answers our wish-list, we could get a triple signature of the dark matter particle, by seeing the annihilations, by directly detecting it, by producing it at the LHC - all three of these methods are sensitive across the same mass range."
Randall said: "In these experiments the question is when do you have antimatter that could be explained by astrophysical sources, and when do you have something that really could be an indication that you have something new?"
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