Darkish matter thriller deepens with demise of reported WIMP detection

First of two components

In thriller tales, the chief suspect nearly all the time will get exonerated earlier than the top of the e-book. Sometimes as a result of a key piece of proof turned out to be flawed.

In science, key proof is meant to be proper. However typically it’s not. Within the thriller of the invisible “darkish matter” in house, proof implicating one chief suspect has now been instantly debunked. WIMPs, tiny particles extensively thought to be prime darkish matter candidates, have failed to look in an experiment designed particularly to check the lone earlier examine claiming to detect them.

For many years, physicists have realized that a lot of the universe’s matter is nothing like earthly matter, which is made principally from protons and neutrons. Gravitational influences on seen matter (stars and galaxies) point out that some darkish stuff of unknown identification pervades the cosmos. Atypical matter accounts for lower than 20 % of the cosmic matter abundance.

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For unrelated causes, theorists have additionally lengthy instructed that nature possesses mysterious kinds of tiny particles predicted by a theoretical mathematical framework often called supersymmetry, or SUSY for brief. These particles could be large by subatomic requirements however would work together solely weakly with different matter, and so are often called Weakly Interacting Large particles, therefore WIMPs.

Of the various attainable species of WIMPs, one (presumably the lightest one) ought to have the properties essential to clarify the darkish matter messing with the movement of stars and galaxies (SN: 12/27/12). Method again within the final century, searches started for WIMPs in an effort to exhibit their existence and establish which species made up the darkish matter.

In1998, one analysis staff introduced obvious success. An experiment referred to as DAMA (for DArk MAtter, get it?), consisting of a particle detector buried underneath the Italian Alps, seemingly did detect particles with properties matching some physicists’ expectations for a darkish matter sign.

It was a difficult experiment to carry out, counting on the premise that house is stuffed with swarms of WIMPs. A detector containing chunks of sodium iodide ought to give off a flash of sunshine when hit by a WIMP. However different particles from pure radioactive substances would additionally produce flashes of sunshine even when WIMPs are a delusion.

So the experimenters adopted a intelligent suggestion proposed earlier by physicists Katherine Freese, David Spergel and Andrzej Drukier, recognized formally as an annual modulation check. However let’s simply name it the June-December method.

Because the Earth orbits the solar, the solar additionally strikes, touring across the Milky Method galaxy, carried by a spiral arm within the path of the constellation Cygnus. If the galaxy actually is stuffed with WIMPs, the solar must be always plowing by way of them, producing a “WIMP wind.” (It’s just like the wind you are feeling for those who stick your head out of the window of a transferring automotive.) In June, the Earth’s orbit strikes it in the identical path because the solar’s movement across the galaxy — into the wind. However in December, the Earth strikes the wrong way, away from the wind. So extra WIMPs must be hanging the Earth in June than in December. It’s similar to the best way your automotive windshield smashes into extra raindrops when driving ahead than when stepping into reverse.

Because the solar strikes by way of house, it ought to collide with darkish matter particles referred to as WIMPs, in the event that they exist. When the Earth’s revolution carries it in the identical path because the solar, in summer season, the ensuing “WIMP wind” ought to seem stronger, with extra WIMP collisions detected in June than in December.GEOATLAS/GRAPHI-OGRE, ADAPTED BY T. DUBÉAt an astrophysics convention in Paris in December 1998, Pierluigi Belli of the DAMA staff reported a transparent sign (or at the least a powerful trace) that extra particles arrived in June than December. (Extra exactly, the outcomes confirmed an annual modulation in frequency of sunshine flashes, peaking round June with a minimal in December.) The DAMA knowledge indicated a WIMP weighing in at 59 billion electron volts, roughly 60 occasions the mass of a proton.

However some consultants had considerations in regards to the DAMA staff’s knowledge evaluation. And different searches for WIMPs, with totally different detectors and methods, ought to have discovered WIMPs if DAMA was proper — however didn’t. Nonetheless, DAMA endured. A complicated model of the experiment, DAMA/LIBRA, continued to search out the June-December disparity.

Maybe DAMA was extra delicate to WIMPs than different experiments. In spite of everything, the opposite searches didn’t duplicate DAMA’s strategies. Some used substances apart from sodium iodide as a detecting materials, or watched for slight temperature will increase as an indication of a WIMP collision relatively than flashes of sunshine.

For that matter, WIMPs may not be what theorists initially thought. DAMA initially reported 60 proton-mass WIMPs primarily based on the idea that the WIMPs collided with iodine atoms. However later knowledge instructed that maybe the WIMPs had been hitting sodium atoms, implying a a lot lighter WIMP mass — lighter than different experiments had been optimally designed to detect. Yet one more risk: Possibly hint quantities of the metallic aspect thallium (a lot heavier atoms than both iodine or sodium) had been the WIMP targets. However a current evaluate of that proposal discovered as soon as once more that the DAMA outcomes couldn’t be reconciled with the absence of a sign in different experiments.

And now DAMA’s hope for vindication has been additional dashed by a brand new underground experiment, this one in Spain. Scientists with the ANAIS collaboration have repeated the June-December methodology with sodium iodide, in an effort to breed DAMA’s outcomes with the identical methodology and supplies. After three years of operation, the ANAIS staff experiences no signal of WIMPs.

To be honest, the no-WIMP conclusion depends on a whole lot of significantly refined technical evaluation. It’s not only a matter of counting gentle flashes. It’s important to gather rigorous knowledge on the conduct of 9 totally different sodium iodide modules. It’s important to appropriate for the presence of uncommon radioactive isotopes generated by cosmic ray collisions whereas the modules had been nonetheless underneath building. After which the statistical evaluation wanted to discern a winter-summer sign distinction just isn’t one thing it is best to strive at dwelling (until you’re totally versed in issues just like the least-square periodogram or the Lomb-Scargle approach). Plus, ANAIS it nonetheless going, with plans to gather two extra years of knowledge earlier than issuing a ultimate evaluation. So the judgment on DAMA’s WIMPs just isn’t essentially ultimate.

Nonetheless, it doesn’t look good for WIMPs, at the least for the WIMPs motivated by perception in supersymmetry.   

Sadly for SUSY followers, searches for WIMPs from house will not be the one dangerous information. Makes an attempt to provide WIMPs in particle accelerators have additionally thus far failed. Darkish matter would possibly simply end up to encompass another type of subatomic particle.

If that’s the case, it could be a plot twist worthy of Agatha Christie, type of like Poirot turning out to be the killer. For symmetry has lengthy been physicists’ most dependable pal, guiding many nice successes, from Einstein’s relativity concept to the usual mannequin of particles and forces.

Nonetheless, failure to search out SUSY particles thus far doesn’t essentially imply they don’t exist. Supersymmetry simply is likely to be not so simple as it first appeared. And SUSY particles would possibly simply be tougher to detect than scientists initially surmised. But when supersymmetry does end up to not be so tremendous, scientists would possibly must mirror on the ways in which religion in symmetry can lead them astray.

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