91��ɫ

A 91��ɫ researcher and her graduate student are working to shed some light on one of the big questions in physics – what is dark matter and why can’t we see it?

Professor , a particle physicist in the Faculty of Science & Engineering, who joined the faculty this past summer, and graduate student Hiro Sato are co-authors with Andrea De Simone, PhD candidate at the Center for Theoretical Physics, Massachusetts Institute of Technology, in Cambridge, Mass. of a paper published in the American Physical Society’s Physical Review Letters, one of the world's foremost scientific journals.

Above: Veronica Sanz, professor of particle physics in 91��ɫ's Faculty of Science & Engineering, with graduate student Hiro Sato

The article, “Pseudo-Dirac Dark Matter Leaves a Trace”, suggests a method for detecting traces of a dark matter particle that are produced in a collider. Normally, dark matter can only be detected by inference – by “not seeing” it in experiments designed to observe the missing energy that was carried away by the dark matter particle. The method described in the paper would allow experimentalists to see evidence of dark matter, thought to make up 23 per cent of the observable universe, for the first time.

“Everybody wants to get this piece of the cake, they want to explain this 23-per-cent part of the universe,” says Sanz, who began her studies of dark matter during graduate studies at Harvard University and MIT, and continued them in her post-doctoral research at Boston University.

Dark matter was first postulated by Swiss astronomer Fritz Zwicky in 1934 to help explain anomalies in the orbital velocities of galaxies in clusters.

What led Sanz to the idea that she and her co-authors have described was a growing lack of confidence in WIMPS.

No, not weaklings on the beach – these WIMPS are the hypothetical “weakly interacting massive particles” that represent one theory about dark matter. In a technique used by the cryogenic dark matter search (CDMS) detector at Minnesota’s Soudan Underground Laboratory, scientists thought they would be able to detect dark matter, but found nothing.

“This is very worrisome,” says Sanz, “because our WIMP idea of dark matter doesn’t explain why CDMS didn’t see anything. So, we sat down and tried to think of another way of leading to dark matter.”

Experimentalists will now test the 91��ɫ team’s theory in a collider by smashing protons and watching for the observable remnants. “We are asking them to look at particles displaced from the collision point. This is not what they were going to do, they were going to look for [evidence of ] nothing.”

These questions are the subject of a larger debate about the fundamental concept of gravity and the make-up of the universe, says Sato, a master’s student who came to 91��ɫ from the University of Toronto so he could work on dark matter with Sanz.

“People thought maybe something’s wrong with gravity theory – that there’s some fundamental problem with how Einstein or Newton thought about what gravity was. The other way of thinking about it is, no, there is matter, it’s just invisible. There’s nothing wrong with our gravity theory, it’s actually some kind of matter.”

“Pseudo-Dirac Dark Matter Leaves a Trace” was published in the Sept. 16 edition of the Letters and is available as a at the website for those with access, which is included on many 91��ɫ computers via 91��ɫ Libraries.

By David Fuller, YFile contributing writer

Republished courtesy of YFile– 91��ɫ’s daily e-bulletin