Friday, April 10, 2015
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From symmetry

Seeing dark matter without seeing

Indirect detection experiments might be the key to discovering invisible dark matter. Image courtesy of NASA; ESA; M. J. Jee and H. Ford, Johns Hopkins University

The race for the discovery of dark matter is on. Several experiments worldwide are searching for the mysterious substance and pushing the limits on the properties it may have.

Scientists know that dark matter exists because it has a gravitational effect on visible objects made of ordinary matter. And they know that there is a lot of it; dark matter is thought to be about five times as prevalent as other matter in the universe. Yet, dark matter has managed to evade detection so far.

Similar to normal matter, dark matter is commonly believed to be composed of particles. Scientists' current best guess is that these particles are WIMPs: weakly interacting massive particles. These particles would pass right through ordinary matter. That's because they would interact only through the weak nuclear force — which works only over short distances — and gravity.

Scientists are trying to create WIMPs in collisions at the Large Hadron Collider. But it could be that they are too massive to produce in such an accelerator. Scientists are also trying to find WIMPs with detectors deep underground. But so far they haven't appeared.

That's why scientists also search for dark matter indirectly — rather than trying to catch the WIMPs themselves, they look for other signs that they're around. These signs could come in the form of extra gamma rays, cosmic rays or neutrinos, or in patterns imprinted on the cosmic microwave background radiation left over from just after the big bang.

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Manuel Gnida

In Brief

NIU President Baker and Senator Durbin meet with Fermilab senior management

Northern Illinois University president met with Fermilab leadership and Senator Durbin on Wednesday. From left: Fermilab Chief Operating Officer Tim Meyer, Northern Illinois University Professor and Director of Accelerator Science Swapan Chattopadhyay, Fermilab Director Nigel Lockyer, Senator Dick Durbin, NIU Vice President for Research and Innovation Lesley Rigg, NIU President Doug Baker. Photo courtesy of Shanthi Muthuswamy, NIU

On Wednesday, Northern Illinois University President Doug Baker convened a small roundtable discussion on federal support for research with special guest Senator Dick Durbin.

The group discussed the value of research to the future of the United States and the need to address support for science in an atmosphere of intense budget pressures. The U.S. commitment to a strong science, technology and innovation enterprise influences young people and can attract the world's top talent.

Photo of the Day

Disappearing in the distance

Power lines disappear into Wednesday's fog. Photo: Sharan Kalwani, SCD
In the News

Particle physics: A weighty mass difference

From Nature, April 8, 2015

Nuclear physics, and many major aspects of the physical world as we know it, hinges on the 0.14 percent difference in mass between neutrons and protons. Theoretically, that mass difference ought to be a calculable consequence of the quantum theory of the strong nuclear force (quantum chromodynamics; QCD) and the electromagnetic force (quantum electrodynamics; QED). But the required calculations are technically difficult and have long hovered out of reach. In a paper published in Science, Borsanyi et al. report breakthrough progress on this problem.

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Frontier Science Result: CMS

Two Higgs are better than one

Finding pairs of Higgs bosons might be a path to discovering something entirely new. Because Higgs bosons are unstable, what experimenters really do is look for the presence of four b (bottom) quarks.

It seems like only yesterday that scientists were combing diligently through their data looking for single Higgs bosons. Now that we know not only that Higgs bosons exist, but also their mass, scientists have been looking for pairs of Higgs bosons, which could be useful in searching for new physics phenomena.

For example, in an attempt to understand why gravity is so much weaker than other forces, scientists are using Higgs bosons to investigate the idea that there are more dimensions of space than the familiar three. (If you're interested in a little more details of the basic idea, take a look at this video.)

One class of the extra-dimension model predicts the existence of heavy gravitons. (The physics-savvy reader may wonder about the idea of heavy gravitons when gravitons are supposed to be massless. If extra dimensions exist, then it is possible that at least some forms of gravitons might be massive.)

Heavy gravitons would decay, and one of the possible ways in which they could decay would be into two Higgs bosons. Higgs bosons are themselves unstable and they also decay most commonly into pairs of bottom quarks, or b quarks. CMS scientists searched for events in which four bottom quarks were produced, two from each of two Higgs bosons. This is a difficult analysis given the LHC can easily generate collisions in which four quarks (and even four bottom quarks in particular) are produced.

After carefully inspecting the data, what scientists found was completely consistent with the predictions of the Standard Model: They found no heavy gravitons this time around. However, the LHC will resume operations in the near future, and researchers will use the increased collision energy to look for even heavier gravitons.

Don Lincoln

These physicists contributed to this analysis.
These physicists and technical professionals are playing a crucial role in testing prototype modules for the CMS forward silicon pixel detector upgrade.
In the News

Dark matter and muons are ruled out as DAMA signal source

From Physics World, April 9, 2015

A controversial and unconfirmed observation of dark matter made by the DAMA group in Italy may have an even stranger source than previously thought, according to physicists in the UK. Their research suggests that the signal seen by DAMA is neither from dark matter nor from background radiation. Instead, they say that the signal could be the result of a fault in the DAMA detector's data-collecting apparatus.

Starting in 1998, the DAMA-LIBRA experiment — nestled deep underground at the Gran Sasso National Laboratory in Italy — has reported an annual oscillation in the signal from its dark-matter detector. Some physicists believe that this variation is the first direct detection of dark matter and is a result of the Earth moving throughout the galaxy's halo of dark matter. Further data collected by the collaboration over the past 17 years has given the measurement a statistical significance at 9.3σ — well beyond the 5σ that usually signifies a discovery in particle physics. But apart from the CoGENT dark-matter experiment in the US, no other dark-matter searches across the globe have detected a similar effect, calling the claim of the first direct detection of dark matter into question.

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