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	Thursday, Nov. 12, 2015

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The light side of dark matter

New technology and new thinking are pushing the dark matter hunt to lower and lower masses. Photo: Karoline Schäffner/MPP

It's a seemingly paradoxical but important question in particle physics: Can dark matter be light?

Light in this case refers to the mass of the as-yet undiscovered particle or group of particles that may make up dark matter, the unseen stuff that accounts for about 85 percent of all matter in the universe.

Ever more sensitive particle detectors, experimental hints and evolving theories about the makeup of dark matter are driving this expanding search for lighter and lighter particles — even below the mass of a single proton — with several experiments giving chase.

An alternative to WIMPs?

Theorized weakly interacting massive particles, or WIMPs, are counted among the leading candidates for dark matter particles. They most tidily fit some of the leading models.

Many scientists expected WIMPs might have a mass of around 100 billion electronvolts — about 100 times the mass of a proton. The fact that they haven't definitively showed up in searches covering a range from about 10 billion electronvolts to 1 trillion electronvolts has cracked the door to alternative theories about WIMPs and other candidate dark matter particles.

Possible low-energy signals measured at underground dark matter experiments CoGeNT in Minnesota and DAMA/LIBRA in Italy, along with earlier hints of dark matter particles in space observations of our galaxy's center by the Fermi Gamma-ray Space Telescope, excited interest in a mass range below about 11 billion electronvolts — roughly 11 times the mass of a proton.

Such low-energy particles could be thought of as lighter, "wimpier" WIMPs, or they could be a different kind of particles: light dark matter.

SuperCDMS, an WIMP-hunting experiment in the Soudan Underground Laboratory in Minnesota, created a special search mode, called CDMSlite, to make its detectors sensitive to particles with mass reaching below 5 billion electronvolts. With planned upgrades, CDMSlite should eventually be able to stretch down to detect particles with a mass about 50 times less than this.

In September, the CDMS collaboration released results that narrow the parameters used to search for light WIMPs in a mass range of 1.6 billion to 5.5 billion electronvolts.

—Glenn Roberts Jr.

Photo of the Day

A night's FAST

NML, now part of the Fermilab Accelerator Science and Technology Facility, is striking at night. Photo: Giulio Stancari, AD

In the News

Einstein's true biggest blunder (op-ed)

From Space.com, Nov. 6, 2015

It has been a century since Albert Einstein published his first papers laying out his crowning intellectual achievement, the theory of general relativity. This theory showed that space was malleable and could twist and distort under the influence of matter. Since a) the shape of space is affected by the distribution of matter and energy, and b) matter moves around, this further means that the shape of space is dynamic — twisting and bending and changing with time. This idea was truly revolutionary.

In the early days, the implications of this theory were not completely obvious and necessary data were missing. This led to some miscues and changes to the theory as scientists of the time developed greater understanding. One such incident is particularly interesting.

Frontier Science Result: DZero

Two for the price of one

The recent double-parton scattering result for the effective cross-section from DZero, in blue, is compared to previously published results, in red.

Disponible en español

The two mesons whose discovery arguably did the most to advance our understanding of what a meson really is are the J/ψ (pronounced "J sigh") and the Υ (Upsilon). The discovery of the J/ψ in 1974 at Brookhaven National Laboratory and SLAC National Accelerator Laboratory was the discovery that convinced physicists that the quark model was actually right. The discovery of the Υ here at Fermilab in 1977 was the discovery that made it clear that there should be (at least) six different flavors of quark.

With enough energy, such as one had in the Tevatron, it is possible to create both a J/ψ and an Υ simultaneously. What we learn from these events has a lot to do with the way quarks are stuck together inside the proton.

The gluon is the particle that carries the force that holds quarks together. When a proton collides with another proton, a quark or gluon from one proton can collide directly with a quark or gluon from the other proton. Occasionally, there will be two such direct collisions at the same time. Perhaps there will be two quark-quark collisions, or two gluon-quark collisions, or a gluon-gluon collision with a quark-gluon collision; all the possible combinations occur. This "double parton" process does not involve new forms of energy or matter, but it can look like that, and so they are important to understand.

The study of double-parton collisions has a long history; see the Feb. 6, 2014, and Sept. 10, 2015, editions of Fermilab Today. Double-parton collisions that produce a J/ψ and an Υ are almost always a result of two gluon-gluon collisions. By comparing these collisions with other double-parton collisions involving quarks, we can find out if the distribution of gluons in a proton differs from the distribution of quarks in a proton. We use a number called σ_eff, the effective cross section. If it is small, the two collisions tend to happen close to each other.

Recently, DZero made the first measurement of the effective cross section in J/ψ-plus-Υ events, getting the result shown in blue in the above figure. A previous DZero result in which two J/ψ mesons are produced predominantly in gluon collisions also gave a low value of σ_eff, unlike the higher value of σ_eff for double collisions involving quarks.

It is fairly inescapable: The gluons in the proton tend to be clumped together more compactly than the quarks!

—Leo Bellantoni

From top left to bottom right: Volodymyr Aushev, Olga Gogota (both of Taras Shevchenko National University of Kiev, Ukraine), Dmitri Bandurin, Peter Svoisky (both of University of Virginia), Aleksei Popov (IHEP, Russia) and George Vazmin (JINR, Russia) are the primary analysts for this measurement.

In the News

$3 million Breakthrough Prize: Why mutable neutrinos won the day

From Christian Science Monitor, Nov. 9, 2015

The physicists wore gowns and tuxes, and the red carpet was rolled out Sunday night a little north of Hollywood – at NASA's Ames Research Center in Mountain View, Calif., where for the third year the Breakthrough Prizes were handed out to commemorate achievements in mathematics and science at an Oscar-like awards show.

The big winner of the evening were neutrinos, which have had a successful awards season, having also picked up a Nobel prize this year.