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From symmetry

Small teams, big dreams

A small group of determined scientists can make big contributions to physics. Photo: Sandbox Studio with Ana Kova

Particle physics is the realm of billion-dollar machines and teams of thousands of scientists, all working together to explore the smallest components of the universe.

But not all physics experiments are huge, as the scientists of DAMIC, Project 8, SPIDER and ATRAP can attest. Each of their groups could fit in a single Greyhound bus, with seats to spare.

Don't let their size fool you; their numbers may be small, but their ambitions are not.

Smaller machines
Small detectors play an important role in searching for difficult-to-find particles.

Take dark matter, for example. Because no one knows what exactly dark matter is or what the mass of a dark matter particle might be, detection experiments need to cover all the bases.

DAMIC is an experiment that aims to observe dark matter particles that larger detectors can't see.

The standard strategy used in most experiments is scaling up the size of the detector to increase the number of potential targets for dark matter particles to hit. DAMIC takes another approach: eliminating all sources of background noise to allow the detector to see potential dark matter particle interactions of lower and lower energies.

The detector sits in a dust-free room 2 kilometers below ground at SNOLAB in Sudbury, Canada. To eliminate as much noise as possible, it is held in 10 tons of lead at around minus 240 degrees Fahrenheit. Its small size allows scientists to shield it more easily than they could a larger instrument.

DAMIC is currently the smallest dark matter detection experiment — both in the size of apparatus and the number of people on the team. While many dark matter detectors use more than a hundred thousand grams of active material, the current version of DAMIC runs on a mere five grams, and the full detector will have 100 grams. Its team is made up of around 10 scientists and students.

"What's really nice is that even though this is a small experiment, it has the potential of making a huge contribution and having a big impact," says DAMIC member Javier Tiffenberg, a postdoctoral fellow at Fermilab.

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Diana Kwon

Photos of the Day

Bug sightings

The spider that lives outside the 10th floor of Wilson Hall recently posed for the photographer by positioning itself at the center of front lawn quadrupole. Photo: Reynier Cruz Torres, University of Florida
This mottled tortoise beetle was spotted near CZero. Photo: Tony Busch, AD
Frontier Science Result:
South Pole Telescope

Gravitational lensing of the cosmic microwave background by galaxy clusters

Click to enlarge. The left panel shows a simulated map of an unlensed cosmic microwave background. The center panel shows the same map if a large galaxy cluster were along the line of sight. Note that the scale on these two panels goes to 100 microKelvin. The right panel shows the difference between the first two panels. The scale is now down to 10 microKelvin. (Plots are in units of arcminutes.) Image: Antony Lewis and Lindsay King, Institute of Astronomy

The photons that make up the cosmic microwave background (CMB) have traversed the universe almost freely for 13.8 billion years, thereby carrying information about the state of the universe when it was only 380,000 years old. "Almost freely" refers to two ways that these photons are disturbed along their long journeys: They are sometimes scattered by hot electrons and they are deflected by deep gravitational wells.

It is this latter deflection, called gravitational lensing, that offers immense promise as a tool to weigh massive objects such as galaxy clusters. Clusters are very important because their abundance offers insight into why the universe is currently accelerating. Extracting this insight, though, requires careful estimates of the masses of clusters. There are currently several techniques in play: X-ray emission, galaxy counts in the clusters, distortions of the shapes of background galaxies and the signal imprinted on the CMB by hot electrons in clusters.

Lensing of the CMB provides a new way to measure cluster masses, one that has just been demonstrated. A simulated signal from one cluster is shown above. Each panel represents about 35 square arcminutes, about 20 times smaller than the moon, so a CMB experiment must have excellent resolution to see the effect. Cluster lensing is the difference between the left and center panels, shown in the right panel. The signal is roughly several microKelvin, much smaller than the typical hot and cold spots that have made the CMB famous. So the resolution must be coupled with exquisite sensitivity.

Large ground-based telescopes such as the 10-meter South Pole Telescope are beginning to attain this dual capability. The noise levels are still too high to measure lensing by a single cluster, so the SPT team performed a likelihood analysis using 513 clusters, detected over three years of the telescope's operation, to measure the weighted mass. The result was a 3-sigma measurement of the lensing of the CMB, with the mass consistent with those obtained with other methods. A paper on this result has recently been accepted for publication in The Astrophysical Journal.

The team is now optimistic that this effect will lead to competitive constraints on cluster masses with upcoming surveys, such as SPT-3G and CMB-S4.

Scott Dodelson

Eric Baxter (University of Pennsylvania), Ryan Keisler (Stanford University), Scott Dodelson (Fermilab) and the South Pole Telescope collaboration performed this work.
In the News

Synopsis: Spatial tests of dark matter

From Physics, May 20, 2015

Dark matter constitutes roughly one quarter of the energy density of the Universe, but its composition remains unknown. Indirect searches aim to determine the presence and nature of dark matter by detecting photons produced when it decays or annihilates. Recent observations of a spectral line at 3.5 kiloelectronvolts from certain galaxy clusters and of a gamma-ray excess from the Galactic Center have been interpreted as possible dark matter signatures. However, more mundane origins of the signals, e.g., pulsars or emission from potassium atoms, have not yet been ruled out. Now, a research group led by Peter Graham at Stanford University, California, has proposed that high-spatial-resolution observations of the sky could provide a "smoking gun" for decaying dark matter.

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