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Measurements of Fundamental Parameters

Electron Electric Dipole Moment

The Eric Cornell and Jun Ye groups are investigating whether the electron has an electric dipole moment (eEDM). An electric dipole moment is a measure of a tiny separation of positive and negative charges in a system. If the electron does have an electric dipole moment, it's a pretty darn small one.

But even a very small eEDM would have large implications for our understanding of fundamental physics that explain how the world works, including why there’s enough matter in the Universe to form galaxies, stars, and planets like Earth. Right now, there are several different theoretical models of fundamental particles (such as the electron) and their interactions that attempt to explain how the Universe came to be. Each one predicts a different value for the eEDM. These values range from a charge separation of 10-25 to <10-40 cm. In 2013, the published eEDM limit of 10-27 meant that if the electron were the size of the Earth, its eEDM would only alter the planet’s roundness by about .022 microns, or the size of a small virus.

The tantalizing idea of measuring something so small inspired Cornell to collaborate with theorist John Bohn on a multiyear project to try and measure the eEDM in ultra cold trapped molecular ions. If the Cornell and Ye groups succeed in detecting an eEDM, it will show that running time forward or backward at the quantum level makes a difference in the behavior of individual particles.

The first step in the JILA search for the tiny eEDM was to scan for electronic transitions in hafnium fluoride ion (HfF+), identified by Cornell and Bohn as a good candidate for the eEDM search. A powerful optical frequency comb spectroscopy allowed the team precisely identified these transitions.

Then in 2013, the Ye and Cornell groups invented a nifty new approach to making the actual eEDM measurement. This approach uses a trapped ball of ~1000 HfF+ ions. The researchers took the ions and aligned them with an electric field that rotated in time. This procedure caused the axes of all the molecular ions to point in the same direction while still keeping them in the trap. The technique is allowing the researchers to probe the interaction of an electron inside an ion with the large electric fields already present inside the HfF+ ions.

The identically aligned electrons in all 1000 ions will help magnify an eEDM signal if there is one. And, because the ions can be trapped in this way for a long time, the signal can be measured for much longer than in other experiments with neutral molecules. The new apparatus represents a major advance in the technology for precision measurement of an eEDM.

The eEDM collaboration is now working to improve their apparatus to look for an eEDM at the level of 10-28 to 10-29 cm, a level that is predicted by at least one major theory. Measuring at this level will test some of the same physics under study at the Large Hadron Collider in Cern.

Fine Structure Constant

The Jun Ye group is using cold molecules for precision tests of fundamental laws of nature, including searches for possible time variations of physical constants such as α, the fine structure constant. This is the "coupling constant," or measure of the electromagnetic force that governs how electrons, muons, and light interact. New models for the basic structure of matter predict that α may have changed over vast spans of cosmic time. However, conventional physics wisdom says α has always been the same. Our basic understanding of our world depends on scientists’ ability to determine whether or not α is an "inconstant" constant.

The Ye group is teaming up with astronomers to solve this mystery by taking advantage of the special properties of highly reactive OH molecules. This molecule is abundant in outer space and naturally emits microwave radiation at four specific frequencies when it returns to its ground state after being excited by radiation from nearby stars or collisions with other interstellar molecules. These natural "masers" can be detected and measured on Earth. And, some of them were created billions of years ago in the early Universe.

Because each OH transition frequency has a different dependence on the value of α, it should be possible to compare the value of a billions of years ago with its value today—provided astronomers and laboratory scientists like those in the Ye group can measure the four transition frequencies precisely enough to detect small differences in α. In 2006, the precision-measurement specialists in the Ye group were able to improve the precision of one OH transition frequency measurement by 25-fold, and the other three by tenfold.

To answer the question whether the value of α has changed over eons of times, the Ye group is waiting for astronomers to make their own precise measurements of the same four OH transitions in a 10-billion-year-old maser. Preliminary measurements of these transitions by the Westerbork Synthesis Radio Telescope in 2004–2005 provided tantalizing, but not statistically significant, hint of a possible small change in α.

Coupling between Quantum Physics and Gravity

Scientists at JILA and the National Institute of Standards and Technology (NIST) recently used their high-accuracy optical atomic clocks to set new, much lower limits on the strength of any coupling that might exist between quantum and gravitational effects. The tighter limits have placed more stringent constraints on advanced theories that include coupling between gravity and quantum effects that come into play between atomic and subatomic particles in close proximity. Physicists want to find evidence of such coupling to develop a single theory that would explain everything about our world.

The idea for an experiment to look for coupling between quantum physics and gravity came from theoretical physicist Victor Flambaum of Australia’s University of New South Wales. Flambaum suggested the use of the exquisite precision of modern atomic clocks to look for evidence of this thus-far elusive effect. He proposed comparing different atomic clocks to see whether the ratio of their clock transition frequencies depends on Earth’s distance from the Sun. This distance varies because Earth travels around the Sun in an elliptical orbit. During a year, the gravitational potential varies by 3.3%.

If coupling between quantum effects and gravity exists, it might manifest as annual variations in atomic-clock transition-frequency ratios; high-precision clock comparisons would either reveal this coupling or place a limit on its strength. Plus, a long-term record of the ratio of different clock frequencies could provide evidence of the constancy, or inconstancy, of such fundamental constants as the fine-structure constant, α, or the electron-proton mass ratio, µ.

In early 2007, NIST researchers reported that they had been unable to detect any variations in fundamental constants related to Earth’s orbital position during a six-year study. Later during a comparison of the Sr-lattice clock at JILA with the Cs primary frequency standard at NIST, the Jun Ye group looked for coupling of gravity with three fundamental constants: α, µ, and the light quark mass; they also enlisted help for this project from colleagues in France and Japan. At the same time, NIST researchers decided to look for evidence of coupling between α and gravity during a comparison of atomic clocks based on single ions of mercury and aluminum.

The researchers found no evidence for coupling between gravity and α, µ, or the light quark mass. If coupling exists, it is still too small to detect with today’s most accurate and precise optical atomic clocks.

Precision Geophysical Measurements

Precision measurements of distance have been carried out in space since the early days of the space program. One effort is the Gravity Recovery and Climate Experiment (GRACE) mission. GRACE, consisting of twin satellites launched in 2002, has been making detailed measurements of the Earth's gravity field. One of GRACE’s main goals is to identify changes in total water storage in different areas of the world. A planned GRACE follow-on mission is expected to make use of precise laser measurements of distance changes between carefully shielded test masses inside two satellites (roughly 100 km apart). Peter Bender works with colleagues in CU’s other departments to understand the spatial resolution that can be achieved for determining changes in total water storage in areas such as medium-sized river basins.

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