Ytterbium's broken symmetry: The largest parity violations ever measured in an atom

Ytterbium's broken symmetry
An atomic beam of ytterbium is generated in the oven at left, then passed through a chamber with magnetic and electric fields arranged at right angles -- the magnetic field colinear with the atomic beam, and the electric field colinear with a laser beam that excites a "forbidden" electron-energy transition. Weak interactions between electron and nucleus contribute to the forbidden transition. Credit: Lawrence Berkeley National Laboratory

Ytterbium was discovered in 1878, but until it recently became useful in atomic clocks, the soft metal rarely made the news. Now ytterbium has a new claim to scientific fame. Measurements with ytterbium-174, an isotope with 70 protons and 104 neutrons, have shown the largest effects of parity violation in an atom ever observed - a hundred times larger than the most precise measurements made so far, with the element cesium.

"Parity" assumes that, on the atomic scale, nature behaves identically when left and right are reversed: interactions that are otherwise the same but whose spatial configurations are switched, as if seen in a mirror, ought to be indistinguishable. Sounds like common sense but, remarkably, this isn't always the case.

"It's the weak force that allows parity violation," says Dmitry Budker, who led the research team. Budker is a member of the Nuclear Science Division at the U.S. Department of Energy's Lawrence Berkeley National Laboratory and a professor of physics at the University of California at Berkeley.

Of the four forces of nature - strong, electromagnetic, weak, and gravitational - the extremely short-range weak force was the last to be discovered. Neutrinos, having no electric charge, are immune to electromagnetism and only interact through the weak force. The weak force also has the startling ability to change the flavor of quarks, and to change protons into neutrons and vice versa.

Violating parity - neutrons and the weak force

Protons on their own last forever, apparently, but a free neutron falls apart in about 15 minutes; it turns into a proton by emitting an electron and an antineutrino, a process called beta decay. What makes beta decay possible is the weak force.

Ytterbium's broken symmetry
Yacov Zel'dovich proposed that the weak force induces electrical currents in the nucleus, which flow like currents in a tokamak. This anapole moment has been detected in nuclear valence protons but not yet in valence neutrons. Credit: Lawrence Berkeley National Laboratory

Scientists long assumed that nature, on the atomic scale, was symmetrical. It would look the same not only if left and right were reversed but also if the electrical charges of particles involved in an interaction were reversed, or even if the whole process ran backwards in time. Charge conjugation is written C, parity P, and time T; nature was thought to be C invariant, P invariant, and T invariant.

In 1957 researchers realized that the weak force didn't play by the rules. When certain kinds of nuclei such as cobalt-60 are placed in a magnetic field to polarize them - line them up - and then allowed to undergo beta decay, they are more likely to emit electrons from their south poles than from their north poles.

This was the first demonstration of parity violation. Before the 1957 cobalt-60 experiment, renowned physicist Richard Feynman had said that if P violation were true - which he doubted - something long thought impossible would be possible after all: "There would be a way to distinguish right from left."

It's now apparent that many atoms exhibit parity violation, although it is not easy to detect. P violation has been measured with the greatest accuracy in cesium atoms, which have 55 protons and 78 neutrons in the nucleus, by using optical methods to observe the effect when atomic electrons are excited to higher energy levels.

The Berkeley researchers designed their own apparatus to detect the much larger parity violation predicted for ytterbium. In their experiment, ytterbium metal is heated to 500 degrees Celsius to produce a beam of atoms, which is sent through a chamber where magnetic and electric fields are oriented at right angles to each other. Inside the chamber the ytterbium atoms are hit by a laser beam, tuned to excite some of their electrons to higher energy states via a "forbidden" (highly unlikely) transition. The electrons then relax to lower energies along different pathways.

Weak interactions between the electron and the nucleus - plus weak interactions within the nucleus of the atom - act to mix some of the electron energy states together, making a small contribution to the forbidden transition. But other, more ordinary electromagnetic processes, which involve apparatus imperfections, also mix the states and blur the signal. The purpose of the chamber's magnetic and electric fields is to amplify the parity-violation effect and to remove or identify these spurious electromagnetic effects.

Upon analyzing their data, the researchers found a clear signal for atomic parity violations, 100 times larger than the similar signal for cesium. With refinements to their experiment, the strength and clarity of the ytterbium signal promise significant advances in the study of weak forces in the nucleus.

Watching the weak force at work

The Budker group's experiments are expected to expose how the weak charge changes in different isotopes of ytterbium, whose nuclei have the same number of protons but different numbers of neutrons, and will reveal how weak currents flow within these nuclei.

The results will also help explain how the neutrons in the nuclei of heavy atoms are distributed, including whether a "skin" of neutrons surrounds the protons in the center, as suggested by many nuclear models.

Ytterbium's broken symmetry
The most common isotope of ytterbium has 70 protons and 104 neutrons in the nucleus. Credit: LBNL

"The neutron skin is very hard to detect with charged probes, such as by electron scattering," says Budker, "because the protons with their large electric charge dominate the interaction."

He adds, "At a small level, the measured atomic parity violation effect depends on how the neutrons are distributed within the nucleus - specifically, their mean square radius. The mean square radius of the protons is well known, but this will be the first evidence of its kind for distribution."

Measurements of parity violation in ytterbium may also reveal "anapole moments" in the outer shell of neutrons in the nucleus (valence neutrons). As predicted by the Russian physicist Yakov Zel'dovich, these electric currents are induced by the weak interaction and circulate within the nucleus like the currents inside the toroidal winding of a tokamak; they have been observed in the valence protons of cesium but not yet in valence neutrons.

Eventually the experiments will lead to sensitive tests of the Standard Model - the theory that, although known to be incomplete, still best describes the interactions of all the subatomic particles so far observed.

"So far, the most precise data about the Standard Model has come from high-energy colliders," says Budker. "The carriers of the weak force, the W and Z bosons, were discovered at CERN by colliding protons and antiprotons, a 'high-momentum-transfer' regime. Atomic parity violation tests of the Standard Model are very different - they're in the low-momentum-transfer regime and are complementary to high-energy tests."

Since 1957, when Zel'dovich first suggested seeking atomic variation in atoms by optical means, researchers have come ever closer to learning how the weak force works in atoms. Parity violation has been detected in many atoms, and its predicted effects, such as anapole moments in the valence protons of , have been seen with ever-increasing clarity. With their new experimental techniques and the observation of a large atomic parity violation in ytterbium, Dmitry Budker and his colleagues have achieved a new landmark, moving closer to fundamental revelations about our asymmetric universe on the atomic scale.

More information: "Observation of a large atomic parity violation in ytterbium," by K. Tsigutkin, D. Dounas-Frazer, A. Family, J. E. Stalnaker, V. V. Yashchuck, and D. Budker, appears in Physical Review Letters and is available online at .

Source: Lawrence Berkeley National Laboratory (news : web)

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Physicists propose new method of measuring the weak interaction

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User comments

Jul 22, 2009
Now if they would move the Ytterbium back and forth through the laser, magnetic field, and DC electrostatic field they will even find something even more revealing and useful.

Jul 22, 2009
Really? And that might be?

How clever you are.

Jul 22, 2009
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Jul 22, 2009
no dont do that! you might create a black hole that would swallow the earth!!

Jul 23, 2009
Higgs mechanism - spontaneously breaking the weak force symmetry and giving masses to particles. No one has seen a Higgs particle yet.

Jul 23, 2009
Experiments like the one described cannot tell us if Nature really treats left and right differently because we cannot isolate them from neutrino background.

Parity symmetry breaking is only present in weak interactions and it just so happens that neutrinos which are poorly understood and abundant here on Earth are only capable of weak interactions.

I find it much more probable that the presence of chiral background neutrinos makes one kind of weak decay more favorable then that left and right differ somehow on the most fundamental level.

Jul 23, 2009
It might be possible to test the above hypothesis by placing the apparatus described in the article in the stream of artificially produced neutrinos and look for an increase in symmetry breaking signal.

Negative result would not completely rule out the hypothesis as the effect may be nonlinear but a positive result would certainly confirm it.

Jul 23, 2009
no dont do that! you might create a black hole that would swallow the earth!!

never mind the earth. it would swallow my milk!

Jul 23, 2009
A good article. Reasonable text and three illustrations to help. Not beyond the grasp of the average mainstream literature reader, and provoked some interesting comments. More like this, please :)

Jul 23, 2009
There have been a few experiments showing a relationship between our orbital distance from the sun and beta-decay half-lives. Perhaps, if this violation is neutrino dependent, it will be possible to find a similar variance.

Jul 25, 2009
Please, please, please..go back to Maxwell's original 20 equations in 20 unknowns that illustrate the vortex motion of the components, not Heaviside's simplistic vectoral summing and then Lorentz's hack job of 'symmetry'. All the original works which are so critical to explain all of these things are contained in Maxwell's original math.

Thank you, and good night.

Jul 28, 2009
AWT model enables us to imagine, what happens here at least conceptually. With compare to neutron stars, the neutrons are collapsed to protons by surface tension forces of tiny droplet which is forming atom nuclei, so that the highest pressure isn't at the center, but near surface, so that the most neutrons is here (in neutron stars neutrons are kept at the center instead).

The parity violation corresponds violation inside of Aether foam, which is forming the vacuum or interior of atom nuclei. This is because both particles, both antiparticles are formed by foam branes (i.e. by surface gradients of foam membranes), while normal particles are formed by gradients/vortices at the inner surface of foam bubbles, while antiparticles are formed by vortices on the outer surface gradients.
While vacuum foam is sparse, the foam membranes are relatively thin here and here's nearly no difference between curvature of gradients on the inner and outer surface of foam membranes and between behavior of particles and antiparticles.

The situation inside of atom nuclei is different because of high mass/energy density and the Aether foam of space-time is very dense like soap foam shaken. Because bubbles of such foam are tiny and spherical, here exists a relatively large difference between curvature of inner and outer gradients and formation of particles is strongly preferred here due their higher surface curvature.


The same situation existed in the universe in its very beginning, where most of matter was formed in symmetric way, but due the lower stability of antimatter under such condition most of antimatter evaporated into smaller particles, while rest of matter has condensed into stars and planets.

Why atoms of ytterbium are so god in demonstration of CP asymmetry I cannot explain exactly in this moment, but we can suppose, just inside of atom nuclei of Yb a maximal mass/energy density exists, because large atoms have lower surface curvature and tension, so they decay more easily and small atoms have low probability of interaction between electrons and atom nuclei, because of low number of protons, so that Yb can become a good compromise.

Aug 03, 2009
The laser was tuned to rare transition between nearly spherical s-orbitals, which are close to atom nuclei and quite stable due their symmetry (excited orbitals of rod-like shape radiate energy like antennae, so that their transitions are much more probable, faster and reversible) - so that the symmetry violation in electron transitions can manifest itself in most pronounced way. The symmetry violation is always most pronounced in irreversible processes.

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