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Grad student opportunities for decays in traps
Grad student opportunities for atomic physics of francium
U.S. grad student opportunities in spin-polarized decay
Our recent review "Standard Model tests with trapped radioactive atoms" published or arXiv
Simple CERN Courier article
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A revolution has recently occurred in atomic physics: using lasers, physicists
can now cool neutral atoms to very low energies and trap them. TRIUMF is
harnessing these new technologies. We are trapping radioactive atoms in a
vacuum, to do precision, low-energy experiments that test the
Standard Model.
We have coupled our laser trap to the copious output of radioactive alkali atoms from TRIUMF's ISAC facility. The weak interaction takes place via the exchange between particles of very heavy bosons, the W+, W- and the Z. |
We are looking for the exchange of new
bosons not now described in the Standard Model. We would not make the heavy
bosons directly, as is done at high-energy colliders like LEP. Instead, we
make careful measurements of the decay products in ways that are sensitive to
the spin and the "handedness" of possible new exchange bosons. In the
ß-decay
experiments, the decay is purely through the weak force, so we are automatically
doing experiments higher than the energy scale determined by the mass of the W
particle, 80 GeV (about 86 proton masses).
SOMEWHAT PRETTIER PICTURES can be found at our description of our Magneto optical trap (MOT), the basis of TRIUMF's neutral atom trap ("TRINAT"). |
The ß/neutrino Direction Correlation: Are there Scalar Bosons?
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The MOT produces a sample of atoms, less than 1 mm in size, held suspended in space
by laser beams and a small magnetic field. If a radioactive atom, "A", held in
such a trap undergoes ß-decay, the products are a "daughter nucleus", A'; a
beta particle (i.e. an electron); and a neutrino (v - the Greek "nu"):
All the products (including A') can freely escape the trap without contacting any nearby materials. We cannot detect the neutrino efficiently, but "conservation of momentum" provides us with a way of getting the information we need. The momenta (mass x velocity) of all the emerging particles must "balance", so by measuring the momentum of both A' and ß, we can deduce the neutrino's momentum; and so we can measure the correlation of the direction of the ß and the neutrino. Imagine detectors for A' and the ß that are back-to-back, with the trapped atoms placed between them. If the neutrino and the ß are emitted in the same direction, then to balance their joint momentum, A' must have high momentum in the opposite direction. On the other hand, if the neutrino and ß are emitted back-to-back, then their momenta will balance each other to some extent; so A' will have very low momentum, merely making up the difference between the other two particles. |
(That's a real (false color) CCD image of
3,000 atoms with 1 second half-life in the middle of the ugly XFIG cartoon.)
How does this help us to deduce the contribution of a scalar boson? Consider the decay of metastable potassium-38, in which both the parent nucleus and the daughter have spin 0. That means the ß and the neutrino between them must carry off a total of zero angular (spinning) momentum. In the Standard Model, where all the exchange bosons are vectors, it turns out that the ß and neutrino have opposite helicity, i.e. the ß spins along its direction of motion, but the neutrino spins opposite to its direction of motion. So, in the Standard Model, the neutrino and ß cannot be emitted back-to-back (because the spins would not add up to zero); therefore A' could not have a low momentum. If we actually see recoils of A' with low momentum, that means a new, spin-zero, scalar boson was exchanged.
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We have shown that a scalar boson is exchanged less than 0.5% of the time, and this is complementary to other higher-energy experiments. We are upgrading the experiment with a goal of 0.1%.
Upgrade details
ß Asymmetry: Is Nature Left-Handed?
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Another powerful tool that atom traps give us is the ability to polarize nuclei to a very high degree. (Polarized particles all have their spin axes parallel, and they spin in the same direction.) One simple method is to turn off the trap and let the atoms fall, and align their spin in one direction by shining a weak beam of circularly polarized light on them. We can use these methods to study parity violation. Parity is the transformation produced by reflecting the universe in a mirror. Under this transformation your left hand, not being symmetrical, changes into your right hand. Similarly, we can give a "handedness" to a nucleus by polarizing it; if the final products have a preference for coming out of a decay in a direction correlated with the spin, then parity is violated. It was believed that parity was a good symmetry until 1957, when Madame Wu at Columbia showed that parity is maximally violated in ß-decay, i.e. the neutrinos come out fully correlated with the nuclear spin. We say now that the weak interaction is "left-handed", that ß-decay is mediated by the exchange of a W boson that only couples to left-handed neutrinos. This is incorporated in the Standard Model. |
So why are we still interested? Because there is no fundamental reason that
Nature should be completely left-handed. This has been put arbitrarily into the
Standard Model merely to match experiment. The goal now is to measure precisely
(to about 0.1% would be complementary to high-energy
colliders) to what extent parity is maximally violated, i.e. is there a
"right-handed" neutrino? The standard model W only couples to
left-handed neutrino's, but there are theories
("left-right symmetric theories") that include an additional W that
couples to right-handed neutrinos.
The additional W's interact with the same
strength at very high energy, but with different strengths at energies
accessible to our experiment.
The world average of ß-decay experiments
currently does not see any evidence for right-handed neutrino's, and neither
do the best muon experiments (like TWIST, which is finishing at TRIUMF).
The muon experiments only involve leptons, while our beta decay experiments involve quarks. If we can achieve similar accuracy to TWIST, we will be able to constrain models that predict semileptonic interactions that are different from leptonic ones: e.g., many supersymmetric models do this. |
Upgrade details
In 1957 Sam Treiman noticed that the emission of the daughter nuclei from spin-polarized nuclear beta decay of a certain type ("Gamow-Teller") would be exactly symmetric in the Standard Model. We have now measured this quantity and set interesting constraints on non-Standard Model tensor interactions. We are finished with this one, unless we get more support from nuclear structure theorists."Tensor interaction constraints from beta decay recoil spin asymmetry of trapped atoms", J.R.A. Pitcairn et al., Physical Review C 79, 015501 (2009) published or arXiv
Atomic Physics of Radioactives
| The cold, confined atoms in the traps are very useful for laser spectroscopy experiments on small numbers of atoms, making them ideal for atomic experiments on radioactives. An atom caught in a trap can be "interrogated" by laser light as long as it lives, at a rate of about 100,000 photons per second. Among the possible experiments are measurements of parity violation in atoms. The main interaction between electrons and a nucleus is, of course, the electromagnetic interaction, which can be thought of as the exchange of virtual photons. Because the weak and electromagnetic interactions are now combined in a unified set of equations, it is also true that virtual Z bosons are exchanged in the atoms; | although this happens at a rate 10e-11 times less, it can be distinguished because it violates parity. This effect has been measured to an accuracy of 0.5% in cesium, the heaviest stable alkali element atom. Such atoms have a simple electronic structure - a single electron outside a closed core - which makes the experiments feasible and the atomic many-body theory (necessary to interpret the experiments) tractable. The heaviest alkali element is francium, where the effect is expected to be 20 times bigger than in cesium. ISAC has now produced francium and will soon have enough for experiments. |
More info on francium experiments
Francium experiments have been approved at TRIUMF to measure parity violating effects in optical transitions as well as the effect in hyperfine transitions of the parity-violating static nuclear anapole moment
Experts looking for source material might enjoy the expanded version of the "symmetries" section of the TRIUMF 5-Year plan before the high-energy editor started over: full version (1 MB), or a pithier less parochial version (0.6MB). These include non-trap and EDM stuff not in the review above.
Direct searches for massive particles
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We can trap nuclear isomers, i.e. nuclei in a long-lived excited state. In the gamma-ray decay of such an isomer, the products produced- the gamma and the nucleus in its ground state- have equal and opposite momentum (see ugly picture). If instead of a gamma-ray, a massive particle were produced, it would have less momentum. So the momentum of the final nucleus, if it is ever smaller than usual (the small bump below the peak in the simulation), would signal the emission of an exotic massive particle. We are looking for spin-0 and spin-1 massive particles with masses between 20 and 550 keV in these experiments. Such exotic particles could help explain the surplus of annihilation radiation at the galactic center by INTEGRAL (see the ESA image at the right), along with models of dark matter decay suggested by high-energy positrons seen by PAMELA. (The INTEGRAL people have counted longer and ID'd potential astrophysics sources for the extra positrons... but we'll do our experiment anyway.) If we reach our planned sensitivity of part per million, the constraints would be useful. We may also use electron capture decay to look for massive extra sterile neutrinos. There has been experimental evidence for these at various times, though nothing has been confirmed. They play havoc with standard astrophysics. If we could make them go away, 99% of polled theorists say they would appreciate that. If we found some, they might like it more. |
 
The galaxy pictured in 511 keV gammas. (No, it's not the 1 mm trap cloud :) )
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Recent conference proceeding on direct searches for massive particles (0.3 Mb pdf file), and TRIUMF proposal
| ISAC Research Areas |