Experimental Nuclear Physics
What is the origin of the spin of the proton?Physicist have been investigating the spin of the proton for more than 70 years. You might think that we should be done by now, but each set of experiments seems to uncover yet another mystery.
The first experiments of Frisch and Stern in 1933, and Kellogg, Rabi, and Zacharias in 1936, showed that the magnetic moment of the proton was about 2.5 times larger than the expected value. This surprise was later seen to be a consequence of the fact that the proton is composed of quarks.
It was natural then to expect that the spins of the quarks must combine to become the spin of the proton. Remarkably, when this common-sense hypothesis was tested in 1988 by the EMC collaboration using polarized lepton-hadron scattering, the stunning result was that the quark contribution to the spin of the proton was only 12 +/- 17%. Relativistic models of the proton had predicted the quarks carry at least 60% of the spin. This discovery that the spin of the proton could not be accounted for by quarks was called the ''Spin Crisis'', and it launched a new series of experiments at SLAC, CERN, DESY, and Jefferson Lab.
The mystery deepened when, with improved precision, these experiments came to essentially the same conclusion as before - quarks only contribute 25% of the total spin of the proton. Where is the remainder?
A more sophisticated treatment of the proton's spin was necessary, including the influence of the sea of virtual quarks and anti-quarks that pop in an out of existence inside the proton. The angular momentum from the motion of the proton's constituents also contributes to its spin. Finally, a contribution to the spin is expected from the gluons that bind the quarks composing the proton together.
A significant step towards resolving the spin puzzle requires us to measure the contribution of the gluons to the spin of the proton. To do this, our group at UMass has become a member institution of the PHENIX Collaboration, which operates one of the two major detectors at the the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
RHIC that has the unique ability to collide high energy beams of protons whose spins are parallel or antiparallel. Such collisions can be modeled as collisions between a component from one proton - a quark, anti-quark, or gluon, and a component from the other. The probability that they scatter is sensitive to whether the spins of these elementary constituents are aligned or anti-aligned. By looking for the dependence of the scattering by-products on the initial alignment of the spins of the protons, we can unravel the contributions of the individual quarks, anti-quarks, and gluons to the spin of the proton in a manner not possible before. This new approach is significant in that, unlike in lepton-hadron scattering, the gluon participates at the lowest order, yielding unique sensitivity to its spin contribution to the proton.
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Electric Dipole Moments
What are the fundamental symmetries of nature?Symmetry considerations have had an enormous role in the development of physics. In 1918, the mathematician Emmy Noether showed that conservation laws can be derived from continuous symmetries. The law of conservation of energy can actually be derived from the symmetry requirement that nature behaves the same today as yesterday. Such considerations are used by theoretical physicists to restrict the form of the equations used to describe the physical universe.
In addition to continuous symmetries, there exists discrete symmetries such as parity and time reversal. These symmetries explore whether nature looks the same when viewed in a mirror and if we can distinguish between a microscopic event, such as the collision of two electrons, and a movie of the same event run backwards.
Observing a permanent electric dipole moments (EDM) of a fundamental particle would be interesting because a non-zero dipole moment requires that both parity and time reversal symmetries are violated. This can be seen in the figure at the right where we model an electron with its spin direction and dipole moment direction indicated by S and D respectively. Under parity, the dipole moment is flipped about the horizontal axis, but the spin direction remains unchanged. Under time reversal, the spin direction reverses, but the dipole moment direction is unchanged.
If the dipole moment is identically zero, we can't distinguish between the upper and lower pictures, so we can't distinguish the parity or time reversed picture from the unreversed picture, preserving the symmetries. However, if the dipole is non-zero, the two pictures are different, and we can say whether we're in the regular universe, or one in which time or space have been reversed, breaking the symmetries.
Despite five decades of increasingly sensitive experiments, the EDMs of fundamental particles have all been consistent with zero. That may change. Many new mainstream theories of elementary particle physics predict dipole moments within reach of the next generation of EDM experiments. If dipole moments are observed and these fundamental symmetries are broken, it would be tremendously exciting - the Standard Model of physics would be overturned definitively. Conversely, if the new searches still see nothing, a whole class of theories will have to be discarded - an equally important outcome.
These considerations go beyond the academic. The most astounding symmetry violation is responsible for our existence - the fact that the universe is composed of matter and not antimatter. There is still no convincing explanation for why this is so, but its resolution hints at the existence of dipole moments whose magnitudes may be detectable by a new generation of EDM experiments.
Proton and Deuteron EDM EffortsWe are involved in the development of exciting new experiments to search for EDMs in the proton and deuteron. The proton and deuteron experiments are being developed in the U.S. by the Storage Ring Electric Dipole Moment Collaboration at Brookhaven National Laboratory, and in Europe at the Institute for Nuclear Physics at Jülich. These experiments are still at the proposal stage. We hope to have CD0 approval in 2012.
Electron EDM EffortsAt UMass we are developing an experiment to determine the potential sensitivity of an electron EDM search in an excited state of the diatomic molecule lead oxide (PbO), in a buffer gas of neon. In addition, we are looking into the possibility of an electron EDM search in molecular ions trapped in an electrostatic storage ring. A poster describing this approach can be found here. Back to Top
Experimental Atomic Physics
Measurement of the Depolarizing Collision Cross-Section of PbO* in NeonWith a series of talented undergraduates and startup funds from UMass, we are mounting an experiment to measure the depolarization cross-section of an excited state of PbO in a buffer gas of neon. If the cross-section is not too large (of the order of 5 x 10-15 cm2), a new experiment to measure the EDM of the electron with more than an order of magnitude in sensitivity may be possible.
We will measure this cross-section using the Hanle effect. The steps are to ablate PbO in a cell of neon at 12 K in a magnetic field oriented along z. A y-polarized external cavity diode laser propagating in the x direction at 406.5 nm excites the X(v"=0, J"=0+) state to a superposition of B(v'=5, J'=1, m' = +/- 1).
At zero magnetic field there is no precession and no fluorescence in the y direction. At large fields (several Gauss) the coherent state precesses quickly and it can decay with the emission of a photon along y, leading to a maximum in the fluorescence signal. At intermediate fields the fluorescence signal is determined by the probability that the coherent states survives in the buffer gas long enough to precess enough to emit a photon along y. The width of the fluorescence signal versus magnetic field can be analyzed to extract the depolarizing cross-section.
We will also measure this cross-section in helium, and then decide which is the best route to an exciting new measurement of the EDM of the electron.
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