Physicists with the STAR Collaboration at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) have produced definitive evidence for two phenomena predicted more than eight decades ago: production of matter and antimatter from photon collisions and polarization-dependent light-bending (birefringence) in a vacuum.
Two gold ions (red) move in opposite direction at 99.995% of the speed of light (v, for velocity, = approximately c, the speed of light). As the ions pass one another without colliding, two photons (γ) from the electromagnetic cloud surrounding the ions can interact with each other to create a matter-antimatter pair: an electron (e-) and positron (e+). Image credit: Brookhaven National Laboratory.
When an electron at rest annihilates with its antimatter counterpart, a positron, the process results in the emission of two photons.
In 1934, physicists Gregory Breit and John A. Wheeler studied the theory of the reverse process of ‘collision of two light quanta’ to create electron-positron pairs.
In their original study, the researchers realized the near impossibility of achieving gamma-ray collisions in existing Earth-based experiments and proposed an alternative approach with photon collisions originating from highly charged nuclei passing each other at ultra-relativistic speeds.
“In their paper, Breit and Wheeler already realized this is almost impossible to do,” said Dr. Zhangbu Xu, a physicist at DOE’s Brookhaven National Laboratory and a member of the STAR Collaboration.
“Lasers didn’t even exist yet! But Breit and Wheeler proposed an alternative: accelerating heavy ions. And their alternative is exactly what we are doing at RHIC.”
A gold ion, with 79 protons, carries a powerful positive charge. Accelerating such a charged heavy ion to very high speeds generates a powerful magnetic field that spirals around the speeding particle as it travels — like current flowing through a wire.
“If the speed is high enough, the strength of the circular magnetic field can be equal to the strength of the perpendicular electric field,” Dr. Xu said.
“And that arrangement of perpendicular electric and magnetic fields of equal strength is exactly what a photon is a quantized particle of light.”
“So, when the ions are moving close to the speed of light, there are a bunch of photons surrounding the gold nucleus, traveling with it like a cloud.”
At RHIC, the scientists accelerate gold (Au) ions to 99.995% of the speed of light in two accelerator rings.
“We have two clouds of photons moving in opposite directions with enough energy and intensity that when the two ions graze past each other without colliding, those photon fields can interact,” Dr. Xu said.
The STAR team observed a total of 6,085 exclusive electron-positron pairs in Au+Au collisions at RHIC.
“Our results provide clear evidence of direct, one-step creation of matter-antimatter pairs from collisions of light as originally predicted by Breit and Wheeler,” said Dr. Daniel Brandenburg, a physicist at DOE’s Brookhaven National Laboratory and a member of the STAR Collaboration.
STAR’s ability to measure the tiny deflections of electrons and positrons produced almost back-to-back in these events also gave the physicists a way to study how light particles interact with the powerful magnetic fields generated by the accelerated ions.
“The cloud of photons surrounding the gold ions in one of RHIC’s beams is shooting into the strong circular magnetic field produced by the accelerated ions in the other gold beam,” said Dr. Chi Yang, a physicist at Shandong University and a member of the STAR Collaboration.
“Looking at the distribution of particles that come out tells us how polarized light interacts with the magnetic field.”
German physicists Werner Heisenberg and Hans Heinrich Euler in 1936, and American physicist John Toll in the 1950s, predicted that a vacuum of empty space could be polarized by a powerful magnetic field and that such a polarized vacuum should deflect the paths of photons depending on photon polarization.
Toll also detailed how light absorption by a magnetic field depends on polarization and its connection to the refractive index of light in a vacuum.
At RHIC, the authors measured how the polarization of the light affected whether the light was ‘absorbed’ by the magnetic field.
“This is similar to the way polarized sunglasses block certain rays from passing through if they don’t match the polarization of the lenses,” Dr. Yang said.
“When we look at the products produced by photon-photon interactions at RHIC, we see that the angular distribution of the products depends on the angle of the polarization of the light.”
“This indicates that the absorption (or passing) of light depends on its polarization.”
“This is the first Earth-based experimental observation that polarization affects the interactions of light with the magnetic field in the vacuum — the vacuum birefringence predicted in 1936.”
The team’s paper was published in the journal Physical Review Letters.
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J. Adam et al. (STAR Collaboration). 2021. Measurement of e+e− Momentum and Angular Distributions from Linearly Polarized Photon Collisions. Phys. Rev. Lett 127, 052302; doi: 10.1103/PhysRevLett.127.052302
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