Scientists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) – the US Department of Energy Science Office’s user center for nuclear physics research at the US Department of Energy’s Brookhaven National Laboratory – have provided compelling evidence for two physical phenomena predicted over 80 years ago. … The results were obtained from a detailed analysis of more than 6,000 pairs of electrons and positrons formed in the collision of gliding particles at the RHIC, and are published in Physical Review Letters.
The main discovery is that pairs of electrons and positrons – particles of matter and antimatter – can be created directly by the collision of highly energetic photons, which are quantum “packets” of light. This transformation of energetic light into matter is a direct consequence of Einstein’s famous equation E = mc 2, which says that energy and matter (or mass) are interchangeable. Nuclear reactions in the sun and at nuclear power plants regularly convert matter into energy. Scientists have now converted light energy directly into matter in one step.
The second result shows that the path of light passing through a magnetic field in a vacuum bends differently depending on how the light is polarized. This polarization-dependent deflection (known as birefringence) occurs when light passes through certain materials. (This effect is similar to how wavelength-dependent deflection separates white light into rainbows.) But this is the first demonstration of polarization-dependent deflection of light in a vacuum.
Both results depend on the ability of the RHIC STAR – Solenoid Tracker at RHIC – to measure the angular distribution of particles produced by grazing collisions of gold ions moving at near the speed of light.
Meeting clouds of photons
Such possibilities did not exist when, in 1934, physicists Gregory Breit and John A. Wheeler first described the hypothetical possibility of light particles colliding to form pairs of electrons and their antimatter counterparts known as positrons.
“In their paper, Breit and Wheeler have already realized that this is nearly impossible to do,” said Zhangbu Xu, a physicist at Brookhaven Laboratory, a member of the STAR Collaboration RHIC. “Lasers didn’t even exist yet! But Breit and Wheeler proposed an alternative: accelerating heavy ions. And their alternative is exactly what we do at RHIC. “
An ion is essentially a bare atom devoid of electrons. The gold ion with 79 protons carries a powerful positive charge. Accelerating such a charged heavy ion to very high speeds creates a powerful magnetic field that spirals around the accelerating particle as it moves – like a 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,” Xu said. And this arrangement of perpendicular electric and magnetic fields of equal strength is a photon – a quantized “particle” of light. “So, when the ions move at a speed close to the speed of light, there is a beam of photons that surround the gold core, which move with it like a cloud.”
At RHIC, scientists accelerate gold ions to 99.995% the speed of light in two rings of accelerators.
“We have two clouds of photons moving in opposite directions with enough energy and intensity so that when two ions slide past each other without colliding, these photon fields can interact,” Xu said.
STAR physicists tracked interactions and looked for predicted electron-positron pairs.
But such pairs of particles can be created using a number of processes in RHIC, including using “virtual” photons, a state of a photon that exists for a short time and carries an effective mass. To be sure that matter-antimatter pairs come from real photons, scientists must demonstrate that the contribution of “virtual” photons does not affect the outcome of the experiment.
To do this, STAR scientists analyzed the patterns of the angular distribution of each electron in relation to its partner positron. These patterns are different for pairs formed by the interaction of real photons and virtual photons.
“We also measured all energies, mass distributions and quantum numbers of the systems. They are consistent with theoretical calculations of what might happen to real photons, ”said Daniel Brandenburg, a Goldhaber researcher at Brookhaven Lab, who analyzed STAR data on the discovery.
Other scientists have tried to create electron-positron pairs from collisions of light using powerful lasers – focused beams of intense light. But the individual photons in these intense beams don’t have enough energy yet, Brandenburg said.
One experiment at the SLAC National Accelerator Laboratory in 1997 was successful using a nonlinear process. Scientists first had to increase the photon energy in one laser beam by hitting it with a powerful electron beam. The collisions of amplified photons with multiple photons simultaneously in a huge electromagnetic field created by another laser produced matter and antimatter.
“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,” Brandenburg said. “With the RHIC’s high-energy heavy ion beam and the large acceptance and precision measurements of the STAR detector, we can analyze all kinematic distributions with high statistics to determine that the experimental results do indeed agree with real photon collisions.”
Bending light in a vacuum
STAR’s ability to measure the tiny deflections of electrons and positrons that occur almost one after the other in these events has also given physicists the opportunity to study how light particles interact with the powerful magnetic fields created by accelerated ions.
“A cloud of photons surrounding gold ions in one of the RHIC beams shoots into a strong circular magnetic field created by accelerated ions in another gold beam,” said Chi Yang, a longtime STAR fellow at Shandong University who has spent his entire career studying the electron positron pairs obtained as a result of various processes in the RHIC. “By looking at the distribution of the outgoing particles, we learn how polarized light interacts with a magnetic field.”
Werner Heisenberg and Hans Heinrich Euler in 1936 and John Tall in the 1950s predicted that the vacuum of empty space could be polarized by a powerful magnetic field, and that such a polarized vacuum would deflect the paths of photons depending on the polarization of the photons. Toll in his dissertation also described in detail how the absorption of light by a magnetic field depends on polarization and its relationship with the refractive index of light in a vacuum. This polarization-dependent deflection or birefringence has been observed in many types of crystals. It has also recently been reported that light emanating from a neutron star is bent in this way, presumably due to its interaction with the star’s magnetic field. But no terrestrial experiment has found birefringence in a vacuum.
At RHIC, scientists measured how the polarization of light affects whether light is “absorbed” by a magnetic field.
Yang explained that this is similar to how polarized sunglasses block the passage of certain rays if they don’t match the polarization of the lenses. In the case of sunglasses, in addition to reducing light transmission, you can in principle measure the temperature rise of the lens material as it absorbs the energy of the blocked light. In RHIC, the absorbed light energy is what creates electron-positron pairs.
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