Collisions of light produce matter/antimatter from pure energy

Date:
July 29, 2021
Source:
DOE/Brookhaven National Laboratory
Summary:
Scientists studying particle collisions have produced definitive
evidence for two physics phenomena predicted more than 80 years
ago: that matter/ antimatter can be created directly by colliding
photons and that a magnetic field can bend polarized light along
different paths in a vacuum.



FULL STORY ========================================================================== Scientists studying particle collisions at the Relativistic Heavy
Ion Collider (RHIC) -- a U.S. Department of Energy Office of Science
user facility for nuclear physics research at DOE's Brookhaven National Laboratory -- have produced definitive evidence for two physics phenomena predicted more than 80 years ago. The results were derived from a detailed analysis of more than 6,000 pairs of electrons and positrons produced
in glancing particle collisions at RHIC and are published in Physical
Review Letters.


==========================================================================
The primary finding is that pairs of electrons and positrons -- particles
of matter and antimatter -- can be created directly by colliding very
energetic photons, which are quantum "packets" of light. This conversion
of energetic light into matter is a direct consequence of Einstein's
famous E=mc2 equation, which states that energy and matter (or mass)
are interchangeable. Nuclear reactions in the sun and at nuclear power
plants regularly convert matter into energy. Now scientists have converted light energy directly into matter in a single step.

The second result shows that the path of light traveling through a
magnetic field in a vacuum bends differently depending on how that light
is polarized.

Such polarization-dependent deflection (known as birefringence) occurs
when light travels through certain materials. (This effect is similar
to the way wavelength-dependent deflection splits white light into
rainbows.) But this is the first demonstration of polarization-dependent light-bending in a vacuum.

Both results depend on the ability of RHIC's STAR detector -- the
Solenoid Tracker at RHIC -- to measure the angular distribution of
particles produced in glancing collisions of gold ions moving at nearly
the speed of light.

Colliding clouds of photons Such capabilities didn't exist when physicists Gregory Breit and John A.

Wheeler first described the hypothetical possibility of colliding light particles to create pairs of electrons and their antimatter counterparts,
known as positrons, in 1934.



==========================================================================
"In their paper, Breit and Wheeler already realized this is almost
impossible to do," said Brookhaven Lab physicist Zhangbu Xu, a member
of RHIC's 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." An ion is essentially
a naked atom, stripped of its electrons. 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,"
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, scientists accelerate gold
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," Xu said.



==========================================================================
STAR physicists tracked the interactions and looked for the predicted
electron- positron pairs.

But such particle pairs can be created by a range of processes at RHIC, including through "virtual" photons, a state of photon that exists
briefly and carries an effective mass. To be sure the matter-antimatter
pairs were coming from real photons, scientists have to demonstrate
that the contribution of "virtual" photons does not change the outcome
of the experiment.

To do that, the STAR scientists analyzed the angular distribution patterns
of each electron relative to its partner positron. These patterns differ
for pairs produced by real photon interactions versus virtual photons.

"We also measured all the energy, mass distributions, and quantum numbers
of the systems. They are consistent with theory calculations for what
would happen with real photons," said Daniel Brandenburg, a Goldhaber
Fellow at Brookhaven Lab, who analyzed the STAR data on this 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 within those intense beams don't have
enough energy yet, Brandenburg said.

One experiment at the SLAC National Accelerator Laboratory in 1997
succeeded by using a nonlinear process. Scientists there first had to
boost the energy of the photons in one laser beam by colliding it with
a powerful electron beam.

Collisions of the boosted photons with multiple photons simultaneously
in an enormous 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. "Thanks to RHIC's high-energy
heavy ion beam and the STAR detector's large acceptance and precision measurements, we are able to analyze all the kinematic distributions
with high statistics to determine that the experimental results are
indeed consistent with real photon collisions." Bending light in a
vacuum 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 Chi Yang, a
long-time STAR collaborator from Shandong University who spent his
entire career studying electron-positron pairs produced from various
processes at RHIC. "Looking at the distribution of particles that come
out tells us how polarized light interacts with the magnetic field."
Werner Heisenberg and Hans Heinrich Euler in 1936, and 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, in his
thesis, also detailed how light absorption by a magnetic field depends
on polarization and its connection to the refractive index of light in
a vacuum. This polarization- dependent deflection, or birefringence, has
been observed in many types of crystals. There was also a recent report
of the light coming from a neutron star bending this way, presumably
because of its interactions with the star's magnetic field. But no
Earth-based experiment has detected birefringence in a vacuum.

At RHIC, the scientists 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,
Yang explained. In the case of the sunglasses, in addition to seeing
less light get through, you could, in principle, measure an increase in
the temperature of the lens material as it absorbs the energy of the
blocked light. At RHIC, the absorbed light energy is what creates the electron-positron pairs.

"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," Yang said.

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.

"Both of these findings build on predictions made by some of the
great physicists in the early 20th century," said Frank Geurts,
a professor at Rice University, whose team built and operated
the state-of-the-art "Time-of-Flight" detector components of
STAR that were necessary for this measurement. "They are based
on fundamental measurements made possible only recently with the
technologies and analysis techniques we have developed at RHIC." ========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


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Physical Review Letters, 2021; 127 (5) DOI: 10.1103/
PhysRevLett.127.052302 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210729183606.htm

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