Understanding photon collisions could aid search for physics beyond the
Standard Model.
Hot on the heels of proving an 87-year-old prediction that matter can be
generated directly from light, Rice University physicists and their
colleagues have detailed how that process may impact future studies of
primordial plasma and physics beyond the Standard Model.
“We are essentially looking at collisions of light,” said Wei Li, an
associate professor of physics and astronomy at Rice and co-author of the
study published in Physical Review Letters.
“We know from Einstein that energy can be converted into mass,” said Li, a
particle physicist who collaborates with hundreds of colleagues on
experiments at high-energy particle accelerators like the European
Organization for Nuclear Research’s Large Hadron Collider (LHC) and
Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC).
Accelerators like RHIC and LHC routinely turn energy into matter by
accelerating pieces of atoms near the speed of light and smashing them into
one another. The 2012 discovery of the Higgs particle at the LHC is a
notable example. At the time, the Higgs was the final unobserved particle in
the Standard Model, a theory that describes the fundamental forces and
building blocks of atoms.
Impressive as it is, physicists know the Standard Model explains only about
4% of the matter and energy in the universe. Li said this week’s study,
which was lead-authored by Rice postdoctoral researcher Shuai Yang, has
implications for the search for physics beyond the Standard Model.
“There are papers predicting that you can create new particles from these
ion collisions, that we have such a high density of photons in these
collisions that these photon-photon interactions can create new physics
beyond in the Standard Model,” Li said.
Yang said, “To look for new physics, one must understand Standard Model
processes very precisely. The effect that we’ve seen here has not been
previously considered when people have suggested using photon-photon
interactions to look for new physics. And it’s extremely important to take
that into account.”
The effect Yang and colleagues detailed occurs when physicists accelerate
opposing beams of heavy ions in opposite directions and point the beams at
one another. The ions are nuclei of massive elements like gold or lead, and
ion accelerators are particularly useful for studying the strong force,
which binds fundamental building blocks called quarks in the neutrons and
protons of atomic nuclei. Physicists have used heavy ion collisions to
overcome those interactions and observe both quarks and gluons, the
particles quarks exchange when they interact via the strong force.
But nuclei aren’t the only things that collide in heavy ion accelerators.
Ion beams also produce electric and magnetic fields that shroud each nuclei
in the beam with its own cloud of light. These clouds move with the nuclei,
and when clouds from opposing beams meet, individual particles of light
called photons can meet head-on.
In a PRL
study published in July, Yang and colleagues used data from RHIC to show photon-photon collisions
produce matter from pure energy. In the experiments, the light smashups
occurred along with nuclei collisions that created a primordial soup called
quark-gluon plasma, or QGP.
“At RHIC, you can have the photon-photon collision create its mass at the
same time as the formation of quark-gluon plasma,” Yang said. “So, you’re
creating this new mass inside the quark-gluon plasma.”
Yang’s Ph.D. thesis work on the RHIC data
published in PRL in 2018
suggested photon collisions might be affecting the plasma in a slight but
measurable way. Li said this was both intriguing and surprising, because the
photon collisions are an electromagnetic phenomena, and quark-gluon plasmas
are dominated by the strong force, which is far more powerful than the
electromagnetic force.
“To interact strongly with quark-gluon plasma, only having electric charge
is not enough,” Li said. “You don’t expect it to interact very strongly with
quark-gluon plasma.”
He said a variety of theories were offered to explain Yang’s unexpected
findings.
“One proposed explanation is that the photon-photon interaction will look
different not because of quark-gluon plasma, but because the two ions just
get closer to each other,” Li said. “It’s related to quantum effects and how
the photons interact with each other.”
If quantum effects had caused the anomalies, Yang surmised, they could
create detectable interference patterns when ions narrowly missed one
another but photons from their respective light clouds collided.
“So the two ions, they do not strike each other directly,” Yang said. “They
actually pass by. It’s called an ultraperipheral collision, because the
photons collide but the ions don’t hit each other.”
Theory suggested quantum interference patterns from ultraperipheral
photon-photon collisions should vary in direct proportion to the distance
between the passing ions. Using data from the LHC’s Compact Muon Solenoid
(CMS) experiment, Yang, Li and colleagues found they could determine this
distance, or impact parameter, by measuring something wholly different.
“The two ions, as they get closer, there’s a higher probability the ion can
get excited and start to emit neutrons, which go straight down the beam
line,” Li said. “We have a detector for this at CMS.”
Each ultraperipheral photon-photon collision produces a pair of particles
called muons that typically fly from the collision in opposite directions.
As predicted by theory, Yang, Li and colleagues found that quantum
interference distorted the departure angle of the muons. And the shorter the
distance between the near-miss ions, the greater the distortion.
Li said the effect arises from the motion of the colliding photons. Although
each is moving in the direction of the beam with its host ion, photons can
also move away from their hosts.
“The photons have motion in the perpendicular direction, too,” he said. “And
it turns out, exactly, that that perpendicular motion gets stronger as the
impact parameter gets smaller and smaller.
“This makes it appear like something’s modifying the muons,” Li said. “It
looks like one is going at a different angle from the other, but it’s really
not. It’s an artifact of the way the photon’s motion was changing,
perpendicular to the beam direction, before the collision that made the
muons.”
Yang said the study explains most of the anomalies he previously identified.
Meanwhile, the study established a novel experimental tool for controlling
the impact parameter of photon interactions that will have far-reaching
impacts.
“We can comfortably say that the majority came from this QED effect,” he
said. “But that doesn’t rule out that there are still effects that relate to
the quark-gluon plasma. This work gives us a very precise baseline, but we
need more precise data. We still have at least 15 years to gather QGP data
at CMS, and the precision of the data will get higher and higher.”
Reference:
Observation of Forward Neutron Multiplicity Dependence of Dimuon
Acoplanarity in Ultraperipheral Pb-Pb Collisions at √sNN=5.02 TeV by A. M.
Sirunyan et al. (CMS Collaboration), 17 September 2021, Physical Review
Letters. DOI: 10.1103/PhysRevLett.127.122001
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