Physicists probe light smashups to guide future research
Understanding photon collisions could aid search for physics beyond the Standard Model

Date:
September 20, 2021
Source:
Rice University
Summary:
Light has no mass, but Europe's Large Hadron Collider (LHC) can
convert light's energy into massive particles. Physicists studied
matter- generating collisions of light and showed the departure
angle of their debris is subtly distorted by quantum interference
patterns in the light prior to collision. Their findings will help
physicists accurately interpret future experiments aimed at finding
'new physics' beyond the Standard Model.



FULL STORY ==========================================================================
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.


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"We are essentially looking at collisions of light," said Wei Li, an
associate professor of physics and astronomy at Riceand 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."
LHC and CMS are supported by the European Organization for Nuclear
Research, the Department of Energy, the National Science Foundation
and scientific funding agencies in Austria, Belgium, Brazil, Bulgaria,
China, Colombia, Croatia, Cyprus, Ecuador, Estonia, Finland, France,
Germany, Greece, Hungary, India, Iran, Ireland, Italy, South Korea,
Latvia, Lithuania, Malaysia, Mexico, Montenegro, New Zealand, Pakistan,
Poland, Portugal, Russia, Serbia, Spain, Sri Lanka, Switzerland, Taiwan, Thailand, Turkey, Ukraine and the United Kingdom.

========================================================================== Story Source: Materials provided by Rice_University. Original written
by Jade Boyd. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. A. M. Sirunyan et al. Observation of Forward Neutron
Multiplicity
Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb
Collisions at sNN=5.02  TeV. Physical Review Letters,
2021; 127 (12) DOI: 10.1103/PhysRevLett.127.122001 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210920152005.htm

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