[ Interactions with the Higgs field only generate about 1 percent
of ordinary mass. The other 99 percent comes from phenomena associated
with the strong force, the fundamental force that binds quarks into
protons and neutrons that comprise the nucleus of the atoms of
ordinary matter.]
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THE STRENGTH OF THE STRONG FORCE  
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Chris Patrick
August 3, 2022
Phys.org [[link removed]]

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_ Interactions with the Higgs field only generate about 1 percent of
ordinary mass. The other 99 percent comes from phenomena associated
with the strong force, the fundamental force that binds quarks into
protons and neutrons that comprise the nucleus of the atoms of
ordinary matter. _

, Thomas Jefferson National Accelerator Facility

 

Much ado was made about the Higgs boson when this elusive particle was
discovered in 2012. Though it was touted as giving ordinary matter
mass, interactions with the Higgs field only generate about 1 percent
of ordinary mass. The other 99 percent comes from phenomena associated
with the strong force, the fundamental force that binds smaller
particles called quarks into larger particles called protons and
neutrons that comprise the nucleus of the atoms of ordinary matter.

Now, researchers at the U.S. Department of Energy's Thomas Jefferson
National Accelerator Facility have experimentally extracted the
strength of the strong force [[link removed]], a
quantity that firmly supports theories explaining how most of the mass
or ordinary matter in the universe is generated.

This quantity, known as the coupling of the strong force, describes
how strongly two bodies interact or "couple" under this force. Strong
force coupling varies with distance between the particles affected by
the force. Prior to this research, theories disagreed on how strong
force coupling should behave at large distance: some predicted it
should grow with distance, some that it should decrease, and some that
it should become constant.

With Jefferson Lab data, the physicists were able to determine the
strong force coupling at the largest distances yet. Their results,
which provide experimental support for theoretical predictions, were
recently featured on the cover of the journal _Particles_.

"We are happy and excited to see our effort get recognized," said
Jian-Ping Chen, senior staff scientist at Jefferson Lab and a
co-author of the paper.

Though this paper is the culmination of years of data collection and
analysis, it wasn't entirely intentional at first.

A SPINOFF OF A SPIN EXPERIMENT

At smaller distances between quarks, strong force coupling is small,
and physicists can solve for it with a standard iterative method. At
larger distances, however, strong force coupling becomes so big that
the iterative method doesn't work anymore.

"This is both a curse and a blessing," said Alexandre Deur, a staff
scientist at Jefferson Lab and a co-author of the paper. "While we
have to use more complicated techniques to compute this quantity, its
sheer value unleashes a host of very important emerging phenomena."

This includes a mechanism that accounts for 99 percent of the ordinary
mass in the universe. (But we'll get to that in a bit.)

Despite the challenge of not being able to use the iterative method,
Deur, Chen and their co-authors extracted strong force coupling at the
largest distances between affected bodies ever.

They extracted this value from a handful of Jefferson Lab experiments
that were actually designed to study something completely different:
proton and neutron spin.

These experiments were conducted in the lab's Continuous Electron Beam
Accelerator Facility, a DOE user facility. CEBAF is capable of
providing polarized electron beams, which can be directed onto
specialized targets containing polarized protons and neutrons in the
experimental halls. When an electron beam
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a majority of the electrons are all spinning in the same direction.

These experiments shot Jefferson Lab's polarized electron beam at
polarized proton or neutron targets. During the several years of data
analysis afterward, the researchers realized they could combine
information gathered about the proton and neutron to extract strong
force coupling at larger distances.

"Only Jefferson Lab's high-performance polarized electron beam, in
combination with developments in polarized targets and detection
systems allowed us to get such data," Chen said.

They found that as distance increases between affected bodies, strong
force coupling grows quickly before leveling off and becoming
constant.

"There are some theories that predicted that this should be the case,
but this is the first time experimentally that we actually saw this,"
Chen said. "This gives us detail on how the strong force, at the scale
of the quarks forming protons and neutrons, actually works."

LEVELING OFF SUPPORTS MASSIVE THEORIES

These experiments were conducted about 10 years ago, when Jefferson
Lab's electron beam was capable of providing electrons at up to 6 GeV
in energy (it's now capable of up to 12 GeV). The lower-energy
electron beam was required to examine the strong force at these larger
distances: a lower-energy probe allows access to longer time scales
and, therefore, larger distances between affected particles.

Similarly, a higher-energy probe is essential for zooming in for views
of shorter timescales and smaller distances between particles. Labs
with higher-energy beams, such as CERN, Fermi National Accelerator
Laboratory, and SLAC National Accelerator Laboratory, have already
examined strong force coupling at these smaller spacetime scales, when
this value is relatively small.

The zoomed-in view offered by higher-energy beams has shown that mass
of a quark is small, only a few MeV. At least, that's their textbook
mass. But when quarks are probed with lower energy, their mass
effectively grows to 300 MeV.

This is because the quarks gather a cloud of gluons, the particle that
carries the strong force, as they move across larger distances. The
mass-generating effect of this cloud accounts for most of the mass in
the universe—without this additional mass, the textbook mass of
quarks can only account for about 1% of the mass of protons and
neutrons. The other 99% comes from this acquired mass.

Similarly, a theory posits that gluons are massless at short distances
but effectively acquire mass as they travel further. The leveling of
strong force coupling at large distances supports this theory.

"If gluons remained massless at long range, strong force coupling
would keep growing unchecked," Deur said. "Our measurements show that
strong force coupling becomes constant as the distance
[[link removed]] probed gets larger, which is a sign
that gluons have acquired mass through the same mechanism that gives
99% of mass to the proton and the neutron."

This means strong force coupling at large distances is important for
understanding this mass [[link removed]] generation
mechanism. These results also help verify new ways to solve equations
for quantum chromodynamics (QCD), the accepted theory describing the
strong force.

For instance, the flattening of the strong force coupling at large
distances provides evidence that physicists can apply a new,
cutting-edge technique called Anti-de Sitter/Conformal Field Theory
(AdS/CFT) duality. The AdS/CFT technique allows physicists to solve
equations non-iteratively, which can help with strong force
calculations at large distances where iterative methods fail.

The conformal in "Conformal Field Theory" means the technique is based
on a theory that behaves the same at all spacetime scales. Because
strong force coupling levels off at larger distances, it is no longer
dependent on spacetime scale, meaning the strong force
[[link removed]] is conformal and AdS/CFT can be
applied. While theorists have already been applying AdS/CFT to QCD,
this data supports use of the technique.

"AdS/CFT has allowed us to solve problems of QCD or quantum gravity
that were hitherto intractable or addressed very roughly using not
very rigorous models," Deur said. "This has yielded many exciting
insights into fundamental physics
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So, while these results were generated by experimentalists, they
affect theorists the most.

"I believe that these results are a true breakthrough for the
advancement of quantum chromodynamics
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said Stanley Brodsky, emeritus professor at SLAC National Accelerator
Laboratory and a QCD theorist. "I congratulate the Jefferson Lab
physics community, particularly, Dr. Alexandre Deur, for this major
advance in physics."

Years have passed since the experiments that accidentally bore these
results were conducted. A whole new suite of experiments now use
Jefferson Lab's higher energy 12 GeV beam to explore nuclear physics.

"One thing I'm very happy about with all these older experiments is
that we trained many young students and they have now become leaders
of future experiments," Chen said.

Only time will tell which theories these new experiments support.

MORE INFORMATION: Alexandre Deur et al, Experimental Determination of
the QCD Effective Charge αg1(Q), _Particles_ (2022). DOI:
10.3390/particles5020015
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