New calculation refines comparison of matter with antimatter
Theorists publish improved prediction for the tiny difference in kaon
decays observed by experiments

Date:
September 17, 2020
Source:
DOE/Brookhaven National Laboratory
Summary:
An international collaboration of theoretical physicists
has published a new calculation relevant to the search for an
explanation of the predominance of matter over antimatter in our
universe. The new calculation gives a more accurate prediction for
the likelihood with which kaons decay into a pair of electrically
charged pions vs. a pair of neutral pions.



FULL STORY ==========================================================================
An international collaboration of theoretical physicists -- including scientists from the U.S. Department of Energy's (DOE) Brookhaven
National Laboratory (BNL) and the RIKEN-BNL Research Center (RBRC) -- has published a new calculation relevant to the search for an explanation
of the predominance of matter over antimatter in our universe. The collaboration, known as RBC- UKQCD, also includes scientists from CERN
(the European particle physics laboratory), Columbia University, the
University of Connecticut, the University of Edinburgh, the Massachusetts Institute of Technology, the University of Regensburg, and the University
of Southampton. They describe their result in a paper to be published in
the journal Physical Review D and has been highlighted as an "editor's suggestion."

========================================================================== Scientists first observed a slight difference in the behavior of matter
and antimatter -- known as a violation of "CP symmetry" -- while studying
the decays of subatomic particles called kaons in a Nobel Prize winning experiment at Brookhaven Lab in 1963. While the Standard Model of particle physics was pieced together soon after that, understanding whether the
observed CP violation in kaon decays agreed with the Standard Model has
proved elusive due to the complexity of the required calculations.

The new calculation gives a more accurate prediction for the likelihood
with which kaons decay into a pair of electrically charged pions vs. a
pair of neutral pions. Understanding these decays and comparing the
prediction with more recent state-of-the-art experimental measurements
made at CERN and DOE's Fermi National Accelerator Laboratory gives
scientists a way to test for tiny differences between matter and
antimatter, and search for effects that cannot be explained by the
Standard Model.

The new calculation represents a significant improvement over the group's previous result, published in Physical Review Letters in 2015. Based
on the Standard Model, it gives a range of values for what is called
"direct CP symmetry violation" in kaon decays that is consistent with the experimentally measured results. That means the observed CP violation
is now, to the best of our knowledge, explained by the Standard Model,
but the uncertainty in the prediction needs to be further improved since
there is also an opportunity to reveal any sources of matter/antimatter asymmetry lying beyond the current theory's description of our world.

"An even more accurate theoretical calculation of the Standard Model may
yet lie outside of the experimentally measured range. It is therefore
of great importance that we continue our progress, and refine our
calculations, so that we can provide an even stronger test of our
fundamental understanding," said Brookhaven Lab theorist Amarjit Soni.

Matter/antimatter imbalance "The need for a difference between matter and antimatter is built into the modern theory of the cosmos," said Norman
Christ of Columbia University. "Our current understanding is that the
present universe was created with nearly equal amounts of matter and antimatter. Except for the tiny effects being studied here, matter and antimatter should be identical in every way, beyond conventional choices
such as assigning negative charge to one particle and positive charge to
its anti-particle. Some difference in how these two types of particles
operate must have tipped the balance to favor matter over antimatter,"
he said.



==========================================================================
"Any differences in matter and antimatter that have been observed to
date are far too weak to explain the predominance of matter found in our current universe," he continued. "Finding a significant discrepancy
between an experimental observation and predictions based on the
Standard Model would potentially point the way to new mechanisms of
particle interactions that lie beyond our current understanding -- and
which we hope to find to help to explain this imbalance." Modeling quark interactions All of the experiments that show a difference between matter
and antimatter involve particles made of quarks, the subatomic building
blocks that bind through the strong force to form protons, neutrons, and
atomic nuclei -- and also less-familiar particles like kaons and pions.

"Each kaon and pion is made of a quark and an antiquark, surrounded by
a cloud of virtual quark-antiquark pairs, and bound together by force
carriers called gluons," explained Christopher Kelly, of Brookhaven
National Laboratory.

The Standard Model-based calculations of how these particles behave
must therefore include all the possible interactions of the quarks and
gluons, as described by the modern theory of strong interactions, known
as quantum chromodynamics (QCD).



==========================================================================
In addition, these bound particles move at close to the speed of
light. That means the calculations must also include the principles
of relativity and quantum theory, which govern such near-light-speed
particle interactions.

"Because of the huge number of variables involved, these are some of
the most complicated calculations in all of physics," noted Tianle Wang,
of Columbia University.

Computational challenge To conquer the challenge, the theorists used
a computing approach called lattice QCD, which "places" the particles
on a four-dimensional space-time lattice (three spatial dimensions
plus time). This box-like lattice allows them to map out all the
possible quantum paths for the initial kaon to decay to the final two
pions. The result becomes more accurate as the number of lattice points increases. Wang noted that the "Feynman integral" for the calculation
reported here involved integrating 67 million variables! These complex calculations were done by using cutting-edge supercomputers. The first
part of the work, generating samples or snapshots of the most likely
quark and gluon fields, was performed on supercomputers located in the
US, Japan, and the UK. The second and most complex step of extracting
the actual kaon decay amplitudes was performed at the National Energy
Research Scientific Computing Center (NERSC), a DOE Office of Science
user facility at DOE's Lawrence Berkeley National Laboratory.

But using the fastest computers is not enough; these calculations are
still only possible even on these computers when using highly optimized computer codes, developed for the calculation by the authors.

"The precision of our results cannot be increased significantly by
simply performing more calculations," Kelly said. "Instead, in order to
tighten our test of the Standard Model we must now overcome a number of
more fundamental theoretical challenges. Our collaboration has already
made significant strides in resolving these issues and coupled with improvements in computational techniques and the power of near-future
DOE supercomputers, we expect to achieve much improved results within
the next three to five years."

========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. R. Abbott, T. Blum, P. A. Boyle, M. Bruno,
N. H. Christ, D.

Hoying, C. Jung, C. Kelly, C. Lehner, R. D. Mawhinney,
D. J. Murphy, C. T. Sachrajda, A. Soni, M. Tomii,
T. Wang.

Direct CP violation and the DI=1/2 rule in K->pp decay from
the standard model. Physical Review D, 2020; 102 (5) DOI:
10.1103/PhysRevD.102.054509 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200917122822.htm

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