Muons continue to defy Standard Model
An international team of
physicists has reported what it describes as the most
significant deviation to date between experiment and
theory in particle physics. Measurements of how muons
wobble in a magnetic field offer the clearest hint so far
of new physics beyond the Standard Model of particle
physics. The findings were announced today (8 January) by
the muon g-2 collaboration at Brookhaven National
Laboratory in the US.
The muon g-2 experiment measures the "g-factor"
that relates the spin of the muon - a particle that is
208 times heavier than an electron - to its magnetic
moment. Simple quantum theories predict that g=2 for
particles such as electrons and muons. However, radiative
corrections caused by the continuous emission and re-absorption
of short-lived virtual particles means that g is not
exactly equal to 2.
These corrections can be caused by particles that are
part of the Standard Model, or by more exotic particles
that are not included in the model. Probing the
differences between experimental results and theoretical
predictions is therefore a good way to search for new
physics beyond the Standard Model. The leading candidate
for such new physics is supersymmetry - a theory which
predicts that all the particles in the Standard Model
have so-called superpartners.
The Brookhaven experiment has already observed such
differences for the g-2 value of positive muons, which
have now been confirmed by the first g-2 measurements for
negative muons. The latest measurement, which matches the
combined precision of the previous results, differs from
theory by 2.9 standard deviations. If all three results
are combined, the difference between theory and
experiment is 2.8 standard deviations.
"The fact that our measurement continues to deviate
from theory may be an indication that we are seeing new
physics beyond the Standard Model," said Lee Roberts
of Boston University, spokesperson for the experiment.
"Our experiment is now 14 times more precise than
the first muon g-2 experiment performed at CERN in the
1970s - and this precision places important restrictions
on potential new theories".
"The recent g-2 result strengthens the case for new
physics effects, with supersymmetry a leading candidate,
but it is by no means definitive," says William
Marciano, a theorist at Brookhaven. "Continued
scrutiny of theory and further running of the experiment
The muon g-2 team - which includes physicists from the
US, Russia, Japan, the Netherlands and Germany - has
submitted its result to Physical Review Letters.
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