A fresh measurement from an old experiment may have just given us a major clue to several major physics mysteries. The Collider Detector at Fermilab (CDF), a particle accelerator experiment that ran until 2011, recently made headlines when it re-measured the mass of a particle known as the "W boson."
Each of
the four fundamental forces (strong force, weak force, electromagnetism, and
gravity) has corresponding particles that 'transport' the force: the photon – a
particle of light – is a carrier of the electromagnetic force, for example. The
W boson is one of the weak force's carriers.
It's
unusual for an experiment that hasn't collected data in almost a decade to
pique people's curiosity. The reasons are nuanced, but powerful. To understand
why, consider where we are currently in our understanding of the fundamental
forces and elements of matter, as stated in the so-called "Standard
Model." The Standard Model encompasses all known basic particles as well
as the strong, weak, and electromagnetic forces.
The theory
explains the mass of the W (and all other fundamental particles), as well as
the presence of the Higgs boson, which was found in 2012 at CERN. The Standard
Model is thus 'completed,' although important questions remain unsolved. How
does gravity (a conspicuous omission from the model!) fit in, for example?
Why is
there so much so-called "dark matter" in the Universe, according to
astrophysical data, and what is it? Why is it that matter outnumbers antimatter
by such a large margin? The Standard Model is clearly not complete, and several
variations have been proposed.
The
Standard Model, on the other hand, is a subtle structure. In quantum physics'
microscopic universe, particles influence one another even when there isn't
enough energy for them to exist. They can travel through tiny loops, forming
and dissolving before being seen. We refer to them as "virtual"
particles, yet their impact is extremely real and quantifiable. They have an
effect on particle masses, thus while the Standard Model does not predict the
absolute values of particle masses, it does predict – very exactly – some of
the correlations between them.
Then it's
back to the W mass. It is possible to measure it directly, as CDF has done (and
more on that in a moment). It can also be estimated using all of the previous
data we've taken, as well as the Standard Model's mass relationships.
Something
is incorrect if the directly measured value does not correspond with the
estimated value. Intriguingly, new, beyond-the-Standard-Model virtual particles
could be involved in those cycles. The new CDF mass measurement does not agree
with the estimated W mass, which has piqued interest.
If the
Standard Model is right, you'd only expect one discrepancy this large if you
made this CDF measurement a million million times. There are certain reasons to
be cautious, as there usually are. W bosons created in high-energy collisions
between protons and anti-protons were measured by CDF.
Because
being that accurate takes a long time, the measurement has taken over a decade.
When a W is created, it decays instantly, and one of the byproducts is a
neutrino, which CDF is unable to detect. The neutrino's mass (and so the W
mass) is computed based on the assumption that it must balance everything else
produced in the collision.
This means
that many other sources of uncertainty, such as particle dispersion inside the
proton, extraneous background particles, and, of course, the precise geometry
and accuracy of the detector itself, might have a substantial impact.
Even so,
faults can never be totally ruled out, and the current result is slightly
different from previous measurements, including those taken by CDF. Now that
the result is public, it will be scrutinised like few other measurements, with
other experiments, particularly those at CERN, attempting to match its
precision and confirm or dispute the discrepancy.
However,
this is a strong indication that answers to some of the big problems left
unanswered by the Standard Model may be close at hand, especially as the Large
Hadron Collider enters its third working period and increases the accuracy with
which it can explore the energy frontier.
Reference: Science.org
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