A new measurement by a student and professor at the University of Rochester
has shed new light on the limits of scientists’ standard model of physics.
Doctoral student Ben Kilminster and Kevin McFarland, professor of physics,
used the particle accelerator at Fermilab to conduct the first measurement
ever done with enough precision to discern certain characteristics of how
the top quark, the heaviest particle in known physics, decays. The work is
reported in today’s issue of Physical Review D.
The findings suggest there is no connection between the top quark and the
weak nuclear force-an idea that had been attractive because the unusually
high and similar energies of the top quark and the weak force’s particle,
the W-boson, stood out from the rest of the known particles. A link between
the quark and the boson would have strongly suggested that the top quark
held a special place in the quantum world, perhaps as a kind of “father” of
the weak force, which is responsible for the characteristics of all known
matter.
“No one has made this kind of measurement as precisely, and the findings
are laying another brick in our knowledge of how the universe works,” says
McFarland. “People are trying desperately to understand why the weak force
is weak. At the beginning of the universe, it and the force that is
responsible for light, among other things, were essentially one and the
same; but now, light can cross the cosmos, but the weak force can’t even
cross an atom. We’ve come up with a lot of theories as to why this is, but
these new findings mean that a lot of those theories are going to have to
be crossed off.”
To understand the critical connection between the top quark and the weak
force’s W-boson, McFarland and Kilminster designed a test to measure the
top quark’s parity. Parity is a property of quantum particles that
describes how they act if their directions in space are reversed, as
though they were being viewed in a mirror. Imagine lobbing a tennis ball
over the net to your opponent. You would expect that when your opponent
hits the ball back, reversing its direction, that the ball will behave
identically-falling to the court, bouncing off your racket, etc. Some
particles, however, change their properties when “mirrored.” It would be
as if returning tennis balls ignore gravity and pass right through your
racket. Physicists discovered, to their amazement, that the reason for
this lack of parity is that the weak force only seems to act on particles
that have a certain kind of characteristic, called “left-handed spin.”
Imagine the tennis ball again, flying over the net, spinning on an axis
that’s pointed at your opponent, so it appears to be spinning clockwise
from your position behind it. If your opponent returns the ball
perfectly-reflecting it as if in a mirror-the direction of the ball has
reversed, but its spin will still appear to be moving clockwise as it flies
toward you. If you were to point your right thumb in the direction the ball
was flying as you smacked it over the net, your fingers would naturally
curl in a clockwise direction. When it returned to you, and you aimed your
left thumb in its direction of travel, your fingers would curl in that
same
clockwise direction, but with your thumb pointing toward you. Scientists
use this as a quickie method to label the spin of particles as “left-” and
“right-handed.” For some reason, which physicists are still puzzling over,
the weak force only ever affects left-handed particles.
McFarland and Kilminster knew that if they could show that the tremendously
heavy top quark was left-handed, then the weak force would likely act on it
the same way it did to all the other known quarks.
“In our quest to understand the nature of the weak force, physicists have
come up with some theories that link the weak force’s extremely heavy
W-boson with the extremely heavy top quark,” says McFarland. “In those
theories, the top quark holds a special place in the universe, including the
possibility of the universe being filled with top-quark pairs that create a
drag on other particles, and hence give them mass. If the W-boson and top
quark were intimately linked, it would have ramifications for all of
physics.”
Measuring the handedness of the top quark with precision had never been
accomplished before. Since there is no known way to make the measurement of
the top quark directly, the Rochester team decided to look at the particles
the top quark decays into. One of the principal functions of the weak force
is to “break down” heavier particles, like the top quark, into lighter
quarks from which nearly everything in the universe is constructed. In the
Fermilab accelerator, the team let a soup of top quarks decay into their
constituent, lighter particles. Those particles would shoot out in certain
directions, based on their spin. Connecting which particles came from which
top quarks in the ensuing collision had always been the stumbling block for
physicists, but Kilminster developed a program that essentially picked out
all the particles that might have been produced by a top quark decay, and
statistically figured out which top quark they came from.
The results showed that for the majority of the time, the top quark’s decay
spattered its resulting particles in a pattern that strongly suggested the
top quark decay is not symmetric, and thus that the weak force likely
interacts with it in the same basic manner that it interacts with all
particles. The theories that were based on the idea that the top quark and
the weak force were linked to each other in a unique fashion were shown to
be highly unlikely.
“As it happens, it seems the fact that the top quark and the W-boson have
around the same mass may just be a coincidence,” says McFarland. “Models
that rely on a link between the two are becoming more and more implausible.
The theories are really a last ditch effort to make do with the
fundamentally flawed Standard Model of physics. If these theories keep
getting disproved, we’re going to have to go on to an entirely new model of
the universe’s workings.”
This research was funded by the National Science Foundation and the U. S.
Department of Energy.
