Professor proposes theory of unparticle physics 
Georgi, a highly regarded physicist well-known for his pioneering work in areas including supersymmetry, quantum chromodynamics, and grand unified theories, explains that the low-energy physics of nontrivial scale-invariance cannot be described in terms of particles. In this initial investigation of the idea, he gives a quantitative scenario of the production of unparticle stuff, and predicts how it could be experimentally detected in the upcoming Large Hadron Collider (LHC), the most powerful particle accelerator that will open in early 2008.
In scale-invariant theory—where objects don’t change
when their dimensional qualities are multiplied by a rescaling
parameter—the concept of particles doesn’t work because
most particles have a definite nonzero mass. In quantum
mechanics, this isn’t a problem because the standard model
does not have scale-invariance. But Georgi suggests that
there could be an undiscovered sector of the standard model
that is exactly scale-invariant.
“I have been having a lot of fun with this,” Georgi told
PhysOrg.com. “It is a phenomenon that has been understood
mathematically for a long time, in the sense that we know
of theories that have the peculiar property of scale-invariance.
It is hard to describe this because it is so different from
what we are used to. For us it makes a big difference whether
we measure masses in grams or kilograms. But in a scale-invariant
world, it makes no difference at all.”
Georgi explains that photons, which are particles of light,
have the property of scale invariance because they have
zero mass. Multiplying all the photon energies by a factor
of 1000 would make them look exactly the same.
“Clever theorists (like Ken Wilson) showed long ago that
there were crazier possibilities which do not involve particles
with zero mass, but still have the property that energies
can be multiplied by any factor to give a physically equivalent
theory,” Georgi said. “[But] this is impossible if there
are particles with any definite nonzero mass. That is why
I called this ‘unparticle’ stuff.”
This scale-invariant sector would interact very weakly with
the rest of the standard model, making it possible to observe
evidence for unparticle stuff, if it exists. The unparticle
theory is a high-energy theory that contains both standard
model fields and “Banks-Zaks” fields (which has scale-invariance
at an infrared point). The two fields can interact through
the interactions of ordinary particles under high enough
machine energy or a low enough mass scale.
“If all of the stuff that is scale-invariant couples to
all the stuff that isn't in a way that gets weaker and weaker
as the energy gets lower, then it could be that, at the
energies we can probe today, we just don't see the unparticle
stuff at all,” Georgi explained. “There could be a scale-invariant
world separate from our own that is hidden from us at low
energies because its interactions with us are so weak.”
These particle interactions would appear to have missing
energy and momentum distributions. Georgi has calculated
the peculiar distributions of missing energy for the decay
of a top quark, which would signify the production of unparticle
stuff.
“The very confusing question of ‘What does unparticle suff
look like?’ gets replaced by a simpler question: ‘How does
unparticle stuff begin to show up as the energy of our experiments
is increased?’” he said.
He explained that a good way of understanding unparticle
stuff is with neutrinos. Neutrinos have some properties
in common with unparticle stuff. For example, neutrinos
are nearly massless and therefore nearly scale invariant.
They couple very weakly to ordinary matter at low energies,
and the effect of the coupling increases as the energy increases.
“Very often, in a scattering experiment, we can infer the
existence of neutrinos by adding up the energy and momentum
of the colliding particles and subtracting the energy and
momentum of all the particles we can see to get the energy
and momentum of the ‘missing’ (which just means that we
don’t see them because they escape our detectors without
interacting) neutrinos,” he said. “By doing the scattering
many times, we can measure a probability distribution for
the missing energy and momentum. And by looking at the distribution,
we can tell whether there is one or two or more neutrinos
missing in the particular process we are studying.
“An interesting result of my analysis is that such a distribution
for a process that produces unparticles looks like the distribution
for a fractional
number of massless particles,” he added. “This is weird,
but it follows very simply from the scale invariance of
the unparticles. It is the first glimmer of an answer to
the question of how unparticles begin to show up.”
Because the signatures of unparticle stuff would be very
distinct, LHC experiments have the potential to verify the
existence of unparticle stuff immediately. Georgi says that,
in his opinion, unparticle stuff would be a more striking
discovery than supersymmetry or extra dimensions, both of
which point to just more new particles. Unparticle stuff,
on the other hand, would be a different concept altogether.
“I, and a number of other researchers, am now trying to
push these ideas harder,” Georgi said. “Other weird properties
of unparticles have already emerged. I expect more. It is
great fun. Of course, it would be even greater fun if we
actually saw stuff like this at the LHC. But even if we
don’t, I believe that analyses like this are useful because
they can shake us out of preconceptions that could cause
us to miss important physics as the energy of our machines
grows.”
Citation: Georgi, Howard. “Unparticle Physics.” Physical
Review Letters 98, 221601 (2007).
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