Scientists track particles, search for origin and fate of universe
By Jennifer Lauren Lee
Sept. 21, 2002 9:00 p.m.
Tucked between a video store and a Westwood parking lot, UCLA
physicists are assembling the parts to a machine that may help
explain why your pizza doesn’t fall through the table when
you set it down to read this newspaper.
The machine, called the Compact Muon Solenoid, will be fully
assembled in spring of 2007 when researchers will begin using it to
track two subatomic particles.
The first is the Higgs particle, which is believed to give
matter its mass, and is theoretically responsible for everything
from the structure of galaxies to the mass of textbooks. The second
particle it will track are called supersymmetric particles, which
are the missing sisters to the particles we’ve already found,
and whose discovery could answer questions about the origin of the
universe ““ and its fate.
By theory, both particles should exist, but neither has ever
been detected before. Now, UCLA physicists are joining other
experimental physicists from around the world in an international
effort to confirm the predictions of the theorists who postulated
the existence of these particles decades ago.
But before they can detect these particles, physicists must find
a way to create them. The Higgs and supersymmetric particles are
extremely “heavy,” requiring immense amounts of energy
just to call them into existence. The largest particle accelerator
in the world, the Large Hadron Collider, is currently being built
in Switzerland at the site where the CMS experiment will take
place. When complete, it will be capable of producing up to 14,000
times more energy than that of a nuclear reaction.
Even if scientists create the particles, they won’t last
long enough for the detectors to pick up their traces
directly. Like an ice cream cone with too many scoops, a heavy
particle is unstable; it decays soon after it is created, exploding
into new particles which stream out of the collision like shrapnel
from a bomb. So instead of setting their traps for the particles
they seek, physicists will infer the existence of those particles
by looking at the direction and speed of the shrapnel.
As its name implies, the Compact Muon Solenoid will be detecting
not Higgs or supersymmetric particles, but muons ““
high-energy particles which are among the shrapnel that the Higgs
and supersymmetric particles shoot out when they decay.
“They’re essentially like fingerprints,”
explains Jay Hauser, UCLA professor of physics and head of the team
of physicists that is designing and building many of the
electronics that will be used on the detectors.
Another team of UCLA researchers is working on the Final
Assembly and Testing of the muon detection chambers. On the ground
floor of the dimly lit Science and Technology Research Building,
located just a few blocks south of campus, Russian physicist Dr.
Mikhail Ignatenko prepares to ship the first batch of assembled
muon chambers to Switzerland. There, the 11-foot-long,
wedge-shaped, copper-plated chambers will be assembled into the
circular “endcaps” of the CMS (see diagram).
Muon detectors are located as far as possible from the center of
the CMS system, where the Higgs and supersymmetric particles will
be created, in order to give the high-energy muons time to slow
down so the equipment can detect them.
“That’s why (the Compact Muon Solenoid) has to be so
big,” said Dr. Ignatenko. When completed, the CMS will be an
enormous barrel stretching 21 meters from endcap to endcap and 14.5
meters in diameter.
To test the chambers, Ignatenko and his team use muons streaming
in from space in the form of cosmic rays, since it is much cheaper
than making their own muons with an accelerator. These muons are
not as energetic as the ones created from the decay of heavy
particles such as the Higgs, but the detectors still pick them
up.
The detectors are activated when a passing muon
“hits” the wire and copper strips inside the muon
chambers, leaving a trail of higher charges behind it like tracks
of smashed grass behind a bicycle. The trail curves as the muon,
which has a negative charge, is affected by a huge cylindrical
magnet, or solenoid, which is built into the CMS detector. A
straighter track indicates that the particle was strong enough to
keep going in the same direction before the magnet could affect
it.
Once triggered, the electronics designed by UCLA select the best
muon tracks and send them off to a computer which determines
whether their properties match those expected from a muon shot off
by a decayed Higgs or supersymmetric particle.
“”˜Best’ in this case means straightest,”
Hauser explains. “The muon track of a lighter particle will
curl significantly, whereas a more energetic (muon) won’t
curl as much. So we look for very straight tracks.”
The chambers may detect as many as a million muons per second,
but most of these will probably have come from sources other than
those which concern CMS. Consequently, the electronics designed by
UCLA physicists must be sensitive enough to recognize when a
certain shower of particles should have been produced by the Higgs
or supersymmetric particles in particular, and not by another heavy
particle that produces muons.
The CMS project involves at least 42 countries, according to
Martin Von der Mey, supervisor of the lab working on the
electronics. A $500 million budget is provided by the U.S.
Department of Energy and the National Science Foundation for the
United States faction of the CMS project.
Even the failure of the experiment to detect a single Higgs or
supersymmetric particle could benefit physics. It might prompt
theorists to come up with new explanations for why that slice of
pizza has mass, or it might convince the experimentalists to look
for the Higgs and supersymmetric particles using other methods.
Either way, Ignatenko is convinced that the walls of physics
will not crumble ““ at least as far as the theorists are
concerned. “Theorists go further and further,”
Ignatenko said with a smile. “They don’t wait for us
(experimental physicists) to catch up.”
