Expecting the Extraordinary
Carnegie Mellon High-Energy Particle Physicists Seek Nature's Subtle Secrets
Beneath an Illinois pastureland where bison graze, beams of protons and antiprotons zoom around inside 4 miles of circular pipe, accelerating until they hurtle along at nearly the speed of light. The particle beams smash into each other, resulting in collisions that scatter debris in every direction.
This demolition derby of the tiniest proportions at Fermilab is one way high-energy particle physicists look for clues to solve a cosmic mystery: What happened to antimatter?
Antimatter has existed in the minds of science ﬁction writers for decades. The Starship Enterprise is powered by antimatter, and Isaac Asimov gave his ﬁctional robots brains made out of antimatter particles called positrons. But antimatter is not just the stuff of science ﬁction.
Most physicists believe that matter and antimatter were created in equal amounts during the Big Bang, a ﬁery explosion that formed the universe more than 13 billion years ago. At that time, in theory, every particle of matter should have had a counterpart-an antimatter particle of opposite charge. If that were the case, they would have annihilated each other and released bursts of energy. But for some reason, all antimatter apparently disappeared nanoseconds after the Big Bang, while some matter remained and formed planets, stars and galaxies. The big question is: Why?
The ‘why' behind the matter-antimatter imbalance in the universe is one of nine big questions in a report compiled by the High Energy Physics Advisory Panel (HEPAP) to the Department of Energy and the National Science Foundation. Fred Gilman, head of the Department of Physics at Carnegie Mellon and chair of HEPAP, has been instrumental in HEPAP's quest to address these questions that lie at the heart of particle physics today.
High-energy particle physicists at Carnegie Mellon have a long history of using theory and experiment to reconstruct pieces of the matter-antimatter puzzle. Their experiments, conducted at particle accelerators around the world, may shed light on this cosmic conundrum and uncover new principles of physics.
"At accelerators, we can produce conditions like those in a time-slice of the universe right after the Big Bang," said Gilman. "By recreating the conditions and components of the universe shortly after the Big Bang, we ultimately hope to explain why our present universe is made out of matter and not antimatter."
What's the Matter with Antimatter?
Matter, as we encounter it in the world immediately around us, comprises three elementary particles: the electron, the up quark and the down quark. These quarks are components of the proton and neutron. The electron belongs to a class of particles known as leptons. Although appearing identical to matter particles, antimatter particles have an opposite charge. For example, matter's corresponding antimatter particles are the positron, the anti-up quark and the anti-down quark, each bearing opposite charges.
By carrying out an elegant mathematical procedure-applying two operations known as
Charge Conjugation and Parity-physicists can mathematically transform a particle into its antiparticle and predict its behavior. Based on these considerations, ﬁrst made in the 1950s, theoretical physicists had expected particles and their antiparticles to decay (or change into other particles) at the same rate, thereby exhibiting a perfect symmetry, dubbed CP symmetry.
A surprise came in 1964. At Brookhaven National Laboratory, James Cronin of the University of Chicago and Val Fitch of Princeton University led a collaboration of physicists who discovered that an exotic particle called a K meson (a subatomic particle comprising a down quark and an anti-strange quark) did not decay in the same way as its antiparticle. This behavior violated the predicted CP symmetry and offered a possible explanation for the apparent dominance of matter over antimatter in the universe.
"Without CP violation, all matter and antimatter would have almost certainly annihilated each other after the Big Bang, leaving nothing behind but cosmic background radiation," said Manfred Paulini, associate professor of physics at Carnegie Mellon and a member of the Collider Detector at Fermilab (CDF) collaboration.
An Imperfect Fit
The Nobel Prize-winning Cronin-Fitch experiment turned particle physics on its head and sent theoretical physicists back to the chalkboard. For the next 40 years, they worked to incorporate "CP violation" into the Standard Model, a framework that organizes the soup of fundamental subatomic particles (quarks and leptons) and how they interact. To test whether their theoretical predictions governing CP violation are consistent with experimental ﬁndings, particle physicists today design experiments using the Standard Model. It now accommodates CP violation, but the ﬁt is far from perfect.
"The Standard Model predicts the CP violation we've measured so far," said Roy Briere, associate professor of physics at Carnegie Mellon and co-spokesperson of the CLEO collaboration at Cornell University. "But the CP violation we know about fails by a factor of one million to explain what happened to antimatter in the early universe."
Either the Standard Model is altogether wrong or the physicists have yet to ﬁnd some missing element in it that would account for these extraordinary discrepancies. Through their experiments, Carnegie Mellon physicists are looking for a surprise that they ultimately hope will explain the abundance of matter and the dearth of antimatter.
"One way to test the Standard Model's prediction of CP violation is to measure the sides and angles of the unitary triangle," said Helmut Vogel, professor of physics at Carnegie Mellon and a member of the CLEO collaboration.
The unitary triangle is a geometric representation of the Cabibbo-Kabayashi-Maskawa (CKM) matrix. Part of the Standard Model, the CKM matrix outlines how different quarks can transform into one another, which is key to predicting their behavior and interaction. By developing parameters that clearly and conveniently describe this matrix, Lincoln Wolfenstein, emeritus professor of physics at Carnegie Mellon, brought an understanding of the CKM matrix to experimentalists, according to Vogel.
Represented geometrically, the CKM matrix takes the shape of the unitary triangle with speciﬁc proportions and measurements. The triangle's angles correspond to the amount of CP violation that occurs in certain particle interactions, and the lengths of the triangle's sides are proportional to the probability that one type of quark will decay into another.
Even after decades of experimentation, physicists still haven't established the exact length of each triangle leg and angle, so they can't say for certain whether some as yet undiscovered feature of the Standard Model could account for the million-fold difference between the CP violation they've measured and the CP violation needed to explain the matter-antimatter imbalance in the universe. To make more measurements, they are conducting experiments at accelerators around the world to determine how fast subatomic particles decay and what they become.
Accounting for CP Violation through Studies of Exotic Particles
In 1999, two new accelerators began operating at the Stanford Linear Accelerator Center in Stanford, California, and at the Japanese National Accelerator Center. They were constructed principally to investigate CP violation in the decays of B mesons (exotic particles consisting of a beauty quark and either an anti- up, -down or -strange). These "B factories" produce B mesons to carry out experiments that measure certain sides and angles of the unitary triangle.
"I think the hope was that the B factories would measure something that didn't agree with the theory, which might have told us more about CP violation and maybe pointed the way to the eventual asymmetry in the universe. But it looks like, so far, this darn model is too good," said Tom Ferguson, professor of physics at Carnegie Mellon and a member of the CLEO collaboration.
As part of the CDF collaboration, Paulini and James Russ, professor of physics at Carnegie Mellon, are taking a slightly different approach than the B factories in hopes of measuring more precisely one side of the unitary triangle. They are studying another type of B meson at Fermilab's Tevatron, the world's highest-energy particle accelerator currently in operation.
"To constrain the length of one side, you have to use Bs0 mesons, particles with a beauty quark and an anti-strange quark. The Tevatron at Fermilab is the only accelerator that produces these particles right now," said Paulini.
The Tevatron collides beams of protons and antiprotons at nearly 2 trillion electron volts (2 TeV), which is about 100 million times the energy of the electron beam in an old- fashioned television's picture tube. The CDF experiment detects the byproducts of these high-energy collisions, including Bs0 mesons and their antiparticles. By measuring and comparing how these particles and antiparticles decay, Paulini and Russ hope to detect something extraordinary.
"The hope is that this side of the triangle is going to be a little shorter than the length the Standard Model predicts," said Russ. "Then we can go back to the theorists and say, something is not right. What could be going on with the theory?"
The studies of Bs0 mesons under way at Fermilab ultimately could hold the key to understanding how matter changes into antimatter and provide the next big piece of evidence for CP Violation in the universe, according to Gilman.
Lowering Energy in Pursuit of Charm
The theories outlined in the Standard Model make speciﬁc predictions about particles and their behavior. But how do we know that the theory is correct? One way to determine whether the current theory measures up to experimental results is to slow things down. Carnegie Mellon faculty Briere, Ferguson and Vogel, members of the CLEO collaboration, are testing the Standard Model in what's called the charm sector.
The CLEO collaboration, a group of about 140 scientists from 22 universities, was a major player in B physics for more than 20 years. With the advent of the B factories and their high intensity experiments, CLEO could no longer compete in making measurements of B meson decays. So the group did something never done before with an accelerator-it lowered its energy.
"When particle beams collide at higher energies, a lot of particles are produced, and you have to carefully search through the data to ﬁnd what you are looking for," said Vogel. "At CLEO, we use just the right amount of energy needed to create charm mesons. No stray particles are produced."
Because they are able to study the decay of charm mesons (particles that contain a charm quark and either an anti-up, -down, or -strange quark) without any contaminating particles, they can reduce uncertainties and make extremely precise measurements. The more precise the measurements, the more stringent the test of the theory, according to Vogel.
CLEO experiments measure charm meson decays, which can then be compared to theoretical calculations based on a branch of the Standard Model called Quantum Chromodynamics (QCD).
Because quarks are the building blocks that combine to form a host of exotic particles, including charm mesons and B mesons, fully understanding QCD is key to making conﬁdent predictions about meson behavior. High-speed computational power is needed, said Colin Morningstar, an assistant professor of physics at Carnegie Mellon, who performs calculations to extract observables from QCD and to better understand its inner workings.
Theoretical predictions from QCD studies of charm meson decays should help experimentalists make better measurements of B meson decay properties.
Testing the QCD calculations against experimental results in charm systems will al- low physicists to apply them with conﬁdence in studies of B meson decays. Such checks will be a very important ingredient in using B mesons to probe physics beyond the Standard Model in our search to understand the matter-antimatter asymmetry of our universe.
Higher Energies, New Physics
When it comes down to it, more than 40 years of theory and experiments at various energies have yet to establish the reason why antimatter isn't here. Perhaps the missing piece of the puzzle is some other aspect of physics that has yet to be discovered, something that could upend the Standard Model altogether. And this new physics might be revealed by colliding subatomic particles at unprecedented energies.
Gilman has high hopes that 2007, which marks the opening of the Large Hadron Collider (LHC) at the European Laboratory for Particle Physics (CERN), will usher in a new era of particle physics. Perhaps the LHC's 14 TeV will give the subatomic world the burst of energy needed to shake loose its secrets.
"The LHC is the next big step worldwide in ﬁnding new particles, interactions and symmetries," said Gilman. "The TeV energy scale will project even further back to the Big Bang."
Colliding protons at such high energies should produce bits of matter (and antimatter) smaller than anything anyone has seen before, according to Gilman. Four detectors will record some of the 800 million collisions that occur every second. Ferguson, Vogel and Briere are contributing to the construction of one of these detectors, the Compact Muon Solenoid (CMS). Speciﬁcally, the Carnegie Mellon team is constructing state-of-the-art electronics for the endcap muon system of the CMS detector. Consisting of about 150,000 electronic channels, the end cap muon system will detect muons, fundamental particles that be- long to the lepton family. By analyzing muons and other particles produced in these high-energy collisions, physicists hope to ﬁnd evidence of new particles and symmetries that so far have only existed in theory.
Gilman is excited about the future. "At the end of the 20th century, we thought we knew the fundamental particles and interactions in the universe. This was not true. Since 95 percent of the universe is not like the matter immediately around us, we are beginning a new round of exploration that includes ﬁnding the missing pieces needed to understand the matter- antimatter imbalance in the universe."