Top quark chefs

September 2, 2009 | 11:52 am

The top quark is the heaviest fundamental particle.

Fundamental particles are the most basic building blocks of matter; they cannot be broken into parts. Even though it is so small, the top quark is as heavy as a gold atom, which is made up in part of almost 200 protons and neutrons.

“We think the top quark might be special because it is so massive,” said Bernd Stelzer, an experimental physicist on the CDF collaboration. “We want to look at it from all angles.”

Fermilab physicists are examining the production, properties, and decay of top quarks to gain the most complete picture of the particle possible. They compare their observations to predictions made in the Standard Model of physics and in theories that build on that model.

The CDF and DZero collaborations at Fermilab released 25 new experimental results this summer that pushed top quark measurements to higher and higher precision, Stelzer reported at the Physics in Collision conference in Kobe, Japan, this week.

The tiny particle with a huge mass has inspired and factored into many theories, some of them wild.

In one theory, the three-dimensional world we experience is only one layer of a four-dimensional universe. The closer a particle that lives in the warped fourth dimension comes to our slice of life, the closer it is to the generation of mass. So, according to the theory, the top quark is so heavy because it is the closest fundamental particle.

Other physicists have suggested that the Higgs, the theoretical particle that gives other particles mass, is actually made up of pairs of heavy top quarks bound together.

Fermilab physicists recently made strides toward understanding the curious particle.

Fourteen years ago, the CDF and DZero collaborations discovered the top quark produced in particle-antiparticle pairs in Fermi National Accelerator Laboratory’s particle accelerator, the Tevatron. In March, CDF and DZero scientists announced the first observation of particle collisions resulting in single top quarks.

“The new single top quark sample can serve as a new compass to point us to new physics,” Stelzer said.

The collisions that create top quark pairs always behave the same way, but single top quarks are created in two distinct ways. The Standard Model predicts how often each of the two processes will create a single top quark, as do the many exotic theories about the tiny, tubby particle. Measuring the rates of these processes precisely will allow physicists to determine which theories came up with the right prediction.

A virtual helping hand

Collisions of protons and antiprotons in the Tevatron convert energy into mass, as described by Einstein’s famous equation, E=mc². Accelerator operators accelerate particles to high energies and smash them together, which creates a spray of short-lived particles with a mass that corresponds to that energy.

When the collision energy of the machine corresponds to the mass of a particle, the rate of producing that particle in the collisions shoots up rapidly. This is called resonance production.

At an energy of about 80 GeV, the Tevatron produces a plethora of W bosons. When turned up to about 91 GeV, it begins to create Z bosons.

But there is no such recipe for top quarks.

The collisions that create both pairs of top quarks and single tops borrow energy from virtual particles. Virtual particles wink in and out of existence for only brief moments, but they affect interactions between particles around them.

The more massive a particle, the more infrequently it appears as a virtual particle. But even very massive particles can appear this way. That’s important, because an interaction needs a very large virtual particle to create a heavy top quark.

Physicists cannot predict the emergence of a virtual particle by the energy of the machine. Top quarks appear to lack resonance production.

At the conference, Stelzer also discussed efforts at the Tevatron to find evidence for resonance production of top quarks. This would point to top quarks being created in a new way not predicted by the Standard Model, without the help of virtual particles.

“That would be fantastic,” Stelzer said. “It would be an unambiguous sign of new physics and would likely allow us to reveal the origin of the enormous mass of the top quark.”

If CDF and DZero do not find this evidence of new physics, researchers will continue to search for it at experiments at the Large Hadron Collider at CERN.

Kathryn Grim

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Particle plushie designer digs Fermilab

September 2, 2009 | 8:57 am

There are Beanie Babies, plush, tiny stuffed animals with heart-shaped tags. And then there are subatomic particle plushies, the stuffed versions of the constituents of Beanie Babies–soft, cuddly representations of the hadrons and leptons that make up all matter in the universe.

Really, what budding young scientist wouldn’t want to take one to bed instead of a Teddy bear?

That’s what Julie Peasley was betting on, sort of, when she designed the Particle Zoo line of plushies after seeing hand-made stuffed animals at craft shows.

“I love physics and I love crafts, so I thought why not combine them,” she says. “I was sure someone had done it already, but they hadn’t.”

To maintain the scientific accuracy of her artwork, she strives to learn about particle physics and the people that make it their livelihood.  So recently, she visited two of the top-ranking high-energy particle detectors in the world:  the CMS experiment at CERN and the CDF experiment at Fermilab. While at Fermilab, she also got a tour of the NuMI tunnel, where a neutrino beam starts its path to the underground MINOS detector in Minnesota.

“I have an art degree,” she says. “All my physics knowledge has come from popular science books and science communicators helping expand my understanding.”

At CERN, she toured the CMS cavern and walked the platform that put her eye-level with the top portion of the seven-story detector. It was awe inspiring, she says.

At Fermilab, what the CDF detector–at a mere four stories–lacked in stature was made up by its up-close and personal viewing. Peasley’s visit coincided with the detector’s maintenance checkup, which occurs every one or two years. The detector was pulled apart so crews could make repairs, which also allowed Peasley to view the thousands of wires and layers of metal and scintillating plastic needed to record the data and possible signals of new physics out of millions of proton-antiproton collisions a second. She got to stand next to the muon chamber and peer inside the open detector to see the beam pipe and central tracking chamber.

“Here it is more intimate, more enclosed and smaller, but, honestly, bigger than I thought it would be,” she says. “It does seem more complex up close.”

Heidi Schellman, a professor at Northwestern University who works with the DZero collaboration at Fermilab, gave Peasley a tour of Fermilab from top to bottom. Schellman had commissioned Peasley to make a strange bottom meson plushie as gifts for three of her PhD students who were studying the particle.

Schellman also asked for a plushie to commemorate the first observation of the single top quark, announced in March by DZero and CDF scientists. The single top quark decays into a bottom quark and a W boson, which in turn decays into an electron and a neutrino or a muon and a neutrino. Peasley created a top quark that decays to its final state by unzipping it, revealing two smaller plush toys: an antimuon and a neutrino. The top quark then can be reversed into a bottom quark. You can see this “decay” in this video.

Special requests such as these have been on the rise, Peasley says.

She has branched off from the basic Standard Model particles to include theorized particles, including, of course, the Higgs boson and antiparticles. Also new is an astrophysics line that includes a brane with five strings attached and a cosmic microwave background plushie that resembles a blob of lava.

Arguably, Peasley chose one of the more visually abstract fields of physics to turn into art.  But it was that very nature that of high-energy particle physics that captured her imagination.

“I think it is the most interesting science out there right now because of the mystery,” Peasley says. “It is all impressive. CERN is impressive. This is impressive.”

Tona Kunz

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ATLAS pops up on bookshelves

September 1, 2009 | 5:04 pm

Writing a book about a complex physics experiment is one thing; making the concepts pop out at readers is quite another.

Writer Emma Sanders and paper engineer Anton Radevsky worked with ATLAS scientists to create Voyage to the Heart of Matter: The ATLAS Experiment at CERN, an 8 page pop-up book about ATLAS (from the same publishers who brought you the ATLAS coffee-table book). The book features 3-D images of the big bang, the French/Swiss countryside that reveals ATLAS under the surface, and the ATLAS detector, which readers must put together themselves.

“There are all kinds of possibilities with a pop up book that aren’t possible with flat text,” says Sanders.

The idea for the pop-up book first came from a handful of ATLAS scientists, who passed the idea on to the CERN outreach department, and who eventually presented Sanders with the opportunity to write it.

Sanders, who is head of Microcosm, CERN’s exhibition center, had never done a pop-up book, nor a children’s book before. She joined forces with Radevsky, a long-time paper engineer located in Sofia, Bulgaria, who would lead the paper construction. Sanders visited Radevsky in Sofia, where she spent time learning about paper folding from Radevsky, in his paper strewn office.

“As I walked back to my hotel room, I saw everything around me as folded paper,” says Sanders. “I said that to Anton and he said, ‘I’ve spent my whole life like that.’”

Sanders worked closely with about fifty scientists on designing the book, and she received input from many more sources. She says she wanted to reveal how the ATLAS detector worked, and how it tracked subatomic particles; but she also wanted to address some of the larger questions that ATLAS is trying to answer.

“One of the most difficult pages was perhaps one of the simplest, which was representing matter and antimatter meeting,” she says. “We tried all kinds of complex pop-ups for that and every time we showed the scientists something they said ‘No, that doesn’t work.’ That was probably the most difficult one to make.

The final product is a beautiful feat of pop-up engineering, good for young readers and adults alike. It features multiple tabs, flaps, and pop-ups per page page, and segments that must be constructed by readers.

“I think it’s just a really fun way of discovering the subject,” says Sanders, “I really hope it brings a whole new audience to the exciting science that’s going on here.”

The book will be available from the publisher or through amazon.com starting November 9.

Calla Cofield

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Building a mystery with the g-2 experiment

September 1, 2009 | 10:28 am

Experimental physicists love a mystery. When an experiment does not turn out the way theorists predict, it can mean the discovery of new physics.

So it might seem contradictory that one thing a physicist simply cannot tolerate is uncertainty.

Experiments studying subatomic particles called muons recently illustrated the difference between the two and spurred experimentalists to call for further study.

The trouble with uncertainty

Muons are the heavy cousins of electrons. Both muons and electrons have a property called a magnetic moment, which determines how they act in a magnetic field. The equation to calculate the magnetic moment of a muon depends on a number represented by the letter g, which early theory had predicted was equal to 2. Subsequent experiments found a different value for g.

In 2004, an experiment called g-2 (g minus two) at Brookhaven National Laboratory on Long Island in New York made the most precise measurement of g-2 until that point.

But the results were uncertain. The theoretical prediction had an amount of uncertainty. The experimental result had an amount of uncertainty. Both amounts were smaller than one part in a million.

The difference between the two numbers and the combined uncertainty just barely left scientists without a clear answer as to whether they had actually discovered a difference between theory and experiment.

“The goal of the experimentalist is to be able to say something meaningful, so we hate these small-significance deviations,” said Denis Bernard, experimental physicist at Ecole Polytechnique in France. “Each time we have a small-significance deviation, we work to improve the precision and eliminate a mere statistical fluctuation–or better, discover the next big thing.”

In a lecture at the Physics in Collision conference in Kobe, Japan, this week, Bernard presented the results of a new method of determining the theoretical measure of g-2.

Physicists in the BaBar collaboration at SLAC National Accelerator Laboratory submitted the results of the new measurement to Physical Review Letters last week.

The new measurement was more precise, and the result brought the theoretical prediction for g-2 and its experimental measurement closer together.

However, the significance of the difference between the two numbers remains maddeningly the same.

It works like a poll. If Candidate A is predicted to earn 60 percent of the vote and Candidate B is predicted to earn 40 percent, but the margin of error is 11 percentage points, the poll does not actually predict a winner.

But if the two percentages are closer together–say, 51 percent to 49 percent–the margin of error can be lower–say, 2 percentage points–and still render the prediction of who will win just as meaningless.

Bernard said a new g-2 experiment, which some physicists hope might take place at Fermi National Accelerator Laboratory in Illinois or at High Energy Accelerator Research oragnization (KEK) laboratory in Japan, could finally put an end to the frustration.

Either the theory and experimental results correspond or the numbers are far enough apart that scientists can officially declare a discrepancy. If theory and experiment do not match, physicists may have their mystery.

“Theory could be incomplete,” Bernard said. “We might be seeing new physics. Who knows?”

Theory through experiment

The least precisely understood ingredient of the theoretical prediction for g-2 is related to the strong force. Physicists cannot compute this number using quantum chromodynamics, the study of how quarks and gluons interact through the strong force. So they must find it through experiment.

Physicists from the BaBar collaboration at SLAC made this measurement by studying the production of a pair of subatomic particles called pions in collisions of electrons and their antiparticles, positrons, at a wide range of energies.

The physicists used a new technique to study collisions at different energies. They focused on electron-positron collisions in which the electron or positron released a high-energy photon, a particle of light, before colliding. The photons took different amounts of energy with them and therefore reduced the energy of the collisions by different amounts. So researchers were able to study different kinds of collisions without changing the energy at which they ran the accelerator.

The number of pion pairs created in these collisions depended on the energy of the collisions. Determining the relationship between the energy level and the number of events that resulted in pion pairs helped the physicists predict the value of g-2.

g-2, part II

In the old g-2 experiment at Brookhaven, physicists fed spinning muons into a ring-shaped magnetic field almost 46 feet in circumference.

A muon’s spin axis changed by a set number of degrees at each turn as it circulated around the ring, like a horse on a merry-go-round that spins on its pole as the entire ride moves in a circle.

Eventually the muon would decay, breaking apart into an electron and neutrinos. The speed at which the escaping subatomic particles moved depended upon the direction in which the muon had been spinning.

Imagine you are about to fling yourself dramatically from a speeding car as it jumps the gap between two sides of an opening drawbridge. Do you try to jump backward to land on the near side of the bridge? Or do you try to jump forward onto the far side?

Even if you try to jump backward, the momentum you have from traveling in the speeding car will pull you forward. The force of flinging yourself backward will only slow your forward momentum, not reverse it, so you will probably land in the water below.

If you try to jump forward, however, your effort will combine with the forward momentum you already have, and you may just make it to the other side.

Similarly, a muon with a backward spin will not fling its electron as far forward as a muon with a forward spin.

Using this knowledge, scientists measured the energy of electrons to determine the direction in which their parent muons were spinning. Electrons with higher energy, the ones that made it to the other side of the metaphorical bridge, came from muons with forward spins. And electrons with lower energy, the ones that wound up in the water, came from muons with backward spins.

Some researchers have suggested physically moving the g-2 experiment from Brookhaven National Laboratory to Fermilab, in Batavia, Illinois, where facilities could produce a larger number of muons. Others have suggested designing a new g-2 experiment at KEK laboratory in Tsukuba, Japan. Both laboratories could be capable of finding more precise results.

Then, Bernard said, physicists could finally solve the mystery. Or create a new one.

Kathryn Grim

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