Illustrations: Sandbox Studio Illustrations: Sandbox Studio 14 Chasing charm inCHINA symmetry | volume 06 | issue 02 may 09 American scientists are flocking to the Beijing Electron Positron Collider, whose recent upgrades make it the premier place to study charm quarks and their kin. By Kelen Tuttle 15 The flight plan of today’s particle physicist can be “This was a good deal for all,” says University of a dizzying thing. Like migratory creatures, research- Minnesota Professor Ron Poling, whose group ers circle the globe in search of the best data, was one of the first in this new wave of American working on one experiment for several years and collaborators. “Beijing had an upgraded acceler- then moving on to the next, all in the hope of ator and a new detector, and we had physics answering fundamental questions about the way we wanted to do on just such a machine—but the world works. nowhere to do it. We couldn’t have made this One of particle physics’ most recent migrations transition more seamless if we had planned it from involves researchers from the United States. For the beginning.” several years, scientists with Cornell University’s CLEO-c experiment in New York studied charm Cranking out the charm quarks produced in collisions of electrons with About 200 miles northwest of the buzz of New positrons. Yet just as they began to find intriguing York City, the CLEO-c detector operated at a hints of the unexpected, CLEO-c reached the low hum from 2005 to 2008. The machine’s energy end of its allotted funding and shut down. was tuned to produce particles containing charm Now the researchers are refocusing their work quarks against a limited background of other pro- on the Beijing Electron Spectrometer, located cesses, allowing for very precise studies of charm at China’s Beijing Electron Positron Collider. When quark decays. These decays offer researchers the machine ramps up to full strength after a a means to test the Standard Model of particle recent upgrade, it will be the world’s premier instru- physics, which describes the interaction of all ment for studying particles that contain a charm visible matter in the universe. The Standard Model quark, as well as for many other types of physics. has been validated in many experiments; by look- This is not the first time Americans have ing at exceedingly precise data like that from migrated to Beijing collider experiments. They CLEO-c, researchers check whether the model have come and gone over the facility’s two holds true there, too. decades, although in recent years it was more Charm factories like those at Cornell and of the latter, with all of the American collabora- Beijing tend not to make front-page discoveries tors—except a core group from the University of as often as their high-energy cousins, such as Hawaii—leaving Beijing to join experiments in Fermilab’s Tevatron. Charm factories don’t operate the United States. But recently, as US electron– at energies high enough to produce never-before- positron colliders shut down, the Americans seen, super-heavy particles. But despite their lower have returned. profiles, charm factories do groundbreaking work. 16 At the Beijing Electron Positron Collider, the real action takes place in shielded tunnels. The 200-meter-long linear accelerator, beneath the long, skinny building on the right, accelerates electrons and positrons and injects them into an underground storage ring, left, that is 240 meters around. TheThe BeijingBeijing Electron Spectrometer Spectrometer detector detector records recordsthose collisions, those collisions, which offerwhich clues offer toclues the tonature the nature of subatomic of subatomic processes processes and particles, and particles, including including the charm the charm quark quarkand its and kin. its kin. By taking very precise measurements, charm fac- using the theory of Quantum Chromodynamics, tories can very accurately test theories, see the or QCD for short. QCD works well at high energies, minuscule secondary effects of new physics, and but calculations of what exactly goes on at even discover new low-mass particles. the lower energies at play within a meson are “If a rare process shows up at an abnormally exceedingly complex—so complex, in fact, that large rate or you see something forbidden by the even the world’s most powerful computers find it Standard Model, it’s evidence of new physics,” says impossible to make these calculations with University of Minnesota Professor Dan Cronin- a high degree of precision. Hennessy, who worked on the CLEO experiment So researchers simplify those dynamics with for more than a decade. a method called lattice quantum chromodynamics, One of the ways that researchers at CLEO-c or LQCD. It envisions particles interacting not tested the Standard Model and searched for this within space and time as we experience them, new physics was by observing decays of mesons but in finite increments—as if the particles existed containing charm quarks. only on the vertices of a three-dimensional grid, with time ticking forward in discrete clicks. By Hint of new physics running computer simulations of this grid-world, Mesons are subatomic particles that each contain physicists can apply QCD to lower-energy situa- a quark, an antiquark, and some gluons, the ele- tions and make increasingly precise predictions. mentary particles that help bind them together. At CLEO-c, researchers did find a small dis- The inside of a meson is a tumultuous place. agreement between those predictions and the Quarks constantly exchange gluons and those observed decay of Ds mesons. Was this proof gluons constantly exchange other gluons. To of new physics? A flaw in the LQCD simulations? make things even more complicated, the laws of Or nothing more than a blip in the data? symmetry | volume 06 | issue 02 may 09 quantum mechanics govern this swarm of parti- cles, which means not only that researchers can A timely switch never know precisely where a particle is located, Before they could answer these questions, funding but also that particles in this dynamic mix appear for CLEO-c dried up. The machine stopped taking and disappear in the blink of an eye. data in the spring of 2008. Nonetheless, researchers strive to understand “ CLEO -c was very successful, but we didn’t get the world around them, right down to the chaos the accelerator performance that we had hoped, within the meson, and so have found a way to and our goals are not yet fully met,” says University explain the interaction between quarks and gluons of Rochester Professor Ed Thorndike, who has 17 worked on data from a series of collider experi- the original accelerator tunnel and infrastruc- ments at Cornell for more than two decades. ture, but replaced nearly everything else. Instead “We very much want to repeat these measurements of accelerating single electron and positron at another machine to see what’s going on.” bunches inside one storage ring, the new machine China’s accelerator offers just such an accelerates 93 electron and 93 positron bunches opportunity. at a time inside two storage rings. A pristine Just about the time that CLEO-c started run- superconducting magnet, the first of its kind built ning, the Chinese Institute of High Energy in China, combines with the shiny new BES-III Physics started major improvements to the Beijing detector to more precisely measure particles’ collider and began to construct a new Beijing energies and speeds. And an innovative calibra- Spectrometer detector, BES-III. tion system involving a diode laser built by the “When we started the upgrade construction, we University of Hawaii ensures that the time-of- were hoping that more US groups would get flight detector system, which identifies particles, involved,” Yifang Wang, spokesman for BES-III, is performing as designed. These and other says. “There’s a complementary function between improvements make collisions at the Beijing accel- BES-III work and CLEO-c work. It’s beneficial erator the best in the world for studying physics to the physics and to the community when we can in this energy region. collaborate in this way.” “ BES has a long history of very important physics,” says University of Hawaii Professor Fred Ten times more data Harris, who served as co-spokesperson of BES-II The goal of the upgrade was to increase the and continues that role for BES-III. In its previous detector’s sensitivity and the collider’s luminosity, incarnations, the Beijing detector made ground- a measure of the number of particles in each breaking precision measurements of the tau collision. To do this, the Chinese institute retained particle mass and the R value, which measures The charm quark is one of 16 types of elementary particles observed by experimenters. 18 the likelihood that electron–positron collisions will is at play. If, on the other hand, the LQCD calcula- create particles made of quarks. The R value tions prove accurate, researchers will know that helped refine the prediction of the mass of the they understand the intricacies of the chaos within Higgs, the as-yet-unseen particle thought to the meson well enough to predict how it behaves. lend elementary particles their mass. “Understanding this is an essential ingredient With a design intensity 100 times that of the for particle physics,” Poling says. original Beijing Electron Positron Collider, Harris says, the upgraded accelerator “promises A multipurpose tool even more significant contributions.” While testing the CLEO-c results is one of the Once the upgraded machine reaches full major goals of the Americans who moved to luminosity—which should happen in two to three Beijing to work with the new detector, it is not years—it will produce significantly more data by any means the only physics that will be of the type sought by the CLEO-c physicists.