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Illustrations: Sandbox Studio

Illustrations: Sandbox Studio

14 Chasing charm inCHINA symmetry | volume 06 | issue 02 may 09

American scientists are flocking to the Beijing , 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 ’ 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 with About 200 miles northwest of the buzz of New . 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 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 shut down, the Americans seen, super-heavy particles. But despite their lower have returned. profiles, charm factories do groundbreaking work.

16 for morethanadecade. explain the interaction between quarks and gluons explain theinteractionbetween quarks andgluons within themeson, andsohavefoundawayto the worldaround them,rightdowntothechaos and disappearintheblinkofan eye. but also that particles in this dynamic mix appear never know precisely where a particle is located, cles, which meansnotonlythatresearchers can quantum mechanics governthisswarmofparti- make thingsevenmorecomplicated,thelawsof To gluons. other exchange constantly gluons those and gluons exchange constantly Quarks place. tumultuous a is meson a of inside The together. them bind help that particles mentary a quark,anantiquark,andsomegluons,theele- Mesons are subatomic particles that each contain Hint ofnewphysics charm quarks. containing new decays physics of was mesons by observing Modelandsearched forthis tested theStandard Hennessy, whoworkedonthe ProfessorUniversity ofMinnesota DanCronin- Model,it’sevidenceofnewphysics,”Standard says large rate or you see something forbidden by the even discovernewlow-massparticles. minuscule secondary effects of new physics, and tories can very accurately test theories, see the precisemeasurements,charm fac- very By taking One ofthewaysthatresearchers at Nonetheless, researchers strive to understand “If arareprocessshowsupatanabnormally to thenatureofsubatomicprocessesandparticles,includingcharmclues quarkanditskin. meters around.The BeijingSpectrometer detector recordsthosecollisions,which offer clues around. The Beijing Electron Spectrometer detector electrons andpositronsinjectsthemintoanundergroundstoragering,left,records thatis240 those collisions, which offer 200-meter-long linearaccelerator, beneaththelong,skinnybuildingonright,accelerates At the Beijing Electron Positron Collider, the real action takes place in shielded tunnels. The CLEO experiment CLEO -c -c 17 17 or using the theory of , physicists canapply grid-world, this of simulations computer running indiscretewith timeticking clicks. forward By only ontheverticesofathree-dimensionalgrid, but in finite increments—as if the particles existed within spaceandtimeasweexperience them, or a method called lattice quantum chromodynamics, a highdegreeofprecision. with calculations these make to impossible even the world’s most powerful computers find it exceedingly complex—so complex, infact,that are meson a within play at energies lower the at on goes exactly what of calculations but of newphysics?Aflawinthe data inthespringof2008. data tions andmakeincreasinglyprecisepredictions. observed decayofD observed the and predictions those between agreement for Before they could answer these questions, funding A timelyswitch Or nothingmorethanablipinthedata? of Rochester Professor Ed Thorndike, whohas and ourgoalsare notyet fullymet,” saysUniversity the accelerator performance that we had hoped, QCD LQCD CLEO So So researchers simplify those dynamics with “ At At CLEO CLEO forshort. . Itenvisionsparticlesinteractingnot -c was very successful, but we didn’t get -c dried up. The machine stopped taking -c, researchers didfindasmall dis- QCD QCD s works well at high energies, workswellathighenergies, mesons.Was thisproof tolower-energysitua- LQCD simulations? simulations?

symmetry | volume 06 | issue 02 | may 09 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 collision. To do this, the Chinese institute retained particle mass and the value, which measures

The 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. The done there. new detector is noticeably more sensitive than The upgraded Beijing collider operates in an BES-II and when commissioning is complete, will exciting energy range called the tau/charm region. produce collisions at a rate more than 10 times Here, electron–positron collisions are energetic higher than at CLEO-c, thanks in large part to the enough to form the charm quarks that interest new storage rings. This increase in power and the CLEO-c researchers. But by changing the sensitivity should lead to a better understanding of beam energy in this region, scientists can also the disagreement previously glimpsed at CLEO-c. produce very large numbers of J/psi particles as “With limited statistics, you can find a hint that well as excited forms of J/psi called psi' and psi" maybe something is going on,” Thorndike says. (pronounced “psi prime” and “psi double prime”). “But with ten times more data you can see if it’s By observing the decays of these particles, BES just a fluctuation or if it’s a real effect and you’ve collaborators from around the world will continue found something exciting.” their explorations of many unresolved topics in physics, including searches for gluons bound Forging new partnerships into a difficult-to-observe particle called a glue- In July 2008, just a few months after CLEO-c shut ball—something predicted by QCD but not yet down, the new BES detector recorded a beam unambiguously seen. Researchers will also look for from the upgraded collider for the first time. The new physics within the decays of the J/psi, psi' Americans were ready and waiting; with encour- and psi". And, as with charm quarks, they will use agement from University of Hawaii researchers these three particles to test LQCD calculations. and BES management, the universities of Binding together this varied research is the Minnesota, Florida, and Rochester and Carnegie desire to understand the minute workings of the Mellon University had officially joined the collab- physical universe and the knowledge that, as oration earlier that year. the years pass, fewer and fewer facilities around The newcomers are now working to integrate the world will offer researchers the opportunity themselves into the collaboration. For instance, to perform these studies. the University of Minnesota repurposed a com- “What’s nice is that you can do many types of puting farm, originally built to run simulations physics at a single machine,” Cronin-Hennessy for CLEO-c, to serve as a North American data says. “We’re approaching this in different ways, hub for BES-III. Now the university will serve but we have the same final goal: to understand as a middleman, importing large chunks of data the fundamental forces of nature.” from Beijing and making it available to US Right now, the Beijing accelerator is focused researchers, who will analyze data with this same on churning out psi’ particles, and has already computing farm. recorded the world’s largest sample of psi’ data “Working remotely has always been difficult, ever produced at an electron–positron collider. but it’s getting easier,” Carnegie Mellon Professor Once researchers have tripled that data set, they Roy Briere says. “Spotty connections make trans- plan to move on to other energy ranges and ferring data from China rather difficult; instead other particles. In two to three years, the collab- of every group trying to transfer the data oration plans to begin producing the Ds mesons and run the software individually, we do it in one that will allow researchers to continue the research symmetry | volume 06 | issue 02 may 09 centralized location.” begun at CLEO-c. With the Beijing data, US scientists will dive “Thanks to the Chinese, we may be able to back into much of the research they conducted at answer some of the most compelling questions in CLEO-c, searching for new physics and testing particle physics,” Poling says. “The Chinese have LQCD. If researchers confirm the earlierCLEO -c built it, and we have come.” results that seemed to disagree with theory, theo- rists will have a lot of thinking to do; it would mean that either the LQCD calculations are flawed, or some sort of physics beyond the Standard Model

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