Tracking particle smashups and detector conditions from half a world away, scientists seek answers to big physics mysteries.
If you think keeping track of the photos on your mobile phone is a challenge, imagine how daunting the job would be if your camera were taking thousands of photos every second. That’s the task faced by particle physicists working on the Belle II experiment at Japan’s SuperKEKB particle accelerator, which started its first physics run in late March. Belle II physicists will sift through “snapshots” of millions of subatomic smashups per day—as well as data on the conditions of the “camera” at the time of each collision—to seek answers to some of the biggest questions in physics.
A key part of the experiment is taking place half a world away, using computing resources and expertise at the U.S. Department of Energy’s Brookhaven National Laboratory, the lead laboratory for U.S. collaborators on Belle II.
“We store an entire copy of the Belle II data, and we have the computing resources to process that data and make it available to collaborators around the world,” said Benedikt Hegner, a physicist in Brookhaven Lab’s Computational Sciences Initiative. To date, Brookhaven’s Scientific Data and Computing Center (SDCC) has handled up to 95 percent of the experiment’s entire computing workload—reconstructing particles from simulated events prior to the experiment’s startup, and since late March, from live collision events. SDCC will continue that role for the experiment’s first three years, thereafter maintaining some 30 percent of the data-transfer and storage responsibility while transitioning the rest to other Belle II member nations that have powerful GRID computing capabilities.
“We are developing the data-distribution software, working not only with Belle II colleagues but also with colleagues at CERN, the European laboratory for particle physics research, learning from their experience managing datasets from the Large Hadron Collider (LHC)—as well as our own experience at the RHIC & ATLAS Computing Center,” Hegner said.
Brookhaven also hosts Belle II’s “conditions database”—an archive of the detector’s conditions at the time of each recorded collision. This database tracks millions of variables—for example, the detector’s level of electronic noise, millimeter-scale movements of the detector due to the strong magnetic field, and variations in electronic response due to small temperature changes—all of which need to be properly taken into account to make sense of Belle II’s measurements.
“This is the first time a particle physics experiment’s conditions database is being hosted at a distant location,” Hegner noted. Tracking the conditions helps calibrate the detector and even feeds input to the “trigger” systems that decide which collisions to record. “If we’re having trouble with our system, Belle II will eventually see that during data collection. So, the reliability of our services is essential,” Hegner said.
But Brookhaven’s involvement in Belle II goes beyond cataloging collisions and crunching the numbers. Physicists and engineers in the Laboratory’s Superconducting Magnet Division made contributions essential to upgrading the KEK accelerator, helping to transform it into SuperKEKB, and members of Brookhaven Lab’s physics department are looking forward to analyzing Belle II data and being part of the upgraded facility’s discoveries.
Improved magnets, more collisions, “new physics”?
Like its predecessor, SuperKEKB collides electrons with their antimatter counterparts, known as positrons. To keep collision rates high, these beams must be tightly focused. But the magnetic fields guiding the particles in one beam can have unwanted effects in the adjacent beam, causing the particles to spread. To fine-tune the fields of the accelerator magnets and counteract these adjacent-beam effects, Brookhaven’s magnet division constructed 43 custom-designed corrector magnets. These corrector magnets are installed on each side of the Belle II detector, making adjustments to both the incoming and outgoing beams to maintain high beam intensity, or “luminosity.” High luminosity results in higher collision rates, so physicists at Brookhaven and around the world will have more data to analyze.
“Belle II will accumulate more than 50 times the data sample of the original Belle experiment at KEK,” said Brookhaven physicist David Jaffe, who is coordinating Brookhaven Lab scientists’ involvement in the project.
By scouring reconstructed images of the particles emerging from these electron-positron collisions, physicists will search for signs of “new physics”—something that cannot be explained by the particles and forces already included in the Standard Model, the world’s reigning (and well-tested) theory of particle physics.
One particular area of interest is the decay of beauty and charm mesons—particles made of two quarks, one of which is a heavy “beauty” or “charm” quark. These “heavy flavor” mesons are created in abundance in electron-positron collisions at the SuperKEKB accelerator.
“SuperKEKB is called a ‘B factory’ because it is optimized for the production of beauty mesons. It also produces an abundance of charm mesons,” Jaffe said. “While many physicists on Belle II will be investigating the behavior of beauty mesons, the Brookhaven team will be exploiting the huge sample of charm mesons to look for possible discoveries.”
For example, if heavy flavor mesons measured by Belle II decay (transform into other particles) differently than predicted by the Standard Model, such a discrepancy would be an indication that some new, previously undiscovered particle might be taking part in the action.
Evidence of new particles might help account for the mysterious dark matter that makes up some 27 percent of the universe, or offer clues about dark energy, which accounts for another 68 percent (with the remaining 5 percent made of the ordinary matter we see around us). Such a discovery might also help explain why today’s universe is made of matter rather than a mix of matter and antimatter, even though scientists believe both were created in equal amounts at the very beginning of time.
To grasp how shocking this matter-antimatter asymmetry is, think of the common laundry experience of losing a random sock in the dryer. But imagine if every time you did the laundry—even a billion loads, each with a billion pairs of socks labeled “left” and “right”—you always ended up with a single unpaired left sock and never a lone right sock. That’s what it’s like for physicists trying to understand why the universe ended up with only matter. There must be some difference in the way matter and antimatter behave to explain this anomaly.
There is evidence that matter and antimatter behave differently from several well-known experiments studying meson decays. These include a Nobel Prize-winning experiment at Brookhaven’s Alternating Gradient Synchrotron, which studied the decay of mesons containing a strange quark in the 1960s. More recently, several experiments studying beauty meson decays at other B factories—the original Belle at KEK, the BaBar experiment at the SLAC National Accelerator Laboratory in the U.S., and the LHCb experiment at CERN—observed similar asymmetries. But thus far, the matter-antimatter asymmetry observed in beauty and strange mesons follows the pattern predicted by the Standard Model, and is not sufficient to explain the matter-antimatter asymmetry of the universe.
LHCb also recently observed a smaller level of matter-antimatter asymmetry in charm meson decays for the first time. It is unclear if this new observation is consistent with the Standard Model or due to new particles that preferentially interact with charm quarks. Additional measurements are needed to solve this mystery.
“What we’ll do at Belle II is like many, many trips to the laundromat where we carefully launder our `charmed’ socks and use different methods to dry them. We’ll use our observations from these different loads of charmed laundry to map out what happens in charm meson decays to higher precision than ever before,” explained Jaffe. “Then we’ll compare those observations to our expectations from the Standard Model to see if we’ve found evidence for new particles.”
The Belle II experiment, Jaffe noted, complements LHCb. “Belle II has a different range of features that enable contrasting studies of the charm mesons,” he said. “We are starting to accumulate large data samples to help us make the precision measurements we need to resolve these questions. Once we’ve confirmed the technical capabilities of the experiment, we will move on to data analysis and the possibility of discovery.”