Brookhaven and the Belle II Experiment
Tracking particle smashups and detector conditions from half a world away, scientists seek answers to big physics mysteries
The Belle II experiment at Japan’s SuperKEKB particle accelerator, which started its first physics run in March 2019, takes “snapshots” of the decay products streaming from collisions of electrons and positrons 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.
The Goal of Belle II
The purpose of the upgraded Belle II experiment is to search for new physics phenomena 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—while also making precision measurements of known phenomena.
One particular area of interest is the decay of “heavy flavor“ mesons. These are particles made of two quarks, one of which is a heavy “beauty“ or “charm“ quark. The study of the decay process (transformation into other lighter particles) of heavy flavor mesons has established that matter and antimatter behave differently. The observed differences are, however, not sufficiently large to explain why today’s universe is made of matter rather than a mix of matter and antimatter. Additional sources of matter-antimatter asymmetry must exist and, if observed, could indicate that some new, previously undiscovered particles might be taking part in the action. To identify such unobserved phenomena, scientists must analyze many times more heavy flavor meson decays than have ever been produced at past electron-positron facilities. Belle II will accumulate more than 50 times the data sample of the original Belle experiment at KEK, which itself produced 760 million such events.
Physicists at Brookhaven Lab play a leading role in searches for new sources of matter-antimatter asymmetry by studying decays of charm mesons. While the existence of matter-antimatter differences in beauty mesons have been known since decades, they have been observed only recently in charm decays by the LHCb experiment at CERN, the European laboratory for particle physics research. Studies at Belle II will complement those ongoing at LHCb and will allow to understand whether the observed matter-antimatter asymmetry can be explained by the Standard Model or it is caused by new phenomena.
The Belle II experiment produces multiple peta-bytes of data each year. Brookhaven Lab’s Scientific Data and Computing Center (SDCC) hosts an entire copy of the raw data and a huge fraction of the simulation and selection data, together with an archive of the detector’s conditions at the time of each recorded collision. The SDCC also provides the computing resources necessary to reconstruct and analyze the data. The team at Brookhaven is also developing the data-distribution software for sharing Belle II data with collaborators around the world, working with Belle II colleagues and colleagues at CERN, learning from their experience managing datasets from the Large Hadron Collider as well as Brookhaven’s own experience at the RHIC & ATLAS Computing Center.
To keep collision rates high, the electron and position beams that circulate inside SuperKEKB 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.