Bottom quarks misbehave in LHC experiment

Data suggest need to revise calculations or reexamine understanding of protons

illustration of LHCb proton collision

QUARK QUIRK  New data from the Large Hadron Collider’s higher-energy proton collisions show that particles made of b quarks flew off at angles more often than expected. Scientists from the LHCb experiment, which observes the aftermath of proton collisions in its detector (visualization shown), reported the result August 4.


CHICAGO — Theoretical physicists are scratching their heads after scientists presented surprising new studies of a particle known as the bottom quark.

At the new, higher energies recently reached at the Large Hadron Collider particle accelerator, particles containing bottom quarks flew off at an angle more often than expected. Scientists reported the result August 4 at the International Conference on High Energy Physics.

Quarks make up larger particles like the proton and neutron. At the LHC, near Geneva, scientists smash together protons to produce new particles, including bottom quarks.

Those bottom quarks are bound together with other quarks into larger particles known as b hadrons. Scientists with LHCb, an experiment at the LHC, found an unexpected behavior in b hadrons that sped off at an angle from beams of colliding protons, rather than continuing on a nearly parallel trajectory. At high energies, the number of b hadrons flying off at an angle, relative to those at lower energies, was almost twice as large as expected.

The discrepancy could point to a problem with scientists’ predictions of how the particles should behave. Such predictions are based on the theory of how quarks interact, known as quantum chromodynamics, or QCD, which is important for grasping the inner workings of protons and neutrons. “Understanding QCD really sets the basis of our understanding of nature,” says LHCb member Marina Artuso of Syracuse University in New York.

Scientists who make predictions for how b hadrons should behave have had trouble explaining the discrepancy. “Whichever way you turn it, it’s really weird. Which to me, personally, makes it extremely exciting,” says theoretical physicist Michelangelo Mangano of CERN, the European particle physics lab that operates the LHC.

But, he cautioned, it’s unlikely to be an indication of phenomena that would upend the standard model of particle physics. Rather, it may be that calculations need further refinement, or that scientists need to tweak their understanding of the proton, by altering estimates of the momentum carried by the various particles found inside it.

The issue could also lie with LHCb’s measurement, but the scientists say they are very confident in their result. The team continues to study the data to better characterize the effect.

Emily Conover

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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