Protons can be intrinsically charming.
Subatomic particles are a mixture of three lighter particles called quarks: two of the type known as up quarks and one down quark. But physicists have speculated for decades that protons may also host more massive quarks, called “intrinsic” charm quarks. A new analysis support that ideaphysicists report on August 18 Nature.
Charm quarks are much heavier than up or down quarks. So heavy that, surprisingly, “you can have a component of the proton that is heavier than the proton itself,” says theoretical physicist Juan Rojo of the Vrije Universiteit Amsterdam.
Red and his colleagues combined a variety of experimental results and theoretical calculations in hopes of revealing the hypothetical charm of the proton. Measuring this feature is key to fully understanding one of the most important particles in the universe, says Rojo.
Physicists know that the deeper you probe a proton, the more complicated it appears. When observed at very high energies, as in collisions at particle accelerators like the Large Hadron Collider, or LHC, near Geneva, protons contain a motley crew of transient quarks and their antimatter equivalents, the antiquarks (Serial number: 04/18/17). These “extrinsic” quarks are created when gluons, particles that help “glue” quarks into protons, split into quark-antiquark pairs.
The extrinsic quarks are not fundamental to the identity of the proton. They are simply the result of how gluons behave at high energies. But charm quarks could exist inside protons even at low energies, in a more persistent and deeper form.
In quantum physics, particles do not acquire a definite state until they are measured; instead, they are described by probabilities. If protons contain an intrinsic charm, there would be a small probability of finding within a proton not only two up quarks and one down quark, but also a charm quark and an antiquark. Since protons are not well-defined collections of individual particles, a proton mass is not a simple sum of its parts (Serial number: 11/26/18). The small probability means that the entire mass of the charm quark and antiquark does not add up to the weight of the proton, which explains how the proton can contain particles heavier than itself.
Using thousands of measurements from experiments at the LHC and other particle accelerators, combined with theoretical calculations, the team found evidence for the intrinsic charm of the proton at a statistical level called 3 sigma. Intrinsic charm quarks carry about 0.6 percent of a proton’s momentum, the researchers report.
But normally 5 sigma is required to get a conclusive result. “The data and analysis are not yet sufficient … to go from ‘evidence for’ to ‘discovery’ of intrinsic charm,” says Ramona Vogt, a theoretical physicist at Lawrence Livermore National Laboratory in California, who wrote a perspective article about the study for Nature.
Furthermore, defining what “intrinsic charm” means is not straightforward, which confounds comparison of the new finding with previous results from different groups. “Previous studies have found different limits on intrinsic charm in part because they have used different definitions and schemes,” says theoretical physicist Wally Melnitchouk of the Jefferson Lab in Newport News, Virginia.
In particular, the new analysis incorporates the results of the LHCb collaboration, which potentially consistent reported measurements with intrinsic charm on the proton on February 25 Physical Review Letters. Including such data in the analysis is “what’s really new,” says theoretical physicist C.-P. Yuan from Michigan State University in East Lansing. But Yuan has reservations about the type of calculation used to interpret the data. “It’s not done in what we now call cutting-edge analysis.”
Scientists need to pin down the intrinsic charm content of the proton to better understand the results at the LHC and other facilities that break up protons and observe what comes out. Investigators must be able to gauge the ins and outs of the objects they collide with.
Data from future accelerators, like the planned Electron-Ion Collider, could help, says theoretical physicist Tim Hobbs of Fermilab in Batavia, Illinois. For now, the proton remains a mystery. “The problem is still ours; It’s still very challenging.”