The Wire
July 14, 2015

An artist's impression of five strongly bonded quarks in a pentaquark. Credit: CERN/LHCb Collaboration
An artist’s impression of five strongly bonded quarks in a pentaquark. Credit: CERN/LHCb Collaboration

“When you have eliminated the impossible, whatever remains, however improbable, must be the truth” – thus spake Sherlock Holmes. Particle physicists at the Large Hadron Collider today announced the discovery of a new particle after an investigation following in the steps of Holmes’ wisdom. The particle is exceedingly rare in the books of fundamental physics, called a pentaquark. It’s named for the fact that it’s composed of five quarks, indivisible particles that in their leagues make up all known matter.

However, this is the first time experimental physicists have observed five quarks coming together to make a bigger particle. They commonly manifest as protons and neutrons, which are clumps of three quarks each.

The collaboration of scientists and engineers of the LHCb detector – which spotted the pentaquarks – uploaded a paper to the arXiv preprint server on July 13 and submitted a copy to the journal Physical Review Letters for publication. The abstract describes two resonances – or unstable particles – at masses 4,380 MeV and 4,449.8 MeV (to compare, a proton weighs 938 MeV), not including uncertainties in the range 40-110 MeV. The have been temporarily designated Pc(4380)+ and Pc(4450)+.

The LHCb detector spotted the pentaquarks during the particle decays of another particle called Λb (read Lambda b). However, instead of discerning their presence by a spike in the data, the scientists spotted them by accounting for all other data points and then figuring out one consistent explanation for what was leftover. And the explanation called for conceding that the scientists had finally spotted the elusive pentaquark. “Benefitting from the large data set provided by the LHC, and the excellent precision of our detector, we have examined all possibilities for these signals, and conclude that they can only be explained by pentaquark states”, said LHCb physicist Tomasz Skwarnicki of Syracuse University in a statement.

According to the pre-print paper, the chances of the observation being a fluke, or due to some other process that could’ve mimicked the production of pentaquarks, are less than 1-in-3.5-million. As a result, the observations are sufficiently reliable and make for a discovery – even if the particle wasn’t observed as much as its unique shadow. At the same time, because the history of the experimental pursuit of pentaquarks is dotted with shepherds crying wolves, the data will be subjected to further scrutiny. In the most recent and famous case in 2003, four research labs from around the world (TJNAF, AITEP, SPring-8, ELSA) claimed to have spotted pentaquarks, only to be disproved by tests at the Istituto Nazionale di Fisica Nucleare in Genova in April 2005.

The LHC, which produces the high-energy collisions that detectors like the LHCb study in detail, shut down in early 2013 for a series of upgrades and reawakened in May 2015. The pentaquark was found in data gathered during the first run, when the LHC produced collisions at an energy of 8 TeV (1 TeV is 1,000 MeV). In the second run, the collision energy has been hiked to 13 TeV, which increases the frequency with which exotic particles like pentaquarks could be produced.

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