Serendipitous discovery (Image: CERN / LHCb Collaboration)
It鈥檚 a particle so elusive that even the world鈥檚 largest physics experiment could only discover it by accident. The pentaquark has at last been found.
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鈥淚t鈥檚 not something we鈥檝e been actively looking for,鈥� says Patrick Koppenburg, a Physics Coordinator at the Large Hadron Collider at CERN near Geneva, Switzerland. 鈥淚t鈥檚 more something we鈥檝e stumbled across.鈥�
The pentaquark represents a fundamentally new arrangement of quarks, indivisible particles that are some of the building blocks of matter.
Normally observed in sets of two or three, groupings of four or even five quarks were predicted in the 1960s. In 2003 pentaquarks seemed to pop up in a few particle accelerators, only to be declared phantoms in 2005.
Then a tetraquark 鈥� made of four quarks 鈥� was sighted, and confirmed in 2013, but the pentaquarks remained stubbornly concealed.
鈥淒efinitely a particle鈥�
鈥淧entaquarks have been reported several times in the past,鈥� says Koppenburg, 鈥渂ut they all appeared to be fakes.鈥�
So when the team at the LHCb experiment detected what looked like the signature of a pentaquark, they were determined to take their time with the analysis. Now they are pretty certain they have it. The signal has a 鈥�10 sigma鈥� stamp of accuracy, meaning that there is only a 1 in 1022 chance of it being a statistical fluctuation in the data.
鈥淭his is definitely a particle,鈥� says Koppenburg.
The pentaquark was observed by studying the decay of particles called Lambda b baryons, which normally occurs in one smooth step. However, the physics of the strong force, which governs the way quarks behave, means there are sometimes intermediate states along the way. During one such not so smooth decay, an elusive five-quark particle popped up with a signature that tallied with that of the theorised pentaquark.
鈥淭his is a decay that physicists know and love,鈥� says Guy Wilkinson, the spokesperson for the LHCb collaboration, 鈥渟o in that sense it was certainly a serendipitous discovery鈥�.
Smells like a pentaquark
Although some physicists believe that the five quarks are tightly bound into a single well-defined particle, others propose that it would be better described as a mini-molecule, consisting of a two-quark meson and a three-quark baryon held together by the strong nuclear force.
According to Wilkinson, such a state could still justifiably be called a particle. 鈥淚t鈥檚 an alternative way in which the particle can be bound,鈥� he says. 鈥淎ll the parallel studies enabled us to see that it smelled, behaved and felt like a pentaquark.鈥�
Because the particle decays almost immediately, more experiments will be needed to determine further information about its size and mass.
鈥淥ur experiment is beautifully designed for understanding these sorts of things,鈥� says Wilkinson. 鈥淲e have the perfect microscope for looking more closely. This will certainly not be the last pentaquark discovered.鈥�
Journal reference: ArXiv, arxiv.org/abs/1507.03414
