An experiment of the Large Hadron Collider reports observation of particles composed by five quarks

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. Image: CERN / Collaboration LHCb
Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. Image: CERN / Collaboration LHCb
Research
(15/07/2015)

The LHCb experiment of the Large Hadron Collider (LHC), located at the headquarters of the European Organization for Nuclear Research (CERN), in Switzerland, has reported the discovery of a class of particles known as pentaquarks. The international collaboration of the experimente, composed by researchers from the University of Barcelona (UB), the Ramon Llull University, the University of Santiago de Compostela (USC) and the Instituto de Física Corpuscular (IFIC, CSIC-UV), has submitted a paper reporting these findings to the journal Physical Review Letters, which has published it on the digital repository arXiv.

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. Image: CERN / Collaboration LHCb
Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. Image: CERN / Collaboration LHCb
Research
15/07/2015

The LHCb experiment of the Large Hadron Collider (LHC), located at the headquarters of the European Organization for Nuclear Research (CERN), in Switzerland, has reported the discovery of a class of particles known as pentaquarks. The international collaboration of the experimente, composed by researchers from the University of Barcelona (UB), the Ramon Llull University, the University of Santiago de Compostela (USC) and the Instituto de Física Corpuscular (IFIC, CSIC-UV), has submitted a paper reporting these findings to the journal Physical Review Letters, which has published it on the digital repository arXiv.

“The pentaquark is not just a new particle”, said LHCb spokesperson Guy Wilkinson. “It represents a way to aggregate quarks, namely the fundamental constituents of ordinary protons and neutrons, in a pattern that has never been observed before in over fifty years of experimental searches. Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which weʼre all made, is constituted”.

Our understanding of the structure of matter was revolutionized in 1964 when North-American physicist Murray Gell-Mann, proposed that a category of particles known as baryons, which includes protons and neutrons, are comprised of three fractionally charged objects called quarks, and that another category, mesons, are formed of quark-antiquark pairs. Gell-Mann was awarded the Nobel Prize in Physics for this work in 1969.

This quark model also allows the existence of other quark composite states, such as pentaquarks composed of four quarks and an antiquark. Until now, however, no conclusive evidence for pentaquarks had been seen.

Juan Saborido, head of the USC research group that participates in the LHCb, explains that "the quark model suggested more than fifty years ago does not exclude the existence of particles composed by more than three quarks. However, exotic hadrons began to show signs of their existence few years ago. The researcher considers that the discovery of these new particles composed by five quarks "does not refuse the standar model, but constitutes a very important finding to understand hadron structure".

Eugeni Graugés, member of the UB research team that collaborates in the experiment LHCb, affirms: "This result is important to validate quantum chromodynamics (QCD), a theory that proves the existence of five-quark bound states". And he explains: "Like if a meson (two quarks) and a baryon (three quarks) would make a bound state. Or like molecules composed by different atoms".

Fernando Martínez Vidal, IFIC researcher, highlights: "Although we know that some particles are composed by two or three quarks since 1964, there is anything in the nature that forces interactions to be like that. Therefore, we have been carrying out experiments to find particles that present other kind of quark aggregates. This finding is a reward for the effort made. 

LHCb researchers looked for pentaquark states by examining the decay of a baryon known as Lambda b into three other particles, a J-psi meson, a proton and a charged kaon. The study of the spectrum of masses of the two first particles revealed that intermediate states were sometimes involved in their production. These have been named Pc(4450)+ and Pc(4380)+, the former being clearly visible as a peak in the data, with the latter being required to describe the data fully.

“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”, affirms LHCb physicist Tomasz Skwarnicki of Syracuse University. "More precisely the states must be formed of two up quarks, one down quark, one charm quark and one anti-charm quark", points out the expert.

Earlier experiments that have searched for pentaquarks have proved inconclusive. Where the LHCb experiment differs is that it has been able to look for pentaquarks from many perspectives, with all pointing to the same conclusion. Itʼs as if the previous searches were looking for silhouettes in the dark, whereas LHCb conducted the search with the lights on, and from all angles. The next step in the analysis will be to study how the quarks are bound together within the pentaquarks. "The quarks could be tightly bound said LHCb physicist Liming Zhang of Tsinghua University (China)or they could be loosely bound in a sort of meson-baryon molecule, in which the meson and baryon feel a residual strong force similar to the one binding protons and neutrons to form nuclei”.

More studies will be needed to distinguish between these possibilities, and to see what else pentaquarks can teach us. The new data that LHCb will collect in LHC run 2 will allow progress to be made on these questions.