First predicted in the 1960s, like the Higgs boson before it, the pentaquark eluded science for decades until its recent detection at CERN’s LHCb collaboration. The discovery amounts to finding a new form of matter…
In 1964, physicists Murray Gell Mann and George Zweig independently proposed the existence of the subatomic particles known as quarks. Both postulated that the key properties of particles – baryons and mesons – were best explained if they were in turn made up of other constituent particles. Zweig coined the term “aces” for the three new hypothesised building blocks, but it was Gell-Mann’s term of “quark” that stuck.
The Standard Model of Particle Physics is a theory about the electromagnetic, weak and strong nuclear interactions, developed throughout the mid-to-late 20th century, as a worldwide collaborative effort.
Upon experimental confirmation of the existence of quarks, the theory is finalised in the mid-1970s. Ever since that time, further evidence of its validity have been provided by successive discoveries of the other predicted particles, such as the bottom quark (1977), the top quark (1995), the tau neutrino (2000) and even more recently, the Higgs boson (2012) to complete the whole set.
All in all, the Standard Model has 61 elementary particles.
It also allowed for other quark states.
One of them is the pentaquark. This purely theoretical particle was composed of four quarks and an antiquark (the anti-matter equivalent of an ordinary quark).
Theoretical. That is… Until now!
Results from LHCb Collaboration
The findings have been submitted to the journal Physical Review Letters in an article titled ‘Observation of J/ψp resonances consistent with pentaquark states in Λ0b → J /ψ K-p -decays:
Observations of exotic structures in the $ J/ψp$ channel, that we refer to as pentaquark-charmonium states, in Λ0b → J/ψ K-p$ -decays are presented. The data sample corresponds to an integrated luminosity of 3/fb acquired with the LHCb detector from 7 and 8 TeV pp collisions. An amplitude analysis is performed on the three-body final-state that reproduces the two-body mass and angular distributions.
To obtain a satisfactory fit of the structures seen in the J/ψp mass spectrum, it is necessary to include two Breit-Wigner amplitudes that each describe a resonant state. The significance of each of these resonances is more than 9 standard deviations.
One has a mass of 4380 ± 8 ±29 MeV and a width of 205 ± 18 ± 86 MeV, while the second is narrower, with a mass of 4449.8 ± 1.7 ± 2.5 MeV and a width of 39 ± 5 ± 19 MeV. The preferred Jp assignments are of opposite parity, with one state having spin 3/2 and the other 5/2.
Physicists studied the way a sub-atomic particle, called Lambda b $ \Lambda_b$ decayed – or transformed – into three other particles inside LHCb. Their analysis revealed that intermediate states were sometimes involved in the production of the three particles. After examining all possibilities for these signals, they concluded that they can only be explained by pentaquark states.
These intermediate states have been named Pc(4450)+ and Pc(4380)+.
During the mid-2000s, several teams claimed to have detected pentaquarks, but their discoveries were subsequently undermined by other experiments.
In Particle Physics, the word ‘pentaquark’ seems almost cursed because many past discoveries were then superseded by new results that showed that the previous ones were actually just fluctuations, not real signals…
Strong evidence for the pentaquark came from experiments at the Jefferson Lab in Newport News, Virginia during 2003. The experiments involved multi-GeV photons impacting a deuterium target. The evidence showed a five-quark baryon state at a mass of 1.54 GeV with a narrow width of 22 MeV.
But, previous experiments only measured the so-called mass distribution, where a statistical peak may appear against the background “noise” – the possible signature of a novel particle.
The Large Hadron Collider (LHC) powered up again in April 2015 following a two-year shutdown to complete a programme of repairs and upgrades. The LHC enabled researchers to look at the data from additional perspectives, namely the four angles defined by the different directions of travel taken by particles within LHCb.
Getting in a State…
LHCb spokesperson Guy Wilkinson commented:
“The pentaquark is not just any new particle… 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 we’re all made, is constituted.”
“There is no way that what we see could be due to something else other than the addition of a new particle that was not observed before.”