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Pentaquark are particles made out of 4 quarks and 1 antiquark, unlike mesons (quark+antiquark) and baryons (3 quarks). This is by far not the first claim of a pentaquark discovery, but I think it is the most convincing one so far. We'll see which alternative interpretations will come up in the next weeks.
If confirmed, it is a completely new class of hadrons.
Some experimental background:
LHCb looked at the decays of ##\Lambda_b^0 \to J/\psi p K^-##. Those four particles are well-known and the mother particle lives long enough to see its decay length: the three daughter particles come from a common point (the decay vertex) that is well separated from the primary interaction point. That allows to reduce the background significantly. LHCb then asked "could this be a decay chain? ##\Lambda_b^0 \to X K^-##, ##X \to J/\psi p## with a new particle X"? This would give a peak in the invariant mass distribution of the ##J/\psi p## pair. And indeed they found such a peak.
Not every peak in such a distribution is a particle, which is a serious issue for all discovery claims. There are many effects in a three-body decay that have to be considered because they can lead to bumps in the spectrum.
I see three good arguments why this peak should be a pentaquark (or something even more exotic):
The arguments are weaker for the second particle which is wider and does not have a clear Argand diagram.
LHCb will certainly try to measure more decay modes, or to find other similar particles.
This discovery is related to the measurement of Z(4430)+, a charged state decaying to ##\Psi' \pi^-##.
References:
LHCb press release
arXiv preprint
Abstract of preprint:
If confirmed, it is a completely new class of hadrons.
Some experimental background:
LHCb looked at the decays of ##\Lambda_b^0 \to J/\psi p K^-##. Those four particles are well-known and the mother particle lives long enough to see its decay length: the three daughter particles come from a common point (the decay vertex) that is well separated from the primary interaction point. That allows to reduce the background significantly. LHCb then asked "could this be a decay chain? ##\Lambda_b^0 \to X K^-##, ##X \to J/\psi p## with a new particle X"? This would give a peak in the invariant mass distribution of the ##J/\psi p## pair. And indeed they found such a peak.
Not every peak in such a distribution is a particle, which is a serious issue for all discovery claims. There are many effects in a three-body decay that have to be considered because they can lead to bumps in the spectrum.
I see three good arguments why this peak should be a pentaquark (or something even more exotic):
- the peak is quite narrow (first plot in first reference). You can see several other particles contributing to the spectrum in the invariant mass distribution plot, but they are all wide, and even with interference effects it is unlikely that they could give such a narrow peak.
- the Argand diagram (second plot in first reference). It is a plot of the complex amplitude of the unknown contribution as function of the mass. You expect a circle for a particle, and indeed the points are on a circle. There is no reason why it should be circular without a new particle.
- the quark content: the particle is too light to contain a b-quark, but it needs a charm and anticharm quark to produce a ##J/\psi## in the decay. It also needs three (valence) quarks more than antiquarks to be baryon-like. Both together requires at least five quarks.
The arguments are weaker for the second particle which is wider and does not have a clear Argand diagram.
LHCb will certainly try to measure more decay modes, or to find other similar particles.
This discovery is related to the measurement of Z(4430)+, a charged state decaying to ##\Psi' \pi^-##.
References:
LHCb press release
arXiv preprint
Abstract of preprint:
Observations of exotic structures in the J/ψp channel, that we refer to as pentaquark-charmonium states, in ##\Lambda_b^0 \to J/\psi p K^-## 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.