Physicists on CERN’s LHCb collaboration say they’ve observed three new exotic particles – X(4274), X(4500) and X(4700) – and also confirmed the existence of a fourth one, X(4140). According to the scientists, each of these particles contains two quarks and two antiquarks.
View of the LHCb detector. Image credit: CERN.
The quark model, proposed in 1964 by Murray Gell-Mann and George Zweig, is the most valid classification scheme of hadrons that has been found so far.
In this model, hadrons are classified according to their quark content.
However, it has been for a long-held mystery that all observed hadrons were formed either by a pair of quark-antiquark or by three quarks only.
But, in the last decade physicists have found evidence of the existence of particles formed by more than three quarks.
For example, in 2009 physicists working on the Collider Detector at Fermilab (CDF) experimental collaboration found one of these – the X(4140) tetraquark particle.
This result was then confirmed later by a new CDF analysis, and by CERN’s CMS collaboration and Fermilab’s DØ experiment.
Nevertheless, until now, the X(4140) quantum numbers were not fully determined, and this ambiguity exposed the theoretical explanation to uncertainty.
The LHCb (standing for ‘Large Hadron Collider beauty’) physicists has determined the X(4140) quantum numbers with high precision.
This result has a large impact on the possible theoretical interpretations, and indeed it excludes some of the previously proposed theories on its nature.
The observation of the X(4274), X(4500) and X(4700) tetraquark particles has been announced for the first time.
This image shows the data (black dots) of the mass distribution resulting from the association of the J/ψ and φ mesons, where the contribution of the X(4140), X(4274), X(4500) and X(4700) exotic particles is put into evidence by the four peaking structures at the bottom. Image credit: CERN.
Even though X(4140), X(4274), X(4500) and X(4700) contain the same quark composition, they each have a unique internal structure, mass and their own sets of quantum numbers.
The interest in these four states is also that they are the only known exotic candidates which do not contain u and d quarks, which are the lightest quarks and those which human beings and the matter around us are made of. As such, they may be more tightly bound than other exotic particles.
The results are based on a detailed analysis of the decay of a B+ meson into mesons called J/ψ, φ and K+, where the new particles appear as intermediate ones decaying to a pair of J/ψ and φ mesons.
To perform this research, the LHCb scientists used the full set of data collected during the first LHC run, from 2010 to 2012. The large signal yield efficiently collected with the LHCb detector has allowed the scientists to discover those three new particles that were peaking out from the data.
“The LHCb analysis yields a clear observation of the X(4140), and indicates a particle with similar mass but larger width to the earlier measurements from CDF, CMS and DØ,” the scientists said.
“It is important to emphasize that simple ‘bump-hunting’ in the mass spectra is not sufficient to learn about the nature of such complicated hadronic structures. Rather, a multidimensional full amplitude analysis is crucial for data interpretation, and has allowed LHCb to characterize fully the particles, and to determine their quantum numbers,” they explained.
The researchers discussed their results in a pair of papers (paper1 & paper2) submitted to the journal Physical Review Letters and the journal Physical Review D.
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R. Aaij et al (LHCb collaboration). 2016. Observation of J/ψφ structures consistent with exotic states from amplitude analysis of B+→J/ψφK+ decays. Physical Review Letters, submitted for publication; arXiv: 1606.07895
R. Aaij et al (LHCb collaboration). 2016. Amplitude analysis of B+→J/ψφK+ decays. Physical Review D, submitted for publication; arXiv: 1606.07898
