Z(4430): CERN Physicists Confirm Existence of Exotic Hadrons

Apr 10, 2014 by Sci-News.com

Scientists from CERN’s Large Hadron Collider beauty collaboration have confirmed the unambiguous observation of the long-sought exotic hadron Z(4430), a particle that cannot be classified within the traditional quark model.

The Large Hadron Collider, the world's largest and most powerful particle accelerator. Image credit: CERN.

The Large Hadron Collider, the world’s largest and most powerful particle accelerator. Image credit: CERN.

Hadrons are subatomic particles that can take part in the strong interaction – the force that binds protons inside the nuclei of atoms.

In the traditional quark model, hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons).

Particle physicists were searching for exotic hadrons since the 1960s. Many candidates have been proposed but up to now there has not been unambiguous proof of their existence.

Now the Large Hadron Collider beauty (LHCb) collaboration has published results of precise measurements of properties of the Z(4430), a particle that appears to be made of a charm, an anti-charm, a down and an anti up quarks.

Physicists at the BELLE experiment reported the first evidence for the Z(4430) in 2008. They found a tantalizing peak in the mass distribution of particles that result from the decays of B mesons.

They later confirmed the existence of the Z(4430) with a significance of 5.2 sigma on the scale that particle physicists use to describe the certainty of a result.

In a paper submitted to the journal Physical Review Letters (arXiv.org version), the LHCb scientists report a more detailed measurement of the Z(4430) that confirms that it is unambiguously a particle, and a long-sought exotic hadron at that.

The squared mass distribution for the 25,200 B meson decays to ψ’ π- found by LHCb in their entire data set. The black points represent the data, the red curve the result of the simulation when including the presence of the Z(4430)- state. The dashed light brown curve below shows that the simulation fails to reproduce the data if no contribution from Z(4430)- is included, establishing the clear presence of this particle with 13.9 sigma. Image credit: Large Hadron Collider beauty collaboration.

The squared mass distribution for the 25,200 B meson decays to ψ’ π- found by LHCb in their entire data set. The black points represent the data, the red curve the result of the simulation when including the presence of the Z(4430)- state. The dashed light brown curve below shows that the simulation fails to reproduce the data if no contribution from Z(4430)- is included, establishing the clear presence of this particle with 13.9 sigma. Image credit: Large Hadron Collider beauty collaboration.

They analyzed more than 25,000 decays of B mesons selected from data from 180 trillion proton-proton collisions in the Large Hadron Collider.

“The significance of the Z (4430) signal is overwhelming – at least 13.9 sigma – confirming the existence of this state,” said LHCb spokesperson Dr Pierluigi Campana.

“The LHCb analysis establishes the resonant nature of the observed structure, proving that this is really a particle, and not some special feature of the data.”

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R. Aaij et al. 2014. Observation of the resonant character of the Z(4430)− state. Physical Review Letters, submitted for publication; arXiv: 1404.1903