Physics

Existence of Exotic Hadron Confirmed

April 9, 2014 | by Janet Fang

Photo credit: The LHCb experiment's 4,500-metric-ton detector is located in a cavern 100 meters beneath the French countryside / LHCb
 
A CERN collaboration has just announced results that confirm the existence of exotic hadrons -- a type of subatomic particle that can’t be classified using the traditional quark model. This very exotic state looks strangely like a particle made up of four quarks. 
 
Let’s step back a bit. Quarks are one of the subatomic particles that make up protons and neutrons; they’re classified in 6 ways (up, down, strange, charm, bottom, or top), each with a corresponding antiparticle. Hadrons are made up of bound quarks and antiquarks, and they take part in the strong forces that bind protons inside the nuclei of atoms. Mesons are hadrons formed from quark-antiquark pairs; the rest are baryons, which are made of three quarks. 
 
But ever since it was proposed in the 1960s, physicists have found several particles that just don’t fit within this hadron structure scheme. In 2003, for example, physicists found something that looked like a bound state of four quarks. 
 
Now, an international collaboration has published the “unambiguous observation” of an exotic particle called Z(4430). The announcement comes from the Large Hadron Collider beauty (LHCb) experiment, which explores what happened after the Big Bang by studying “beauty" (b) quarks. Beauty quarks are absent from the universe now, but were common right after the Big Bang. Hadrons containing these unstable and short-lived b and anti-b quarks are collectively known as B mesons.
 
The first evidence of Z(4430) was reported in 2008, when the Belle Collaboration found a peak in the mass distribution of particles that result from the decays of B mesons. The Belle team later confirmed it with a significance of 5.2 sigma on the scale that particle physicists use to describe how certain they are of a result (basically, you get to declare a discovery with 5 sigma, but anything less than that doesn’t have a high enough level of certainty associated with it).  
 
Now, LHCb has an even more detailed measurement of Z(4430) that confirms it’s a particle, the long-sought exotic hadron -- and not just some special feature of the data. The “Z” shows that it belongs to a strange series of particles that are referred to as XYZ states. The particle gives its mass at 4430 MeV, roughly four times heavier than a proton, Quantum Diaries explains, and it has a negative electric charge. It appears to be made of a charm, an anti-charm, a down, and an anti-up quark.
 
The data sample was large: They analyzed 25,000 decays of B mesons that were selected from data from 180 trillion proton-proton collisions in the LHC. The significance of the Z(4430) signal is at least 13.9 sigma, and scientists are currently working on a new model to describe the newly confirmed state. 
 
 
The results were announced in the LHCb publication. 
 
 
Image: detector and cavern / LHCb experiment
 
 

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