Two collider research teams find evidence of new particle Zc(3900)

June 18, 2013 by Bob Yirka, Phys.org report

The quark wing of the particle zoo includes (a) quark pairs called mesons, (b) quark triplets called baryons, and possibly (c) four-quark combinations that may explain the Zc(3900) observations. Credit: (c) APS/Alan Stonebraker, via Physics Viewpoint, DOI: 10.1103/Physics.6.69
(Phys.org) —Two research teams working independently at two different particle accelerators have found evidence of what appears to be a four-quark particle that has come to be called Zc(3900). Both teams are made up of a large number of researchers affiliated with institutions from around the world and both have published their findings in separate papers in the journal Physical Review Letters.

The discovery of what appears to be a new particle has come about at the two sites (the Belle and BESIII experiments in Japan and China, respectively) as a result of research into Y(4260), a particle discovered in 2005. have been intrigued by Y(4260) as it appears to be made of a charmed quark, an anti-charmed quark and an extra (in addition to the gluons holding the quarks together). In studying the decay of Y(4260)—which is found in the debris after smashing and together in their respective accelerators—both research teams noticed a spike of energy of about 3.9 gigaelectronvolts, which, as it turns out, is approximately four times the weight of a proton. That suggested evidence of a particle with four quarks, something that has never been seen before.

After much more study, both teams concluded that what their data was showing them was something both new and real. Initial indications are that Zc(3900) is indeed a previously unknown type of matter—a particle with four quarks. Combined, the research teams have found 460 examples of the Zc(3900) particle giving serious credence to their actual existence. Thus far, the particle appears to have an electric charge and at least one charm and one anti-charm quark. Both teams suspect that it also has an up and anti-down quark as well, giving it its full complement of four quarks.

Both research teams point out that there might be other explanations for their findings—Zc(3900) could simply be a pair of two-quark particles bound together, for example. Another possibility is that it's a pair of two- particles that are loosely bound, which would mean they are simply interacting for a very short time span.

More research will be conducted by both teams, and of course other physicists around the globe. Studying how Zc(3900) decays should help determine if what the two teams have found is something truly new, or if it is just already known particles behaving in a novel way.

Explore further: Belle discovers new heavy 'exotic hadrons'

More information: 1. Observation of a Charged Charmoniumlike Structure in e+e-→π+π-J/ψ at √s=4.26  GeV, Phys. Rev. Lett. 110, 252001 (2013) http://prl.aps.org/abstract/PRL/v110/i25/e252001 (free PDF)

Abstract
We study the process e+e-→π+π-J/ψ at a center-of-mass energy of 4.260 GeV using a 525  pb-1 data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be (62.9±1.9±3.7)  pb, consistent with the production of the Y(4260). We observe a structure at around 3.9  GeV/c2 in the π±J/ψ mass spectrum, which we refer to as the Zc(3900). If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the π±J/ψ invariant mass spectrum, neglecting interference, results in a mass of (3899.0±3.6±4.9)  MeV/c2 and a width of (46±10±20)  MeV. Its production ratio is measured to be R=(σ(e+e-→π±Zc(3900)∓→π+π-J/ψ)/σ(e+e-→π+π-J/ψ))=(21.5±3.3±7.5)%. In all measurements the first errors are statistical and the second are systematic.

2. Study of e+e-→π+π-J/ψ and Observation of a Charged Charmoniumlike State at Belle, Phys. Rev. Lett. 110, 252002 (2013) http://prl.aps.org/abstract/PRL/v110/i25/e252002 (free PDF)

Abstract
The cross section for e+e-→π+π-J/ψ between 3.8 and 5.5 GeV is measured with a 967  fb-1 data sample collected by the Belle detector at or near the Υ(nS) (n=1,2,…,5) resonances. The Y(4260) state is observed, and its resonance parameters are determined. In addition, an excess of π+π-J/ψ production around 4 GeV is observed. This feature can be described by a Breit-Wigner parametrization with properties that are consistent with the Y(4008) state that was previously reported by Belle. In a study of Y(4260)→π+π-J/ψ decays, a structure is observed in the M(π±J/ψ) mass spectrum with 5.2σ significance, with mass M=(3894.5±6.6±4.5)  MeV/c2 and width Γ=(63±24±26)  MeV/c2, where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmoniumlike state.

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geokstr
1 / 5 (2) Jun 18, 2013
Eventually the quark itself will be split, and then that particle will also.
vacuum-mechanics
1 / 5 (7) Jun 18, 2013
Physicists have been intrigued by Y(4260) as it appears to be made of a charmed quark, an anti-charmed quark and an extra gluon (in addition to the gluons holding the quarks together). In studying the decay of Y(4260)—which is found in the debris after smashing electrons and positrons together in their respective accelerators—both research teams noticed a spike of energy of about 3.9 gigaelectronvolts, which, as it turns out, is approximately four times the weight of a proton….

It is interesting to note that we start with a small mass of electron and positron then (after the collision) what we got is an unknown unstable type of matter which is heavier than the weight of a proton, how could it is something like that? Maybe what we 'temporary' found is not the matter, but instead it is …
http://www.vacuum...=9〈=en
RealScience
5 / 5 (3) Jun 18, 2013
...we start with a small mass of electron and positron then (after the collision) what we got is an unknown unstable type of matter which is heavier than the weight of a proton, how could it is something like that?


Vac-mech - which part of E=MC² do you not understand?
Silverhill
5 / 5 (6) Jun 19, 2013
@LarryD
e- & p+ appear to be very stable so it would be interesting to find out just what levels of 'packaged' g.r. produces stability then other levels would be unstable.
Note that e+ and p- are also very stable, at least when there is no normal matter nearby to react with.

@vacuum-mechanics
It is interesting to note that we start with a small mass of electron and positron then (after the collision) what we got is an unknown unstable type of matter which is heavier than the weight of a proton, how could it is something like that?
Note that the electrons and positrons mentioned in the article abstracts had energies in the 3.8 - 5.5 GeV range. They therefore did not have a small mass; they had a large relativistic mass, whose energy equivalent could condense into relatively high-mass, short-lived particles.

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