An international team of scientists that operate the BESIII
Experiment at the Beijing Electron Positron Collider in China recently began a
series of specialized studies aiming at an understanding of the anomalous
"Y(4260)" particle. As a
striking and unexpected first observation from these new studies, the
collaboration has reported that the Y(4260) particle in fact decays to a new,
and perhaps even more mysterious, particle that they named the
"Z_c(3900)."
Since its 2005 discovery by the BaBar experiment at the SLAC
National Laboratory in Stanford California, the Y(4260) particle has continued
to mystify researchers. While other particles that share certain similarities to
the Y(4260) have long been successfully explained as examples of a charmed
quark and anti-charmed quark paired together by the strong force of particle
physics, attempts to incorporate the Y(4260) into this model have failed, and
its underlying nature remains unknown.
In late December of 2012, the BESIII team embarked on a
program of research to produce large numbers of Y(4260) particles by
annihilating electrons and anti-electrons (positrons) with a total energy that corresponds
to the mass of the Y(4260). Once produced, the Y(4260) quicklydecays, and its
decay products are measured with the BESIII particle detector. According to
Prof. Xiaoyan Shen, the spokesperson of
the BESIII experiment, from the Institute of High Energy Physics, Chinese
Academy of Sciences, "the goal of our program is to understand the various
processes by which the Y(4260) decays with the hope that this will provide
clues about its internal structure, and thereby yield new insights into the workings
of the strong force, which is responsible for holding quarks together inside
subatomic particles."
While commonly known subatomic particles, such as the proton
and the neutron, are comprised of the relatively lightweight up- and down-
quarks, the BESIII Experiment is specialized for the study of matter that
contains the heavier charmed quarks. The J/psi particle, for example, which is known
to be composed of a charmed quark and an anti-charmed quark bound together by
the strong force, can be copiously produced at the collider in Beijing.
"To date, BESIII has directly produced more than a billion J/psi particles
in these electron-positron annihilations," according to Prof.Fred Harris
from the University of Hawaii, the co-spokesperson of the BESIII experiment.
The J/psi particle forms the cornerstone of what has been thought to be a well
understood system of various possible configurations of charmed and
anti-charmed quarks, called the "charmonium" mesons, that are the
simplest and considered to be among the most easily understood subatomic
particles. But the recent discoveries of several new particles, including the
Y(4260) and now the Z_c(3900), have cast doubt on these optimistic assessments
and suggest that more complex structures have to be considered.
Previous studies of the Y(4260) used electron and positron
beams with a total energy that was well above that which corresponds to the
mass of the Y(4260). In these
experiments, the Y(4260) mesons were produced via the relatively rare process
in which either the original electron or positron beam particle first radiated
a high energy gamma-ray, thereby lowering the total annihilation energy to the
Y(4260) mass region. When the electronsand
positrons collided with an energy corresponding to the Y(4260) mass, the
Y(4260) could be formed, and this, in fact, led to its initial discovery.
"In contrast, the BESIII Experiment exploits the unique
energy range that is accessible at the Beijing Electron Positron Collider to
produce the Y(4260) directly and more efficiently by tuning its beam energies
to exactly match the Y(4260)'s mass",
Prof. Yangheng Zheng, from the University of Chinese Academy of Sciences
and a co-spokesperson of the BESIII experiment, said. In the first two weeks of
this program, BESIII had already collected the world's largest sample of
Y(4260) decays. Then, by the end of the first month, evidence pointing to the
existence of the Z_c(3900) was already very strong.
The anomalous particles of charmonium, such as the Y(4260)
and, now, theZ_c(3900), appear to be members of a new class of recently
discovered subatomic particles, called the XYZ mesons, that are adding new
dimensions to the study of the strong forces that quarks and antiquarks exert
on each other. In the most widely
accepted theory of these forces, Quantum Chromodynamics (QCD), there are in
fact more possibilities for charmonium mesons than simply a charmed quark bound
to an anti-charmed quark. Some theories
predict that gluons, the particles that mediate the forces between quarks, may
themselves exist inside mesons in an excited state, a configuration referred to
as "hybrid charmonium."
Another proposed possibility is that more than just a charmed and
anti-charmed quark may be bound together to form "tetraquark" or
molecule-like mesons.
In principle, QCD could be used to determine the properties
of these more exotic configurations. The
problem is that when QCD is applied to situations like these, the equations
that ensue are impossible to solve, at least not by normal techniques. Some progress has been made recently using
numerical methods with very high-powered computers to solve the applicable QCD
equations, and there are indications that these methods, referred to as
"lattice QCD," may ultimately be able to account for the existence of
the Y(4260) as a state of hybrid charmonium.
However, the hybrid picture cannot explain the newly
discovered Z_c(3900), which decays into an electrically charged pi-meson plus a
neutral J/psi and, thus, must itself carry an electric charge. Since it decays to a J/psi, the Z_c(3900),
which has a mass slightly higher than that of a helium atom, must contain a
charmed quark and an anti-charmed quark.
But this configuration is electrically neutral. Adding a gluon to form a hybrid does not
help, because gluons are also electrically neutral. In order to have a non-zero electric charge,
the Z_c(3900) must also contain lighter quarks.
Different theoretical models have been proposed that attempt to explain
how this could come about. The
positively charged Z_c(3900) particle could be a tightly bound four quark
composite of a charmed and anti-charmed quark plus an additional up quark and
anti-down quark. Or, perhaps, the Z_c(3900) is a molecule-like structure
comprised of two mesons, each of which contain a charmed quark (or anti-charmed
quark) bound to a lighter anti-quark (or quark). Another scenario is that the
Z_c(3900) is an artifact of the interaction between these two mesons. In any case, the appearance of such an exotic
state in the decay of another exotic state was unanticipated by most
researchers.
At present, the ball is clearly in the experimenters' court
and there is much hope, by theorists and experimenters alike, that with moreexperimental
data, the veil that continues to shroud these mysterious particles can be
lifted. "We are very excited about
this", commented Yifang Wang, Director of the Institute of High Energy
Physics at Beijing."With our Beijing collider, we can accumulate a lot
more data that will permit more comprehensive investigations of the nature of
this unusual, electrically charged charmonium state. When all of these results are usedas inputs
to theory, we may begin to open the door toward a fuller understanding of the
XYZ particles discovered in recent years."
-The Beijing Spectrometer (BESIII) experiment at the Beijing
Electron Positron Collider is composed of about 350 physicists from 50
institutions in 11 countries.
-The discovery was reported in arXiv :1303.5949 (http://xxx.lanl.gov/abs/1303.5949)
and was submitted to Physical Review Letters.
Contact:
BESIII Spokesperson: Prof. SHEN Xiaoyan
Email: shenxy@ihep.ac.cn
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