Latest CERN experiment confirms existence of exotic particle
11 Apr 2014 04:21 PM
Scientists at
CERN’s Large Hadron Collider (LHC) have confirmed the existence of a new
class of subatomic particles, exotic hadrons.
A new measurement performed by
the LHCb collaboration, one of the four large experiments at the LHC at CERN,
has confirmed the existence of the exotic object labelled the Z(4430)-. This
particle does not fit into the pattern of particles we have seen up to now. The
LHCb result confirms an observation made by the Belle collaboration in 2008
that was later questioned and so resolves this previously unclear
situation.
We and everything around us are
made of atoms, and atoms are made in turn of smaller constituents. Atomic
nuclei are orbited by electrons. The protons and neutrons that form atomic
nuclei consist of three fundamental particles, called quarks, bound together.
Other combinations of quarks can occur - particles containing two bound quarks
(mesons) are also seen in nature. However, until now all particles containing
quarks (hadrons) have conformed to one of these two types; quarks seem to like
to come in twos or threes.
However, the underlying theory
of quantum chromodynamics (QCD) that describes the behaviour of quarks allows
for many different quark combinations, such as four quark states, to bind
together into hadrons. Over the last forty years many searches for such exotic
states have been performed but until now there was no conclusive proof of their
existence. Several more mundane explanations for the Z(4430)- signal seen by
Belle had been put forward, but the LHCb result establishes that for the first
time we have seen the “smoking gun” signal for resonant behaviour
of a particle that contains at least four quarks/antiquarks.
The “4430” refers to
the (approximate) mass of this state, corresponding to roughly four times the
mass of a proton.
Dr. Greig Cowan, STFC Ernest
Rutherford Fellow at the University of Edinburgh and one of the lead analysts
on this project says,
“This is a fantastic
result from the LHCb collaboration. It confirms previous signs of this exotic
state and shows, for the first time, that it has has the characteristic
behaviour of a resonance. In addition we have also been able to pin down the
quantum numbers and properties of this state with higher precision than
previous experiments."
Speaking about what this means
for particle physics research Professor Tara Shears, LHCb lead for the
University of Liverpool, said
"We've always taken the
existence of two and three quark particle states for granted, but there's
no reason why more complicated versions shouldn't occur. LHCb's
observation and measurement of the Z(4430)- is going to help us explore this
feature of matter. LHCb's measurement also demonstates the experiment's
versatility - who would have thought that an experiment designed to investigate
the strange features of antimatter could also help us understand QCD and matter
better?"
END
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Notes to
editors
The work from LHCb for this
project has built on the work that the Belle Collaboration did in 2008. At that
time Belle reported evidence for an exotic structure, the Z(4430)-, that did
not fit into the normal classification scheme. This state has 1 unit of
electric charge and must contain a minimum of four quarks. Given this is a
smoking gun for an exotic meson it has always been deemed important to confirm
the Belle observations and understand whether the Z(4430)- is a real particle.
Detailed studies show that the LHCb data can only be explained by the inclusion
of the Z(4430)- and this state shows behaviour that is characteristic of a
resonance ('phase motion across the peak').
The next steps are to search for
other signs of this particle in other decays of B hadrons so that we can
further study its properties. This may give signs of this same particle,
allowing complementary ways to understand the nature of this state. This
information will be essential to help understand what the nature of this state
really is.
The theories give different
predictions for rates of decays into different channels. They also have
different predictions for sets of additional molecules or tetraquarks. This
opens an exciting (but challenging) field that LHCb will be pursuing
intensively in the coming years.
The result was presented for the
first time on April 8th 2014 at the SM@LHC conference in Madrid,
Spain.
The black points at the first
image above show the ψ’π- invariant mass squared distribution of
the data. The blue histogram shows the Z(4430) contribution. The second image
shows the so called Argand diagram proving to the experts that the Z(4430)
structure seen in the data (black points) represents really the resonant
particle production and decay, since it follows approximately a circular path
(red circle).
LHCb is an
experiment set up to explore what happened after the Big Bang that allowed
matter to survive and build the Universe we inhabit today. Located in a vast
underground cavern, 100 metres beneath the French countryside, LHCb is one of
four large experiments based at the CERN laboratory near Geneva, Switzerland.
The experiment, which involves about 700 scientists from 52 institutions around
the world, has recorded the particles produced by the first circulating LHC
proton beam on September 10th, 2008, and will run for at least 10
years.
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experimental detectors and played a central role in much of the research that
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