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The juices of
life
QUARKS have
captured the scientific imagination, at least since physicist Murray
Gell-Mann’s prediction about them in the early 1960s. Quarks are
tiny particles that make up the protons and neutrons usually found
in the nuclei of atoms. Well, the German word for cottage cheese is
“quark”, but though we’re all, including cheese, made up of quarks,
the name has no special connection with cheese.
Gell-Mann
adopted the name for the basic bits of matter after a line from the
novel, Finnegans Wake by James Joyce — three quarks for Muster Mark!
Interestingly, quarks were then thought to exist only in units of
three.
For a long time, scientists were puzzled as to why quarks
didn’t exist in formation of more than three. In 1997, Russian
theorists Maxim Polyakov, Dmitri Diakonov and Victor Petrov
predicted the combination of five quarks called “pentaquark”. Now
scientists have begun to find the experimental evidence of
pentaquarks and other combinations of more than three quarks. Quarks
are too small to see, even with a powerful microscope. So scientists
have to use giant atom-smashing devices such as the Continuous
Electron Beam Accelerator to study how they interact.
Some people
don’t like the idea of carrying out resource-intensive research
studies that don’t have a direct practical value to society.
Expressing his concern about too much funds going into research on
weird things like quarks, Bernard Levin, a senior science journalist
of The Times, once posed a general question to scientists: “Can you
eat quarks?” To which Cambridge scientist Sir Alan Cottrell replied
by way of a brief letter to the editor: “Sir: Mr Bernard Levin asks
‘can you eat quarks?’ I estimate that he eats
500,000,000,000,000,000,000 quarks a day.”
Scientists didn’t
bother about criticism from whistleblowers and enthusiastically
continued in their endeavour to understand the fundamental building
blocks of matter. There are hundreds of known subatomic particles
and most are composites of simpler particles. They all fit into two
categories — baryons and mesons.
Baryons represent stable
particles such as protons and neutrons. These are made up of three
quarks, which are held together by a strong nuclear force called
“glue.” Quarks are known to exist in six flavours, grouped into
pairs: up/down, charm/strange, and top/bottom.
A proton is made
up of two up quarks and one down quark. In short-hand notation, the
proton has the configuration uud. Similarly, the neutron has the
configuration udd.
Mesons comprise two quarks — a quark and an
anti-quark. The classic example of a meson is the positive pi-meson,
or just pi+, with one up quark and one anti-down quark,
configuration u d-bar. Mesons are unstable particles that vanish in
a split second.
Composed of two up quarks, two down quarks and
one anti-quark, Dmitri Diakonov estimated the mass-energy of a
pentaquark to be about 1.5 times as heavy as a proton. The
mass-energy of a proton or neutron at rest is approximately 1 Giga
Electron Volt. Thus, particle physicists in their billiard
experiments looked for a particle with mass-energy in the vicinity
of 1.5 GeV. One GeV is the amount of energy an electron gains when
it moves through a potential difference of one volt (in a vacuum).
In 2002, the first tentative evidence of the pentaquark — a
formation of five quarks — was announced at an international
scientific conference in Japan. In their landmark work carried out
at the SPring-8 accelerator in Hyogo, Japan, a team of researchers
led by Dr Takashi Nakano zapped carbon atoms with high-energy gamma
rays. As gamma ray photons “crashed” into the neutrons, a few
neutrons transformed into a five-quark particle with a mass of 1.54
GeV.
Detecting a pentaquark is difficult because its life is
extremely short. Before decaying into other particles, it lasts a
tiny fraction of a second. Irrelevant reactions or “debris” produced
during the experiment further impede the detection of any new
particles. In fact, the team found the particle in three-year-old
data, after they were told what types of energy peaks to look for by
Diakonov.
In May 2003, Valery Kubarovsky, a research scientist at
the Rensselaer Polytechnic Institute in New York, and his colleagues
announced a more convincing account of the existence of the
pentaquark at an international physics conference in New York City.
They also published the results in the 23 January 2004 issue of the
journal, Physical Review Letters.
The research was carried out at
the US department of energy’s Thomas Jefferson National Accelerator
Facility by the CLAS (CEBEF Large Acceptance Spectrometer) under
international collaboration. To limit the debris, the CLAS team
selected the simplest target. Since they could not isolate a single
neutron — stable neutrons don’t exist freely — they turned to the
single proton as the target.
Protons are available in abundance.
In fact the nucleus of hydrogen, the simplest element known so far,
is made up of one proton. In their experiment, the Jefferson lab
team liquefied hydrogen at a temperature that reached a few degrees
above absolute zero before zapping the element with gamma rays.
“Shifting our focus from neutrons to protons dramatically
altered our results,” Kubarovsky said. “We strongly increased the
previous success rates for detecting pentaquarks.” The independent
experiments in Japan, the USA and Russia detected the energy peak in
a few instances out of several billion collisions. The CLAS team
reported a detection count of about 45, which is the most
significant in the world.
Physicists in Germany have detected
the first “pentaquark” to contain a charm quark. The 6.3 km long
Hadron-Electron Ring Accelerator facility at DESY is the world’s
first and only storage ring in which two different types of matter
particles collide: protons and electrons (or their antiparticles,
the positrons).
The H1 team, an international collaboration, has
found evidence of a charmed pentaquark with a mass of 3099 MeV in
electron-proton collisions at the HERA accelerator. This work has
been submitted to Physics Letters B for publication.
Now as many
laboratories have announced the experimental evidence of the
existence of pentaquarks and even skeptics are convinced that these
particles exist. But scientists believe that there is much more to
learn about these particles. They emphasise the need to further
increase the pentaquark detection rate per particle
collision.
Scientists are also trying to learn the nature of
these newly discovered particles. For example, it is not yet clear
how the five quarks are glued together – tightly or otherwise.
Diakonov’s theory further predicts the existence of a beast particle
called the “decuplet”, made up of 10 pentaquarks. Whether he is
right remains to be seen.
Our journey in search of the origin of
matter — and the origin of the Universe itself — has taken us deeper
and deeper inside atoms. The discovery of electrons has practically
led to the gadgets that we use every day, including televisions and
computers. Larger groupings of quarks and antiquarks may have
existed in the early universe. A better understanding of the new,
exotic particles is positively going to change the way we perceive
our universe.
Raj Kaushik
(The author, a former project
coordinator with the National Council of Science Museums, India, now
works as a senior server developer in Toronto.)
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