This appeared in Deccan Herald (SPECTRUM) on 27 sep 2016
PENTAQUARK - A NEW PARTICLE
Palahalli R Vishwanath
New results from the LHC experiment confirm last year's discovery
that quarks can combine into groups of five.
Today, the micro world is very well explained by the very
elegant Standard Model of Particle Physics. The basic tenets of this
model are conservation laws which give raise to various types of
symmetries and four types of interaction ( gravitation,
electromagnetic, nuclear and weak ) which govern the physical world.
Further all particles are divided into two types depending on its
value of what is called the Quantum mechanical spin which can be
visualized as akin to the way earth spins. The particles with half
integer values for spin are called Fermions and the rest as Bosons .
The model requires several bosons like the photon as carriers of
different type of interaction whereas fermions called quarks and
leptons are considered as building blocks of matter. Leptons
consist of the familiar electron and unfamiliar particles like
neutrinos and muons. However, well known particles like protons
and neutrons are NOT in the list of the elementary particles. This is
because they are considered as mixtures of quarks. How quarks
came to be regarded as fundamental is an integral part of history of
particle physics
The beginning of 20th century saw man take a deeper look
into atoms which had been postulated long time ago . It was found
that the atom consists of 3 particles - electron, proton and
neutron -which were all considered elementary. However, from
1930s onwards there were many more particles detected by cosmic ray
experiments in the next few decades: (a) the Positron, the anti
particle of electron and an example of symmetry in nature (b)
Medium mass ( between that of electron and proton) particles called
Pi mesons , the carrier of the nuclear forces and a seemingly
purposeless particle called the muon which could penetrate a large
amount of matter (c) Strange particles which were later categorized
as K mesons, Hyperons (also classified as baryons along with proton
and neutron) etc. But with time the accelerators came on the scene
and a plethora pf particles were detected which consisted of
hitherto unknown mesons and baryons. These were not stable like the
proton and were also short lived. With so many new particles, it was
natural to wonder whether they were all really fundamental . The
new discoveries were parodied when it was said " that each
discovery should be fined 10000 dollars !"
It was Murray Gell-mann who brought order into this world of
chaos in 1964.He seasoned that the basis of all the baryons and
mesons is a triplet which he called Quarks after a phrase in the
great Irish writer James Joyce's Finnegan’s Wake. The
revolutionary aspect was that these Quarks have non integer
charge like 1/3 and 2/3 unlike particles like proton, electron etc ;
however, they
would have spin half like other fermions. The three quarks were
called UP, DOWN and STRANGE with their antiparticles (UP BAR etc
) to conserve symmetry. Protons are made up of three ( 2 UP and 1
DOWN ) quarks while Pi plus Meson two (1 UP and 1 DOWN BAR)
quarks . While these particles need only 2 types of quarks, K
mesons , Hyperons etc need at least one strange quark . With time 3
more types of quarks were proposed - C(CHARM), B(BOTTOM )and T (TOP)
- and they have all been detected in the laboratory. Top, the
heaviest quark with about 175 times the mass of the proton was
discovered 20 years ago in Fermilab in Chicago. The standard model
has also been deemed complete after the discovery of the famous
Higgs Boson in 2013 in LHC in Geneva.
Quarks were considered as just mathematical entities for
quite some time since many experiments to detect quarks gave null
results . However scattering experiments at Stanford showed that
just like atom has nucleus as a hard constituent , proton also has
three hard constituents which could be identified with quarks (also
called Partons by Feynman) .There are also models of exotic forces
to explain why quarks cannot be isolated and thus confined to be
within baryons and mesons. Apart from electric charge, quarks also
have another charge called the Color charge which can have three
values represented by red, blue and green. These three “colors”
add up to colorless particles like protons and neutrons in
the same way that red, green, and blue light combine to create a
white glow. Thus color is not seen in the outside world . Because
of color , there are actually 36 quarks !
PENTAQUARKS
At the time of the original theory itself ,
possible mixtures of more than 3 quarks had been suggested. The
strong interaction theory also does not forbid exotic type of
particles like Tetra (4), Penta(5) and Hexa(6) quarks. While no
search till recently had given conclusive results about such
particles, last
year a group working on the CERN’s Large Hadron Collider(LHC)
claimed detection of Pentaquarks with high significance. The data
have been further analyzed and two new studies published this month
show that evidence for Pentaquarks is robust.
Since then the same experiment
has also detected few Tetraquarks, a combination of 4 quarks.
In the LHC, two protons collide at very
high energy to create various new particles and lot of normal
particle debris . Thus any new search is better done slightly away
from the debris. The present experiment showed that the collision
occasionally produces bottom quarks which travel a short distance
and then decay into a Pentaquark plus other particles all of which
are recognized and registered in a a series of detectors. The
experiment has evidence for two new Pentaquarks with masses of
about 4.4 GeV, four times that of a proton.
There are two ways to envision the
Pentaquark: it could be thought of as a baryon and a meson
(specifically, proton and a J/psi meson) 'molecule' bound together
or as a mixture of 4 quarks and an anti quark ( specifically 2
UP,1 DOWN , 1 CHARM and 1 ANTI CHARM ). It is considered colorless.
Scientists
believe that this discovery of the Pentaquark is just the tip of
the iceberg . According to them it is not just another new
particle and thus studying its properties allows one to understand
better even ordinary matter apart from complex forces between quarks
. Gell-mann , the discoverer of the
quarks ,said that " ..In the future
they may find more and more combinations.." It is also expected
to throw light on interactions at the core of neutron stars and the
possible existence of Quark Stars.
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(text has
about 1090 words)
Pl try to
include these figures, at least Fig 2
Fig 1 - The
fundamental particles as of today
Fig 2 :
Various quark mixtures
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