Download What are we are made of?

What are we are made of?
Vinay B Kamble
(Former Adviser, DST & Former Director, Vigyan Prasar, New Delhi)
Email: [email protected]
Just a few particles!
Today we know that molecules are made of atoms. Atoms are made of particles
called protons, neutrons, and electrons. Protons and neutrons are made of even smaller
particles called quarks and gluons. How about quarks? They are regarded as
fundamental particles; they do not show any substructure, and hence, they cannot be
further broken up into still smaller particles. Electrons and neutrinos are regarded as
fundamental particles too. Thus, electrons, neutrinos, and quarks are indivisible.
Protons and neutrons, and hence the atomic nucleus consists of two kinds of
quarks, up quarks and down quarks. We thus need three elementary particles for all
matter to exist: electrons, up quarks and down quarks. But during the 1950s and 1960s,
new particles were unexpectedly observed in both cosmic radiation and at newly
constructed accelerators, the new siblings of electrons and quarks.
In addition to the matter particles, there are also force particles for each of
nature’s four forces - gravitation, electromagnetism, the weak force and the strong
force. Gravitation and electromagnetism are the most well-known. The strong force acts
upon quarks and holds protons and neutrons together in the nucleus, whereas the weak
force is responsible for radioactive decay, which is necessary, for instance, for nuclear
processes inside the Sun.
Over the years, the physicists have worked out a mathematical model - The
Standard Model - that describes the known fundamental particles that make up the
matter we are familiar with (classified as fermions after Enrico Fermi who proposed
them); and the particles that transmit the forces (classified as bosons after Satyendra
Nath Bose who proposed them (see Figure 1). As it turns out, there are only 17 particles
in the Standard Model. Of these, 6 are fermions such as quarks that make up neutrons
and protons in nuclei; and 6 fermions called leptons (“light” particles), such as electrons
that go around these nuclei. Quarks and leptons are the particles that make up the
matter. Four particles are called gauge bosons. These are the particles that transmit
forces and thus allow fermions to interact. Finally, there is the Higgs boson – the heart
of the Standard Model! If it were not for this particle, the Standard Model would just fall
apart. It is required not to transmit force, but to give mass to other particles. But, before
we continue with the story, let us describe the Standard Model and why it is important
not only to particle physicists, but also for everyone and everything we come across in
nature.
Towards Order: The Standard Model
The Standard Model is a theory concerning the three fundamental interactions
called the electromagnetic, strong and weak nuclear interactions that mediate between
the subatomic particles that are the building blocks of matter. It does not include gravity.
Often the terms interaction and force are used interchangeably. The Standard Model
was driven forward sometimes by new experimental discoveries and sometimes by
theoretical advances; and spanned many decades and many continents. The current
formulation was worked out in the mid 1970s when the existence of quarks was
experimentally confirmed. Since then, discoveries of the bottom quark (1977), the top
quark (1995), and the tau neutrino (2000) have given further credence to the Standard
Model.
It turns out that all forms of matter are made from just twelve fundamental
particles! These particles can be divided into two distinct groups – quarks and leptons.
They are distinguished by the different ways in which they react to the fundamental
forces. There are 6 quarks and 6 leptons. The six quarks are called up, down, charm,
strange, bottom and top (in order of mass). The six leptons are the electron, electronneutrino, the muon, muon-neutrino, tau and tau neutrino. Though the familiar electron is
a fundamental particle, proton and neutron are not. Proton is composed of two up
quarks and one down quark. A neutron is composed of one up and two down quarks.
The charge of up quark is +2/3, while that of down quark is -1/3. The charge of the
electron, however, is -1! Quarks and leptons have half-integral spin (1/2, 3/2 etc), a
property of a class of particles called fermions that follow Fermi-Dirac statistics abiding
by Pauli’s exclusion principle allowing only one particle in one quantum state.
Gravity and electromagnetism are familiar to us. The weak and strong forces are
relatively new. Electromagnetism is responsible for current in wires, electricity in our
homes, radio, television and telecommunication, including chemical binding in atoms
and molecules and chemical reactions. The strong nuclear force binds atomic nuclei
together and makes them stable, and it acts only through quarks. Weak interactions are
most noticeable when particles undergo beta decay (radioactivity in which electrons or
positrons are emitted) from nuclei, and in the production of deuterium and then helium
from hydrogen that powers the sun's thermonuclear process. This is how the sun shines
and we get energy. The weak force is felt by both quarks and leptons, unlike strong
force which is felt only by quarks. If two leptons come within the range of weak force, it
is possible for them to change into other leptons. While gravity and the electromagnetic
force have infinite range, strong and weak nuclear forces have short ranges of
interaction, 10-15 metres and 10-17 metres respectively. In terms of their strengths,
gravity is the weakest force, and then come electromagnetic, weak and strong force.
Three Generations
Figure 2 shows the standard way in which the quarks and leptons are grouped
into families. Each column is referred to as a generation. The up and down quarks
belong to 1st generation, strange and charm to 2nd generation; and top and bottom
quarks to 3rd generation. Most commonly found quarks in universe are up and down
quarks, since atomic nuclei are composed of protons and neutrons. The others are
more massive and rarer.
Now consider the leptons. Most familiar of them is, of course, the electron.
Indeed, the properties of electron are mirrored in muon and tau – but only they can
decay into other particles. They have the same electrical charge and respond to
fundamental forces in the same way. The electron is a stable particle. The other three
leptons are called neutrinos, as they are electrically neutral. Unlike neutrons, they are
fundamental particles. We say that neutrons are charge zero, while neutrinos are
neutral. Neutrinos have extremely small masses. For example, electron neutrino has
mass less than one thousandth of an electron! Electron and electron-neutrino belong to
the 1st generation; muon and muon-neutrino belong to the 2nd generation, while tau
and tau-neutrino belong to the 3rd generation along with their quark counterparts. We
may note that quarks and leptons are fermions with spin ½ and follow Fermi-Dirac
statistics. Fermions follow Pauli’s exclusion principle, that is, only one particle can be
accommodated in a particular quantum state.
Let us now consider the last column indicated by bosons, viz., photon, gluon, and
weak force. These are called gauge bosons and are the force carriers that mediate the
electromagnetic (γ), strong (g), and weak interactions (Z0, W+ and W-). Incidentally,
bosons are particles with integral spin (0, 1, 2 etc) and follow Bose-Einstein statistics.
They do not follow Pauli’s exclusion principle, and hence any number of particles can be
accommodated in a particular quantum state.
Interactions in physics are the ways in which particles influence other particles.
At macroscopic level, electromagnetism allows particles to interact with one another via
electric and magnetic fields. The Standard Model explains such forces as resulting from
matter particles exchanging other mediating particles, known as force mediating
particles. When a force-mediating particle is exchanged between two particles, at a
macroscopic level the effect is equivalent to a force influencing both of them, and the
particle is therefore said to have mediated that force. Photons mediate the
electromagnetic force between electrically charged particles. The photon is massless.
The Z0, W+ and W- gauge bosons have non-zero mass and yet mediate the weak
interactions between all quarks and leptons. They are massive, with the Z0 being more
massive than the W+ and W-. The gluons mediate the strong interaction between the
quarks. Gluons are massless. The gauge bosons of the Standard Model all have spin 1,
thus making them bosons.
Finally, we come to the Higgs boson shown separately. Why do we show it
separately? Higgs is not a gauge boson. Physicists do not need it to transmit force, but
to give mass to other particles. Two of the 16 other fundamental particles, the photon
and the gluon, are massless. But without the Higgs, there is no explanation of where the
mass of other particles comes from. Without it, the masses of all particles would be
zero, and they would travel with the speed of light, as demanded by the theory of
relativity.
Apparently, the discovery of the particle announced by CERN on 04 July 2012
appears to have properties of the much sought after Higgs boson. However, further
investigations have boosted our confidence that this must be the Higgs particle as
required by the Standard Model. Then, this was the last piece of the jigsaw puzzle of the
standard model!
So, how many particles?
We may note that the 17 particles we have described here are the basic particles
of the Standard Model. We note that every fundamental particle has an antiparticle
which carries an electric charge that is opposite of the charge on the particle. Thus we
have 6 quarks and 6 antiquarks; and 6 leptons and 6 antileptons. As regards the bosons
- photon, gluon, Z0 and the Higgs Boson H0 are their own antiparticles, while antiparticle
of and W- is W+. Together, they are responsible for some 200 composite particles
(protons, neutrons, mesons etc) that we observe in various circumstances through
different combinations and at different energies. If we include graviton (spin 2 and
hence a boson) which is supposed to be the carrier of gravitational interaction and its
own antiparticle, though not included in the list of the Standard Model, the total number
of the fundamental particles thus would add up to 31.
The fundamental particles described above and others that are composed by
various combinations of these particles are classified into several groups depending on
their properties. For example, Hadrons (Heavy particles) include mesons (kaons, pions
etc) which are bosons; and baryons which include protons, neutrons and other heavy
particles that are fermions. These are schematically depicted in Figure 3.
Colour Charge
We shall briefly discuss yet another aspect of the Standard Model, the Colour
Charge on quarks. Leptons, such as the electron, possess the property of the electric
charge, which comes in two types, positive and negative, and allows them to feel the
electromagnetic force. Quarks have fractional electric charge and feel the
electromagnetic force. Bound as baryons in a nucleus, unlike leptons, quarks also feel
the strong force. This implies that quarks must have a property that enables them to feel
strong force and also binds them together in threes in nucleons (protons and neutrons),
or as pairs in mesons. This strong force analogy of electric charge is called the colour.
Quark colours come in three shades, red (r), green (g), and blue (b). Surely, this has
nothing to do with real colours! They only describe a property of the quarks that
determines the strength of their interactions with each other through strong force.
Any stable baryon (baryon or meson) must have an overall ‘colour charge’ that is
colourless. This is similar to combinations of the red, blue and green primary colours in
light that mix together to make white. Baryons such as protons must have a red, blue
and green quark: u(r), u(b), and u(g). Antiquarks have the corresponding anticolour,
anti-red, anti-blue, and anti-green. In mesons, the colours would cancel out in quarkantiquark pairs. For example, a π+ meson would be composed of an up quark with red
colour charge, and a down antiquark with anti-red colour and is overall colourless, since
the red is cancelled out by anti-red.
Colour is a central concept in the theory of force that holds quarks together and
acts as the source of strong force. The colour force between quarks is described by a
gauge theory called quantum chromodynamics. It describes an exchange force
involving particles of the colour field that carry the force from one quark to another. The
carrier particles are the gluons that have zero mass and spin, and hence are bosons.
The colour field is analogous to the electromagnetic field and the gluon is analogous to
the photon in quantum electrodynamics (QED). Incidentally, quantum electrodynamics
combines electromagnetism, relativity, and quantum mechanics. The strong force,
which the nuclear physicists observe as binding the nucleus together and mediated by
the exchange of mesons, actually has a more fundamental origin in the colour force that
binds the quarks together!
When colour force is introduced, the number of quarks and antiquarks in the
Standard Model would triple (12 x 3). But the number of gluons would increase to 8 from
just 1! Thus there would be 36 quarks, 12 leptons, 8 gluons, photon, Z0, W- , W+ and the
Higgs Boson H0, adding up to 61 particles! With graviton it would be 62! Thus we have a
large family that has proliferated from just 17 basic fundamental particles!
Fields! Fields! Fields!
What the Standard Model of particle physics does is that it unites the
fundamental building blocks of nature and three of the four forces known to us – except
gravitation. Gravitation remains outside the Standard Model. For long, it was an enigma
how these forces actually work. For instance, how does the piece of metal that is
attracted to the magnet know that the magnet is lying there, a bit further away? And how
does the Moon feel the gravity of Earth? The explanation offered by physics is that
space is filled with many invisible fields. The gravitational field, the electromagnetic field,
the quark field and all the other fields fill space, or rather, the four dimensional spacetime, an abstract space where the theory plays out. The Standard Model is a quantum
field theory in which fields and particles are the essential building blocks of the universe.
In quantum physics, everything is seen as a collection of vibrations in quantum
fields. These vibrations are carried through the field in small packages, quanta, which
appear to us as particles. Two kinds of fields exist: matter fields with matter particles,
and force fields with force particles - the mediators of forces. Incidentally, the Higgs
particle, too, is a vibration of its field - often referred to as the Higgs field. Without this
field the Standard Model would collapse like a house of cards, because quantum field
theory brings infinities that have to be reined in.
The Standard Model would only work if particles did not have mass. As for the
electromagnetic force, with its massless photons as mediators, there was no problem.
The weak force, however, is mediated by three massive particles; two electrically
charged W particles and one Z particle. They did not sit well with the light-footed
photon. How could the electroweak force, which unifies electromagnetic and weak
forces, come about? The Standard Model was threatened. This is where Englert, Brout
and Higgs entered the stage with the ingenious mechanism for particles to acquire
mass that managed to rescue the Standard Model.
Symmetry and the Standard Model
The Higgs field is a very special kind of field. All other fields vary in strength and
become zero at their lowest energy level. Not the Higgs field. Even empty space would
still be filled by a field - the Higgs Field - that does not become zero. We do not notice it;
just the way we do not notice air. But without it we would not exist, because particles
acquire mass only in contact with the Higgs field. Particles that do not pay attention to
the Higgs field do not acquire mass, those that interact weakly become light, and those
that interact intensely become heavy. If the Higgs field suddenly disappeared, all matter
would collapse as the suddenly massless electrons would disperse at the speed of light.
So what makes the Higgs field so special? It breaks the intrinsic symmetry of the
world. In nature, symmetry abounds; faces, flowers and snowflakes exhibit various
kinds of geometric symmetries. We come across other kind of symmetries in physics
that describe the world we live in, though on a deeper level. For example, one such,
relatively simple, symmetry stipulates that it does not matter for the results if a
laboratory experiment is carried out in anywhere in the world, or even at any time.
Special theory of relativity deals with symmetries in space and time, and has become a
model for many other theories, such as the Standard Model of particle physics. It turns
out that the equations of the Standard Model are symmetric; in the same way that a ball
looks the same from whatever angle you look at it, the equations of the Standard Model
remain unchanged even if the perspective that defines them is changed.
Principles of symmetry play an important role in describing physical phenomena.
In 1918, the German mathematician Emmy Noether showed that the conservation laws
of physics, such as the laws of conservation of energy, or conservation of momentum,
and conservation of electrical charge, also have their origin in symmetry. Symmetry,
however, dictates certain requirements to be fulfilled. A ball has to be perfectly round;
the tiniest hump will break the symmetry. For equations other criteria apply. And one of
the symmetries of the Standard Model prohibits particles from having mass. Now, this is
apparently not the case in our world, so the particles must have acquired their mass
from somewhere. This is where the Higgs mechanism plays a crucial role for symmetry
to both exist and simultaneously be hidden from view.
How the Higgs boson was postulated
By 1930s, it became clear that the ranges of the fundamental forces would be
inversely proportional to the masses of the particles that transmit force. These,
incidentally, can be regarded as the quantum excitations of the all pervasive force fields.
For example photon is an excitation of the electromagnetic field. Hence, barring the
massless photon (which transmits the electromagnetic force), the carriers of strong and
weak nuclear forces need to be massive particles. Now, symmetry of equations of
electromagnetism makes photon massless making electromagnetic force have an
infinite range. When this idea was extended to strong and weak interactions, the
extended symmetry operation implied that the excitations of these force fields when
included in the theory should also be massless! Thus the symmetry required that the
matter particles in the theory – quarks and leptons – should also be massless.
In 1964, Peter Higgs and other particle physicists came up with a solution. They
demonstrated that by introducing a “scalar field” (whose particles have zero value for
quantum spin) and incorporating the mechanism of “spontaneous symmetry
breaking”(See Figure 4), the problem of massless particles could be solved in theories
with gauge invariance as the underlying mathematical symmetry. In physics, gauge
invariance (also called gauge symmetry) is the property of a field theory in which
different configurations of the underlying fields - which are not themselves directly
observable - result in identical observable quantities. A theory with such a property is
called a gauge theory. A transformation from one such field configuration to another is
called a gauge transformation. The Higgs mechanism when incorporated into the field
equations, would allow particles to have masses.
Integrating Higgs mechanism into the Standard Model allowed scientists to make
predictions of various quantities, including the mass of the heaviest known particle, the
top quark. Experimentalists found this particle just where equations using the Higgs
mechanism predicted it should be.
The Higgs mechanism works as a medium that exists everywhere in space.
Particles gain mass by interacting with this medium. Peter Higgs pointed out that the
mechanism required the existence of an unseen particle, which we now call the Higgs
boson. The Higgs boson is the fundamental component of the Higgs medium, much as
the photon is the fundamental component of light. From where does Higgs particle itself
get its mass? The Higgs boson gets its mass from the self-interaction in the Higgs field,
which arises through a mechanism known as “spontaneous symmetry breaking” by the
Higgs field of a certain universal symmetry that prevailed at the time of Big Bang. The
Higgs mechanism does not predict the mass of the Higgs boson itself but rather a range
of masses. Fortunately, the Higgs boson when it decays would leave a unique particle
footprint depending on its mass. So it would be possible for scientists to know what to
look for and they would be able to calculate its mass from the particles they saw in the
detector.
As discussed earlier, the electromagnetic force is carried by photons which have
no mass, whereas the weak force is carried by particles called W and Z, which do have
mass. The W and Z particles were discovered in a Nobel prize winning enterprise at
CERN in the 1980s, but the mechanism that gives rise to their mass was not yet
experimentally identified; and that is where the Higgs particle comes in (See Figure 5).
The Higgs mechanism in its basic form is the simplest theoretical model that could
account for the mass difference between photons and the W and Z particles, and by
extension could account for the masses of a range this mechanism is not the only
possible explanation.
The experimental confirmation of the Higgs boson would place the last piece of
jigsaw puzzle that Standard Model is into its designated slot. This explains the euphoria
when a candidate for the Standard Model Higgs boson was discovered in two
independent experiments at LHC on 04 July 2012. By now we feel reasonably confident
that this was the particle we were looking for!
Yet, all is not well with the Standard Model
The Standard Model developed in 1960s and 1970s does not include answers to
many basic questions such as how to unify electroweak forces with strong or
gravitational forces. It cannot even explain mysteries of the Universe, answers to which
have their roots in the world of fundamental particles. At the time of the Big Bang, the
Universe had no dimensions at all. How did the Universe, infinitely dense at the time of
the Big Bang, evolve into a vast Universe full of stars and planets we live in today? As
the early universe expanded, energy should have condensed into equal amounts of
matter and antimatter, which would then annihilate each other on contact, reverting to
pure energy. Thus, the universe should really be empty! But, there is more matter than
antimatter and therefore we exist! Given the energy and the temperatures at which the
LHC works, the experiments planned may help physicists understand why the Universe
grew with just enough more matter than antimatter.
How about the dark matter? What is so ‘dark’ about it? Observations of the
motion of distant galaxies indicates that they are subject to more gravity than their
visible matter could possibly account for, implying existence of some exotic hidden
matter in the mix. Where did this dark matter come from? In fact, out of the total matter
and energy content in the universe, what we observe is just about 4 per cent matter in
the form of stars and galaxies. What we do not observe is the 22 per cent of dark matter
and remaining 74 per cent being “dark energy”. Higgs mechanism hence may account
for only the 4 per cent of the matter that we observe! How do we explain the dark matter
and dark energy, then? A theory called Supersymmetry could possibly explain this.
According to it, every fundamental particle had a much more massive counterpart in the
early Universe. Indeed, the electron might have had a massive partner that physicists
refer to as the ‘selectron’. Similarly, the muon might have had the ‘smuon’, and the
quark might have had ‘squark’. Many of those supersymmetric partners would be
unstable. However, one kind of particles may have been just stable enough to survive
till today without interacting with any other particles. Could they might be dark matter?
By smashing particles like protons at energies and temperatures that existed at the
earliest moments of the Universe, the LHC could reveal the particles and forces that
were responsible for everything that followed. Supersymmetry also predicts a family of
Higgs bosons. Where does the Higgs boson observed at LHC fit in? Let us wait and
watch.
Late, but not too late!
François Englert and Peter W. Higgs were jointly awarded the Nobel Prize in
Physics in 2013 for the theory of how particles acquire mass. As we noted earlier, in
1964 they proposed the theory independently of each other (Englert together with his
now deceased colleague Robert Brout). In 2012, their ideas were confirmed by the
discovery of a so called Higgs particle at the CERN laboratory outside Geneva in
Switzerland. Both François Englert and Peter Higgs were young scientists when they
independently of each other put forward a theory that rescued the Standard Model from
collapse. Almost half a century later, on Wednesday 4 July 2012, they were both in the
audience at the European Laboratory for Particle Physics, CERN, outside Geneva,
when the discovery of a Higgs particle that finally confirmed the theory was announced
to the world. Robert Brout, however, did not live to be a part of the audience on that
day, nor be a recipient of the 2013 Nobel Prize.
References:
1.
2.
3.
4.
5.
6.
Particle Physics by Christopher Bishop 2002 Pub: John Murray
Concepts of Modern Physics by Arthur Beiser 2003 Pub: Tata Mc Grow- Hill
Introduction to Elementary Physics by David Griffiths 1987 Pub: John Wiley
CERN Website: home.web.search.ch Numerous articles in Wikipedia Nobel Prize website: www.nobelprize.org Satyendra Nath Bose
Figure 1: Satyendra Nath Bose with wife Usha Bala
Particles are of two types - fermions and bosons. Fermions have a spin of half integrals
(1/2, 3/2 etc) while bosons have integral spin (0, 1, 2 etc). The Higgs particle is a boson.
Satyendra nath Bose is best known for his work with Albert Einstein that resulted into
“Bose-Einstein Statistics’ that describes the behaviour of the particles with integral spin.
Two or more bosons can occupy the same place at the same time, but no two fermions
can occupy the same state. This is why electrons which are fermions, have to stay away
from each other and be in different orbital states as in an atom. Einstein extended
Bose's ideas to material particles (or matter), and predicted what is known as the BoseEinstein condensation. Several scientists who worked on these ideas received Nobel
prizes. Satyendra Nath Bose was never considered by the Nobel committee.
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4 Spontaneous Symm
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To understand the Higgs mechanism, imagine that a room full of physicists quietly
chattering is like space filled only with the Higgs field....
... a well known scientist walks in, creating a disturbance as he moves across the room,
and attracting a cluster of admirers with each step ...
... this increases his resistance to movement, in other words, he acquires mass, just like
a particle moving through the Higgs field ... In this analogy, these clusters are the Higgs
particles.
Figure 5: How particles get mass through Higgs Mechanism (Courtesy CERN)