Why the Name?
The
massive media ‘higgsteria’ created last week by the announcement of the
discovery of a new fundamental particle of nature, called the Higgs boson, would not have happened if
the particle had not been described mischievously as the ‘God Particle’. However much the scientific community may resent
the tag that has now become part of popular scientific lore, it was paradoxically
a famous physicist who had a role in it.
In 1993 Leon Lederman, 1988 Nobel laureate in physics and Director of
the Fermi National Accelerator Lab (FNAL of Fermilab for short) near Chicago in
USA wrote a humorous popular book on the still elusive Higgs boson and, when
the question of a suitable title confronted him, wanted to call it the goddamn particle, reflecting the sheer
frustration of the particle physics community.
His publisher wouldn’t agree to such an explosively damaging title and
chose to shorten it to ‘god particle’, with Lederman acquiescing meekly,
perhaps even gladly. The book was
published under the title “The God
Particle”, with the intriguing sub-title, “If the Universe is the Answer, What is the Question?”! One can readily imagine the impact on the
sales figures if the main title had been replaced by a very prosaic ‘Higgs
Boson’.
A peep into the past
Some
of the key concepts and ideas of quantum physics central to understanding the significance
of the new discovery have been presented in a purely qualitative way in three
of my previous articles, numbered 30, 32, and 36 respectively. They highlight the revolutionary contributions
of India’s Satyendranath Bose after whom the boson is named, the great Albert
Einstein whose collaboration with Bose led to radically new insights into the behavior
of matter at the subatomic level, India-born astrophysicist Subramanyan
Chandrasekhar whose theory of white dwarf stars built a bridge between quantum
physics and astrophysics, quantum physicists Wolfgang Pauli, Enrico Fermi and
Paul Dirac who made seminal contributions to quantum physics, and a trio of
last year’s Nobel Prize winning cosmologists who showed that not only is the
universe expanding but doing so at an ever increasing rate. The reader will find it useful to go through
them before continuing. Even otherwise, he
should be able to appreciate the implications of the latest discovery and why
it is regarded as so vitally important to unraveling the secrets of nature both
at the subatomic and cosmic scales. This
article attempts to minimize the hype and present the story succinctly and
objectively.
The
universe came into being with a ‘big bang’, a cataclysmic explosion in which
both space and time had their beginnings, which is now known to have occurred
13.7 billion years ago. Incredible as it
may seem, most of the decisive events that shaped the future course of the
universe occurred within about a quadrillionth of a second after the primordial
explosion when the universe consisted of pure energy. Everything else evolved from here onward in
accordance with the laws of physics codified in what has come to be known as
the ‘Standard Model’ as applied to fundamental particles and their interactions. The recent discovery may well be the
culmination of this edifice though many of its rough edges remain to be ironed
out in due course.
Structure of the Atom
The
idea that all matter is made up of some ‘fundamental’ building blocks dates
back to ancient civilizations who regarded these to be familiar things like
air, water, fire, earth, etc. By early
twentieth century the picture had changed dramatically, influenced by Dalton’s
atomic theory of matter a century before.
The building blogs had expanded to 92 stable elements ranging from
hydrogen to uranium and classifiable into recognizable groups within the
periodic table of elements conceived by Mendeleev. Each element was found to consist of specific
numbers of more fundamental ‘particles’ called protons, neutrons, and
electrons, with the protons and neutrons bound tightly together into a nucleus
at the centre of the atom and a cloud of electrons spread around the nucleus very far away, but still within the atom.
This means that the atom is an enormous
void, with an incredibly small fraction of its volume accounted for by the
constituent particles.
Practically
all the mass of the atom could be attributed to the nucleus. The proton was found to be about two thousand
times the mass of the electron, with the (free) neutron being very slightly
more massive than the proton.
Electric
charge was recognized as an important
property of the electrons and protons, each electron having the same amount of
charge as the proton, but of the opposite sign (negative). The element as a whole is normally
electrically neutral since the number of electrons in it is the same as the
number of protons, the chemical property of the element depending crucially on
this number and the distribution of electrons in the electron cloud surrounding
the nucleus.
The Particle Zoo
The
best way to understand the physics of particles is to let energetic charged
particles like protons collide with other particles and study what happens in
the process. This can be done in two
ways – one is to let nature do the job for us in the upper atmosphere where
highly energetic primary cosmic rays (mostly protons) coming from distant space
interact with the constituents of air and produce a wide variety of collision
products that include some exotic particles as well as high energy photons
which themselves normally trigger other similar processes. Since such cosmic ray events are relatively
rare, one needs a copious source of high energy charged particles with
well-defined and precisely measurable energies and momenta. This is where particle accelerators come in a
big way and have revolutionized our understanding of particle physics. The Large Hadron Collider (LHC) at CERN near
Geneva which has been so much in the limelight recently is easily the world’s
biggest and most expensive particle accelerator. A bit more about it later.
Over
a period of several decades, intensive experimental studies of this nature have
produced a whole new world of particles, indeed hundreds of particles, most of
them called mesons, giving rise to a veritable
‘particle zoo’, which was a cause for utter confusion among particle physicists
in the early years. It was quickly
realized that most of the particles in this zoo were really secondary products,
only some of them fitting the description of ‘fundamental’.
Particle Attributes
At
whatever level, particles have several key attributes like mass, charge, and
spin. The masses of particles are
generally expressed in equivalent energy units since mass and energy are basically
the same according to Einstein’s theory of relativity. The unit employed is the electron volt (eV) which is the energy gained by an electron in
crossing a potential difference of one volt.
Since it is a very small unit, energies are expressed in units of keV,
MeV, GeV, and TeV. Accordingly the mass
of an electron is 0.511 MeV and that of a proton 938 MeV or 0.938 GeV. The
charge of a particle may be positive (+), negative (-) or zero. Spin is a purely quantum mechanical property,
loosely analogous to rotation of a rigid body.
For reasons yet to be deciphered, nature makes a huge distinction
between particles of half integral spin, called fermions, and particles of integral spin (including zero), called bosons.
The consequences of this are dealt with in two of my previous articles
referred to earlier.
The Standard Model
The
Standard Model is a very comprehensive theory that attempts to explain
everything in the known universe in terms of just a few truly fundamental
particles and the forces of interaction among them. These are:
- 6 leptons
- 6 quarks, and
- 4 forces of interaction mediated by their
corresponding carrier particles
For
the most part, it has been a highly successful theory, with many of its
predictions verified to extraordinary precision. But it doesn’t explain everything. In particular, it excludes gravity which has
so far defied all attempts to be brought under the fold of a single unified
theory of everything, the holy grail of theoretical physics.
Before
proceeding further, it is necessary to point out that for every particle there
also exists a corresponding antimatter
particle, or antiparticle. The antiparticle behaves exactly like its
corresponding normal particle, except that it has an opposite charge where
charged particles are concerned (opposite magnetic moment for uncharged particles). Thus we have antiprotons and antielectrons,
the latter better known as positrons. When a matter particle collides with its
antiparticle the result is a mutual and total annihilation of each other into
pure energy. Fortunately for us,
antimatter is both extremely rare and scarce, posing little credible threat to
our material world. There is reason to
believe that matter and antimatter had been produced in nearly equal amounts during the early phase of the big bang, but a
slight excess of matter over antimatter at that stage meant an eventual
elimination of most antimatter in course of time.
Leptons
The
six leptons of the standard model are (all fermions with spin ½):
- the familiar electron,
- the muon (0.106 GeV),
- the tau particle (1.777 GeV), and
- their associated neutrinos – electron neutrino, muon
neutrino and tau neutrino
Neutrinos
are produced inside stars (including of course the Sun), supernovae, nuclear
reactors and in accelerators during particle collisions as well as other
processes. They are virtually massless
particles travelling at the speed of light and interacting with other particles
to such an incredibly low degree that trillions of them can pass right through
the earth’s crust each second without any obstruction whatever. Nevertheless, their discovery was possible
only through a study of such exceptionally rare interaction with other
particles of matter.
Quarks
The
six quarks of the standard model are (all fermions with spin ½):
- up
(0.002 GeV, 2/3 charge)
- down
(0.005 GeV, -1/3 charge)
- charm
(1.3 GeV , 2/3 charge)
- strange (0.1
GeV, -1/3 charge)
- top (173
GeV, 2/3 charge)
- bottom (4.2
GeV, -1/3 charge)
An
impressive array of circumstantial evidence has piled up for the quarks which
cannot exist independently outside the nucleons whose constituents they
are. Their names are typical of the silly
and arbitrary manner in which names are conceived in many areas of science,
especially in particle physics. The name
‘quark’ itself is drawn from a nonsensical word from a novel by Irish writer
James Joyce. The most massive of these,
the top quark, was discovered in 1995 at Fermilab with its Tevatron, the
world’s largest particle accelerator at that time. Leon Lederman was a key player in the discovery.
Hadrons
Hadrons,
which include protons and neutrons, are particles with integral electric charge
produced by appropriate combinations of two or three quarks/antiquarks confined
inside. There are two categories of
hadrons – baryons, like neutrons and
protons made of three quarks and mesons
made of one quark and one antiquark. Thus
the proton is a hadron corresponding to [(uud); net charge +1] and the neutron is
also a hadron, corresponding to [(udd); net charge 0].
The
pi+ meson
(also called pion) is an example of a
meson made of an up quark and a down antiquark, giving a net charge of +1. Similarly, the pi_ meson (antipion)
is made of a down quark and an up antiquark. Most mesons are unstable and decay
into other particles because of their very short ‘shelf’ life.
Fundamental Interactions and Forces
It
is fair to say that, at the most fundamental level, everything is made of
quarks and leptons. An even more
important question is what holds them together.
The answer is that they do so through interaction with each other.
There are four fundamental interactions between these particles, and all
forces can be attributed to these interactions.
Any force we can think of – friction, electrical attraction or repulsion
that hold molecules together in matter in any form, magnetic attraction or
repulsion, gravity (always attractive), particle decay, forces that bind nuclei
together – is caused by one of these four fundamental interactions, viz., gravity, electromagnetic,
weak and strong. The first two are
something we encounter very commonly in our everyday life. The last two show up only under special
circumstances.
A
subtle distinction is made between forces and interactions though they are
often used synonymously. The interactions
of a particle include all the forces affecting it, but also all the decay
processes and annihilations they may undergo.
At
a fundamental level, a force is not just something that happens to
particles. It involves the exchange of
something between two particles, the exchange of what are called carrier particles.
Of
the four fundamental forces, gravity has practically no role to play at the
sub-atomic level because of the extremely small masses involved. However, it is vitally important at the
cosmic scale, responsible for large scale aggregates of matter such as planets,
stars, galaxies, etc.
The
strong force holds quarks together to form hadrons and its carrier particles are
called gluons. The force has an extremely short range since
quarks are confined inside baryons and mesons.
It is strong, in fact extremely strong, because it has to overcome the
very high repulsive electric forces between the constituent positively charged
protons and hold the nucleons together in the nucleus as a stable entity.
Weak interactions are responsible for the decay
of massive quarks and leptons into lighter quarks and leptons. When fundamental particles decay, we observe
the particle vanishing and being replaced by two or more different particles. Though the total of mass and energy is
conserved, some of the original particle's mass is converted into kinetic
energy, and the resulting particles always have less mass than the original
particle that decayed. The
only matter around us that is stable is made up of the smallest quarks and
leptons, which cannot decay any further.
Weak interactions are mediated by the W+, W-, and the Z particles. The first two are electrically charged and
the Z is neutral. The Standard Model has
united electromagnetic interactions and weak interactions into one unified
interaction called electroweak.
The carrier particle for the electromagnetic force is
the photon. Photon energies may range from the very
energetic gamma rays to very low energy radio waves. Photons are massless and always travel at the
speed of light.
The carrier particles are all bosons (spin=1). The gluon is massless as well as chargeless,
as is the photon. However, the W and Z
bosons are quite different. The W has a
large mass of 80.39 GeV with both positive and negative charge versions. The Z boson has an even larger mass of 91.19
GeV and is chargeless.
The Higgs Boson
One
of the conundrums in the early days of the Standard Model was how to account
for the formation of material particles from the primordial soup and how they
formed more complex particles, atoms, molecules, aggregates of matter like
planets, stars, galaxies, etc., and even life itself from then on. Without such a mechanism, the initial
products of the big bang should all have flown off at the speed of light since
they were all massless. In other words,
something was needed to impart mass
to the particles, in addition to other attributes, thereby the inertia required
to slow them down and participate in the interactions outlined earlier. Peter Higgs, a British theoretical physicist,
was one of a small band of people who addressed this issue in the early sixties
and came up with an explanation complimenting the other findings of the
Standard Model. What he proposed was an
all pervasive force field, later called the Higgs field, in which a mediating
particle called the Higgs boson acted
as the carrier particle much like the other bosons and accounted for the
observed masses of the fundamental particles and their subsequent
products.
The
Higgs proposition was intellectually satisfying and a much needed cog in the
wheel of the Standard Model. However,
consistent with the basic nature of science itself, only
observational/experimental evidence would settle the matter decisively and this
is what took nearly half a century to secure.
Unlike the huge number of particles discovered with numerous
accelerators in controlled particle-particle collisions, including other
carrier bosons, nothing corresponding to the expected properties of the
hypothetical higgs particle was discovered until recently, adding to the mounting
frustration of particle physicists. It
was recognized early on that this particle could be produced only in very high
energy collision events, would itself have a very large mass, and therefore the
discovery had to wait the availability of super colliders like the FNAL
Tevatron and the CERN LHC. The rest is
history unveiled so dramatically earlier this month, with the LHC taking the
cake.
The LHC
The
Large Hadron Collider (LHC) is quite simply the most complex and sophisticated
instrument built till now for cutting edge scientific research. Apart from the successful search for a Higgs
boson, it is addressing some of the most fundamental questions of physics,
advancing the understanding of nature at its deepest levels. For a change, it is a European rather than an
American enterprise, though its reach has been truly global.
The
LHC lies at least a 100 m below ground level in a 3.8 m wide circular tunnel of
27 km circumference straddling the borders of Switzerland and France at CERN,
the European Centre for Nuclear Research near Geneva. The tunnel houses two adjacent parallel beam
pipes, intersecting at four points where gargantuan detectors capture the
aftermath of the collisions (presently) between beams of protons travelling
close to the speed of light in opposite directions. Two of these detectors, CMS and Atlas,
figured prominently in the recent discovery of the new particle. The pipes are enveloped by 9300 huge superconducting
magnets which operate close to absolute zero on the temperature scale using
liquid helium for cooling. Up to 4 TeV
of energy can now be attained in each proton beam and this will go up to 7 TeV
in due course. Such high energies are
needed to recreate through particle-particle collisions the conditions that are
theorized to have existed in the primordial universe immediately after the big
bang.
About
600 million collisions can be captured each second and their imprints recorded
and analyzed in real time with the detectors using fully automated state of the
art techniques that required a radically new computing technology called grid computing developed for the purpose
at CERN (CERN was also the birthplace of the all too familiar World Wide Web
through Tim Berners-Lee in 1989). Only
an incredibly small fraction of these collisions end up producing the kind of
decay events that point to the involvement of the higgs particle at some
stage. This is the reason why the LHC
has to be run for very long periods of time before a statistically significant
number of expected ‘signature events’ accumulate for processing and
analysis.
Thousands
of scientists and engineers from all over the world are involved in the
operations and studies at CERN headquarters as well as a worldwide network of
centres. The LHC at CERN easily dwarfs
its American predecessor, the Tevatron at Fermilab, whose underground tunnel circumference
is only 6.3 km and peak proton energy only 1 TeV, and which is heading towards permanent
shutdown soon.
Incidentally,
when the American scientific community failed to secure its government’s support
for its ambitious Superconducting Super Collider project as a successor to its
Tevatron, an 80 km circumference behemoth far more ambitiously conceived than
the European CERN effort, the mantle of leadership in Particle Physics and
Cosmology passed from its traditional stronghold in USA to Europe.
Unveiling the New Particle
CERN
had given enough hints about a formal announcement of the discovery to be made
at its headquarters on 4th of July. A
two hour webcast of presentations and discussions involving key representatives
of the CMS and Atlas groups was masterfully orchestrated by its Director-General
Rolf Heuer. What had not been made known
in advance was that these presentations would be purely technical in nature,
meant mainly for professionals in the field.
This was a huge dampener, especially for TV channels like NDTV 24x7 that
had planned a full live coverage and had to hastily involve its own experts in
a larger role to lighten the tedium. Even
viewers with a decent background of contemporary physics must have felt let
down as was the case with this writer.
However, most of the disappointment was made up in a lengthy and live
media Q&A session where the media representatives posed numerous highly
relevant and knowledgeable questions that drew the kind of non-technical
responses they were really looking for.
This is when most of the lay viewers got a real picture of the implications,
importance, and enormity of the discovery. Both CMS and Atlas experiments had
discovered the particle independently, quite a massive boson with a mass of
about 125 GeV, to a five sigma level of confidence, which meant that there was
less than one in a 3.7 million possibility of a mistaken finding.
Rolf
Heuer and his associates made some guarded statements, falling just short of
claiming the discovery as that of the
Higgs boson. They said this might only be
a Higgs boson, leaving open the
possibility that this may be just one of many such bosons if a more refined
version of the Standard Model, called Supersymmetry theory, gathers further
support. Heuer took particular care to
stress that the discovery marked just the beginning of a series of investigations
with the LHC in the coming years, addressing even deeper questions. It is interesting and even heartening to note
that the tag of ‘god particle’ was referred to in only one question and
cleverly sidestepped by Heuer and company; similarly sidestepped was a question
about a possible Nobel Prize for the discovery.
The programme took a nostalgic turn when the venerable 83-year
old Peter Higgs himself was introduced from among the front benchers in the
audience and stood up for a prolonged round of applause. Later he said; “I'm rather surprised that it happened in my
lifetime - I certainly had no idea it would happen in my lifetime at the
beginning, more than 40 years ago, because at the beginning people had no idea
about where to look for it, so it's really amazing for me to find out that it's
really enough... for a discovery claim.
I think it shows amazing dedication by the young people involved with
these colossal collaborations to persist in this way, on what is a really a
very difficult task. I congratulate
them.”
Incidentally, Higgs is known to be an atheist, and displeased that the
Higgs particle is nicknamed the "God particle", as he believes the term "might
offend people who are religious".
What Next?
Rolf
Heuer could not have put it more accurately when he said that the present
discovery is just the beginning of a series of investigations with the LHC over
the next 10-15 years. There are many
crucial questions crying about the universe around us to be answered and LHC is
expected to at the very least show the way for some of the answers.
Perhaps
the most crucial question facing physicists arises from the fact that only
about 4% of the universe is composed of matter as we know it. Dark matter (23%) and dark energy (73%) are
believed to make up the rest, but they are at present hopelessly difficult to
detect and study, other than to some extent through the gravitational forces
they exert on the cosmic scale. Another
question is why nature is biased in favour of matter over antimatter. Yet another is the nature and properties of
the immensely hot and dense quark-gluon plasma believed to have existed
immediately after the big bang.
Recreating this is one of the goals of the LHC.
Come September…
…the Nobel Prize Committee will have to
decide on the people to whom this year’s honors should be bestowed. Peter Higgs has emerged as a strong candidate
for the Physics award, perhaps a certainty.
He has already been proposed by no less a person than Stephen Hawking,
arguably the greatest physicist the world has known since Einstein and
Feynman. Rather ironically, the Nobel
Committee is yet to honor Hawking who held the same position as Newton in
Cambridge and perhaps with comparable distinction in many respects. Somewhat
surprisingly, Hawking had offered a bet of £100 that the Higgs particle would never be
found and has admitted losing it.
Nobel prizes are normally given to utmost
three people in a discipline each year.
If Higgs ends up at the top of the list who will be the other one or two? In deciding this, can the committee ignore
the monumental experimental work that led to the new discovery at CERN? But then, excepting the Peace Prize, no organization has ever been given a Nobel
Prize. Is it not time to change this
policy and award this year’s prize in Physics jointly to Peter Higgs and CERN
itself, thus honoring the extraordinary contributions of thousands of
scientists and engineers who made the discovery possible?
