Sunday, July 15, 2012

The Exasperating God Particle – Missing piece in a cosmic jigsaw puzzle


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  One can readily imagine the impact on the sales figures if the main title had been replaced by a very prosaic ‘Higgs Boson’.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.
 
For a wonderfully detailed and not too technical chart of the Standard Model, see: http://www.cpepweb.org/images/chart_2006_4.jpg  (This chart has no reference to the higgs boson).  It is part of a multimedia educational package under the caption ‘The Particle Adventure – The Fundamentals of Matter and Force’, available at http://particleadventure.org/.
 
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?  

6 comments:

A V G Rao said...

Well written, understandable by lay readers

A V G Rao said...

An afterthought: please provide direct links to your previous articles wherever they are referred.

Kedarnath Modi said...

I never understood many terms earlier by news and articles in papers but this article revealing the entire thing is really appreciable. Nice job Professor.

The one thing that always worries me is why only foreign countries conduct such experiments and not India.

Dr S N Prasad said...

Regarding Mr A V G Rao's suggestion, links to all my articles appear in a chronological order immediately to the right of the blog text and below my profile.

Regarding Mr Kedarnath Modi's comment, experimental research in frontier areas of particle physics/cosmology are frightfully expensive and affordable only by international consortia like CERN in which countries like India do have a prominent presence.

Arvind said...
This comment has been removed by the author.
Arvind said...

Thank you for the article - I enjoyed reading it.