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God particle? But the devil is in the details

July 10, 2012 08:38 IST
With the confirmation of the Standard Model or perhaps a modification of it, we have a reason to remain hopeful that maybe we are a step closer towards realising the holy grail of physics, says Vidya Ramanathan

If you have been on the internet since July 4 or picked up any newspaper since, it is impossible that you would have escaped reading about the (external linkbig discovery at the Large Hadron Collider at the CERN (European Organization for Nuclear Research) in Geneva, Switzerland. Scientists have discovered a new massive particle and they are calling it the 'Higgs-like' particle for now. It has received almost the same press, if not more, as the impending TomKat divorce!

So what is all the fuss about the discovery of this brand new particle that the media has dubbed the 'God particle'?

Surely this must mean a great deal to particle physicists, but what does it mean to the rest of us?

I must admit that in spite of having attended a particle physics course in graduate school (and, more embarrassingly, with a PhD in physics), the Higgs boson didn't make much sense when I (external linkfirst read about it in the New York Times. This article is an effort on my part to understand its significance from a non-particle physicist, or still better from a non-physicist point of view. 

By now we have all heard that this latest discovery explains how elementary particles acquire mass. Up until now there were only theories (such as the Higgs mechanism) that explained why elementary particles have the masses they do. The Higgs mechanism postulates that particles gain mass through interaction with an all-pervasive Higgs field. So what is this Higgs field and how does this latest discovery fit into all this discussion about mass of a particle?

Let us take a step back and focus on elementary particles. So this new Higgs-like particle makes its way into the elementary particle hall of fame or, simply put, the Standard Model. The Standard Model describes 12 elementary particles of which 11 are already known, with the Higgs boson being the only one missing up until now. Within the Standard Model the particles interact with each other through three of the four fundamental forces of nature, viz, the electromagnetic, strong and the weak forces (gravity as the fourth fundamental force is not accounted for in the Standard Model).

We deal with electromagnetic force on a daily basis through our exposure to light, heat, microwaves, radio waves, etc. But the weak and the strong forces only exist at the microscopic or subatomic level.

The 12 elementary particles are broadly classified as fermions and bosons. Fermions like electrons are the building blocks of matter. These building blocks need some sort of mortar to bind them into more complex forms of matter such as nucleus, atoms and molecules. Bosons such as photons are the particles that provide this binding agent.

How do these bosons function as a binding agent? Forces or fields governed by quantum mechanics (laws of physics at the subatomic level) have packets of energy or quanta associated with them. A quantum mechanical field such as the electromagnetic field permeates all space and its quantum (a photon) is the messenger that carries with it a minimum amount of energy associated with this field. Interactions or fundamental forces between different elementary particles are due to absorption or emission of these packets of energy. So essentially fundamental particles that are bosons are force carriers.

For example, we see visible light and feel the heat from hot surfaces as our eye or skin absorbs photons associated with the respective electromagnetic fields. In the Standard Model, there is a force carrier or a boson associated with each of the three fundamental forces. They are:

Photons (mass-less) carry electromagnetic interactions.

W and Z bosons (non-zero mass) carry the weak interaction; hold atoms together to form molecules that eventually form macroscopic objects. They are also among the heaviest of elementary particles.

Gluons (mass-less) carry the strong interactions; hold the constituents of the nuclei together.

This is where it gets interesting: why are some force carriers massive while the others have no mass at all?! Also, the range of the fundamental force is determined by the mass (or the lack thereof) of its corresponding force carrier. This explains the infinite range of the electromagnetic and the strong forces as compared to the weak interaction.

Over the years the Standard Model has gained wider acceptance with the discovery of each new elementary particle.  At the same time, there has been a compelling reason to understand how the elementary particles acquire their measured masses. Three teams of scientists in 1964 came up with different but related approaches to understand how exactly energy in the universe gets converted to mass. These three, now famous, papers were written by Robert Brout and François Englert (1,2) Peter Higgs (3) and Gerald Guralnik, C Richard Hagen, and Tom Kibble (4,5) and are credited with the prediction of the Higgs mechanism which explains how the force carrying bosons acquire mass. 

The mechanism speculates the existence of a ubiquitous field, known as the Higgs field that fills the universe. As all quantum fields have quanta associated with them, the quanta of the Higgs field is the Higgs boson. To explain how certain particles attain mass as they move through a Higgs field, consider this oft-cited example

Imagine a room that is evenly filled with paparazzi; the term 'evenly filled' implies uniformity or a symmetry. Now a popular movie star enters through one of the doors and the people closest to this entrance gather around her as she moves, thereby breaking the underlying symmetry of the room. By gathering people around her she gains momentum which is an indication of mass. She is now harder to slow down than she would be without the crowd around her. Also, once she stops, it's harder to get her going again. How far she gets into the room depends on the size of the crowd around her.

On the other hand, a layman walking into the room would escape the notice of the people present and he can quickly wade in and out of the room without gathering any 'mass'. This clustering effect is the Higgs mechanism. The field has no effect on a travelling photon, just like our layman entering a room filled with paparazzi, and hence it gathers no mass and renders an infinite range for the electromagnetic interaction.

The W and Z bosons on the other hand are the movie stars. They strongly interact with the Higgs field to gain a huge amount of mass and this is why the weak force is shorter in range as compared to the electromagnetic force.

This latest discovery at CERN can easily be hailed as the mother of all discoveries because if indeed this new particle is the Higgs boson, then it not only completes and validates the Standard Model but it also confirms the existence of the Higgs field itself!

Despite its stupendous achievements the Standard Model isn't the holy grail of everything and all that is known to us. Gravity, the fourth fundamental force, is not included in the Standard Model (more on why there isn't a quantum theory of gravity yet). Einstein's theory of gravity is not compatible with quantum mechanics and cannot be explained within the realm of the Standard Model.

Another significant gap in our understanding is the expansion of the universe. There is evidence to believe that galaxies are hurtling away from each other faster than the speed that existing theories suggest. To explain this rather puzzling observation, scientists have suggested that the universe is filled with 'dark matter' that contains 'dark energy' that tends to accelerate its growth. The unexplained entity 'dark energy' currently accounts for 73 per cent of the total mass of the universe!

But, for now, with the confirmation of the Standard Model or perhaps a modification of it, we have a reason to remain hopeful that maybe we are a step closer towards realising the holy grail of physics, a theory of everything that encompasses all the four fundamental forces, dark matter and the concept of dark energy. And this in itself is a reason to celebrate!

References:

F Englert, R Brout (1964). 'Broken Symmetry and the Mass of Gauge Vector Mesons'. Physical Review Letters 13 (9): 321–323. Bibcode 1964PhRvL..13..321E.DOI:10.1103/PhysRevLett.13.321.

R Brout, F Englert (1998). 'Spontaneous Symmetry Breaking in Gauge Theories: A Historical Survey'. arXiv:hep-th/9802142 [hep-th].

PW Higgs (1964). 'Broken Symmetries and the Masses of Gauge Bosons'. Physical Review Letters 13 (16): 508–509. Bibcode 1964PhRvL..13..508H.DOI:10.1103/PhysRevLett.13.508.

GS Guralnik, CR Hagen, TWB Kibble (1964). 'Global Conservation Laws and Massless Particles'. Physical Review Letters 13 (20): 585–587. Bibcode 1964PhRvL..13..585G.DOI:10.1103/PhysRevLett.13.585.

GS Guralnik (2009). The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles'. International Journal of Modern Physics A 24 (14): 2601 2627. arXiv:0907.3466Bibcode 2009IJMPA..24.2601GDOI:10.1142/S0217751X09045431.

Vidya Ramanathan has a PhD in experimental physics from the University of Florida, Gainesville, Florida, and is currently a postdoctoral research associate at the Rensselaer Polytechnic Institute, Troy, New York
Vidya Ramanathan