Wednesday, 11 July 2012

Understanding Higgs Boson

by Ed Harding


The recent discovery of the Higgs boson has been widely reported by the mainstream media, the story planting itself in a prominent position on the homepage of most news websites. But despite the furore in the media, most of us still do not understand what it really is and the scientists are still unsure as to whether they have actually discovered the Higgs boson itself. Being known as "the God particle" will have helped the Higgs boson to conjure up such a large media following. Though this name is a hyperbole, and many scientists certainly do not approve of the term, the Higgs is an important particle indeed. It gets special attention because, if it is discovered, the standard model of particle physics will be complete. However, even when we are sure that it definitely exists, there will still be many unanswered questions.
The other reason for such excitement is that the discovery was made at CERN. The Large Hadron Collider is certainly impressive, with a circumference of 27 km and estimated cost of $10 Billion. CERN has been attracting media attention ever since the Large Hadron Collider was powered up, partly due to the controversial cost and partly due to some people who, excited by the idea of Doomsday, claimed that the collisions would produce a large black hole that would swallow up the Earth. They did not know much about physics.
Professor Peter Higgs
(source: Guardian)
The scientists at CERN say that they have discovered a particle of mass 125.3GeV (gigaelectronvolts), which is what is to be expected for a Higgs boson. Also the decay patterns produced are consistent with what is to be expected. The find has been given a 5-sigma rating, which means that there is only a one in a 3.5 million chance that this data would be produced if no particle was present. The five sigma rating means that the find can officially be classed as a discovery of a new particle. However CERN are keen to stress that this newly discovered particle is not necessarily the Higgs boson, and that much work needs to be done before the existence of the Higgs can be confirmed.
To discover the new particle, high energy protons were collided and the resulting fallout of particles recorded. To record these collisions, the different voltages around the collision are recorded and computers analyse the data, determining the known particles and their energies. The trajectories of the particles are projected backwards so that the physicists can see what happens immediately after the collision, and then identify the new particle, which decays too quickly to be picked up by the detector.
So now we can move on to the physics. What exactly is a Higgs boson?

Particle theory in physics is summarised by the standard model, which describes all the particles that make up the universe (disregarding dark matter, dark energy and gravity). The particles are separated into two classes, fermions and bosons. The fermions are the quarks which make up protons and neutrons, and the leptons which are things such as electrons and neutrinos. The bosons are the force carriers. The W and Z bosons are responsible for the weak interaction, the gluon is responsible for the strong interaction, and the photon is responsible for the electromagnetic field. The standard model can be reduced to a set of mathematical equations, which relate all of the fundamental particles and their properties. However it was discovered that, if mass was fed in as a property of the fundamental particles, the equations no longer worked. So in 1964 Peter Higgs and his colleagues proposed what is now known as the Higgs field and its associated boson. It was claimed that the Higgs field permeates throughout all of space, is what gave particles their mass, and this was found to be consistent with the standard model.
To first understand the Higgs field we must understand what we mean by mass. Mass can be defined to be an object’s resistance to acceleration, or change of motion. To get a grasp of what is going on with the Higgs field we can use a simple analogy. Imagine a snowfield, deep powder stretching on to infinity. This is our Higgs field that we must travel through. Particles have different masses, and photons have no mass at all, so obviously the effect of this field on particles is not constant. We can compare our photon to a skier. The skier glides over the snow, not hindered by it, travelling at the speed of light. Now consider a light particle, an electron say, and a person wearing snowshoes. This person is obviously hindered by the snow, and will move more slowly than the skier, hindered by the powder around them, just like the electron in the Higgs field. And finally we can think of a person with no equipment whatsoever; they will sink deep into the snow, and make very slow process. They are equivalent to our heavy particles, such as top quarks. “OK, but what about this particle?” you may be wondering.  Well, stretching the analogy to its limit, we can say that, very simply, the Higgs boson is like a snowflake, and lots of them make up the snowfield, or Higgs field. More technically the Higgs boson is produced by quantum excitations of the Higgs field, just like photons are produced by quantum excitation of the electromagnetic field.
So what are the implications of this discovery? Well for the layman, it unfortunately does not mean a lot. For physics, however, the confirmation of the existence of the Higgs is significant. We will finally have a complete standard model of particle physics. The Higgs boson is also involved in the weak force and W and Z bosons, which determines nuclear decay in atoms. Furthermore, discovery of the Higgs particle was one of the main aims of the Large Hadron Collider when it was built, and this discovery will justify the often disputed cost.
The discovery is also important for Peter Higgs. Speaking at the conference in CERN, where the discovery was announced, Higgs said "It's really an incredible thing that it's happened in my lifetime." It is expected that, if the particle is confirmed to be the Higgs boson, Peter Higgs and the five other members of the team who first proposed the particle will receive the Noble Prize. They certainly deserve it.




1 comment:

  1. Mr Thomas (Physics Dept) writes:

    "Nicely written, Ed. I heard a good analogy of the ‘quantum fluctuations’ in the field too – i.e. why should the Higgs exist on its own? Imagine a small gust of wind blowing across the snow field; this might cause a vortex where the snow flakes would be trapped – this would b the Higgs Boson coming into spontaneous existence. Presumably it is such a disturbance that has been caused in the Higgs field by the LHC collisions?

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