Higgs Boson and the Standard Model | By Vedran Jelic

On July 4th 2012 an international collaboration of physicists from CERN announced the discovery of a new particle. Strong evidence suggests that this particle is indeed the Higgs boson which we sought out to find with the Large Hadron Collider (LHC) – a 27 km underground tunnel designed to smash protons together. The discovery of a new particle that exists for less than 10-20 seconds is so far removed from most people’s everyday lives that in order to fully appreciate the discovery a basic understanding of particle physics is required.

The world around us is made up of atoms – spherically shaped structures with radii around 10-10 meters. Every piece of visible matter is created from atoms; yet atoms themselves are only composed of three subatomic particles: protons, electrons and neutrons. Protons and neutrons lie within the nucleus (10-15 m) at the center of the atom while electrons form probabilistic clouds around the atom. Protons and electrons attract each other while electrons repel themselves, as do protons. In order to explain protons bunching up in the nucleus, physicists proposed a new force of nature called the strong force.

It is incredible that the entire visible universe is almost exclusively made up of these three particles. However, when we look to nature, we see vastly different physical and chemical properties. Electrons form bound states around protons, but two electrons cannot be in the same particular bound state. Subsequent electrons must take on a new energy configuration; this is why an element like helium can be an unreactive gas while lithium is a highly reactive solid, even though there is only one proton difference between the two elements (and one electron plus two neutrons).

The significance of the Higgs boson lies even deeper: protons and neutrons are not elementary particles, they are composed of a quarks – a new set of particles theorized in 1964. The interaction of quarks with themselves is mediated by the strong force. There are six types of quarks, each with a very particular mass; however, heavy quarks are not stable and decay very quickly into lighter quarks. The decay of quarks is governed by yet another fundamental force of nature – the weak force. The strong, weak and electromagnetic forces all interact with the environment through particles called force carriers. In more precise terms, the strong force is mediated by gluons, the weak force is mediated by W and Z bosons, while the electromagnetic force is mediated by more commonly known photons.

The existence of four distinct forces (including gravity) remains as an outstanding problem in science. The holy grail of physics would be a grand unifying theory that describes how all four of these forces are manifestations of one force. To that end, physicists have combined three of these forces rather successfully. The first historical unification of forces occurred just before the 20th century when James Clerk Maxwell combined the electric and magnetic forces into the electromagnetic. At the time, this was the second fundamental force known (after gravity) and it wasn’t until after the 1920’s quantum revolution that we theorized the need for weak (1933) and strong (1964) fundamental forces. In 1968, the weak force was combined with the electromagnetic into what is now called the electroweak interaction. The interpretation behind this is that at extremely high energies (like the early universe) the two forces behave as one, but as we lower the energy to everyday occurrences we see two distinct manifestations of the electroweak force. The combination of the strong and electroweak forces is known as the Standard Model (see figure) – our modern framework which accounts for nearly everything in the observable universe (except gravity).

The set of elementary particles described by the Standard Model of Physics

If these three forces are all manifestations of one force, then why do they all have different force carriers, each with very different masses (gluons and photons are massless while W and Z bosons are very heavy). The answer became known as the Higgs Mechanism, and was added to the electroweak interaction (before the standard model) by Peter Higgs1 and his colleagues2,3. The Higgs Mechanism describes a Higgs field which permeates through all of space and has been proposed as the energy of the vacuum from which all else came. The Higgs field does not generate mass, as that would violate energy conservation; instead, it transfers mass/energy (E=mc2) via the Higgs boson into quarks, leptons and W/Z bosons (red, green, and purple respectively in the figure). However, the Higgs field does not interact with gluons and photons, therefore leaving them massless.  It should be noted that most of the quark’s mass comes from the kinetic energy of gluon-gluon interactions; the Higgs field only generates about 1% of the total quark mass. In the figure, all particles are shown to be coming from the Higgs boson (except the massless gluons and photons), this is because the Higgs field couples its energy to other particles and the Higgs boson is what mediates the interaction. Particles couple to the Higgs field with varying strength, the more massive the particle, the stronger the coupling.

Clearly, the Higgs boson is essential to our current understand of subatomic interactions. In order to search for this theorized particle, the Large Hadron Collider (LHC) was constructed. It is capable of accelerating protons to within 3 m/s of the speed of light. When protons collide at extremely high energies, they are capable of producing new particles via mass-energy equivalence (E=mc2). The LHC is designed to generate 14 000 GeV of energy from proton-proton collisions, while the heaviest particle found to date is the top quark at 173 GeV. Most newly produced particles decay incredibly fast and often we can only detect their decay products. The Higgs boson has five common decay paths which can occur; by detecting the byproducts of these decay paths we can determine properties about the Higgs boson (such as mass).

On July 4th 2012 the LHC collaboration reported a mass of 126 GeV for the newly discovered particle. This mass is consistent with the decay paths for the Higgs boson which strongly suggests that this new particle is indeed the Higgs. In order to confidently claim the existence of a new particle, physicists used statistical analysis on trillions of proton-proton collisions. A large number of collisions were required before the signal at 126 GeV was not attributable to signal fluctuations – in fact, based on recent results, there is about a 1 in 3 million chance that the signal is background fluctuations.

Postulating and verifying the existence of a Higgs field is a major step in understanding the nature of reality. By discovering this new particle (which is likely the Higgs boson) we hope to uncover more properties of the Higgs field which may help to solve other outstanding problems in physics such as the origin of Vacuum Energy, Dark Energy and Inflation

[1] Higgs, P. W. (1964). “Broken Symmetries and the Masses of Gauge Bosons.” Physical Review Letters 13(16): 508-509.

[2] Englert, F. and R. Brout (1964). “Broken Symmetry and the Mass of Gauge Vector Mesons.” Physical Review Letters 13(9): 321-323.

[3] Guralnik, G. S., C. R. Hagen, et al. (1964). “Global Conservation Laws and Massless Particles.” Physical Review Letters 13(20): 585-587.

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  • Jess

    I really enjoyed this, thank you

  • Phyllis

    Another incredibly well written article from the brilliant mind of Vedran Jelic. With this work, the author takes you on a wild ride filled with facts, drama and a bit of romance. Seeing him at school I had always assumed he was gifted, but after having the foundations of my world shaken by his mastery over both the written language and the world of physics, there is no doubt in my mind that this man is a genius. This article is not without its flaws however. I don’t know that I agree with the not so subtle political undertones he has chosen to include in his writing, but the way they were worked into the piece at least get his point across in a very easy to read way.



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