The Higgs Story-Part 15: Producing the Higgs


It is time to turn to the issue of how to produce the Higgs particle. Particles that are too short-lived to be found in nature have to be produced in the laboratory. What one does is to use Einstein’s famous relation E=mc2. If one has energy large enough, one can in principle produce any particle in the lab. The larger the mass of the desired particle, the larger the energy required. The way the particle is produced is by accelerating easily obtainable stable particles (i.e., those that do not decay quickly into other particles) like protons and electrons (and their anti-particles) and then colliding them with each other so that their energy of motion is converted into mass energy of the new particle. (For previous posts in this series, click on the Higgs folder just below the blog post title.)

It helps if the mass of the desired particle is known because then one can precisely pick the energies of the colliding particles so that the total exactly matches the mass of the desired particle, thus reducing the number of other particles being produced that just clutter up the detectors (this is called the background) and have to be weeded out. In such cases, it is more advantageous to use electrons and positrons (the anti-particle of the electrons) in your colliding beams because they have no substructure and all the energy of each particle goes into the new particle. Thus you can choose the total energy of the colliding particles to precisely match the mass of the new particle. Electron colliders are like scalpels, great for high-precision detailed studies where you know the mass of the particle being produced.

But the mass of the Higgs was unknown. Also unknown was how it gets its mass. Does it get its value the same way as all the other 18 elementary particles (the quarks and lepton and gauge bosons) by interacting with the Higgs field? The answer is no, although the Higgs particle does interact with itself. The reason is that the masses of all the 18 quarks, leptons, and force (‘gauge’) particles would be zero if the symmetries that give rise to the patterns among them were exact. The fact that they are not zero requires an explanation and the one that has been arrived at is that the symmetries are inexact. They are ‘spontaneously broken’ (in the language of particle physics) by the Higgs field via the Higgs mechanism.

But the Higgs field itself is not the result of any symmetry, broken or otherwise, as far as we know. It just is, a brute fact of nature, and hence there is no a priori reason for the particle associated with it to have any particular value for its mass. The Higgs particle mass is a truly free parameter that could have had any value at all and can only be obtained by measurement.

We were not totally ignorant about the Higgs mass. There is an aspect of theory called unitarity that says that its mass should be below 1 TeV (about a 1000 proton masses) but that was about all we knew. When you are searching for a particle with an unknown mass, it is better to use proton-proton collisions because protons, unlike electrons, are not elementary particles. They are composites containing three quarks and an unknown number of gluons. So when two protons collide, you can visualize it as essentially two bags containing quarks and gluons smashing into each other. The actual reactions that produce new particles are those caused by an individual quark or gluon in one proton bag colliding with an individual quark or gluon in the other proton bag.

Since there will be a spread of energies of the individual quarks and gluons in each bag, a single proton-proton collision involves a range of energies of collision between the constituents of each bag so that unlike with electron-electron collisions you can sample many energies simultaneously in each collision and thus have a better chance of hitting on the exact energy to match the mass of the particle you wish to create. The downside is that the collision produces a huge amount of background that you have to sift through in order to find just those reactions that signal the possible production of a Higgs particle. Whereas electron-positron colliders are like scalpels, proton-proton colliders are like sledgehammers, great for smashing things apart when you are not quite sure exactly what you are looking for. The catch is that you then have to sift through a huge amount of debris.

Clearly carrying this out was not an easy task, as demonstrated by the fact that it has been nearly fifty years since the idea of the Higgs field and particle was originally proposed in 1964, and the original workshop to study the feasibility of building the LHC to search for it was held nearly thirty years ago. Furthermore the LHC accelerators and detectors have cost so far an estimated $10 billion or so and there have been over 3,000 scientists from all over the world working on each of the ATLAS and CMS detectors. Big science doesn’t get much bigger than this.

And we should not forget the ill-fated Superconducting Supercollider (SSC) that was proposed and designed for roughly the same purpose as the LHC, was endorsed by president Ronald Reagan in 1987, and later killed by Congress in 1993 after over $2 billion had been spent on it and construction begun. All that remains of the SSC now is an empty underground tunnel in a remote area of west Texas that is 54 miles in circumference, over three times that of the LHC.

The SSC was supposed to be able to reach energies of 40 TeV, which was 20 times higher than what had been achieved up to that point and three times as much as the targeted energy of the LHC of 14 TeV. The LHC has so far reached 8 TeV and has been shut down for two years to ramp it up to reach the target.

Next: Design challenges of the LHC

Comments

  1. Doug Little says

    Now that we know what the mass of the Higgs is are they going to try and confirm it using electron-electron collisions?

  2. Mano Singham says

    Yes, there are already plans to build an electron-postron collider in Japan to do detailed studies. Money is going to be an issue.

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