Finding the Higgs*
One of the prime objects of the LHC being constructed in the first place was to discover whether the conceptual framework on the previous page but one is correct and whether the Higgs particle does in fact exist as predicted by the Standard model. If the particle is shown to exist, it may provide an access way to the Higgs field.
As witnessed on the previous page, the methodology for finding the Higgs and other phenomena the LHC is looking for is to accelerate two particle beams (hadrons) around a 17-mile ring in opposite directions, the goal being to get them to collide at nearly the speed of light. If a collision does occur, it potentially creates smaller pieces of matter (particles), a scenario similar to that at the beginning of the universe, thereby giving scientists a unique look at the universe’s origins and particles never before observed and the Higgs boson[1]. Since the latter only exists at high energies, and only lasts for fractions of a second and then decays into other particles, scientists cannot observe a Higgs directly and have been looking for trace patterns of decay into lighter particles that indicate the Higgs has made an appearance. Such trace patterns include decay into two photon, a pair of W bosons or Z bosons or a bottom quark and its antiparticle, each of which decays into a tight “jet” of secondary particles or hadrons[2]. Unfortunately many decay modes are indistinguishable from the thunderous din of ordinary background events that result from 500 million proton-proton collision every second, and LHC experiments are designed to spot the occasional interesting events that might evolve from Higgs decay and throw much of the rest away[3]. So how do scientists access this material?
I had always speculated whimsically that when all the particle smashing was over and the dust and the noise had settled, scientists would throw open the door and start rummaging around the debris in their big gum boots and rubber gloves looking to see what new particles they could find. I wondered whether they would actually destroy evidence in the process - farcical I agree - but it turns out that I was not too far out in my speculation about the crudity of the process. Richard Feynman once compared proton collisions to figuring out how Swiss watches work by smashing them together. Another account has him likening the process to smashing garbage cans into each other, which means that a lot of junk comes out, because the process of smashing protons together is very messy[4]. We now know, of course, that all the evidence which survives the trigger is disseminated across the globe for scientists to analyse at their leisure. There is just too much to sift through in one place such as the LHC data centre.
From the above analogies, one should not be surprised to hear that proton collisions are exceptionally complex. Protons are not elementary particles but little balls of quarks and gluons bound together by the strong subnuclear force. When they smash together, quarks can bounce off quarks, quarks off gluons, gluons off gluons. Quarks and gluons can split into still more quarks and gluons. They ultimately clump together in particles that shoot out of the collider in narrow sprays that physicists call jets. Somewhere buried in the resultant mess may be things that humans have never seen before: new particles, new symmetries, maybe even new dimensions of spacetime. But, sifting them out may be difficult, because to our instruments, exotic particles look rather like ordinary ones - the differences are small and easily missed.[5]
The Higgs – at last!
In any event, following a plethora of high energy particle collisions over a number of years with the energy levels being ramped up from time to time, on 4 July 2012 two teams of CERN scientists working independently of each other (ATLAS and CMS) announced that they had observed several dozen events in which two photons came “blazing out” with combined energies of 125 to 126 GeV[6] at the level of 5 sigma. Actually the data revealed striking peaks jutting out at these levels, and this time the experiments revealed more than a dozen extra events in which a heavy particle exploded into four charged leptons at 125 GeV. As a result, the two teams expressed confidence that this indeed represented trace patterns of decay, affording evidence of a new particle: a boson and the heaviest yet found. The reference to 5 sigma means that scientists are 99.9999% sure that the thing they're looking at is a new particle and not something else such as a random fluctuation, 5 sigma being the standard for declaring that a new particle exists. Another way of expressing the same idea is that 5 sigma means there’s a 1 in 3.5 million chance of the result occurring by chance. In other words, it’s a measure of how confident scientists feel their results are[7].
Technically, the results are at a 4.9 sigma level, which is just under that. The particle itself was spotted at an energy level of 125.3 ± 0.6 GeV, which is "in agreement with the standard model [Higgs boson] at a 95% confidence range."[8].
What the scientists observed looked something like this:
The curve on the graph represents the number of decay events at specific energy levels that physicists expect to see when they smash protons together in the LHC. The crosses represent particle collisions at different levels of energy, and the bump at about 125 GeV indicates that something different is going on. |
[1] “What is the Higgs Boson – and will CERN scientists find the God particle”, Christian Science Monitor, undated blog. See also ‘The quest for the God particle and the secrets of the universe”, SMH, 5-6 Feb 2005; “Particle smasher heads for collision course”, SMH, 30 March 2010; Telegraph, London, 31 March 2010. The term God particle stems from the title of a book by Leon Lederman: The God Particle: If the Universe Is the Answer, What Is the Question?
[2] Michael Riordan, Guido Tonelli and Sau Lan Wu, “The Higgs at Last”, Scientific American, October 2012, 58 at 60 -63. This article contains a history of the search for the Higgs extending over various particle smashers and several decades.
[3] Ibid, 62-63.
[4] How the process actually works may be found here: http://www.huffingtonpost.com/2013/08/19/how-particle-accelerators-work-physicists_n_3777290.html?ir=Australia For a visual depiction, see https://www.youtube.com/watch?v=G6mmIzRz_f8 and https://www.youtube.com/watch?v=1sldBwpvGFg
[5] Zvi Bern, Lance Dixon and David Kosower, “Loops, trees and the search for new physics”, op cit, Scientific American, May 2012, 20 at 25-26, where the Feynman anecdote also appears. The article reappears in the Scientific American’s Special Collector’s Edition, August 2013, 28-35.
[6] Recall that a GeV or gigaelectronvolt is a unit of energy equal to a billion electron volts.
[7] http://www.physics.org/article-questions.asp?id=103
[8] “What CERN found”: http://dvice.com/archives/2012/07/cern-announces.php. 125 GeV/c2 is equivalent to about 133 proton masses, in the order of 10−25 kg, which is "consistent with the Higgs boson": Source: http://en.wikipedia.org/wiki/Standard_Model.
What exactly is this new particle?
The particle was also observed decaying into W and Z bosons which is what you would expect from a particle bestowing them with mass. So, the evidence appears to substantiate the existence of a new particle, but what that particle is or what its properties are has yet to be determined. In principle, the Higgs must appear the same everywhere, have no spin at all and even parity (behaving the same when observed in a mirror). Experimental tests are awaited to see if these rigorous standards are capable of substantiation, and by the end of 2012 scientists were expected to have much more data available to them than at the time of the July announcement, but the LHC was then closed down for two years to be refitted so as to collide protons together at even higher energies, reopening in March 2015 so nothing has emerged, at least publicly, in the meantime[1].
But even at this stage, in some respects the revelations do not accord with what physicists had expected to find[1]. Not only is the 125 GeV mass of the new particle vastly less than it should be, it is about as small as it can possibly be; and a Standard Model Higgs should decay not just into force-transmitting bosons, but also to matter-making fermions, and here the situation with the new particle is not so clear. It has also been observed decaying into two photons, which is indirect proof that it interacts with the heaviest form of quark, the top quark. However, it appears to decay into a pair of photons too frequently, about 1 ½ times the rate predicted by the Standard Model. According to theory, the Higgs cannot interact directly with photons because it has no electric charge, and photons have no mass, so it first splits into a pair of top quarks and antiquarks that in turn radiate photons. And if photons are in oversupply, on the other hand there is a dearth of tau leptons. According to the Standard Model, a Higgs boson at about 125 GeV should decay into tau particles about six per cent of the time, but the discovered particle seems to be doing it a lot less than that.
So at present the range of uncertainties and “don’t knows” appears to look something like this:
The Standard Model of particle physics has had all its theoretical predictions confirmed by experiment over the last 40 years. However, physicists realise that it cannot be a complete description of nature. Not only does it fail to incorporate gravity[2], it provides no explanation of dark matter, which influences the motions of galaxies but otherwise does not seem to interact with ordinary matter. It also fails to account for dark energy the force which appears to be accelerating the expansion of the universe[3]. The Higgs is the last missing piece of the Standard Model, but finding something even more exotic will oblige scientists to rethink their theories about how the universe is put together.
* Header: Simulated Large Hadron Collider CMS particle detector data depicting a Higgs boson produced by colliding protons decaying into hadron jets and electrons. Source: https://en.wikipedia.org/wiki/Tevatron
[1] Michael Slezak, “New particle, new questions – The discovery of the world’s most wanted boson could kick-start new physics”, New Scientist, 15 July 2012, 6. See also Michael Chalmers, “The Higgs Problem – What exactly is that particle?” New Scientist, 10 November 2012, 34,35.
[2] Three American scientists “claim to have discovered a way to incorporate gravity into this model, which if correct, this may provide an avenue for uniting general relativity with quantum mechanics: see footnote 27.
[3] “Waiting for the Higgs”, Tim Folger, Scientific American, October 2011, 58-63 at 61-62.