Read more: “Instant Expert 35: The Higgs boson”
The smaller the components you want to break matter into, the more energy you need to smash it apart. The Large Hadron Collider is the latest in a long line of ever more powerful particle accelerators designed to do just that. Its main task was to probe the energy scales at which the standard model predicted the Higgs must exist. Last July, scientists at the LHC announced that they had found something
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The Large Hadron Collider
One consequence of Einstein’s celebrated equation linking energy and mass, E = mc2, is that the energy released when massive particles collide at high speed can be used to make other, highly massive particles. The Large Hadron Collider at CERN near Geneva, Switzerland, has spent two years bashing together protons at record energies of up to 4 teraelectronvolts (TeV) each. Collide two protons with all that extra energy and you can, in principle, make about 8000 more of them.
The LHC is housed in a 27-kilometre-long tunnel. It is commonly described as a ring, but is actually more like an octagon with rounded corners. In the straight sections, intense electromagnetic fields kick two counter-circulating beams of protons every time they come past, increasing their speed. By the time of collision they have reached 99.999999991 per cent of the speed of light.
Bending such a fast-moving beam of particles requires very powerful magnets. Any power lost to electrical resistance would be a brake on performance, and so the magnets are made of supercooled superconducting materials. Even then, they can only achieve gentle curving – hence the LHC’s vast size.
On four of the octagon’s sides, more magnets constrain the proton beams to a fraction of the thickness of a human hair, and bring them into a head-on collision. Four large detectors record the results, one at each point: , , and . ATLAS and CMS are all-purpose detectors, designed to measure whatever pops up – including the fleeting traces of Higgs bosons.
ATLAS and CMS
When two protons collide at the heart of the LHC’s ATLAS and CMS detectors, they split into their constituent quarks and gluons, which decay further into myriad particles spraying out in all directions. It is the detectors’ jobs to measure or identify these collision products.
Each detector is made up of concentric cylinders. Those closest to the collision point are made of a semiconductor. If an electrically charged particle passes through the semiconductor it liberates the material’s loosely bound atomic electrons, creating a pattern of electrical currents that allows a precise measurement of the particle’s path. Magnets surrounding the detector cause the paths of the charged particles to curve, with the degree of curvature representing the particle’s momentum.
The next cylinder outwards consists of detectors filled with liquid argon (in ATLAS’s case) or lead tungstate crystals (at CMS). Collisions with the densely packed atoms in these detectors stop most particles in their tracks, and photons emitted as the particles decelerate can be used to measure the particles’ energy, and so identify them.
The heavier variants of electrons known as muons are not stopped by these detectors, but are identified and measured by the next cylinder of dedicated detectors. The even more reclusive neutrinos are not measured at all. Their presence is instead deduced by totting up the momentum of all the other particles produced in a collision and seeing if anything is left unaccounted for.
The products of many simultaneous proton-proton collisions fly away at close to the speed of light, and collisions that need a closer look need to be picked out as quickly as possible, because within 50 nanoseconds, another bunch of protons will collide at the detector’s heart. This will drop to 25 nanoseconds after the LHC upgrade currently taking place. This is the thicket of data, passed all around the world on a “grid” of interlinked computers, in which a Higgs boson must be identified.
Particle found
A Higgs boson is short-lived, decaying almost instantaneously into other particles. To infer its presence, we must measure these decay products and look for evidence that they came from a Higgs.
Fortunately, the standard model predicts everything we need to know about the Higgs boson, apart from its precise mass. For every possible mass, we can predict the number of Higgs particles that the LHC should produce, and what they will decay into.
For example, the Higgs should sometimes decay into pairs of high-energy photons. Because momentum is conserved in particle decays, the momentum of the photons can be translated into the mass of the particle that produced them. There are many possible sources of a photon pair, but if we concentrate on the likely looking ones and plot their combined momentum on a histogram, an unknown particle will make itself known as a “bump” – an excess of events corresponding to a particular mass (see diagram). This is what both ATLAS and CMS saw at a mass of around 125 gigaelectronvolts, and announced to the world on 4 July 2012 as the Higgs boson.
It wasn’t the only evidence. The Higgs boson is also expected to decay into two Z bosons, which each decay further to two leptons. Combining the momentum of these leptons produced a peak at the same mass visible in the photon data. W bosons added their own strand of evidence. These particles decay into neutrinos, which are not detected, so there is no definite mass bump in this case. Instead we just see more W decays than we would expect if the Higgs did not exist.
All in all, the evidence was just enough to grab a gold-standard “5-sigma” discovery, signifying a probability of 1 in 3.5 million that the finding came about as a result of random statistical noise. Since then, our certainty that something is there has increased still further. But it will take more tests before we are certain it is what we think it is.
This article appeared in print under the headline “A discovery”
