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"It's a great day for particle physics," says Ben Allanach, a theoretical physicist at the University of Cambridge. "It's very exciting, I think we're on the verge of the Higgs discovery." Allanach talks after a webcast from CERN, which held physicists at the University of Cambridge enthralled in complete silence for nearly two hours. New evidence for the Higgs boson might have been CERN's worst kept secret, but scientists from around the world, including those here in Cambridge, were eager to hear the word from the horse's mouth. And indeed, it seems like the Large Hadron Collider at CERN has given particle physics an early Christmas present — compelling evidence that the Higgs boson does exist.
A diagrammatic view of the LHC. Image © CERN.
For 40 years physicists have been building on the standard model of particle physics – a mathematical description of the fundamental particles that make up our Universe and how they interact with each other. This model has been phenomenally successful, describing experiments with tremendous precision, but it has required one major tweak. The standard model predicts that these fundamental particles should not have any mass – and we know that some particles do have mass (experimentally observed both in the laboratory, and in the post office when I posted some surprisingly heavy Christmas presents).
This problem was theoretically resolved by the physicist Peter Higgs in the 1960s. He predicted that fundamental particles gained their mass when they interacted with a mysterious force field, now called the Higgs field, in the early moments after the Big Bang. Some particles experienced more drag than others as they moved through the viscous Higgs field and the stronger the drag, the more mass they gained (see What's happening at the LHC?).
Quantum physics tells us that we can also think of force fields in terms of particles. Allanach describes the Higgs field as a weird sort of quantum jelly that extends through space. "Like any liquid, the jelly can be calm like the surface of an undisturbed lake, or it can ripple. A ripple would look like a particle in experiments — this particle is the notorious Higgs boson." (See Particle hunting at the LHC.)
Catching sight of the Higgs boson is one of the primary aims of the Large Hadron Collider (LHC). The pipes inside this 27km long ring-shaped tunnel are cooled to a temperature colder than that of outer space and contain virtually nothing at all, being kept at an ultrahigh vacuum. Beams of protons are spun in opposite directions around the pipes at ever increasing speeds until they are almost travelling at the speed of light. These beams then intersect, colliding the particles at extremely high energies. The idea is that these particle collisions would produce the Higgs boson. (You can read more about the LHC in our article LHC for dummies and at CERN's LHC website.)
But finding the Higgs isn't a matter of directly catching the beast itself. "If a Higgs boson is produced during a collision at the LHC it will quickly decay into other particles," says Allanach. "The Higgs leaves its signature in the properties of the particles it decays into." When exotic particles, such as the Higgs, are produced during the collisions, their mass (measured in gigaelectronvolts, GeV, of energy thanks to e=mc2) is converted into the mass and kinetic energy of the resulting particles that are then detected by the LHC experiments.
Data from the ATLAS experiment: a spike in the region of 125GeV is evidence for the Higgs boson. (Image CERN)
Physicists have analysed the properties and behaviour of the particles detected by the ATLAS and CMS experiments, the signatures of the exotic particles produced in all the collisions so far. Due to the fantastic predictive power of the standard model they knew exactly how many of these signatures (say pairs of photons with a particular energy) would have been produced if the collisions did not produce any Higgs bosons. They then plotted this against a histogram of the actual counts of particles detected in the experiments.
The data from both the CMS and ATLAS experiments shows a bump in this histogram in the region of 125GeV mass – more pairs of particles were produced than was predicted by the background processes of the standard model. However, this bump in the data would be explained by Higgs bosons of this mass being produced during collisions.
Physicists were hoping that the evidence presented today would weigh in at the 3 sigma level (see Countdown to the Higgs). This would mean that the chance of ordinary background processes in a world without the Higgs producing the fluctuations observed is only one in 250. But what they got instead was a little less. ATLAS produced a 2.3 sigma level while CMS only came up with 1.9 sigma. However, the fact that the two experiments found compatible results drives up the confidence level, and Allanach believes this could well bring it into the 3 sigma region. The holy grail — which would be considered as proof that the Higgs boson exists —is nothing less than a 5 sigma level, which means that the chance of background processes in a Higgs-less world fluctuating as observed is only one in 3.5 million!
Nevertheless, physicists are thrilled with the results announced today. "The evidence is compelling," says David Tong, a theoretical physicist at the University of Cambridge. "It's not definite, but it's likely to turn into something real." Allanach agrees. "I personally feel 95% confident [that the Higgs boson exists at around 125GeV]." He believes that we will have more conclusive results within a year or two. As Fabiola Gianotti, who presented the results of the ATLAS experiment said, it's been an extraordinary time at the LHC.