Dark matter and Higgs

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Gavin Davies, professor and long-time researcher in the field of physics, presented his inaugural lecture to an eager audience. After receiving his degree from Imperial College, Professor Davies continued his career in particle physics at some of the most famous institutions in the discipline – first working at LEP in Geneva from 2000 – 2010, and then at the Tevatron in Chicago, Illinois, before moving on to the LHC at Cern.

Professor Davies started off his talk with some background to the discipline of particle physics; the fundamental building blocks of matter were once thought to be atoms, but research then discovered the nucleons and soon quarks, leptons and the force-mediating bosons burst onto the scene.

Since these revelations last century, physicists have sought to unify each phenomenon into a grand paradigm, and our best attempt so far has been the Standard Model; three generations of quarks and leptons each increasing in energy, and the four bosons controlling electromagnetic and nuclear interactions. But there are still questions raised by this such as where mass and gravitation fit in. This was Professor Davies’s first area of research.

In order to answer this, and other, questions, physicists have looked at the two extreme scales – the farthest reaches of the outer universe and the smallest distances between individual particles. Interestingly, the experiments performed at LHC intend to re-enact the first 10-10 seconds after the Big Bang, by colliding particles with super high energies and observing what decays from it. One focus has been on finding the Higgs boson, first theorised to exist in the 1970s, which is said to give matter its mass.

Particle interactions are understandably difficult to observe, and are best measured by the heat, light and charge they give out. There are also background effects that need to be attenuated so extra sensitive detectors are required. Professor Davies worked with researchers and electromechanical engineers from Birmingham University to design and build liquid xenon scintillation crystals to measure the ionisation and excitation of particles. This was especially challenging as the materials had to be cooled to –110°C, and needed to convert received ultraviolet radiation into a more measureable wavelength. In addition, the slides used were thin and fairly brittle, meaning Professor Davies had to be careful when transporting them by car.

There are many ways to create the Higgs boson and many ways in which it decays. The method chosen was for decay into two photons or two Z-bosons, which then decayed into four leptons. But the probabilities of detecting these particles were minute – for every 250,000 interactions that attempted to produce a Higgs-125 particle, only 400 of them underwent Higgs-to-photon decay and just 10 had Higgs-to-Z boson decay.

There was also a statistical element that took into account background production. A distribution was created for every interaction of the particles observed and their energy, but two statistical questions were left to answer: how compatible were the results with the background flux, denoted by the p-value, and how compatible were the results with the expected values from the Standard Model.

The standard deviation of the resulting spectrum was used as a boundary for classification: a 3σ value (3 in 2000 chance) meant there was evidence to suppose the particle exists, and a 5σ value (1 in 3.5 million) indicated a discovery. This was the benchmark announced on 4th July 2012 when the world’s attention turned to Geneva once more. The ‘Eureka’ moment came when Cern’s Director General, Rolf Heuer, stated: “We have it!”

Results obtained at Cern fit our current model of the universe but, in the grand scheme of things, the matter detected makes up a measly 4.6% of the known universe. Dark matter is said to occupy 23%, and we try to measure it as weakly interacting massive particles, lovingly known as WIMPs by the scientific community. The rest of the universe is made up of dark energy about which we still have little idea.

Professor Davies has high hopes that the discovery of this Higgs-like particle has taken us into a “decade of enlightenment,” and feels that when the LHC reopens in 2015, physicists will continue their particle collision experiments, and continue unlocking their vast mysteries.

 

What’s missing? Searching for dark matter and the Higgs boson was delivered at Imperial College, South Kensington, on 20 February 2013.

 

IMAGE: Anthony Mattox, flickr

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