The Large Hadron Collider

A Talk by Dr Helen Heath of Bristol University 11th February 2015

At the RPEC meeting on the 11th Feb 2015 Dr Helen Heath treated us to a broad overview of the Large Hadron Collider including what it is, the engineering challenges posed by the successful implementation of such a large long term multi-national project and some of the basic underlying physics involved.

Dr Helen Heath studied Physics at Oxford before moving to Bristol to study for a PhD in Particle Physics. She worked on an experiment called NA62 searching for charged particles. Following her PhD she spent two years working for York University in Toronto, Canada on an experiment in Hamburg, Germany which studied the structure of the proton. She returned to Bristol nearly 25 years ago and has worked on a number of projects but mainly the Compact Muon Solenoid (CMS) Experiment which is one of the two general purpose detectors at the CERN Large Hadron Collider. This was one of the two experiments which found the evidence for the existence of the Higgs Boson.

The LHC is constructed in an underground circular tunnel below the French/Swiss border and the business part of the accelerator consists of two circular beam tubes 27Km in length around which protons or other ions are accelerated to speeds approaching that of light. The tubes are pumped down to a very hard vacuum to allow the ions to pass around without unintended collisions whilst they are accelerated and guided by super conducting electromagnets. The Vacuum involved is emptier than deep space and the temperature involved, at 1.9 Kelvin, colder than deep space. There are 1232 electro magnets each 15m long rated at 8.33 Tesla. Each beam tube is seeded from proton generators originally used in earlier CERN experiments and the two proton beams are set up to contra rotate.

At four points around the 27km loop are placed the detectors for particular experiments; these are known as ATLAS, CMS, ALICE and LHCb. Two of the detectors are general purpose particle detectors, CMS and ATLAS and the other two are aimed at making more specialised measurements. The beam formed in each circular tube comprises of proton bunches which can be phased such that the collision point of the bunches takes place within a chosen detector. The beams circulate at 11,254 times per second, produce 100 million collisions per second and produce temperatures, at the point of collision, 1billion times hotter than that the centre of the sun.

As a result of the high energy proton collisions a plethora of fundamental subatomic particles result These are captured within the detector producing to give information as to the energy and characteristics of the particles generated. The resulting signals are analysed with in a worldwide computing network spread around the academic centres of the world.

As Dr. Helen Heath had spent much time working on the CMS detector she moved on to describe the engineering challenges involved with this part of the LHC project. The CMS was constructed above ground to form two large sub-assemblies. Each sub-assembly was craned down very slowly in to the detector hall through an access shaft with very little clearance. The two sub-assemblies were finally integrated in the detector hall. For a successful integration the two sub-assemblies were required to meet fine tolerances. The design must retain mechanical integrity over a very high temperature/pressure range, being built and installed at ambient temperature at atmospheric pressure and operating at temperatures approaching absolute zero in a high vacuum.

Following more than twenty years of preparation and construction, the Large Hadron Collider (LHC) started colliding protons in November 2009. The data taken during 2010 and 2011 retraced the history of particle physics and the developments in our understanding of matter at its most fundamental level. The data collected in 2012 took us into new territory with the discovery of the Higgs Boson.

The LHC was taken down following the discovery of the Higgs Boson for upgrades to the magnets so as to safely allow the use of greater power without the risk of failures. The follow-on programme will be looking to plug outstanding gaps in our knowledge. Finding Dark Matter and Super Symmetric Particles is high on the list of issues to be resolved. Potentially the findings could cause a major rethink of our theories of the makeup of the universe.

For further information about the LHC visit the CERN Web site. LHC

Julian Todman