LHC: The Six Billion Dollar Questions

ATLAS is one of two general purpose detectors at the LHC (the other being CMS). One of the scientific goals for ATLAS is the search for new physics therefore the detector itself is designed to be very versatile. As detectors go, ATLAS is massive, roughly the size of a 5-story building weighing about 7,700 tons. The basic components of the detector are the tracking systems, calorimeters, the muon spectrometer, the magnet systems, the data acquisition system and the trigger. I specifically work on the calorimeters. When the beams collide, there is the possibility to produce new heavy particles. These particles would decay quickly into 'jets' of lighter particles.

The purpose of the calorimeters is to measure the energies of the photons, electrons, protons and neutrons produced in order to study the properties of new particles.

Just how big is the LHC?

The circular tunnel where the beam circulates is roughly 26 kilometers (16.5 miles) in circumference. The tunnels depth underground ranges from 50 meters to 175 meters. (As a side note, one of the best forms of transportation in the tunnel is a bicycle. Although the change of depth is so gradual you can not see it by eye, you can really feel it when peddling around the tunnel!). Inside the tunnel are liquid-helium cooled superconducting magnets which are used to accelerate the protons in the beam. The beams are designed to collide within ATLAS and CMS 40 million times per second.

What is the purpose of the LHC?

When operational, the LHC will be the most power collider to ever run. The accelerator is designed to collide protons on protons. Each beam of protons have an energy of 7 TeV, resulting in a total collision energy of 14 TeV. For comparison the current most-powerful accelerator, the Tevatron at Fermilab, has a total collision energy of 1.96 TeV.

One of the major scientific goals of the LHC and ATLAS is discovery of the Higgs boson. This particle is predicted by the Standard Model, and it is the only particle predicted by this theory which has not yet been observed. Interaction with the Higgs boson is how the other elementary particles (such as the quarks and electrons) acquire mass, so the Higgs is a very important particle in the theory. If the Higgs particle as predicted by the Standard Model exists, it will be observed at the LHC.

Besides finding the elusive Higgs, there are many other open scientific questions that the LHC hopes to probe. The current theoretical description of the elementary particles and their interactions (the Standard Model) has been tested extensively at other collider experiments. But there is much theoretical motivation to suggest that the Standard Model is not complete. Gravity for example is not incorporated into the Standard Model. There are large corrections applied to the particle masses in order for the theory to agree with nature. Several new theories have been proposed to address these open issues. In the theory of Supersymmetry (SUSY), a new symmetry is introduced which relates the known elementary particles to a 'superpartner'. The introduction of the superpartner naturally corrects the particle masses. The existance of SUSY is also predicted in many string theories.

Other open questions include, why is there so much matter over anti-matter in the universe? This question can be addressed at the LHC by studying the charge-parity (CP) violation in the elementary particles and any new particles.

Most of the matter in the universe is 'dark matter'. What is dark matter? SUSY, for example, predicts the existence of new heavy particles which might be candidates for dark matter observed by astronomers.

What was your motivation to join this project, and what do you hope to observe and learn once the LHC is “turned on” in 2008?

My motivation for working on ATLAS is the search for new physics: physics beyond the Standard Model. Part of what makes the LHC so exciting is that we don't know what we are going to see. We have lots of predictions and theories about what to expect but perhaps nature has a curve-ball in store. Who knows? Everyday the excitement and anticipation of discovery at CERN grows. We all have a lot of fundamental questions about nature and the universe and I am hoping that the turn-on of the LHC might lead us to some answers.

Interview by Audrey Wang

Monica Dunford's blog about her time at CERN and ATLAS can be found here