The Janice Emens McAdam Department of Physics

The Little Bang Theory: Smashing Atoms at the LHC

20 April 2016


Dr David Krofchek
Dr David Krofchek

In the twentieth century, two fundamental theories vastly expanded our understanding of matter and energy. Einstein's general theory of relativity explained and integrated concepts underpinning gravity, space, and time. Quantum mechanics focused on the nature and behaviour of matter and energy in terms of probability – with a mathematical function yielding the likely position, momentum, and other physical properties of a particle or photon.

Both theories illuminated murky and mysterious domains. There are, however, incompatibilities – and hence an ongoing search for a unified theory that accounts for all forces in nature under a single mathematical framework. String theory is perhaps the best known attempt at this “theory of everything”. The essential idea of string theory is that all elementary particles can also be modeled as one-dimensional strings, which may or may not be looped. 

Dr David Krofchek is conducting experiments at the Large Hadron Collider (LHC) in Geneva to test competing predictions from each theory.

The details


The string idea has yet to deliver a unified theory, and may never do so. However, scientists studying it have found connections between specific quantum theories and other theories of gravity. The connections are known as “dualities”. These connections provide tools that let us take very hard calculations in theories that describe our world and relate them to easy calculations in models of gravity – albeit, gravity in a universe with five dimensions. (The three spatial dimensions, plus time, are augmented with an esoteric fifth dimension. The fifth dimension is a little hard to imagine, but if you care to try, it’s proposed to be an exceedingly small loop (around 10-33 centimetres). 

Your atomic kitset

Quarks are the building blocks of sub-atomic particles. They combine to form a class of particle known as hadrons, to which both protons and neutrons belong. Gluons are also sub-atomic particles. They contribute to the binding force between quarks.

Dr Krofcheck is conducting experiments to test predictions made for Quantum Chromo Dynamics (QCD), which describes the interactions between quarks, based on a possible dual theory of gravity. To do this, he is using the LHC in Geneva to smash lead atoms together at close to the speed of light. 

Immensely squeezed nuclear matter

This research explores connections between a dual model of gravity that describes a five dimensional universe and the “strong coupling limit” of the QCD quantum field theory. (Both models account in different ways for the force of interaction amongst sub-atomic particles.) 

Artist’s impression of the Quark Epoch
Artist’s impression of the Quark Epoch © Nasa

Dr Krofchek is about to use the upgraded LHC to smash together atomic nuclei (in this case, lead) and recreate the perfect fluid behaviour of immensely squeezed nuclear matter. At the instant of impact, the temperature spikes to approximately 1014 ˚C. 

Under such extraordinary heat, the protons and neutrons in both colliding nuclei melt back into fundamental quarks and gluons, producing a quark-gluon plasma on Earth. This replicates the first microseconds of the Universe’s life, known as the Quark Epoch. It’s believed that in this instant everything consisted of quark-gluon plasma.

Dr Krofcheck’s research exploits the experimental discovery that, rather than being blown apart, the short-lived quark-gluon plasma “flows” – just like water flows out the sides when two water droplets collide.

A heavy B-quark particle-jet passing through this matter will lose a predictable amount of energy according to the dual model of gravity, and a strikingly different amount under QCD quantum field theory. The actual energy loss will be measured and compared with the two predictions. The broad outcome will be an empirical test of a string theory inspired relationship: the duality between the known QCD quantum field theory and a model of gravity in a five-dimensional universe.

the instant of impact between two lead atoms, during which a B-quark particle jet is fired through the resulting quark-gluon plasma.
The instant of impact between two lead atoms, during which a B-quark particle jet is fired through the resulting quark-gluon plasma (QGP). Image © CERN