CMS Physics at RAL

Why is gravity so much weaker than electromagnetism? (The electromagnetic force repelling two electrons is much greater than the gravitational force attracting them). The `Randall-Sundrum’ model could help explain this difference. It hypothesises that electromagnetism (and the other forces of the `Standard Model) is locked inside the 3 spatial dimensions that we see around us, whereas gravity acts not only in these 3 dimensions, but also in one additional one, that we have not yet observed. Gravity is thus weaker than the other forces because its part of its energy is leaking into the 4th spatial dimension.

To understand extra dimensions, consider a tightrope walker. She is confined to a one dimensional world (can only move backwards or forwards), whilst an ant that looks at the rope more closely (and is smaller, so can cling to undersides) and can move in two dimensions, being able to go round the rope. Going round the rope the ant can leave the dimension the tightrope walker can access and continuing round the rope the ant will eventually rejoin the walker in her dimension. A similar analogy can be applied to a particle leaving and returning to our 3 spatial dimensions.

The Z’ boson is a hypothetical particle, predicted in many exotic extensions of the Standard Model. Like the Z boson, it is a force carrying particle (gauge boson), and it can decay to a positron and electron.

Examples of exotic models that predict Z’ are `string theories’ and `extra-dimensional gravitational’ models. Measuring the production rate of the Z’ will help verify or exclude these models.

Since Z’s are anticipated by lots of models and their decay is easy to reconstruct, they make a good candidate for the first observation of physics beyond the Standard Model.

Exotic `extra-dimensional’ theories predict that microscopic black holes could be formed at the LHC. Evidence for these would help solve a long-standing problem of how to merge Einstein’s theory of gravity with quantum field theory.

The mass distribution of electron-positron pairs shows a peak at 91 GeV, corresponding to the Z boson, but no additional peak is seen, meaning that the Z’ boson has not yet been observed.

Illustration of a massive galactic black holeWe look for evidence of a quantum black holes decaying to two particles (an electron and a muon)

There is no evidenced for quantum black holes in the distribution. The data agrees with expected Standard Model events

A massive star can collapse to form a black hole, a region from which gravity prevents anything, including light escaping. Theoretically, black holes arise in general relativity, a classical theory of gravity.

Tightrope walker is limited to a 1-dimensional movement.

An ant on the rope can move in 2 dimensions.

Extra-Dimensional Theories – what are they ?

Z’ Searches Quantum Black Holes

Most of the interesting particles (Higgs bosons, Z’ bosons, `super-symmetric’ particles …) that can be produced at the LHC, decay almost instantly to ordinary particles (electrons, photons …) that we detect with CMS. However, some exotic models predict the existence of longer-lived particles. For example, `super-symmetric’ theories predict that a long-lived particle called the `neutralino’ should exist. It could be related to the mysterious `dark matter’ in the Universe.

We are searching for long-lived exotic particles that live long enough to travel about 1 – 1000 mm before they decay. The picture shows what we would see in CMS if two long-lived particles were produced, and one decayed to electron + positron and the other to muon + anti-muon. We see two tracks coming from each decay point (so displaced from the centre of the detector) with associated energy deposits in the ECAL (red) from the electrons and hits in the Muon Chambers (brown) from the muons.

The search for long-lived exotic particles