My research

My research concerns the planned International Linear Collider (ILC). I am studying ways to improve the physics we can get from ILC by improving its design, and looking in more detail at measurements of the Higgs boson that we will be able to perform (in particular CP violation), and on optimising the design of detectors for the planned International Linear Collider (ILC).

CP violation

The CP symmetry is respected if physics is unchanged when we change particles to anti-particles ("C" for charge), and the coordinates x to -x, y to -y, and z to -z ("P" for parity). We think that in the big bang which created our universe, equal amounts of matter and anti-matter should have been created from "pure energy". However, when we look around us, our universe seems to consist almost only of matter (rather than anti-matter). There are several conditions which are required in order to get rid of all the anti-matter in the universe (formulated by Sakarov). One of these is that CP must be violated. Some small violations of the CP symmetry in particle interactions have been measured, but not enough to produce the universe as we see it. We therefore suspect that other processes should violate CP to a larger extent.

It is not known if such CP violation occurs in processes in which the Higgs boson plays a role. The ILC will allow us to measure the Higgs with sufficient detail to tell if it is a significant source of CP violation. Tau leptons produced in Higgs decay are a powerful tool with which to perform these measurements.

Calorimetry

A particular interest is the development and use of the electromagnetic calorimeter (ECAL) subsystem. The role of the ECAL is primarily to detect and measure photons and electrons produced in the ILC collisions, and also to play a more general role in the reconstruction of events prduced at the ILC.

The ECAL being designed for ILC is different in several important ways to the ECALs in previous experiments. The most striking difference is the high granularity of its readout. Each readout channel corresponds to a volume of less than one cubic cm; in a "typical" ECAL in other experiments, the corresponding volume is 100s of cubic cm. This granularity allows detailed reconstruction of individual particles produced in the collisions, even when produced within tightly-packed jets of particles.

This has the advantage of allowing accurate measurement of jet energies (a "jet" of particles is produced when a quark "hadronises"), allowing excellent measurement of several physics processes which would otherwise give only limited information. It also involves several technical challenges, in particular the reliable reading and collection of signals produced in many 10s of millions of ECAL detector cells.

I am working on a realisation of this technique which uses sensors made of silicon interleaved with tungsten sheets to measure photons and electrons. The tungsten induces these particles to "shower": a single, high energy, photon, electron or positron becomes hundreds or thousands of lower energy photons, electrons, and positrons. By essentially counting the number of electrons and positrons produced in this process, we can estimate the energy of the incident particle.
The role of the silicon sensors, which work as PIN diodes, is to do this counting job. When charged particles (in this case electrons and positrons) cross the silicon sensor, electrons in the valence band are knocked into the silicon conduction band, creating an electron-hole pair. An electric field applied across the sensor results in the e-h pairs creating a current across the sensor, which can be measured by sensitive electronics. The number of such pairs produced in a sensor is determined by the number of particles crossing it, allowing the total number of particles in the shower, and therefore its energy, to be estimated.

I carry out this research within the CALICE collaboration and the ILD detector concept group.

More details are available here.