My research

My research concerns the next generation of experiments we plan to make at the Energy Frontier, often called "Higgs Factories". At present there are several proposals for such an experimental facility (each with their own acronym: ILC, FCC, CEPC, LCF, CLIC...).
The main aim of these experiments is to examine the properties of the Higgs boson as precisely as possible. The Higgs boson holds a unique place in today's "Standard Model" (SM) of particles and their interactions: we want to test if it behaves as predicted in that model, or whether it carries the imprint of a deeper truth which lies beyond the SM, today's description of nature.
I am studying ways to improve the physics output we can get from such an experiment by improving its design, using new technologies, 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 which will record the results of the experiments.

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 future Higgs factory 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.

See these two papers for details:
measuring Higgs CP at ILC (arXiv:1804.01241)
reconstructing taus at ILC (arXiv:1507.01700)

Quantum tomography

The tau lepton is a very useful tool, since we can reconstruct which way it spins by looking at how it decays. In the case of Higgs decays, this spin direction is affected by the CP of the Higgs boson (see above). More generally, the spins of two tau leptons produced in an interaction are expected to be "entangled", due to the magic of Quantum Mechanics. If we can measure the tau lepton spin directions sufficiently well, then we can fully reconstruct the "spin density matrix" which encodes the full information about the entanglement.
I'm currenty working on a method to do this at a Higgs Factory. It's a non-trivial thing to do, since an important part of the information is lost due to our inability to measure the neutrinos produced when the tau decays.

Vertex Detector

The "vertex detector" is placed as close to the interaction point as is possible, around 1 cm in the case of a Higgs Factory. It should measure the position of particles with a precision of a few micro-metres in order to allow the reconstruction of certain types of quarks and leptons which travel a short distance (mm or less) before decaying.
The requirements for a vertex detector at a Higgs factory are not so different than those for the Belle2 experiment which is now operating at KEK. I'm collaborating with colleagues from Belle2 who are designing and preparing to build a new and better vertex detector for their experiment. It is based on MAPS technology, in which CMOS technology is used to make sensors which can both very precisely measure the passage of particles, and analyse the resulting electronic signals in situ, within the sensor itself. This results in a detector with fewer extraneous supporting services, and therefore less material to disturb the passage of particles, thereby giving more precise measurements.

Detector design

The overall design of the detector for a Higgs factory involved deciding which technologies to use to measure various types of particle we expect, how to fit them together, and how to integrate the detector with the accelerator. To make informed decisions about such questions, we rely on computer simulations of the detectors, their subsystems, and the processes which will occur within them.
I'm involved with the International Large Detector (ILD) concept, which is developing a design for a Higgs factory experiment. I'm developing and looking after the simulation model of this detector.
A particular interest of mine is how it fits with the accelerator. Different projects (ILC, FCCee, CLIC, ...) have different boundary conditons for the detector, and different rates of "background" processes which can affect the ability of the detector to operate in an optimal way. Understanding these differences is important, since it may well influence which detector technologies are chosen for the detector.

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.