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Content archived on 2024-05-29

Linear collider research at the University of Edinburgh

Final Activity Report Summary - ILCEDINB (Linear Collider Research at the University of Edinburgh)

This research investigates the properties of the fundamental particles that make up the universe, and the interactions between these particles. The current understanding of these particles and interactions is encapsulated in 'The Standard Model of Particle Physics'. It states there are 12 matter particles, classified into two types: 'quarks' and 'leptons'. Some examples of these are the 'up' and 'down' quarks, which bind together to form protons and neutrons, and the electron, which is one of the leptons. These quarks and leptons interact via three forces: the electromagnetic force, and the strong and weak nuclear forces. The effects of the strong and weak forces are not too apparent at everyday energies - but the electromagnetic force is the same one used to generate electricity, or stick a magnet to a fridge. Almost all the predictions of the Standard Model have been verified - except one - the existence of a particle called the 'Higgs boson'. The Higgs boson (named after Professor Higgs from the University of Edinburgh, who first proposed the particle) is key to the Standard Model as it provides a mechanism for all the massive particles to have mass.

The aim of my project was to investigate how well it is possible to test the properties of the Higgs boson. In particular I want to know how well we can test the hypothesis that the Higgs boson gives mass to the quarks. The Higgs boson - like almost all of the other fundamental particles - does not live for very long. Once produced, it will decay very rapidly into other particles. If it's true that the Higgs boson gives masses to the quarks and leptons then the relative decay rate into any given quark or lepton will be proportional to the quark or lepton mass squared. It is this key property I aim to investigate.

The Large Hadron Collider (LHC) at CERN, will turn on this year. It collides protons and protons. If the Higgs boson exists then the LHC will very likely discover it. However due to the messy collisions at the LHC, it will be challenging to test this key property of the Higgs boson. The challange comes as we need to look at the decays of the Higgs to particular quarks - Higgs decaying into a pair of bottom quarks, and Higgs decaying into a pair of charm quarks. These two signatures: a pair of bottom quarks and a pair of charm quarks look very similar, and it's only by looking at the details is it possible to tell these two decays apart.

Therefore I chose to see these decays could be measured at a different collider: the International Linear Collider (ILC). The ILC, currently, only exists on paper. It would collide electrons and anti-electrons, which is a much cleaner experimental environment for making precise measurements.

My project worked on several different aspects of the ILC and its capability for differentiating between the bottom and charm quark signals:
(1) Developing software to distinguish these two signals at the ILC.
(2) Testing new silicon sensor which could be used at the ILC to facilitate the reconstruction of the bottom and charm quark signals.
(3) Using simulated data to understand how well the ILC could be used to measure the rate which the Higgs boson decays into pairs of charm quarks and pairs of bottom quarks.

This work was done with a post-doctoral researcher, who was funded mainly from this grant, and a graduate student. We worked as part of the Linear Collider Flavour Indentifier collaboration (LCFI) for tasks (1) and (2). We collaborated with colleagues from the University of Bristol for task (3). Further work is needed in developing high performance silicon sensors which can be used at the ILC, but if they can be developed, we found that we can distinguish between bottom and charm quarks well enough to measure the decay rate of the Higgs to bottom quarks to 6% accuracy, and Higgs to charm quarks at 30% accuracy, for a given mass of the Higgs boson. This would allow a reasonable test of the key hypotheses that the interactions Higgs boson with quarks give rise to the mass of the quarks.