Spectator Tagging Experiments to Understand the Structure of Matter

How can we measure a sub-atomic particle which only comes into existence for an exceptionally short time and disappears before we can even detect it? This research aims to design innovative technology enabling us to do this, to push beyond current limits in our vital knowledge of the structure of matter.

We are on the brink of a transformation in our capabilities to observe the inner-dynamics of the nucleus, one of the main building blocks of our universe. 99.9% of the mass of the visible universe resides in atomic nuclei, which are composed of protons and neutrons (nucleons). Nucleons themselves have a composite structure, consisting of partons (quarks and gluons).

Hadrons are sub-atomic particles, including nucleons, which are bound together and interact by the fundamental strong interaction. This interaction is vital in many nuclear processes, for example chain reactions in burning stars, or nuclear reactors for energy. Quantum chromodynamics (QCD) is the theory describing the strong interaction. Understanding it will be key to answering some of the most pressing questions in nuclear physics, like "why don't quarks exist freely in Nature and why are they confined inside hadrons?" or "what is the origin of a hadron's mass?".

To understand QCD we must understand the structure of hadrons in terms of their partons. The arrangement of partons inside the nucleon is constantly changing, with hadrons continuously being created and destroyed inside it. About 20% of the time the nucleon exists in a virtual (very short-lived) state in which it is accompanied by a meson. Mesons are the simplest bound hadrons, comprising one quark and one anti-quark. They are the force mediators between nucleons, holding nuclei together, and are valuable virtual laboratories for QCD. Despite significant theoretical advances concerning their structure, experimental data are extremely sparse.

Through this research, UK scientists will lead experiments aiming to scatter high energy electrons from these virtual meson "clouds" of nucleons. This will shed light on the nucleon's elusive meson content and provide access to the composite structure of two different mesons - the pion and kaon. These mesons differ in their quark/anti-quark contents.

Scattering from the virtual meson cloud itself cannot be measured - the meson fragments during the scattering. To successfully select the correct scattering events, its existence must be inferred by measuring the corresponding nucleon recoiling from the interaction, in coincidence with the scattered electron. If the scattering took place from the meson cloud, the nucleon will travel in certain expected directions, with specific energies. This is spectator tagging and offers access to targets not easily found in Nature.

This research will optimise the design of the detector for the recoiling nucleons and operate it in the experiments. The detector will be filled with gas. When charged particles pass through it, a signal is created. The signals will be constantly recorded by fast electronics, to determine the directions and energies of the particles. This will be a ground breaking piece of technology. It must be capable of withstanding, and disentangling, extremely high rates of particles within its active volume (several millions of particles per second).

The experiments will be performed at the recently upgraded Jefferson Lab (USA), offering an unprecedented high intensity electron beam and opening the frontier to these rare scattering processes. The experiments will provide a gateway to a future landscape of spectator tagging experiments to study hadronic structure further. Techniques developed in this field also have potential impact in several applications which benefit society. For example in medical physics or nuclear waste decommissioning, where cutting edge particle detectors are instrumental, as well as knowledge of the detailed computer simulations required to study their implementation.