Development of the TORCH Time-of-Flight Detector

This proposal describes a two-year research and development programme towards developing a time-of-flight (ToF) detector, named TORCH, for future particle physics experiments. TORCH uses a novel technique to measure the arrival time of a charged particle which passes through it. Because the ToF is proportional to the particle's velocity, the particle type (pion, proton or kaon) can be determined from knowledge of the particle's momentum, measured from the particle's bend in the experiment's magnetic field. TORCH uses a plane of 10 mm thick quartz as a source of Cherenkov light. The photons are emitted when a charged particle traverses a medium in which the particle's velocity exceeds that of light in the medium. The photons radiated in the quartz plate propagate by total internal reflection to the top edge of the plate and are then focused onto an array of Micro-Channel Plate (MCP) photon detectors. A ToF resolution of better than 15 ps per track is to be achieved, which will result in a pi/K/p particle separation up to a momentum of 10 GeV/c and beyond.

A TORCH module will be constructed from a quartz radiator bar and focussing block, to which the MCP detectors will be mounted. The module will then be tested in a charged particle beam at CERN. Software will be written to reconstruct the detected photons, to evaluate their arrival times, and then to calculate the time of flight of the incident particle which radiated them. TORCH will then be moved to the LHCb experiment, to perform a full system test to verify the timing capabilities for charged tracks and photons.

There are several important benefits that this research will bring. The TORCH detector will have a unique capability to provide ToF to experiments where particle identification (PID) is paramount. As an example, the LHCb Upgrade is a so-called flavour-physics experiment which relies heavily on the ability to distinguish all particles in the collision. Also, future experiments at the FCC-ee machine, proposed as a possible European accelerator after the LHC, would also highly benefit from PID measurements made in a compact spatial volume that TORCH facilitates. TORCH can also be used for a ToF detector which will associate charged particles and high-energy gammas with the primary collision point (vertex) from which they originate, differentiating their source from the several vertices present in the crossing of LHC beam bunches.

A key physics aim of future flavour-physics experiments is to understand the origin of CP violation, which remains one of the mysteries of experimental particle physics and is a key piece in the puzzle of the matter-antimatter asymmetry in the universe. New generations of experiments will exploit the large number of B-hadrons (particles containing a b-quark), produced in order to investigate matter-antimatter asymmetries of rare B-hadron decays. Hence TORCH will enable the study of new and unexpected physics, and which can represent a huge step forward in our understanding of nature.

Finally, the TORCH proposal relies heavily on the R&D of novel MCP photon detectors, which are developed by TORCH in collaboration with industry. The MCPs have been quantified for their good lifetime, excellent timing resolution (~20-30 picoseconds) and spatial accuracy. The MCPs are promising detectors for use in particle physics, astro-particle physics experiments, and also space and ground-based astronomy projects. They also have applications in medical imaging, eg. positron emission tomography (PET), and many other applications that require detection of low levels of light with good spatial accuracy and timing.

Economic impact: The collaboration of TORCH with Photek UK to develop Micro Channel Plate Photomultiplier tubes (MCP-PMTs) to our specific specification will result in the manufacture of new type of photon detectors in the commercial sector. These tubes will have the smallest granularity currently on the market: 64 x 64 pixels over an area of 2 x 2 inch2. They will have good tolerance (>5 Ccm-2) to high photon exposure using atomic-layer deposition techniques, and excellent timing resolutions below 30 ps. We will work with Photek to develop a commercially cheap and robust MCP-PMT that can be easily deployed in applications requiring characteristics of speed and high spatial resolution for research areas beyond particle physics and also the commercial sector. As well as benefitting particle physics projects that need single-photon detection, the photodetectors have applications in fields such as positron emission tomography (PET), bringing benefit for the National Health Service. Space and cosmic-shower detection will benefit from MCP-PMTs which detect low levels of light with good spatial accuracy over visible wavelengths.
We will develop optical systems for the TORCH readout system, working closely with companies like Nikon, Japan, to produce cheap optical components (the quartz radiator plate and focusing block). We will also encourage UK companies to become engaged in this programme.

Dissemination of the science and technology: For dissemination of our scientific results, we will contribute to technology conferences (TIPP, RICH, ICHEP, Frontiers in Instrumentation etc) which bring together academia and industry. We will publish in scientific journals such as Nuclear Instruments and Methods and Journal of Instrumentation.

Public Engagement/Outreach: We are fully committed to public outreach and dissemination and we will continue to enthuse the general public in our achievements, and in the goals and aspirations of STFC science. We have made several contributions on local radio stations and World Service promoting our science. We also disseminate our science directly via seminars, masterclasses, café scientifiques and non-university societies. We will continue to participate in events such as the Royal Society summer exhibition, for which our most recent "Antimatter matters" exhibit was extremely successful.

Societal Impact: We have an excellent track record in training generations of graduate students, now employed in diverse industrial occupations, commerce or academia. A future TORCH detector will provide an ideal training ground in photon technology, optics, fast electronics and scientific computing for students, engineering and technical staff.
Finally, a TORCH detector could facilitate the discovery of new unexpected phenomena, for example in flavour physics, that would represent a huge step forward in our understanding of nature. Society in general would hugely benefit from such a discovery, and a new generation of young people would be enthused to study physics.