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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - CHROMIUM (CHROMIUM)

Teaser

Summary

The recent discovery of the Higgs particle has left us in no doubt that the Standard Model (SM) of particle physics is correct in very large part.
This theory has managed to describe almost everything we see involving three of the four known forces of nature: the Strong, Weak and Electromagnetic forces.
However, another slightly less recent discovery, that of neutrino mass, is in tension with the SM\'s successes. In order for the Weak force to be properly described in the theory,
neutrinos were assigned exactly zero mass: however it has been proved that while their masses are very small, they are definitely not zero. Over the last few decades, particle physicists mounted a number of experiments looking for, and finally measuring, a quantity known as Charge Parity (CP) violation in sub-atomic particles called quarks. This manifests itself as a difference in interaction rates between matter and antimatter particles and the reason for looking for this difference was an attempt to explain what happened to the anti-matter which must have existed in the early universe. The simple explanation was that as the Universe expanded, transitions between matter and anti-matter were stopped at some energy density and over time, the small excess of matter that had been produced led to the total annihilation of all the antimatter, leaving just a small amount of matter which is what we see around us in the Universe. Unfortunately, the CP violation observed in the quark sector was much too small .
However, the discovery of neutrino mass gives rise to the possibility of a mechanism which can explain the matter that we see.
The overarching goal of this proposal is to measure, or constrain further, the neutrino CP violating angle dcp,
by measuring the size of the difference between the quantum mechanical flavor oscillations of neutrinos and anti-neutrinos, taking advantage of a
unique, and manifestly overlooked opportunity provided by the NuMI muon neutrino beam, produced at the Fermi National Accelerator Laboratory.

Neutrinos only interact via the Weak Interaction, and this produces an exceptionally acute problem for precision measurements. As the needed precision
increases, so too does the detector mass, until a statistically significant improvement over existing measurements requires either decades of data taking
or a detector that up to now would be beyond acceptable cost. There is a new effort in the USA to develop a $600M experiment to search for this effect along with many
other potentially naturally occurring processes such as proton decay and supernova, in a planned future neutrino beam. This could provide results at the earliest in 2030.
Our approach is to use the existing neutrino beam and to build a vastly cheaper detector with the sole purpose of measuring the beam neutrinos.

The novel detector concept is named CHIPS, for Cherenkov detectors In PitS which will be located in the Wentworth 2W flooded taconite pit near Aurora, Minnesota. This pit intersects the NuMI neutrino beam. The concept pushes on costs of the detector by using the natural body of water to support the detector volume, avoiding a very strong and costly mechanical structure. It uses the water overburden to shield from cosmic rays, making use of the time window that the beam is delivered and the knowledge of the neutrinos direction, to avoid having to be positioned under a very large overburden of rock. The first year will deploy a 20~kt volume vessel. Neutrinos from the NuMI beam interact in the large volume of water and produce charged particles which in turn produce Cherenkov radiation in the water volume. This is predominantly blue light which can be detected by sensitive light detectors called Photo-Multiplier Tubes (PMTs). Funds from Templeton Foundation could enable the full instrumentation of the vessel within 2 years, enabling physics measurements to start very quickly. The NuMI beam will deliver neutrinos for the next 7-8 years and so time is of t

Work performed

\"The global project has accelerated over the past year in order to try to make up the lost time from the grant start. The detector can only be deployed seasonally and we did not succeed with the 2018 deployment before season end October 1st. There are several work strands where there has been progress, and several progressional developments outside of the ERC-funded part of the detector which are important.

section{ Design and Simulation : Work Strand 1}
Final design has been verified by detailed simulations. This has included overall layout and full simulation of the light cones (to increase effective photocathode area). Deep learning techniques have been applied to our reconstruction which have resulted in significant gains in efficiency, as shown in the Figures called vtxX_energy.png, vxY_energy.png, vtxZ_energy.png and energy_energy.png.

{PMT Plane Design - work strand 2}.
The original design outlined in the ERC proposal called for 31 PMTs per plane, the plane was constructed of perspex sheets as shown in Figure~ref{fig:nikhefplane}(left). During the summer of 2015 it was concluded that this was not an optimal design due to difficulty with maintaining water tightness across the large number of flat joints. A different approach was developed on the fly during that prototype deployment, shown in Figure ref{fig:nikhefplane} (right) which has been carried through to the final design. The design and specifics of the detector planes are complete. The design of 4 types of planes is complete and construction of the planes has been completed. Factories were set up in Minnesota and in Wisconsin. 3500/5500 PMTs and bases for the front and end-cap sections have been delivered and potted and all other subsystems for detector planes have been delivered. 10 planes have been fully tested with readout during the winter months in a dark room at the site. Data shows dark rates of the PMTs is acceptable reaching about 100Hz/tube at 4C. All the endcap planes have been constructed, cabled and water tested. A small-format White Rabbit (WR) board has been developed at Nikhef for insertion into the central water tight container shown in the figure PK-switch.png.
We originally foresaw veto PMTs in the top and side \"\"veto-volumes\"\" to aid with cosmic ray rejection. However, it was realised that not only did that require at least an extra 2m in diameter for the whole structure along with the associated costs but it would increase the complexity of the water circulation system as well as the construction itself. The main use of veto counters will be on the top-cap of the detector to veto cosmic rays entering the top of the detector.

{bf Veto and Rear PMTs - work strand 3}. A design based on the front plane design has been developed using borrowed PMTs from our French colleagues. The electronics for these PMTs has been developed from scratch using innovative design using components that are commercially available and present in many mobile phones. The rear wall instrumentation was originally foreseen to be 100 large PMTS, instrumented with the PARISROC readout system for which there were funds for hardware and technical effort at UCL. The PMTs were to be donated by UCL. However, we were lent 500 3\"\" Hamamatsu R6091\'s from our colleagues from the NEMO-3 collaboration and together with
electronics developed at UW we have developed a novel idea of a distributed readout system for particle physics based on commercially available electronic components. There are three distinct aspects of this hardware development: a small-format Cockroft-Walton (CW) positive HV generator base (based on a design from the COUPP experiment at FNAL) to drive the PMTs; a \"\"microdaq\"\" board which contains a STM32F446 microprocessor and sits on the PMT, delivering self-triggered time-over-threshold information to the single-board computer as well as control signals to the CW board; and the WR signal fan-out board to deliver the absolute time to every microdaq. The CW is contr\"

Final results

The development of the electronics for the loaned PMTs has produced an unforseen innovation.
The idea is for a distributed system that can be used for large arrays of PMTs based on commercially available electronic components. It can be seen as part of a movement to distribute intelligence towards the detector, a natural way forward as electronic components gets cheaper and smarter. This approach of the microprocessor on the PMT has been pioneered at UW, with a more sophisticated microDAQ prototype (V2) deployed at the South Pole this winter (2017/18) for IceTop tests.
The MicroDAQ relies on commercially tested components and could potentially be sold as a commercial system with the timing fan-out and CW base. The CW base is not necessary, but can easily be modified for different PMTs if needed as it works together and is controlled by the MicroDAQ.