The AQUA-RICH project

Abstract

The AQUA-RICH project, a long baseline and atmospheric neutrino experiment proposed by Tom Ypsilantis during the last decade of the last century, is briefly reviewed. The detector proposal is discussed within the context of the searches for neutrino oscillations at that time and as part of the longstanding seminal ideas and key contributions provided by Tom Ypsilantis in the development of the RICH technique.

Introduction

AQUA-RICH [1], [2], [3], [4], [5] was one of the last projects proposed by Tom Ypsilantis during his long career. For this effort he served also as senior director of research at INFN Bologna during the second half of the ’90s. Even if still engaged in important work for the future LHCb detector at the LHC, during the ’90s the study of the neutrino oscillations attracted increasingly Tom’s interest. It was a field developing rapidly in those years, reaching first clear experimental evidences.

At the time the first proposal of the AQUA-RICH detector was circulated in a written form, in 1996, the Super Kamiokande detector was starting operations. The Nobel Prize for neutrino oscillation discovery was awarded in 2015 to Takaaki Kajita and Artur Mac Donald for their work studying atmospheric and solar neutrinos, with experiments – Super-Kamiokande and SNO – taking data since the second half of ’90s. Remarkably, in those years, Tom Ypsilantis was indeed proposing innovative detectors for the study of solar and atmospheric neutrinos. His HELLAZ proposal [6] was a track projection chamber based on 2000 cubic meters of a cold helium-methane gas mixture to detect pp and ${}^7$Be neutrinos from the Sun. The AQUA-RICH proposal, discussed in this paper, aimed to measure neutrino oscillations with high accuracy in a large water tank, through the emission of Cherenkov light. The detector was suitable both for detection of atmospheric neutrinos or from a beam source. The real innovative part, with respect to all other massive neutrino water Cherenkov detectors – as Super-Kamiokande, or its predecessor Kamiokande – is that the AQUA-RICH was proposed with a focusing mirror technique. The AQUA-RICH proposal was keeping together Tom’s growing interest for neutrino physics and, at the same time, one of his main contributions to experimental physics, that is the introduction of new type of optics, that allows for measurements of focused Cherenkov images over a large solid angle. The invention of Cherenkov RICH imaging dates back to 1977 [7]: almost twenty years later the same two authors, Tom and Jacques Séguinot, were proposing a giant application of their seminal idea to study neutrino oscillations.

The original proposal for a large water Cherenkov detector, authored by Tom Ypsilantis, Jacques Séguinot and Antonino Zichichi, is in a paper [1] summarizing a lecture given by Tom in 1995 at the SLAC Summer Institute on Particle Physics. During 1995 a similar talk was also given by Tom at the International Workshop on Ring-Imaging Cherenkov Detectors, in Uppsala. The nickname “AQUA-RICH” was not yet proposed, but the purpose and the main idea of the project were crystal clear from the title: “A Long Baseline RICH with a 27-Kiloton Water Target and Radiation for Detection of Neutrino Oscillations”. Later in 1996, a paper on Nuclear Instruments and Methods, signed by Tom and other collaborators [2] as proceedings of the Uppsala talk, further detailed the idea, presenting some results of early simulations. Behind the water radiator, in this design a 20 m thick gas RICH was also proposed to measure momentum of high energy muons. In January 1996 a letter of intent [3] was submitted to the INFN Gran Sasso Laboratories (LNGS) on the occasion of their call for long baseline proposals. However, and interestingly, an initial proposal for a long baseline neutrino detector, based on the RICH technology but using argon gas as target and radiator, dates back to 1994 [8].

This paper is organized as follows. Section 2 briefly summarizes the status of the atmospheric neutrino oscillations studies at the mid of the ’90s of the previous century (for a full review see for example [10]) and the rationale of the innovative AQUA-RICH proposal in that context: several projects were submitted at that time as part of the effort to set up a research program associated to a long baseline neutrino beam from CERN to the LNGS. In Section 3 the actual proposal is then reviewed, covering its main ideas, the foreseen parameters of the final detector in its variants and its expected performance. A brief review of the main critical points of the proposal and its legacy are then described.