[Paper Review] PTOLEMY: A Proposal for Thermal Relic Detection of Massive Neutrinos and Directional Detection of MeV Dark Matter

E. Baracchini, Maria Grazia Betti+49 more|arXiv (Cornell University)|Aug 6, 2018

Superconducting and THz Device TechnologyPhysics and Astronomy47 references64 citations

TL;DR

PTOLEMY proposes a proof-of-principle underground experiment to directly detect the Cosmic Neutrino Background using a graphene-based tritium target combined with a high-resolution TES calorimeter, and to explore directional MeV dark matter detection with graphene.

ABSTRACT

We propose to achieve the proof-of-principle of the PTOLEMY project to directly detect the Cosmic Neutrino Background (CNB). Each of the technological challenges described in [1,2] will be targeted and hopefully solved by the use of the latest experimental developments and profiting from the low background environment provided by the LNGS underground site. The first phase will focus on the graphene technology for a tritium target and the demonstration of TES microcalorimetry with an energy resolution of better than 0.05 eV for low energy electrons. These technologies will be evaluated using the PTOLEMY prototype, proposed for underground installation, using precision HV controls to step down the kinematic energy of endpoint electrons to match the calorimeter dynamic range and rate capabilities. The second phase will produce a novel implementation of the EM filter that is scalable to the full target size and which demonstrates intrinsic triggering capability for selecting endpoint electrons. Concurrent with the CNB program, we plan to exploit and develop the unique properties of graphene to implement an intermediate program for direct directional detection of MeV dark matter [3,4]. This program will evaluate the radio-purity and scalability of the graphene fabrication process with the goal of using recently identified ultra-high radio-purity CO2 sources. The direct detection of the CNB is a snapshot of early universe dynamics recorded by the thermal relic neutrino yield taken at a time that predates the epochs of Big Bang Nucleosynthesis, the Cosmic Microwave Background and the recession of galaxies (Hubble Expansion). Big Bang neutrinos are believed to have a central role in the evolution of the Universe and a direct measurement with PTOLEMY will unequivocally establish the extent to which these predictions match present-day neutrino densities.

Motivation & Objective

• Demonstrate direct detection of the Cosmic Neutrino Background via neutrino capture on tritium using a graphene-based target.
• Develop and validate a high-resolution (≤0.05 eV) TES calorimeter coupled to a precision HV system for endpoint electron energy measurement.
• Establish a low-background, ultra-radio-pure graphene target and electromagnetic filtering to suppress backgrounds.
• Explore directional detection of MeV-scale dark matter using graphene-based and CNT-based detector concepts.
• Assess scalability toward a full-scale Cosmic Neutrino Telescope and pave the path for a global deployment of multiple telescopes.

Proposed method

• Use a graphene-coated tritium target to enable neutrino capture on tritium and detect the mono-energetic electron signal above the beta-decay endpoint.
• Employ a Transition Edge Sensor (TES) calorimeter with sub-0.05 eV energy resolution for low-energy electrons.
• Implement a MAC-E style electromagnetic filter to achieve ~1% energy resolution in the filtering process.
• Develop high-stability, precision HV controls and an RF triggering system to identify single electrons in coincidence with energy measurements.
• Undertake underground, low-background operations at LNGS to validate background levels and background suppression strategies.
• Pursue graphene-based MeV dark matter directional detection through PTOLEMY-G3 (G-FET sensors) and PTOLEMY-CNT concepts to assess sensitivity and angular information.

Experimental results

Research questions

• RQ1 Can the CNB be directly detected via neutrino capture on tritium with a graphene-target setup at LNGS?
• RQ2 What is the achievable energy resolution and background level required to separate CNB signal from tritium beta-decay end-point background?
• RQ3 How does a high-purity graphene target impact 14C and other backgrounds relevant for CNB measurements?
• RQ4 Is it feasible to achieve directional detection for MeV-scale dark matter using graphene-based sensors or carbon nanotube targets?
• RQ5 What is the pathway to a scalable, full-scale Cosmic Neutrino Telescope architecture based on the prototype results?

Key findings

• Energy resolution better than 0.05 eV demonstrated for low-energy electrons with TES calorimetry (prototype goal).
• Underground LNGS deployment planned to achieve ~10^6 background reduction factor for CNB-relevant measurements.
• PTOLEMY-G3 targets sensitivity around rac{\bar{\sigma}_e}{ }\sim 10^{-33} cm^2 for MeV dark matter at 4 MeV with a 1e3 cm^3 fiducial volume, illustrating directional detection potential.
• PTOLEMY-CNT concept shows potential reach of \bar{\sigma}_e ~ 10^-37 cm^2 for 5 MeV DM in appropriate exposure, leveraging CNT geometry for directional information.
• High radio-purity graphene targets and CO2-derived graphene production are being developed to suppress 14C backgrounds to levels suitable for CNB and MeV-DM searches.