CosmoPhys

Slides from talk by Chris Tully at the Opening New Windows to the Universe forum at Brookhaven National Labs on 2021-11-03: PTOLEMY: Relic Neutrino Detection New developments since this post was originally created: Cosmologist Sunny Vagnozzi shares some updated info on the PTOLEMY project status in a short thread here. He also mentions that he is collaborating on a paper about PTOLEMY with Stefano Gariazzo that will soon (as of March 2020) be posted on the arXiv. -—

This is about paper 1902.05508, posted to the arXiv on 2/14/2019.

I was mostly unfamiliar with this groundbreaking project so this new paper provided a reading-up opportunity, leading to these general overview notes. I added the bolding for emphasis.

The PTOLEMY project¹ aims to directly detect relic neutrinos from the cosmic neutrino background (CNB or CνB), along with a impressive broader set of capabilities or opportunities^2a. The project is described as the “the first of its kind and the only one^2b conceived that can look directly at the image of the Universe encoded in neutrino background produced in the first second after the Big Bang”.³ (pg2) Achieving the project's goal “would profoundly confront and extend the sensitivity of precision cosmology data.”(pg5) This paper addresses the theoretical aspects of the project, its physics goals, and an outline of the project's scope of work to be done in the next three years. An earlier paper 1808.01892 gives more details on three phases of the project: proof-of-principle demonstrator, scalable prototype realization and tests, and full detector construction.

The technology is based on neutrino capture on beta decaying nuclei (NCB)⁵, with tritium (³H) determined as the best choice. The capture results in a tiny boost of energy to the electrons emitted in tritium decay, so there'll be a peak in the electron spectrum above the β-decay endpoint⁴. The planned target is ∼100g of tritium atomically bound to a radio-pure graphene substrate (they refer to it as “tritiated graphene”). They expect ∼10 CνB capture events per year, depending on the mass hierarchy and the Dirac versus Majorana nature of the neutrinos; the rate is half as large for non-relativistic Dirac neutrinos^2a. The anticipated energy resolution is ∼0.05eV, “an order of magnitude beyond the original target and the highest resolution of any calorimeter.” [source]

This paper was reviewed prior to release at an international conference⁶ of principal collaborators in Nov 2018. Chris Tully leads the experimental effort⁷. A 2016 article about him and this project is here.

I searched for a project website but did not find one. For supplemental details on this project, there's a video of a Chris Tully talk. Tully gives an indication of the magnitude of the effort at this point near the end of the talk⁹. Slides from another of his talks are here.

The eventual plans are for a world-wide array of detectors to reach discovery sensitivity for the CNB. The number and deployment will depend on the next phase of PTOLEMY developments after the 3-year study described in the two papers. ___ Note: page numbers in the footnotes refer to pages in the PDF, not page numbers as shown on the page footer

¹ The acronym stands for the PonTecorvo Observatory for Light, Early-universe Massive-neutrino Yield. Originally the facilities were at Princeton, with the leading P and T stand for Princeton Tritium but that was changed with the move to LNGS. (I'm not clear on whether the move is planned or actually underway or even completed. I found nothing online about that.) The meeting room for the recent conference⁶ was in the Pontecorvo Room at LNGS. Chris Tully has a humorous anecdote about his renaming dilemma in this video clip.

^2a Broader capabilities or potential opportunities include (1) MeV dark matter searches which are anticipated to begin at the end of 2019 with data collection through 2020 [1808.01892, pg6], (2) checking the neutrino mass hierarchy “from a different and independent approach than oscillation experiments” (pg20), (3) measuring the standard neutrino absolute mass scale (pg20), (4) determining the neutrino mass ordering (pg12), and (5) checking for the “scenario of an extra sterile neutrino with mass in the eV range coupled to the active states via oscillations” (see section 6 beginning on pg 15). There's a discussion on pg7 about the capture event count rate being different (~double) for non-relativistic Majorana neutrinos vs Dirac, along with a discussion of whether the experiment could distinguish between the two cases (characterized as “difficult to achieve”).

^2b “Despite the demanding technological issues in dealing with a large tritium mass, as well as in achieving high energy resolutions (which, however, are not so far from present values), we note that there are no other feasible approaches to directly unveil the CNB. The only other plausible one, the Stodolsky effect, is much more challenging to be detected, if not simply impossible, if cosmological neutrinos have zero (or exceedingly small) chemical potential.” (pg20)

³ Neutrinos decoupled ~1 second after the end of inflation, being in thermal equilibrium prior to that. [neutrino decoupling] In contrast, photons did not begin freely streaming until ~380,000 years later, prior to that they were constantly scattered by free electrons until the universe cooled enough for neutral atoms to form.

⁴ For more in-depth details on ³H beta decay and NCB, see [pg12 in PTOLEMY: A Proposal for Thermal Relic Detection of Massive Neutrinos and Directional Detection of MeV Dark Matter 1808.01892, slides 4 – 6 in footnote⁸, and footnote⁵. Wikipedia also has a quickie overview.

⁵ For more details on NCB, see Measuring anisotropies in the cosmic neutrino background (PhysRevD.90.073006)

⁶ The International Conference on Cosmic Neutrino Background Detection and Dark Matter Searches with PTOLEMY. Slides and materials from that conference are available at the link.

⁷ Tully and Marcello Messina are the spokespersons at the top of the project's organization chart.⁸ (slide 39 in footnote 8)

⁸ See slides from Messina's talk at the 2018 conference event⁶ site: The PTOLEMY project – from an idea to a real experiment for detecting Cosmological Relic Neutrinos.

⁹ “I remember giving a talk at Stony Brook a couple years back and a good colleague of mine Paul Grannis said 'look, your experiment involves atomic physics, nuclear physics, condensed matter physics, cosmology particle physics, you know, you and what army is going to take on this.' And you know if anything maybe my role is to show that there's something worth fighting for but I think that I see you know around the world there's a growing and growing larger number of people who are interested in tackling these problems and so I wouldn't be surprised that you see a lot of these observatories springing up. I think that this idea of starting with the light Dark Matter scattering is a very promising way to get that going.”

Tags: #cosmology #neutrinos #physics