Nucleation efficiency of nuclear recoils in bubble chambers

Bubble chambers using liquid xenon (and liquid argon) have been operated (resp. planned) by the Scintillating Bubble Chamber (SBC) collaboration for GeV-scale dark matter searches and CEνNS from reactors. This will require a robust calibration program of the nucleation efficiency of low-energy nuclear recoils in these target media. Such a program has been carried out by the PICO collaboration, which aims to directly detect dark matter using C3F8 bubble chambers. Neutron calibration data from mono-energetic neutron beam and Am-Be source has been collected and analyzed, leading to a global fit of a generic nucleation efficiency model for carbon and fluorine recoils, at thermodynamic thresholds of 2.45 and 3.29 keV. Fitting the many-dimensional model to the data (34 free parameters) is a non-trivial computational challenge, addressed with a custom Markov Chain Monte Carlo approach, which will be presented. Parametric MC studies undertaken to validate this methodology are also discussed. This fit paradigm demonstrated for the PICO calibration will be applied to existing and future scintillating bubble chamber calibration data.

The First Droplet in a Cloud Chamber Track

In a cloud chamber, the quantum measurement problem amounts to explaining the first droplet in a charged-particle track; subsequent droplets are explained by Mott’s 1929 wave-theoretic argument about collision-induced wavefunction collimation. I formulate a mechanism for how the first droplet in a cloud chamber track arises, making no reference to quantum measurement axioms. I look specifically at tracks of charged particles emitted in the simplest slow decays, because I can reason about rather than guess the form that wave packets take. The first visible droplet occurs when a randomly occurring, barely-subcritical vapor droplet is pushed past criticality by ionization triggered by the faint wavefunction of the emitted charged particle. This is possible because potential energy incurred when an ionized vapor molecule polarizes the other molecules in a droplet can balance the excitation energy needed for the emitted charged particle to create the ion in the first place. This degeneracy is a singular condition for Coulombic scattering, leading to infinite or near-infinite ionization cross sections, and from there to an emergent Born rule in position space, but not an operator projection as in the projection postulate. Analogous mechanisms may explain canonical quantum measurement behavior in detectors such as ionization chambers, proportional counters, photomultiplier tubes or bubble chambers. This work is important because attempts to understand canonical quantum measurement behavior and its limitations have become urgent in view of worldwide investment in quantum computing and in searches for super-rare processes (e.g., proton decay).

Non-electronic detectors

This chapter presents the non-electronic detector types cloud chamber, bubble chamber and photoemulsions with which the trajectories of ionizing particles can be made visible. Of these ‘classical’ detectors cloud and bubble chambers have today no or at most only minor relevance in research because of their relatively cumbersome data acquisition. However, photoemulsions–despite their laborious data analysis–are still employed in modern experiments when it comes to achieving position resolutions in the micrometer regime. Therefore deployment and analysis of photoemulsions are described in some more detail. Cloud chambers are today only used for demonstration purposes to make radioactivity and cosmic radiation visible. Bubble chamber pictures are frequently drawn on to display reaction chains and event topologies.

Searching for Dark Matter: Background Discrimination in the PICO detector

The PICO experiment uses superheated bubble chambers located at SNOLAB for direct detection of Weakly Interacting Massive Particles (WIMPs), one of the candidate particles for dark matter. Bubbles form in the detector when a particle interacts with a nucleus of the target fluid, and the recoiling deposits enough energy to nucleate a bubble in the superheated fluid. Much of the data analysis for PICO focuses on determining what type of particle caused a bubble to form. The differentiation is made by analysing signals from pressure sensors, piezoelectric acoustic sensors, and stereoscopic cameras. This talk will present an overview of the sensors and analysis which are used to discriminate between WIMP interactions and background events in the PICO 2L detector, with a focus on the role of image analysis and the potential sensitivity of the detector if good discrimination can be realized.