• cmb
• cosmic rays
• LiteBIRD
• noise
• polarization

Photons of the Cosmic Microwave Background (CMB) have been the subject of cosmological study since they were discovered by Penzias and Wilson in 1965, as they offer insight on the state of the early Universe. These photons, in fact, last scattered off electrons about 300000 years after the Big Bang, at a redshift z~1100 and have since traveled freely through Universe, preserving the characteristics they had at the time of last scattering. In particular, the mapping of temperature fluctuations and polarization of photons is of interest for CMB experiments.
The CMB features a practically perfect black body spectrum with a temperature of ~2.725 K which is equal throughout the Sky to better than a part in 10^4. The most convincing explanation for this phenomenon is in support of a cosmological model that is nowadays widely accepted: the inflationary model.
The study of polarization aims at confirming the inflationary model through the detection of the characteristic polarization pattern of primordial gravitational waves, that is the B modes. As a matter of fact, the CMB polarization can be analyzed in terms of E and B modes , that can be derived from the Stokes parameters through a linear transformation.
While the mapping of temperature fluctuations in the CMB renders a frozen image of the Universe at decoupling, the CMB polarization yields information further back in time, up to 10^-36 - 10^-32 seconds after the Big Bang. The CMB polarization experiments can be ground-based, balloon-borne or satellites. The next frontier for Space missions is represented by LiteBIRD, an international collaboration, led by the Japanese Aerospace Exploration Agency (JAXA), which is scheduled to be launched in the late 2020s and is intended to perform its measurements on the span of three years, when it will map, from the Lagrange point L2, polarized fluctuations in the CMB in search of B modes. It will carry three telescopes: the ”Low-Frequency Telescope” (LFT), the ”Mid-Frequency Telescope” (MFT) and the ”High-Frequency Telescope” (HFT) and it will deploy bolometers whose temperature variations induced by a deposited optical power will be measured by Transition Edge Sensors (TES). LiteBIRD will have the scientific goal of measuring the CMB polarization with a sensitivity <0.001 on the tensor to scalar ratio, r. In general this kind of measurements are very challenging, because the current instrumentation is so sensitive that the studies are limited both by instrumental and by physical effects like the incidence of cosmic rays. In L2 the satellite will be exposed to a particularly intense primary cosmic rays flux during the span of the mission, due to the modulation induced by the solar cycle, which will be at its minimum in those years, allowing for a greater flux of primary cosmic rays (mainly protons of ~ GeV energy) to penetrate the heliosphere. The cosmic rays noise deserves to be carefully investigated because it has proved to be particularly challenging even for past missions, such as Planck. This thesis aims at addressing this problem for the HFT telescope. In this regard, a program was developed that simulates the operation of an HFT TES by receiving in input the optical power and the cosmic rays noise. Optical power data were extracted from a CMB temperature fluctuations map by simulating LiteBIRD scanning strategy via the LiteBIRD Simulation Framework, which is a set of Python modules that are being developed to model the instruments onboard. The cosmic rays noise, on the other hand, was generated through a Geant4 Monte Carlo simulation that reproduced the energy loss of protons of kinetic energy between 1 and 3 GeV in the TES and in its silicon wafer. Among other quantities, the TES program returned a current signal which was initially decimated through a CIC filter algorithm, then converted to power units and used to obtain a Noise Equivalent Power (NEP) estimate. Although the obtained results depend on some characteristic parameters of the TES that have been fixed in the program, these can be modified if necessary, therefore the developed program represents a versatile tool for assessing the entity of cosmic rays noise in LiteBIRD TES bolometers.