D. Veberič

Search for dark matter in the hidden-photon sector with a large spherical mirror

If dark matter consists of hidden-sector photons which kinetically mix with regular photons, a tiny oscillating electric-field component is present wherever we have dark matter. In the surface of conducting materials this induces a small probability to emit single photons almost perpendicular to the surface, with the corresponding photon frequency matching the mass of the hidden photons. We report on a construction of an experimental setup with a large ~14 m2 spherical metallic mirror that will allow for searches of hidden-photon dark matter in the eV and sub-eV range by application of different electromagnetic radiation detectors. We discuss sensitivity and accessible regions in the dark matter parameter space.

A likelihood method to cross-calibrate air-shower detectors

Abstract We present a detailed statistical treatment of the energy calibration of hybrid air-shower detectors, which combine a surface detector array and a fluorescence detector, to obtain an unbiased estimate of the calibration curve. The special features of calibration data from air showers prevent unbiased results, if a standard least-squares fit is applied to the problem. We develop a general maximum-likelihood approach, based on the detailed statistical model, to solve the problem. Our approach was developed for the Pierre Auger Observatory, but the applied principles are general and can be transferred to other air-shower experiments, even to the cross-calibration of other observables. Since our general likelihood function is expensive to compute, we derive two approximations with significantly smaller computational cost. In the recent years both have been used to calibrate data of the Pierre Auger Observatory. We demonstrate that these approximations introduce negligible bias when they are applied to simulated toy experiments, which mimic realistic experimental conditions.

An integrated general purpose SiPM based optical module with a high dynamic range

Silicon photomultipliers (SiPMs) are semiconductor-based light-sensors offering a high gain, a mechanically and optically robust design and high photon detection efficiency. Due to these characteristics, they started to replace conventional photomultiplier tubes in many applications in recent years. This paper presents an optical module based on SiPMs designed for the application in scintillators as well as lab measurements. The module hosts the SiPM bias voltage supply and three pre-amplifiers with different gain levels to exploit the full dynamic range of the SiPMs. Two SiPMs, read-out in parallel, are equipped with light guides to increase the sensitive area. The light guides are optimized for the read-out of wavelength shifting fibers as used in many plastic scintillator detectors. The optical and electrical performance of the module is characterized in detail in laboratory measurements. Prototypes have been installed and tested in a modified version of the Scintillator Surface Detector developed for AugerPrime, the upgrade of the Pierre Auger Observatory. The SiPM module is operated in the Argentinian Pampas and first data proves its usability in such harsh environments.

Heitler-Matthews model with leading-particle effect

It is well known that the Heitler model and its extensions (like the Heitler-Matthews model) describe qualitatively many fundamental properties of extensive air showers initiated in the atmosphere by the high-energy cosmic rays. Typically, only the secondary particle multiplicity and the fraction of neutral-to-charged pions are considered as analytically treatable parameters of hadronic interactions. Other relevant parameters of hadronic interactions such as inelasticity and the lifetime of the leading particle are taken into account only in detailed Monte Carlo simulations. Here we present an extension of the Heitler-Matthews model that includes the leading particle effect and analytically derive the number of muons produced in air showers in a self-consistent way. In a second step we apply this model to calculate the dependence of the depth of the shower center-of-gravity on various model parameters. In this way, we obtain predictions for the depth of shower maximum, which is a well known observable sensitive to the mass of primary particles and is regularly used in cosmic-ray research.

SiPMs – A revolution for high dynamic range applications

In a wide range of applications, semi-conductor photo sensors (SiPMs) are increasingly replacing classical photo multiplier tubes (PMT). They have the advantage of an easier handling due to their significantly lower bias voltage and a long life time without aging. Usually, detectors need an adapted design for the application of SiPMs due to their smaller size compared to PMTs. n nWhile the linear dynamic range of a PMT is inherently limited and usually depends strongly on the individual PMTs, SiPMs promise a dynamic range which only depends on the SiPM type applied and not on the individual sensor. n nSiPMs are compiled from individual Avalanche Photo Diodes operated in Geiger-mode (G-APD). Every of these diodes is only capable of the detection of a single photon at a time. Thus, the number of G-APDs inherently limits the dynamic range of a SiPM. Strictly speaking, a SiPM is non-linear starting from the first detected photon. If this non-linearity is taken into account, the dynamic range for todays sensors can reach 10^6 for coincident photons. A complication arises for extended pulses from the fact that typical re-charge times of individual cells are in the order of several nano-second. n nWith a 3.8 sqm scintillator detector developed for the upgrade of the Pierre-Auger Observatory, it has been shown that SiPMs can nowadays act as an ideal replacement even in applications which require a high dynamic range. This has been successfully proven by operating two identical detectors on top of each other, one read out with SiPMs and one by a PMT. It is demonstrated that even at very strong illumination the SiPM response is still understood. Furthermore, laboratory measurements confirm that individual sensors are, within the systematic errors, exhibiting identical response. Given the precision of the devices and their advantages in operation, including the possibility of characterizing their response during measurement without any additional calibration device, the application of SiPMs will be a revolution for high dynamic-range applications, significantly reducing systematic uncertainties due to improved stability.