The final results of the search for the lepton flavour violating decay $$\mathrm {\mu }^+ \rightarrow \mathrm {e}^+ \mathrm {\gamma }$$
 based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling $$7.5\times 10^{14}$$
 stopped muons on target are presented. No significant excess of events is observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of $$ \mathcal{B} (\mu ^+ \rightarrow \mathrm{e}^+ \gamma ) < 4.2 \times 10^{-13}$$
 (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date.

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Search for the lepton flavour violating decay μ + → e + γ with the full dataset of the MEG experiment

The XENON1T experiment is currently in the commissioning phase at the Laboratori Nazionali del Gran Sasso, Italy. In this article we study the experiment's expected sensitivity to the spin-independent WIMP-nucleon interaction cross section, based on Monte Carlo predictions of the electronic and nuclear recoil backgrounds. The total electronic recoil background in 1 tonne fiducial volume and (1, 12) keV electronic recoil equivalent energy region, before applying any selection to discriminate between electronic and nuclear recoils, is (1.80 ± 0.15) · 10(−)(4) (kg·day·keV)(−)(1), mainly due to the decay of (222)Rn daughters inside the xenon target. The nuclear recoil background in the corresponding nuclear recoil equivalent energy region (4, 50) keV, is composed of (0.6 ± 0.1) (t·y)(−)(1) from radiogenic neutrons, (1.8 ± 0.3) · 10(−)(2) (t·y)(−)(1) from coherent scattering of neutrinos, and less than 0.01 (t·y)(−)(1) from muon-induced neutrons. The sensitivity of XENON1T is calculated with the Profile Likelihood Ratio method, after converting the deposited energy of electronic and nuclear recoils into the scintillation and ionization signals seen in the detector. We take into account the systematic uncertainties on the photon and electron emission model, and on the estimation of the backgrounds, treated as nuisance parameters. The main contribution comes from the relative scintillation efficiency Script L(eff), which affects both the signal from WIMPs and the nuclear recoil backgrounds. After a 2 y measurement in 1 t fiducial volume, the sensitivity reaches a minimum cross section of 1.6 · 10(−)(47) cm(2) at m(χ) = 50 GeV/c(2).

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Physics reach of the XENON1T dark matter experiment

This article reviews the progress made over the last 20 years in the development and applications of liquid xenon detectors in particle physics, astrophysics, and medical imaging experiments. A summary of the fundamental properties of liquid xenon as radiation detection medium, in light of the most current theoretical and experimental information is first provided. After an introduction of the different type of liquid xenon detectors, a review of past, current, and future experiments using liquid xenon to search for rare processes and to image radiation in space and in medicine is given. Each application is introduced with a survey of the underlying scientific motivation and experimental requirements before reviewing the basic characteristics and expected performance of each experiment. Within this decade it appears likely that large volume liquid xenon detectors operated in different modes will contribute to answering some of the most fundamental questions in particle physics, astrophysics, and cosmology, fulfilling the most demanding detection challenges. From detectors based solely on liquid xenon (LXe) scintillation, such as in the MEG experiment for the search of the rare ''{mu}{yields}e{gamma}'' decay, currently the largest liquid xenon detector in operation, and in the XMASS experiment for dark matter detection, to the class ofmore » time projection chambers which exploit both scintillation and ionization of LXe, such as in the XENON dark matter search experiment and in the Enriched Xenon Observatory for neutrinoless double beta decay, unrivaled performance and important contributions to physics in the next few years are anticipated.« less

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Liquid Xenon Detectors for Particle Physics and Astrophysics

We review the current status of liquid noble gas radiation detectors with energy threshold in the keV range, which are of interest for direct dark matter searches, measurement of coherent neutrino scattering and other low energy particle physics experiments. Emphasis is given to the operation principles and the most important instrumentation aspects of these detectors, principally of those operated in the double-phase mode. Recent technological advances and relevant developments in photon detection and charge readout are discussed in the context of their applicability to those experiments.

Liquid noble gas detectors for low energy particle physics

The XENON100 dark matter experiment

An 800 l liquid xenon scintillation γ -ray detector is being developed for the MEG experiment which will search for μ + → e + γ  decay at the Paul Scherrer Institut Absorption of scintillation light of xenon by impurities might possibly limit the performance of such a detector We used a 100 l prototype with an active volume of 372 × 372 × 496 mm 3 to study the scintillation light absorption We have developed a method to evaluate the light absorption, separately from elastic scattering of light, by measuring cosmic rays and α sources By using a suitable purification technique, an absorption length longer than 100 cm has been achieved The effects of the light absorption on the energy resolution are estimated by Monte Carlo simulation

Absorption of scintillation light in a 100 l liquid xenon γ-ray detector and expected detector performance

Capacitive level meter for liquid rare gases

A novel gamma-ray detector based on a liquid xenon scintillator is under development for a new experiment to search for the lepton flavor violating process μ + →e + γ at the Paul Scherrer Institute (PSI). The experiment is designed to have a sensitivity down to 10 −14 branching ratio. We constructed a prototype detector with an active volume of 69 l to demonstrate the performance of the proposed detector for high-energy gamma rays of around 50 MeV. After a test of the cryogenic system for the prototype detector, we have begun studies with the prototype using a gamma-beam up to 40 MeV from the laser Compton backscattering facility at the TERAS electron storage ring of the National Institute of Advanced Industrial Science and Technology (AIST).

Development of a liquid xenon scintillation detector for a new experiment to search for μ→eγ decays

We are developing a liquid xenon calorimeter for the MEG experiment. This experiment, to be performed at the Paul Scherrer Institut (PSI) in Villigen (CH), was designed by an Italian-Japanese-Russian-Swiss collaboration to search for the /spl mu//sup +//spl rarr/e/sup +//spl gamma/ decay, forbidden in the standard model of electroweak and strong interactions. The photon four momentum is to be measured by an innovative C-shaped 800 litres liquid xenon calorimeter read by more than 800 photo-multiplier tubes. An absorption length for Xe scintillation light larger than 100 cm has been measured. The performance of such a detector has been measured using a large volume prototype (100 litres) in a test beam at PSI, using high energy photons from decays of neutral pions from a charge exchange reaction of negative pions on protons. A resolution of 5% FWHM was obtained.

T. Yoshimura

Papers

Absorption of scintillation light in a 100 l liquid xenon γ-ray detector and expected detector performance

Capacitive level meter for liquid rare gases

Development of a liquid xenon scintillation detector for a new experiment to search for μ→eγ decays

Transparency of a 100 liter liquid xenon scintillation calorimeter prototype and measurement of its energy resolution for 55 MeV photons