arXiv subjects

G. Chiarello

Publications and source records attributed to G. Chiarello.

At least 19 recordsLinked to original sources

arXiv2025

Enhancing Particle Identification in Helium-Based Drift Chambers Using Cluster Counting Insights from Beam Test Studies

Particle identification in gaseous detectors traditionally relies on energy loss measurements (dE/dx); however, uncertainties in total energy deposition limit its resolution. The cluster counting technique (dN/dx) offers an alternative approach by exploiting the Poisson-distributed nature of primary ionization, providing a statistically robust method for mass determination. Simulation studies with Garfield++ and Geant4 indicate that dN/dx can achieve twice the resolution of dE/dx in helium-based drift chambers. However, experimental implementation is challenging due to signal overlap in the time domain, complicating the identification of electron peaks and ionization clusters. This paper presents novel algorithms and modern computational techniques to address these challenges, facilitating accurate cluster recognition in experimental data. The effectiveness of these algorithms is validated through four beam tests conducted at CERN, utilizing various helium gas mixtures, gas gains, and wire orientations relative to ionizing tracks. The experiments employ a muon beam (1 GeV/c to 180 GeV/c) with drift tubes of different sizes and sense wire diameters. The analysis explores the Poisson nature of cluster formation, evaluates the performance of different clustering algorithms, and examines the dependence of counting efficiency on the beam particle impact parameter. Furthermore, a comparative study of the resolution achieved using dN/dx and dE/dx is presented.

physics.ins-det↗

arXiv2024

Performances of a new generation tracking detector: the MEG II cylindrical drfit chamber

The cylindrical drift chamber is the most innovative part of the MEG~II detector, the upgraded version of the MEG experiment. The MEG~II chamber differs from the MEG one because it is a single volume cylindrical structure, instead of a segmented one, chosen to improve its resolutions and efficiency in detecting low energy positrons from muon decays at rest. In this paper, we show the characteristics and performances of this fundamental part of the MEG~II apparatus and we discuss the impact of its higher resolution and efficiency on the sensitivity of the MEG~II experiment. Because of its innovative structure and high quality resolution and efficiency the MEG~II cylindrical drift chamber will be a cornerstone in the development of an ideal tracking detector for future positron-electron collider machines.

physics.ins-det↗

arXiv2024

Operation and performance of MEG II detector

The MEG II experiment, located at the Paul Scherrer Institut (PSI) in Switzerland, is the successor to the MEG experiment, which completed data taking in 2013. MEG II started fully operational data taking in 2021, with the goal of improving the sensitivity of the mu+ -> e+ gamma decay down to 6e-14 almost an order of magnitude better than the current limit. In this paper, we describe the operation and performance of the experiment and give a new estimate of its sensitivity versus data acquisition time.

physics.ins-det↗

arXiv2024

A search for $μ^+\to e^+γ$ with the first dataset of the MEG II experiment

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay $μ^+\to e^+γ$ from data taken in the first physics run in 2021. No excess of events over the expected background is observed, yielding an upper limit on the branching ratio of B($μ^+\to e^+γ$) < $7.5 \times 10^{-13}$ (90% C.L.). The combination of this result and the limit obtained by MEG gives B($μ^+\to e^+γ$) < $3.1 \times 10^{-13}$ (90% C.L.), which is the most stringent limit to date. A ten-fold larger sample of data is being collected during the years 2022-2023, and data-taking will continue in the coming years.

hep-ex↗

arXiv2022

The measuring systems of the wire tension for the MEG II Drift Chamber by means of the resonant frequency technique

The ultra-low mass cylindrical drift chamber designed for the MEG II experiment is a challenging apparatus made of 1728 phi = 20 micron gold plated tungsten sense wires, 7680 phi = 40 micron and 2496 phi = 50 micron silver plated aluminum field wires. Because of electrostatic stability requirements all the wires have to be stretched at mechanical tensions of about 25, 19 and 29 g respectively which must be controlled at a level better than 0.5 g. This chamber is presently in acquisition, but during its construction about 100 field wires broke, because of chemical corrosion induced by the atmospheric humidity. On the basis of the experience gained with this chamber we decided to build a new one, equipped with a different type of wires less sensitive to corrosion. The choice of the new wire required a deep inspection of its characteristics and one of the main tools for doing this is a system for measuring the wire tension by means of the resonant frequency technique, which is described in this paper. The system forces the wires to oscillate by applying a sinusoidal signal at a known frequency, and then measures the variation of the capacitance between a wire and a common ground plane as a function of the external signal frequency. We present the details of the measuring system and the results obtained by scanning the mechanical tensions of two samples of MEG II cylindrical drift chamber wires and discuss the possible improvements of the experimental apparatus and of the measuring technique.

physics.ins-det↗

arXiv2021

Detailed analysis of chemical corrosion of ultra-thin wires used in drift chamber detectors

Ultra-thin metallic anodic and cathodic wires are frequently employed in low-mass gaseous detectors for precision experiments, where the amount of material crossed by charged particles must be minimised. We present here the results of an analysis of the mechanical stress and chemical corrosion effects observed in $40$ and $50~{\rm{μm}}$ diameter silver plated aluminum wires mounted within the volume of the MEG\,II drift chamber, which caused the breaking of about one hundred wires (over a total of $\approx 12000$). This analysis is based on the accurate inspection of the broken wires by means of optical and electronic microscopes and on a detailed recording of all breaking accidents. We present a simple empirical model which relates the number of broken wires to their exposure time to atmospheric humidity and to their mechanical tension, which is necessary for mechanical stability in the presence of electrostatic fields of several kV/cm. Finally we discuss how wire breakings can be avoided or at least strongly reduced by operating in controlled atmosphere during the mounting stages of the wires within the drift chamber and by choosing a $25\,\%$ thicker wire diameter, which has very small effects on the detector resolution and efficiency and can be obtained by using a safer fabrication technique.

physics.ins-det↗

arXiv2021

A photogrammetric method for target monitoring inside the MEG II detector

An automatic target monitoring method based on photographs taken by a CMOS photo-camera has been developed for the MEG II detector. The technique could be adapted for other fixed-target experiments requiring good knowledge of their target position to avoid biases and systematic errors in measuring the trajectories of the outcoming particles. A CMOS-based, high resolution, high radiation tolerant and high magnetic field resistant photo-camera was mounted inside the MEG II detector at the Paul Scherrer Institute (Switzerland). MEG II is used to search for lepton flavour violation in muon decays. The photogrammetric method's challenges, affecting measurements of low momentum particles' tracks, are high magnetic field of the spectrometer, high radiation levels, tight space constraints, and the need to limit the material budget in the tracking volume. The camera is focused on dot pattern drawn on the thin MEG II target, about 1 m away from the detector endcaps where the photo-camera is placed. Target movements and deformations are monitored by comparing images of the dots taken at various times during the measurement. The images are acquired with a Raspberry board and analyzed using a custom software. Global alignment to the spectrometer is guaranteed by corner cubes placed on the target support. As a result, the target monitoring fulfils the needs of the experiment.

physics.ins-det↗

arXiv2020

Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment

We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, $μ^+ \to \mathrm{e}^+\mathrm{X}, \mathrm{X} \to γγ$. This search uses a dataset resulting from $7.5\times 10^{14}$ stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009--2013. No significant excess is found in the mass region 20--45 MeV/c$^2$ for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20--40 MeV/c$^2$, down to $\mathcal{O}(10^{-11})$ at 90\% confidence level.

hep-ex↗

arXiv2020

A 10-3 drift velocity monitoring chamber

The MEG-II experiment searches for the lepton flavor violating decay: mu in electron and gamma. The reconstruction of the positron trajectory uses a cylindrical drift chamber operated with a mixture of He and iC4H10 gas. It is important to provide a stable performance of the detector in terms of its electron transport parameters, avalanche multiplication, composition and purity of the gas mixture. In order to have a continuous monitoring of the quality of gas, we plan to install a small drift chamber, with a simple geometry that allows to measure very precisely the electron drift velocity in a prompt way. This monitoring chamber will be supplied with gas coming from the inlet and the outlet of the detector to determine if gas contaminations originate inside the main chamber or in the gas supply system. The chamber is a small box with cathode walls, that define a highly uniform electric field inside two adjacent drift cells. Along the axis separating the two drift cells, four staggered sense wires alternated with five guard wires collect the drifting electrons. The trigger is provided by two 90Sr weak calibration radioactive sources placed on top of a two thin scintillator tiles telescope. The whole system is designed to give a prompt response (within a minute) about drift velocity variations at the 0.001 level.

physics.ins-det↗

arXiv2020

The Drift Chamber of the MEG II experiment

The MEG experiment at the Paul Scherrer Institut searches for the charged-Lepton-Flavor-Violating mu+ -> e+ gamma decay. MEG has already set the world best upper limit on the branching ratio: BR<4.2x10^-13 @ 90% C.l. An upgrade (MEG II) of the whole detector has been approved to obtain a substantial increase of sensitivity. Currently MEG II is completing the upgrade of the various detectors, an engineering run and a pre-commissioning run were carried out during 2018 and 2019. The new positron tracker is a unique volume, ultra-light He based cylindrical drift chamber (CDCH), with high granularity: 9 layers of 192 square drift cells, ~6-9 mm wide, consist of ~12000 wires in a full stereo configuration. To ensure the electrostatic stability of the drift cells a new wiring strategy should be developed due to the high wire density (12 wires/cm^2 ), the stringent precision requirements on the wire position and uniformity of the wire mechanical tension (better than 0.5 g) The basic idea is to create multiwire frames, by soldering a set of (16 or 32) wires on 40 um thick custom wire-PCBs. Multiwire frames and PEEK spacers are overlapped alternately along the radius, to set the proper cell width, in each of the twelve sectors defined by the spokes of the rudder wheel shaped end-plates. Despite to the conceptual simplicity of the assembling strategies, the building of the multiwire frames, with the set requirements, imposes a use of an automatic wiring system. The MEG II CDCH is the first cylindrical drift chamber ever designed and built in a modular way and it will allow to track positrons, with a momentum greater than 45 MeV/c, with high efficiency by using a very small amount of material, 1.5x10^-3 X0 . We describe the CDCH design and construction, the wiring phase at INFN-Lecce, the choice of the wires, their mechanical properties, the assembly and sealing at INFN-Pisa and the commissioning.

physics.ins-det↗

arXiv2020

The new drift chamber of the MEG II experiment

This article presents the MEG II Cylindrical Drift CHamber (CDCH), a key detector for the phase 2 of MEG, which aims at reaching a sensitivity level of the order of $6 \times 10^{-14}$ for the charged Lepton Flavour Violating $μ^+ \rightarrow \mbox{e}^+ γ$ decay. CDCH is designed to overcome the limitations of the MEG $\mbox{e}^+$ tracker and guarantee the proper operation at high rates with long-term detector stability. CDCH is a low-mass unique volume detector with high granularity: 9 layers of 192 drift cells, few mm wide, defined by $\approx 12000$ wires in a stereo configuration for longitudinal hit localization. The total radiation length is $1.5 \times 10^{-3}$ $\mbox{X}_0$, thus minimizing the Multiple Coulomb Scattering (MCS) contribution and allowing for a single-hit resolution of 110 $μ$m and a momentum resolution of 130 keV/c. CDCH integration into the MEG II experimental apparatus will start in this year.

physics.ins-det↗

arXiv2020

Commissioning of the MEG II tracker system

The MEG experiment at the Paul Scherrer Institut (PSI) represents the state of the art in the search for the charged Lepton Flavour Violating (cLFV) $μ^+ \rightarrow e^+ γ$ decay. With the phase 1, MEG set the new world best upper limit on the $\mbox{BR}(μ^+ \rightarrow e^+ γ) < 4.2 \times 10^{-13}$ (90% C.L.). With the phase 2, MEG II, the experiment aims at reaching a sensitivity enhancement of about one order of magnitude compared to the previous MEG result. The new Cylindrical Drift CHamber (CDCH) is a key detector for MEG II. CDCH is a low-mass single volume detector with high granularity: 9 layers of 192 drift cells, few mm wide, defined by $\sim 12000$ wires in a stereo configuration for longitudinal hit localization. The filling gas mixture is Helium:Isobutane (90:10). The total radiation length is $1.5 \times 10^{-3}$ $\mbox{X}_0$, thus minimizing the Multiple Coulomb Scattering (MCS) contribution and allowing for a single-hit resolution $< 120$ $μ$m and an angular and momentum resolutions of 6 mrad and 90 keV/c respectively. This article presents the CDCH commissioning activities at PSI after the wiring phase at INFN Lecce and the assembly phase at INFN Pisa. The endcaps preparation, HV tests and conditioning of the chamber are described, aiming at reaching the final stable working point. The integration into the MEG II experimental apparatus is described, in view of the first data taking with cosmic rays and $μ^+$ beam during the 2018 and 2019 engineering runs. The first gas gain results are also shown. A full engineering run with all the upgraded detectors and the complete DAQ electronics is expected to start in 2020, followed by three years of physics data taking.

physics.ins-det↗

arXiv2018

The role of surface chemical reactivity in the stability of electronic nanodevices based on two-dimensional materials "beyond graphene" and topological insulators

Here, we examine the influence of surface chemical reactivity toward ambient gases on the performance of nanodevices based on two-dimensional materials "beyond graphene" and novel topological phases of matter. While surface oxidation in ambient conditions was observed for silicene and phosphorene with subsequent reduction of the mobility of charge carriers, nanodevices with active channels of indium selenide, bismuth chalcogenides and transition-metal dichalcogenides are stable in air. However, air-exposed indium selenide suffers of p-type doping due to water decomposition on Se vacancies, whereas the low mobility of charge carriers in transition-metal dichalcogenides increases the response time of nanodevices. Conversely, bismuth chalcogenides require a control of crystalline quality, which could represent a serious hurdle for up scaling.

cond-mat.mtrl-sci↗

arXiv2018

Gas Distribution and Monitoring for the Drift Chamber of the MEG-II Experiment

The reconstruction of the positron trajectory in the MEG-II experiment searching for the $μ^+ \to e^+ γ$ decay uses a cylindrical drift chamber operated with a helium-isobutane gas mixture. A stable performance of the detector in terms of its electron drift properties, avalanche multiplication, and with a gas mixture of controlled composition and purity has to be provided and continuously monitored. In this paper we describe the strategies adopted to meet the requirements imposed by the target sensitivity of MEG-II, including the construction and commissioning of a small chamber for an online monitoring of the gas quality.

physics.ins-det↗

arXiv2018

The design of the MEG II experiment

The MEG experiment, designed to search for the mu+->e+ gamma decay at a 10^-13 sensitivity level, completed data taking in 2013. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6 x 10-14 for the branching ratio, a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance.

physics.ins-det↗

arXiv2016

Single-hit resolution measurement with MEG II drift chamber prototypes

Drift chambers operated with helium-based gas mixtures represent a common solution for tracking charged particles keeping the material budget in the sensitive volume to a minimum. The drawback of this solution is the worsening of the spatial resolution due to primary ionisation fluctuations, which is a limiting factor for high granularity drift chambers like the MEG II tracker. We report on the measurements performed on three different prototypes of the MEG II drift chamber aimed at determining the achievable single-hit resolution. The prototypes were operated with helium/isobutane gas mixtures and exposed to cosmic rays, electron beams and radioactive sources. Direct measurements of the single hit resolution performed with an external tracker returned a value of 110 $μ$m, consistent with the values obtained with indirect measurements performed with the other prototypes.

physics.ins-det↗

arXiv2016

Muon polarization in the MEG experiment: predictions and measurements

The MEG experiment makes use of one of the world's most intense low energy muon beams, in order to search for the lepton flavour violating process $μ^{+} \rightarrow {\rm e}^{+} γ$. We determined the residual beam polarization at the thin stopping target, by measuring the asymmetry of the angular distribution of Michel decay positrons as a function of energy. The initial muon beam polarization at the production is predicted to be $P_μ = -1$ by the Standard Model (SM) with massless neutrinos. We estimated our residual muon polarization to be $P_μ = -0.85 \pm 0.03 ~ {\rm (stat)} ~ { }^{+ 0.04}_{-0.05} ~ {\rm (syst)}$ at the stopping target, which is consistent with the SM predictions when the depolarizing effects occurring during the muon production, propagation and moderation in the target are taken into account. The knowledge of beam polarization is of fundamental importance in order to model the background of our ${\megsign}$ search induced by the muon radiative decay: $μ^{+} \rightarrow {\rm e}^{+} \barν_μ ν_{\rm e} γ$.

hep-ex↗

arXiv2016

Measurement of the radiative decay of polarized muons in the MEG experiment

We studied the radiative muon decay $μ^+ \to e^+ν\barνγ$ by using for the first time an almost fully polarized muon source. We identified a large sample (~13000) of these decays in a total sample of 1.8x10^14 positive muon decays collected in the MEG experiment in the years 2009--2010 and measured the branching ratio B($μ^+ \to e^+ν\barνγ$) = (6.03+-0.14(stat.)+-0.53(sys.))x10^-8 for E_e > 45 MeV and E_γ > 40 MeV, consistent with the Standard Model prediction. The precise measurement of this decay mode provides a basic tool for the timing calibration, a normalization channel, and a strong quality check of the complete MEG experiment in the search for $μ^+ \to e^+γ$ process.

hep-ex↗