The observation of a resonance structure in the opening angle of the electron-positron pairs in the Li-7(p,e(+)e(-)) 8Be reaction was claimed and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, consistent with this particle, were later observed in processes involving He-4 and C-12 nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the mu (+) -> e(+)gamma decay, can be exploited to investigate the existence of this particle and study its nature. Protons from a Cockroft-Walton accelerator, with an energy up to 1.1 MeV, were delivered on a dedicated Li-based target. The gamma and the e(+)e(-)pair emerging from the Be-8(& lowast;) transitions were studied with calorimeters and a spectrometer, featuring a broader angular acceptance than previous experiments. We present in this paper the analysis of a four-week data-taking in 2023 with a beam energy of 1080 keV, resulting in the excitation of two different resonances with Q-value 17.6 and 18.1 MeV. No significant signal was found, and limits at 90% C.L. on the branching ratios (relative to the gamma emission) of the two resonances to X17 were set, R-17.6 < 1.8 x 10(-6 )and R-18.1 < 1.2 x 10(-5) in the mass range between 16.5 MeV/c(2) and 17.1 MeV/c(2).

We present the ongoing development of an algorithm for the software alignment of the MEG II drift chamber based on the MillePede global approach. This method uses cosmic rays data collected during the 2022 data taking period to perform wire-by-wire alignment. We discuss first results obtained on data.

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.

The MEG II experiment searches for the charged lepton flavor violating decay mu (+) -> e(+)gamma at the Paul Scherrer Institute in Switzerland. To achieve an order of magnitude sensitivity improvement with respect to MEG, it relies on a liquid XEnon Calorimeter (XEC), a 1000 L liquid xenon C-shaped tank equipped with PMTs and SiPMs to collect the Vacuum UltraViolet scintillation light from the 52.8 MeV signal gamma. Among the various calibration methods of the LXe calorimeter, we developed one to extract the detector performance at an energy close to the signal gamma's. To do so, a beam of negative pions is sent towards a liquid hydrogen target in order to produce neutral pions via the charge exchange reaction pi(-)p -> r(0)n. Neutral pions decay into a pair of gammas with energies following a flat spectrum between 54.9 MeV and 82.9 MeV in the lab frame. The 55 MeV gammas are selected by requiring a back-to-back topology of the gammas using an auxiliary detector based on BGO facing the XEC. This gamma source is used to extract the energy and timing resolutions of the detector. The liquid hydrogen (LH2) target has stringent requirements in order to match the MEG II design, reach temperatures below 20 K and allow fast liquefaction. The latest design of the liquid hydrogen target and its performances are presented here.

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay ^+ →e^+ 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 ( ^+ →e^+ ) < 7.5 × 10^-13 (90 ℬ ( ^+ →e^+ ) < 3.1 × 10^-13 (90

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.

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay $\mu^+\to e^+\gamma$ 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($\mu^+\to e^+\gamma$)<$7.5 \times 10^{-13}$ (90% C.L.). The combination of this result and the limit obtained by MEG gives B($\mu^+\to e^+\gamma$)<$3.1 \times 10^{-13}$ (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.

In the panorama of the state-of-the-art searches for extremely rare Charged Lepton Flavor Violating (CLFV) processes, the Mu-E-Gamma (MEG) experiment is definitely a reference point in the intensity frontier of modern physics research, setting the best upper limit on the μ + → e + γ decay. The upgrade of MEG, MEG II, wants to give further impetus to the CLFV searches with muons. MEG II relies on a series of upgrades: on the photon side we point up improvements of the γ detector resolutions and acceptance; on the positron side we rely on completely brand new detectors with better acceptance, efficiency and performances; on the Trigger and Data Acquisition (DAQ) side we are able to exploit a higher muon beam intensity despite the increased number of read out channels thanks to a new and optimized electronics. After three years of commissioning, in 2021 the MEG II experiment finally entered the physics data taking phase. An overview of the MEG II physics and experimental contexts is presented, together with the current detector performances based on data. Thanks to the new experimental apparatus the final sensitivity goal is expected to be one order of magnitude better than the first phase of MEG.

The MEG experiment at the Paul Scherrer Institut (PSI) represents the state of the art in the search for the charged Lepton Flavor Violating mu(+)-> e(+)gamma decay, setting the most stringent upper limit on the BR (mu(+) -> e(+)gamma) <= 4.2 x 10(-13) (90% C.L.). An upgrade of MEG, MEG II, was designed, commissioned and recently started the physics data taking. Its goal is to reach a sensitivity level of 6 x 10(-14). In order to reconstruct the positron momentum vector a Cylindrical Drift CHamber (CDCH) with unprecedented peculiarities was built, featuring angular and momentum resolutions at the 6.5 mrad and 100 keV/c level. The CDCH is a 2-meter long, 60 cm in diameter, low-mass, single volume detector with high granularity: 9 layers of 192 drift cells, few mm wide, defined by similar to 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 x 10(-3) X-0, thus minimizing the Multiple Coulomb Scattering and allowing for a single-hit resolution < 120 mu m. After the assembly at INFN Pisa, the CDCH was transported to PSI and integrated into the MEG II experimental apparatus since 2018. The commissioning phase lasted for the past three years until the operational stability was reached in 2020. The analysis software is continuously developing and the tuning of the reconstruction algorithms is one of the main activities. The latest updates on the positron momentum vector resolutions and tracking efficiency are presented.

The MEG II experiment aims to improve the sensitivity to μ+→e+γ down to 6×10−14, surpassing the 4.2×10−13 UL by MEG. To achieve this sensitivity the detector performances need to be assessed and monitored via dedicated calibrations. To calibrate the Liquid Xenon Calorimeter near the signal energy (52.8 MeV), photons are produced through the Charge EXchange (CEX) process π−p→nπ0(→γγ). Here we present the liquid Hydrogen target used for the CEX run 2021, during the first MEG II physics run.

Two very promising materials as the BrilLanCe (Cerium doped Lanthanum Bromide, LaBr3(Ce) and the LYSO (Lutetium Yttrium OxyorthoSilicate, Lu2(1-x) Y2x SiO5 (Ce)) coupled to Silicon photomultipliers (MPPC/SiPM) could represent an appealing option for the future calorimetry. The response of both LaBr3(Ce) and LYSO detectors having MPPC as photosensors have been studied via detailed Monte Carlo (MC) simulations. The impinging gammas are in the range of 50-100 MeV. The MC simulations are based on GEANT4, including the full electronic chain up to the waveform digitizer and finally the reconstruction algorithms. The results have been obtained are very promising. For a detector based on a (radius R = 4.45 cm, length L = 20.3 cm) LaBr3(Ce) crystal an energy resolution of..../.. [%] = 2.3(1) and a timing resolution of.... [ps] = 35(1) have been predicted. The energy resolution can be further improved by using larger crystals (either R = 6.35 cm or R = 7.6 cm, L = 20.3 cm) approaching respectively a.... /.. [%] = 1.20(3) or a.... /.. [%] = 0.91(1). Detector based on LYSO crystal of similar size performs even better, thanks to the shorter LYSO Moliere radius compared to the LaBr3(Ce) one. For a detector based on a (R = 3.5 cm, L = 16 cm) LYSO crystal an energy resolution of.... /.. [%] = 1.7(1)% can be obtained, and that can be further improved using bigger crystals (R = 6.5 cm, L = 25 cm,..../.. [%] = 0.74(1)%. Energy resolution approaching.... /.. [%] = 0.3(1)% can be addressed for both crystals with ultimate sizes (R = 20-23 cm, L = 17-32 cm), complemented by timing and position resolutions in the range of.(30) ps and.(a few mm) respectively. Such results put these future high energy calorimeters at the detector forefront at intensity frontiers.

In the MEG II detector, the measurement of the momentum of the charged particle is performed by a high transparency single volume, full stereo cylindrical Drift Chamber (CDCH). It is composed by 9 concentric layers, each consisting of 192 drift cells. The single drift cell is approximately squared, with a 20 mu m gold plate tungsten sense wire surrounded by 40 mu m/50 mu m silver plated aluminum field wires in a ratio of 5:1. During the construction of the first CDCH, we observed the breaking of about hundred cathode wires: 97 of these were 40 mu m aluminum wires, while 10 were 50 mu m wires. Since the number of broken cathodes is less than 1% of the total, one can expect the influence on the track reconstruction efficiency to be not so dramatic. We verified by means of simulations that the loss of one cathode does not change the cell electric field appreciably. Here we present the results of the analysis of the effects of mechanical stress and chemical corrosion observed on these broken wires. Finally, we show the studies carried out on new wires to overcome the weaknesses found and the process that will be used for the construction of the new drift chamber (CDCH2). It will be built with the same modular technique, as for the previous one, the use of the wiring robot will be optimized to improve some weaker step in the procedure, new wires will be adopted with a 25% thicker diameter, which has very little effects on the resolution and efficiency of the detector. Furthermore these wires are made with a manufacturing process different from that used previously.

The physics programme of the MEG II experiment can be extended with the search for new invisible particles produced in rare muon decays. The hunt for such elusive signals requires accurate simulations to characterise the detector response and estimate the experimental sensitivity. This work presents an improved simulation of muon decay in MEG II, based on McMule and Geant4.

Intending to improve the current sensitivity on mu -> e gamma decay by one order of magnitude, the MEG II experiment at Paul Scherrer Institute completed the integration phase in 2021 with all detectors successfully operated throughout the subsequent beamtime. Earlier in 2021, the WaveDAQ integrated Trigger and Data Acquisition (TDAQ) system, developed for the readout of the experiment, was completely commissioned. Receiving almost 9000 channels from the detectors, the MEG II TDAQ system is the largest WaveDAQ deployment so far, proving the scalability of the overall design, from bench-top setup through various smaller-size experiments. We will describe how MEG II trigger system reduces the similar to 10(7) muon decays at the experiment target down to a 10 Hz event rate by exploiting the signal event characteristics at the online level. The trigger system performs the calorimetric reconstruction of the photon shower and then compares the timing and direction with positron candidates within a 600 ns hard latency time. The first release of the online reconstruction, deployed in 2021, achieved a 2.4 % photon energy resolution at the signal energy of 52.8 MeV and a similar to 2 ns coincidence time resolution among the child particles.

In the quest for Lepton Flavor Violation (LFV) the MEG experiment at the Paul Scherrer Institut (PSI) represents the state of the art in the search for the charged LFV decay mu(+ )-> e(+) gamma, setting the most stringent upper limit on the BR(mu(+ )-> e(+) gamma) <= 4.2 x 10(-13) (90% C.L.). An upgrade of MEG, MEG II, was designed and it recently started the physics data taking, with the aim to reach a sensitivity level of 6 x 10(-14). The Cylindrical Drift CHamber (CDCH) is a key detector in order to improve the e(+) angular and momentum resolutions at the 6.5 mrad and 100 keV/c level. The CDCH is a low-mass single volume detector with high granularity: 9 layers of 192 drift cells each, few mm wide, defined by 12000 wires in a stereo configuration for longitudinal hit localization. After the assembly, the CDCH was transported to PSI for the commissioning phase and it has been integrated into the MEG II experimental apparatus since 2018. The operational stability was reached in 2020 and the complete readout electronics was tested for the first time in 2021. A preliminary analysis of 2020-2021 data is presented.

The complete MEG II Trigger and Data Acquisition System, named WaveDAQ, was installed and commissioned in Spring 2021 and successfully carried out the data taking campaign planned for the same year (Chiappini et al., 2021 [1]). It consists of 544 custom made 16-channel acquisition boards which contain the Domino Ring Sampler 4 chips for the analog sampling of the detector signals at > 1 GHz frequency. This paper presents how the MEG II physics objectives shaped the design and operation of the WaveDAQ, the result is a flexible and scalable trigger and data acquisition system.

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating μ+→e+γ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR (μ+→e+γ)≤4.2×10−13 at 90% confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of 6×10−14. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (7×107 μ+/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years.

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 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 ≈ 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.

The MEG experiment took data at the Paul Scherrer Institute in the years 2009–2013 to test the violation of the lepton flavor conservation law, which originates from an accidental symmetry that the Standard Model of elementary particle physics has, and published the most stringent limit on the charged lepton flavor violating decay μ+→e+γ: BR(μ+→e+γ) <4.2×10−13 at 90% confidence level. The MEG detector has been upgraded in order to reach a sensitivity of 6×10−14. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute (7×107 muons/s) with an upgraded detector. The main improvements are better rate capability of all sub-detectors and improved resolutions while keeping the same detector concept. In this paper, we present the current status of the preparation, integration and commissioning of the MEG II detector in the recent engineering runs.

We present a field-programmable gate array (FPGA)-based technique for the on-line identification of highly ionizing particles in a liquid xenon (LXe) detector. The method was developed and successfully exploited to select $\alpha $ particles emitted by 241 Am sources submerged in LXe in an overwhelming, mostly beam-related, $\gamma $ -ray background. After revising the main features of xenon and other liquid noble gases as ultra-violet (UV) scintillating media, we describe the algorithm idea and its firmware implementation. We then present the results in terms of efficiency and background suppression for the real-time $\alpha $ -particle tagging and the limits of the MEG trigger configuration. Finally, we show that in MEG II we are going to overcome the main issues and further improve the performances.