MEG Experiment Research Articles

Towards a new generation of Charged Lepton Flavour Violation searches at the Paul Scherrer Institut: The MEG upgrade and the Mu3e experiment

The MEG experiment has recently set a new upper limit on the branching ratio of the µ+ → e+γ decay, ℬ(µ+ → e+γ) < 4.2 × 10−13 (at 90% confidence level) and un upgrade of the experiment (the MEGII experiment) is ongoing with the aim of improving the single event sensitivity (SES) by one order of magnitude with respect to the previous MEG experiment’s SES. The strong scientific motivation associated with the charged Lepton Flavour Violation (cLFV) searches pushes also towards searching for the complementary muon cLFV µ+ → e+e+e− decay with a completely new apparatus, the Mu3e experiment, aiming at a SES improved by at least three orders of magnitude with respect to the previous SINDRUM experiment’s SES (Mu3e phase I). An ultimate SES of few ×10−16 is foreseen requiring 109 µ/s (Mu3e phase II). Both experiments will be hosted at the Paul Scherrer Institut which currently delivers the most intense continuous low energy muon beam in the world up to few ×108 µ/s. The status of both the MEGII and Mu3e phase I experiments is given.

Full system of positron timing counter in MEG II having time resolution below 40 ps with fast plastic scintillator readout by SiPMs

A positron timing counter (TC) with 30–40 ps time resolution for 50 MeV/c positrons required by the MEG II experiment has been developed. We employed a highly segmented design with 512 scintillator plates (120 × 40 × 5 mm3 and 120 × 50 × 5 mm3) read out by 6-SiPM-arrays at the both ends. Pile up is reduced by the segmented design and multi-counter measurement improves the overall timing resolution. All the single counters were assembled and their resolutions were measured to be below 100 ps in a pre-test with a 90Sr source. The construction and installation were completed in 2017. An engineering run was performed at the end of 2017 at the πE5 muon beam line in PSI, which supplies the most intense muon beam (7 × 107 stops/s on a target) in the world. We successfully operated the full system of TC in the MEG II environment and achieved a time resolution below 40 ps with more than 6 counter hits. The overall resolution for the signal positrons is estimated to be 38.5 ps by weighting the obtained resolutions with the number of hits distribution of the signal positrons obtained by a Monte Carlo simulation. The time resolution of the TC is improved by a factor of 2 with respect to the MEG experiment.

The ultra light Drift Chamber of the MEG II experiment

The MEG experiment at the Paul Scherrer Institute searches for the charged Lepton Flavor Violating μ+→e+γ decay. MEG has already determined in a first data taking phase the world best upper limit on the branching ratio: BR(μ+→e+γ)<4.2×10−13 An upgrade of the whole detector has been approved to obtain a substantial increase in sensitivity. Currently MEG is in upgrade phases, this phase involves all the detectors. The new positron tracker is a high transparency single volume, fully stereo cylindrical Drift Chamber (CDCH), immersed in a non uniform longitudinal B-field, with length of 1.93 m, internal radius of 17 cm and external radius of 30 cm. It is composed of 9 concentric layers, divided into 12 identical sector of 16 drift cells. The single drift cell is approximately square, with a 20 μm gold plated W sense wire surrounded by 40 μm silver plated Al field wires in a ratio of 5:1. The total number of wires amounts to 11904 for an equivalent radiation length per track turn of about 1.5x10−3 X0 when the chamber is filled with a gas mixture of helium and iso-butane. Due to the high wire density (12wires∕cm2), the use of the classical feed-through technique as wire anchoring system could hardly be implemented and therefore it was necessary to develop new wiring strategies. The number of wires and the stringent requirements impose the use of an automatic system (wiring robot) to operate the wiring procedures. Several tests have been performed in different prototypes of the drift chamber, exposed to cosmic rays, test beams and radioactive sources, to fulfill the requirement on the spatial resolution to be less than 110 μm. In this paper we describe the CDCH construction and the first tests during the 2018 Engineering Run.

Grammatical class modulates the (left) inferior frontal gyrus within 100 milliseconds when syntactic context is predictive

The current study set out to examine the spatiotemporal dynamics of predictive processing during syntactic processing. To do so, we conducted an MEG experiment in which we contrasted MRI-constrained sources elicited by nouns and verbs when they were preceded by a predictive syntactic context (i.e., possessive pronouns for nouns, and personal pronouns for verbs) versus a non-predictive syntactic context (visually matched symbols). The results showed rapid (from ~80 ms onwards) noun-verb differences in the left and (to a lesser extent) right inferior frontal gyri (IFG), but only when those nouns and verbs were preceded by the syntactically predictive context (i.e. their corresponding pronoun). Furthermore, the contrast between possessive and personal pronouns that preceded the rapid noun-verb modulations in the (L)IFG also produced differences in source activation in various regions of the prefrontal cortex (the superior frontal and orbitofrontal cortex). We suggest the data show that syntactic unification manifests very early on during processing in the LIFG. The speed of such syntactic unification operations is hypothesized to be driven by predictive top-down activations stemming from a domain-general network in the prefrontal cortex.

MEG sensor patterns reflect perceptual but not categorical similarity of animate and inanimate objects

Human high-level visual cortex shows a distinction between animate and inanimate objects, as revealed by fMRI. Recent studies have shown that object animacy can similarly be decoded from MEG sensor patterns. Which object properties drive this decoding? Here, we disentangled the influence of perceptual and categorical object properties by presenting perceptually matched objects (e.g., snake and rope) that were nonetheless easily recognizable as being animate or inanimate. In a series of behavioral experiments, three aspects of perceptual dissimilarity of these objects were quantified: overall dissimilarity, outline dissimilarity, and texture dissimilarity. Neural dissimilarity of MEG sensor patterns was modeled using regression analysis, in which perceptual dissimilarity (from the behavioral experiments) and categorical dissimilarity served as predictors of neural dissimilarity. We found that perceptual dissimilarity was strongly reflected in MEG sensor patterns from 80 ms after stimulus onset, with separable contributions of outline and texture dissimilarity. Surprisingly, when controlling for perceptual dissimilarity, MEG patterns did not carry information about object category (animate vs inanimate) at any time point. Nearly identical results were found in a second MEG experiment that required basic-level object recognition. This is in contrast to results observed in fMRI using the same stimuli, task, and analysis approach: fMRI voxel patterns in object-selective cortex showed a highly reliable categorical distinction even when controlling for perceptual dissimilarity. These results suggest that MEG sensor patterns do not capture object animacy independently of perceptual differences between animate and inanimate objects.

MEG final results and progress towards MEG II

The MEG experiment, which is to search for lepton flavor violating muon decay, had been successfully finished in 2013. The final sensitivity of the experiment was \boldsymbol{5.3 \times 10^{-13}}5.3×10−13, and since the experiment did not find any signal, the upper limit of the branching ratio of the \boldsymbol{\mu^{+} \rightarrow \mathrm{e}^{+} \gamma}𝛍+→e+𝛄 was set to be \boldsymbol{4.2 \times 10^{-13}}4.2×10−13 at 90% CL. The MEG II experiment will improve the sensitivity by an order of magnitude with three years data taking, and the target sensitivity is \boldsymbol{6 \times 10^{-14}}6×10−14. In 2018 after the detector integration, the muon beam data taking will be planned with limited number of readout channels. In 2019, the engineering run and the physics run will be started.

Does tACS entrain neural oscillations?

Despite the widespread use of tACS for manipulating neuronal oscillations, it is still not clear whether this method is able to entrain neuronal oscillations. This is mainly because stimulation artifacts interfere with simultaneously recorded brain signals and make it hard to search for such possible entrainments. In my talk, I will first review our current knowledge about these artifacts and provide a mathematical model for them. Next, I present our recent results from an MEG experiment, in which we search for the echoes of tACS entrainment after stimulation offset.

Status of the MEG II experiment at PSI

The MEG experiment, phase I, has recently established the best experimental upper limit on the branching ratio of the µ → eγ decay of 4.2 × 10−13 (90% Confidence Level). According to the standard model (SM) of particle physics, such a decay should definitely be non observable while several model of new physics (NP) beyond the SM (BSM) predict it should happen at an experimentally detectable rate. In this talk I summarize the theoretical motivations for searching for this decay and I present the status of the second phase of the experiment, MEG II, which will be able to increase the sensitivity to the µ → eγ decay by about one order of magnitude in few years of data taking.

MEG II and Mu3e status and plan

The MEG experiment searching for lepton flavor violating muon decay, finished data taking successfully in 2013. The final sensitivity of the experiment was 5.3 × 10−13, and since the experiment did not find any signal, the upper limit of the branching ratio of the µ → e+γ was set to be 4.2 × 10−13 at 90% C.L.. The MEG II experiment will have a target sensitivity improved by an order of magnitude down to 6 ×10−14 with three years data taking. In 2018 after the detector integration, the muon beam data taking took place with a limited number of readout channels. In 2019, the engineering run and the physics run will be started. The Mu3e experiment will search for the lepton flavor violating decay µ+→ e+e−e+ with unprecedented sensitivity. Construction of the detector is going to start in 2020 and for the first phase a single event sensitivity of 2 × 10−15 is expected.

The construction technique of the new MEG II tracker

The MEG experiment, at the PSI, aims to search for the charged lepton flavor violating decay μ + → e + γ . MEG has established the world best upper limit for the branching ratio: BR ( μ + → e + γ ) < 4 . 2 × 1 0 − 13 at 90% CL. An upgrade (MEG II) of the whole detector has been approved to obtain a substantial increase in sensitivity. The new positron tracker is a single volume, full stereo cylindrical Drift Chamber (CDCH), filled with a gas mixture of 85: 15 H e ∕ i − C 4 H 10 . Due to their high wires density (12 wires/cm 2 ) and the stringent requirements on spatial resolution, the use of the classical feed-through technique as wire anchoring system can hardly be implemented and therefore it is necessary to develop an automatic system (wiring robot) with a high-performing control. A wiring robot, designed and built at the INFN Lecce and University of Salento laboratories was used for the wiring phase of the MEG CDCH. The new CDCH is currently in the test phase at INFN Lecce and Pisa, and later it will be delivered to PSI for commissioning.

Low latency serial communication for MEG II trigger system

In the framework of the upgrade of the MEG experiment a new trigger system with demanding performances in terms of latency was designed. To achieve the goal of 512 ns latency the serial connections between trigger boards have been optimized so that they only rely on serial to parallel shift registers for the data flow, minimizing the amount of logic to be crossed by data. A dedicated Finite State Machine (FSM) was designed to select the sampling point of the serial stream with respect to an external precision clock used by both transmitter and receiver. As a proof of concept a 16 000 channel weighted waveform sum including pedestal suppression, was computed, resulting in 700 ns of latency.

The mott based double turn method for the MEG II spectrometer characterisation

The MEG experiment has set the latest limit of 4.2 × 10 −13 at 90 % Confidence Level (C.L.) to the branching ratio of the charged Lepton Flavour Violating (cLFV) decay μ + → e + γ . The MEG II experiment will be more sensitive by an order of magnitude. High resolutions in terms of the kinematic variables used to identify a μ + → e + γ event are mandatory among other aspects to achieve this sensitivity. Thus careful calibration and monitoring of the experimental apparatus are required. One of the dedicated calibration methods involves the Mott scattering process of a mono-energetic positron beam on the MEG II target. As a result positrons with energies around the MEG II signal energy get scattered into the drift chamber. The calibration and extraction of resolution can then be done by the double turn method presented in this article. • Simulation of Mott events provides similar results as the expected MEG II signal. • Beam quality and details of scattering process hardly affect the outcome. • The Double Turn Method provides the detector resolution up to an expected factor. • Resolution in momentum and angular variables can be estimated.

The temporal dynamics of structure and content in sentence comprehension: Evidence from fMRI-constrained MEG.

Humans have a striking capacity to combine words into sentences that express new meanings. Previous research has identified key brain regions involved in this capacity, but little is known about the time course of activity in these regions, as hemodynamic methods such as fMRI provide little insight into temporal dynamics of neural activation. We performed an MEG experiment to elucidate the temporal dynamics of structure and content processing within four brain regions implicated by fMRI data from the same experiment: the temporo-parietal junction (TPJ), the posterior temporal lobe (PTL), the anterior temporal lobe (ATL), and the anterior inferior frontal gyrus (IFG). The TPJ showed increased activity for both structure and content near the end of the sentence, consistent with a role in incremental interpretation of event semantics. The PTL, a region not often associated with core aspects of syntax, showed a strong early effect of structure, consistent with predictive parsing models, and both structural and semantic context effects on function words. These results provide converging evidence that the PTL plays an important role in lexicalized syntactic processing. The ATL and IFG, regions traditionally associated with syntax, showed minimal effects of sentence structure. The ATL, PTL and IFG all showed effects of semantic content: increased activation for real words relative to nonwords. Our fMRI-guided MEG investigation therefore helps identify syntactic and semantic aspects of sentence comprehension in the brain in both spatial and temporal dimensions.

In Spoken Word Recognition, the Future Predicts the Past.

Speech is an inherently noisy and ambiguous signal. To fluently derive meaning, a listener must integrate contextual information to guide interpretations of the sensory input. Although many studies have demonstrated the influence of prior context on speech perception, the neural mechanisms supporting the integration of subsequent context remain unknown. Using MEG to record from human auditory cortex, we analyzed responses to spoken words with a varyingly ambiguous onset phoneme, the identity of which is later disambiguated at the lexical uniqueness point. Fifty participants (both male and female) were recruited across two MEG experiments. Our findings suggest that primary auditory cortex is sensitive to phonological ambiguity very early during processing at just 50 ms after onset. Subphonemic detail is preserved in auditory cortex over long timescales and re-evoked at subsequent phoneme positions. Commitments to phonological categories occur in parallel, resolving on the shorter timescale of ∼450 ms. These findings provide evidence that future input determines the perception of earlier speech sounds by maintaining sensory features until they can be integrated with top-down lexical information.SIGNIFICANCE STATEMENT The perception of a speech sound is determined by its surrounding context in the form of words, sentences, and other speech sounds. Often, such contextual information becomes available later than the sensory input. The present study is the first to unveil how the brain uses this subsequent information to aid speech comprehension. Concretely, we found that the auditory system actively maintains the acoustic signal in auditory cortex while concurrently making guesses about the identity of the words being said. Such a processing strategy allows the content of the message to be accessed quickly while also permitting reanalysis of the acoustic signal to minimize parsing mistakes.

Temporal Expectation Modulates the Cortical Dynamics of Short-Term Memory.

Increased memory load is often signified by enhanced neural oscillatory power in the alpha range (8-13 Hz), which is taken to reflect inhibition of task-irrelevant brain regions. The corresponding neural correlates of memory decay, however, are not yet well understood. In the current study, we investigated auditory short-term memory decay in humans using a delayed matching-to-sample task with pure-tone sequences. First, in a behavioral experiment, we modeled memory performance over six different delay-phase durations. Second, in a MEG experiment, we assessed alpha-power modulations over three different delay-phase durations. In both experiments, the temporal expectation for the to-be-remembered sound was manipulated so that it was either temporally expected or not. In both studies, memory performance declined over time, but this decline was weaker when the onset time of the to-be-remembered sound was expected. Similarly, patterns of alpha power in and alpha-tuned connectivity between sensory cortices changed parametrically with delay duration (i.e., decrease in occipitoparietal regions, increase in temporal regions). Temporal expectation not only counteracted alpha-power decline in heteromodal brain areas (i.e., supramarginal gyrus), but also had a beneficial effect on memory decay, counteracting memory performance decline. Correspondingly, temporal expectation also boosted alpha connectivity within attention networks known to play an active role during memory maintenance. The present data show how patterns of alpha power orchestrate short-term memory decay and encourage a more nuanced perspective on alpha power across brain space and time beyond its inhibitory role.SIGNIFICANCE STATEMENT Our sensory memories of the physical world fade quickly. We show here that this decay of short-term memory can be counteracted by so-called temporal expectation; that is, knowledge of when to expect a sensory event that an individual must remember. We also show that neural oscillations in the "alpha" (8-13 Hz) range index both the degree of memory decay (for brief sound patterns) and the respective memory benefit from temporal expectation. Spatially distributed cortical patterns of alpha power show opposing effects in auditory versus visual sensory cortices. Moreover, alpha-tuned connectivity changes within supramodal attention networks reflect the allocation of neural resources as short-term memory representations fade.

The design of the MEG\xa0II experiment

The MEG experiment, designed to search for the {mu ^+ rightarrow hbox {e}^+ gamma } decay, completed data-taking in 2013 reaching a sensitivity level of {5.3times 10^{-13}} for the branching ratio. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6times 10^{-14}, 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.

Charged lepton flavour violation searches at the Paul Scherrer Institut: Status of the MEGII and Mu3e experiments

The MEG experiment has recently set a new upper limit on the branching ratio of the μ+ → e+γ decay, B(μ+ → e+γ) &lt; 4.2 × 10-13 (at 90% confidence level) and un upgrade of the experiment (the MEGII experiment) is ongoing with the aim of improving the single event sensitivity (SES) by one order of magnitude with respect to the previous MEG experiment’s SES. The strong scientific motivation associated with the charged Lepton Flavour Violation (cLFV) searches pushes also towards searching for the complementary muon cLFV μ+ → e+e+e- decay with the Mu3e experiment aiming at a SES improved by at least three orders of magnitude with respect to the previous SINDRUM experiment’s SES (phase I) up to an ultimate SES of few ×10-16. Both experiments will be hosted at the Paul Scherrer Institut which delivers the most intense continuous low energy muon beam in the world up to few ×108 μ/s. The status of both the MEGII and Mu3e experiments is given.

Choice of Magnetometers and Gradiometers after Signal Space Separation.

Background: Modern Elekta Neuromag MEG devices include 102 sensor triplets containing one magnetometer and two planar gradiometers. The first processing step is often a signal space separation (SSS), which provides a powerful noise reduction. A question commonly raised by researchers and reviewers relates to which data should be employed in analyses: (1) magnetometers only, (2) gradiometers only, (3) magnetometers and gradiometers together. The MEG community is currently divided with regard to the proper answer. Methods: First, we provide theoretical evidence that both gradiometers and magnetometers result from the backprojection of the same SSS components. Then, we compare resting state and task-related sensor and source estimations from magnetometers and gradiometers in real MEG recordings before and after SSS. Results: SSS introduced a strong increase in the similarity between source time series derived from magnetometers and gradiometers (r2 = 0.3–0.8 before SSS and r2 > 0.80 after SSS). After SSS, resting state power spectrum and functional connectivity, as well as visual evoked responses, derived from both magnetometers and gradiometers were highly similar (Intraclass Correlation Coefficient > 0.8, r2 > 0.8). Conclusions: After SSS, magnetometer and gradiometer data are estimated from a single set of SSS components (usually ≤ 80). Equivalent results can be obtained with both sensor types in typical MEG experiments.

MEG II liquid xenon scintillation calorimeter to detect 52.8 MeV [formula omitted] with large area VUV-sensitive MPPCs

The MEG II experiment will search for new physics like SUSY-GUT/SUSY-seesaw through the lepton flavor violating μ+→e+γ decay with ten times better sensitivity than the MEG experiment. The MEG experiment published the result of B(μ+→e+γ)<4.2×10−13 at 90% C.L. in 2016, which was thirty times better result than the previous limit. Because the sensitivity of the MEG experiment was already limited by accidental backgrounds, the MEG detector must be upgraded to reach one order of magnitude better sensitivity.The MEG experiment utilized 846 2 inch PMTs to detect scintillation light in a 900 L liquid xenon gamma calorimeter. On the γ incident face, 216 2 inch PMTs will be replaced with 4092 12 × 12 mm2 MPPCs to improve the energy and position resolution for γ events which interact near the incident face due to finer granularity and better uniformity of the scintillation light collection efficiency. We are now constructing the MEG II liquid xenon calorimeter. The detector calibration, purification, monitoring, and commissioning are in progress. Here the detector status will be described together with the expected detector performance.

Corrigendum to “Learning transforms functional organization for Mandarin lexical tone discrimination in the brain: Evidence from a MEG experiment on second language learning” [J. Neurolinguistics, 42 (2017) 124–139

Corrigendum to “Learning transforms functional organization for Mandarin lexical tone discrimination in the brain: Evidence from a MEG experiment on second language learning” [J. Neurolinguistics, 42 (2017) 124–139