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G. Dal Maso

Publications and source records attributed to G. Dal Maso.

12 recordsLinked to original sources
arXiv2025

New limit on the μ+->e+γdecay with the MEG II experiment

This letter reports the result of the search for the decay μ+->e+γundertaken at the Paul Scherrer Institut in Switzerland with the MEG II experiment using the data collected in the 2021- 2022 physics runs. The sensitivity of this search is 2.2x10-13, a factor of 2.4 better than that of the full MEG dataset and obtained in a data taking period of about one fourth that of MEG, thanks to the superior performances of the new detector. The result is consistent with the expected background, yielding an upper limit on the branching ratio of B(μ+->e+γ)<1.5 x 10-13 (90 % C.L.). Additional improvements are expected with the data collected during the years 2023-2024. The data-taking will continue in the coming years.

hep-ex↗
arXiv2024

Search for the X17 particle in $^{7}\mathrm{Li}(\mathrm{p},\mathrm{e}^+ \mathrm{e}^{-}) ^{8}\mathrm{Be}$ processes with the MEG II detector

The observation of a resonance structure in the opening angle of the electron-positron pairs in the $^{7}$Li(p,\ee) $^{8}$Be 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 $^{4}$He and $^{12}$C nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the $μ^+ \rightarrow \mathrm{e}^+ γ$ 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 $γ$ and the e$^{+}$e$^{-}$ pair emerging from the $^8\mathrm{Be}^*$ 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 \SI{17.6}{\mega\electronvolt} and \SI{18.1}{\mega\electronvolt}. No significant signal was found, and limits at \SI{90}{\percent} C.L. on the branching ratios (relative to the $γ$ emission) of the two resonances to X17 were set, $R_{17.6} < 1.8 \times 10^{-6} $ and $R_{18.1} < 1.2 \times 10^{-5} $.

nucl-ex↗
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↗
arXiv2021

Science Case for the new High-Intensity Muon Beams HIMB at PSI

In April 2021, scientists active in muon physics met to discuss and work out the physics case for the new High-Intensity Muon Beams (HIMB) project at PSI that could deliver of order $10^{10}$\,s$^{-1}$ surface muons to experiments. Ideas and concrete proposals were further substantiated over the following months and assembled in the present document. The high intensities will allow for completely new experiments with considerable discovery potential and unique sensitivities. The physics case is outstanding and extremely rich, ranging from fundamental particle physics via chemistry to condensed matter research and applications in energy research and elemental analysis. In all these fields, HIMB will ensure that the facilities S$μ$S and CHRISP on PSI's High Intensity Proton Accelerator complex HIPA remain world-leading, despite the competition of muon facilities elsewhere.

hep-ex↗
arXiv2014

Attainment results for nematic elastomers

We consider a class of non-quasiconvex frame indifferent energy densities which includes Ogden-type energy densities for nematic elastomers. For the corresponding geometrically linear problem we provide an explicit minimizer of the energy functional satisfying a nontrivial boundary condition. Other attainment results, both for the nonlinear and the linearized model, are obtained by using the theory of convex integration introduced by Müller and Sverák in the context of crystalline solids.

math.AP↗
arXiv2005

Time-dependent systems of generalized Young measures

In this paper some new tools for the study of evolution problems in the framework of Young measures are introduced. A suitable notion of time-dependent system of generalized Young measures is defined, which allows to extend the classical notions of total variation and absolute continuity with respect to time, as well as the notion of time derivative. The main results are a Helly type theorem for sequences of systems of generalized Young measures and a theorem about the existence of the time derivative for systems with bounded variation with respect to time.

math.FA↗
arXiv2000

Some properties of the solutions of obstacle problems with measure data

We study some properties of the obstacle reactions associated with the solutions of unilateral obstacle problems with measure data. These results allow us to prove that, under very weak assumptions on the obstacles, the solutions do not depend on the components of the negative parts of the data which are concentrated on sets of capacity zero. The proof is based on a careful analysis of the behaviour of the potentials of two mutually singular measures near the points where both potentials tend to infinity.

math.FA↗
arXiv2000

Value Functions for Bolza Problems with Discontinuous Lagrangians and Hamilton-Jacobi Inequalities

We investigate the value function of the Bolza problem of the Calculus of Variations $$ V (t,x)=\inf \{\int_{0}^{t} L (y(s),y'(s))ds + ϕ(y(t)) : y \in W^{1,1} (0,t; R^n) ; y(0)=x \}, $$ with a lower semicontinuous Lagrangian $L$ and a final cost $ϕ$, and show that it is locally Lipschitz for $t>0$ whenever $L$ is locally bounded. It also satisfies Hamilton-Jacobi inequalities in a generalized sense. When the Lagrangian is continuous, then the value function is the unique lower semicontinuous solution to the corresponding Hamilton-Jacobi equation, while for discontinuous Lagrangian we characterize the value function by using the so called contingent inequalities.

math.AP↗
arXiv2000

Uniqueness of solutions to Hamilton-Jacobi equations arising in the Calculus of Variations

We prove the uniqueness of the viscosity solution to the Hamilton-Jacobi equation associated with a Bolza problem of the Calculus of Variations, assuming that the Lagrangian is autonomous, continuous, superlinear, and satisfies the usual convexity hypothesis. Under the same assumptions we prove also the uniqueness, in a class of lower semicontinuous functions, of a slightly different notion of solution, where classical derivatives are replaced only by subdifferentials. These results follow from a new comparison theorem for lower semicontinuous viscosity supersolutions of the Hamilton-Jacobi equation, that is proved in the general case of lower semicontinuous Lagrangians.

math.AP↗
arXiv1995

Capacity theory for monotone operators

If $Au=-div(a(x,Du))$ is a monotone operator defined on the Sobolev space $W^{1,p}(R^n)$, $1<p<+\infty$, with $a(x,0)=0$ for a.e. $x\in R^n$, the capacity $C_A(E,F)$ relative to $A$ can be defined for every pair $(E,F)$ of bounded sets in $R^n$ with $E\subset F$. We prove that $C_A(E,F)$ is increasing and countably subadditive with respect to $E$ and decreasing with respect to $F$. Moreover we investigate the continuity properties of $C_A(E,F)$ with respect to $E$ and $F$.

funct-an↗