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Laser test: Dependence of the time resolution on the electron-peak charge for anode voltages of 450\,V (red circles, drift voltages between 300 and 425\,V), 475\,V (green squares, drift voltages between 300 and 400\,V), 500\,V (blue triangles, drift voltages between 275 and 400\,V), and 525\,V (magenta inverted triangles, drift voltages between 200 and 350\,V). The continuous lines are the result of fitting the functional form of Eq.~\ref{eq:slewing} to the experimental points for the same anode voltage (see text). Statistical uncertainties are shown.

Beam test: Dependence of the time resolution on the drift and anode voltage for a PICOSEC detector irradiated by 150\,GeV muons. For each curve at a given anode voltage, the maximum drift voltage corresponds to the maximum gain at which the detector can work in stable conditions. Statistical uncertainties are shown.

Laser test: Mean of the SAT values (left) and time resolution (right) as a function of the electron-peak charge in case of single photoelectron data, for an anode voltage of 525\,V and drift voltages between 200 and 350\,V. The solid curves in the left distribution are the result of fitting the functional form (see text, Eq.~\ref{eq:slewing}) to the experimental points for each drift voltage, with the constraint that the parameters \textit{b} and \textit{w} must be the same for all drift voltages. Meanwhile, the solid curve in the right distribution is the result of fitting the same equation to all experimental points, without any distinction of the drift voltage. Statistical uncertainties are shown.

Beam test: Dependence of the signal arrival time (left) and the time resolution (right) on the electron-peak charge for 150\,GeV muons, for anode (A) voltages between 250\,V and 300\,V and drift voltages (D) between 400\,V and 500\,V. Statistical uncertainties are shown.

Laser test: Mean of the SAT values (left) and time resolution (right) as a function of the electron-peak charge in case of single photoelectron data, for an anode voltage of 525\,V and drift voltages between 200 and 350\,V. The solid curves in the left distribution are the result of fitting the functional form (see text, Eq.~\ref{eq:slewing}) to the experimental points for each drift voltage, with the constraint that the parameters \textit{b} and \textit{w} must be the same for all drift voltages. Meanwhile, the solid curve in the right distribution is the result of fitting the same equation to all experimental points, without any distinction of the drift voltage. Statistical uncertainties are shown.

Laser test: Average of the electron-peak shape normalized to unity for electron-peak charges of 1.0-1.1\,pC (continuous red line), 2.0-2.5\,pC (segmented green line) and 3-4\,pC (dashed blue line). The figure shows a zoom to the leading edge, while the inset shows the complete electron-peak component. The detector was operated at an anode voltage of 450\,V and a drift voltage of 350\,V with the ``COMPASS gas'' at 1\,bar absolute pressure.

The PICOSEC detection concept. The passage of a charged particle through the Cherenkov radiator produces UV photons, which are then absorbed at the photocathode and partially converted into electrons. These electrons are subsequently preamplified and then amplified in the two high-field drift stages, and induce a signal which is measured between the anode and the mesh.

Sketch of the first prototype of the PICOSEC detector, described in detail in the text. The scale of some components is exaggerated for clarity.

Beam test: An example of the signal arrival time distribution for 150\,GeV muons, and the superimposed fit with a two Gaussian function (red line for the combination and dashed blue and magenta lines for each Gaussian function), for an anode and drift voltage of 275\,V and 475\,V, respectively. Statistical uncertainties are shown.

Laser test: Two examples of the electron-peak charge distributions generated by single photoelectrons: one biased by the PICOSEC detector threshold (left), and another unbiased- using only the reference photodetector in the trigger chain (right). The voltage settings in both cases are 450\,V for the anode and 350\,V for the drift. In both cases, the charge distribution is fit by a Polya function (red line), and with a separate noise contribution (blue line in the right plot). Statistical and systematic uncertainties are shown.

Laser test: Two examples of the electron-peak charge distributions generated by single photoelectrons: one biased by the PICOSEC detector threshold (left), and another unbiased- using only the reference photodetector in the trigger chain (right). The voltage settings in both cases are 450\,V for the anode and 350\,V for the drift. In both cases, the charge distribution is fit by a Polya function (red line), and with a separate noise contribution (blue line in the right plot). Statistical and systematic uncertainties are shown.

Beam test: Dependence of the signal arrival time (left) and the time resolution (right) on the electron-peak charge for 150\,GeV muons, for anode (A) voltages between 250\,V and 300\,V and drift voltages (D) between 400\,V and 500\,V. Statistical uncertainties are shown.

Schematic of the experimental setup during the laser tests, described in detail in the text.

Laser test: Dependence of the corrected time resolution on the drift voltage for anode voltages of 450\,V (red circles), 475\,V (green squares), 500\,V (blue triangles) and 525\,V (magenta inverted triangles). Statistical uncertainties are shown.

Layout of the experimental setup (not to scale) during the beam tests. The incoming beam enters from the right side of the figure; events are triggered by the coincidence of two $5\times 5$\,mm$^2$ scintillators in anti-coincidence with a ``veto'' scintillator. Three GEM detectors provide tracking information of the incoming charged particles, and the timing information is measured in three PICOSEC detectors (Pos0, Pos1, Pos2). Details are given in the text.

Beam test: An example of the signal arrival time distribution for 150\,GeV muons, and the superimposed fit with a two Gaussian function (red line for the combination and dashed blue and magenta lines for each Gaussian function), for an anode and drift voltage of 275\,V and 475\,V, respectively. Statistical uncertainties are shown.

Beam test: An example of the signal arrival time distribution for 150\,GeV muons, and the superimposed fit with a two Gaussian function (red line for the combination and dashed blue and magenta lines for each Gaussian function), for an anode and drift voltage of 275\,V and 475\,V, respectively. Statistical uncertainties are shown.

Beam test: An example of the electron-peak charge distribution (black points) generated by 150\,GeV muons and compared to the statistical prediction (red line) obtained from a maximum likelihood method. Inset: the electron-peak distribution generated by a signal from the UV lamp (black points) is fit by the single electron-peak distribution (red line) described by Eq.~\ref{eq:Polyafunction} plus a noise contribution (blue line). The settings are 275\,V for the anode and 475\,V for the drift voltages, in both cases. Statistical uncertainties are shown.

An example of an induced signal from the PICOSEC detector generated by 150\,GeV muons (blue points), recorded together with the timing reference of the microchannel plate MCP signal (red points) discussed in the text. The PICOSEC signal contains a fast component produced by the electrons, and a slower component generated by the ion drift. The fast electron-peak amplitude and the Signal Arrival Time, defined in the waveform analysis of Sec.~\ref{sec:PulseAnalysis}, are also shown.

Photograph of the readout structure of the bulk Micromegas detector in the first PICOSEC prototype. Six pillars, arranged in a hexagonal pattern, support the mesh in the central region of the amplification gap. The mesh and anode voltages are supplied by the two visible strip-lines onto which two coaxial cables are soldered outside the active volume. Their shielding is soldered to the solid copper ground layer on the lower side of the readout PCB.