Roberto Messi

A model for RPC detectors operating at high rate

Abstract We present a simple model to describe the behavior of Resistive Plate Chambers (RPC) exposed to a high particle flux. We show that the RPC current, I, saturates at large flux values and we explain why the dependence of I on the applied voltage V0 is essentially linear. We show that in the saturated regime the current is controlled by the value of the bulk electrode resitivity, ρ, which is directly related to the performance of the detector at high particle rates. Measuring the I−V0 curve under these conditions offers a simple and direct method to obtain ρ and to monitor its possible variations.

Performance of low-resistivity single and dual-gap RPCs for LHCb

Abstract Resistive plate chambers (RPC) are strong candidates for the outer regions of the LHCb muon detector. We have tested single-gap and dual-gap detectors built with low-resistivity phenolic plates (ρ=9×10 9 Ω cm ) and operated in avalanche mode. Measurements have been performed over a wide range of beam intensities and on the GIF at CERN. The results are presented and discussed, with special emphasis on the detection efficiency.

A new limit on the light speed isotropy from the GRAAL experiment at the ESRF

When the electrons stored in the ring of the European Synchrotron Radiation Facility (ESRF, Grenoble) scatter on a laser beam (Compton scattering in flight) the lower energy of the scattered electron spectra, the Compton Edge (CE), is given by the two body photon-electron relativistic kinematics and depends on the velocity of light. A precision measurement of the position of this CE as a function of the daily variations of the direction of the electron beam in an absolute reference frame provides a one-way test of Relativistic Kinematics and the isotropy of the velocity of light. The results of GRAAL-ESRF measurements improve the previously existing one-way limits, thus showing the efficiency of this method and the interest of further studies in this direction.

The characterization and application of a low resource FPGA-based time to digital converter☆

Abstract Time to Digital Converters (TDCs) are very common devices in particles physics experiments. A lot of “off-the-shelf” TDCs can be employed but the necessity of a custom DAta acQuisition (DAQ) system makes the TDCs implemented on the Field-Programmable Gate Arrays (FPGAs) desirable. Most of the architectures developed so far are based on the tapped delay lines with precision down to 10xa0ps, obtained with high FPGA resources usage and non-linearity issues to be managed. Often such precision is not necessary; in this case TDC architectures with low resources occupancy are preferable allowing the implementation of data processing systems and of other utilities on the same device. In order to reconstruct γγ physics events tagged with High Energy Tagger (HET) in the KLOE-2 (K LOng Experiment 2), we need to measure the Time Of Flight (TOF) of the electrons and positrons from the KLOE-2 Interaction Point (IP) to our tagging stations (11xa0m apart). The required resolution must be better than the bunch spacing (2.7xa0ns). We have developed and implemented on a Xilinx Virtex-5 FPGA a 32 channel TDC with a precision of 255xa0ps and low non-linearity effects along with an embedded data acquisition system and the interface to the online FARM of KLOE-2. The TDC is based on a low resources occupancy technique: the 4×Oversampling technique which, in this work, is pushed to its best resolution and its performances were exhaustively measured.

First results from an aging test of a prototype RPC for the LHCb Muon System

Abstract Recent results of an aging test performed at the CERN Gamma Irradiation Facility on a single-gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge of about 0.45 C/cm 2 , corresponding to about 4 years of LHCb running at the highest background rate. The performance of the chamber has been studied, under several photon flux values, exploiting a muon beam. A degradation of the rate capability above 1 kHz/cm 2 is observed, which can be correlated to a sizeable increase of resistivity of the chamber plates. An increase of the chamber dark current is also observed. The chamber performance is found to fulfill the LHCb operation requirements.

Readout optimization for the KLOE QCAL tile calorimeters

Abstract A tile calorimeter (QCAL) has been designed for the KLOE experiment. Optimization studies have been conducted by testing the coupling between different types of scintillators and WLS fibers. Results on light yield, attenuation length of fibers and time resolution are presented.

Offline analysis of HEP events by “dynamic perceptron” neural network

In this paper we start from a critical analysis of the fundamental problems of the parallel calculus in linear structures and of their extension to the partial solutions obtained with non-linear architectures. Then, we present shortly a new dynamic architecture able to solve the limitations of the previous architectures through an automatic redefinition of the topology. This architecture is applied to real-time recognition of particle tracks in high-energy accelerators.

Preliminary results of an aging test of RPC chambers for the LHCb Muon System

Abstract The preliminary results of an aging test performed at the CERN Gamma Irradiation Facility on a single-gap RPC prototype developed for the LHCb Muon System are presented. The results are based on an accumulated charge density of 0.42 C / cm 2 , corresponding to about 4 years of LHCb running at the highest background rate. We observe a rise in the dark current and noise measured with source off. The current drawn with source on steadily decreased, possibly indicating an increase of resistivity of the chamber plates. The performance of the chamber, studied with a muon beam under several photon flux values, is found to still fulfill the LHCb operation requirements.

Principles of computational dynamics: applications to parallel and neural computations

In this paper, starting from a general discussion on neural network dynamics from the standpoint of statistical mechanics, we discuss three different strategies to deal with the problem of pattern recognition in neural nets. Particularly we emphasized the role of matching the intrinsic correlations within the input patterns, to solve the problem of the optimal pattern recognition. In this context, the first two strategies, we applied to different problems and we discuss in this paper, consist essentially in adding either white noise or colored noise (deterministic chaos) on the input pattern pre-processing, to make easier for a classical backpropagation algorithm the class separation, respectively because the input patterns are too correlated among themselves or, on the contrary, are too noisy. The third more radical strategy, we applied to very hard pattern recognition problems in HEP experiments, consists in an automatic (dynamic) redefinition of the same net topology on the inner correlations of the inputs.

Search for light-speed anisotropies using Compton scattering of high-energy electrons

Based on the high sensitivity of Compton scattering off ultra relativistic electrons, the possibility of anisotropies in the speed of light is investigated. The result discussed in this contribution is based on the -ray beam of the ESRF’s GRAAL facility (Grenoble, France) and the search for sidereal variations in the energy of the Compton-edge photons. The absence of oscillations yields the two-sided limit of 1.6 × 10 14 at 95 % confidence level on a combination of photon and electron coefficients of the minimal Standard Model Extension (mSME). This new constraint provides an improvement over previous bounds by one order of magnitude.