1 Detector simulation

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1 1 Detector simulation Detector simulation begins with the tracking of the generated particles in the CMS sensitive volume. For this purpose, CMS uses the GEANT4 package [1], which takes into account the geometry and material of the detector and simulates the physics processes that occur as particles cross each sensitive volume: multiple scattering, energy loss, the bending of the track due to the magnetic field, and the production of secondary particles. GEANT4 version 6.2 was used in the present study. The output are GEANT SimHits, which carry information about the passage of a track through the sensitive detector volumes. In the muon system, the sensitive region is defined as an active gas volume comprising one CSC chamber, one DT cell or a RPC single gap module (double gap modules are approximated with a single active active volume, called a roll). The SimHit contains the particle type and energy, the position of entry and exit in the gas volume, as well as the energy loss in the gas. Subsystem-specific Digitizer packages turn these SimHits into digital signals for individual readout channels, called Digis. They represent different object for the three muon detectors: for CSCs, they correspond to individual readout channels on the strip and wire group channels; for DTs, each digi consists of a TDC time associated with an individual drift cell; for RPCs, a digi is a pair of values: the fired strip code and the corresponding bunch crossing. These digis are then packed into raw data format and stored. 1.1 CSC Simulation The CSC simulation starts by making a line between the entry and exit points of the SimHit, and by doing a simulation of the ionization and delta ray creation along this line. The output of this simulation is a line of 80 ionization clusters. Next, each ionization cluster is drifted to the nearest wire. The drifing is done according to a parametrization of the path length, drift time, and drift distance, derived from more detailed simulations (FIXME cite). The parametrization is function of three variables: the magnetic field and the distance to the wire in both the direction of the wire plane and the direction 1

2 perpendicular to the chamber. Two sets of parametrizations were used, one set for the ME1/1 chambers, which are thinner and operate in high magnetic fields, and one set for the other chambers. ME1/1 chambers have typical drift times of ns, and RMS drift distances of 100 µm, while the other chambers have drift times of ns and drift distances of 150 µm. When an ionization cluster reaches a wire, an avalanche of charge is created, according to a parametrized distribution, with factors to account for charge collection efficiency. This charge avalanche has two effects. Firstly, the charge is amplified and shaped into a signal on the wire group s readout channel. Secondly, image charges are induced on the five nearest cathode strips, according to the so-called Gatti distribution (FIXME cite), which are in turn amplified into signals on the strips. The signals, which are represented as histograms of charge, are superimposed for all ionization clusters, and all SimHits. Wire digis consist of 16 bits, each representing whether the wiregroup signal was over threshold for a bin of 25 nanoseconds. Noise is simulated by jittering the threshold. The cathode simulation is more complex. Pedestal noise is correlated between time bins, so we read the correlations from the CMS conditions database for each channel, and apply them to the four time bins where the signals should peak. The other time bins use uncorrelated pedestal noise. Crosstalk between channels is also applied, using channel-specific constants obtained from the conditions database. Crosstalk results in a 10% transfer of charge from one strip to a neighboring strip. Since our crosstalk has a component proportional to the slope of the neighboring signal, it typically causes neighboring signals to peak earlier than the main signal. The cathode signal is split into two paths, with the trigger path resulting in comparator digis, and the offline path resulting in strip digis. The timing of the wire, strip, and comparator signals are tuned by first subtracting a time corresponding to the distance from the center of the readout element to the interaction point, divided by the speed of light. The speed of signal propagation from the hit position to the readout is measured in data, and applied. Finally, we fine-tune the timing using configurable constants for each ring of chambers. Next, the strip and comparators digis are used as input to the Level 1 trigger algorithms, which create digis representing the LCTs. Finally, we pack all our digis into the CSC raw data format. Some zero suppression of the strip signals is applied, based on the pre-trigger results in the cathode LCT simulation. Subsequent reconstruction starts from the raw data format. 2

3 1.2 DT simulation The determination of the position of particles in the DT detectors is based on the measurement of the drift time of ionization electrons. An accurate modeling of the cell response, including the TDC measurement of the drift time information, is essential for a reliable simulation of physics events. The simulation of the cell response consists of the computation of the drift time of the electrons produced by ionization. The simulation must reproduce not only the average behavior of the cell as a function of the track parameters and of the magnetic field, but also the smearing of the drift times which determines the cell resolution. This is achieved with a parametrization of the drift cell behavior based on a detailed simulation with the GARFIELD package [2]. The drift time is parameterized as a function of the distance of the simulated hit from the wire, the incidence angle of the muon, and the magnetic field components parallel and orthogonal to the wire. This time is smeared according to a double-half-gaussian distribution, where the two sigmas are also parameterized. For the case of several hits in the same cell, the drift times are computed independently, but separate digis are created only if the difference in drift times is larger than the configurable dead time. Hits from energetic delta rays and any other secondaries (e.g. e + e from pair production) crossing the whole cell are handled in the same way as muons. Delta rays produced in the gas are ignored, since they are already generated by GARFIELD and their effect is included (statistically) in the parameterization. Soft electrons stopping in the gas cannot be handled by the parameterization and are currently ignored. Most of these hits cover a very short path and are not expected to produce a detectable signal. Finally, since the DTs are time-measuring devices, the delays contributing to the TDC measurement must be accounted for. The time of flight of the muons from the interaction point to the cell and the propagation time of the signal along the anode wire are added to the simulated time. 1.3 RPC simulation The RPC digitization is heavily parametrized. The digitization task is to assign none, one or few Digis to a SimHit and to simulate the detector noise. The SimHits are processed independetly. The RPC digitization algorithm uses the following parameters from dedicated DB : efficiency parameter for each readout strip 3

4 noise rate parameter for each readout strip timing parameter for each roll cluster size distribution for each roll If the roll is hit by an ionizing particle, the digitizing algorithm will assign a Digi to it with a given probability, defined by the efficiency parameter. The number of fired adjacent strips is calculated using an empirical cluster size distribution for each roll. The impact point position is used to decide the Digi coordinates. The signal propagation time and the timing parameter are used to decide the bunch crossing. A Poisson distribution is used to evaluate the probability that a certain strip gives a spurious signal (noise) during a given bunch crossing. Inter roll crosstalk is negligible. The real detectors have some dead or masked strips, which are treated in a very simple way by digitization algorithm: the corresponding efficiency and noise rate parameters are set to zero in the DB. Three sets of simulation parameters are used: the ideal conditions (e.g. efficiency 95%) the parameters estimated using the CRAFT data [3] the parameters estimated by the proton-proton collisions data However the statistics is not enough to estimate correctly the efficiency value of each strip, but the overall roll efficiency is well estimated. Thus the efficiency averaged over a roll is assigned to each strip in the roll. The efficiency needs statistics to be correctly estimated and is relatively constant over long periods of time. Thus efficiency parameters should be updated few times per year. The noise and dead strips, however may change run by run and should be constantly monitored. However the simulation do not use time-dependent conditions. All of the simulation parameters are estimated using dedicated detector performance analysis. The same analysis is used to validate the simulation and to compare it to the real data. The simulation is in very good agreement with the data. References [1] The Geant4 Collaboration, GEANT - Detector Description and Simulation Tool, 4

5 [2] R. Veenhof, Garfield, a Drift-Chamber Simulation Program user s guide, CERN Program Library W5050, 1994 [3] Serguei Chatrchyan et al., Performance Study of the CMS Barrel Resistive Plate Chambers with Cosmic Rays, JINST 5:T03017,