The next generation of lepton colliders, such as the International Linear Collider (ILC) a jet energy resolution of the order of 3-4% at the Z boson mass is required for the clear separation of hadronic decays of the W and Z bosons. Within the ILC community, Particle Flow calorimetry has been identified as the most promising way to achieve this level of precision and was successfully tested on Monte Carlo simulations. The key factor in Particle Flow calorimetry is the single particle separability within a jet, which requires a high granularity of the calorimeters. The CALICE collaboration is performing studies of several designs of highly granular calorimeters, with sensitive layers finely segmented into cells, which are individually read out. The fine segmentation of the sensitive layers and the high sampling frequency allow for an excellent reconstruction of the spatial development of hadronic showers. A comparison between data and Monte Carlo simulations is presented, concerning both the longitudinal and lateral development of hadronic showers and the global response of the calorimeter. The performance of several GEANT4 physics lists with respect to these observables is evaluated.
Since 2006 several beam tests of the CALICE prototypes have been conducted at the Deutsches Elektronen-Synchrotron (DESY), at the European Organization for Nuclear Research (CERN) and at the Fermi National Accelerator Laboratory (FNAL). The paper focuses on data collected at CERN in 2007, when the experimental set-up consisted of a silicon-tungsten electromagnetic sampling calorimeter (SiW-ECAL), a scintillator-steel hadron sampling calorimeter with analog readout (AHCAL) and a scintillator-steel tail catcher and muon tracker (TCMT). The prototypes were exposed to pion beams in the energy range from 8 GeV to 100 GeV, provided by the CERN SPS H6 beam line.
The SiW-ECAL is divided into stacks composed of 30 modules of alternating tungsten and silicon layers. The silicon layers are segmented into PIN diodes of 1 × 1 cm2, for a total number of about 9700 read-out cells. The AHCAL is a sampling structure of 38 modules, each consisting of a ∼2 cm thick steel absorber plate and a sensitive layer instrumented with 0.5 cm thick scintillator tiles. The total depth of the prototype is 1.2 m, translating into about 5.3 interaction lengths, while the lateral dimensions are approximately 1 × 1 m2. Each sensitive layer is composed of scintillator tiles of different sizes. The 30 × 30 cm2 core has a granularity of 3 × 3 cm2, while the outer region is equipped with tiles of increasing sizes (6 × 6 cm2 and 12 × 12 cm2). The scintillation light from each tile is read out individually by a Silicon PhotoMultiplier (SiPM) coupled to the scintillator via a WaveLength Shifting fiber (WLS). The TCMT is positioned downstream with respect to the AHCAL in order to absorb the tails of the showers leaking out of the AHCAL. The interactions of hadrons with matter cannot be modeled from first principles alone; several phe- nomenological models, working with different approximations, exist. Several so called “physics lists” are available in GEANT4, which combine different models in different energy ranges, with a random choice of which model is used for overlapping energy regions.
The fine segmentation of the CALICE calorimeters allowed to investigate properties of pion showers especially in longitudinal and tranverse profiles. As an example of the data sensitivity we present the radial shower properties in the figure below. An accurate modelling of the transverse shower profile is particularly important for a successful development of particle flow algorithms, since it affects the degree of overlap between showers and therefore the efficiency in disentangling single particles within jets.
Measurement of the average centre of gravity
From comparisons paper concludes that FTFP_BERT and CHIPS are the physics lists that best agree with observables presented. Most observables fluctuate by ±2-6%. The considered radial observables present more significant changes of the order of 10% and show a better agreement with data for older version of the physics list. Since these tests several improvements have been implemented in the simulation.
Physicists and engineers from FZU were responsible for the design, production and testing of silicon PIN diodes for the SiW-ECAL calorimeter and the calibration system of the AHCAL calorimeter. They participated in the data taking, data analysis and presentation of results at conferences.
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