Study of the performance of hadron calorimeter using Monte Carlo techniques

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1 Indian Journal of Pure & Applied Physics Vol. 53, January 2015, pp Study of the performance of hadron calorimeter using Monte Carlo techniques Y Elmahroug 1, *, B Tellili 1,2 & C Souga 3 1 Université de Tunis ELManar, Faculté des Sciences de Tunis, Unité de Recherche de Physique Nucléaire et des Hautes Energies, 2092 Tunis, Tunisie 2 Université de Tunis El Manar, Institut Supérieur des Technologies Médicales de Tunis, 1006 Tunis, Tunisie 3 Université de Carthage, Ecole Polytechnique de Tunisie, B.P La Marsa, Tunisie * youssef_phy@hotmail.fr Received 5 June 2014; revised 24 September 2014; accepted 8 November 2014 In the present paper, the linearity and the energy resolution of a hadron calorimeter based on Glass Resistive Plate Chambers (GRPCs) as an active element and lead as an absorber are examined for two operation modes, analog and digital by using Geant4 simulation toolkit. This investigation has been carried out in order to develop a hadron calorimeter for the future linear collider International Linear Collider. Keywords: Calorimetry, Resistive plate chambers, International linear collider, GEANT4 1 Introduction The Standard Model has gradually imposed to describe the interactions between fundamental particles. This model has enabled almost perfectly describe the electroweak and strong interactions as well as the properties of quarks, leptons and bosons. But it has several shortcomings. Indeed, it is not possible to unify into a single theory the strong and electroweak interactions, or to obtain a satisfactory description of quantum gravity or to explain the origin of dark matter. To complement and possibly beyond the limits imposed by the Standard Model, CERN has built the new proton-proton accelerator LHC. With this new collider, four experiments are installed: ALICE, ATLAS, CMS and LHCb. This collider opens the way for fundamental research in physics at energy scales never reached. For the same purposes, physicists are already working on next generation accelerators. Thus, the project named ILC or International Linear Collider was born. Measuring more than 30 km long 1. It is much more powerful than the previous electronpositron colliders and will clarify many of the discoveries at the LHC and especially the searching for one or more Higgs particles and the study of their characteristics 1. Many interesting physics processes will be provided by the electron-positron collisions at the center-of-mass energies between 500 GeV and 1 TeV in ILC such as W, Z and H heavy bosons processes 1. These processes will be characterized by multi-jet. A very good energy resolution (30%/E) is required to the reconstruction of heavy bosons (W, Z, H) in hadronic final states 2,3. The most efficient method to achieve this resolution is Particle Flow Algorithm (PFA). This method allows to separate,respectively the deposited energies from photons and due to hadrons and the deposited energies from the charged hadrons and due to neutral hadrons in a jet. The optimal application of this technique requires highly granular hadronic calorimeter in order to separate the shower 3. Accordingly, the study of a calorimeter performance is an essential step. In this context, several kinds of hadron calorimeters are under construction and testing 4-7. Among the best hadron calorimeters, there are those which use Resistive Plate Chambers (RPCs) as active elements. This is due to their several advantages as compared to other detection technologies, such as their strength and very good performances in terms of temporal and spatial resolutions Glass RPCs were chosen instead of bakelite RPCs due to their long-term stability, they do not need oil surface treatment with linseed oil, their fabrication is simple with low production cost and their homogeneity In the present paper, a study based on Geant4 simulation is conducted to investigate the performance of a hadron calorimeter using lead as an absorber material and Resistive Plate Chambers (RPCs) as active elements for the future linear

2 8 INDIAN J PURE & APPL PHYS, VOL 53, JANUARY 2015 collider ILC. The present paper aims to study and simulate the parameters which characterize the functioning of a hadron calorimeter as the energy resolution, the linearity and the reconstructed energy and also to make a comparative study between digital and analog methods. 2 Methodology To simulate the operation of a hadron calorimeter, a Monte Carlo simulation program 18,19 was developed based on the GEANT4 (Geometry and Tracking) version 9.2.p04. This program is inspired from many examples of Geant4 and uses the Linear Collider Physics List to simulate the interactions of particles with matter 20. The information collected from this program are handled by another programs based on software Root 21. The calorimeter used in the present study is composed of alternating passive layers called absorber and active layers called sensitive material. The dimensions of calorimeters are cm 3 and 95% of hadronic shower cascades energy is deposited in the calorimeter 22,23. The whole of calorimeter consists of forty layers (absorber + sensitive material). Each active layer is segmented into 1cm 1cm readout cells (pads) and a total of channels in 40 layers. The absorber is a layer of 2 cm of lead and the sensitive detector is a Glass Resistive Plate Chambers (GRPCs). The structure of RPC is shown in Fig. 1, for more details about the RPC structure see Refs (14, 24). To study the performance of calorimeter, we generated typical events occurring in hadronic interactions. Negative pions with incident energies varying between 5 and 100 GeV, in the orthogonal direction to the face of the calorimeter were chosen events of negative pions were generated for each level of energy of 5 GeV. The high number of events was necessary to reduce the statistical errors. The CPU time required to process one configuration varies from a few hours for lower energies and can reach 2 days for high energies as the case of the primary pions with energies of 100 GeV. The objective of this simulation is to reconstruct the energy of the incident pion based solely on the information collected at the active part of calorimeter. 3 Results and Discussion 3.1 Energy Resolution and Linearity The first step for analysing the data simulated by Geant4, is to measure the total deposited energy by a primary pion in the active medium (the resistive plate chambers) for the analog mode and the digital mode, instead of using the deposited energy, we counted the total number of hits in the active medium. Indeed, the analog mode is based on the following principle, each cell activated by the passage of a secondary particle sends analog signals to the acquisition chain proportional to the energy deposited in the cell. As against, in the digital mode, a secondary particle passing through the active part is detected by a triggering signal. If the energy measured by an RPC is above the threshold, the cell is enabled and a binary signal is sent to the read chain. In this case, an event is characterized by a number of affected cells (hits). The second step for data simulation is plotting and fitting the distributions of the measured energy and number of hits. From the fit parameters, we determined the linearity and the resolution. The distribution of the deposited energy and the distribution of the hits number for primary pion with an energy of 50 GeV are shown in Figs 2 and 3, respectively. The determination of the proportionality factor between signal and the primary pions energy is divided into two main stages. In the first step, we took the average value of deposited energy and the hits number. The second step consists in plotting these values as a function of primary pions energy and then a linear interpolation is performed by a polynomial function of first degree (p = p 0 + p 1.x). Fig. 1 Schematic view of a glass RPC (Refs14, 24) Fig. 2 Distributions of deposited energy in the active layers of analog hadron calorimeter (GRPCs) for 50 GeV pions

3 ELMAHROUG et al.: PERFORMANCE OF HADRON CALORIMETER 9 Fig. 3 Distributions of hits number for 50 GeV pions Fig. 6 Comparison between the energy resolution of the digital readout and that of the analog (p 0 =C and P 1 =S) Table 1 Parameters of resolution Mode Parameters of Resolution Stochastic(%) Constant (%) Digital 67.75± ± Analog 48.73± ±0.168 Fig. 4 Linearity between the hits number and the primary pions energy for digital readout Fig. 5 Linearity between the deposited energy and the primary pions energy for analog readout Linearity The linearity between the calorimeter response and the primary pions energy for digital and analog modes are shown in Figs 4 and 5, respectively. The deposited energy and the hits number are proportional to the incident energy. By taking into account the fit parameters, the deposited energy and the hits number are as follows: E dep (MeV) = ( ± ) E(GeV) (0.0708± )...(1) N hit = (9.878± ) E(GeV) +(1.452± )...(2) Resolution of the Deposited Energy and the Hits Number The resolution is defined as the ratio between the width at half height and the mean value. We determined it, then plotted as a function of the primary pion energy. The same method used to estimate linearity was used to determine the energy resolution. The fit parameters of the deposited energy distribution or hits number were used. The obtained values are plotted as a function of primary pions energy. The fitting was done by the following resolution function: σ S N = + + C (3) E E E where S is the term of stochastic, C a constant and N is the electronic noise term which is not taken into account because there is no electronic noise in Monte Carlo simulations. The energy resolution for digital and analog modes is shown in Fig. 6. The fits values (p0=c and p1=s) are presented in Table 1. At low energies, the energy resolution in the digital mode is better than in the analog mode, this can be explained by the fact that in an analog calorimeter, the

4 10 INDIAN J PURE & APPL PHYS, VOL 53, JANUARY 2015 deposited energy by a pion in RPCs presents a large tail due to Landau fluctuations. These fluctuations increase the energy resolution for the analog readout as compared to the digital case, because these effects are suppressed in the digital mode. However, for high energies, we observe the degradation of the resolution in digital mode which is caused by saturation effects of hits number. Indeed, at high incident energy, the shower particle density will be very high, and this increases the probability that a cell will be crossed by several secondary particles, but these particles will be counted as one hit. energy distribution and also the reconstructed energy distribution using the hits number has the same shape as that the hits number distribution Response and Linearity The linearity between reconstructed energy and primary pions energy is obtained by plotting the mean values of the reconstructed energy distribution according to the incident energy. These points are fitted by a polynomial of degree one. The linearity curves for digital and analog modes are shown in Figs 9 and 10, respectively. 3.2 Reconstructed Energy Distribution of Reconstructed Energy The linear relationship between the calorimeter response and the incident pion energy can be translated by Eqs (1) and (2). Using these two equations and replacing E by E rec, the reconstructed energy can be written as follows: E (76.98 ± 0.62) Edep + (5.45 ± 0.49) Analog (GeV)= (0.10 ± ) N 4 + (0.15 ± ) Digital (4) rec 5 From Eq. (4), we determined the reconstructed energy of each primary pion. Figures 7 and 8 show the reconstructed energy distributions using the deposited energy and the hits number for primary pions with energies of 100 GeV. It is clear that the reconstructed energy distribution using the deposited energy has the same shape as that of the deposited Fig. 8 Distributions of reconstructed energy for 100 GeV pions for digital readout Fig. 9 Linearity between the reconstructed energy and the primary pions energy for digital readout Fig. 7 Distributions of reconstructed energy for 100 GeV pions for analog readout Fig. 10 Linearity between the reconstructed energy and the primary pions energy for analog readout

5 ELMAHROUG et al.: PERFORMANCE OF HADRON CALORIMETER 11 Fig. 11 Comparison between the energy resolution of the digital readout and that of the analog, using reconstructed energy (p 0 = S and p 1 = C). From the fit parameters, we can deduce that the reconstructed energy can be written in the following form: ( ± ) E(GeV) +( ± ) Analog Erec (GeV)= (5) (1.002 ± ) E (GeV) +( ± ) Digital The linearity degradation in the digital mode is due to the saturation phenomenon. As explained, at high incident energies, the number of counted hits is saturated, for this, it does not follow the augmentation of incident energy Resolution of Reconstructed Energy To determine the energy resolution (σe rec /E rec ), the same procedure was used to calculate and plot the resolutions (σ N /N) and (σe dep /E dep ). The resolution for digital and analog modes is shown in Fig. 11, respectively. As well as the deposited energy resolution (σe dep /E dep ) and the hits number (σ N /N) at low energies, the digital mode provides a better resolution (σe rec /E rec ). This is due to Landau fluctuations of the reconstructed energy in the analog case. At high energies, in the analog mode, the resolution is better than in the digital case because of the effects of hits number saturation 25,26. This resolution can be written as follows: σ E E rec rec (61.04 ± 3.984)% + (5.141 ± %) Analog E(GeV) = (44.1± 2.444)% + (9.524 ± )% Digital E(GeV) (6) 4 Conclusions The simulation study of the hadron calorimeter characteristics which uses lead as an absorber and resistive plate chambers as an active medium, shows that at low energies, the energy resolution is better for the digital mode than for the analog mode, and is opposite for high energies. However, the calorimeter linearity response is better for the analog mode than for the digital mode. The analog resolution is dominated only by Landau fluctuations, but in the digital mode the resolution and linearity are degraded by the saturation effect which can be suppressed by applying two or three thresholds. References 1 ILC Reference Design Report (RDR), ILC Collaboration, ILC- REPORT ILC Homepage: 2 Doroba K, Acta Phys Pol B, 35 (2004) Thomson M A, Nucl Instrum Methods Phys Res A, 611 (2009) Ammosov V, Nucl Instrum Methods Phys Res A, 494 (2002) Ammosov V, Nucl Instrum Methods Phys Res A, 533 (2004) Andreev V, Balagura V, Bobchenko B & Buzhan P, Nucl Instrum Methods Phys Res A, 540 (2005) Andreev V, Cvach J, Danilov M & Devitsin E, Nucl Instrum Methods Phys Res A, 564 (2006) Repond J, Nucl Instrum Methods Phys Res A, 572 (2007) Drake G, Repond J, Underwood S & Xia L, Nucl Instrum Methods Phys Res A, 578 (2007) Bilki B, Butler J, Cundiff T & Drake G, JINST, 3 (2008) Bilki B, Butler J, May E, Mavromanolakis G, Norbeck E, Repond J, Underwood D, Xia L & Zhang Q, JINST, 4 (2009) Calcaterra A, Sangro R de, Gamba D & Mannocchi G, Nucl Instrum Methods Phys Res A, 533 (2004) Akindinov AV, Alici A & Anselmo A, Nucl Instrum Methods Phys Res A, 533 (2004) Ammosov V, Gapienko V, Ivanilov A, Semak A, Sviridov Yu & Zaets V, IHEP, P (2007) Va vra J, Nucl Instrum Methods Phys Res A, 515 (2003) Ambrosio M, Candela A, Deo Mde, D Incecco M, Gamba D, Giuliano A, Gustavino C, Lindozzi M, Morganti S, Redaelli N, Tonazzo A, Trapani P & Trinchero G C, IEEE Trans Nucl Sci, 50 (2003) Fonte P, Nucl Sci, 49 (2002) Agostinelli S, Allison J & Amako K, Nucl Instrum Methods Phys Res A, 506 (2003) The Geant4 web page: 20 Linear Collider Physics List Homepage: sics\_lists/ilc/ilc\_physics\_list.html. 21 ROOT, an object-oriented data analysis framework, version CERN, (2011) 22 Adloff C Blaha J, Blaising J, Chefdeville M, Espargilière A & Karyotakis Y, JINST, 4 (2009) Leroy C, Rancoita P G, Rep Prog Phys, 63 (2000) Bedjidian M, Belkadhi K, Boudry V & Combaret C, JINST, 6 (2010) 02001; ST, 6 (2010) P02001.