A Geant4 validation study for the ALICE experiment at the LHC Kevin Nicholas Barends Department of Physics University of Cape Town Supervisor: Dr Alexander Kalweit Co-supervisor: Dr Sandro Wenzel 04 August 2017 Abstract This report depicts the work I have done during my 8 week period as a summer student. ALICE officially uses GEANT3 but needs to switch to Geant4 as it is advantageous. During this study, we observed that GEANT3 describes the experimental data more accurately at higher momentum in certain regions of the energy loss in the TPC whereas Geant4 describes the data more accurately at lower momentum. We also observed that Geant4 has a higher tracking efficiency for the proton, pion, kaon and antiproton at low momentum in the TPC and TOF. At mid-momentum, GEANT3 and Geant4 is observed to have the same efficiency. However, further study is needed.
1 Introduction The main focus of the ALICE experiment is to study the aftermath of high energy heavy-nuclei collisions (such as Pb-Pb). The dynamics of these collisions are dominated by the strong interaction (QCD) and ALICE is particularly interested in one physics process known as the quark-gluon plasma (QGP). In order to study QGP, ALICE needs to be able to fully reconstruct the events that occur from the interaction point. Thus, ALICE uses all the known particle tracking and identification techniques [1]. The experiment itself is not enough to study the physics of the strong force. There needs to be a theoretical reference on which to compare. Computer simulation software such as Monte Carlo generators and GEANT are used to recreate the experiment and provide this theoretical reference. ALICE uses many different event generators (each one specific to a certain type of experiment) to generate the collisions and GEANT3 to simulate the particle transport and detector response. However, there is a newer version of GEANT known as Geant4 which has many advantages over GEANT3, i.e. [2] Geant4 has a more accurate model of hadronic interactions at low momentum Geant4 allows to switch and customize different models for certain physics processes (called physics list ) Geant4 is well maintained and supported Despite these advantages, ALICE cannot just switch to Geant4 because it is a complete rewrite of GEANT3 in a different programming language. GEANT3 is written in Fortran which is a procedural programming language while Geant4 is written in C++ which is an object-oriented programming language. This complete rewrite in a different programming language could potentially bring about many bugs and, therefore, ALICE needs to do validation studies before making the decision to switch. This project serves to assist ALICE in its validation study by specifically looking at how well GEANT3 and Geant4 compares to experimental data between different particle energy losses through the time-projection chamber (TPC) and comparing the tracking efficiency between GEANT3 and Geant4 for certain particles through various detectors in the experiment. The experimental dataset that will be used is from 2010 pass4 (LHC10b) while the simulation datasets that will be used are LHC17c4d for GEANT3 and LHC17c4b for Geant4 as they have the same detector design. 2 Specific energy loss in the TPC The TPC in the ALICE experiment can provide particle tracking and identification at very high precision [3]. It was filled with a gas which consisted of 90 Ne/ 10 CO 2 / 5 N 2 for the dataset which we considered. When a charged particle traverses through the gas, it will excite and ionize the atoms in the gas along its trajectory. Thus, the charged particle loses an amount of energy per unit track length (de/dx) which is unique to each particle. This energy loss is described by the Bethe-Bloch formula, de dx = 4πNe4 mc 2 z 2 β 2 (ln 2mc2 β 2 γ 2 I 2 β 2 δ(β) 2 where mc 2 is the rest mass energy of the electron, z the charge of the particle, N the number density of electrons in the matter traversed, e the elementary charge, β the velocity of the particle and I the mean excitation energy of the atom. Each atom in the gas that is ionized by the passing charged particle releases an electron and these electrons are then directed towards the read-out chamber, producing the signal and track of the charge particle (see Fig. 1). The TPC schematic shown in Fig. 1 allows for a 3D reconstruction of the track of the passing charged particle. Thus, we can measure the momentum and energy loss of the incoming charged particle and then determine its mass (and therefore its identity). ) 1
Figure 1: A diagram that depicts the physical processes of the TPC [3]. 2.1 Analysis From the Bethe-Bloch formula we know that each particle has a unique amount of energy loss per unit track length and therefore we can study the differences (or similarities) between GEANT3 and Geant4 when compared to experimental data around these particle regions. In this study the minimum ionizing, kaon, proton and electron regions were considered. We decided to look at the energy loss against momentum in the TPC while applying some cuts to ensure that we only extract the information from the TPC, i.e. p < 10 GeV/c η < 0.8 The energy loss diagram in the TPC for experimental data is shown below (including the different particle regions) Figure 2: de/dx spectrum of the TPC [3] 2
2.2 Results We first compared the de/dx spectrum of the TPC for both GEANT3 and Geant4 as shown in Figure 3. Figure 3: Comparison of energy loss signal in GEANT3 and Geant4. When studying the figures above, we don t see much difference. Thus, we considered studying specific regions within the diagram, i.e. The Minimum Ionizing region: Figure 4: Comparison of energy loss in GEANT3, Geant4 and experimental data in the minimum ionizing region. From Fig. 4 we observe that at the lower momentum section, i.e. p < 0.55 GeV/c, Geant4 describes the experimental data more accurately whereas at the higher momentum section, i.e. 0.55 GeV/c < p < 1.0 GeV/c, GEANT3 describes the experimental data more accurately. The value 0.55 GeV/c is not arbitrary. There is a minimum which lies in between 0.5 GeV/c and 0.6 GeV/c and we projected this region onto the energy loss axis to compare the position of this minimum between GEANT3, Geant4 and experimental data, i.e. 3
Figure 5: Comparison of the mean minimum position in GEANT3, Geant4 and experimental data. From Fig. 5 we observe that the experimental data s minimum position occurs approximately midway between the Geant4 and GEANT3 minimum positions. The Kaon and Proton regions: Figure 6: Comparison of energy loss in GEANT3, Geant4 and experimental data in the kaon (left) and proton (right) regions. From Fig. 6 (left), we observe that Geant4 describes the experimental data more accurately across the momentum considered. From Fig. 6 (right), we observe that Geant4 describes the experimental data more accurately in the lower momentum region, i.e. 0.4 GeV/c < p 0.45 GeV/c, and as the momentum increases from 0.45 GeV/c to 0.6 GeV/c, GEANT3 describes the experimental data more accurately. 4
The Electron region: Figure 7: Comparison of energy loss in GEANT3, Geant4 and experimental data in the electron region. From Fig. 7, we observe that GEANT3 describes the experimental data more accurately across the momentum considered except for a few outliers (which should be fixed when we consider more statistics). We can also note that the electron region is fairly linear and therefore we can project this region onto the momentum axis and compare the position of the electron, i.e. Figure 8: Comparison of the mean electron position in GEANT3, Geant4 and experimental data. The above conclusion drawn from Fig. 7 is further validated in Fig. 8. In Fig. 8, we see that the mean position of the electron in the experimental data is closer to the mean position of the electron in GEANT3 than in Geant4. 5
3 Tracking Efficiency ALICE uses many event generators, such as Pythia, HIJING etc (each one is used for a specific experiment), to simulate the collisions and the aftermath thereof. Many different particles are produced in different directions. These particles are known as the generated particles. Once the particles are generated, ALICE implements GEANT3 to transport the particles through the different detectors in the experiment and simulates each detector s response. However, this response is influenced by many different aspects such as detector material, detector resolution and many more, and therefore we would like to know how efficient are GEANT3 and Geant4 in tracking the generated particles to a response in the detectors. We only considered the response in the TPC and the TOF in this study for tracking protons, pions, kaons and antiprotons. As ALICE would like to know whether the collaboration should switch to Geant4, we compared the efficiencies of GEANT3 and Geant4 for each of the particles mentioned before for both detectors. Below is diagram showing the ALICE construction for the datasets we used 3.1 Results Proton: Figure 9: Comparison of the proton efficiency in the TPC (left) and TOF (right) between GEANT3 (red) and Geant4 (blue). 6
From Fig. 9, we see that for both detectors GEANT3 and Geant4 are comparable (this is further validated in Fig. 10 with an exception). Moreover, we see that the efficiency in the TPC is higher than the efficiency in the TOF. This could be a result of the resolution of the different detectors being poor as it transports from the TPC to the TOF and/or the TRD, which is in between in the TPC and the TOF, which could be absorbing some of the particles and therefore they never reach the TOF. Figure 10: Proton efficiency ratio between GEANT3 and Geant4 for the TPC (left) and the TOF (right). In Fig. 10, below 0.5 GeV/c we see that the efficiency between GEANT3 and Geant4 is lower than 1 for both detectors. This means that the efficiency in Geant4 is greater than in GEANT3 at low momentum. Pion: Figure 11: Comparison of the pion efficiency in the TPC (left) and TOF (right) between GEANT3 (red) and Geant4 (blue). We see a very similar observation for the pion as we did for the proton except, now in the pion case, both GEANT3 and Geant4 are comparable across the momentum considered (see Fig. 12). 7
Figure 12: Pion efficiency ratio between GEANT3 and Geant4 for the TPC (left) and the TOF (right). Kaon: Figure 13: Comparison of the kaon efficiency in the TPC (left) and TOF (right) between GEANT3 (red) and Geant4 (blue). From Fig. 13, we see that for both detectors GEANT3 and Geant4 are comparable (this is further validated in Fig. 14). Once again, we can note that the efficiency drops when going from the TPC to the TOF. The same observation is drawn as in the proton case. 8
Figure 14: Kaon efficiency ratio between GEANT3 and Geant4 for the TPC (left) and the TOF (right). From Fig. 14, we observe that at low momentum Geant4 has a higher efficiency whereas at high momentum the efficiencies between GEANT3 and Geant4 fluctuate around 1. The fluctuations are, however, not too wide spread and therefore the two simulation programmes are fairly comparable at high momentum. Antiproton: Figure 15: Comparison of the antiproton efficiency in the TPC (left) and TOF (right) between GEANT3 (red) and Geant4 (blue). In Fig. 15 (left), we observe that there is a noticeable discrepancy just after 0.5 GeV/c betwen GEANT3 and Geant4, i.e. Geant4 has a higher efficiency than GEANT3. Other than this discrepancy, it appears that GEANT3 and Geant4 are fairly comparable in the TPC. However, when extrapolating to the TOF (right), there are noticeable differences in the efficiencies between GEANT3 and Geant4. Although, the usual reduction in efficiency is observed. 9
Figure 16: Antiproton efficiency ratio between GEANT3 and Geant4 for the TPC (left) and the TOF (right). In Fig. 16, we observe that around and below 0.5 GeV/c Geant4 has a higher efficiency than GEANT3 in the TPC (left). After the extrapolation to the TOF (right), we observe the efficiency between GEANT3 and Geant4 fluctuate greatly starting from 0.5 GeV/c. 4 Conclusion This project is meant to assist ALICE in their decision concerning the switch to Geant4. We studied specific outputs from GEANT3 and Geant4 and compared the two. We also compared the outputs with experimental data. One of the specific outputs we studied was the energy loss in the TPC. The overall result (Fig. 3) yielded no noticeable differences. However, we were not satisfied with this observation and dug deeper. We studied specific regions within this diagram to make a more comprehensive comparison. We chose to study the minimum ionizing, kaon, proton and electron regions. Each region yielded a different conclusion when comparing GEANT3 and Geant4. For the minimum ionizing (Fig. 4 & 5) and proton region (right plot in Fig. 6), Geant4 describes the experimental data more accurately at low momentum (relative to the momentum considered) whereas GEANT3 describes the experimental data more accurately at high momentum (relative to the momentum considered). For the kaon region (left plot in Fig. 6), Geant4 describes the experimental data more accurately across the momentum considered. And, lastly, for the electron region (Fig. 7 & 8), GEANT3 describes the experimental data more accurately across the momentum considered. Another output we studied was the tracking efficiency in the TPC and TOF. When comparing GEANT3 with Geant4 for protons (Fig. 10) and pions (Fig. 12), they are comparable across the momentum considered except that in the proton case, Geant4 yields a higher efficiency below 0.5 GeV/c. However, more discrepancies arise when comparing the efficiencies of the kaon (Fig. 14) and the antiproton (Fig. 16). At low momentum (below 0.5 GeV/c) Geant4 has a higher efficiency but at high momentum (p 1.5 GeV/c) the efficiency between GEANT3 and Geant4 fluctuate at very noticeable amounts. There is one common conclusion we can draw from each particle is that there is a reduction in efficiency when extrapolating from the TPC and TOF which means that the TRD has an effect on tracking efficiency and this needs to be considered when conducting analysis. The above analysis shows that both GEANT3 and Geant4 seems to describe better or worse different aspects of the experimental data. They differ in many respects but are also similar in many others. Depending on the experiment that is being conducted, caution needs to be taken into account when deciding whether to use GEANT3 or Geant4. 10
Acknowledgements I would like thank Alexander Kalweit and Sandro Wenzel for their continuous support, guidance and motivation throughout this project. I would like to thank the CERN Summer School team and ALICE for the opportunity to be able to assist ALICE in reaching their goal. I would like to thank SA-CERN (Ithemba Labs) for the financial support. References [1] ALICE Collaboration (2008) The ALICE experiment at the CERN LHC, JINST 3 S08002. Online: http://iopscience.iop.org/article/10.1088/1748-0221/3/08/s08002/meta [2] Geant4 Collaboration (2003) GEANT4: a simulation toolkit, Nucl. Instrum. Meth. A 506 250-303. Online: https://inspirehep.net/record/593382/files/fermilab-pub-03-339.pdf [3] Kalweit, A. (2012) Production of light flavour hadrons and anti-nuclei at the LHC, Doctoral Thesis. [4] Thaeder, J. (2010) 2010-ALICE Cross-Section. Online: https://aliceinfo.cern.ch/figure/node/3396 11