Monte Carlo Simulations of the Stopped Muon Monitor at LBNE using Geant 4 Daniel Poulson Senior Honors Thesis University of Colorado at Boulder, Department of Physics Defended 11/7/2013 Honors Committee: Thesis Advisor: Prof. Eric D. Zimmerman Physics Honors Advisor: Prof. James Thompson Physics Honors Committee Member: Prof. Andrew Hamilton APS This thesis entitled: Monte Carlo simulations of the Stopped Muon Monitor at LBNE using Geant 4 written by Daniel C. Poulson has been approved for the Department of Physics Prof. Eric D. Zimmerman Prof. James Thompson Prof. Andrew Hamilton Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. Abstract The purpose of the Long–Baseline Neutrino Experiment is to make measurements of neutrino oscillations. The stopped muon monitors are one part of the experiment, and are designed to measure absolute flux and composition of the tertiary muon beam. The detectors operate by detecting µ+ decay in an interior water volume, and 12B decay produced by µ− capture on a graphite shell. The design of the stopped muon monitors is simulated with a Monte Carlo software package called Geant 4. The detector’s response to µ+, µ−, 12B, as well as fast and thermal neutron backgrounds are reviewed and analysed. The inability of the stopped muon monitor to detect most 12B decays, as well as the overlap in signal of neutron interactions and 12B decay, require design modifications. The graphite shell is removed and should be exchanged for a neutron shielding material, while the water volume should be replaced with mineral oil. A scintillating veto is also included to identify events which originate, and are fully contained by the mineral oil. Follow-up simulations indicate that the 12B signal is significantly improved and the scintillating veto may be able to identify fully contained events. Contents 1 Introduction 1 1.1 Context .................................... 1 1.2 Overview.................................... 1 2 Neutrino Oscillations 3 2.1 NeutrinosinMassandFlavorBases. 3 2.2 ThePMNSNeutrinoMixingMatrix. 5 2.3 Charge-Parity Violation and Mass Effects . 7 3 The Long-Baseline Neutrino Experiment 8 3.1 AcceleratorExperiments ........................... 8 3.2 LBNE ..................................... 12 3.3 TheMuonAlcove............................... 14 4 The Stopped Muon Monitor Design and Simulation 17 4.1 TheStoppedMuonMonitor ......................... 17 4.2 Geant4 .................................... 19 5 Simulations of the Stopped Muon Monitor 22 5.1 Checking Physical Processes . 22 5.2 Simulations using the CDR Design . 27 5.3 IssueswiththeCDRDesignoftheSMM . 30 5.4 Simulations with the Revised Design . 33 5.4.1 Light Yield Changes . 33 5.4.2 VetoStudies.............................. 39 5.5 AdditionalNeutronBackground . 40 6 LBNE Simulations and Future Analysis 42 6.1 G4LBNESimulations............................. 42 6.2 FutureAnalysis ................................ 43 i 7 Conclusion 46 ii List of Figures 2.1 The two possible options for mass hierarchy, NH (a) and IH (b), depending 2 on the sign of ∆m32. This scale shows only the differences in masses. It does not show the bottom of the scale since neutrino oscillations may only measure mass differences and not absolute neutrino mass. There is a 95% confidence level that m . (0.3 1.3) eV/c2 putting the scale here in j j − 2 4 the sub eV /c [1]. ..............................P 6 3.1 A simple diagram of the creation of the ν beam. Imagetakenfrom[2] . 9 3.2 Mean rate of energy loss in different materials for different muon momen- tum. Radiative effects such as Bremsstrahlung are not included. Plot takenfrom[1]................................. 11 3.3 An aerial photon of Fermilab with markers for the future LBNE installa- tions.Imagetakenfrom[2]. .. .. 12 3.4 The side-view of the near site. The proton beam is angled down towards the Homestake Mine in Lead, SD (to the left). Image taken from [2] . .. 13 3.5 The liquid-argon time projection chamber to be used at the Homestake mine. This detector will provide data on the composition of the neutrino beam.Imagestakenfrom[3]. 14 3.6 The layout of the Muon Alcove. Each muon detector measures a different characteristic of the muon beam. For scale the distance between the and of the absorber and the beginning of the blue blocks is approximately 6 m. Imagemodifiedfrom[4] ........................... 16 3.7 The three types of muon detectors to be used in the Muon Alcove for LBNE.Imagestakenfrom[4] . .. .. 16 4.1 Two basic Feynman diagrams showing muon interactions. .. 18 4.2 A top view of the Muon Alcove. The SMMs are laid out with a 5x5 cross of detectors at the first layer, followed by several three detector high columns. Each layer is separated by a wall of shielding blue-blocks. Image takenfrom[4]................................. 19 iii 4.3 Comparing the mass attenuation coefficient of Al for γ rays of two Geant 4 physics models (Low Energy [ ]; Standard [ ]) against actual data taken • ◦ fromtheNISTdatabase[—]. Imagetakenfrom[5] . 21 5.1 The top and side view of the simulated SMM. Regions I, II, III, and IV correspond to graphite, PMTs, water, and aluminum respectively. .... 23 5.2 The starting x and y-positions of all simulated µ+. It can be seen that the positions are uniformly distributed throughout the volume. .. 24 5.3 The number of hits recorded at the PMTs (per 150,000 µ+ decays) vs z and r2 positions. The radius is squared in order to accommodate greater areabeingsweptoutasradiusincreases. 25 5.4 Comparison of signals from positive and negative muon decays. Zero hit eventsarenotincluded. ........................... 26 5.5 Comparison of signals from positive and negative muon decays including zero hit events. The peak at zero corresponds to µ− capture on nuclei. 26 5.6 The Geant 4 simulated 12B decay energy spectrum. The histogram has beennormalizedsothatsumofentriesis1. 27 5.7 The number of optical photons at PMTs for neutrons starting in the water volumefrom10MeVto10eV. ....................... 28 5.8 The number of optical photons at PMTs for neutrons starting in the water volumefrom10MeVto10eVonalogscale. 29 5.9 The number of optical photons at PMTs for µ+ uniformily distributed throughtheentiredetectorvolume. 30 5.10 The number of optical photons at PMTs for 12B simulated throughout the graphitevolumeoftheSMM. .. .. 31 5.11 Position dependence of the number of hits detected vs z and r2 positions. 32 5.12 The number of optical photons at PMTs for 12B simulated throughout the watervolumeoftheSMM. ......................... 32 5.13 The top and side view of the revised SMM. Regions I, II, III, IV, V corre- spond to graphite, PMTs, mineral oil, the scintillating veto, and aluminum respectively. .................................. 34 5.14 Simulations of 177,000 µ+ inwaterandMarcol7. 34 5.15 Simulations of 177,000 12BinwaterandMarcol7. 35 5.16 Simulations of 177,000 10 eV neutrons in water and Marcol 7. ... 35 5.17 Simulations of 177,000 µ+ in mineral oil with varying light yield. 36 5.18 Simulations of 177,000 12B in mineral oil with varying light yield. 36 5.19 Simulations of 177,000 10 eV neutrons in mineral oil with varying light yield. 37 iv 5.20 Simulations of 10 MeV to 10 eV neutrons to see the effect of Marcol 7 on the neutron background. Zero hit events are not included. ... 38 5.21 A comparison of 10 eV neutrons with 2.2 MeV photons, both histograms do not inclue zero hit events. The region of overlap is the first part of the neutronhistogram. .............................. 38 5.22 Part of the distibution of optical photons created in the scintillating veto. 39 5.23 Comparisons of all µ+ decays against the veto cut and the events which started in the mineral oil. Events with zero hits are suppressed. ... 40 6.1 Negative muon energy after the absorber with parentage information. 44 6.2 Negative muon energy after the absorber with parentage information. Ver- ticallogscale.................................. 44 6.3 Positive muon energy after the absorber with parentage information. 45 6.4 Positive muon energy after the absorber with parentage information. Ver- ticallogscale.................................. 45 v List of Tables 2.1 A 2013 update on the status of experimental best fits for the neutrino 2 mixing parameters. Since the sign of ∆m31 is ambiguous, some parameters have two rows for fits of the normal and inverted hierarchies. Data taken from[6]..................................... 6 3.1 The main decay modes for K and π, which are produced after the proton beam collides with the target. This table also holds for all charges replaced 0 0 with their charge conjugates. The K is a superposition of 50 % KS 0 and 50 % KL. It can be seen that there are potential decays which may contaminate the νµ beam in the form of νe, νµ, and νe. Data taken from [1] 10 5.1 A list of the simulated concentric cylinders used to construct the SMM. Larger cylinders encapsulate the cylinders which are smaller. The height of each cylinder spans one radius length above and one radius length below thecenterofthedetector. .. .. 23 vi Chapter 1 Introduction 1.1 Context The standard model of particle physics contains all of the particles which are currently known and describes their interaction. This model has been highly successful in its theoretical predictions when compared with experiment. However, there are still some particle interactions which are not included within the framework of the standard model. These interactions fall into the category of new physics. Describing the underlying phys- ical processes which describe new physics is the next step expanding understanding of sub-atomic physics. One example of new physics which goes beyond the standard model focuses on one of the subgroups of particles called neutrinos, and the phenomenon of neutrino mixing which is called neutrino oscillations. Understanding of neutrino oscillations has increased in recent decades due to several large scale neutrino measuring experiments.