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Neutrino Induced Charged Current Π Production at the T2K Near Detector

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Neutrino Induced Charged Current Π Production at the T2K Near Detector Imperial College London Department of Physics Neutrino Induced Charged Current π+ Production at the T2K Near Detector James Edward Young Dobson June 2012 Submitted in part fulfilment of the requirements for the degree of Doctor of Philosophy in Physics of Imperial College London and the Diploma of Imperial College London 1 2 Abstract + A study of νµ-induced charged current (CC) π production at the T2K off-axis near de- tector (ND280) is presented. Using Monte Carlo (MC) data studies event selections for both CC-inclusive and enriched CC-π+ samples have been developed using the ND280 tracker-region and surrounding ECals. Two types of CC-π+ selections were developed| one using the TPC to identify the pion and the other using a new ECal PID based on the deposited charge per unit length. Data/MC ratios are calculated and compared with the associated detector, neutrino interaction and flux simulation systematics. The predicted neutrino interaction rate was based on v2.6.2 of the GENIE MC generator and on T2Ks tuned 11a JNUBEAM flux simulation. The data used was collected between Nov. 2010 and March 2011 during the Run 2 data taking period and corresponds to a total integrated POT of 7:83 1019. For the ν -CC-Inclusive selection which selects ν -CC × µ µ interactions with a purity of 88:1% we find: CCIncl CCIncl +0:032 +0:112 +0:093 NData =NMC = 1:021 0:015(stat) 0:031(det) 0:097(xsec) 0:093(flux). ± − − − For the TPC-based νµ-CC-π± selection which selects νµ-CC interactions with at least one π+ in the final state with a purity of 81:3% we find: CCπ± CCπ± +0:044 +0:125 +0:208 NData =NMC = 1:041 0:057(stat) 0:044(det) 0:127(xsec) 0:190(flux). ± − − − For the ECal-based νµ-CC-π± selection which selects νµ-CC interactions with at least one π+ in the final state with a purity of 67:9% we find: CCπ± CCπ± +0:074 +0:119 +0:205 NData =NMC = 0:985 0:070(stat) 0:1 (det) 0:118(xsec) 0:187(flux). ± − − − These show that the current measured and predicted rates for both the inclusive rate of νµ neutrino interactions and those with at least one π+ in the final state agree to within the systematic uncertainties associated with neutrino interaction and flux simulation. More- over, these selections lay the groundwork for future analyses, using larger data sets, that can be used to constrain these sources of uncertainty. 3 4 Declaration As is typical for a thesis performed within the context of a large collaborative physics experiment the work presented here relies critically on the output of a large body of work, which in the case of T2K was performed by around 500 people over the last 10 years. The purpose of this declaration is to make clear my personal contributions. Chapter 1 gives an overview of the current status of neutrino oscillation physics and describes the modelling of neutrino interactions in the few-GeV region using the GENIE Monte Carlo generator. This information was synthesised from a number of sources includ- ing textbooks, review articles and publications and is referenced appropriately. Through- out my time on T2K I have also been an active member of the GENIE collaboration. This involved acting as the contact point between T2K and GENIE as well as work on the validation and optimisation of the T2K specific event generation programs, a short summary of this work is given in 1.3.5. x Chapter 2 describes T2K and is largely based upon the published descriptions of the experiment. The motivation for studying neutrino induced charged current π+ produc- tion at the T2K near detector discussed in 2.5 is based on the output of internal studies x performed as part of the T2K disappearance measurement and is also referenced appro- priately. I contributed to these studies through work on the validation and development of GENIE specific reweighting libraries. In addition to my work within the GENIE collabo- ration my own personal contributions to T2K come mainly from my involvement with the UK designed and built ND280 ECal and my role as manager, inherited from my supervisor Dr Yoshi Uchida1, of the oaAnalysis software package. oaAnalysis is the important final stage in the ND280 software chain which distills and collates the information necessary to allow individual users to perform ND280-based analyses. My work on the ND280 ECal included an early test bench, performed under the supervision of Dr Antonin Vacheret2, which provided the first demonstration of the complete scintillator bar readout chain, as well as work on the in situ calibration of the ECal using cosmic tracks resulting in the provision of routines to equalise the response of all channels on the Ds- and Barrel-ECal. I also contributed to the testing of the Ds-ECal in the CERN T9 test beam and its com- missioning in the ND280 detector in J-PARC. The main body of my own work is presented in chapters 3 and 4. In chapter 3 the event selections I developed to study neutrino induced π+ production at the T2K near detector are described. These are based on the output of the sub-detector and global reconstruc- 1Of Imperial College London. 2At the time at Imperial College London but now at the University of Oxford. 5 tion, and in particular the global vertexing algorithm, developed by others. The data-MC comparisons and validation work presented is my own. In general I use the particle iden- tification (PID) variables provided by the various sub-detectors, the only exception is the ECal-based dQ=dL PID which I developed specifically for this analysis. The evaluation of the detector systematics associated with these selections was performed by myself but draws heavily on the output of internal studies performed within the T2K NuMu physics working group. In chapter 4 I evaluate the systematics associated with neutrino flux and neutrino inter- action simulation using the T2KReWeight package. T2KReWeight was originally devel- oped by my co-supervisor, Dr Costas Andreopoulos3, and myself with the aim of providing a GENIE specific reweighting tool. The generalisation of T2KReWeight to act as an in- terface to not only the GENIE reweighting libraries but also to the NEUT MC generator and JNUBEAM flux reweighting libraries was my own work. This involved providing the general interface and implementations for both the NEUT and JNUBEAM reweighting libraries in the form of example templates which were then filled out by the relevant ex- perts. I herewith certify that the above and all other material in this dissertation which is not my own work has been properly acknowledged. James Edward Young Dobson 3Of the Rutherford Appleton Laboratory, UK. 6 Acknowledgments Over the past four years I had more than my fair share of the knowledge, guidance and support that access to not one, but two, excellent mentors brings. I would like to thank my supervisor at Imperial, Dr. Yoshi Uchida|who, in addition to providing the expertise and advice only possible from someone with such a broad background in particle physics, also encouraged me to work on multiple aspects of the experiment, which helped me develop as a researcher. I would especially like to express my thanks to him for the generous level of support and encouragement I received towards the end of my PhD. I would also like to thank Dr. Costas Andreopoulos, my co-supervisor at the Rutherford Appleton Laboratory (RAL). Through frequent trips to RAL I benefited from his extensive knowledge of both neutrino cross section and oscillation physics. The invaluable physics discussions, often philosophical, on the many lifts to and from Didcot train station were a delight, and contributed significantly to my motivation over the course of my doctorate. The T2K Imperial group has been an excellent place to study. The feedback received at the weekly group meetings was central to developing the work presented in this thesis. Many thanks to all the students and staff for their contributions. In particular I would like to acknowledge the guidance and suggestions of Dr. Morgan Wascko and Dr. Antonin Vacheret. T2K is a large experiment and it almost goes without saying that the work presented here would not have been possible without the effort of many people. I am very grateful for the feedback and suggestions I received from the NuMu physics group regarding the near detector selections I developed: in particular I would like to acknowledge suggestions and clarifications I received from Dr. Thomas Lindner, Dr. Federico Sanchez, Dr. Anthony Hillairet, Javier Caravaca, Prof. Scott Oser, Dr. Mark Hartz and Dr. Anselmo Cervera when evaluating the detector and flux systematics. I am grateful to Gustav Wikstrom who developed the global vertexing algorithm upon which this thesis relies so heavily. In addition I thank my GENIE and T2K collaborator Prof. Steve Dytman for all the useful discussions and suggestions with regards to charged current π production and the methodology of my analysis. Finally I would like to thank all of my family and close friends who have been so supportive over the last four years. In particular I would like to thank my mother Dor, stepmom Biddy and my wonderful girlfriend Martina for their dedicated support and understanding over the last year, without which I would not have been able to finish. 7 8 Contents 1 Neutrino Oscillation Physics 15 1.1 Neutrino Oscillation Formalism . 16 1.1.1 Matter Effects . 20 1.2 Overview of Current Knowledge . 21 1.3 Neutrino Interactions: The GENIE Monte Carlo Generator .
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