Measuring the disappearance of muon neutrinos with the MINOS detectors

TLDR: In this article, the authors describe the most recent measurement of muon neutrino disappearance in the NuMI beam using the MINOS experiment using a pair of steel scintillator tracking calorimeters.

Abstract: MINOS is a long baseline neutrino oscillation experiment. It measures the flux from the predominately muon neutrino NuMI beam first 1 km from beam start and then again 735 km later using a pair of steel scintillator tracking calorimeters. The comparison of measured neutrino energy spectra at our Far Detector with the prediction based on our Near Detector measurement allows for a measurement of the parameters which define neutrino oscillations. This thesis will describe the most recent measurement of muon neutrino disappearance in the NuMI muon neutrino beam using the MINOS experiment. \\\\ The general method of a disappearance analysis at the MINOS experiment will be outlined, the selection of events, extrapolation between detectors, and fitting the data to the atmospheric mixing parameters. An analysis of the full MINOS Forward Horn Current charged current muon neutrino interactions sample is detailed, with a best fit to the atmospheric mixing parameters in a two flavour approximation of $\Delta |m^2_{atm}| = 2.42\times10^{-3}~\mathrm{eV}^2$ and $\sin^2(2\theta_{23}) = 0.936$. The change to a three flavor analysis with matter effects from a simple two flavour approximation is described, with a very slight preference of $-2\Delta \log(L)=0.01$ for the inverted hiearchy found. A study of the NuMI beam is also shown, with potential locations for new oscillation experiments in the NuMI beam discussed.

Figures

Table 3.1: Perecentage contribution of different decays in the target hall to the neutrino event rate in the MINOS Near Detector during low energy forward horn current running after the addition of helium to the pipe. Numbers from truth information in Monte Carlo (MC).

Table 5.1: Run Periods showing accumulated PoT in the Far Detector. Beam types as described in Section 3.2.

Figure 4.1: Left, Far Detector νµ data and predictions for the no oscillations hypothesis (red histogram) and with the best-fit oscillation parameters from the best fit to the oscillations hypothesis (blue histogram). The band around the oscillated prediction represents the total systematic uncertainty described in section 4.3.1. Total background in the oscillated prediction is also displayed (gray shaded histogram). Right, ratio of Far Detector νµ data to a null oscillation prediction (black) and a ratio of a Far Detector best-fit to oscillation prediction to a null oscillation prediction (blue).

Figure 3.12: Shown is the true energy lost per scintillator plane for a MC cosmic muon and the window in track length used for calibration. Figure taken from [69].

Figure 4.12: The steps in the extrapolation of a Near Detector measurement to a Far Detector prediction.

Figure 5.8: Reconstructed energy distribution of events selected as neutrinos in the Near Detector. The red histogram represents the Monte Carlo expectation with the systematic error, the blue histogram represents the total neutral current background, and black points represent data.

Full text

Measuring the Disappearance of
Muon Neutrinos with the MINOS
Detectors
A thesis submitted to University College London for the degree of
Doctor of Philosophy
in the Faculty of Physical Sciences
August 2013
Alexander Radovic
Department of Physics and Astronomy

The last few years have been a long and exciting adventure, and I am in the
debt of a great many friends, colleagues and family members. I have had a great
many mentors, not least of all my supervisor Prof. Jenny Thomas who has been
a constant source of sage wisdom, often when I didn’t even know I needed it.
Thank you for you honest advice and support over these past 3 years.
I still remember my first fledgling months working with MINOS within the “dis-
appearance group”, and I would like to thank Justin Evans for his support, guid-
ance, and patience as I learned the ropes of running an analysis. Thanks also to
Ryan Nichol who has been putting up with me since I first worked on a summer
project at UCL with Anita in 2009. Thanks to Robert Hatcher and Art Kreymer,
I suspect I have learnt more from overheard snippets of conversation between the
two of you than I could find in the best of computing textbooks. Thanks to the
many great scientists at Fermilab, UCL, and within MINOS who have shared
their wisdom with me along the way.
I have been blessed with a great many friends scattered across the globe and
should like to thank them all for making the last few years the rare pleasure that
it has been. Jamie, Liam and Chris. You remain the closest thing to brothers
that I have, and I wish you all the very best in your own adventures. Particular
thanks go to my very dear friends Cindy Joe and Joseph Zennamo who braved
my terrible spelling and grammar to help edit the thesis you are reading now.
Finally, none of this would have been possible without the loving support of
my family. Mum, Gran, Brana, and Anna- thank you so very much.
2

Contents
Abstract 21
Declaration 23
1 Introduction 24
2 History and Theory of Neutrino Physics 27
2.1 Inception and Discovery of the Neutrino . . . . . . . . . . . . . . 27
2.2 The Weak Interaction . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.3 The Number of Neutrino Generations . . . . . . . . . . . . . . . . 32
2.4 Evidence for Neutrino Oscillations . . . . . . . . . . . . . . . . . . 34
2.4.1 The Solar Neutrino Anomaly . . . . . . . . . . . . . . . . 35
2.4.2 Atmospheric Neutrino Anomaly . . . . . . . . . . . . . . . 39
2.5 The Theory of Neutrino Oscillations . . . . . . . . . . . . . . . . 40
2.5.1 Three flavor oscillations . . . . . . . . . . . . . . . . . . . 43
2.5.2 The Two Flavor Approximation . . . . . . . . . . . . . . . 44
2.5.3 Matter Effects . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.6 Measurement of the Neutrino Oscillation Mixing Parameters so Far 48
2.6.1 Solar Neutrino Oscillations . . . . . . . . . . . . . . . . . . 48
2.6.2 Atmospheric Neutrino Oscillations . . . . . . . . . . . . . 51
2.6.3 Reactor Neutrino Oscillations . . . . . . . . . . . . . . . . 56
3

2.7 Ongoing and Future Efforts . . . . . . . . . . . . . . . . . . . . . 58
2.7.1 Majorana Neutrinos . . . . . . . . . . . . . . . . . . . . . 58
2.7.2 The Resolution of the Mass Hierarchy . . . . . . . . . . . . 58
2.7.3 Measurement of δ
cp
. . . . . . . . . . . . . . . . . . . . . . 60
2.7.4 Sterile Neutrinos . . . . . . . . . . . . . . . . . . . . . . . 60
3 The MINOS Experiment 63
3.1 Physics Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.2 The NuMI Neutrino Beam . . . . . . . . . . . . . . . . . . . . . . 65
3.2.1 Neutrino and Antineutrino Production . . . . . . . . . . . 65
3.3 MINOS Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.3.2 Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.3.3 Scintillating Strips . . . . . . . . . . . . . . . . . . . . . . 75
3.3.4 Photomultiplier Tubes . . . . . . . . . . . . . . . . . . . . 79
3.3.5 Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . 81
3.3.6 Electronics and Readout . . . . . . . . . . . . . . . . . . . 83
3.4 Light Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.5 Triggering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.6 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.6.1 Absolute Track and Shower Energy . . . . . . . . . . . . . 91
3.7 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.7.1 Beam Simulation . . . . . . . . . . . . . . . . . . . . . . . 91
3.7.2 Detector Simulation . . . . . . . . . . . . . . . . . . . . . 93
3.8 Neutrino Events in the MINOS Detectors . . . . . . . . . . . . . . 95
3.8.1 Event Topologies . . . . . . . . . . . . . . . . . . . . . . . 95
3.8.2 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . 97
4

4 Studies of Muon Neutrino Disappearance through Oscillations
at MINOS 101
4.1 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.1.1 Preselection . . . . . . . . . . . . . . . . . . . . . . . . . . 103
4.1.2 Analysis Selection . . . . . . . . . . . . . . . . . . . . . . . 107
4.2 Extrapolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.2.1 Unfolding . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
4.2.2 Beam Matrix . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.3 Systematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
4.3.1 Evaluating the Impact Of Different Systematics . . . . . . 124
4.4 Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
4.4.1 Systematic Uncertainty . . . . . . . . . . . . . . . . . . . . 129
5 The Charged Current Analysis 131
5.1 Analysis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
5.1.1 Resolution Binning . . . . . . . . . . . . . . . . . . . . . . 132
5.1.2 The Rock and Antifiducial Sample . . . . . . . . . . . . . 134
5.1.3 Addition of the ND Coil Hole Selection Criteria . . . . . . 136
5.2 Data Epochs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.2.1 MC Reweighting . . . . . . . . . . . . . . . . . . . . . . . 139
5.3 CC Analysis Fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
5.3.1 Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6 Three Flavour Oscillations 148
6.1 Analysis Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.2 The Exact Three Flavour Oscillation Formula . . . . . . . . . . . 149
6.3 New Truth Binning . . . . . . . . . . . . . . . . . . . . . . . . . . 150
6.4 Fitting Choices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
5