2 (2) & sin
2.6 Neutrino-to-Antineutrino Transitions
In addition to standard oscillations, this thesis also presents a search for νµ → ν¯µ transitions.
Transitions between neutrinos and antineutrinos were Bruno Pontecorvo’s original idea for neutrino oscillations in the 1960’s [15]. The idea resurfaced in the literature around 1980 in the context of understanding the consequences of Majorana neutrinos [104, 105]. More recently, several possible models for neutrino-to-antineutrino transitions within the simplest Standard Model extensions for neutrino mass are catalogued in [106]. The transitions can be helicity-violating, mediated either by a Majorana mass term or by a magnetic moment, or they can be helicity-conserving, producing a sterile left-handed antineutrino. The analysis in this thesis is sensitive only to transitions like the ones mediated by the Majorana mass: the helicity-conserving transitions produce sterile neutrinos which cannot be observed in the MINOS detectors and the magnetic moment transition requires a change of flavor (e.g.νµ →¯νe). However, since the Majorana mass-mediated transition requires creating wrong-helicity states, its amplitude is suppressed by a factor proportional to (mν/Eν)2and consequently is expected to occur below the 10−7level in a high-energyνµbeam, making the process effectively unobservable [107]. Thus, any observation ofνµ→ν¯µ would require new physics.
Such transitions could be introduced by the V+A currents that arise in left-right symmetric models [108], but stringent limits have been set on leptonic V+A interactions by studies of the end point spectrum in polarizedµ+ decay [109], by studies of inverse muon decay [110], and by studies of the high-y dependence of νµ/¯νµ-nuclei interactions [111, 112]. Scalar (S) and pseudoscalar (P) interactions can also introduce spin-flips and change the lepton helicity [113], but contributions from these interactions were limited to less than 7% (95% C.L.) by investigations of the polarization of µ+’s in ¯νµinteractions at the CHARM experiment [114]. Neutrino-to-antineutrino transitions could also be introduced by certainCPT-violating parameters in the Kosteleck´y parameterization [94].
In the early 1980’s the BEBC bubble chamber in the CERN SPS neutrino beam was able to set limits on transitions to ¯νe from νµ and νe [115]. The best limit on anomalous µ+ production
in a νµ beam comes from the CCFR experiment: the fraction of µ+ relative to the CCνµ rate was limited to 1.6×10−4 for y < 0.5 and 3.1×10−4 for y >0.5 [116]. However, there have been no measurements of transitions to ¯νµ’s at atmospheric oscillation length scales, or at such low energies:
the CCFR experiment used 120 GeV neutrinos and a baseline of 1.1 km. Searching for νµ → ¯νµ
requires a detector capable of identifying individualµ+’s among manyµ−’s produced by the muon neutrino beam, a unique capability of MINOS among long-baseline experiments. In fact, a recent analysis used the limit described later in this thesis to improve limits on the effective Majorana muon-neutrino mass,|hmµµi|[117].
The search for neutrino-to-antineutrino transitions in this thesis uses an empirical parameteriza- tion, based on the knowledge thatνµ’s are known to be disappearing [67] in the NuMI beam with an energy-dependence described by Equation 2.136 and the supposition that some fraction, α, of thoseνµ’s are transitioning to ¯νµ’s instead of oscillating toντ’s. Thus, the appearance probability takes on the form
P(νµ →ν¯µ) =αsin2(2θ23) sin2
∆m2atm
L 4E
(2.165) Note that the oscillation parameters above are those for neutrinos, not those for antineutrinos.
44 Physics of Neutrinos and Antineutrinos
Chapter 3
The MINOS Experiment
The Main Injector Neutrino Oscillation Search (MINOS) is a long-baseline neutrino-oscillation ex- periment. Its main components are the NuMI neutrino beam and two detectors. The NuMI beam is located at the Fermi National Accelerator Laboratory (Fermilab or FNAL), where 120 GeV protons from the Main Injector are directed at a fixed target to produce mesons which decay to produce the neutrino or antineutrino beam. Oscillations are measured by sampling the primarilyνµ or ¯νµ
beam at two locations: one close to the neutrino source before oscillations have occurred (the Near Detector) and one located approximately a quarter of the atmospheric oscillation length (see Equa- tion 2.158) away, where oscillations will be near maximal (the Far Detector). Like the NuMI beam, the Near Detector is situated at Fermilab, 1 km downstream of the neutrino target. The Far Detec- tor is situated 735 km from the neutrino source in the Soudan Underground Laboratory in northern Minnesota.
The two-detector design makes the measurement of oscillations less dependent on simulation and significantly more robust against a range of systematic uncertainties, especially the neutrino flux from NuMI and the poorly known low-energy neutrino interaction cross section. Since these
10 km
12 km 735 km
Fermilab
Soudan
Figure 3.1: Schematic views of the components of the MINOS experiment: the NuMI beam and Near Detector at Fermilab and the Far Detector in Soudan, MN.
46 The MINOS Experiment
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Figure 3.2: Cross-sectional view of the NuMI beam complex and MINOS Near Detector Hall at Fermilab.
Figure taken from [118].
systematics affect both detectors in the same way, they are effectively cancelled out when both detectors are used to measure oscillations. Section 5.3 describes how this process works in practice.
The rest of this chapter gives detailed descriptions of the main components of the experiment.
It describes the MINOS data, from the raw detector output through calibration and reconstruction.
It also includes a description of how the data is simulated.