1. Characteristic Signal of Neutron-Antineutron Oscillation in Argon Nuclei at DUNE
Motivations for Experimental Detection of 𝑛— 𝑛 Oscillations in Argon
The Deep Underground Neutrino Experiment and Proposed 𝑛— 𝑛 Search
Current Work in 𝜈 MonteCarlo Simulation and Analysis
Modern cosmological models propose Baryon Number Violation (BNV) as an explanation
for the observed imbalance of matter and antimatter in the universe. BNV (Δ𝐵 ≠ 0) decays
can exist while still maintaining Δ(𝐵 − 𝐿) = 0, a requirement of the standard model.
Proposed BNV mechanisms, such as proton decay (Δ𝐵 = 1) and leptogenesis models,
occur at very high energies (GUT scales), and any imbalance arising from the former would
be erased during the electroweak (EW) sphaleron phase transition [2]. Thus, solely Δ(𝐵 −
𝐿) = 0 would not be a suitable explanation of baryogenesis. Δ𝐵 = 2 processes like neutron-
antineutron (𝑛— 𝑛) oscillations probe energies above LHC, but still below the EW phase
transition energy. Observation of the oscillation would confirm the need to probe higher
scales still, or rule out Post-Sphaleron Baryogenesis (PSB) [1,3]. Majorana particles [4]
provide a usable phenomenology for development of an experiment to observe BNV [5].
Fermilab and DUNE have completed seven MonteCarlo Challenges (MCCs, using GENIE) to
furnish experimentalists with simulated data for analysis. For 𝑛— 𝑛, we must also complete
atmospheric 𝜈 background analysis, where the hope is improved software, particle ID, momentum
and energy reconstruction can push it to zero. If an 𝑛 − 𝑛 signal is still thought to be viable after this
analysis, we will also consider the effects of cosmogenic 𝜇’s and fast 𝑛’s; 𝑛— 𝑛 generators for Ar
are currently under construction by two groups and analysis is forthcoming.
Two-level, time-dependent systems can oscillate between two states based on
the off-diagonal mixing factor 𝛿𝑚. This factor distinguishes the two states from
one another, and provides insight to their suppression modes; in this case, it
pertains to the magnetic dipole energy of each particle. This assumes that the 𝑛
and 𝑛 masses are equivalent and that their dipole moments are equal and
opposite, which can be shown as a consequence of the CPT Theorem. In a
short time limit and low-magnetic field configuration, the probability of free
transition is:
We would like to thank the DUNE collaboration, our colleagues, for all of
their aid and discussions in this matter.
We could not do this work without their input and expertise.
This research was funded by Department of Energy-High Energy Physics
A distribution of proposed PSB models as a function of
predicted free oscillation time [3]. Blue line shows horizontal
beamline oscillation time (ESS, 3 yr, ~500X ILL); red shows
DUNE/LBNE (10 years, no background, ~13,500X ILL).
References
[1] K.S.Babu et al., “Working Group Report Baryon Number Violation,'‘ arXiv:1311.5285 [hep-ph].
[2] V.A. Kuzmin, V.A. Rubakov and M.E. Shaposhnikov,”On the Anomalous Electroweak Baryon Number Nonconservation in the Early Universe,” Phys.Lett.B 155, 36
(1985).
[3] K.S. Babu, P.S. Bhupal Dev, E.C.F.S. Fortes and R. N. Mohapatra, “Post-Sphaleron Baryogenesis and an Upper Limit on the Neutron-Antineutron Oscillation
Time,'' Phys. Rev. D 87, 115019 (2013).
[4] E. Majorana, “Theory of the Symmetry of Electrons and Positrons,'' Nuovo Cim. 14, 171 (1937).
[5] D.G. Phillips, II et al., “Neutron-Antineutron Oscillations: Theoretical Status and Experimental Prospects,'' Phys. Rept. 612, 1 (2016)
[6] K. Abe et al.,”Search for 𝑛 − 𝑛 oscillation in Super Kamiokande,” Phys. Rev. D 91, 072006 (2015).
Previous Simulations and Experimental Results from Super-Kamiokande
𝑃𝑓𝑟𝑒𝑒 𝑛 → 𝑛 ≅ 𝛿𝑚 𝑡 2
=
𝑡
𝜏 𝑓𝑟𝑒𝑒
2
→ 𝜏 𝑓𝑟𝑒𝑒 =
𝜏 𝑏𝑜𝑢𝑛𝑑
𝑅
DUNE is an international collaboration of over 800 scientists based at Fermilab. DUNE, together with the Long-Baseline Neutrino
Facility (LBNF), will construct the most intense neutrino beam in the world along with near and far detectors. The far detector will
utilize Liquid Argon Time Projection Chambers (LArTPCs) with a fiducial volume of roughly 40 kilotons. It is hoped that LArTPC’s
superior tracking and particle identification abilities will ultimately reduce effects of background in the search for 𝑛— 𝑛 oscillation
events, permitting the chance to observe such a process [5].
Using 𝑛 bound in Ar, DUNE currently plans to include 𝑛 − 𝑛 events in their nucleon
decay searches. Using GENIE, modeling is underway on intranuclear interactions
mimicking 𝑛 − 𝑛 annihilation in Ar nuclei. Eliminating atmospheric neutrino (𝜈)
background from such events will be a challenge for LArTPCs at DUNE, so simulation
work must be considered for 𝜈 interactions in Ar nuclei, which may produce similar
signals to 𝑛 − 𝑛 annihilation.
Key to understanding possible experimental signals will be the integration of these two for
a proper robust analysis, which will determine the viability of any detection of this process
above background levels. One proposed signature of the event is release of roughly 2 GeV
of energy (excluding hadronic rest masses) while particles on the whole maintain zero total
momentum. Similarly, within the nucleus, between 2-6 pions can be made from the
annihilation event, some of these eventually escaping. While deeply inelastically scattering
𝜈’s can also produce pions, the kinematical signatures of the event differ.
Previous searches for 𝑛— 𝑛 occurred at Super-K, where 24 candidate events were observed and an
atmospheric 𝜈 background of 24.1 events were expected. Limitations in the resolution of Cherenkov
radiation detector technologies cause large systematic error shifts in expected energy and
momentum ranges; this is not thought to be an issue with LArTPCs, given current known results from
MicroBOONE, ArgoNeuT, etc. Using similar technology proposed for DUNE, ProtoDUNE will continue
down MicroBOONE’s path for in-depth LArTPC testing at CERN, scheduled to be commissioned by
2018. We hope to complete similar MCs and analysis over the next two years for DUNE.
Our analysis uses ROOT macros
analyzing LArSoft output Trees of
GEANT4 truths and reconstructed
events. Detector simulation is an
integral part of the event generation
process.
A 10 kt geometry is used to save
on computational time in MCC’s for
the collaboration.
Here we are using similar cuts to
SuperK to attempt to understand
the atm. 𝜈 events, though we are
not yet including any detector
effects. As the primary particles
produced in 𝑛 − 𝑛 are thought to be
pions, the outgoing (visible) pion
spectra will be important. This
follows from considerations at the
generator level, where we know
that the only background we have
will be those events with no
leptonic signature (neutral current
events with 2-6 pions).
Table 1: Atmospheric Neutrinos Pion Signal
(100,000 events, ~10,000 per year; see above)
Approx. Count Per Year
~𝑝 − 𝑛 % ~𝑛 − 𝑛 % ~𝑝 − 𝑛 ~𝑛 − 𝑛
π⁺π° 74.7 π⁺π⁻ 47.0 ~28 ~42
π⁺2π° 13.5 2π° 21.9 ~5 ~19
π⁺3π° 2.2 π⁺π⁻π° 18.4 ~1 ~16
2π⁺π⁻π° 7.5 π⁺π⁻2π° 5.6 ~3 ~5
2π⁺π⁻2π° 1.9 π⁺π⁻3π° 0.9 ~1 ~1
2π⁺π⁻2ω 0? 2π⁺2π⁻ 3.9 0? ~4
3π⁺2π⁻2π° 0.3 2π⁺2π⁻π° 1.8 ~0 ~2
π⁺π⁻ω 0? 0?
2π⁺2π⁻2π° 0.5 ~0
We expect DUNE LArTPCs to have much greater background
suppression and particle ID capabilities than previous experiments
The Deep Underground Neutrino Experiment’s effort to resolve the matter-antimatter asymmetry mechanism of the early universe
Joshua Barrow – Department of Physics and Astronomy
Here, 𝑅 ≅ 5 ⋅ 1022
𝑠−1
is the nuclear suppression factor, derived in zero-point
motion with a quasi-free condition for neutron motion within the nucleus.
a
b
c
a) Atm. 𝜈 events producing 2+ outgoing pions with no
leptons produced in primary interactions with trackable
high energy protons. b) All atm. 𝜈 events producing 2+
outgoing with visible pions, muons, electrons, and
protons. Green circles show ~area of interest for 𝑛 − 𝑛.TotalMomentum()TotalMomentum()
Invariant Mass (𝐺𝑒𝑉)
Invariant Mass (𝐺𝑒𝑉)
Table 1) Comparison table for identical Super
Kamiokande pion channels. c) Spectra for individual
pions channels from T1 using total energy of pions.
Lines mark 2 GeV of total energy and ~1 count per year.