Development of a New Search for Neutron/Anti-neutron Oscillation at the European Spallation Source Matthew Frost UTK HEP Seminar – April 5, 2017
Why study NNbar? • Novel Observation: The spontaneous transmutation of a neutron to an anti-neutron would be the first experimental observation of Baryon number violation. • SUSY and BSM Physics: Reveals new physics that would exist in constructs beyond the Standard model, setting energy scales for these new physical phenomena. -> Δ 𝐶 − 𝑀 ; Δ𝐶 = 2 • Astronomical Observations: Justification of observed matter/anti-matter imbalance in the universe. • Historical Vindication: Ettore Majorana proposed that charge-neutral fermions were in fact their own anti-particle. 2
Neutron Oscillation • For a two-level system the probability of oscillation is For free neutrons, 𝑊 ≪ 1 , and 𝑢 ≪ 1 yielding 2 𝑢 ℏ 𝑄 𝑜 ~ , where the oscillation time 𝜐 𝑜−𝑜 = 𝜐 𝑜−𝑜 𝛽 Additional details in Josh Barrow’s March 8 presentation: www.phys.utk.edu/research/hep/seminar-slides/2017-spring/barrow-march08.pdf 3
Experimental Figure of Merit • Used to optimize experiment design in simulation. Figure of Merit = Φ 𝑢 2 • Φ → Total neutrons on detector (scales with source intensity) 𝑢 2 → Square of Mean Flight Time (scales with wavelength) • • Sensitivity units are in “ILL/year” • Compared to last observation attempt at ILL in 1990. 4
Baseline Experiment Geometry The proposed NNbar experiment at the European Spallation Source (ESS) entails four main components: • A large view of ESS cold neutron moderator systems. • An Ellipsoidal super-mirror reflector about 40 meters in length. • An ultra-high vacuum tube with magnetic field shielding about 200 meters in length. • A 2 meter diameter carbon foil annihilation target surrounded by a particle tracker detection system. CTUB 5
Baseline Experiment Geometry This optimization has shown performance gains ~100x beyond the ILL experiment with other simulated cold neutron sources, and thus provides a good configuration to test with other cold source concepts. • Super Mirror Reflectivity m=6 • Minor Axis b=2 m • Major Axis c=100 m • Start/Stop reflector position 10-50 m • ± 5 ° Acceptance Angle • Detector Efficiency 50% CTUB 6
The European Spallation Source • A pulsed source of cold (<25meV) neutrons designed particularly for neutron scattering instrumentation used in studies of advanced materials. (Condensed Matter, Engineering materials, Biological structures) • Proposed startup in 2019. 7
ESS Experiment Features 5MW, 14Hz, 3.2ms pulse width Competitive with other sources. Time Averaged brightness comparable to ILL cold source. Long flight paths are already planned for scattering instrumentation. 8
Simulation Sensitivity History Preliminary investigations of experiment sensitivity with various proposed source designs proved useful in determining whether to pursue development of the experiment at ESS. Moderator TDR 2013 LD2 Pancake H 2 Baseline FOM/yr 250 550 200 9
Neutron Beam Phase Space • Beam trajectory phase spaces have lower dimensionality and distinct correlations between those dimensions, and thus are easy to represent via distributions that are developed from a statistically relevant set of MC data. • Source design and conceptual development is performed using MCNP • MCNP output events are investigated via correlation and histograming • Space and trajectory distributions are fit and weighted against the calculated correlation • The result is a suitable subroutine that provides events describing the complete phase space • In development for high intensity LD2 source to advance experimental sensitivity. 𝜍(𝑦, 𝑧, 𝑤 𝑦 , 𝑤 𝑧 , 𝑤, 𝑢) 10
BF2 Moderator Concept Be Reflector Enclosure • The ESS will move forward with Upper BF2 the “Butterfly” shaped hybrid Spallation Target moderator design for Mark-I of the Lower BF2 source moderator/reflector Be Reflector Enclosure system. • This design incorporates elements of both thermal and cold moderating sources of neutrons for scattering instrumentation. To Nnbar/HIBEAM Experiment 11
Large Beam Port • A Large Beam Port was designed to accommodate high-intensity experiments like NNbar. • Equivalent to three traditional beam ports in horizontal. • Allows a view of both the top and bottom moderator systems. • Enables placement of optical devices closer to the source. 12
Reflector Geometry for BF2 • Once the BF2 configuration was finalized, further optimization of the position of super-mirror reflector was pursued. • Due to the spatial distribution of the cold neutron emitting surfaces of the BF2, the baseline experiment ellipsoid will not be the most effective means by which to transport the cold intensity. Baseline ellipsoid 313 centered on lower cold spot Baseline ellipsoid 188 centered on middle of lower BF2 Baseline ellipsoid centered 201 on middle of both BF2 13
Quadruple Focusing (Lobed) Ellipse • Using a more complex parameterized lobed reflector b top model, an optimized geometry x offset specifically tailored to BF2 can be y top determined • “Clover” reflector y bottom • z 0 , z end , m(z) b bottom • b top , b bottom , y top , y bottom , x offset Cross Section of Clover Reflector 14
Segmentation of Reflectors • • Initial simulations are performed Initial results suggest that angular using an ideal ellipsoid, but this segmentation near focal points ultimately will prove to be and around beam trajectory has impractical. much greater impact on transport as compared to along the axis • A method will be developed to • most economically segment the With modern super-mirror reflector, while minimally substrate technologies, a hybrid impacting the overall sensitivity design can be conceived. contribution. • Current super-mirror guide geometries are constructed of many surfaces approximately 50cm in length, and 5-10cm wide 15
Segmentation of Reflectors 300 • Initial results suggest that angular 250 segmentation near focal points and around beam trajectory has 200 much greater impact on transport Ideal Ellipse (No Segmentation) Sensitivity as compared to along the axis 150 • With modern super-mirror substrate technologies, a hybrid 100 design can be conceived. 50 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Inverse Number of Segments 16
Reflectivity Optimization • Cold neutron intensity could be enhanced with high reflectivity super-mirrors • Higher reflectivity increases cost, and may provide little benefit closer to reflector entrance. Target Source • Benefit strongly depends on reflector geometry, and is included in optimization Reflector parameter space. 17
Nested Ellipse Geometry • Significant decrease in length, with little compromise on reflector illumination, yielding a higher overall FOM. • Resources can be saved with a smaller reflector area, however the system is further complicated by suspension of optics and smaller, more complex segmentation • First order calculations show a possible 30% increase in sensitivity with nested optics 18
Multi-SANS for Neutron Reflection Model Dependencies: • Particle Radius Distribution • Scattering amplitudes • Absorption probability • Bulk Density and Depth R. Cubitt et al. / Nuclear Instruments and Methods in Physics Research A 622 (2010) 182 – 185 • Macroscopic determination of free-path lengths • Incident Particle Velocity • Nano-particle temperature • Down/Up inelastic scattering 19
Multi-SANS for Neutron Reflection • Increase in experiment sensitivity due to • Divergence redirection More neutrons on specular reflector • In-elastic down-scattering Longer Free Flight Time • If particles are actively cooled Sub-Thermal Neutron Phase Space Map at R = 2755 mm to be used for further analysis. 20
Preliminary Hardware R&D H igh I ntensity B aryon E xtraction A nd M easurement • The ESS will startup at low power and slowly ramp to full power as planned over a 2-3 year period. • One moderator will be used in the first generation reflector/moderator system • HIBEAM provides an opportunity to test experiment concepts applicable to a final NNbar experiment. • Novel Optics • Annihilation Detector Systems • Magnetic Shielding • Background investigations • Radiological Shielding • Other Fundamental Neutron Physics endeavors can be pursed as well. 21
Questions? 22
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