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 𝐵 − 𝐿 ; Δ𝐵 = 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.

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Neutron Oscillation

• For a two-level system the probability of oscillation is

For free neutrons, 𝑉 ≪ 1, and 𝑡 ≪ 1 yielding

𝑃𝑛~𝑡

𝜏𝑛−𝑛

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, where the oscillation time 𝜏𝑛−𝑛 =ℏ

𝛼

Additional details in Josh Barrow’s March 8 presentation:www.phys.utk.edu/research/hep/seminar-slides/2017-spring/barrow-march08.pdf

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Experimental Figure of Merit

• Used to optimize experiment design in simulation.

Figure of Merit = Φ 𝑡2

• Φ →Total neutrons on detector (scales with source intensity)

• 𝑡2 →Square of Mean Flight Time (scales with wavelength)

• Sensitivity units are in “ILL/year”

• Compared to last observation attempt at ILL in 1990.

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Baseline Experiment GeometryThe 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

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Baseline Experiment GeometryThis 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

• Acceptance Angle ±5°

• Detector Efficiency 50%

CTUB

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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.

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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.

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Simulation Sensitivity History

Moderator TDR 2013 LD2 Pancake H2

Baseline FOM/yr 250 550 200

Preliminary investigations of experiment sensitivity with various

proposed source designs proved useful in determining whether to

pursue development of the experiment at ESS.

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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.

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𝜌(𝑥, 𝑦, 𝑣𝑥, 𝑣𝑦, 𝑣, 𝑡)

BF2 Moderator Concept• The ESS will move forward with

the “Butterfly” shaped hybrid

moderator design for Mark-I of the

source moderator/reflector

system.

• This design incorporates elements

of both thermal and cold

moderating sources of neutrons

for scattering instrumentation.

Be Reflector Enclosure

Be Reflector Enclosure

Spallation Target

Upper BF2

Lower BF2

To Nnbar/HIBEAM Experiment

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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.

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• 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.

Reflector Geometry for BF2

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Baseline ellipsoid

centered on lower cold spot 313

Baseline ellipsoid

centered on middle of

lower BF2

188

Baseline ellipsoid centered

on middle of both BF2 201

• Using a more complex

parameterized lobed reflector

model, an optimized geometry

specifically tailored to BF2 can be

determined

• “Clover” reflector

• z0, zend, m(z)

• btop, bbottom, ytop, ybottom, xoffset

Quadruple Focusing (Lobed) Ellipse

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btop

bbottom

ybottom

ytop

xoffset

Cross Section of Clover Reflector

Segmentation of Reflectors• Initial simulations are performed

using an ideal ellipsoid, but this ultimately will prove to be impractical.

• A method will be developed to most economically segment the reflector, while minimally impacting the overall sensitivity contribution.

• Current super-mirror guide geometries are constructed of many surfaces approximately 50cm in length, and 5-10cm wide

• Initial results suggest that angular segmentation near focal points and around beam trajectory has much greater impact on transport as compared to along the axis

• With modern super-mirror substrate technologies, a hybrid design can be conceived.

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Segmentation of Reflectors

0

50

100

150

200

250

300

0 0.05 0.1 0.15 0.2 0.25 0.3

Inverse Number of Segments

Sen

siti

vity Ideal Ellipse (No Segmentation)

• Initial results suggest that angular segmentation near focal points and around beam trajectory has much greater impact on transport as compared to along the axis

• With modern super-mirror substrate technologies, a hybrid design can be conceived.

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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.

• Benefit strongly depends on

reflector geometry, and is

included in optimization

parameter space.

Target

Reflector

Source

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• 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

Nested Ellipse Geometry

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R. Cubitt et al. / Nuclear Instruments and Methods inPhysics Research A 622 (2010) 182–185

Multi-SANS for Neutron Reflection

Model Dependencies:• Particle Radius Distribution

• Scattering amplitudes

• Absorption probability

• Bulk Density and Depth• Macroscopic determination of free-path lengths

• Incident Particle Velocity

• Nano-particle temperature• Down/Up inelastic scattering

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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.

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Preliminary Hardware R&D

• 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.

High Intensity Baryon Extraction And Measurement

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Questions?

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