Neutron EDM

It turns out that the “roundness” of the neutron’s electric charge distribution (measured by the Electric Dipole Moment, EDM) is connected at a deep level to a rather fundamental question – “Why is there any matter in the Universe?” Detecting non-zero nEDM would mean a huge breakthrough in the search for Beyond the Standard Model Physics.

Matter and anti-matter were created in equal amounts during the Big Bang. Almost all of it (all but about half a part per billion) was annihilated in subsequent matter/anti-matter collisions. Clearly (and thankfully) a small excess of matter survived to form our Universe. Theories attempting to explain this almost invariably predict that the neutron is not quite perfectly round.

There are two CP violation (CPV) mechanisms present in the Standard Model (SM). In the electroweak sector, there is a CP-odd phase in the Cabibbo-Kobayashi-Maskawa (CKM) matrix, resulting in the observed CPV in B- and K-meson decays. This phase predicts a practically unobservable nEDM value at the 10-32 e·cm level. In the strong SM sector, i.e. QCD Lagrangian, there is a static CPV phase θQCD which is free to take any value between zero and 2π and allows a nEDM as large as 10-16 e·cm. In reality, the present experimental limit of the nEDM places θQCD < 10-10, giving rise to what is typically referred to as the “strong CP problem” because of the small value. Therefore, neither electroweak nor QCD sectors of the SM can provide the required level of CP violation without fine tuning parameters. That strongly motivates a search for new CPV sources. Low energy observation of non-zero EDMs is one of the most sensitive tools for physics beyond the SM with physics reaches competitive with high-energy physics frontiers.

A solution to the strong CP problem was found by introducing a dynamic CPV field with a new particle called the axion instead of the static θQCD. The axion ground state dynamically forces QCD theory to be CP-symmetric by setting θQCD to zero. Nevertheless, one can not exclude the possibility of a non-zero nEDM due to a non-zero θQCD term. nEDM measurements, in combination with complementary EDM measurements in other systems, can directly shed light on the strong CP problem.

Proposed initially for solving the strong CP problem, axions also became viable candidates for solving the dark matter puzzle. As result, the axion theoretical and experimental research field exploded, considering many kinds of axion-like particles (ALP)[1]. While QCD axion searches rely on high-energy accelerator and astrophysics capabilities, light ALPs can be searched for using existing nEDM setups utilizing polarized neutrons or nuclei [roccia2021indirect]. Axion interaction with polarized neutrons can be observed as an oscillating EDM [2]. An observed nEDM value can also restrict allowed parameter space for axions. Thus observation of a non-zero nEDM has the potential to be a breakthrough for baryogenesis, CPV new physics, the strong CP problem, and dark matter searches.

At present, the most precise nEDM result is  d_n < 1.8 x 10-26e·cm (90~\% C.L.) which was published in 2022 by the PSI nEDM collaboration  [3]. Several groups worldwide are building experiments sensitive at the 10-27 e·cm level, with the most advanced being the n2EDM located at PSI [4], TUCAN located at TRIUMF[5], LANL nEDM [6], and PanEDM at ILL [7]. Realizing an order of magnitude improvement in sensitivity requires improving not only systematics related to magnetic field stability and homogeneity, but also statistical uncertainties; in the PSI result [3], the statistical uncertainty of 1.1stat·10-26 e·cm exceeds the systematic uncertainty 0.2sys·10-26 e·cm. Apart from increasing the electric field strength and observation time, increasing the available UCN density is the most straightforward means of improving the statistical sensitivity. An increase in the number of UCN can be also be accomplished by through improvements in transport efficiency and by increasing the size of the measurement region, however the latter requires stability and homogeneity of the magnetic field over a larger region which could impact systematic uncertainties.

Experiments with potential reach into the 10-28 e·cm range and below may be possible with a new approach to measuring the nEDM using polarized UCN in LHe doped with polarized 3He[8]. An attempt to develop this experiment was undertaken by the nEDM@SNS collaboration, which made significant progress in technical development of the experimental components [9] until the funding support was terminated. NC State/TUNL played a key role in the development of the Systematic and Operational Studies Apparatus (SOSA) which focused on the development of a cryogenic test-bed for exploring unique operational procedures and studying systematic effects related to nEDM measurements. The test-bed did not require an electric field, but is designed to have magnetic shielding and nuclear magnetic resonance (NMR) capabilities that allow for precise spin manipulation of polarized 3He and neutrons.

Ramping down nEDM@SNS and kicking off nEDMSF

At present we are near completion of SOSA commissioning for the first phase of systematic studies involving only polarized 3He. We are assembling the B0 and spin flip BRF superconducting magnetic coils, which are the last of the non-completed components of the setup.  The scientific program includes testing three possible methods of measuring the 3He correlation function. Next steps will be testing of the 3He spin dressing modes with and without modulations  and looking into the light axion-like particle search.

SOS@TUNL apparatus 

When US funding agencies ceased support of the nEDM@SNS project, the collaboration began discussions with European partners to explore revitalization of a similar nEDM experiment (nEDM SuperFluid, nEDMSF) at the European Spallation Source (ESS). The proposal was well-received and we are developing an approach that is less aggressive than the previous effort. Plans are underway to begin two prototyping studies utilizing the ILL cold beam PF1B including recently submitting a collaborative proposal for beam time with Caltech, Heidelburg, ILL, Laboratoire de Physique Subatomique \& Cosmologie, Montclair, ORNL, Rutherford Appleton Laboratory, Stockholm Univ., TU Munich, U.\ of Bern, UIUC, U.\ of Kentucky, U.\ of Lund, U.\ of Sussex, and Yale Univ. A a kick-off meeting for the nEDMSF project will be held at ILL on Dec 9-10, 2024.

The first proposed experiment involves testing a cryogenic cell coupled with a light collection system to investigate both UCN production and the efficiency of light collection. Existing cryostat from LANL will be re-purposed for the test. This test will prove the ability to generate and detect high UCN densities required for a 10-28 e·cm statistical sensitivity. The second experiment at ILL proposes to demonstrate the feasibility of the nEDM experimental techniques using the SOSA with a tentative schedule of 2029. By this time, studies with polarized 3He will be complete with sufficient time to modify the apparatus for operation using a cold beam.

The SOSA apparatus can also be used to search for light ALPs using polarized 3He. Such a search for axion dark matter was performed by the PSI group by observing the output of their 199Hg magnetometer for 13 hr. The target signal was a frequency modulation of the output by the axion field oscillation frequency, Ωa ~ ma, the axion mass. The axion field would result in sidebands to the precession frequency (ΩL ± Ωa) with ΩL the Larmor frequency. Their results are shown in Fig. below. The extrapolated sensitivity for the improved n2EDM apparatus at PSI is also shown. In any event, including the mass implies the effect would be 66 times larger for 199Hg than the 3He that could be used in an SOSA attempt to duplicate the experiment. The vapor pressure of 199Hg however was 4.4·10-7 mbar, corresponding to 6·1011 atoms in a 50~cc beam volume. Our MEOP system on the other hand can produce ~ 3·1018 polarized 3He atoms per 10 min. cycle, which yields a concentration of 1.5·10-7 in a 1 liter measurement cell. At this concentration, the 3He-3He spin interaction is negligible. Note that the 3He magnetic moment is 4 times larger than that of 199Hg. Comparing sensitivity, we lose a factor of 66 from the mass ratio but gain a factor of 3 in magnetic moment and √(107/2) from the number of spins observed, resulting in a factor of 50 better sensitivity. See Axion mass limits from polarized atom experiments. This is attractive for the present SOSA configuration as it can be carried out with no E field and without neutrons.

TUCAN nEDM efforts

 

The TUCAN nEDM setup is shown above. UCN are produced in the spherical production volume surrounded by liquid deuteriumD at 20 K and heavy water at 300 K. UCN are transported to the nEDM setup first through a ~ 2.5 m long cryogenic section filled with isotopically pure LHe-4, and then after a short rising S-shaped section, enter a room temperature guide. UCN are polarized by a magnet and enter a splitting guide leading to two measurement cells. Except for the liquid deuterium cryostat that will be delivered in spring of 2025, the UCN source has passed all cryogenic tests and is ready for operation. The magnetically shielded room has been assembled and additional components of the apparatus are in development at collaborating institutions. The primary goal of the TUCAN collaboration for 2025 is to produce UCN.

Since joining, the primary efforts of the TUNL team has focused on cryogenic aspects of the project. Our team’s unique experience in both cryogenics and the physics of UCN production in LHe and solid deuterium enable us to contribute across many aspects of the experiment. We propose to increase our involvement as the experiment moves to UCN production. Locally, we plan to participate in UCN guide development using the PULSTAR UCN source and to perform two stand along experiments using our SOSA cryostat that are related to possible improvements of the LHe source operation. One is development of a super leak filter to produce an isotopically pure 4-He. another is to measure thermal conductivity of turbulent  LHe in the 1 K – 0.5 K temperature range.

References:

    1. 1. Experimental searches for the axion and axion-like particles; P.W. Graham et al, Annual Review of Nuclear and Particle Science v 65 (2015)485-514
    1. 2. Search for a new interaction mediated by axion-like particles; Pin-Jung Pin-Jung,, PhD thesis, ETH Zurich, 2021
    1. 3. Measurement of the permanent electric dipole moment of the neutron, C. Abel et al; PRL 124 (2020) 081803
    1. 4. The design of the n2EDM experiment: nEDM Collaboration; N.J. Ayres et al; European Physical Journal C81 (2021) 512
    1. 5. The Precision nEDM Measurement with UltraCold Neutrons at TRIUMF; R. Matsumiya et al; Proceedings of the 24th International Spin Symposium (SPIN2021), (2021) 020701
    1. 6. Performance of the upgraded ultracold neutron source at Los Alamos National Laboratory and its implication for a possible neutron electric dipole moment experiment; T.M. Ito et al; Phys. Rev. C97 (2018)12501
    1. 7. The PanEDM neutron electric dipole moment experiment at the ILL; D. Wurm et al,; EPJ Web of Conferences 219 (2019) 2006.
    1. 8. Neutron electric-dipole moment, ultracold neutrons and polarized 3He; R Golub, SK Lamoreaux – Physics Reports, 1994.
    1. 9. A new cryogenic apparatus to search for the neutron electric dipole moment; M.W. Ahmed et al; Journal of Instrumentation 14 (2019) P11017

Our team

P. R. Huffman, R. Golub, E. Korobkina, I. Berkutov , C. Hickman, C. Teander, M. Morano

Bob Golub is co-founder of nEDM@SNS experiment  and principal scientist.

Ekaterina Korobkina and Paul Huffman are subsystem managers for nEDM@SNS (SOS apparatus and Assembly and commissioning subsystems). In addition, Paul Huffman is Technical Coordinator for the entire project.

Igor Berkutov is chief cryogenic person for SOS apparatus

Our current and recently graduated grad students:

Matt Morano developed very precise and reproducible digital power source for coils, developed system for active magnetic field cancelation, wrote a spin tracking software, which will help us to analyze spin data and theoretically predict how to minimize systematic errors, integrated NMR electronic components in one system ready for operation; developed a code to obtain a complicated NMR pulse for precise spin manipulation of two different spins . He graduated in 2023 and stays with us as a Caltech postdoc.

Clark Hickman is working on development of superconducting components for magnetic coils and is actively involved in measurements and data analyze at LANL in UCN storage.

Cole Teander  is working on UCN source at PULSTAR reactor. He is also actively involved in measurements and UCN transport simulations for UCN storage.

Our current undergrad students:

Our group always welcomes undergrad students, who wants to be hands on in the lab as well as learn COMSOL, SolidWork and other engineering tools.

Blake Devis  is learning Pentrack UCN transport simulation package and using it to  verify different options of UCN guide geometry. He is also helping at the reactor with UCN source assembly/disassembly tasks and how to work with vacuum equipment.

Benjamin Hanestad has joint our group recently and will be working on design and fabrication of the UCN detector for the first UCN production test at UCN source at PULSTAR reactor.

Collaboration

We are working in close collaboration with Caltech (magnetic coils/shields, spin dressing technique), University of Kentucky (magnetic coils/shields), Montclair University (measurement cell, UCN storage), ORNL (scintillation light collection with DAQ), LANL ( UCN storage), UNAM (MEOP coil). 

Our posters

– What makes nEDM@SNS experiment very special and why we need Systematic and Operational Study testbed.

 – how we polarize 3He nuclei using MEtastable OPtical exchange (MEOP)

  – about SQUIDs

  – Berry phase and frequency shift

 

Our publications

  1. A New Cryogenic Apparatus to Search for the Neutron Electric Dipole Moment,” M.W. Ahmed et al.J. Inst. 14 P11017 (2019)
  2. “The neutron electric dipole moment experiment at the Spallation Neutron Source,” K.K.H Leung et al., EPJ Web of Conferences 219, 02005 (2019)
  3. “Geometry and design of origami bellows with tunable response,” A. Reid, F. Lechenault, S. Rica and M. Adda-Bedia, Phys. Rev. E95 013002 (2017).
  4. “Random walks with thermalizing collisions in bounded regions: Physical applications valid from the ballistic to diffusive regimes,” C.M. Swank, A. K. Petukhov and R. Golub, Phys. Rev. A93 062703 (2016).
  5. “Frequency shifts and relaxation rates for spin-1/2 particles moving in electromagnetic fields,” G. Pignol, M. Guigue, A. Petukhov, R. Golub, Phys. Rev. A92 053407 (2015).
  6. “Geometric phases in electric dipole searches with trapped spin-1/2 particles in general fields and measurement cells of arbitrary shape with smooth or rough walls,” R. Golub, C. Kaufman, G. Muller, A. Steyerl, Phys. Rev. A92 062123 (2015).
  7. “Fundamental neutron physics beamline at the spallation neutron source at ORNL,” N. Fomin, G. Greene, R. Allen, V. Cianciolo, C. Crawford, T. Ito, P. Huffman, E. Iverson, R. Mahurin, W. Snow, Nucl. Inst. Meth. A773 45 (2015).
  8. “Calculation of geometric phases in electric dipole searches with trapped spin-1/2 particles based on direct solution of the Schrodinger equation,” A. Steyerl, C. Kaufman, G. Mueller, S.S. Malik, A.M. Desai, R. Golub, Phys. Rev. A89 052129 (2014).
  9. “Ultracold neutron source at the PULSTAR reactor: Engineering design and cryogenic testing,” E. Korobkina, G. Medlin, B. Wehring, A.I. Hawari, P.R. Huffman, A.R. Young, B. Beaumont, G. Palmquist, Nucl. Inst. Meth. A767 169 (2014)
  10. “Experimental Searches for the Neutron Electric Dipole Moment,” S.K. Lamoreaux and R. Golub, J. Phys. G36 104002 (2009).
  11. “Relaxation of spin-polarized 3He in mixtures of 3He and 4He at about 330 mK,” Q. Ye, H. Gao, W. Zheng, D. Dutta, F. Dubose, R. Golub, P. Huffman, E. Korobkina, C. Swank, Phys. Rev. A80 023403 (2009)
  12. “Relaxation of spin polarized 3He in mixtures of 3He and 4He below the 4He lambda point,” Q. Ye, D. Dutta, H. Gao, K. Kramer, X. Qian, X. Zong, L. Hannelius, R. McKeown, B. Heyburn, S. Singer, R. Golub, E. Korobkina, Phys. Rev. A77 053408 (2008)
  13. “Electric dipole moment searches: Effect of linear electric field frequency shifts induced in confined gases,” A.L. Barabanov, R. Golub and S.K. Lamoreaux, Phys. Rev. A74 052115 (2006)
  14. “Evaporative Isotopic Purification of Superfluid Helium-4,” M.E. Hayden, S.K. Lamoreaux and R. Golub, Am. Inst. Phys. Conf. Proc. 850 147 (2006)
  15. “Characterization of Scintillation Light produced in Superfluid Helium-4,” G. Archibald, J. Boissevain, R. Golub, C.R. Gould, M.E. Hayden, E. Korobkina, W.S. Wilburn, J. Zou, Am. Inst. Phys. Conf. Proc. 850 143 (2006)
  16. Detailed discussion of a linear electric field frequency shift induced in confined gases by a magnetic field gradient: Implications for neutron electric-dipole-moment experiments, SK Lamoreaux, R Golub, Physical Review A 71 (3), 032104 (2005)

Books by Prof. Bob Golub

  1. “Ultra-cold neutrons”, R. Golub, D. Richardson, S.K. Lamoreaux, CRC Press – 1991
  2. “The historical and physical foundations of Quantum mechanics”, R. Golub, S.K. Lamoreaux, Oxford University press, 2023