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W. M. Snow Physics Department Indiana University Center for the Exploration of Energy and Matter

Nuclear/Particle/Astrophysics with Slow Neutrons. W. M. Snow Physics Department Indiana University Center for the Exploration of Energy and Matter wsnow@indiana.edu. 1. Neutron technology and slow neutrons 2. Neutron lifetime 3. NN weak interaction: parity violation

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W. M. Snow Physics Department Indiana University Center for the Exploration of Energy and Matter

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  1. Nuclear/Particle/Astrophysics with Slow Neutrons W. M. Snow Physics Department Indiana University Center for the Exploration of Energy and Matter wsnow@indiana.edu 1. Neutron technology and slow neutrons 2. Neutron lifetime 3. NN weak interaction: parity violation 4. Search for neutron electric dipole moment: time reversal violation 5. Neutron gravitational bound states and search for extra dimensions For more: J. Nico and W. M. Snow, Annual Reviews of Nuclear and Particle Science 55, 27-69 (2005) Thanks for slides to: Geoff Greene, Chen-Yu Liu, Jen-chieh Peng, Philip Harris, Hartmut Abele, Hiro Shimizu, T. Soldner,…

  2. Nuclear/Particle/Astrophysics with Slow Neutrons... is “Nuclear” physics, but with an “isotope of nothing” is Particle Physics:but at an energy of 10-20 TeV, using a low energy decelerator employs a particle which, according to Big Bang Cosmology, is lucky to be alive relies on experimental techniques and ideas from nuclear, particle, atomic, and condensed matterphysics is pursued at facilities built mainly for chemistry, materials science, and biology

  3. Neutron Properties Electric charge: qn=0, electrically neutral [qn<10-21e] Size: rn~10-5Angstrom=1 Fermi [area~ 10-25 cm2=0.1 “barn”] Internal Structure: quarks [ddu, md~ mu~few MeV ] + gluons Spin: sn= 1/2 [Fermi statistics] Magnetic Dipole Moment:n/ p = -0.68497935(17) Electric Dipole Moment: zero[dn < 3x10-26 e-cm] Mass: mn=939.56536(8) MeV [mn> mp+ me, neutrons can decay] Lifetime:n=885.7 +/- 0.8 seconds

  4. Why is it such hard work to get slow neutrons? E E=0 Neutrons are bound in nuclei, need several MeV for liberation. We want E~kT~25 meV (room temperature) or less VNN p n How to slow down a heavy neutral particle with Mn= Mp ? Lots of collisions… [1/2]N=(1 MeV)/(25 meV) for N collisions E 0 E/2 Neutrons are unstable when free->they can’t be accumulated easily

  5. The ILL reactor Cold Source

  6. H4 H9 H7 cold thermal inclined tube IH hot HS HCS VCS H2O D2O ILL7 1 m IH2 IH3 ILL22 H5 H10 IH1 H1 H12 H11 IH4 Inside the ILL Reactor H2 H3 H6 H8 H13

  7. The Spallation Neutron Source • $1.4B--1GeV protons at 2MW, started in 2007. • Short (~1 usec) pulse– mainly for high TOF resolution

  8. Moderators Target

  9. n ν β n 2MeV 235U 235U n 0.1eV γ ν 30K γ n β 235U “Slow” Neutrons: MeV to neV Nuclear reactor/ Spallation source 300K Neutron Moderator (LH2, LD2) neV meV

  10. Neutron Energy, Momentum, and Wavelength

  11. Neutrons in Condensed Matter for a “thermal” neutron (EK=mv2/2=3/2 kBT, T=300K-> EK=25 meV) the de Broglie wavelength of the neutron is l≈ 2 Angstroms |p> |k> |p> |k> phonon diffraction Thermal neutrons have the right energies and momenta to match excitations (phonons, spin waves, molecular rotations…) and static structures (crystals, molecular shapes,…) in condensed media

  12. Many Condensed Matter Phenomena lie within the Ranges of Length & Time Seen by Neutron Scattering

  13. Potential step -> neutron index of refraction Neutron kinetic energy If a > 0, total external reflection <Vstrong>=2h2bs/m, ~+/- 100 neV <Vmag>=B, ~+/- 60 neV/Tesla <Vgrav>=mgz~100 neV/m <Vweak>=[2h2bw/m]s·k/|k|~10-7<Vstrong> All forces contribute to the neutron optical potential:

  14. Neutron optical guides at ILL/Grenoble (top view)

  15. Cold Neutron Guide Hall at NIST n

  16. What methods are used to polarize neutrons? B gradients (Stern-Gerlach, sextupole magnets) electromagnetic F=()B B Reflection from magnetic mirror: electromagnetic+ strong f=a(strong) +/- a(EM) with | a(strong)|=| a(EM)| f+=2a, f-=0 B L  Transmission through polarized nuclei: strong ≠ -  T ≠ T Spin Filter:T=exp[-L]

  17. Ultra-Cold Neutrons (UCN) (Fermi/Zeldovich) • What are UCN ? • Very slow neutrons (v < 8 m/s, l > 500 Å, E<Voptical ) that cannot penetrate into certain materials Neutrons can be trapped in material bottles or by magnetic fields

  18. The weak interaction: just like EM, but not really: one EM photon 3 “weak photons” [W+, W-, Z0], can change quark type e- e- e- d u e- u u g Z0 W+- Z0 e- e- e- e- u d u u ‘V’(r)≈[e2/r]exp(-Mr) , MZ,W≈ 80-90 GeV V(r)=e2/r, mg=0 “Empty” space (vacuum) is a weak interaction superconductor |B| weak field penetration depth 1/ MZ,W r r vacuum superconductor our “vacuum”

  19. The weak interaction violates mirror symmetry and changes quark type weak interaction = eigenstates  d [CKM] quark mass eigenstates W+- u e Vud in n decay Matrix must be unitary Only the weak interaction breaks mirror symmetry: not understood Discovered by C.S.Wu r->-r in mirror, but s->+s

  20. Neutron -decay Input for theory of Big Bang element creation Clean extraction of fundamental parameters of the electroweak theory.

  21. 885.7(8)s 878.5(8)s Influence of : Shift  by 9 (1%) = 885.7(8)s Yp = 0.2479(6)  = 878.5(8)s  Yp = 0.2463(6) Astrophysical observations: large systematic uncertainties, Yp = 0.238(2)(5), Yp = 0.232 … 0.258, … Neutron LifetimeEffect on Primordial 4HeAbundance inUniverse

  22. Neutron Lifetime Measurement with a Proton Trap and Flux Monitor (Dewey et al) dN(t)/dt=-N(t)/, measure decay rate and total # of neutrons in a known beam volume Protons from neutron decay trapped in a Penning trap and counted Neutron # in trap inferred from flux monitor

  23. In-Beam Lifetime Apparatus @ NIST

  24. 1.8 1.7 1.6 1.5 Stor-1 * 1000 [ s-1] 1.4 1.3 1.2 1.1 0 10 20 30 40  [s-1] Neutron Lifetime Using UCN Bottle Lifetime extrapolated to S/V->0 limit Measure storage time, vary S/V

  25. “History” of Neutron Lifetime Measurements World average [PDG] Last (single) measurement

  26. N-N Weak Interaction: Size and Mechanism ~1 fm NN repulsive core → 1 fm range for NN strong force = valence + sea quarks + gluons + … NN strong force at low energy mediated by mesons QCD possesses only vector quark-gluon couplings → conserves parity Both W and Z exchange possess much smaller range [~1/100 fm] weak Relative strength of weak / strong amplitudes: NN weak amplitudes first-order sensitive to qq correlations Weak interaction violates parity. Use parity violation to isolate the weak contribution to the NN interaction.

  27. Asymmetry A of gamma angular distribution upon polarized neutron capture due to weak NN interaction [from snp] Goal: 1x10-8 at SNS/7000 MW-hours Asymmetry depends mainly on the weak pion coupling f for n-p PV gamma asymmetry in n+D->3H+ also possible (SNS letter of intent) PV Gamma Asymmetry in Polarized Neutron Capture

  28. NPDGamma Experimental Setup at Los Alamos (LANSCE) B0=10 gauss

  29. Parity Violating Asymmetry in n+p->D+gamma at LANSCE

  30. Analogous to optical rotation in an “handed” medium. Transversely-polarized neutrons corkscrew due to the NN weak interaction PV Spin Angle is independent of incident neutron energy in cold neutron regime, In combination with n-p experiment, determine weak interactions between nucleons and use it to calculate parity violation in atoms Another Parity-Violating Observable: Neutron Spin Rotation f Refractive index dependent on neutron helicity

  31. n Cross section of Spin Rotation Apparatus Side View

  32. Liquid Helium Motion Control System Nonmagnetic cryostat: target feedthroughs and liquid motion control system

  33. Distribution of Raw Asymmetries Data under analysis Result consistent with zero at 1E-6 rad/m level Future experiments in H,D, and 4He possible

  34. Diffuse -ray flux expected from annihilation Cohen, De Rujula, Glashow; astro-ph/9707087 Where’s the antimatter in the universe? In the lab we make equal amounts of matter and antimatter So why is the universe lopsided? Is it just an accident?

  35. Matter/Antimatter Asymmetry in the Universe in Big Bang, starting from zero Sakharov Criteria to generate matter/antimatter asymmetry from the laws of physics • Baryon Number Violation (not yet seen) • C and CP Violation (seen but too small by ~1010) • Departure from Thermal Equilibrium (no problem?) A.D. Sakharov, JETP Lett. 5, 24-27, 1967 Relevant neutron experimental efforts Neutron-antineutron oscillations (B) Electric Dipole Moment searches (T=CP)

  36. Neutron Electric Dipole Moment Non-zero dn violates both P and T Under a parity operation: Under a time-reversal operation:

  37. Electro- 10-20 10-20 magnetic electron: 10-22 neutron: Experimental Limit on d (e cm) 10-24 Multi SUSY Higgs f ~ 1 Left-Right f ~ a/p 10-30 10-30 2000 1960 1970 1980 1990 10-32 10-34 Standard Model 10-36 10-38 EDM limits: the first 50 years Factor ~10 per 8 years Barr: Int. J. Mod Phys. A8 208 (1993)

  38. ... current EDM limit on neutron would correspond to charge separation of x  10 +e x -e Reality check If neutron were the size of the Earth...

  39. EDM Measurement Principle B0 B0 B0 E E () – () = – 4 E d/ h assuming B unchanged when E is reversed. <Sz> = + h/2 h(0) h() h() <Sz> = - h/2 Present EDM limit corresponds to energy difference of 1E-22 eV!

  40. Four-layer mu-metal shield High voltage lead Quartz insulating cylinder Coil for 10 mG magnetic field Upper electrode Main storage cell Hg u.v. lamp PMT to detect Hg u.v. light Vacuum wall Mercury prepolarising cell RF coil to flip spins Hg u.v. lamp Magnet S N UCN guide changeover UCN polarising foil Ultracold neutrons (UCN) UCN detector nEDM apparatusat ILLusingUCN • Use 199Hg co-magnetometer to sample the variation of B-field in the UCN storage cell • Limited by low UCN flux of ~ 5 UCN/cm3 • Figure–of-merit ~ E(NΤ)1/2

  41. Dispersion curve for free neutrons Landau-Feynman dispersion curve for 4He excitations ln = 8.9 Å; E = 1.03 meV UCN production in liquid helium • 1.03 meV (11 K) neutrons downscatter by emission of phonon in liquid helium at 0.5 K • Upscattering suppressed: Boltzmann factor e-E/kT is small if T<<11K R. Golub and J.M. Pendlebury Phys. Lett. 53A (1975), Phys. Lett. 62A (1977)

  42. nEDM experiment at SNS, ~2014 GOAL: ~1E-28 e-cm Figure–of-merit ~ E(NΤ)1/2 New approach aims at N  100 N, T  5 T, E  5 E

  43. Classical/QM Bouncing Neutrons

  44. Neutron Probability Distributions Above the Mirror

  45. General scheme of the experiment (flow-through integral mode) Vhorizont~4-15 m/s Vvertic~2 cm/s Selection of vertical and horizontal velocity components

  46. Experimental Apparatus

  47. the Experiment

  48. Observation/Comparison to Theory

  49. R compact dimension Infinite dimension Large Extra Dimensions of Spacetime?New Forces of nature? Why not? Maybe gravity is so weak because some of its flux flows into extra “compact” dimensions of size  If so, V is not 1/r if r~

  50. Limits for alpha and lambda Green: Neutron Limits

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