Neutron-Antineutron Oscillations
W. M. Snow Indiana University/CEEM Neutron summer school 2015 What are neutron-antineutron oscillations?
Why are they interesting to search for?
How do you look for them?
How well can you do?
Recent developements
Students: might be useful to get out pencil and paper n n oscillations —can we add them to the list? Neutral meson |qq〉 states oscillate -
2nd order weak K0, B0 K0, B0
And neutral fermions can oscillate too -
… νµ νe So why not - New ? n physics n
-21 Neutron is a long-lived neutral particle (qn<10 e) and can oscillate into an antineutron. No oscillations have been seen yet.
Need interaction beyond the Standard Model that violates Baryon number (B) by 2 units. No experimental observation of B violation yet. But we expect B violation at some level Question for students: why do I need zero electric charge to get oscillations? Neutron-An neutron Oscilla ons: Formalism
" n% Ψ = n-nbar state vector #$ n&' α≠0 allows oscilla ons
" En α % H = $ ' Hamiltonian of n-nbar system # α E n & p2 p2 En = mn + + Un ; En = mn + + Un 2mn 2mn Note : • α real (assuming T)
• mn = mn (assuming CPT) U U in matter and in external B [ n n from CPT] • n ≠ n µ( ) = −µ( ) Question for students: calculate the eigenvalues and eigenvectors for U=0 B,L are Probably Not Conserved Baryon Asymmetry of Universe (BAU) is not zero. If B(t=after inflation)< Both B and L conservation are “accidental” global symmetries: given SU(3)⊗SU(2)⊗U(1) gauge theory and matter content, no dimension-4 term in Standard Model Lagrangian violates B or L in perturbation theory. Nonperturbative EW gauge field fluctuations (sphaelerons) present in SM, VIOLATE B, L, B+L, but conserve B-L. Very important process for trying to understand the physics of the baryon asymmetry in the early universe. Neutron-antineutron oscillations generically access scales not far above the electroweak scale: physics is completely different from the GUT scales accessed in proton decay No evidence that either B or L is a gauge theory like Q: where is the macroscopic B/L force? (not seen in equivalence principle tests). Question for students: why are tests of the equivalence principle sensitive to a macro B force? Ma er/An ma er Asymmetry in the Universe in Big Bang, star ng from zero Sakharov Criteria to generate ma er/an ma er asymmetry from the laws of physics – Baryon Number Violation (not yet seen in the lab) – C and CP Violation (seen, but too small by ~1010) – Departure from Thermal Equilibrium (no problem?) A.D. Sakharov, JETP Le . 5, 24-27, 1967 Relevant neutron experimental efforts Neutron-antineutron oscillations (B) Electric Dipole Moment searches (T=CP) T Violation in Polarized Neutron Optics (T=CP) P, CP, T, and CPT Detector • Parity viola on (1956, or arguably earlier) – only in weak interac on • CP viola on (1964) 60Co – parametrized but not understood e- – seen in K0 & B0 systems – Doesn’t seem to be responsible for baryon asymmetry of universe • T viola on (1999) π – CPT is good symmetry so far: T ↔ CP π KL π π KS π Searches for B Violation (Nucleon Decay and Neutron- Antineutron Oscillation) Probe Different Physics Mode Nucleon decay N-Nbar oscillations effect on B ΔB=1, ΔL=1, ΔB=2, ΔL=0, and L others Δ(B-L)=2 Δ(B-L)=0,2,… Effective g g L = QQQL L = QQQQQQ operator M 2 M 5 Mass scale Grand Unified >electroweak probed (GUT) scale scale (< Question for students: how do I get those powers of M in L? Nucleon Decay LBNE Physics Working Group Report J. L. Raaf @ IF-2011 arXiv:1110.6249v1 [hep-ex] Question for students: how do you set an experimental limit of 1E33 years if the age of the universe of 1E10 years? 8 Very Large detectors Long exposures From E. Kearns Neutron-An neutron Oscilla ons: Formalism " n% Ψ = n-nbar state vector #$ n&' α≠0 allows oscilla ons " En α % H = $ ' Hamiltonian of n-nbar system # α E n & p2 p2 En = mn + + Un ; En = mn + + Un 2mn 2mn Note : • α real (assuming T) • mn = mn (assuming CPT) U U in matter and in external B [ n n from CPT] • n ≠ n µ( ) = −µ( ) Question for students: calculate the eigenvalues for finite U Neutron-An neutron Oscilla ons with Free Neutrons: The experimental figure-of-merit # E + V α & α 2 + α 2 + V 2 . For H = P (t) = × sin2 - t 0 % E V ( n→n 2 2 $ α − ' α + V ,- /0 where V is the potential difference for neutron and anti-neutron. −23 Present limit on α ≤ 10 eV Contributions to V: 2 2 " α 2 + V 2 % * α - * t - For $ t ' <<1 ("quasifree condition") Pn→n = , × t/ = + . +, τ ./ #$ &' nn 2 Figure of merit (with no background)= N=#neutrons, T=NT “quasifree” observa on me How to Search for N-Nbar Oscilla ons Figure of merit for probability: 2 N=total # of free neutrons observed NT T= observation time per neutron while in “quasifree” condition When neutrons are in matter or in nucleus, n-nbar potential difference is large->quasifree observation time is short B field must be suppressed to maintain quasifree condition due to opposite magnetic moments for neutron and antineutron (1) n-nbar transitions in nuclei in underground detectors (2) Cold and Ultracold neutrons εnn π π Nucleus A A* + n nN pions Bound neutron N-Nbar search experiments 32 Experiment Year A n⋅year (10 ) Det. eff. Candid. Bkgr. τ nucl , yr (90% CL) Kamiokande 1986 O 3.0 33% 0 0.9/yr >0.43×1032 Frejus 1990 Fe 5.0 30% 0 4 >0.65×1032 Soudan-2 2002 Fe 21.9 18% 5 4.5 >0.72×1032 SNO * 2010 D 0.54 41% 2 4.75 >0.301×1032 Super-K 2011 O 245 12.1% 24 24.1 >1.89×1032 * Preliminary • From Kamiokande to Super-K atmospheric ν background is about the same ~ 2.5 /kt/yr. • Large D2O, Fe, H2O detectors are dominated by backgrounds; LAr detectors are unexplored • Observed improvement is weaker than SQRT due to irreducible background and uncertainties of efficiency and background. Previous n-nbar search experiment with free neutrons Schematic layout of At ILL/Grenoble reactor in 89-91 by Heidelberg-ILL-Padova-Pavia Collabora on Heidelberg - ILL - Padova - Pavia nn search experiment M. Baldo-Ceolinat Grenoble et al., Z. Phys., 89-91 C63 (1994) 409 (not to scale) Cold n-source Top view of horizontal experiment 25Κ D2 fast n, γ background 58 HFR @ ILL Bended n-guide Ni coated, 57 MW L ~ 63m, 6 x 12 cm 2 H53 n-beam ~1.7. 1011 n/s Focusing reflector 33.6 m Flight path 76 m No background observed! < TOF> ~ 0.109 s Detector: No candidates observed. Magnetically Tracking& Limit set for a year of running: shielded Calorimetry Discovery potential : 95 m vacuum tube 8 2 9 τ nn >0.86×10Nns⋅t = 1.5⋅10 sec vn ~700 m/s with L ~ 76 m aMeasurednd t = 0. 1limit09 s e:c Annihilation measured P <1.606 ×10−18 7 nn τnn ≥ 8.6⋅10 sec target ∅1.1m sensitivity: N ⋅t 2 =1.5×109 s2 s ΔE~1.8 GeV Beam dump ~1.25 1011 n/s "ILL sensitivity unit" 14 Better Slow Neutron NNbar Experiment: What do we want? (HOW DIFFICULT IS IT?) While still keeping quasifree condition, one wants more 2 with nbar detector watching the annihilation surface NT • higher cold neutron brightness from moderator? POSSIBLE • slower cold neutron energy spectrum? DIFFICULT • more efficient extraction of cold neutrons with optics to quasifree flight/detector? YES: GREAT PROGRESS SINCE ILL NNBAR • longer “quasifree” flight time? YES • longer experiment operation time? YES (ILL only ran 1 year) How to Improve the Experiment? Max neutron flux/brightness from production source/target: ~unchanged for ~4 decades 1E+18 US-SNS IBR-2 ISIS JSNS-1 HFIR ILL NRU MLNSC ESS /s) 2 MTR 1E+15 FRM-II NRX HFBR SINQ-III SINQ-I SINQ-II X-10 1E+12 IPNS IBR-30 KENS Thermal Flux (n/cm ThermalFlux 1E+09 CP-2 Tohoku Linac CP-1 Berkeley 37 inch cyclotron 1E+06 Fux of pulsed Fissions reactor 0,35mCi Ra-Be source sources pulsed reactor continoues spallation source peak pulsed spallation source 1E+03 Trend line of reactorsl Trend of spallation sources (average) Chadwick average Trend of spallation sources (peak) 1E+00 Year 1900 1920 1940 1960 1980 2000 2020 Neutron flux is increasing only slowly with time R. Eichler, PSI Projected ESS Peak Slow Neutron Brilliance (from ESS website) relative to other sources Upon me-averaging, Neutrons E kin T,K Velocity Wavelength Fast ~ 1 MeV ~ 1010 ~ 0.046 c ~ 0.0003 Å Thermal ~ 25 meV ~ 300 ~ 2.2 km/s ~ 1.8 Å Cold ~ 3 meV ~ 35 ~ 760 m/s ~ 5 Å Very Cold (VCN) ~ 1 meV ~ 10 ~ 430 m/s ~ 9 Å Ultra Cold (UCN) ~ 250 neV ~ 0.003 ~ 8 m/s ~ 600 Å Number Neutron Moderator (H2, D2,…) Why is it hard to make colder neutron spectra from neutron moderators? 1. Cryogenic engineering (heat loads, conductivity/heat capacity…) 2. For cold neutrons with λ>d, σel >> σin, most collisions elastic 3. Frozen materials required to lower neutron T also freeze inelastic modes 4. Radiation damage causes cryogenic solids to decompose or explode 5. Phonon phase space decreases as ω3, need to find other inelastic modes Research on better slow neutron moderators in progress Quasifree Condition: B Shielding and Vacuum µBt<<ћ ILL achieved |B|<10 nT over 1m diameter, 80 m beam,one layer 1mm shield in SS vacuum tank, 1% reduction in oscillation efficiency (Bitter et al, NIM A309, 521 (1991). For new experiment need |B|<~1 nT NEW RESULT: this seems not to be needed anymore (SVG arXiv) Voptt<<ћ: Need vacuum to eliminate neutron-antineutron optical potential difference. P<10-5 Pa is good enough, much less stringent than LIGO Antineutron detector for “zero background” condition n + A → 〈5〉 pions (1.8 GeV) Annihilation target: ~100µ thick Carbon film σannihilation ∼ 4 Kb σnC capture ∼ 4 mb vertex precisely defined. No background was observed at ILL Better Free Neutron Experiment (Horizontal geometry) need slow neutrons from high flux source, illuminate phase space acceptance of neutron focusing reflector, free flight path of ~200m Improvement on ILL experiment by factor of >~500-1000 in transition probability is possible at ESS with existing n optics technology, sources, and moderators for a reasonable running time. D ~ 4 m L ~ 200 m concept of neutron supermirrors: Swiss Neutronics neutron reflection at grazing incidence (< ≈2°) @ smooth surfaces @ multilayer @ supermirror refractive index n < 1 total external reflection e.g. Ni θc = 0.1 °/Å 2d sin 1.0 1.0 λ = θ 0.9 0.9 d3>d2>d1 0.8 0.8 1..0 0.7 0.7 0..9 0.6 0.6 0..8 0.5 0.5 0..7 reflectivity 0.4 reflectivity 0.4 0..6 0.3 0.3 0..5 0.2 reflectivity reflectivity 0.2 reflectivity 0..4 0.1 0.1 0..3 λ = 5 Å λ = 5 Å 0.0 0.0 0..2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0..1 = 5 Å θ [°] θ [°] 0..0 λ 0..0 0..2 0..4 0..6 0..8 1..0 1..2 1..4 θ [[°]] m=1 θcritical →mθcritical m=2 “Supermirrors”: θcritical →mθcritical Commercial supermirror neutron mirrors are available with m >6 ! Phase space acceptance for straight guide ∝m2, more with focusing reflector From P. Boeni, Swiss Neutronics ILL experiment used m~1 op cs Use supermirror ellip cal focusing reflector close to the source to increase transverse phase space acceptance for cold neutrons within fixed solid angle ΔΩ and direct them to annihila on target ΔΩ Annihila on Target Cold source L Super-mirror focusing reflector Y. Kamyshkov et al., Use of cold source and large reflector mirror guide for neutron-an neutron oscilla on search (proposal), Proceedings of the ICANS-XIII mee ng of the Interna onal Collabora on on Advanced Neutron Sources, Paul Scherrer Ins tute, Villigen, Switzerland, October 11-14, p. 843 (1995). Supermirror Neutron Optics: Future Possibilities At a pulsed neutron source like ESS, neutrons of a given speed reach the mirror at a known me. We can therefore imagine an array of mirrors ling an ellipse and phased to the source to condi on the beam Piezodrivers “In the future one may consider varying the shape of the guides actively by means of piezo actuators. If used at pulsed sources, beam size and therefore the divergence for each wavelength during a neutron pulse can be optimized. The combination of fast mechanical actuators with supermirror technology may become useful for active phase space transformation.” Peter Boeni, Nucl. Inst. Meth. A586, 1 (2008). Question for students: does this process obey Liouville’s theorem? Summary New physics beyond the Standard Model can be discovered by NNbar Experiments with free neutrons can possess very low backgrounds (sharp vertex localiza on): possibility for a crisp discovery Sensi vity of free neutron experiment for NNbar transi on rate can be improved by factor of >~500-1000 using exis ng technology [Combina on of improvements in neutron op cs technology, longer observa on me/larger-scale experiment, and source design op miza on at green-field facility]. Further improvements in a free neutron experiment can come from further neutron op cs technology/ moderator/reflector development