Neutron beta decay frontier in Europe Stefan Bae ler Inst. Nucl. - PowerPoint PPT Presentation

Neutron beta decay frontier in Europe Stefan Bae β ler Inst. Nucl. Part. Phys. N.B.: I am mostly presenting other people’s work. I thank for slides from K. Bodek, W. Heil, C. Schmidt, O. Zimmer, G. Konrad, D. Moser, S. Ivanov, D. Geisbauer, B. Märkisch. Errors are all due to my rearranging and compressing.

Observ rvable les in in Neutron Be Beta Decay Observables in neutron beta decay, as a function of e - 𝜏 𝑜 Ԧ generally possible coupling constants (assuming only Lorentz-Invariance): p n Jackson et al., PR 106, 517 (1957), C.F. v.Weizsäcker, Z. f. Phys. 102,572 (1936), M. Fierz, Z. f. Phys. 105, 553 (1937) 𝜉 𝑓 ഥ          dw 2  p p m   2   2  2       2     G F V 1 3 E  e e d E 1 3 1 a b u d e dE e  E E E e  b  e e Fierz interference term 0     p p p p         e e  + A B D n   E E E E Neutrino-Electron-Correlation   e e   2 1  a   2 1 3    2 Re   Beta-Asymmetry A 2 Neutrino-Asymmetry 𝐶   2 1 3    2          2 1 2 2 G V 1 3 E Neutron lifetime n F u d e (Equations in SM, where 𝜇 = 𝑕 𝐵 /𝑕 𝑊 ) 2

Neutron Lifetime Measurements Beam: Decay rate: 𝑒𝑂 𝑒𝑢 = 𝑂 𝜐 𝑜 Bottle: Neutron counts : 𝑂 = 𝑂 0 𝑓 − 𝑢 𝜐 𝑓𝑔𝑔 with 1 𝜐 𝑓𝑔𝑔 = 1 1 𝜐 𝑜 + 𝜐 𝑥𝑏𝑚𝑚 UCN material bottle not used magnetic bottle beam 895 Storage bottle(s) Neutron lifetime [s] 890 885 UCN Shutter detector 880 UCN from source 875 1985 1990 1995 2000 2005 2010 2015 2020 Experiment publication Many new experiments. In Europe… • Improved material bottles (e.g. Big GRAVITRAP, Serebrov et al.) • Magnetic bottles (e.g. UCN τ , C.-Y. Liu et al., LANL; τ SPECT, W. Heil, M. Beck et al., TRIGA Mainz; HOPE, O. Zimmer et al., ILL Grenoble, PENELOPE, S. Paul et al., TU München; V. Ezhov et al. PNPI ) 3 • Beam Lifetime (only at NIST)

Neutron lifetime in material bottle - Big GRAVITRAP FIG. 1. Basic scheme of inner part of the apparatus (a) with conceptual scheme for the measuring procedures (b). Idea: Measure Neutron count rate after storage: 𝑂 𝑢, 𝐹 = 𝑂 0 𝑓 −𝑢/𝜐 𝑡𝑢 (𝐹) −1 𝐹 = 𝜐 𝑜 −1 + 𝜐 𝑚𝑝𝑡𝑡 −1 with storage lifetime 𝜐 𝑡𝑢 𝐹 −1 and 𝜐 𝑚𝑝𝑡𝑡 𝐹 = 𝜈 𝑈, 𝐹 𝜉 𝐹 = 𝜃 𝑈 𝛿(𝐹) Effective collision frequency Loss coefficient If one measures two situations with different 𝛿 , and computes 𝛿 2 (𝐹)/𝛿 1 (𝐹) , one gets −1 = 𝜐 1 −1 − −1 − 𝜐 1 𝛿 2 𝐹 −1 𝜐 𝑜 𝜐 2 ൗ 𝛿 1 𝐹 − 1 This can be done by varying trap geometry (better), or by varying neutron energy. Result: 𝜐 𝑜 = 881.5 ± 0.7 𝑡𝑢𝑏𝑢. ± 0.6 𝑡𝑧𝑡𝑢. (Phys. Rev. C 97, 055503 (2018)) 4 S. Ivanov, A.P. Serebrov et al. (PNPI Gatchina et al.)

Neutron lifetime in magnetic trap - 𝜐𝑇𝑄𝐹𝐷𝑈 Octupole magnet aSPECT superconducting Halbach-configuration magnet (see later) radial axial 𝑊 = − Ԧ 𝜈 𝑜 ⋅ 𝐶 𝑊 𝑞𝑝𝑢 ∼ 50 neV permanent magnets Sm 2 Co 17 Oktupole superconducting HFS AFP magnet Goal: magnet LFS Spin-Flip Δ𝜐 𝑜 ≤ 2 s (soon) UCN-guide Δ𝜐 𝑜 ≤ 0.3 s 𝑪 𝟏 (2023?) p-detector 5

Neutron lifetime in magnetic trap (2) - HOPE Measurement procedure: Start with well established “fill and empty” method. Use lower end coil as magnetic shutter. Feature: Full-bore access from top and bottom: • insertion of diffusive paddle and absorber • monitoring of depolarisation • detection of marginally trapped neutrons • later proton detection possible at top Couple experiment to superfluid-helium UCN source SUN-2 at ILL (Magnetic shutter) (pessimistic estimate: 3000 UCN/fill)  n ~ 0.7 s in 50 days (statistical) Experiments @ PF2 performed in fall 2014 ( 𝜀𝜐 = 52.7 s in about 1 day) Experiments @ SUN-2 in preparation L. Babin 6 Oliver Zimmer (ILL Grenoble) et al.

Neutron lifetime in magnetic trap (3) – PENELOPE absorber • Will be located at the Forschungs- movement mechanism Neutronenquelle Heinz Maier-Leibnitz (FRM II) • Magneto-gravitational trap for proton detector ultra-cold neutrons 2.5 m • Filling: Insert UCN, Ramp up magnetic field, remove high field outer pressure seekers (spin-down neutrons) vessel with absorber) • Aiming for a precision of ± 0.1 s helium vessel • Measuring protons (during storage) and neutrons (after storage) storage walls (electropolished) 7 Dominic Gaisbauer (TU München)

Determination of ratio 𝜇 of V,A coupling constants −2 Re 𝜇 + 𝜇 2 1 + 3 𝜇 2 Determination of ratio 𝜇 = 𝑕 𝐵 𝑕 𝑊 from 𝐵 = Τ Τ 1 − 𝜇 2 1 + 3 𝜇 2 from experiment: Τ or 𝑏 = UCNA (2018) Ideogram of current experiments: Δ𝜇/𝜇 = 0.03% aCORN (2017) (Nab goal) PERKEO II (2013) 𝜇 from 𝐵 UCNA (2013) ( ) 𝜇 from 𝑏 UCNA (2010) ( ) Byrne (2002) 𝜇 from 𝐵/𝐶 ( ) PERKEO II (2002) My current average: Mostovoi (2001) 𝜇 = −1.2756(11) Yerozolimskii (1997) PERKEO II (1997) ( ) Liaud (1997) PERKEO I (1986) Stratowa (1978) −1.30 −1.28 −1.26 −1.24 𝜇 = 𝑕 𝐵 /𝑕 𝑊 (There is a shift to the left, since 20 years, confirmed with increasing accuracy over the years) 8

The Beta Asymmetry – general idea Electron Detector (Plastic Scintillator) 𝜏 𝑜 Ԧ e - p n 𝜉 𝑓 ഥ Decay Electrons 1 + 𝑐 𝑛 𝑓 𝜏 𝑜 ⋅ p 𝑓 𝑒Γ ∝ 𝜛 𝐹 𝑓 + 𝐵 Ԧ 𝐹 𝑓 𝐹 𝑓 Polarized Neutrons Experimental Reality: • Flip neutron spin, don’t compare Split Pair Magnet detectors! • Two detectors still needed to suppress electron backscattering. Magnetic Field PERKEO II  N N v     up down A cos p , Pf  e n N N c up down Perkeo 2 - Beam time Result Publication 1995 A = -0.1189(12) Phys. Lett. B 407, 212 (1997) 1997 A = -0.1189(7) Phys. Rev. Lett. 88, 211801 (2002) 9 A = -0.11926 +47 2004 Phys. Rev. Lett. 110, 172502 (2013) -53

The Beta Asymmetry – PERKEO III Detector 1 Total length: 8 m electrons Pulsed Detector 2 Cold Neutron B = 90 mT B = 150 mT Beam (homogeneous) Background subtraction with pulsed beam Neutron Beamstop NB: At PPNS Grenoble, 2018, B. Märkisch presented a result with Δ𝐵 = 2.1 ⋅ 10 −4 . The number is unpublished so far, and was not part of the slides I was given. 10 B. Märkisch (TU München), D. Dubbers (U Heidelberg), H. Abele (TU Wien) et al.

aSPECT @ ILL Grenoble e - n 1 + 𝑏 𝑞 𝑓 𝑒Γ ∝ cos 𝜄 𝑓𝜉 𝜄 𝑓𝜉 p 𝐹 𝑓 𝜉 𝑓 ഥ p  p p Proton Detector p p Decay rate w(E) Magnetic p field p  e p e Spectrum for a = +0.3 … for a = -0.103 [PDG2016] 0 200 400 600 Analyzing Plane Proton kinetic energy E [eV] Electrode Protons Protons 780 V at No voltage at Analyzing plane Analyzing plane Neutron Decay No protons Electrons Electrons Noise Noise Preliminary result (PPNS Grenoble, 2018): 𝑏 = −0.10603(91) 11 W. Heil, M. Beck, C. Schmidt (U Mainz) et al.

The Beta Asymmetry – next step (PERC) Motivation: • Use idea from PERKEO III to reduce beam-related background (pulsed beam) • Increase count rate by looking at decay volume in neutron guide • Increase sensitivity through use of magnetic filter (see also A. Serebrov, Nucl. Inst. Meth. 505, 344 (2005)) For asymmetry 𝑒Γ ∝ (1 + 𝐵 cos 𝜄) 𝑞 ∥ /deg. 1 80 𝑞 ⊥ 0.8 Count rate 𝜄 𝑑 60 0.6 Crit. Angle conversion Adiabatic 40 Magnetic Field 0.4 20 0.2 0 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 𝐶 𝑒𝑓𝑑𝑏𝑧 /𝐶 𝑔𝑗𝑚𝑢𝑓𝑠 𝐶 𝑒𝑓𝑑𝑏𝑧 /𝐶 𝑔𝑗𝑚𝑢𝑓𝑠 Electron or Proton Count rate asymmetry 𝑞 ∥ 2 1.2 Stat. Figure of merit Trajectory 1.75 1 1.5 0.8 𝑞 ⊥ 1.25 0.6 1 0.75 0.4 If 𝐶 𝑔𝑗𝑚𝑢𝑓𝑠 > 𝐶 𝑒𝑓𝑑𝑏𝑧 , detector behind filter 0.5 0.2 0.25 detects only particles with angle to field 𝜄 0 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 𝐶 𝑒𝑓𝑑𝑏𝑧 𝐶 𝑒𝑓𝑑𝑏𝑧 /𝐶 𝑔𝑗𝑚𝑢𝑓𝑠 𝐶 𝑒𝑓𝑑𝑏𝑧 /𝐶 𝑔𝑗𝑚𝑢𝑓𝑠 less than crit. angle 𝜄 𝑑 with sin 𝜄 𝑑 = 𝐶 𝑔𝑗𝑚𝑢𝑓𝑠 12 See D. Dubbers et al., Nucl. Instr. Meth. A 596, 238 (2008)

The PERC facility @ FRM II Magnetic Filter 𝑪 𝒈𝒋𝒎𝒖𝒇𝒔 𝐶 𝑒𝑓𝑑𝑏𝑧 𝐶 𝑒𝑓𝑢𝑓𝑑𝑢𝑝𝑠 Cryostat Active volume in a 8 m long neutron-guide, 𝐶 𝑒𝑓𝑑𝑏𝑧 ≤ 1.5 T: (statistics, phase space density (S/B !), smaller detectors) Magnetic Filter, 𝑪 𝒈𝒋𝒎𝒖𝒇𝒔 ≤ 𝟕 T (can select 𝐶 𝑔𝑗𝑚𝑢𝑓𝑠 𝐶 𝑒𝑓𝑑𝑏𝑧 = 2 … 12 ): phase space selection, systematics Τ … (choice of solid angle, backscatter suppression) Source for specialized spectrometers MAC-E filter R × B spectrometer Electron or proton (as in “ aSPECT ” – (NOMOS) detector (Plastic see later) scintillator, silicon) 13 B. Märkisch (TU München), D. Dubbers (U Heidelberg), H. Abele (TU Wien) et al.