Nab: precise experimental study of unpolarized neutron beta decay Dinko Poˇ cani´ c, (for the Nab collaboration) University of Virginia 2015 Fall Meeting of the APS Division of Nuclear Physics Santa Fe, NM 28 – 31 October 2015
Neutron beta decay observables (SM) d w ≃ p e E e ( E 0 − E e ) 2 d E e dΩ e dΩ ν � 1 + a � p e · � p ν + b m � A � p e + B � p ν � � × + � � σ n � · + . . . E e E ν E e E e E ν where in SM: A = − 2 | λ | 2 + Re ( λ ) a = 1 − | λ | 2 1 + 3 | λ | 2 1 + 3 | λ | 2 B = 2 | λ | 2 − Re ( λ ) λ = G A (with τ n ⇒ CKM V ud ) 1 + 3 | λ | 2 G V also proton asymmetry: C = κ ( A + B ) where κ ≃ 0 . 275 . D. Poˇ cani´ c (UVa) The Nab experiment : Basic definitions 30 Oct ’15/DNP2015 2 / 14
Neutron beta decay observables (SM) d w ≃ p e E e ( E 0 − E e ) 2 d E e dΩ e dΩ ν � 1 + a � p e · � p ν + b m � A � p e + B � p ν � � × + � � σ n � · + . . . E e E ν E e E e E ν where in SM: A = − 2 | λ | 2 + Re ( λ ) a = 1 − | λ | 2 1 + 3 | λ | 2 1 + 3 | λ | 2 B = 2 | λ | 2 − Re ( λ ) λ = G A (with τ n ⇒ CKM V ud ) 1 + 3 | λ | 2 G V also proton asymmetry: C = κ ( A + B ) where κ ≃ 0 . 275 . ⇒ SM overconstrains a , A , B observables in n β decay! Fierz interf. term b brings add’l. sensitivity to non-SM processes! D. Poˇ cani´ c (UVa) The Nab experiment : Basic definitions 30 Oct ’15/DNP2015 2 / 14
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 50 × better than: − 0 . 1054 ± 0 . 0055 Byrne et al ’02 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al ’78 − 0 . 091 ± 0 . 039 Grigorev et al ’68 D. Poˇ cani´ c (UVa) The Nab experiment : Goals and motivation 30 Oct ’15/DNP2015 3 / 14
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 50 × better than: − 0 . 1054 ± 0 . 0055 Byrne et al ’02 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al ’78 − 0 . 091 ± 0 . 039 Grigorev et al ’68 ◮ Measure b (Fierz interf. term) in n decay with ∆ b ≃ 3 × 10 − 3 current results: none (not yet reported for n decay) D. Poˇ cani´ c (UVa) The Nab experiment : Goals and motivation 30 Oct ’15/DNP2015 3 / 14
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 50 × better than: − 0 . 1054 ± 0 . 0055 Byrne et al ’02 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al ’78 − 0 . 091 ± 0 . 039 Grigorev et al ’68 ◮ Measure b (Fierz interf. term) in n decay with ∆ b ≃ 3 × 10 − 3 current results: none (not yet reported for n decay) ◮ Nab will be followed by the ABba/PANDA polarized program to measure A , electron, and B / C , neutrino/proton, asymmetries with ≃ 10 − 3 relative precision. D. Poˇ cani´ c (UVa) The Nab experiment : Goals and motivation 30 Oct ’15/DNP2015 3 / 14
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 50 × better than: − 0 . 1054 ± 0 . 0055 Byrne et al ’02 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al ’78 − 0 . 091 ± 0 . 039 Grigorev et al ’68 ◮ Measure b (Fierz interf. term) in n decay with ∆ b ≃ 3 × 10 − 3 current results: none (not yet reported for n decay) ◮ Nab will be followed by the ABba/PANDA polarized program to measure A , electron, and B / C , neutrino/proton, asymmetries with ≃ 10 − 3 relative precision. Motivation: ◦ multiple independent determinations of λ (test of CKM unitarity), ◦ independent and competitive limits on S , T currents (BSM). D. Poˇ cani´ c (UVa) The Nab experiment : Goals and motivation 30 Oct ’15/DNP2015 3 / 14
Electron–neutrino angle from E e and E p p Conservation of momentum in n beta decay, � p p + � p e + � p ν = 0 , yields p 2 p = p 2 e + 2 p e p ν cos θ e ν + p 2 ν . n Neglecting proton recoil energy, E e + E ν = E 0 , so that p ν = E 0 − E e . Therefore: e cos θ e ν is uniquely determined by mea- θ e ν suring E e and E p (or p p ⇒ TOF p ). − ν D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 4 / 14
Nab measurement principles: proton phase space Yield (arb. units) proton phase space 1.5 cos θ e ν = 1 E e = 100 keV 1 2 (MeV 2 /c 2 ) 300 keV cos θ e ν = 0 cos θ e ν = -1 500 keV p 0.5 p 700 keV 0 0 0.2 0.4 0.6 0.8 E e (MeV) NB: For a given E e , cos θ e ν is a function of p 2 p only. D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 5 / 14
Nab measurement principles: proton phase space Yield (arb. units) proton phase space 1.5 cos θ e ν = 1 E e = 100 keV ❆ ❑ 1 2 (MeV 2 /c 2 ) ❆ 300 keV ❆ ❆ cos θ e ν = 0 cos θ e ν = -1 500 keV ❆ p 0.5 ❆ p 700 keV ❑ ❆ ❆ ❆ ❆ ❑ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❑ 0 ❆ ❆ ❆ ❆ 0 0.2 0.4 0.6 0.8 ❆ ❆ ❆ ❆ E e (MeV) ❆ ❆ ❆ ❆ NB: For a given E e , cos θ e ν is a function of p 2 Slope ∝ a p only. Numerous consistency checks are built-in! D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 5 / 14
Nab principles of measurement ◮ Collect and detect ✻ ✣ ✡ both electrons and ✡ ✡ protons from neutron ✡ ❖ ❈ beta decay. ❈ ✡ ✡ ❖ ❈ ❈ ◮ Measure E e and TOF p ✡ ❖ ❈ ❈ and reconstruct decay ✡ ✡ kinematics ✟✟✟✟✟✡ ❖ ❈ ◮ Segmented Si det’s: ❈ ❖ ❈ ❈ ❑ ❆ P ✐ P ✛ ✐ P PPPP P q LANL/Micron development D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 6 / 14
2 z [m] 0 1 10 20 30 3 4 5 fiducial volume lower detector upper detector z [cm] 4 Magnetic field B [T] 0 1 2 3 4 fiducial volume B (on axis) z B (off axis) z -1 3 -30-20 -10 0 LDET ’ upper detector lower detector NBL NBU F TOF T OF ’ NBU ’ NBL ’ UDET ’ UDET LDET neutron beam 2 z [m] r [m] 0 0 0.2 0.4 5 -1 3.5 m flight path omitted Magnetic field B [T] 0 1 Filter Spectrometer Coil design and � B field profile D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 7 / 14
Nab Si detectors (LANL-Micron development) Front ◮ 15 cm diameter ◮ full thickness: 2 mm ◮ dead layer ≤ 100 nm ◮ 127 pixels Back D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 8 / 14
Nab Si detectors (LANL-Micron development) Front ◮ 15 cm diameter ◮ full thickness: 2 mm ◮ dead layer ≤ 100 nm ◮ 127 pixels Back How well do they work? D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 8 / 14
Nab Si detectors (LANL-Micron development) Front ◮ 15 cm diameter ◮ full thickness: 2 mm ◮ dead layer ≤ 100 nm ◮ 127 pixels Back 15000 Noise 1mm thick Si det. Proton normalized yield A. Salas-Bacci et al., NIM A 735 (2014) 408 15 25 30 35 kV 20 10000 5000 0 0 10 20 30 40 50 60 70 80 Channels D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 8 / 14
e E = 150 keV e E = 600 keV e E = 450 keV e E = 300 keV e 0.008 2 0.006 0.004 0.002 0 Yield 2 p 1/ t ’ [1/ μ s ] E = 750 keV Analysis strategy (methods “A” and “B”) ◮ Use edges to determine and verify shape of detection function Φ( p p , 1 / t p ); ◮ Use central part of P t (1 / t 2 p ) ( ∼ 70%) to extract a . A. parametrize edges and width of Φ( p p , 1 / t p ) by fitting; use central part of Φ ( ∼ 70%) to extract a in a multiparameter fit, and B. specify accessible parameters of Φ by direct measurement; ⇒ treat a , µ = 1 / t 2 p (p p ) , and N decays as free parameters in a two-step fit, ◮ as well as a HYBRID OF THE TWO METHODS . D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 9 / 14
e E = 150 keV e E = 600 keV e E = 450 keV e E = 300 keV e 0.008 2 0.006 0.004 0.002 0 Yield 2 p 1/ t ’ [1/ μ s ] E = 750 keV Analysis strategy (methods “A” and “B”) ◮ Use edges to determine and verify shape of detection function Φ( p p , 1 / t p ); ◮ Use central part of P t (1 / t 2 p ) ( ∼ 70%) to extract a . A. parametrize edges and width of Φ( p p , 1 / t p ) by fitting; use central part of Φ ( ∼ 70%) to extract a in a multiparameter fit, and B. specify accessible parameters of Φ by direct measurement; ⇒ treat a , µ = 1 / t 2 p (p p ) , and N decays as free parameters in a two-step fit, ◮ as well as a HYBRID OF THE TWO METHODS . D. Poˇ cani´ c (UVa) The Nab experiment : Principles of measurement 30 Oct ’15/DNP2015 9 / 14
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