Nab: a precise study of unpolarized neutron beta decay Dinko Poˇ cani´ c , (for the Nab Collaboration) Institute for Nuclear and Particle Physics, University of Virginia 27 October 2016 2017 Fall Meeting of the APS Division of Nuclear Physics Pittsburgh, PA 25 – 28 October 2017
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) Nab—neutron β decay : Basic definitions 27 Oct ’17/DNP17 2/ 15
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) Nab—neutron β decay : Basic definitions 27 Oct ’17/DNP17 2/ 15
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 40 × better than: − 0 . 1090 ± 0 . 0041 Darius et al 2017 (aCORN) − 0 . 1054 ± 0 . 0055 Byrne et al 2002 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al 1978 − 0 . 091 ± 0 . 039 Grigorev et al 1968 forthcoming: aCORN, aSPECT, both project ∆ a / a ≃ 1%. D. Poˇ cani´ c (UVa) Nab—neutron β decay : Goals and motivation 27 Oct ’17/DNP17 3/ 15
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 40 × better than: − 0 . 1090 ± 0 . 0041 Darius et al 2017 (aCORN) − 0 . 1054 ± 0 . 0055 Byrne et al 2002 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al 1978 − 0 . 091 ± 0 . 039 Grigorev et al 1968 forthcoming: aCORN, aSPECT, both project ∆ a / a ≃ 1%. ◮ Measure b (Fierz interf. term) in n decay with ∆ b ≃ 3 × 10 − 3 b n = 0 . 067 +93 current results: − 66 (UCNA 2017)! D. Poˇ cani´ c (UVa) Nab—neutron β decay : Goals and motivation 27 Oct ’17/DNP17 3/ 15
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 40 × better than: − 0 . 1090 ± 0 . 0041 Darius et al 2017 (aCORN) − 0 . 1054 ± 0 . 0055 Byrne et al 2002 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al 1978 − 0 . 091 ± 0 . 039 Grigorev et al 1968 forthcoming: aCORN, aSPECT, both project ∆ a / a ≃ 1%. ◮ Measure b (Fierz interf. term) in n decay with ∆ b ≃ 3 × 10 − 3 b n = 0 . 067 +93 current results: − 66 (UCNA 2017)! ◮ 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) Nab—neutron β decay : Goals and motivation 27 Oct ’17/DNP17 3/ 15
Goals of the Nab experiment (at SNS, ORNL) ◮ Measure the e– ν correlation a in neutron decay with precision ∆ a / a ≃ 10 − 3 or ∼ 40 × better than: − 0 . 1090 ± 0 . 0041 Darius et al 2017 (aCORN) − 0 . 1054 ± 0 . 0055 Byrne et al 2002 current results: − 0 . 1017 ± 0 . 0051 Stratowa et al 1978 − 0 . 091 ± 0 . 039 Grigorev et al 1968 forthcoming: aCORN, aSPECT, both project ∆ a / a ≃ 1%. ◮ Measure b (Fierz interf. term) in n decay with ∆ b ≃ 3 × 10 − 3 b n = 0 . 067 +93 current results: − 66 (UCNA 2017)! ◮ 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) Nab—neutron β decay : Goals and motivation 27 Oct ’17/DNP17 3/ 15
Current status of V ud and λ , from n decay . . . remains an unresolved mess: aCORN (2017) 0.98 � � � � �������� � ft(0 + → 0 + ) [ Hardy17 ] ���������������� ������������� PIBETA [ Pocanic04 ] ����������� Very low CL! ����������������������������� 0.975 ������������ [ Satula12 ] �������� ��������������������������� ����������� PKO1 DD-ME2 CKM unitarity ��������������� PC-F1 PC-PK1 [ Liang09 ] [ ZhaoXi11 ] [ PDG 2016 ] V ud 0.97 λ [ PERKEO II 2013 ] �������������������� ������������������������������ τ n [ UCN τ 2017 ] ������������ ��������������� τ n [ beam avg 2016 ] 0.965 λ [ PDG 2016 ] ��������������� τ n [ PDG 2016 ] λ [ UCNA 2013 ] ����� ����� ����� ∆ λ ≃ 0 . 27∆ a ≃ 0 . 24∆ A 0.96 -1.29 -1.28 -1.27 -1.26 λ a A λ = g A /g V λ sensitivity to a , A is similar . τ − 1 ∝ | V ud | 2 | g V | 2 (1 + 3 | λ | 2 ) n ◮ Nab+abBA ⇒ several independent ∼ 0 . 03% determinations of λ , ◮ Combined with b ⇒ new limits on non-SM terms, esp. Tensor. D. Poˇ cani´ c (UVa) Nab—neutron β decay : Goals and motivation 27 Oct ’17/DNP17 4/ 15
Limits on T , S couplings from beta decay CURRENT CONSTRAINTS PROSPECTIVE CONSTRAINTS Low-energy: Low-energy: Low-energy: Low-energy: g S,T from quark model g S,T from lattice g S,T from quark model g S,T from lattice (with | b | < 10 − 3 ) LHC: LHC: √ s = 8 TeV √ s = 14 TeV L = 20, fb -1 L = 10, 300 fb -1 Measurement of b with δ b < 10 − 3 ⇒ > 4-fold improvement on the current limit for ǫ T from π + → e + νγ decay [Bychkov et al, PRL 103 (2009) 051802] . Also see: T. Bhattacharya, et al. Phys. Rev. D 94 (2016) 054508, Mart´ ın Gonz´ alez-Alonso, arXiv:1209.0689, G. Konrad, et al., arXiv:1007.3027, S. Baeßler, et al., J. Phys. G: Nucl. Part. Phys. 41 (2014) 114003. D. Poˇ cani´ c (UVa) Nab—neutron β decay : Goals and motivation 27 Oct ’17/DNP17 5/ 15
How to accomplish the goals of Nab? Measure: ∆ ❛ ≃ 10 − 3 and ∆ ❜ ≃ 3 × 10 − 3 . ❛ D. Poˇ cani´ c (UVa) Nab—neutron β decay : The method 27 Oct ’17/DNP17 6/ 15
How to accomplish the goals of Nab? Measure: ∆ ❛ ≃ 10 − 3 and ∆ ❜ ≃ 3 × 10 − 3 . ❛ (n → p + e − + ¯ Basic approach: ν e ) ◮ Detect electrons directly, in Si detectors, ◮ Measure electron energy in Si detectors, ◮ Detect protons, after acceleration, in Si detectors, ◮ Determine proton momentum from TOF over a long flightpath (electron provides start pulse). D. Poˇ cani´ c (UVa) Nab—neutron β decay : The method 27 Oct ’17/DNP17 6/ 15
How to accomplish the goals of Nab? Measure: ∆ ❛ ≃ 10 − 3 and ∆ ❜ ≃ 3 × 10 − 3 . ❛ (n → p + e − + ¯ Basic approach: ν e ) ◮ Detect electrons directly, in Si detectors, ◮ Measure electron energy in Si detectors, ◮ Detect protons, after acceleration, in Si detectors, ◮ Determine proton momentum from TOF over a long flightpath (electron provides start pulse). A complex magneto-electrostatic apparatus is required to guide particles (nearly) adiabatically to detectors. Location: FnPB at SNS D. Poˇ cani´ c (UVa) Nab—neutron β decay : The method 27 Oct ’17/DNP17 6/ 15
Electron–neutrino angle from E e and E p Conservation of momentum in n beta decay, p 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 , ≃ p 2 e + 2 p e ( E 0 − E e ) cos θ e ν + ( E 0 − E e ) 2 n neglecting proton recoil energy: E e + E ν ≃ E 0 , e so that p ν = E 0 − E e . Therefore: θ e ν cos θ e ν is uniquely determined by mea- suring E e and E p (or p p ⇒ TOF p ). − ν D. Poˇ cani´ c (UVa) Nab—neutron β decay : The method 27 Oct ’17/DNP17 7/ 15
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 ∝ ❛ p only. Numerous consistency checks are built-in! D. Poˇ cani´ c (UVa) Nab—neutron β decay : The method 27 Oct ’17/DNP17 8/ 15
Nab principles of operation ◮ Collect and detect both electron and proton from n decay . ◮ Measure E e and TOF p and reconstruct decay kinematics D. Poˇ cani´ c (UVa) Nab—neutron β decay : Basic principles 27 Oct ’17/DNP17 9/ 15
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