Axial form factor measurements: current status and plans Carlos Mu - PowerPoint PPT Presentation

Axial form factor measurements: current status and plans Carlos Mu˜ noz Camacho* IPN-Orsay, CNRS/IN2P3 (France) IPPP/NuSTEC topical meeting on neutrino-nucleus scattering Durham (U.K.), April 18–20 (2017) *in collaboration with A. Deur, S. ˇ Sirca and ˇ C. Harej Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 1 / 18

Introduction Outline Introduction to nucleon form factors Experimental ways to measure G A Current status New proposal to measure G A through inverse β decay Summary Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 2 / 18

Introduction Nucleon form factors Electromagnetic Form Factors (FF) G E ( Q 2 ) and G M ( Q 2 ) parametrize the electromagnetic current operator: Well-known over a wide range of Q 2 through eN scattering Fourier transforms of nucleon charge and magnetization distributions Proton Neutron Isovector axial-vector current form factors are less known: γ µ G A ( Q 2 )+( p ′ − p ) µ q γ µ γ 5 τ a � � τ a G P ( Q 2 ) � N ( p ′ ) | ¯ u ( p ′ ) 2 q | N ( p ) � = ¯ γ 5 2 u ( p ) 2 m Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 3 / 18

Axial FF Axial form factor G A ( Q 2 ) Probes the spin distribution of the nucleon Usually parametrized using a “dipole” expansion: � g A axial-vector coupling constant g A G A ( Q 2 ) = ( 1 − Q 2 / M A 2 ) 2 M A : adjustable axial mass . form inspired by early (old) fits of electromagnetic FF. It assumes exponential spatial distributions, but w/o strong theoretical justification Measurements of the nucleon axial FF (Quasi-)elastic (anti-)neutrino scattering off protons or nuclei 1 Threshold charged pion electroproduction 2 Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 4 / 18

Axial FF Quasi-elastic ν scattering Elastic: ν p → ν p ν p → l + n Quasi-elastic: ν n → l − p , ¯ m 2 cos θ C d σ ( ν p , ¯ ν p ) = G 2 + C ( Q 2 )( s − m ) 2 � � A ( Q 2 ) ∓ B ( Q 2 ) s − m F dQ 2 8 π E 2 m 2 m 4 ν A ( Q 2 )  = f ( G E , G M , G A ) G A ( Q 2 ) extracted by fitting the  B ( Q 2 ) = f ′ ( G E , G M , G A ) Q 2 − dependence of the cross section C ( Q 2 ) = f ′′ ( G E , G M , G A )  M A obtained using the dipole approximation for G A ( Q 2 ) Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 5 / 18

Axial FF Pion electroproduction � d σ T d σ d σ L � eN → e ′ π + N ′ = Γ v + ǫ L e d Ω ′ e d Ω π d Ω π d Ω π dE ′ d σ T /d Ω π * [µ b/sr ] 8 d σ T (0) fixed, M A fitted d σ T (0) and M A fitted 6 d σ T extracted by Rosenbluth 4 separation M A fitted to different models of the Q 2 − dependence of d σ T 2 0 0 0.1 0.2 0.3 0.4 Q 2 [ GeV 2 /c 2 ] - Model-dependent extraction Phys. Lett. B468, 20 (1999) - Assumptions needed for other model parameters Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 6 / 18

Axial FF Experimental situation ν − scattering: � M A � = 1 . 026 ± 0 . 009 π electroproduction: � M A � = 1 . 062 ± 0 . 015 CERN HLBC 64 CERN HLBC 67 CERN SC 68 CERN HLBC 69 Frascati 70 ANL 69 ANL 73 Frascati 72 ANL 77 DESY 73 CERN GGM 77 Daresbury 75 CERN GGM 79 Daresbury 76 BNL 80 BNL 81 DESY 76 ANL 82 Kharkov 78 IHEP 82 Olsson 78 Fermilab 83 Saclay 93 Fermilab 84 Mainz 99 IHEP 85 BNL 88 AVERAGE IHEP SKAT 88 0.6 0.8 1 1.2 1.4 1.6 CERN BEBC 90 BNL 90 M A [GeV] IHEP SKAT 90 NuTeV 04 K2K SciFi 06 MiniBoone 07 K2K SciBar 08 2 . 4 σ difference on average M A NOMAD 08/09 MiniBoone 10 But large individual MINERvA 13 AVERAGE uncertainties & discrepancies 0.4 0.6 0.8 1 1.2 1.4 1.6 M A [GeV] Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 7 / 18

Axial FF Q 2 − dependence of G A 1 Frascati 70/72 SLAC 73 DESY 73 0.8 Daresbury 75/76 DESY 76 Kharkov 78 Saclay 93 G A (Q 2 )/G A (0) 0.6 0.4 M A = 1.30 1.15 0.2 1 . 0 0 (anti)neutrinos various M A 0 0 0.5 1 1.5 2 Q 2 [GeV 2 ] Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 8 / 18

New proposal Clean measurements of axial FF by inverse β decay Weak charge current reaction: d ω ′ = MG 2 cos 2 θ c d σ ω ′ � � 2 f 1 + ω + ω ′ � � cos 2 ( θ l / 2 ) f 2 + sin ( θ l / 2 ) f 3 π ω M e + p → ν + n f 1 = f 1 ( G A , G p M , G n M ) f 2 = f 2 ( G A , G p M , G p M , G n E , G n E ) f 3 = f 3 ( G A , G p M , G n M ) Model-independent extraction of G A ( Q 2 ) ! High stat. & syst. precision possible Donnelly, Kronenberg & Norum (1996) Pauchy Hwang (1996) Deur, JLab PAC25 LOI Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 9 / 18

New proposal Experimental challenges Neutron detection with accurate kinematics 1 Small cross section ! ( ∼ 10 − 40 cm 2 /sr) 2 (Very) large electromagnetic backgrounds 3 Strategy: Backward kinematics to enhance Weak/EM x-sections (forward n ) High intensity (JLab/Mainz) electron beam + long LH2 target Low energy ( < 120 MeV) beam to stay below π production threshold Polarized beam for background cleanup: � Weak reaction asymmetry: 100% pulse(+) to pulse(-) subtraction: EM background asymmetry is 0 clean cancellation of background Pulsed beam to remove prompt EM background & TOF for n Kinematic identification of the elastic reaction Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 10 / 18

New proposal Experimental setup Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 11 / 18

New proposal Experimental setup Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 11 / 18

New proposal Potential experimental facilities MESA at Mainz: High luminosity, good beam energy, polarized beam Beam pulse structure, beam energy flexibility? FEL at JLab: Good energy Mainly a FEL facility Unpolarized electrons, currently no experimental Hall Hall D tagger at JLab: Long TOF distance (80 m) Possibility of 100 MeV beam, but invasive to Nuc. Phys. program No cryogenic capability currently 5 µ A CW beam limitation JLab injector: High intensity pulsed beam, polarized electrons Space constraints may limit TOF distance Possible interference with to Nuc. Phys. program Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 12 / 18

New proposal Background simulation Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 13 / 18

New proposal Primary sources of background Prompt EM ( γ flash, electrons): can be reduced by timing cuts Windows: Be + e → n + e + X : can be reduced with thin windows and backwards veto detector Scattered electrons (Møller, nuclear scattering): small after sweeping magnet Preliminary background estimates, detailed MC simulation now underway. . . Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 14 / 18

New proposal Cross section projections 6 days at 110 MeV 7 days at 90 MeV 17 days at 70 MeV 30 days at 50 MeV – 100% efficiency and no background assumed Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 15 / 18

New proposal Projected G A ( Q 2 ) results G A (Q 2 ) relative to M A =1.064 GeV dipole 1.2 M A = 0.84 GeV M A = 1.35 GeV This proposal 1.1 1 0.9 0.8 0 0.05 0.1 0.15 0.2 0.25 Q 2 [GeV 2 ] Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 16 / 18

New proposal Status of the project Extensive MC simulations ongoing to EM understand backgrounds Optimization of experimental setup: detector location, shieldings, etc Full experimental JLab proposal expected by 2018 New collaborators welcome! Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 17 / 18

Outlook Summary and conclusions Measurements of G A have large uncertainties and dispersion Still some discrepancy between ν and e scattering experiments Inverse β decay → G A ( Q 2 ) accurately and model-independently High precision measurement will check the dipole approximation Low E energy experiment relatively easy and clean Large EM background supression under investigation Experimental JLab proposal expected next year Stepping stone to a higher energy experiment (up to Q 2 = 4 GeV 2 ) Additional inelastic EM background Full Q 2 mapping of G A Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 18 / 18