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 GA Current status New proposal to measure GA 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) GE(Q2) and GM(Q2) parametrize the electromagnetic current operator:

Well-known over a wide range of Q2 through eN scattering Fourier transforms of nucleon charge and magnetization distributions

Proton Neutron

Isovector axial-vector current form factors are less known: N(p′)|¯ qγµγ5 τ a 2 q|N(p) = ¯ u(p′)

• γµGA(Q2)+(p′ − p)µ

2m GP(Q2)

• γ5

τ a 2 u(p)

Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 3 / 18

Axial FF

Axial form factor GA(Q2)

Probes the spin distribution of the nucleon Usually parametrized using a “dipole” expansion: GA(Q2) = gA (1 − Q2/MA2)2

• gA axial-vector coupling constant

MA: 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

1

(Quasi-)elastic (anti-)neutrino scattering off protons or nuclei

2

Threshold charged pion electroproduction

Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 4 / 18

Axial FF

Quasi-elastic ν scattering

Elastic: νp → νp Quasi-elastic: νn → l−p, ¯ νp → l+n dσ(νp,¯

νp)

dQ2 = G2

F

8π m2 cos θC E2

ν

• A(Q2) ∓ B(Q2)s − m

m2 + C(Q2)(s − m)2 m4

• A(Q2)

= f(GE, GM, GA) B(Q2) = f ′(GE, GM, GA) C(Q2) = f ′′(GE, GM, GA)    GA(Q2) extracted by fitting the Q2−dependence of the cross section MA obtained using the dipole approximation for GA(Q2)

Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 5 / 18

Axial FF

Pion electroproduction

eN → e′π + N′ dσ dE′

edΩ′ edΩπ

= Γv dσT dΩπ + ǫL dσL dΩπ

• dσT extracted by Rosenbluth

separation MA fitted to different models of the Q2−dependence of dσT

2 4 6 8 0.1 0.2 0.3 0.4

Q2 [GeV2/c2] dσT/dΩπ* [µb/sr]

dσT(0) fixed, MA fitted dσT(0) and MA fitted

• Phys. Lett. B468, 20 (1999)
• Model-dependent extraction
• Assumptions needed for other model parameters

Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 6 / 18

Axial FF

Experimental situation

ν−scattering: MA = 1.026 ± 0.009

CERN HLBC 64 CERN HLBC 67 CERN SC 68 CERN HLBC 69 ANL 69 ANL 73 ANL 77 CERN GGM 77 CERN GGM 79 BNL 80 BNL 81 ANL 82 IHEP 82 Fermilab 83 Fermilab 84 IHEP 85 BNL 88 IHEP SKAT 88 CERN BEBC 90 BNL 90 IHEP SKAT 90 NuTeV 04 K2K SciFi 06 MiniBoone 07 K2K SciBar 08 NOMAD 08/09 MiniBoone 10 MINERvA 13 AVERAGE 0.4 0.6 0.8 1 1.2 1.4 1.6 MA [GeV]

π electroproduction: MA = 1.062 ± 0.015

Frascati 70 Frascati 72 DESY 73 Daresbury 75 Daresbury 76 DESY 76 Kharkov 78 Olsson 78 Saclay 93 Mainz 99 AVERAGE 0.6 0.8 1 1.2 1.4 1.6 MA [GeV]

2.4 σ difference on average MA But large individual uncertainties & discrepancies

Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 7 / 18

Axial FF

Q2−dependence of GA

0.2 0.4 0.6 0.8 1 0.5 1 1.5 2 GA(Q2)/GA(0) Q2 [GeV2] Frascati 70/72 SLAC 73 DESY 73 Daresbury 75/76 DESY 76 Kharkov 78 Saclay 93 (anti)neutrinos various MA MA = 1.30 1.15 1 .

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: dσ dω′ = MG2 cos2 θc π ω′ ω

• cos2 (θl/2)f2 +
• 2f1 + ω + ω′

M f3

• sin (θl/2)
• f1 = f1(GA, Gp

M, Gn M)

f2 = f2(GA, Gp

M, Gn M, Gp E, Gn E)

f3 = f3(GA, Gp

M, Gn M)

Model-independent extraction of GA(Q2)! High stat. & syst. precision possible e + p → ν + n

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

1

Neutron detection with accurate kinematics

2

Small cross section ! (∼ 10−40 cm2/sr)

3

(Very) large electromagnetic backgrounds 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% EM background asymmetry is 0

• pulse(+) to pulse(-) subtraction:

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 µA 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 GA(Q2) results

0.8 0.9 1 1.1 1.2 0.05 0.1 0.15 0.2 0.25 GA(Q2) relative to MA=1.064 GeV dipole Q2 [GeV2] MA = 0.84 GeV MA = 1.35 GeV This proposal

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 GA have large uncertainties and dispersion Still some discrepancy between ν and e scattering experiments Inverse β decay → GA(Q2) 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 Q2 = 4 GeV2)

Additional inelastic EM background Full Q2 mapping of GA

Carlos Mu˜ noz Camacho (IPN-Orsay) Axial FF IPPP/NuSTEC 18 / 18