Parity-Violating and Parity-Conserving Asymmetries in ep and eN Scattering in the Qweak Experiment Wouter Deconinck September 29, 2017 Electroweak Box Workshop, Amherst Center for Fundamental Interactions Supported by the National Science Foundation under Grant Nos. PHY-1405857, PHY-1714792.
Parity-Violating Asymmetries are Typically Small Z The Qweak Experiment Electroweak Box Z Z M 2 PV M 2 1 Q 2 Interference of photon and weak boson exchange 2 2 with Asymmetry between + and − incoming electron helicity � � � � e e ′ e e ′ � � A PV = σ + − σ − � � � � σ = + + . . . γ � � σ + + σ − � � � � q q ′ q q ′ � � � � M EM ∝ 1 M NC PV ∝ Z + Q 2 A PV = σ + − σ − ∝ M NC ∝ G F Q 2 ≈ O ( ppm , ppb ) when Q 2 ≪ M 2 M EM ∝ Q 2 σ + + σ −
Parity-Violating Asymmetry to Access Electroweak Parameters Electroweak Box The Qweak Experiment 3
Strategy to Measure Parts-Per-Billion: Integration Integrating or current mode The Qweak Experiment Electroweak Box 2.5 GHz; P2 up to 0.5 THz MOLLER segment rates up to events possible after the fact long as detectors are linear … 0 100 ns Event or counting mode time at least 1 day for 1 ppm precision to 10 MHz per detector segment; based on event characteristics digitized and read-out 0 100 ns time 4 µ A µ A • Each event individually detected, • Very high event rates possible, as • Selection or rejection possible • But no rejection of background • 100 ns pulse separation limits rate • Q Weak segment rates 800 MHz;
Parity-Violating Asymmetry to Access Electroweak Parameters A The Qweak Experiment Electroweak Box Q p 2 Electroweak measurements with protons (elastic scattering) 5 2 M − ( 1 − 4 sin 2 θ W ) ǫ ′ G M G Z � � A PV ( p ) = − G F Q 2 ǫ G E G Z E + τ G M G Z √ ǫ ( G E ) 2 + τ ( G M ) 2 4 πα In the forward elastic limit Q 2 → 0, θ → 0 (plane wave): → − G F Q 2 W + Q 2 · B ( Q 2 ) A PV ( p ) Q 2 → 0 � � − − − − √ ∝ Q p W when Q 2 small 4 πα Precision electroweak Standard Model test of sin 2 θ W : A PV ( p ) ∝ − 1 + 4 sin 2 θ W
Determination of the Weak Charge of the Proton Pushing the envelope of intensity (more detected electrons) Pushing the envelope of precision (better measurements) Electroweak Box The Qweak Experiment 6 • Higher beam current (180 µ A versus usually < 100 µ A) • Longer cryo-target (35 cm versus 20 cm, 2.5 kW in 20 K LH2) • Higher event rates up to 800 MHz (integrating mode) � L dt = 1 ab − 1 • Typical luminosity of 1 . 7 × 10 39 cm − 2 s − 1 , • Electron beam polarimetry precision of 1% at 1 GeV • Helicity-correlated asymmetries at ppb level (beam position at nm level) • Determination of Q 2 since A PV ∝ Q 2 • Isolate elastic scattering from background processes ( f i , A i )
Determination of the Weak Charge of the Proton 1 The Qweak Apparatus, NIM A 781, 105 (2015) Electroweak Box The Qweak Experiment 7
Determination of the Weak Charge of the Proton 1 The Qweak Apparatus, NIM A 781, 105 (2015) The Qweak Experiment Electroweak Box 8 8 Quartz Bar Detectors Trigger Scintillators 8 Segment Toroidal Magnet LH Target 2 Drift Chambers Triple Pb Collimator System High Density Shield Wall
Determination of the Weak Charge of the Proton 1 The Qweak Apparatus, NIM A 781, 105 (2015) Electroweak Box The Qweak Experiment 9
Determination of the Weak Charge of the Proton Azimuthal array of Čerenkov detector Electroweak Box The Qweak Experiment 10 • 8 fused silica radiators, 2 m long × 18 cm × 1.25 cm • Pb preradiator tiles to suppress low-energy/neutral yield • 5 inch PMTs with gain of 2000, low dark current • 800 MHz electron rate per bar, defjnes counting noise
Determination of the Weak Charge of the Proton First experiment with direct access to proton’s weak charge spectrometer and integrating quartz detectors Long awaited fjnal results are now here 1 First Determination of the Weak Charge of the Proton, Phys. Rev. Lett. 111, 141803 (2013) Electroweak Box The Qweak Experiment 11 • Experiment collected data between 2010 and 2012 with toroidal • Preliminary results were published in 2013 based on commissioning data 1 (4% compared to the independent full data set) • Unblinding on March 31, 2017 • Release of unblinded result at PANIC’17 in Beijing: • Sunday September 3, 2017, at PANIC in plenary session • Friday September 8, 2017, at Jefgerson Lab • Publication to be submitted in October 2017
Determination of the Weak Charge of the Proton Background treatment in integrating experiments The Qweak Experiment Electroweak Box transverse spin azimuthal asymmetry when scattering in lead pre-radiators collimator downstream of target and into detector acceptance Unprecedented precision comes with inevitable surprises P A msr 12 • Measured asymmetry A msr corrected for all background contributions • with their own parity-violating asymmetry A i (ppm-level) • and their dilution in the measured asymmetry f i (%-level) − � f i A i A PV = R total 1 − � f i • Discovered qualitatively new “beamline background” • Generated by scattering of helicity-dependent beam halo on clean-up • Discovered qualitatively new “rescattering bias” • Spin precession of scattered electrons in spectrometer, followed by nuclear
Determination of the Weak Charge of the Proton 5.8 factors All uncertainties in ppb (1.2) 1.3 3.4 2.5 Total systematic uncertainty 10.1 5.6 Total statistical uncertainty (1.9) 15.0 8.3 7.3 Total combined uncertainty 18.0 10.0 9.3 (p = 86%) Electroweak Box The Qweak Experiment 1.9 Kinematics: R Q 2 Al target windows: A b 1 Beam Asymmetries: A beam Run 1 Run 2 Combined Charge Normalization: A BCM 5.1 2.3 Beamline Background: A BB between 1.2 5.1 4.7 correlations (1.2) 2.2 1.2 Beam Polarization: P 13 3.4 3.4 Rescattering bias: A bias Note: Total of others < 5 % , incl () A PV ( 4% ) = − 279 ± 31(syst) ± 35(stat) = − 279 ± 47(total) A PV ( full ) = − 226 . 5 ± 5 . 8(syst) ± 7 . 3(stat) = − 226 . 5 ± 9 . 3(total)
14 5% 37% Beam Polarization: P 2.2 5% Al target windows: A b 1 1.9 12% Kinematics: R Q 2 1.3 Total of others Run 1 3.4 11% 2.5 20% Combined in quadrature 10.1 5.6 Electroweak Box The Qweak Experiment 3.4 11% 3.4 25% Charge Normalization: A BCM 5.1 25% 2.3 17% Beamline Background: A BB Rescattering bias: A bias 5.1 1.2 fraction All uncertainties in ppb 5% Run 2 Beam Asymmetries: A beam 4.7 22% 1.2 5% fraction Q Weak : Largest Uncertainties in Precision Q Weak Result δ ( A PV ) δ ( A PV ) < 5% < 5% < 5%
First Determination of the Weak Charge of the Proton 2 The Qweak Experiment Electroweak Box 2 First Determination of the Weak Charge of the Proton, Phys. Rev. Lett. 111, 141803 (2013) 1 R. Young, R. Carlini, A.W. Thomas, J. Roche, Phys. Rev. Lett. 99, 122003 (2007) A 15 with Intercept of A PV at Q 2 → 0 gives weak charge ( Q 2 = 0 . 025 GeV 2 ) W + Q 2 · B ( Q 2 , θ = 0 ) A PV = A PV A 0 = Q p A 0 = − G F Q 2 √ 4 πα Global fjt 1 of all parity-violating electron scattering with 4% data 2 • Fit of parity-violating asymmetry data on H, D, 4 He, Q 2 < 0 . 63 GeV 2 • Free parameters are C 1 u , C 1 d , strange charge radius ρ s and magnetic E , M ∝ G D ), and isovector axial form factor G Z , T = 1 moment µ s ( G s W ( SM ) = 0 . 0710 ± 0 . 0007 (theoretical expectation) • Q p W ( PVES ) = 0 . 064 ± 0 . 012 (global fjt of 4% data 2 ) • Q p • After combination with atomic parity-violation on Cs: • C 1 u = − 0 . 1835 ± 0 . 0054 • C 1 d = 0 . 3355 ± 0 . 0050
Determination of the Weak Vector Charge of the Proton Q p The Qweak Experiment Electroweak Box A New global fjt of all parity-violating electron scattering with full data set 16 A • Fit of parity-violating asymmetry data on H, D, 4 He, Q 2 < 0 . 63 GeV 2 • Free parameters were C 1 u , C 1 d , strange charge radius ρ s and magnetic E , M ∝ G D ), and isovector axial form factor G Z , T = 1 moment µ s ( G s W ( PVES )) = 0 . 0719 ± 0 . 0045 sin 2 θ W = 0 . 2382 ± 0 . 0011 ρ s = 0 . 19 ± 0 . 11 µ s = − 0 . 18 ± 0 . 15 G Z , T = 1 = − 0 . 67 ± 0 . 33 • After combination with atomic parity-violation on Cs: • C 1 u = − 0 . 1874 ± 0 . 0022 • C 1 d = 0 . 3389 ± 0 . 0025
Determination of the Weak Vector Charge of the Proton Electroweak Box The Qweak Experiment 17
Determination of the Weak Vector Charge of the Proton Electroweak Box The Qweak Experiment 18
Determination of the Weak Vector Charge of the Proton Using lattice QCD in the extraction The Qweak Experiment Electroweak Box 1 J. Green et al, Phys. Rev. D92, 031501 (2015) 19 • It is possible to add the lattice strangeness form factor to the global fjt. W ( LQCD )) = 0 . 0684 ± 0 . 0039 • Q p 0 . 15 G0 A4 lattice QCD (this work, m π = 317 MeV) HAPPEX lattice lattice QCD (this work, physical point) lattice QCD [17] 0 . 10 connected LQCD + octet µ from expt. [16] M E + η G s . . . same, with quenched lattice QCD [29] 0 . 05 finite-range-regularized chiral model [30] G s light-front model + deep inelastic scattering data [31] 0 . 00 perturbative chiral quark model [32] dispersion analysis [33] parity-violating elastic scattering [34] − 0 . 05 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 − 0 . 5 − 0 . 4 − 0 . 3 − 0 . 2 − 0 . 1 0 . 0 0 . 1 µ s ( µ N ) Q 2 (GeV 2 )
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