Measurement of the beam normal single-spin asymmetry of 12 C 54 th - - PowerPoint PPT Presentation

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Measurement of the beam normal single-spin asymmetry of 12 C 54 th - - PowerPoint PPT Presentation

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Measurement of the beam normal single-spin asymmetry of 12 C 54 th Int. Winter Meeting, Bormio, 27.01.2016 Dr. Anselm Esser Weak Mixing Angle Weinberg angle / weak mixing angle:

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SLIDE 1

Measurement of the beam normal single-spin asymmetry of 12C

54th Int. Winter Meeting, Bormio, 27.01.2016 Dr. Anselm Esser

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SLIDE 2

Weak Mixing Angle

  • Weinberg angle / weak mixing angle:
  • Important parameter in standard model
  • Relative coupling strength of weak and electromagnetic force
  • Measurement by Parity Violation (PV):
  • Polarised electrons, scattered on protons
  • Cross section dominated by electromagnetic interaction
  • Small contribution from Z0 exchange → Parity violation
  • Measurement of PV asymmetry → Z0 contribution

Parity conserving Large cross section Parity violating small cross section +

e=g⋅sin θW

Z e e' p p'

γ e e' p p'

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SLIDE 3

Neutron Skin

QW

n ≈−0.99

QW

p ≈0.07

  • Heavy nuclei contain more neutrons than protons
  • Spacial distribution of neutrons might be larger
  • Z0 Boson couples more strongly to Neutron
  • Measurement by parity violation:
  • Electron scattering on nuclei
  • Parity violating contribution to

cross section from Z0 exchange

  • Measurement of PV asymmetry

→ Neutron distribution

γ e e' p n

ρ/N r protons neutrons

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SLIDE 4

Beam Normal Asymmetry

  • Difficulties of PV measurements:
  • Large electromagnetic cross section, small asymmetry ~ 10-6
  • Long run times
  • Necessary: Good understanding of background
  • Especially: Helicity correlated background
  • Beam normal (single spin) asymmetry:
  • Helicity correlated background contribution
  • Caused by transversal polarisation component
  • Necessary to measure for all targets used in PV experiment

direction of movement transverse component α

electron beam

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SLIDE 5

Measurement at PREX

  • Measurement of beam normal single spin asymmetry at PREX

EBeam = 1 – 3 GeV

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SLIDE 6

Theoretical Predictions

EBeam = 850 MeV

  • Origin of asymmetry
  • Interference of 1 and 2 photon exchange
  • Calculations:
  • Gorchtein & Horowitz

[Phys. Rev. C77, 044606 (2008)]

– Two photon exchange

approximation

– Including full range of inter-

mediate excitation states

  • Cooper & Horowitz

[Phys. Rev. C72, 034602 (2005)]

– All orders of photon exchange – Coulomb distortion effects – Only elastic intermediate state

=> No consistent Theory but

  • Contribution to every PV experiment
  • Contribution to other measurements (e.g. proton radius)
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SLIDE 7

Mainz Measurement

  • Measurement of beam normal asymmetry on 12C
  • EBeam = 570 MeV
  • Scattering angles = 15° - 26°
  • Q² = 0.02 – 0.05 GeV²/c²

(Q = 0.14 – 0.22 GeV/c)

  • Requirements:
  • High quality transversely polarised electron beam of known polarisation
  • High rate capable detector system

e e'

12C

kinematic range of this experiment

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SLIDE 8

MAMI Accelerator

Møller polarimeter race track microtrons polarised & thermal electron source injection linac Mott- & Compton polarimeter

  • MAinz MIcrotron
  • 5-Stage electron accelerator
  • Continuous wave beam:

E = 180 MeV – 1.6 GeV Imax = 100 µA spectrometer A spectrometer B

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SLIDE 9
  • No polarimeter for direct vertical transversal polarisation

measurement available

  • Mott:

horizontal transversal @ source

  • Compton: longitudinal @ source
  • Møller:

longitudinal @ target

  • Polarimetry:
  • Maximise and measure longitudinal polarisation at target
  • Maximise transversal horizontal component at source
  • Minimise longitudinal and horizontal component at source and target

Polarised Electron Beam

B B E Wien-fjlter solenoid Mott & Compton polarimeter Møller polarimeter polarised electron source experiment electron spin direction spin precession in microtrons

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SLIDE 10

Experimental Set-up

  • Electron Beam:
  • E = 570 MeV
  • I = 20 µA
  • Target:
  • 10 mm 12C
  • Magnetic Spectrometers:
  • Define angular acceptance

(angles 15.11° - 25.9°)

  • Select elastic events
  • Detectors:
  • Quartz-Cherenkov-Detectors
  • Reduced amplification

→ High rate capability

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SLIDE 11

Low rate particle tracking mode:

Precise positioning of detectors & magnetic field setting → Only elastic line in detector acceptance

Benefits of the Spectrometers

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SLIDE 12

Minimising False Asymmetries

Beam related sources:

  • beam current, energy,

position, angle => beam stabilisation

  • Remaining asymmetry:

beam current: ~ 1 ppm

  • ther parameters: < 0.1 ppm

=> Correction in offline analysis Non beam related sources:

  • Ground noise,
  • Gate length fluctuations,
  • Electrical cross talk
  • Hardware suppression
  • Synchronised with power grid
  • Random polarity sequence
  • Inversions of general sign

=> Offline corrections

100 200 300 400 500 600 700

  • 4000
  • 2000

2000 4000 counts Current Asymmetry [ppm] without stabilisation with stabilisation (scaled) 2000 4000 6000 8000 10000

  • 200
  • 150
  • 100
  • 50

50 100 150 200 counts Gate Length Asym. [ppm] mean = -0.036 ± 0.091

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SLIDE 13
  • 100
  • 50

50 100 Asymmetry (ppm)

Results

Inversion of general sign Runs with equal spectrometer angles run number

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SLIDE 14

Results

spectrometer B spectrometer A

  • 35
  • 30
  • 25
  • 20
  • 15
  • 10
  • 5

0.01 0.02 0.03 0.04 0.05 0.06 Transverse Beam Asymmetry [ppm] Q2 [GeV2/c2

  • M. Gorchtein et al.

] PREX (EBeam = 1 - 3 GeV)

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SLIDE 15

Implications

  • Observations
  • Data points don't

agree with theory

  • Data shows

different slope

  • Theory limitations
  • Only 2 photon exchange
  • No Coulomb distortion

effects included

  • Nuclear structure for

heavy nuclei similar to hydrogen

  • Scattering angle: Θ ≈ 0

=> Theory present in many physical measurements needs to be improved

spectrometer B spectrometer A

  • 35
  • 30
  • 25
  • 20
  • 15
  • 10
  • 5

0.01 0.02 0.03 0.04 0.05 0.06 Transverse Beam Asymmetry [ppm] Q2 [GeV2/c2

  • M. Gorchtein et al.

] PREX (EBeam = 1 - 3 GeV)

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SLIDE 16
  • Parity violation experiments allow measurement of
  • Weinberg angle
  • Neutron Skin
  • Beam-normal asymmetry:
  • important background
  • Direct probe for two-photon exchange
  • Experiment:
  • Vertically polarised electron beam & Elaborate polarisation measurement
  • Spectrometers to select elastic events & Quartz Cherenkov detectors
  • Suppression & Correction for false asymmetries
  • Disagreement between theoretical prediction and measurement
  • Continuation of program:
  • Upcoming beam time in April:

– Energy dependence of asymmetry – Different target material: Silicon

Summary & Outlook

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SLIDE 17

Backup

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SLIDE 18

Theoretical Calculations

EBeam = 1 – 3 GeV EBeam = 850 MeV

  • Cooper and Horowitz

[Phys Rev C 72, 034602 (2005)]

  • All orders of photon exchanges
  • Coulomb distortion effects
  • Only elastic intermediate state

Gorchtein and Horowitz [Phys Rev C 77, 044606 (2008)]

  • Two photon exchange approximation
  • Including full range of intermediate

excited states Data from: HAPPEX / PREX @ J-Lab kinematic range of this experiment

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SLIDE 19
  • Intrinsic reduction of false asymmetries:
  • Spin flip synchronised with power grid frequency

→ ground noise

  • Polarity patterns: ↑↓↓↑ or ↓↑↑↓

→ low frequency noise, monotonous changes

  • Random sequence of Polarity patterns

→ monotonous changes

  • Inversion of … pola inverter every 5 minutes

→ electrical cross-talk in DAQ electronics

  • Inversion of absolute sign every day

→ Unknown sources of false asymmetries

  • Random Variations of beam parameters cancel out
  • Offline correction of remaining false asymmetries

Prevention of False Asymmetries

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SLIDE 20

Beam Stability

−2000 −1000 1000 2000 −2000 −1000 1000 2000

A [ppm] Beam Current Asymmetry ∆I/I

dA/dI = 1.04 ± 0.01

Current stabilisation disabled Position stabilisation disabled

Active beam stabilisation:

  • Current (AC / DC)
  • Position (AC / DC)
  • Energy

2.1 µm rms with stabil. 22 ppm rms with stabil. Correlation of asymmetries in both spectrometers

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SLIDE 21

Polarity Correlated Beam Variations

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SLIDE 22
  • Polarity-correlated variations cause false asymmetries:
  • Beam-current: directly influences measure Asymmetry
  • Beam-energy & beam-angle influence cross-section
  • Beam position on target influences Spectrometer-acceptance
  • Correction factors:
  • Calculated: Current, Energy, Angle
  • Simulated: Beam Positions

Correction of False Asymmetries

Correction Factor Mean Value Correction [ppm] Beam Current 1 ppm / ppm

  • 0.94 ppm
  • 0.94

Beam Energy

  • 3.517 ppm/keV

0.0023 keV

  • 0.0079
  • Hor. Position
  • 19.9 ppm / µm
  • 0.002 µm

0.0398

  • Vert. Position

0.061 ppm /µm

  • 0.013 µm
  • 0.0008
  • Hor. Angle
  • 8.95 ppm/µrad
  • 0.0007 µrad

0.006

  • Vert. Angle

0 ppm / µrad

  • 0.011 µrad