The G 0 Experiment: Backangle Running Riad Suleiman Virginia Tech - PowerPoint PPT Presentation

The G 0 Experiment: Backangle Running Riad Suleiman Virginia Tech November 02, 2006

OUTLINE • The Structure of the Proton and the Goal of the G 0 Experiment • Parity Violation in Electron-Nucleon Interaction • The G 0 Experiment • The Backangle Running • Controlling the Helicity-Correlated Beam Properties • Parity Quality of G 0 Beam • Results

Inside the Nucleon: The Building Blocks of Matter Meson: quark + antiquark Baryon: quark + quark + quark proton: u u d valence quarks neutron: u d d

Quarks in More Detail • Mass: range from ~10x electron mass (up quark) to that of a Tungsten atom (top quark) * No internal structure (< 10 -19 m) • • Electric charge: +2/3 (u,c,t) and -1/3 (d,s,b) Proton: • No free quarks: Mass (MeV/c 2 ) Quark Charge (e) Up +2/3 1.5 – 4 Down -1/3 4 – 8 Strange -1/3 80 – 130 gluon Charm +2/3 1150 – 1350 Bottom -1/3 4100 – 4400 * The mass of an electron is 0.5 MeV/c 2 = 9.1x10 -31 kg Top +2/3 171400 ± 2100

Strange Quarks In Particular quark and antiquark pair • Sea of quark and antiquark pairs – Made up of Up, Down, and Strange quarks – Up & Down quarks in sea valence difficult to distinguish quark from valence Up and gluon Down quarks – Strange quark provides a unique window

The Goal of the G 0 Experiment To determine the contribution of the strange quark to the electric and magnetic properties of the proton and neutron. Moving charges → electric current → magnetic field Quarks move around so the proton has a charge Quarks and gluons both distributed over its size. have spin, leading to a magnetic moment and magnetization distribution. • Form Factors: The most fundamental dynamical quantity for describing the inner properties of a composite particle. – Electric (G E ): provides detailed information about the spatial distribution of charges in the particle. – Magnetic (G M ): “ “ “ magnetization in the particle. – Axial (G A ): “ “ “ spin in the particle.

Electron and Nucleon Interactions • Electromagnetic Force (binds electrons to nuclei) – Carrier particle: photon – Parity-conserving • Why an electron • Weak Force (radioactive probe? decay) – No internal structure – Carrier particles: W + , W - and Z – Electromagnetic bosons (particles with integer interaction well spin) understood – Z 0 interaction is parity-violating – Electrons penetrate deep inside a nucleus

What is Parity-Violation? Parity-conservation : strength of Parity-violation : strength of particle interaction is same for particle interaction is different for mirror image mirror image Sun on Sun on right left Sun exerts the same pull on the earth. The bean family twine to form a right-handed spiral. Left-handed spirals do not exist.

Parity Violation mirror Right−Handed (R) (+ helicity) Left−Handed (L) (− helicity) Spin Parity Reversal (Space Inversion) Momentum is equivalent to This is equivalent to: Spin Reversal Electron Electromagnetic force is parity-conserving. Electrons' helicity will not affect the number of electrons scattered. Weak force is parity-violating. Electrons' helicity will affect the number of electrons scattered. A The relative difference in these counting rates meas tells us how big the weak interaction piece is.

Electron and Proton Interactions Revisited γ = + Z Amplitude of electron-proton interaction M M M ( ) ( ⎡ ⎤ * ) = + + 2 2 2 2 Re M M M M M ⎢ ⎥ ⎣ ⎦ γ γ z z Probability: Cross sections: 2 2 σ ∝ + σ ∝ + M M M M γ γ R z R L z L Asymmetry: γ Ζ 2 2 + − + * σ − σ M M M M e M M p e p γ γ γ = = z z = 2 z R L R L A σ + σ 2 2 2 + + + 2 M M M M M γ R L γ γ γ z z R L

Parity-Violating Electron Scattering ⎡− ⎤ σ − σ + + 2 G Q A A A = = R L F E M A ⎢ ⎥ A σ + σ σ πα 2 4 2 ⎣ ⎦ R L u 1 equation 3 unknowns Z ( Q 2 ) G E γ ( Q 2 ) A E = ε ( θ ) G E Z ( Q 2 ) G M γ ( Q 2 ) A M = τ ( Q 2 ) G M Requires 3 measurements at a given Q 2 : Forward angle e + p (elastic) e ( Q 2 ) G M γ ( Q 2 ) A A = − (1 − 4sin 2 θ W ) ′ ε G A Backward angle e + p (elastic) Backward angle e + d (quasi-elastic) 2 + τ G M ( ) ( ) 2 γ γ 2 σ u = ε G E Probes same hadronic flavor structure, with different couplings: 2 1 1 γ = − − u d s G G G G / / / / E M E M E M E M 3 3 3 ⎛ − 8 ⎞ ⎛ − 4 ⎞ ⎛ − 4 ⎞ = 2 θ − 2 θ − 2 θ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ 1 sin 1 sin 1 sin Z u d s G G G G / / / / E M ⎝ 3 W ⎠ E M ⎝ 3 W ⎠ E M ⎝ 3 W ⎠ E M

Strange Quark Form Factors Neglecting trivial breaking due to Coulomb force, one expects the neutron to be an isospin rotation of the proton: = = = , , , , , , , , p u n d p d n u p s n s G G G G G G / / / / / / E M E M E M E M E M E M 2 1 1 γ , = − − p u d s G G G G 2 1 1 / / / / γ = − − E M E M E M E M , 3 3 3 n d u s G G G G / / / / E M 3 E M 3 E M 3 E M G γ, p G u G p Well E,M E,M E,M Measured G γ, n Charge G d Pick ‘n G n E,M E,M E,M symmetry Choose <N| s γ μ s |N> G Ζ, p G s G s E,M E,M E,M σ − σ = = + η + η + η s s eN R L A A G G G 0 σ + σ E E M M A A R L

“Rosenbluth” type of Separation σ − σ = = + η + η + η s s eN R L A A G G G 0 σ + σ E E M M A A R L Vary both the targets (LH 2 , LD 2 , 4 He) and the kinematics World data at Q 2 ~ 0.1 (GeV/c) 2 Θ e η E η M η A Exp Target E beam A o (GeV) (deg) (ppm) (ppm) (ppm) (ppm) SAMPLE LD 2 0.2 150 -7 1.6 0.8 1.8 SAMPLE LH 2 0.2 150 -6 2.1 3.5 1.6 HAPPEx 4 He 3 6 7 20.0 0 0 PVA4 LH 2 0.6 35 -2 10.1 1.0 0.3 G 0 LH 2 3 6 -2 12.0 1.2 0.1

The G 0 Experiment • Forward and backward angle parity-violating e-p elastic and e- d (quasi-elastic) in JLab Hall C , and separated s s e G G G E M A − 2 2 over range ~ 0 . 1 1 . 0 (GeV/c) Q • Superconducting toroidal magnet • Scattered particles detected in segmented scintillator arrays in spectrometer focal plane (FPD) • Custom electronics count and process scattered particles (proton at forward angle and electrons at backward angle ) • Forward angle run completed • Backward angle � March 06 - February 07

What does G 0 mean? When this experiment was proposed 12 years ago, people were interested in this combination of form factors, Charge (magnetization) form so it was named G 0 factor of the proton associated with Z 0 interaction ⎡ ⎤ ⎛ ⎞ 1 γ = − θ − 0 , 2 , , ⎜ ⎟ 4 sin p p p Z G ⎢ G G ⎥ / / / E M W E M E M ⎝ 2 ⎠ ⎣ ⎦ Charge (magnetization) form factor of the proton associated with γ exchange

G 0 Forward Angle Results 2 2 πα ε + τ ) ( ) 4 2 p p G G + η = − s s ( E M G G A A 2 ε + ( 0 ) E M phys NVS 1 p G Q G R F E V • Use the anapole contribution computed by Zhu et al. 04 Examining full data set, probability that s ≠ 0 s + η G M G E is 89% D.S. Armstrong et al, PRL 95 (2005) 092001

G 0 Backward Angle •Electron detection: θ = 108° •Both LH 2 and LD 2 targets •Add Cryostat Exit Detectors (CED) to define electron trajectory •Aerogel Cherenkov detector for π /e separation (p π < 380 MeV/c) CED + Cherenkov Q 2 (GeV/c) 2 E e (MeV) 362 0.23 FPD 686 0.62 e - beam target Common Q 2 with HAPPEX-III and PVA4 (both at forward angles)

Experiment Schematic Forward Angle mode Backward Angle mode e – p magnet magnet beam beam target target

G 0 in Hall C : Superconducting Magnet The key elements (SMS) Halo Detectors G 0 beam monitoring FPD Detectors CED+Cherenkov Detectors Spokesman

G 0 Backangle Run • March 15 – May 1 (687 MeV): – 200 hours LH 2 , 50 hours LD 2 (at 10 μ A) – 80 hours “parity quality” data w/ LH 2 at 60 μ A • May 15 – May 18 (362 MeV): – first look at LD 2 at low beam current High singles rates in the Cherenkov Detector PM tubes from neutrons. Change borosilicate window PM tubes to quartz window PM tubes. – outstanding beam delivery July 19 – Sept 1 (362 MeV): Production data w/ LH 2 at 60 μ A • Sept 22 – Nov 1 (687 MeV): Production data w/ LH 2 at 60 μ A • Nov 1 – Dec 22 (687 MeV): Production data w/ LD 2 at 60 μ A • Jan 5 – Feb 18 (362 MeV): Production data w/ LD 2 at 60 μ A • Great Beam at very low energy, THANKS!

Basic Principles of Parity-Violation Experiments + − − Y Y • How do we carry out parity-violation = meas meas A experiment? + − + meas – We scatter longitudinally Y Y polarized electrons off un- meas meas polarized protons within a hydrogen target r measured rate meas = Y – We reverse the helicity of the electron beam and measure the Q beam charge relative difference in detected signal: • Here’s the catch: – The experimental asymmetry is very small (1-50 ppm). * – The challenge is controlling the false asymmetries. * Four drops of ink in a 55-gallon barrel of water would produce an "ink concentration" of 1 ppm.