Parity Violation in Diatomic Molecules

Transcription

1 Parity Violation in Diatomic Molecules Jeff Ammon, E. Altuntas, S.B. Cahn, R. Paolino*, D. DeMille Physics Department, Yale University *Physics Department, US Coast Guard Academy DeMille Group Funding: NSF

2 Outline 1. Introduction: Parity violating effects, motivation 2. Why Diatomic Molecules? 3. The Experiment: Schematic, signal 4. Level Crossing Spectroscopy in 138 BaF 5. First PV Data with 138 BaF: Evidence of systematics 6. E & B Field Systematics: How to measure & eliminate 7. New Data 8. Future Work

3 Parity Violation Nuclear Spin Independent (NSI) Every nucleon contributes H NSI 3 ( σ p) δ ( r ) Best measurement: Wieman 133 Cs Nuclear Spin Dependent (NSD) Only unpaired nucleons contribute 3 H ( σ I )( σ p) δ r NSD NSD PV smaller, more difficult to measure. ( ) σ : electron spin p : electron mom. I : nuclear spin r : electron pos. Uncertainties: NSI 1% NSD 14% Also: 133 Cs is the only nonzero measurement of NSD in atoms Using diatomic molecules enhances NSD

4 H NSD NSD PV Effects ( κ + κ )(σ I )(σ Z a 3 p)δ ( r ) e V e + A e e H NSD = Z 0 + γ N V N + A N Z 0 exchange Independent of nuclear mass A Z 0, W N Anapole Moment Proportional to A 2/3 Can differentiate effects by measuring multiple nuclear species

5 Anapole Moment Simple model for nuclear anapole (valence nucleon + constant-density core): Current loop (dipole) orbital = + momentum around core Current helix (anapole) spin tilted along momentum due to weak interaction ( σ p helicity term) Causes delta function vector potential at origin

6 Motivation Z 0 Exchange Electron nucleon weak coupling constants (C 2N, C 2P ) Related to fundamental electron-quark weak coupling constants (C 2u, C 2d ) Complementary to PVDIS measurements (different linear combinations of C 2 constants) Anapole Moment Nucleon nucleon coupling constants Nuclear structure Anapole Moment Table - unique signatures for each nuclear species Measurement of parity violation in electron-quark scattering The Jefferson Lab PVDIS Collaboration Nature 506, 67-70

7 Outline 1. Introduction: Parity violating effects, motivation 2. Why Diatomic Molecules? 3. The Experiment: Schematic, signal 4. Level Crossing Spectroscopy in 138 BaF 5. First PV Data with 138 BaF: Evidence of systematics 6. E & B Field Systematics: How to measure & eliminate 7. New Data 8. Future Work

8 Even / Odd State Mixing Pˆ Pˆ + = + = Atomic, molecular states are parity eigenstates δ Weak interaction mixes even & odd states δ = + E + H W E = iw Δ ~ 10Hz ~ Hz δ is very small 1eV

9 Diatomic Molecules Amplify mixing by making small: iw δ = Δ + Δ Diatomic molecules: Large moment of inertia rotational levels have small 2 + I E rot MR 2 2 J = 2 I Energy ~10-5 ev Parity: (-1) J (vs. ~1eV for atoms)

10 Zeeman Tuning Closely spaced levels can be brought to crossing with B-field B-field required: ~ 1 Tesla easy with superconducting magnet Energy How close? 1 part in 10 7 uniformity ~ 10 3 Hz (vs Hz for atoms) + 1 T B Field

11 Viable Nuclei For Anapole Measurement Everything OK (one dot/isotope) Only molecular spectral data needed Only isotopically enriched sample needed Maybe possible with cryogenic beam source

12 Chosen Molecule: 137 BaF Why 137 BaF? Odd neutron ( 133 Cs had odd proton) Heavy larger anapole moment Spectroscopy available BaF molecular beams had been made before Large enough natural abundance don t need enriched source Transitions are diode laser accessible Development & testing: 138 BaF W = 0 (no parity violation) Larger natural abundance (~75% vs ~11% for 137 Ba) Can use same source as 137 Ba

13 Outline 1. Introduction: Parity violating effects, motivation 2. Why Diatomic Molecules? 3. The Experiment: Schematic, signal 4. Level Crossing Spectroscopy in 138 BaF 5. First PV Data with 138 BaF: Evidence of systematics 6. E & B Field Systematics: How to measure & eliminate 7. New Data 8. Future Work

14 Experimental Schematic Superconducting solenoid (1) B E 0 z axis (2) (3) (4) (1) (2) (3) (4) + Laser E 0 Laser

15 Signal Applied E field: E ( t) = E0 sin(ωt) N 0 : # of molecules in lower state : Detuning d: dipole matrix element W: weak matrix element S( E 0 ) π Δ ω de0 W de0 W = 4N 0 sin ω Δ ω Δ Signal: population of upper (detection) state at end Stark Interference term: without this, dependence on W would be second order Second order. Very small. Ignore.

16 Asymmetry + = ω Δ 2 ω ω Δ π sin 4 ) ( de W de N E S ω Δ ) ( ) ( ) ( ) ( de W E S E S E S E S A = = 1) Measure Signal for +E 0 and E 0 2) Form Asymmetry: 3) Solve for W in terms of known quantities.

17 Example Signal & Asymmetry Signal Asymmetry Calculated by numerically solving Schrodinger eqn. Assumes W = 5Hz

18 Outline 1. Introduction: Parity violating effects, motivation 2. Why Diatomic Molecules? 3. The Experiment: Schematic, signal 4. Level Crossing Spectroscopy in 138 BaF 5. First PV Data with 138 BaF: Evidence of systematics 6. E & B Field Systematics: How to measure & eliminate 7. New Data 8. Future Work

19 Multiple Crossings Energy + Multiple crossings because of hyperfine structure 1 T B Field Matrix element W is different at each crossing Underlying PV magnitudes (κ Z & κ a ) are same at all crossings Measure at different crossings as systematic check

20 Level Crossing Spectroscopy in 138 BaF m, m, m S I N S: Electron spin I: Nuclear spin N: Molecule rotational ang. mom. ˆn : molecule internuclear axis W (κ + κ )( nˆ S ) Z a PV magnitudes. Same at all crossings I Angular factor. Different for each crossing. Want to measure PV at multiple crossings as systematic check Need to find B field values where levels cross

21 Finding Crossing Locations Increasing B Field Δ + E-field applied Maximum signal occurs when B-field is set such that =0

22 Spectroscopy Results Zeeman-tuned rotational level-crossing spectroscopy in a diatomic free radical S.B. Cahn et. al. arxiv: Also measured: Soon to be in PRL Dipole matrix elements (d) Polarizabilities (α) Lineshapes

23 Outline 1. Introduction: Parity violating effects, motivation 2. Why Diatomic Molecules? 3. The Experiment: Schematic, signal 4. Level Crossing Spectroscopy in 138 BaF 5. First PV Data with 138 BaF: Evidence of systematics 6. E & B Field Systematics: How to measure & eliminate 7. New Data 8. Future Work

24 New Interaction Region Prisms allow laser delivery for state preparation Old interaction regions: 2 electrodes, 7 electrodes New interaction region: 32 electrodes better control over E field one cycle sine wave for PV measurement

25 First PV Data with 138 BaF 138 Ba Expected: W = 0 Measured: W = Hz (stat) ~2.5 hours of data Previous best sensitivity: 2.9 Hz in 30 hours with atomic Dy* Measured W due to systematic effects *Nguyen, Budker, DeMille, & Zolotorev, PRA 56, 3453 (1997)

26 Evidence of Systematics Data Numerical Calculation Non-reversing E field (E NR ) even part of Asymmetry (e.g. vertical offset) E NR + B gradients (B grad ) odd part of Asymmetry (looks like parity violation!) Need to measure & then eliminate E NR & B grad

27 Outline 1. Introduction: Parity violating effects, motivation 2. Why Diatomic Molecules? 3. The Experiment: Schematic, signal 4. Level Crossing Spectroscopy in 138 BaF 5. First PV Data with 138 BaF: Evidence of systematics 6. E & B Field Systematics: How to measure & eliminate 7. New Data 8. Future Work

28 B field Measurement w/ Molecule Signal Want to flatten B field so that it is uniform Must measure B field first B(z) E(z) z B field only matters where E is non-zero z Measure B(z) by translating E pulse across z and recording Signal

29 B field Measurement w/ Molecule Signal E(z) Signal z1 z2 z3 z B1 B2 B3 B Apply E-field Find signal maximum ( i.e. = 0 ) B(z) B3 B2 B1 z1 z2 z3 z Repeat across interaction region to get B(z)

30 B field Shimming B field measured (a) Shim coils adjusted to flatten field B field measured again (b)

31 E Field Measurement w/ Molecule Signal c E E( t) ( Δ) e iδt dt 1 st order perturbation theory ( F.T. of E(t) ) E NR (t) E R (t) E R (t+τ) t ( Δ) (Δ R ) c E NR + E τ c E + NR iδτ ( Δ) e E (Δ) R shifted E R S E NR 2 2 ( ) + E ( ) + E ( ) [ Re[ E ( ) ] cos( τ) Im[ E ( ) ] sin( τ) ] R R NR NR Interference term ( + E ) S( E ) E ( ) [ Re[ E ( ) ] cos( τ) Im[ E ( ) ] sin( τ) ] S 2 R Signal Difference R R NR NR

32 E Field Measurement w/ Molecule Signal Set = constant Take data over range of τ (applied E location) 2 fit parameters: ( a, b ) ( + E ) S( E ) E ( ) [ Re[ E ( ) ] cos( τ) Im[ E ( ) ] sin( τ) ] S 2 R R R NR NR Sig. Diff. from data data fit function fit function: a * cos( τ) + b * sin ( τ) Repeat at a range of to construct E NR ( ) τ Fourier transform to get E NR (t)

33 E NR (t) Measurement Example Interference Term Fit Repeat over range of E NR ( ) E NR (t) Fourier Transform

34 E NR (t) Shimming Results Before Shimming After 1 Shim Iteration After 2 Shim Iterations Shimming = Applying voltages to counteract E NR

35 Outline 1. Introduction: Parity violating effects, motivation 2. Why Diatomic Molecules? 3. The Experiment: Schematic, signal 4. Level Crossing Spectroscopy in 138 BaF 5. First PV Data with 138 BaF: Evidence of systematics 6. E & B Field Systematics: How to measure & eliminate 7. New Data 8. Future Work

36 Recent Data Measured: W = Hz (stat) Why do systematics remain? Part of E NR lies outside interaction region can t be shimmed out Numerical calculations: This E NR plus reasonable B grad still gives large W

37 Longer E-field Region Plan: Lengthen electrodes can shim E fields and measure B fields further out

38 Future Work Make new interaction region with longer electrodes Shim out E NR and B grad Measure PV in 138 BaF (Should be zero) Improvements in signal (hexapole lens, cryogenic source) Measure PV in 137 BaF (Expected W ~5Hz)