ASK 2011 A Microwave Cavity Axion Experiment for Korea Karl van Bibber Naval Postgraduate School

1. ASK 2011A Microwave Cavity Axion Experiment for Korea Karl van Bibber Naval Postgraduate School Monterey, CAApril 2011

2. Q: What is the most important qualification for an axion hunter? A: The Gift of Immortality Juan Ponce de Leon (1474-1521), Spanish explorer who came to Florida seeking the “Fountain of Youth” (Maybe why Pierre never seems to change?) Looking for the axion may keep us youthful, but in fact physicists are not truly T-invariant! Seriously, this is taking too long. We need to broaden the community of talent, and we need to work together!

3. What this talk will be about This will be a nuts-and-bolts talk about the microwave cavity experiment. What things are important, and what things are hard. It will be oriented to understanding what has been accomplished so far, what are the challenges ahead, and what are the opportunities for an experiment here. A very rough scope of cost and effort will be attempted.

4. The Axion

5. Light cousin of 0: J= 0– 10–8  Horizontal Branch Star limit 10–10 a ga (GeV–1) 10–12 a> 1 Sn1987a ma , gaii fa–1 ga ma 10–14 afa7/6 ma > 1 eV Axion models 10–16 Sn1987a  pulse precludes NNNNa for ma~10–(3–0) eV 100 10–4 10–2 10–6 ma (eV) Horizontal Branch Stars preclude ga > 10–10 GeV–1 G.G. Raffelt “Stars as Laboratories for Fundamental Physics” U. Chicago Press (1996) Good news – Parameter space is bounded Bad news – All couplings are extraordinarily weak Axion basics (What you learn for free)

6. Dark matter Laboratory Solar Axion-photon mixing provides the key [P. Sikivie, PRL 51, 1415 (1983)] Coherent mixing of axions and photons over large spatial regions of strong magnetic fields (a sea of virtual photons) compensates for the extraordinarily small value of ga See Raffelt & Stodolsky for general treatment of axion-photon mixing – PRD 37, 1237 (1988)

7. The cosmological inventory is now well-delineated • But we know neither what the “dark energy” or the “dark matter” is • A particle relic from the Big Bang is strongly implied for DM — WIMPs ? — Axions ? P02552-ljr-u-004

8. E/E ~ 10–11 Nature of axionic dark matter, and principle of the microwave cavity experiment [Pierre Sikivie, PRL 51, 1415 (1983)] Local Milky Way density: halo ~ 450 MeV/cm3 Thus for ma ~ 10 eV: halo ~ 1014 cm–3 virial ~ 10–3 : De Broglie ~ 100 m flow ~ 10–7 : Coherence ~ 1000 km Resonance condition: hn = mac2[ 1 + O(b2~ 10-6) ] Signal power: P  ( B2V Qcav )( g2 ma ra ) ~ 10–23W Key point ! The signal is the Total Energy ( = Mass + Kinetic ) of the axion

9. ADMX stood on the shoulders of giants PRL 80 (1998) 2043 PRD 64 (2001) 092003 ApJ Lett 571 (2002) 27 PRD 69 (2004) 011101(R) PRL 95 (9) 091304 (2005) PRD 74 (2006) 012006 PRL 104 (2010) 041301 From W. Wuensch et al., Phys. Rev. D40 (1989) 3153 + 4 Ph.D. theses We learned much from the first-generation exp’ts (~ liter volume) Already came within a factor of 100-1000 of the desired sensitivity Figure 2

10. Basic formulae Signal power: Scanning rate: QL = Q0/(1+) Loaded Q-value;  coupling f = f - f0Offset from central ClmnCavity form-factor f Cavity bandwidth fstepFrequency tuning steps n Overlapping tuning steps Note both the power and scanning rate depend linearly on QL

11. For scanning at a fixed coupling ga For scanning at a fixed sweep rate Rules-of-thumb for optimizing the experiment Ideally one wants sufficiently low temperature such that one can: Be sensitive to the most pessimistic model axion (e.g. DFSZ ) Which only occupies a fraction of the halo density (e.g. 10% ) Finish the whole works in a tractable time (e.g.10 yrs )

12. h = mc2ma = 4.136 eV . f[GHz] Microwave cavity basics (I) Required/desired features: • Cover ~100 MHz to ~100 GHz • Practical tuning, ± 50% • High quality factor, Q ~ 105 • High cavity form-factor, C = O(1) • Minimal mode-crossings • Minimal mode-localization Simplest – right circular cavity, TM010: • Ez= J0(kr) (empty) • f0= 0.115 GHz / R[m] • C010= 0.69

13. Microwave cavity basics (II) Cavity form-factor Clmn (overlap of E, Bext): Cavity quality, Qlmn: For uniform B = B0: • C(TM010) ~ 0.69 • Much smaller for TM0n0 • TE, TEM identically 0 In high B-field, low-T: • Must be copper (not SC!) • Anomalous skin depth limit Try to use the TM010-like mode for all configurations Q limited to few 105, but we reach the theoretical max

14. Microwave cavity basics (III) – Tuning Mode-crossings Tuning rods, radial offset Ez for TM010 mode; two metal rods half-way from center • Metal - up; dielectric - down • Keep longitudinal symmetry • Keep cavity aspect ratio L/R low • But can ‘walk-around’ crossings

15. dc SQUID basics

16. SQUID amplifier design and performance GaAs Quantum Limit

17. 3 0 - 1 V 0 1 2 3 6 9 2 2 V 4 2 5 N o V a r a c t o r 2 0 Gain (dB) 1 5 1 0 5 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 2 0 0 2 2 0 F r e q u e n c y ( M H z ) Varactor tuning of microstrip SQUID

18. Axion Dark Matter eXperiment (ADMX) University of California, Berkeley John Clarke University of Florida Jeffrey Hoskins, Junseek Hwang, Pierre Sikivie, Neil Sullivan, David Tanner Lawrence Livermore National Laboratory Stephen Asztalos, Gianpaolo Carosi, Christian Hagmann, Darin Kinion, Karl van Bibber National Radio Astronomical Observatory Richard Bradley University of Washington Michael Hotz, Leslie Rosenberg, Gray Rybka, Andrew Wagner

19. Axion hardware ADMX LLNL-UW-Florida-Berkeley-NRAO

20. Axion hardware (cont’d)

21. Dicke Radiometer equation: ADMX is the world’s quietest spectral receiver: Sensitive to one RF photon every two weeks Systematics-limited for signals of 10-26 W – 10-3 of DFSZ axion power. Last signal received from Pioneer 10 (6 billion miles away) ~ 10-21 W.

22. Sample data and candidates Signal maximizes in the wings, and furthermore is episodic → Radio peak Distributed over many subspectra (good), but didn’t repeat → Statistical peak

23. Brief outline of analysis — 100 MHz of data

24. Origin of the non-thermalized component

25. 1-D infall, and the “folding” of phase space

26. Velocity spectrum of axions at our solar system

27. Diurnal and sidereal oscillation of the fine-structure

28. Simulation of one infall model Annual Daily

29. 2b 2a Frequency 1a 1b Time (seconds) x 10-6 Diurnal and sidereal oscillation of late-infall axion peaks

30. 2000 s 52 s Results of a high-resolution analysis PRL 95 (9) 091304 (2005) Measured power in environmental (radio) peak same in Med- & Hi-Res

31. So far, no axion ! (over 1.9 - 3.6 meV) Need to push the experiment on two fronts: Reduce System Temperature ADMX Phase II: add Dilution Refrigerator Go up in Frequency ADMX-HF (High Frequency): smaller microwave cavities ,

32. So where are we going in the future? • Improve sensitivity & scan speed (reduce TS) → ADMX Phase II • Go from “Gen 2.5” to “Gen 3.0” • Increase mass reach (increase frequency ν) → ADMX-HF • “Franchise Model” for ADMX, “One experiment, two sites” • Allows us to attack two decades of mass in parallel, rather than serially ! • More later …

33. ADMX Phase II will be both more sensitive and faster for low masses

34. ADMX-HF at Yale will be a very small experiment ! Design of cavity & magnet Dilution refrigerator above & below deck ADMX-HF will also be a test-bed for innovative concepts, e.g. thin-film superconducting cavities

35. ADMX Phase II & ADMX-HF Coverage f (GHz) 0.5 1 2 5 20 10 10-12 10-13 ADMX - HF KVSZ gagg[GeV-1] 10-14 ADMX (Higher TM) DFSZ ADMX (complete) 10-15 ADMX (Phase II) Wa ~ 0.23 10-16 100 1 10 ma (meV)

36. 100 GHz 10 GHz 1 GHz For the long term, ADMX needs concurrent R&D To get to 10 GHz (40 eV), and ultimately 100 GHz (0.4 meV), we need to: – Develop new RF cavity geometries – Develop new SQUID geometries

37. There is one more thing we need to do to complete the strategy • We also need to go down in frequency ! • One concrete motivation – string theory predicts many axions, but generally much lower in mass, ma ~ neV(Witten, Srvcek; Kim…) • More generally, we cannot really say where the axion is, and must be prepared to look everywhere in m , g Searching in the ma < 1meVrange could be a great opportunity for the Korea Axion Center How would one go about this…?

38. Axion electrodynamics (Sikivie, 1983)

39. Our familiar realization of axion electrodynamics:

40. Case of large fa/N , and magnetic diameter D << l

41. Lumped parameter (LC) resonators Scott Thomas, Rutgers (2006) General idea: a transformer, with a highly permeable core; for a given diameter magnet, the frequency can be much lower than the corresponding microwave cavity. Details omitted here, but: It’s a stretch to reach string axions, but should be very workable for fa ~ 10(12-14), i.e. ma ~ 0.01-1 meV. fa ~ 1015GeV fa ~ 1016GeV

42. Last vexing gap: 100–1000 meV range (l ~ mm) We don’t yet have a good strategy here We should look into F. Casper’s dielectric waveguide scheme Or we may wait for help from above (NGAH) Good new ideas always welcome!

43. What if the axion is found?

44. What is the scale to do it right? Essential elements of a successful program: • Take data with high duty-factor, covering a decade each 3 years • Preparing next series of cavities & amplifiers in parallel with running • Doing modeling of the experiment & “operations research” optimization • Keeping up with the analysis, and preferably done by two teams • Doing concurrent R&D Frankly, ADMX was not able to fully accomplish any of these! Team: 1 Senior experimentalist (Professor) 3 Junior experimentalists (Ass’t Professor) 5 Postdocs 8 Graduate students 10 Undergraduate students 1 Project manager 2 Engineers 4 Technicians 1 Administrative assistant 1 Budget & purchasing agent

45. Capital costs Civil construction 1000 ($K) Will depend on existing facility Microwave electronics 1500 Two setups + R&D lab Magnet 2000 ADMX magnet in FY11 $ ** Dilution fridge (2) 2000 One exp’t, one cavity R&D lab Ancillary equipment 1000 Pumps, etc. Quantum device lab ?? Depends what you have ++ ** Cost of magnets are reasonably linear in stored energy, i.e. ~ B2V; a good rule of thumb ++ Initially one could procure e.g. SQUID MSAs from Berkeley etc... but eventually one should develop an indigenous capability

46. Polarization (g4) Photon regen (g4) Solar search (g4) Resonant reg (g4) Microwave DM (g2) Where we have got to & where we are going

47. Summary and final thoughts • The cavity microwave search for dark matter axions has made excellent progress in 25 years • The 3rd generation experiment will have appropriate sensitivity to do a meaningful search – the challenge is the rate at which we are able to scan in mass • Doing decades of mass serially is not a success-oriented strategy – we must scan decades in parallel • ADMX-II and ADMX-HF are well-poised for 1-10 & 10-100 meV • The field is wide-open for R&D and a major experiment to search down towards string-scale axions – & the 100-1000 meV range too • Should the axion be found (with structure), one could envision VLB-like astronomy involving multiple platforms around the world And many thanks to Prof. Kim for organizing the ASK series of meetings!