Searching for Axions A Symposium in Honor of Helen Quinn SLAC April 16, 2010 Leslie J Rosenberg University of Washington Searching for Axions Outline Recap axion properties (from Roberto’s talk) Selected axion searches (not enough time to talk about them all) 5th force searches Photon regeneration and optical rotation Solar axion searches RF cavity search for dark-matter axions Overall status of axion hunting Axions and axion-like particles (from Roberto’s talk) You could imagine many kinds of scalars and pseudoscalars, e.g., Majoron (from lepton-number breaking…neutrino masses) Familon (from family-symmetry breaking) Dilaton (low-energy state in string theory) Axion (removes CP violation in strong interactions) Axions are well-motivated and their phenomenology is well-understood QCD and CP violation and axions: a very brief history 1973: QCD…a gauge theory of color. QCD theory respected the observed conservation of C, P and CP. 1975: QCD + “instantons” QCD is expected to be hugely CP-violating. “The Strong-CP Problem” QCD on the lattice: CP-violating instantons in a slice of spacetime (sort of) Invention & Properties of the axion Helen and Roberto pondered the strong CP problem. Hmmm. Add in a continuous symmetry, spontaneously broken. One of those new terms containing the vev cancels terms responsible for CP violation (the PecceiQuinn mechanism). Voila. And as we all know, a spontaneously broken continuous symmetry has a boson: this is the axion. Simply: The axion is a light pseudoscalar resulting from the broken “Peccei-Quinn” symmetry to enforce Strong CP conservation fa, the SSB scale the PQ symmetry breaking, is the one important parameter of the theory. Axions and dark matter “…I'm much more optimistic about the dark matter problem. Here we have the unusual situation that two good ideas exist – which, according to William of Occam (the razor guy), is one too many. “The symmetry of the standard model can be enhanced, and some of its aesthetic shortcomings can be overcome, if we extend it to a larger theory. Two proposed extensions, logically independent of one another, are particularly specific and compelling. “One incorporates a symmetry suggested by Roberto Peccei and Helen Quinn in 1977. Peccei-Quinn symmetry rounds out the logical structure of quantum chromodynamics by removing QCD's potential to support strong violation of time-reversal symmetry, which is not observed. This extension predicts the existence of a remarkable new kind of very light, feebly interacting particle: the axion. … This axion can be very useful in unexpected places It was introduced to solve the ‘strong CP problem’ in particle physics but shows up elsewhere. One example: Axions may account for large number of spiral galaxies. (M51) Rector & Ramirez/NOAO (HDF) Harry Furguson/STSci The axion’s contribution to particle, nuclear & astrophysics reminds me of viewing a piontillist painting; when all the bits come into focus, it’s breathtaking. Back to axion masses and couplings 10–8 The axion is a ga (GeV–1) light cousin of 10–10 0: J= 0– Horizontal Branch Star limit a 10–12 a > 1 Sn1987a 10–14 ma , gaii fa–1 ga ma a fa7/6 ma > 1 eV Axion models 10–16 10–6 10–4 m (eV) 10–2 a 100 Good news – Parameter space is bounded Bad news – All couplings are extraordinarily weak Sn1987a pulse precludes NNNNa for ma~10–(3–0) eV Red giant evolution precludes ga > 10–10 GeV–1 5th force searches for distances less than 100 m Axions mediate matter-spin couplings gs 1 a i5gp 2 V ~ 1/rer / rˆ The special role of axion-photon mixing in sensitive searches Lint aga E B Laboratory (“laser”) Dark matter Solar P. Sikivie, PRL 51, 1415 (1983) See Raffelt & Stodolsky for general treatment of axion-photon mixing – PRD 37, 1237 (1988) A class of search: Vacuum birefringence & dichroism Vacuum dichroism ~ N (1/4 gB0L)2 (N = number of passes) FabryPerot B0 Laser Magnet l + Vacuum birefringence = N ·(1/96)·(g B0ma)2·L3/ Maiani, Zavattini, Petronzio, Phys. Lett. B 175 (1986) 359 Example: The PVLAS experiment (INFN Legnaro) E. Zavattini et al., PRL 96 (2006) 110406 Y.Semertzidis et al., PRL 64 (1990) 2998 M = 1/ga Recent PVLAS details & data PVLAS Schematic Phase-Amplitude Plot Rebuilt detector didn’t find signal. Their early value of ga was ostensibly excluded already by 4 orders of magnitude, by solar searches, and stellar evolution (stars would live only a few thousand years) This renewed polarization-rotatation experiments around the world, and much theoretical work Photon regeneration (“shining light through walls”) B0 B0 Photon Detector a Magnet Magnet L L P(a) ~ 1/16 (gB0L)4 ga (GeV-1) Laser Wall Early measurement: g < 6.7 x 10-7 GeV-1 for ma < 1 meV G. Ruoso et al., Z. Phys. C. 56, 505 (1992) & R. Cameron et al., Phys. Rev. D47, 3707 (1993) ma (eV) Several photon regeneration efforts around the world Experiments in various phases of prepation or operation CERN Courier, Vol. 47 No. 2 (March 2007) All of them would still be orders of magnitude away from solar & red-giant limits Resonantly enhanced photon regeneration Basic concept – encompass the production and regeneration magnet regions with Fabry-Perot optical cavities, actively locked in frequency Sikivie et al. PRL (April 27, 2007) Laser Photon Detectors IO Magnet Magnet Matched Fabry-Perots P Re sonant ( a ) 2 P Simple ( a ) 2 2 FF P Simple ( a ) where ’ are the mirror transmissivities & F, F’ are the finesses of the cavities For ~ 10(5-6), the gain in rate is of order 10(10-12) and the limit in ga improves by 10(2.5–3) Solar axion search a Produced by a Primakoff interaction, with a mean energy of 4.2 keV Ze solar-axion spectrum Flux [1010 ma(eV)2 Tcentral = 1.3 keV, but plasma screening suppresses low energy part of spectrum (says G.Raffelt) cm-2 sec-1 keV-1 ] 16 The total flux (for KSVZ axions) at the Earth is given by a 7.44 1011cm2 sec1(ma /1eV)2 0 E [ keV ] 10 The dominant contribution is confined to the central 20% of the Sun’s radius Principle of the solar-axion search experiment Photon Detector B0 a Magnet l a B x 1 2 2 (a ) (ga B0L) F(q) 4 where Sin(qL /2) , F(q) (qL /2) F(0) 1 and q k ka ma /2 2 Example: The CERN Axion Solar Telescope (CAST) a Prototype LHC dipole magnet, double bore, 50 tons, L~10m, B~10T Tracks the Sun for 1.5 hours at dawn & 1.5 hours at dusk Instrumented with: CCD with x-ray lens; Micromegas; TPC CAST results and future CAST has published results equaling the Horizontal Branch Star limit (Red Giant evolution) They are pushing the mass limit up into the region of axion models, 0.11 eV CAST JCAP Plan: Fill the magnet bore with gas (e.g. helium), and tune the pressure When the plasma frequency equals the axion mass, full coherence and conversion probability are restored: K. Zioutas et al., Phys. Rev. Lett. 94, 121301 (2005) p (4Ne /me )1/ 2 m KvB, P. McIntyre, D. Morris, G. Raffelt PRD 39 (1989) 2085 They will go to higher ma with 3He, and a second x-ray optic RF cavity axion-search experiments: Axion and electromagnetic fields exchange energy The axion-photon coupling… ga …is a source in Maxwell’s Equations E2 /2 E B ga aÝE B t So imposing a strong external magnetic field B allows the axion field to pump energy into the cavity. RF cavity: How to detect dark-matter axions Important to lower Ts ADMX: Axion Dark-Matter eXperiment U of Washington, LLNL, University of Florida, UC Berkeley, National Radio Astronomy Observatory/University of Virginia Magnet with insert (side view) Magnet arrives ADMX hardware high-Q cavity experiment insert The axion receiver The world’s quietest radio receiver Systematics-limited for signals of 10-26 W ~10-3 of DFSZ axion power (1/100 yoctoWatt). Recent published data Particle Physics Ap.J Astrophysics These are interesting regimes of particle and astrophysics: probe realistic axion couplings and halo densities Better sensitivity (lower Ts): SQUID Amplifiers IB The basic SQUID amplifier is a fluxto-voltage transducer Vo (t) SQUID noise arises from Nyquist noise in shunt resistance scales linearly with T However, SQUIDs of conventional design are poor amplifiers above 100 MHz (parasitic couplings). Flux-bias to here Noise Temperature (mK) Quantum-limited gigahertz SQUID amplifiers 4 SQUID A2-5, f = 684 MHz SQUID L1-3, f = 642 MHz SQUID K4-2, f = 702 MHz 2 Semiconductor 1000 6 4 TQL 2 100 Clarke and Kinion 6 4 quantum limit 2 2 4 6 8 100 2 4 6 8 1000 2 4 An old idea from antenna design Physical Temperature (mK) (“shunt detuned frequency”) applied to quantum electronics. SQUID commissioning calibration RF-cavity experiment target sensitivity “Definitive” sensitivity over lowest decade in mass (where dark matter axions would likely be) Plus operations into second decade of mass (where unusual axions might be) Overall status of axion hunting CAST SN1987A ADMX Upgrade Conclusions There are lots of axion searches out there, it’s the wild west; most search for “unusual” axion variants. If axions are the “usual” Peccei-Quinn type (“QCD axion”) then ADMX will either find it or exclude it at high confidence. This effort has a 5-year horizon. Of course, there are escape hatches: Are anthropic arguments are correct? Will experiments show WIMPs saturate the local DM halo? Axion or not, the ‘strong CP problem’ is there; it may even be a huge problem (for believers in SUSY). There must be some reason CP violation is suppressed, and I’m guessing Helen and her cohorts got it right. Thank you Helen.