Axions Joaquin Vieira Kavli Institute for Cosmological Physics Enrico Fermi Institute University of Chicago The hypothetical AXION was proposed in 1978 by Steven Weinberg (Physics Nobel Prize 1979 for electroweak force) and Frank Wilczek (Physics Nobel Prize 2004 for QCD) It has not yet been detected after more than twenty five years of experimental searches…

General Overview of Axions Hypothetical point-like neutral pseudo-scalar boson that interacts only rarely with ordinary matter. Arise from the Peccei-Quinn symmetry to explain a long standing problem in the Standard Model. (Yes, THAT Peccei…) One of two best cold dark matter candidates for the missing mass density in the Universe. Has never been detected after more than 25 years of searches. Named after a laundry detergent. “One needed a particle to clean up a problem…” -- Frank Wilczek

Not to be confused with the axion speargun, shoes, motocross helmets, or dvd player Outline: I Strong CP Problem and Neutron EDM II Axion Theory and Phenomenology III (Solar) Axion Experiments IV CERN Axion Solar Telescope V Chicago Calorimeter …with a little symmetry, particle physics, cosmology, and low background experimental physics along the way.

Our story begins: Discovery of subatomic particles: Electron1897 Millikan (first elementary particle) Proton1919Rutherford Neutron1932Chadwick Positron1932Anderson (first anti-matter) Muon1937Anderson (second family) The neutron, being electrically neutral, is very difficult to work with: difficult to produce, accelerate and detect. Neutrons make up nearly half of ordinary matter in the Universe. Made up of d and u quarks, so is not perfectly point-like.

Neutron Electric Dipole Moment All fermions have magnetic dipole moment (maybe not the neutrino…) and the QED calculation of this is one of the greatest achievements in theoretical physics and a triumph of the Standard Model of particle interactions. The EDM for the neutron will arise from an asymmetry in the spatial distribution of the electric charge (carried by quarks) relative to the spin axis. Prior to 1950 it was generally assumed that nuclei and elementary particles could not have an electric dipole moment (EDM), because both P and T would then be violated.

P σ + - d P Reflection of an Electric Dipole Moment Angular momentum is invariant under P transformation

P σ + - σ + - d d Parity symmetry is violated P Reflection of an Electric Dipole Moment Angular momentum is invariant under P transformation

T σ + - d T Reflection of an Electric Dipole Moment

T σ + - σ +- d d Time symmetry is violated (matter dominates universe)

First Experiment to Measure n EDM Purcel and Ramsey pointed out that there was no experimental evidence to support this assumption and they set out to measure the electric dipole moment of the neutron. Before 1957 most predicted values were exactly zero because P symmetry was assumed… Before 1964 most theories assumed T(=CP) symmetry and continued to predict zero…

QCD Prediction for n EDM The QCD Lagrangian This implies a neutron electric dipole moment: Quark mass matrix gluon-gluon interaction term violates CP The underlying theory of strong interactions predicts CP violating interactions, which has only been observed in the neutral K and B systems There is no evidence for the expected QCD-induced CP violation  eff = effective  parameter after the diagonalization of the quark masses G  a = gluon field strength Note that this is ~ because we neglect the other quark families. If any one of the quark masses =0 then EDM=0

Actual estimates of EDM vary from

Prediction of an electric dipole moment for the neutron in QCD: Why is the neutron electric dipole moment so small? Strong CP Problem The smallness of this term implies the numbers are either separately very small or somehow cancel each other to very high accuracy.The smallness of this term implies the numbers are either separately very small or somehow cancel each other to very high accuracy. Why is θ ~ arg Det (M) when θ originates in QCD and the quark mass matrix is set within electroweak physics? The numbers should be completely unrelated, and both could be expected to be ~ O[1]Why is θ ~ arg Det (M) when θ originates in QCD and the quark mass matrix is set within electroweak physics? The numbers should be completely unrelated, and both could be expected to be ~ O[1] STRONG CP PROBLEM  This is the STRONG CP PROBLEM Present experimental limit: More specifically:

Any attempt to explain the smallness of the theta term may be overambitious as long as we do not have an understanding of the origin of the seemingly arbitrary parameters of the standard model Whatever determines the “constants of nature”, the strong CP problem can be elegantly explained by the existence of a new physical field which allows theta to vanish dynamically. In this scheme, the CP-violating Lagrangian is literally switched off by its own force.

How to evade this problem? To make a slight extension of the Standard Model in such a way to introduce a special new symmetry rule => adding the “Peccei-Quinn Symmetry” Peccei-Quinn scaleKinetic term Existence of a new, massless pseudoscalar axion field interacting with the gluon field. Add new term to Lagrangian: Lagrangian (CP conserving) is invariant for adding new axion field term  eff is “absorbed” in the definition of a CP violating effects vanish (  potential energy contains CP violation) Adding PQ symmetry (PQ symmetry spontaneously broken in the vacuum) strong CP problem solved

The standard axion Weinberg PRL Jan Wilczek PRL Jan This new term in the Lagrangian conserves CP   axions assumed to be pseudoscalars (which are odd under CP) Even though axions were constructed to be massless they acquire an effective mass by their interaction with gluons which induces transitions to quark-anti-quark states and thus to neutral pions which means physically axions and neutral pions mix with each other. Predict axion

This model is excluded after extensive experimental searches for such an axion ant 1.8 MeV failed to show anything. This, and other variant models were ruled out by overwhelming astrophysical evidence, as well. Some fine points: The PQ scheme is incompatible with quantum gravity Axions occur naturally in superstring theories

In terms of the Peccei-Quinn scale f A, the axion couplings to photons the ratio of the electromagnetic and color anomaly, model-dependent parameter In the Dine-Fischler-Srednicki-Zhitnitskii(DFSZ) model or Grand Unified Theories axion => E/N=8/3 : DFSZ axions also couple to charged electron In the Kim-Shifman-Vainshtein-Zakharov(KSVZ) model or “hadronic axion” => E/N=0 : KSVZ axions couple to hadrons and photons with no axion-electron coupling The axion-photon coupling constant and the “invisible” axion Introduce a new scale so the axion coupling becomes weaker and the experimental limits are avoided Two photon Primakoff coupling, just like neutral pions

According to their origin...  Cosmological axions  Solar axions How would axions be produced in the Universe?

Astrophysical and cosmological exclusion regions for the axion mass m A, or equivalently the Peccei-Quinn scale f A Exclusion Range Plausible Dark-Matter Range

Sikivie’s great idea The Lagrangian with the axion field a permits the conversion of an axion into a single real photon in an external electromagnetic field.

 Solar axion flux [van Bibber PRD 39 (89)] Solar axions According to their origin...  Cosmological axions  Solar axions Solar physics + Primakoff effect Only one unknown parameter g a  Axion-photon conversion in the detector

Solar axions Principle of detection axions Transverse magnetic field (B) X ray detector AXION PHOTON CONVERSION L COHERENCE

Solar Axion Telescope ANtenna) (aka Solar Axion Telescope ANtenna)

Picture taken 14 th May 2002 Cryogenics installation East-side end of the magnet (looking towards sunset) Cold Box Control room Magnet power supply

Magnet pointing up +8º

Magnet pointing down -8º

Micromegas Very good spatial resolution 350  m X-Y strip pitch Extremely low threshold Low background (high rejection capability from spatial resolution) (Athens/Saclay/CERN) 6.4 keV Calibration spectrum

The CAST X-Ray Telescope: a spare unit from the ABRIXAS Space Mission  27 nested shells  Focal length 1.7 m  Fitted asymmetrically on CAST magnet opening CCD detector with effectively no background Telescope mounted on magnet (mechanical design)

Grazing Incidence X-Ray Telescope (see R. Giacconi, Sci. Amer. 242 (1980) 70) CAST X-Ray Telescope: X-rays from axion – photon conversion in the magnet are focused onto a ~1 mm diameter image at 1.7 m large increase in signal/noise from reduction of region where signal is expected

A high-energy axion detector for CAST (J.I. Collar, D. Miller, J. Vieira, EFI UofChicago)  Goal: extend sensitivity of CAST to axion-induced gammas from few tens of keV to ~150 MeV  Motivation: If new boson couples to nucleons, it can substitute for a  in plasma and nuclear processes [1]. Solar luminosity via axion emission can be as high as few % of total. Search with helioscope has not been performed before.  Weak experimental limits already exist from observed solar  flux below 5.5 MeV (a   following p + d  He + a) [2].  Other reactions of interest exist (e.g., 511 keV from e + + e -  a + , 477 keV from 7 Be+e -  7 Li * + e [3], etc.)  A generic search should not be limited to M1 transitions [4]. Should surpass sensitivity of searches for anomalous production of single  ’s in accelerators [5]. May surpass sensitivity to small branching ratios (~< ) in laboratory searches [6]. (calculation of expected sensitivity in progress)  Must be compact and non-intrusive, yet reach the lowest possible sea-level background and highest efficiency [1] G. Raffelt, "Stars as laboratories for fundamental physics", University of Chicago Press, Chicago and London (1996). [2] G. Raffelt and L. Stodolsky, Phys. Lett. B119, 323 (1982). [3] M. Krcmar et al., Phys. Rev. Lett. (hep-ex/ ) [4] G. Raffelt, Priv. Comm.. [5] C. Hearty et al., Phys. Rev. D 39(1989)3207. [6] A. V. Derbin et al., Phys. At. Nucl. 65 (2002)1335; M. Minowa Phys. Rev. Lett. 71(1993)4120.

Chicago calorimeter Support structure adjustable platform for alignment MicroMegas Detector X-Ray Telescope We have installed a low background gamma ray detector using a scintillating crystal to perform a search for high energy axions with CAST. The calorimeter will operate at sea level and be sensitive to photon energies 0.3 – 200 MeV. While this is out of the range of thermal axions produced inside the sun, the detector will be sensitive to axions emitted in M1 nuclear transitions or electron-positron annihilation with a branching ratio into axion emission.

Large low-background CWO inorganic crystal scintillator (0.6 kg) 200 keV threshold 200 MeV dynamic range (can increase this but efficiency becomes very low) 12.8% resolution at 835 keV measured in CAST area Muon veto efficiency > 97% > 90% livetime (measured with LED pulser) ~ 5 Hz raw counting rate measured in CAST experimental area in February 2004 Careful design and selection of detector and shielding materials

Side View CWO Crystal light guide Low-bckg PMT (< K gamma / day) PMT power base

Front View Side View CWO Crystal light guide Low-bckg PMT (< K gamma / day) PMT power base Ultra-low bckg Pb Incoming gammas (magnet bore) Pb shielding

Front View Side View Plastic Muon Veto CWO Crystal light guide Low-bckg PMT (< K gamma / day) PMT power base Ultra-low bckg Pb Incoming gammas (magnet bore) Pb shielding

Front View Side View Plastic Muon Veto CWO Crystal light guide Low-bckg PMT (< K gamma / day) brass support tube Thin brass end cap and window Boronated thermal n absorber PMT power base Ultra-low bckg Pb Incoming gammas (magnet bore) Rn displacement (N2 purge gas) Pb shielding

(minimal) Electronics, power + DAQ

Detector + Pb shielding+ Muon Veto+ thermal neutron absorber

Use of Pulse-shape background discrimination in lieu of additional shielding

Pulser “Spurious” Pulses Non-Rejected Muons

First Light (axions) from Calorimeter Large non-zero residuals evident before weighting by time spent per position Reasons: –Much more sensitive to gammas from walls and ground than x-ray detectors –Known large dependence of thermal “albedo” neutron flux on height above ground –Muon leackage through front opening in veto (larger above horizon) Ongoing analysis will clarify origin(s)