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We report preliminary results from a search for galactic axions in the frequency range 1.09 < f a < 1.22 GHz. For an axion linewidth V a < 200 Hz we obtain the experimental limit (g arr /m a ) 2 pa <1.4xl0" 41 . The theoretical prediction is (g ar y/m a ) 2 p a = 3.9x 10 ~4 4 with p a =300 MeV/cm 3 . We have also searched for the presence of a continuous spectrum of light pseudoscalar particles; if we assume that the above p a is contained between the upper and lower frequencies of our search, then we find thatg flyr <2xl0-30 MeV ,/2 cm 3/2^1 0" 11 GeV" 1 .PACS numbers: 98.60. Df, 14.80.Gt, 98.80.Es On the basis of rotation curves of galaxies and galactic clusters, it is widely believed today that the total mass in the Universe greatly exceeds the observable luminous matter. Also, arguments concerning primordial nucleosynthesis of the light elements limit the total baryonic matter to less than 0.2 times the critical density required to close the Universe. Little is known about the missing or "dark" matter but many candidate constituent particles have been proposed, some of them exotic.* A leading candidate is a very light pseudoscalar particle, the so-called "invisible axion." 2In 1977 Peccei and Quinn 3 (PQ) introduced a new global symmetry which is spontaneously broken in such a way as to cancel the CP-nonconserving terms that are present in the QCD Lagrangean but not observed. Weinberg 4 and Wilczek 4 pointed out that the breaking of the PQ symmetry must give rise to a pseudoscalar Goldstone boson named the axion. Accelerator searches 5 have failed to detect axions with mass greater than 200 keV, but models with light, weakly interacting axions have been proposed. 2 In these models, the mass of the axion and its coupling to fermions are inversely proportional to the vacuum expectation energy f a at which the PQ symmetry is broken. Specifically,where f n = 93 MeV and m^ = 135 MeV. While m a is not predicted by theory, observations on the cooling rate of ordinary stars 6 imply that m a < 1 eV, while neutron stars 7 imply that m a <4xl0 -2 eV; the best upper limit of m a < 1 x 10 ~2 eV comes from redgiant evolution. 8 At the same time cosmological considerations 9 place a lower limit of m a > 10 ~5 eV. In this window m a~~\ 0~s eV is preferred if the axions are to close the Universe. Axions should have been produced in the early stages of the Universe and later condensed into the galaxies with their present velocity being equal to the virial galactic velocity. Turner has calculated the expected axion density near the Earth on the assumption that galactic halos are due primarily to the presence of axions. 10 He finds that

=5xl0~2 5 g/cm 3^3 00 MeV/cm 3 .The axion couples to fermions and, through a triangle graph, to two photons [see Fig. 1(a)]. Sikivie 11 proposed that, in spite of the extremely weak coupling, axions could be converted to photons in the presence of an intense electromagnetic field via the PrimakofT effect 12 [ Fig. 1(b)]. The interaction Lagrangean is given by Lint = ~~ (#a yy /4;r)E-B(p...

A microwave cavity experiment designed to search for the signal from cosmic axions converting in an external magnetic field covered the mass range (4.5-16.3) X eV, corresponding to the frequency range from 1.09 to 3.93 GHz. Upper limits on the coupling and abundance of nonrelativistic galactic axions have been measured; these limits yield a coupling which is 1-2 orders of magnitude higher than that predicted by the Dine-Fischler-Srednicki model. Also presented are limits on axions with a continuum spectrum, limits on the presence of a wide axion line at the frequency of the 21-cm hydrogen emission line, and limits on the production of pseudoscalar particles by photons in the cavity interacting with the magnetic field.

We have searched for the regeneration of photons propagating in a transverse magnetic field. Such an effect would reveal the existence of light scalar or pseudoscalar particles such as the axion that couple to two photons.We obtain for this coupling the limit gaTr < 1.3 x lo6 GeV)-I, provided the axion mass m, 6 10m3 eV.

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