Scalar fields with a "chameleon" property, in which the effective particle
mass is a function of its local environment, are common to many theories beyond
the standard model and could be responsible for dark energy. If these fields
couple weakly to the photon, they could be detectable through the "afterglow"
effect of photon-chameleon-photon transitions. The ADMX experiment was used in
the first chameleon search with a microwave cavity to set a new limit on scalar
chameleon-photon coupling excluding values between 2*10^9 and 5*10^14 for
effective chameleon masses between 1.9510 and 1.9525 micro-eV.Comment: 4 pages, 3 figure

Asztalos, S. J.

Bradley, R.

Carosi, G.

Clarke, J.

Hagmann, C.

Hoskins, J.

Hotz, M.

Kinion, D.

Martin, C.

Rosenberg, L. J

Rybka, G.

Sikivie, P.

Tanner, D. B.

van Bibber, K.

English

arXiv

The article of record as published may be located at http://dx.doi.org/10.1103/PhysRevLett.105.051801Scalar fields with a "chameleon" property, in which the effective particle mass is a function of its local environment, are common to many theories beyond the standard model and could be responsible for dark energy. If these fields couple weakly to the photon, they could be detectable through the "afterglow" effect of photon-chameleon-photon transitions. The ADMX experiment was used in the first chameleon search with a microwave cavity to set a new limit on scalar chameleon-photon coupling excluding values between 2*10^9 and 5*10^14 for effective chameleon masses between 1.9510 and 1.9525 micro-eV

Rosenberg, L.J.

Asztalos, S.J.

Tanner, D.B.

Calhoun, Institutional Archive of the Naval Postgraduate School

Calhoun: The NPS Institutional ArchiveFaculty and Researcher Publications Faculty and Researcher Publications2010A Search for Scalar Chameleons with ADMXG. Rybkahttp://hdl.handle.net/10945/35434arXiv:1004.5160v1  [astro-ph.CO]  28 Apr 2010A Search for Scalar Chameleons with ADMXG. Rybka, M. Hotz, L. J RosenbergUniversity of Washington, Seattle, Washington 98195S.J. Asztalos∗, G. Carosi, C. Hagmann, D. Kinion and K. van Bibber†Lawrence Livermore National Laboratory, Livermore, California, 94550J. Hoskins, C. Martin, P. Sikivie and D.B. TannerUniversity of Florida, Gainesville, Florida 32611R. BradleyNational Radio Astronomy Observatory, Charlottesville, Virginia 22903J. ClarkeUniversity of California and Lawrence Berkeley National Laboratory, Berkeley, California 94720(Dated: April 30, 2010)Scalar fields with a “chameleon” property, in which the effective particle mass is a function ofits local environment, are common to many theories beyond the standard model and could beresponsible for dark energy. If these fields couple weakly to the photon, they could be detectablethrough the “afterglow” effect of photon-chameleon-photon transitions. The ADMX experiment wasused in the first chameleon search with a microwave cavity to set a new limit on scalar chameleon-photon coupling βγ excluding values between 2 × 109 and 5 × 1014 for effective chameleon massesbetween 1.9510 and 1.9525 µeV.PACS numbers: 14.70.Bh,07.57.Kp,95.36.+xAstrophysical observations from a variety of sources allsuggest that the expansion of the universe is accelerating[1]. The negative pressure required for this phenomenon,under the name dark energy, can be interpreted as anonzero cosmological constant, but could also be the sig-nature of a light scalar field slowly rolling down a shal-low potential [2, 3]. Light scalar fields are ubiquitous inphysics theories beyond the standard model, but havebeen severely constrained by short-range gravity experi-ments [4].It has been suggested, however, that scalar fields withnonlinear self-interactions can have a “chameleon” prop-erty [5] which causes the effective mass of perturbationsto the field to be dependent on the local energy density.This effect can shield all but a thin shell of test massesfrom the new force carried by a scalar field, significantlyrelaxing bounds on couplings from gravity experimentswhile still offering a viable low mass dark energy can-didate on cosmological scales [6–8]. A possible effectivepotential for such a field is [7]Veff(φ, ~x) = Λ4 exp(Λnφn)+eβφMpl ρm(~x)+eβγφMpl ργ(~x). (1)Here φ is the chameleon field, β and βγ are unitless cou-plings to matter and photons, Mpl is the reduced Planck∗Currently at XIA LLC, 31057 Genstar Rd., Hayward CA, 94544.†Currently at Naval Postgraduate School, Monterey, CAmass (2.4 × 1018 GeV), ρm and ργ are the matter andelectromagnetic energy densities, and Λ and n are modelparameters, with Λ ' 3 × 10−12 GeV for dark energy.The field φ minimizes the potential at each location withsome value φ0(~x), and the mass of excitations of the fieldis thenm2φ(~x) '∂2∂φ2V (φ0(~x), ~x). (2)The experimentally accessible parameters are the cou-pling strengths β, βγ and the effective mass of thechameleon mφ inside the experiment.Scalar chameleons may have a different couplingstrength to the electromagnetic field than to matter [9].If the electromagnetic coupling is dominant, electromag-netic experiments searching for dark energy may be morefruitful than gravitational ones. For example, laser ex-periments utilize the unique nature of chameleons to lookfor the “afterglow” of photon-chameleon-photon tran-sitions [10, 11]. In general, these experiments involveshining a laser through a closed, empty container sub-jected to a large magnetic field. In the magnetic field,some photons from the laser mix into chameleons. Formodels in which the chameleon mass inside the walls ofthe container is much greater than that in vacuum, thechameleons are trapped because their effective mass inthe walls is greater than their total energy. If the mixingtime between chameleons and photons is longer than thetime the photons spend in the container, photons maybe detected for a time after the laser is turned off whilethe trapped chameleons mix back into photons, which225 dBmOscillatorReceiverCavityTuning Rod25 dBmOscillatorReceiverCavityTuning Rod25 dBmOscillatorReceiverCavityTuning Rod25 dBmOscillatorReceiverCavityStep 1 Step 2 Step 3 Step 4Electromagnetic modes Pure electromagnetic Afterglow from chameleon Tuning rods moved toexcited by source modes decay modes measured change frequencyFIG. 1: Chameleon search proceduresubsequently escape the container. Current limits fromsuch experiments exclude chameleon-photon couplings of5×1011 < βγ < 6.2×1012 for effective chameleon massesless than 1 meV [12]. The vacuum chameleon massescovered by this type of experiment are model dependent,since the presence of residual gas and the nearby con-tainer walls change the effective chameleon mass insidethe container.An alternative technique is to trap chameleons insidea microwave cavity. Microwave cavities operate at lowerenergies than lasers, but have the advantage that the res-onant nature of the cavity enhances the conversion prob-ability between photons and chameleons. As a result, mi-crowave cavity experiments can potentially be more sen-sitive to βγ . We use the Axion Dark Matter Experiment(ADMX) to demonstrate this potential improvement forthe first time.ADMX is a cavity search for dark matter axions [13].These axions are a consequence of the Peccei-Quinn solu-tion to the strong CP problem [14–16]. A full descriptionof ADMX can be found in Ref. [17], and recent resultsfrom this axion search in Ref. [18]. In brief, ADMX con-sists of a 220 liter cylindrical copper-plated microwavecavity situated inside a 7 tesla magnet. The cavity isheld under vacuum and maintained at a temperature of2 K produced by pumping on liquid helium. Two cop-per rods are used to tune the resonant frequencies of thecavity. When the TM010 resonant frequency of the cavityis tuned to correspond to the axion mass, the resonantmode will be excited by photons produced by the decayof dark matter axions. The excitation of the resonantmode will be detected by an antenna probe inside thecavity and amplified by a microwave receiver.As with axions, chameleons can mix with photonsin the microwave cavity. Unlike axions, the chameleonmechanism traps the chameleon scalars inside the cav-ity along with the photons [19]. In this case the cav-ity will contain both electromagnetic resonances andchameleonic resonances, and the two will mix. Conse-quently, the same technology ADMX uses to search foraxion to photon conversion can be used to search forchameleon afterglow.For the purposes of this analysis, it is assumed thatthe effective chameleon mass in the walls of the mi-crowave cavity is much larger than the effective massinside the cavity, yielding Dirichlet boundary conditionson the wave function. Model dependent effects can mod-ify this assumption, shrinking the effective cavity radiusfor chameleons, and changing their effective mass. Fora detailed analysis of chameleon behavior in cavities, seeRef. [20].Chameleon and photon mode mixing is maximizedwhen the modes have the same frequency. In the presentADMX cavity, this mixing should be most easily achievedbetween the TE011 electromagnetic cavity mode, whichcan be tuned between 850 and 950 MHz, and the lowestchameleon mode, which has a frequency that is the sumin quadrature of the effective chameleon mass (mφ) andwave number (kφ). The position of the tuning rods in-side the cavity can be moved to change the TE011 modefrequency and the lowest chameleon mode wave numberby different amounts, and thus can be used to probe dif-ferent chameleon masses.The procedure used to search for chameleons in ADMXwas as follows (Fig. 1):1. The TE011 electromagnetic mode was excited bydriving the antenna with an external power sourcewith a frequency swept over 20 kHz, roughly thewidth of the cavity resonance, for 10 minutes. Dur-ing this time period, if a chameleon mode were tooverlap with the TE011 mode, some energy wouldbe transferred to the chameleon mode.2. The external source was switched off and the firststage amplifier of the experiment was switched on.In the duration of this switching period (100 ms),the conventional electromagnetic modes decayed.3. The power spectrum within 20 kHz of the TE011cavity resonance was recorded for 10 minutes. If achameleon mode had been excited in the previousstep, its decay could be visible as an electromag-netic mode excitation.4. The tuning rods in the cavity were moved tochange the frequency of the TE011 mode, makingit sensitive to a slightly different range of effectivechameleon masses3FIG. 2: Predicted excess power in the ADMX experimentfrom decaying chameleon modes immediately after turn onof the first stage amplifier as a function of chameleon-photoncoupling strength βγ . Smaller βγ power is limited by couplingstrength, larger βγ power is limited by decay time. Noise floorcorresponds to 3 K system temperature.Following the prescription used in Refs. [13, 21], ifβγ/Mpl is sufficiently small, and the rate of chameleonloss to the cavity walls is negligible, the rate of mixing be-tween the lowest chameleon mode mixing with the TE011mode would beΓ =β2γf2B2Qk2trM2plω3, (3)where βγ andMpl are as defined above, f is a form factor(the overlap between the chameleon mode and the TE011mode which is roughly 0.43 in the case of ADMX), Bis the magnetic field strength, Q is the cavity quality(around 10,000 for ADMX at this mode), ω is the driv-ing frequency (around 900 MHz), and ktr is the wavenumber of the chameleon mode transverse to the appliedmagnetic field. The power detected in the cavity electro-magnetic modes from chameleon decay would bePout = PinpiΓ2b(1− e−Γ2t0)2e−Γt, (4)where Pin is the excitation power, b is the bandwidthover which the driving frequency is swept (20 kHz in thisexperiment), t0 is the duration for which the cavity hasbeen excited, and t is the time elapsed since the cavityexcitation has ceased. This is valid only when the sweepbandwidth is much larger than the chameleon resonancewidth, and the chameleon mode decay rate is smallerthan the electromagnetic mode decay rate.The power excess would appear in the power spec-trum as a peak at frequency ω =√k2φ +m2φ with awidth Γ. The power observed decreases exponentiallywith observation time as the chameleon mode decayswith rate Γ. The expected excess power in the ADMXexperiment immediately after turning on the first stageamplifier as a function of coupling strength is shown inFig. 2. The signal-to-noise ratio of a chameleon signal,which determines its detectability, is given by the sig-nal power (Eq. 4) divided by the system noise tempera-ture of the experiment. The physical temperature of theADMX cavity was 2 K at the time of data taking, and theSQUID (Superconducting Quantum Interference Device)amplifier[22] had a noise temperature of 1 K, yielding a3 K system noise temperature.The above discussion assumes that decay into pho-tons is the dominant energy loss mechanism for excitedchameleon modes. Short range gravity experiments havelimited the effective force between chameleons and mat-ter to be weaker than the gravitational force, making en-ergy loss to the walls negligible [23]. Therefore, the vastmajority of chameleons must eventually decay into pho-tons. As the wavelength of the chameleon mode is sim-ilar to that of the electromagnetic mode, both of whichare much larger than the scale of any penetrations intothe cavity, the bulk of the photons from chameleon de-cay are produced inside the cavity where they can bedetected. For this experiment, this translates to an as-sumption that the chameleon decay rate through meansother than mixing with photons is less than 10−3 Hz, oran unloaded Q of greater than 1012, not far from thatachievable in superconducting microwave cavities [24].There are two ways for a chameleon signal to be missed.First, if the coupling is too weak, too little power is trans-ferred from the electromagnetic mode to the chameleonmode and back to be detected. Second, if the couplingis too strong, the chameleon mode can completely decayaway in the time between the turn-off of the excitationand turn-on of the amplifier, and be indistinguishablefrom the the decay of the electromagnetic mode.The procedure outlined here was followed with theADMX experiment using a 25 dBm oscillator as the ex-citation source with a total integration time of 3.6× 104seconds. The attenuation from the source to the cavityantenna was measured to be 28 dB, making the exci-tation power about 0.5 mW. Given that no statisticallysignificant power excess was observed, chameleon-photoncouplings of 2×109 < βγ < 5×1014 could be excluded at90% confidence over a mass range spanning 1.9510 µeVto 1.9525 µeV, as shown in Fig. 3. As a reminder, themass listed in the figure is the effective mass in the cav-ity, where the magnetic field was 7.1 T and the pressurewas 10−6 torr. The exact relation between this effectivemass and true chameleon vacuum mass depends on one’schoice of model.Compared to previous limits, the limit set by ADMXimproves the lower bounds on chameleon-photon cou-pling by several orders of magnitude, but is valid only fora narrow range of effective masses. The range of massesexplored was limited by the time spent running; with a4FIG. 3: Shaded region: 90% confidence limit excluded pa-rameters for scalar chameleons from ADMX search. Dashedlines: upper and lower exclusion bounds from Ref. [12].longer time, more chameleon masses could be exploredat a rate of 10−3 µeV per day at the same sensitivity.In summary, we used ADMX to demonstrate the via-bility of microwave cavity searches for chameleon scalars.Couplings of 2 × 109 < βγ < 5 × 1014 were excluded forchameleons with an effective mass in the cavity between1.9510 and 1.9525 µeV. This technique is sensitive onlyto a narrow range of masses at each tuning setting, soit is most useful if a precise theoretical prediction canbe made, or to confirm potential positive signals seen inother chameleon searches, such as those performed withlasers or short-range gravity experiments.ADMX will be upgraded soon from a system noise tem-perature of 3 K to an improved noise temperature of200 mK by cooling the cavity to 100 mK, reducing theblack body noise, and by lowering the temperature of theSQUID amplifier to 200 mK [25]. 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A Search for Scalar Chameleons with ADMX

G. Rybka

M. Hotz

L. J Rosenberg

S. J. Asztalos

G. Carosi

C. Hagmann

D. Kinion

K. van Bibber

J. Hoskins

C. Martin

P. Sikivie

D. B. Tanner

R. Bradley

J. Clarke

Crossref

Search for Chameleon Scalar Fields with the Axion Dark Matter Experiment

Scalar fields with a&quot;chameleon&quot; property, in which the effective particle mass is a function of its local environment, are common to many theories beyond the standard model and could be responsible for dark energy. If these fields couple weakly to the photon, they could be detectable through the afterglow effect of photon-chameleon-photon transitions. The ADMX experiment was used in the first chameleon search with a microwave cavity to set a new limit on scalar chameleon-photon coupling beta_gamma excluding values between 2x109 and 5x1014 for effective chameleon masses between 1.9510 and 1:9525 micro eV

UNT Digital Library

A Search for Scalar Chameleons with ADMX

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Calhoun, Institutional Archive of the Naval Postgraduate School

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