Challenges and Opportunities for the Next Generation of Photon Regeneration Experiments
aa r X i v : . [ h e p - ph ] M a r Challenges and Opportunities for the NextGeneration of Photon Regeneration Experiments
Andreas Ringwald
Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany
Abstract.
Photon regeneration experiments searching for signatures of oscillations of photons intohypothetical very weakly interacting ultra-light particles, such as axions, axion-like and hidden-sector particles, have improved their sensitivity considerably in recent years. Important progressin laser and detector technology as well as recycling of available magnets from accelerators mayallow a big further step in sensitivity such that, for the first time, laser light shining through a wallexperiments will explore territory in parameter space that has not been excluded yet by astrophysicsand cosmology. We review these challenges and opportunities for the next generation experiments.
Keywords:
Low energy experimental particle physics, extensions of the Standard Model, axions,extra gauge bosons, hidden-sector matter particles
PACS:
Over the last few years, it became more and more clear that precision experimentsexploiting low-energy photons may yield information on particle physics complemen-tary to experiments at high-energy colliders, in particular on the possible existence ofnew very weakly interacting sub-eV particles (WISPs), such as axions [1], axion-likeparticles (ALPs), and hidden-sector particles (hidden photons [2], minicharged parti-cles [3]), predicted in many extensions of the Standard Model [4, 5]. The report by thelaser polarization experiment PVLAS of the observation of an anomalously large rota-tion of the polarization plane of photons after the passage through a magnetic field [6]– which may be interpreted as evidence for photon disappearance due to conversioninto WISPs [7, 8, 9, 10, 11] – provided the impetus for a number of laser light shin-ing through a wall (LSW) experiments. The latter are searching for photon → WISP → photon conversions (cf. Fig. 1) rather than solely for disappearence, to perform an FIGURE 1.
In LSW experiments, laser photons are sent along a beam onto a wall where they areabsorbed. Some of the photons may converted into WISPs that propagate freely through the wall andreconvert into photons after the wall. LSW may occur due to various processes beyond the StandardModel: g ↔ ALP oscillations in a background magnetic field [12, 13, 14] (left), g ↔ g ′ oscillationsfacilitated by a non-zero mass of the hidden photon ( g ′ ) [2] (middle), and g ↔ g ′ oscillations facilitatedby virtual mini-charged particles in a background magnetic field [10, 11] (right). (From Ref. [4].) independent test of the WISP hypothesis [15, 16, 17, 18, 19, 20, 21] und to improve theconstraints from the pioneering experiment BFRT [22] by about an order of magnitudein the WISP–photon coupling (cf. Fig. 2). Moreover, the momentum gained by theseexperiments towards the establishment of a new low-energy frontier of particle physics MVBFRT
OSQAR
ALPS H preliminary L GammeV - - - - m - @ eV D g - @ G e V - D C ou l o m b F I R A S + h C M B CASTALPS H preliminary L BMVGammeVLIPSS - - - - m Γ ' @ eV D Χ OSQAR
ALPS H preliminary L BFRTBMVGammeVLIPSS - - - - m MCP @ eV D Q FIGURE 2.
Upper bounds from LSW experiments for couplings of pseudoscalar axion-like particles(two photon coupling g ; left panel), massive hidden photons (kinetic mixing c ; middle panel), andmassless hidden photons with an additional minicharged particle (charge Q = c ; right panel). The resultsfrom ALPS are preliminary. Compilation from Ref. [23]. turned out to be conserved even though the original motivation disappeared: the PVLAScollaboration could not reestablish their first observation after an upgrade of their appa-ratus [24]. This is in-line with the finding of the above mentioned LSW experiments.Now, the planning for the next generation of photon regeneration experiments hasstarted. At this stage, it seems to be very helpful to identify targets in WISP parameterspace upon which the next generation of experiments can shoot. In this context, one canclearly identify both • challenges: increase sensitivity beyond astro, cosmo, and other lab bounds, and • opportunities: test WISP interpretation of hints for cosmic photon regeneration,that we will discuss in detail in the following.For hidden photons, laser LSW experiments are in a comfortable position, as is il-lustrated in Figs. 2 (middle) and 3: already by now, they are exploring previously un-touched parameter space, bearing therefore the greatest immediate discovery potential.The cosmo constraint arising from the upper limit on the effective number of relativisticdegrees of freedom contributing to the cosmic radiation density in the era between bigbang nucleosynthesis and recombination [26] (grey area in Fig. 2 (middle)) as well asthe constraint arising from a search for photon regeneration due to solar hidden photonsin the CAST helioscope [27] (purple area in Fig. 2 (middle)) are not competitive withLSW limits in the ∼ meV mass range.This is, however, only true if there is no light physical hidden Higgs particle involved,i.e. if the hidden photon gets its mass from a Stückelberg mechanism. Otherwise, if thehidden photon mass arises via a Higgs mechanism, the physical hidden Higgs effectivelyacts as a minicharged particle, with charge Q = c e h / e , where e h is the gauge couplingof the hidden photon, and the strong astro bound Q . − , for a sub-keV hiddenHiggs mass, inferred from the lifetime of red giants applies [28]. In particularly wellmotivated LARGE volume string compactifications, the gauge coupling of the hiddenphoton can be hyperweak, i.e. diluted due to the volume of the extra dimensions, to e h ∼ − , for a volume corresponding to an intermediate string scale M s ∼ GeV [29].
IGURE 3.
Summary of astrophysical, cosmological and laboratory constraints for hidden photons(kinetic mixing c vs. mass m g ′ ) (Adapted from Ref. [25], where also details can be found.). Therefore, the limit on minicharged particles excludes c & few × − , at low masses, m g ′ ∼ m H h . keV (cf. Fig. 4). Thus, the discovery potential for hidden photons wouldbe increased dramatically if we were able to probe such low values of c with the nextgeneration of laser LSW experiments.Fortunately, this seems doable. The current state-of-the-art LSW experiment ALPS,exploiting an optical resonator at the generation side of the experiment, resulting in apower of ∼ . g → WISP conversions, established an upper limit P LSW . few × − on the LSW probability, corresponding to an upper limit c . few × − in the meV mass range. Exploiting additionally a high finesse ( ∼ )optical resonator also on the regeneration side of the experiment [30, 31] and a single- FIGURE 4.
Prediction of hidden photon kinetic mixing c with the visible photon vs. its mass m g ′ fromLARGE volume string compactifications. The grey area is excluded by hidden photon searches alone.The bright red region predicted for hyperweak hidden photons whose mass arises from a hidden Higgsmechanism takes already into account the astro and cosmo constraints from minicharged particles [28].(Compilation from Ref. [25].). hoton counter, together with an increased power buildup, by a factor of ∼ ∼ + + = c by ∼ / = c ∼ few × − .Such values, at somewhat smaller masses, can also be probed by microwave cavityvariants of the LSW technique [32, 33, 34], which are currently set up [35, 36, 37],and, at somewhat larger masses, by especially designed helioscopes to search for solarhidden photons [38], which are also under consideration (see, e.g., Ref. [39]).Let us turn now to axions and ALPs. Although much less model dependent [40], thevalues of the two photon coupling g of ALPs probed by the current generation of LSWexperiments, g & few × − GeV − , for masses below an meV, falls short, by nearly WD energy loss
LSW 1987a Y ® invisible Γ- burst 1987a e + e - ® invisible HB stars
ADMXCAST + SUMICO B ea m du m p Γ transparency C D M - - - - - - - - - - Log m a @ eV D L og g @ G e V - D FIGURE 5.
Left:
Summary of cosmological and astrophysical constraints for axion-like-particles (twophoton coupling g vs. mass m a of the ALP). Note that the mass region, where the axion can be the colddark matter (the orange regions labeled “CDM"), can be extended towards smaller masses by anthropicreasoning. Also other areas with interesting astrophysical hints, e.g. the one for a non-standard energy lossin white dwarfs [45] or the one for an anomalous g -ray transparency of the universe [46, 47], are markedin orange. The parameter range for the axion is shown hatched. Note that the limit from the microwavecavity axion dark matter search experiment ADMX [48] is valid only under the assumption that the localdensity of ALPs at earth is given by the dark matter density. (Compilation from Ref. [4], where also detailscan be found.) Right:
Prospected sensitivity of a laser LSW experiment exploiting 6+6 Tevatron magnetsand resonantly enhanced photon regeneration [49]. three orders of magnitude, to the strong limits established by lifetime considerations ofhorizontal branch stars [41, 42] and by limits on photon regeneration due to solar ALPsreported by the helioscopes CAST [43] and SUMICO [44] (cf. Fig. 5 (left)). Here, thenext generation of LSW experiments has to gain about three orders of magnitude inthe coupling to start to enter in previously unexplored territory. In addition to abovementioned improvements from the laser and detector side, one has to increase B × L ,the magnitude times the length of the magnetic field region, by one order of magnitudecompared to the current experiments, e.g. by exploiting 5+5 HERA magnets at ALPS,instead of the current 1/2+1/2 configuration. With such improvements, a sensitivity inthe g ∼ few × − GeV − range, for light ALPs, m f ≪ meV, should be achievable [49,50, 51, 52]. For the sensitivity of a similar setup proposed in Ref. [49], exploiting 6+6Tevatron magnets, see Fig. 5 (right).n even wider range of opportunities for discovery would open up if the sensitivityin g can be improved even more, by one order of magnitude, down to g ∼ few × − GeV − , possibly by a combination of laser and magnet upgrades.First of all, ALPs with such a coupling may be motivated from a top-down perspectivearising from string theory. In fact, massless ALPs, with coupling to photons in the g ∼ a / M s ∼ − ÷ − GeV − range, could occur naturally in string compactificationswith an intermediate string scale M s ∼ ÷ GeV.Secondly, there are a number of puzzling astronomical observations which may becommonly explained by cosmic photon oscillations into very light ALPs with g in theabove range (cf. also Fig. 5 (left)). Indeed, photons emitted by distant sources andpropagating through cosmic magnetic fields can oscillate into ALPs, with a numberof consequences in different situations (see, e.g., Refs. [53, 54, 55, 56, 57, 58, 59, 60,61, 62, 63]). Interestingly, ALPs may leave their imprints in luminosity relations ofactive galactic nuclei [64, 65]. In fact, mixing between photons and ALPs in the randommagnetic fields in galaxy clusters induces a characteristic scatter in the relations of X-rayvs. optical luminosities of compact sources in these clusters. Evidence for such an effecthas recently been found in an analysis of luminosity relations of about two hundredactive galactic nuclei, providing a strong hint for the possible existence of a very light, m f . − eV, ALP, with a coupling in the g ∼ − ÷ − GeV − range.This range is also the sensitivity of another astrophysical probe of ALPs, namelythe spectra of cosmologically distant TeV g -ray sources. In fact, recent observationsof a few of them by ground-based gamma ray telescopes have revealed a surprisingdegree of transparency of the universe to very high-energy photons, which seems topoint to less absorption due to pair production, may be due to a less dense extragalacticbackground light and/or to a harder injection spectrum at the sources than initiallythought. However, there is also the intriguing possibility to explain this puzzle throughphoton ↔ ALPs oscillations in the cosmic magnetic fields, again requiring a coupling inthe g ∼ − ÷ − GeV − range [46, 47] (cf. Fig. 5 (left)). The present status of thisaffair is far from conclusive, however. It seems that much more data from many morequite distant TeV gamma sources along different directions in the sky has to be collectedbefore one may be able to perform a systematic search for hints of ALPs [66]. For thisincrease in statistics, we have to wait, however, for the realization of the big TeV gammaray array CTA. It would be great, if we were able to probe the same range of parameterseven earlier in the laboratory, by laser light shining through a wall! REFERENCES
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