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in theoretical extensions of the standard model , all the new particles are not necessarily on the heavy side ; the prediction of very light ( sub - ev ) scalar or pseudoscalar particles that are very weakly interacting is actually quite generic .
looking for signatures of such `` axion - like particles '' ( or alps ) therefore represents another exciting possibility to probe the kind of physics there can be at very high scales , associated with these new degrees of freedom , which is complementary to collider searches .
it is thus not surprising that a lot of ongoing effort is made to scan the parameter space of these hypothetical particles , actively searched for mostly via their electromagnetic coupling .
current experimental developments include for instance light - shining - through - a - wall experiments such as alps , which is being upgraded  @xcite , or projects of next - generation helioscopes such as the international axion observatory  @xcite .
due to their mixing with photons , these spinless particles are moreover not only of interest in particle physics but also in astrophysics , as their existence would change the properties and the propagation of light emitted from distant sources .
a sizeable signal could be expected as the distances involved are huge , even if the alps are only coupled very weakly . in this context , various phenomena have in fact already been interpreted as possible astrophysical hints for the existence of alps , especially in the low - mass region of the parameter space : namely alps with masses @xmath0  ev and couplings to photons @xmath1@xmath2  gev@xmath3 . for a recent summary of the motivations for these particles from theory and of their implications , see refs .
@xcite and references therein .    in this work
@xcite , we consider light coming from quasars .
the fact that these high - luminosity active galactic nuclei ( agn ) are among the brightest and furthest steady light sources known in the universe makes them particularly appealing to look for signals of alps and to constrain their parameter space .
we focus on the window of astrophysical interest and show that constraints can be derived using both linear and circular polarisation data .
polarimetry is an extremely valuable tool to search for axion - like particles as the interaction of these particles with photons , in external magnetic fields for instance , would change the polarisation of light  @xcite .
suitable interaction lagrangians read @xmath4 and lead to similar phenomenological consequences ; henceforth we focus on pseudoscalars . in a nutshell ,
one expects from the mixing that even unpolarised light will develop a non - vanishing degree of linear polarisation and that , in general , linear and circular polarisation will convert at least partially into one another as a consequence of phase - shifts induced by the mixing . as is well - known and readily seen from eq .
, this is because , in an external magnetic field , such spinless particles only couple to one direction of polarisation  @xcite .    the mixing can actually be very efficient at modifying polarisation and can therefore be constrained by precise measurements .
the limits derived from the absence of rotation of the linear polarisation of uv light from agn  @xcite are a recent example of this .    in this work ,
we follow a different idea , which is to consider the spectroscopically defined quasar classes known to have the smallest intrinsic polarisations in visible light , and to compare the predictions of the mixing with observations . as already discussed by harari and sikivie in ref .
@xcite , light from distant sources should be characterised by at least some amount of polarisation if axion - photon mixing happens along the way .
we do this for both linear ( @xmath5 ) and circular ( @xmath6 ) polarisation .
quasar polarisation measurements at optical wavelengths can be found in the literature with uncertainties below 0.1% .
light from quasars is known to be intrinsically linearly polarised , as differences can be seen in different spectroscopic types , and the polarisation is at the 1%-level for the least polarised ones .
if there are many catalogs of linear polarisation measurements , the same can not be said about circular polarisation : it has rarely been studied , despite being measured in the same way as the linear one by simply adding a quarter - wave retarder plate . following the predictions of the mixing , new dedicated observations of quasar circular polarisation in visible light
have therefore been carried out  @xcite .
for observational reasons , most of the objects are located towards the north galactic pole direction , which points to the center of the local supercluster ; see _ e.g. _  @xcite .
no evidence for non - vanishing circular polarisation could be found at the 3@xmath7-level however  @xcite .
we will not repeat here the discussion leading to our subsample ( @xmath5 and @xmath6 ) or the method used to obtain our limits , as all the necessary details can be found in ref .
let us simply emphasise again that we have aimed at being as conservative as possible : taking only into account the influence of the magnetic field in the supercluster , allowing the magnetic field to get a longitudinal component most of the time ( and allowing other quantities to fluctuate ) , as well as considering initially unpolarised light to make sure that the final polarisation due to the mixing is not overestimated for example .    using a conservative method leading to robust results
, we can safely say that the mixing of light with axion - like particles is strongly constrained by the good - quality measurements of polarisation , and especially of circular polarisation , even when bandwidth effects are taken into account  @xcite .
however this might say something about the magnetic fields or the electron density , and not about alps .
this is why we provide here the evolution of our limit with the maximum transverse magnetic field strength and with the mean electron density .
the only reported detection available in the literature for the magnetic field in the local supercluster plane favours a collection of @xmath8 g magnetic field domains that are coherent over @xmath9  kpc  @xcite , averaging to a field 510 times weaker at the supercluster scale .
if we use this result , the 2@xmath7-limit we obtain is @xmath10  gev@xmath3 for @xmath11  ev ; as emphasised in our paper  @xcite , this limit can however be easily rescaled for any other value of the magnetic field strength as it always appears together with the coupling constant in the equations .    on the other hand , in order to give the evolution of our limit with the average electron density , we can take advantage of the fact that , for alp masses much smaller than the plasma frequency @xmath12 the two dimensionless quantities that determine the evolution of the stokes parameters then become independent of @xmath13 , leading to a plateau  @xcite . here , we give in fig .
[ fig : ap_electrondensity ] the evolution of this plateau for various values of the average electron density @xmath14 ; the case illustrated in the paper  @xcite is given by @xmath15  @xmath16 , which is the value usually considered in superclusters .
the information is summarised by saying that points in the parameter space are excluded at 1@xmath7 , 2@xmath7 and 3@xmath7 when the average probability that they contradict the observations is respectively 68.3% , 95.5% and 99.7% .
as anticipated in the paper , the limits on the coupling constant @xmath17 for nearly massless pseudoscalars would be more stringent for values of the plasma frequency smaller than the one we have considered , which reflects the fact that the mixing then becomes more efficient .
conversely , if the electron density in the local supercluster were to actually be much higher than that , then no constraint could be put on such particles as the mixing would then simply not take place in that medium .     on the average electron density .
the maximum transverse magnetic strength used here is @xmath18 g . ]
[ sec : figures ]
we thank the ifpa group for the financial support received to attend this workshop , and acknowledge useful discussions with jean - ren cudell and damien hutsemkers .
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we discuss the constraints derived on the mixing of photons with light pseudoscalars using the distributions of good - quality linear and circular polarisation measurements of light from the least polarised classes of quasars . 
 we also provide the dependence of our limit on the average electron density in the local supercluster for nearly massless particles .