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hidden sectors are often predicted in string theories and contained in various supersymmetric models as the source of susy breaking . as the hidden sector ( hs )
is not charged under the standard model ( sm ) gauge groups and vice versa the two sectors are not directly connected and only interact via messenger particles .
an example for a messenger is the hidden photon @xmath0 which is a frequent feature of sm extensions as extra hidden u(1 ) symmetries for example often remain in the breaking of larger gauge groups or appear in string compactifications .
on the other hand there are several observations in indirect and direct dark matter detection experiments like pamela , fermi , dama and cogent which favour dark matter ( dm ) models with light messenger particles .
such a messenger , despite mediating the dm scattering on nuclei , most importantly ensures that the dm annihilation is at the same time leptophilic and greatly enhanced by the sommerfeld effect .
a gev - scale mass for the hidden photon can be obtained quite naturally both through the stckelberg and the higgs mechanism .
the former , being the simplest mechanism to give mass to abelian gauge bosons , can give for example in certain string compactifications with d7-branes a mass to the hidden photon according to @xmath1 , which depends on the volume of the extra dimension , i.e. the string - scale @xmath2 , and the planck scale @xmath3  @xcite .
for intermediate string - scales @xmath4 gev , which are preferred by the axion decay constant and susy breaking scales , this leads to @xmath5 gev - scale . in the case of the higgs mechanism where the symmetry breaking is transferred by the kinetic mixing from the visible to the hidden sector the hidden photon mass
can be estimated by @xmath6  @xcite .
assuming that the kinetic mixing @xmath7 is generated supersymmetrically at high scales , without light fields charged under both u(1)s , it is of the order of a loop factor  @xcite and we impose the following relation with the hidden gauge coupling @xmath8 @xmath9 where @xmath10 is an @xmath11 factor leading typically to @xmath12 and thus @xmath5 gev - scale .
we consider the most simple hidden sector containing only an extra u(1 ) symmetry and the corresponding hidden photon @xmath0 which kinetically mixes with the ordinary photon .
the most general lagrangian for such a scenario , including a mass - term @xmath13 for the hidden photon and the kinetic mixing between @xmath14 and @xmath0 parameterized by @xmath7 , is given by @xmath15 where @xmath16 is the field strength of the ordinary electromagnetic field @xmath17 and @xmath18 the field strength of the hidden u(1 ) field @xmath19 .
the hidden photon can be constrained by and searched for in experiments through its coupling to sm fermions which is possible due to the kinetic mixing with the photon .
sm precision measurements ( sm pm )  @xcite , the muon and electron anomalous magnetic moment ( @xmath20 )  @xcite and a reinterpretation of the babar search for the @xmath21 decay to a pseudoscalar in the process @xmath22s
@xcite exclude large values of kinetic mixing as shown in grey in the left plot of fig .
[ fig - hplimits ] .
fixed - target experiments where @xmath0 can be produced in    r0.69         bremsstrahlung off @xmath23-beams place additional constraints from the non - observation of the decay @xmath24 .
limits from past @xmath25-beam dump searches that have been studied in  @xcite are shown in the left plot of fig .
[ fig - hplimits ] as grey areas .
the same graph also contains as coloured lines the new limits set by currently running ( mami  @xcite , kloe-2  @xcite , apex  @xcite ) and reanalyzed older experiments ( serpukhov  @xcite , orsay  @xcite ) .
experiments are closing in on the remaining region of the parameter space and more is to be tested in the ( near ) future by presently operating or planned fixed - target experiments at jlab ( apex  @xcite , hps  @xcite , darklight  @xcite ) , mainz ( mami , mesa  @xcite ) and desy ( hips  @xcite ) .
the expected sensitivities of those experiments are shown as coloured lines in the right plot of fig .
[ fig - hplimits ] .
the results of this section are based on an analysis that has been presented in detail in  @xcite .
the simplest possible extension of the hidden sector studied in the previous section is the addition of a dirac fermion as dark matter candidate ( cf .  @xcite ) . as the hidden photon mediates both the dm annihilation and the dm scattering on nuclei it is essential for the determination of the dm relic abundance and direct detection rate respectively .
r0.34        using relation  ( [ eq - kappa ] ) with @xmath26 we find that for a dm mass of 6 gev the correct relic abundance can be obtained on the dark green stripe in fig .
[ fig - hdm - hplimits ] while in the light green area the contribution to the total dm density is only subdominant .
the spin - independent ( si ) scattering on nuclei of the dirac fermion dm can explain the cogent  @xcite signal in the purple band ( 90% cl lighter 99% cl darker purple ) .
the plotted cross section @xmath27 has been rescaled by the relic abundance for subdominant dm and fits the one found in  @xcite to be compatible with cogent for a standard halo model .
constraints from cdms  @xcite and xenon  @xcite are not shown as they do not apply to dm masses as light as 6 gev .
the excluded grey areas and coloured lines for sensitivities of future experiments are the same as in fig .
[ fig - hplimits ] .
a scan over the dm mass allows to fill the complete parameter space as shown in fig .
[ fig - scattplot ] on the left for @xmath28 where dark green corresponds to the correct relic abundance , light green to subdominant and purple to cogent allowed points ( all points are compatible with other direct detection limits ) .
the effects of varying the dm mass , the parameter @xmath10 and the details of the halo model have been studied in more detail in  @xcite .      as a more sophisticated and better motivated model
, we consider three chiral superfields @xmath29 with @xmath30 and @xmath31 charged under the hidden u(1 ) . taking the superpotential @xmath32 and the dimensionless coupling  @xmath33
this is the simplest anomaly - free model possible without adding dimensionful supersymmetric quantities .
while we assume the mssm in the visible sector , the dm phenomenology of the hs depends on the details of the susy breaking .
-2 ( dash - dotted ) similar to fig .
[ fig - hplimits].,title="fig:",scaledwidth=33.0% ] -2 ( dash - dotted ) similar to fig .  [ fig - hplimits].,title="fig:",scaledwidth=33.0% ] -2 ( dash - dotted ) similar to fig .
[ fig - hplimits].,title="fig:",scaledwidth=33.0% ]    if the breaking of the hidden gauge symmetry is via the effective fayet - iliopoulos term induced in the hs by the kinetic mixing with the visible higgs d - term we find that the dm can be either a dirac or a majorana fermion .
the former has similar prospects for si scattering as in the toy model as shown in the middle plot of fig .
[ fig - scattplot ] where the parameter @xmath10 has been scanned over in the range @xmath34 .
the axial couplings of the latter lead to dominantly spin - dependent ( sd ) scattering which is partly constrained by experiments but without any chance of explaining any of the signals in si direct detection experiments . in the case where the hidden gauge symmetry breaking is induced by the yukawa coupling @xmath33 , we find only a majorana fermion as lightest particle in the spectrum . as
before this only shows in sd direct detection experiments , hence the missing purple colour for cogent in the right plot of fig .
[ fig - scattplot ] .
hidden sectors are motivated by various aspects from top - down ( string theory , susy ) and bottom - up ( dm ) and have a potentially rich content like dark forces and dark matter which despite their weak coupling can be phenomenologically interesting .
we reviewed past constraints on hidden photons as dark force and showed the reach of future experiments . a simple toy model for a hs with dm
is found to be consistent with relic abundance and direct detection and additionally provides the correct cross section for explaining cogent .
the better - motivated supersymmetric hidden sectors show some similarities with the toy model but have a more complicated phenomenology where spin - dependent scattering must also be taken into account .
nevertheless , they also give viable models for dm with interesting signatures in experiments .
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hidden sectors in connection with gev - scale dark forces and dark matter are not only a common feature of physics beyond the standard model such as string theory and susy but are also phenomenologically of great interest regarding recent astrophysical observations . 
 the hidden photon in particular is also searched for and constrained by laboratory experiments , the current status of which will be presented here . furthermore , several models of hidden sectors containing in addition a dark matter particle will be examined regarding their consistency with the dark matter relic abundance and direct detection experiments .