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there are currently a number of experimental candidates for light @xmath0 hybrid mesons  @xcite .
the review of the experimental results by page  @xcite , suggests that a @xmath0 state may exist with a mass around @xmath1 gev .
the predictions of lattice qcd , by both milc  @xcite and ukqcd  @xcite predict the lightest @xmath0 state to be around 2 gev ( with large errors ) .
the inclusion of dynamical fermions  @xcite has not produced results substantially different from those of quenched calculations . to start to check for systematic errors , we have repeated our original simulations  @xcite of the hybrid meson spectrum that used wilson fermions , with improved clover quarks .
we have calculated the hybrid meson spectrum at two different @xmath2 values : @xmath3 , with a lattice volume of @xmath4 , and @xmath5 with a lattice volume of @xmath6 . at @xmath7
we used a clover coefficient obtained from tadpole improved perturbation theory , using the plaquette value of @xmath8 . at @xmath9
we used the non - perturbative value of @xmath10 calculated by the alpha collaboration  @xcite .
in table  [ tb : clightresults ] , we report our preliminary results for the mass of the @xmath0 hybrid in the chiral limit . at this stage in our analysis , we have not attempted to estimate the systematic errors in our results from the clover action . to set the scale
, we used the value of @xmath11 from the interpolating formulae published in  @xcite ( this changes the number for the wilson data slightly from our previously quoted number  @xcite ) .    .mass
results for the state ( in mev ) at kappa critical , for the wilson and clover actions , at . [ cols="^,^",options="header " , ]     -8 mm
to study the internal distribution of quarks and glue inside a hybrid meson , we measured @xmath12 where @xmath13 is the field strength tensor and we have suppressed the contraction of @xmath13 with the @xmath14 gamma matrix .
we fix to coulomb gauge , measure a correlator with eq .
[ eq : hybdefn ] at the sink with our standard hybrid meson source  @xcite at time slice @xmath15 .
the operator in eq .
[ eq : hybdefn ] is similar to the one used to measure the wave function of the proton  @xcite , except that one of the quarks is replaced by the field strength tensor .
although the wave function operator is difficult to interpret in terms of constituent gluons , it makes sense in terms of a fock space analysis .
-9 mm    -8 mm    for our preliminary results we use @xmath16 .
however , it will be interesting to measure the wave function with non - zero @xmath17 in eq .
[ eq : hybdefn ] .
one approach to studying the heavy hybrids is to solve the schrdinger equation for the quarks with an excited potential , measured in a lattice simulation  @xcite .
the resulting wave functions do not have any dependence on @xmath17 .
a study of the dependence of the operator in eq .
[ eq : hybdefn ] , on @xmath17 , may provide insight into the validity of the excited potential approach to studying hybrid mesons .    in fig .
[ fig : wave ] we plot the wave functions of the pion , rho and @xmath0 states in coulomb gauge , at @xmath18 and @xmath19 ( corresponding to a vector to pseudoscalar mass ratio of @xmath20 ) , with a sample size of 40 .
the scale on the x axis is set using the chirally extrapolated rho mass .
the wave function in fig .  [ fig : wave ] looks qualitatively similar to those obtained  @xcite by solving the schrdinger equation with an excited lattice potential ( although we are working with lighter quarks ) .
an important issue in the spectroscopy of hybrid mesons is to understand the mixing between a hybrid meson and a 4-quark state with the same quantum numbers .
we studied this issue by using the valence approximation .
the ( naive ) valence approximation  @xcite removes @xmath21 from the quark action , so that the quarks travel forwards in time only ( note that nrqcd  @xcite quarks only propagate forwards in time as well ) . in the quenched approximation ,
it is this term that causes mixing between hybrid and 4-quark states via hairpin diagrams ( see fig . 1 in  @xcite ) .
liu and collaborators  @xcite have developed a valence approximation with an improved non - relativistic limit over the prescription in eq .
[ eq : valence ] suitable for the investigation of the relationship between the quark model and qcd . however , for our purposes , the removal of eq .
[ eq : valence ] from the clover action is adequate to study the effect of the hairpin graph on the hybrid spectrum .    in this preliminary study ,
[ fig : valence ] shows that the effective mass plots of the @xmath0 state , in the valence and quenched approximations , look very similar .
this suggests that at the parameters at which we are working , mixing via hairpin diagrams between the @xmath0 hybrid operator and four quark states is a small effect .
we repeat our original simulations of the hybrid meson spectrum using the clover action , as a check on lattice artifacts . 
 our results for the @xmath0 masses do not substantially change . 
 we present preliminary results for the wave function of the @xmath0 state in coulomb gauge .