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the unitary extension of chiral perturbation theory , u@xmath0pt , has been successfully applied to study many meson - baryon and meson - meson interactions . more recently , it has been used to study the lowest axial vector mesons @xmath3 , @xmath4 , @xmath5 , @xmath6 , @xmath7 , @xmath1 and @xmath8  @xcite . both works generate most of the low - lying axial vector mesons dynamically . however , there is a surprising discovery in ref . @xcite , i.e. , two poles are found in the second riemann sheet in the @xmath9 and @xmath10 channel and both are attributed to the @xmath1 . although the @xmath1 has been observed in various reactions , the most conclusive and high - statistics data of the @xmath1 come from the wa3 experiment at cern that accumulated data on the reaction @xmath11 at 63 gev . these data were analyzed by the accmor collaboration  @xcite . as will be shown in this paper , the two - peak structure , with a peak at lower energy depending drastically on the reaction channel investigated , can be easily explained in our model with two poles for the @xmath1 plus the @xmath8 . with only one pole , as has been noted long time ago  @xcite , there is always a discrepancy for the peak positions observed in the @xmath12 and @xmath13 invariant mass distributions . in the following , we briefly describe the chiral unitary approach , while detailed formalism can be found in refs .  @xcite . in the bethe - salpeter formulation of the unitary chiral perturbation theory  @xcite , one has the following unitarized amplitude : @xmath14^{-1}(-v ) \,\vec{\epsilon}\cdot\vec{\epsilon}\ , ' , \label{bethes}\ ] ]    where @xmath15 is a diagonal matrix with the @xmath16th element , @xmath17 , being the two meson loop function containing a vector and a pseudoscalar meson : @xmath18 with @xmath19 the total incident momentum , which in the center of mass frame is @xmath20 . the loop function @xmath17 can be regularized either by a cutoff or by dimensional regularization . in the former case , one has cutoff values , whereas in the latter , one has subtraction constants as free parameters , which have to be fitted to the data .     and @xmath21 channel.,title="fig : " ] and @xmath21 channel.,title="fig : " ] the tree level amplitudes are calculated using the following interaction lagrangian  @xcite : @xmath22 where @xmath23 means su(3 ) trace and @xmath24 is the covariant derivative defined as @xmath25,\ ] ] where @xmath26 $ ] stands for commutator and @xmath27 is the vector current @xmath28 with @xmath29 in the above equations @xmath30 is the pion decay constant in the chiral limit and @xmath19 and @xmath31 are the su(3 ) matrices containing the pseudoscalar octet of the pion and the vector nonet of the rho . [ fig : gtvpvp ] shows the modulus squared of the @xmath9 , @xmath21 amplitudes multiplied by the corresponding loop functions obtained with @xmath32 mev , @xmath33 and @xmath34 mev . the pole positions and corresponding widths obtained with this set of parameters are shown in table [ tab : poles2 ] . from fig . [ fig : gtvpvp ] , the two poles are clearly seen : the higher pole manifests itself as one relatively narrower resonance around 1.28 gev and the lower pole as a broader resonance at @xmath35 gev . the effective couplings for the coupled channels @xmath36 , @xmath37 , @xmath13 , @xmath38 and @xmath12 , calculated from the residues of the amplitudes at the complex pole positions , are tabulated in table  [ tab : coup ] for both the lower pole and the higher pole , respectively . .effective couplings of the two poles of the @xmath1 to the five channels : @xmath36 , @xmath37 , @xmath13 , @xmath38 and @xmath12 . all the units are in mev.[tab : poles2 ] [ cols="^,^,^,^,^ " , ]     [ tab : coup ]    it is clearly seen that the lower pole couples dominantly to the @xmath12 channel while the higher pole couples more strongly to the @xmath13 channel . if different reaction mechanisms favor one or the other channel , they will see different shapes for the resonance . more importantly , it is to be noted that not only the two poles couple to different channels with different strengths , but also they manifest themselves in different final states . in other words , in the @xmath13 final states , one favors a narrower resonance around 1.28 gev , while in the @xmath12 final states , one would favor a broader resonance at a smaller invariant mass . the reaction @xmath39 can be analyzed by the isobar model as @xmath40 . therefore , one can construct the following amplitudes to simulate this process . assuming @xmath21 dominance for @xmath41 and @xmath42 as suggested by the experiment one has @xmath43 where @xmath44 are the coupled channel amplitudes obtained in section 2 and the clebsch - gordan coefficient @xmath45(@xmath46 ) accounts for projecting the @xmath21 @xmath12 ( @xmath13 ) state into @xmath41(@xmath42 ) . the coefficients @xmath47 and @xmath48 are complex couplings .    to contrast our model with data , it is necessary to take into account the existence of the @xmath8 , which is not dynamically generated in our approach . therefore , we add to the amplitudes in eq . ( [ eq : ts ] ) an explicit contribution of the @xmath8 @xmath49 where @xmath50 and @xmath51 are complex couplings , and @xmath52 and @xmath53 are the mass and width of the @xmath8 with the @xmath54-wave width given by @xmath55 @xmath56 and @xmath57 are calculated by @xmath58    in our model , eq . ( [ bwadd ] ) , we have the following adjustable parameters : @xmath47 , @xmath48 , @xmath59 , @xmath51 , @xmath52 and @xmath60 . in principle , @xmath30 and @xmath61 can also be taken as free parameters . one can then fix these parameters by fitting the wa3 data ( see ref . @xcite for details ) .     and @xmath13 invariant mass distributions . the data are from the wa3 reaction @xmath62 at 63 gev  @xcite . data in the upper panels are for @xmath63 gev@xmath64 , where @xmath65 is the four momentum transfer squared to the recoiling proton . the data are further grouped by @xmath66 followed by the isobar and odd particle . @xmath67 is the total angular momentum , @xmath19 the parity , @xmath68 the orbital angular momentum of the odd particle . @xmath69 denotes the magnetic substate of the @xmath70 system and the naturality of the exchange.,title="fig : " ]   and @xmath13 invariant mass distributions . the data are from the wa3 reaction @xmath62 at 63 gev  @xcite . data in the upper panels are for @xmath63 gev@xmath64 , where @xmath65 is the four momentum transfer squared to the recoiling proton . the data are further grouped by @xmath66 followed by the isobar and odd particle . @xmath67 is the total angular momentum , @xmath19 the parity , @xmath68 the orbital angular momentum of the odd particle . @xmath69 denotes the magnetic substate of the @xmath70 system and the naturality of the exchange.,title="fig : " ]   and @xmath13 invariant mass distributions . the data are from the wa3 reaction @xmath62 at 63 gev  @xcite . data in the upper panels are for @xmath63 gev@xmath64 , where @xmath65 is the four momentum transfer squared to the recoiling proton . the data are further grouped by @xmath66 followed by the isobar and odd particle . @xmath67 is the total angular momentum , @xmath19 the parity , @xmath68 the orbital angular momentum of the odd particle . @xmath69 denotes the magnetic substate of the @xmath70 system and the naturality of the exchange.,title="fig : " ] and @xmath13 invariant mass distributions . the data are from the wa3 reaction @xmath62 at 63 gev  @xcite . data in the upper panels are for @xmath63 gev@xmath64 , where @xmath65 is the four momentum transfer squared to the recoiling proton . the data are further grouped by @xmath66 followed by the isobar and odd particle . @xmath67 is the total angular momentum , @xmath19 the parity , @xmath68 the orbital angular momentum of the odd particle . @xmath69 denotes the magnetic substate of the @xmath70 system and the naturality of the exchange.,title="fig : " ]   and @xmath13 invariant mass distributions . the data are from the wa3 reaction @xmath62 at 63 gev  @xcite . data in the upper panels are for @xmath63 gev@xmath64 , where @xmath65 is the four momentum transfer squared to the recoiling proton . the data are further grouped by @xmath66 followed by the isobar and odd particle . @xmath67 is the total angular momentum , @xmath19 the parity , @xmath68 the orbital angular momentum of the odd particle . @xmath69 denotes the magnetic substate of the @xmath70 system and the naturality of the exchange.,title="fig : " ] and @xmath13 invariant mass distributions . the data are from the wa3 reaction @xmath62 at 63 gev  @xcite . data in the upper panels are for @xmath63 gev@xmath64 , where @xmath65 is the four momentum transfer squared to the recoiling proton . the data are further grouped by @xmath66 followed by the isobar and odd particle . @xmath67 is the total angular momentum , @xmath19 the parity , @xmath68 the orbital angular momentum of the odd particle . @xmath69 denotes the magnetic substate of the @xmath70 system and the naturality of the exchange.,title="fig : " ]    according to ref . @xcite , for an @xmath54-wave resonance , the theoretical differential cross section can be calculated by @xmath71 where @xmath52 is the invariant mass of the @xmath12 or @xmath13 systems , @xmath72 is a normalization constant , @xmath73 is the amplitude specified above for the @xmath12 or @xmath13 channels and @xmath74 is the center of mass three - momentum of @xmath12 or @xmath13 . we have taken @xmath72 to be 1 , or in other words , it has been absorbed into the coupling constants @xmath47 , @xmath48 , @xmath59 and @xmath51 . the theoretical invariant mass distributions calculated with eq . ( [ eq : inv ] ) are shown in fig . [ fig : s1 ] in comparison with the wa3 data  @xcite .    from fig . [ fig : s1 ] , it is clearly seen that our model can fit the data around the peaks very well . in fig . [ fig : s1 ] , the dashed and dotted lines are the separate contributions of the @xmath1 and the @xmath8 . one can easily see that the @xmath8 decays dominantly to @xmath12 , which is consistent with our present understanding of this resonance . it should be mentioned that in our model the lower peak observed in the invariant mass distribution of the @xmath12 channel is due to the contribution of the two poles of the @xmath1 . this is very different from the traditional interpretation . for example , the lower peak observed in the @xmath12 invariant mass distributions of @xmath75 at 13 gev was interpreted as a pure gaussian background by carnegie et al . @xcite , which has a shape similar to the contribution of the @xmath1 as shown in fig . [ fig : s1 ] . on the other hand , the k - matrix approach was adopted to analyze the wa3 data  @xcite and the slac data  @xcite . in this latter approach , the lower peak mostly comes from the so - called deck background , which after unitarization , also has a shape of resonance . as we mentioned in the introduction , even in the original wa3 paper  @xcite , it was noted that their model failed to describe the @xmath76(@xmath12 ) data , in the notation @xmath77 with @xmath78 the naturality of the exchange  @xcite . the predicted peak is 20 mev higher than the data . if the fit were done only to the @xmath12 data , the agreement was much better but then the predicted @xmath1 would be lower by 35 mev than that obtained when other channels were also considered in the fit . it is worth stressing that the @xmath1 peak seen in the upper - left panel of fig . [ fig : s1 ] is significantly broader than that in the upper - right panel . furthermore the peak positions are also different in the two cases ( 1240 mev and 1280 mev respectively ) . both features have a straightforward interpretation in our theoretical description since the first one is dominated by the low - energy ( broader ) @xmath1 state , while the second one is dominated by the higher - energy ( narrower ) @xmath1 state . studies based on unitary chiral perturbation theory obtain two poles in the @xmath10 , @xmath9 , vector - pseudoscalar scattering amplitudes which can be assigned to two @xmath1 resonances . one pole is at @xmath79 mev with a width of @xmath80 mev and the other is at @xmath81 mev with a width of @xmath82 mev . the lower pole couples more to the @xmath12 channel whereas the higher pole couples dominantly to the @xmath13 channel . different reaction mechanisms may prefer different channels and thus this explains the different invariant mass distributions seen in various experiments . we have analyzed the wa3 data on the @xmath83 reaction since it is the most conclusive and high - statistics experiment quoted in the pdg on the @xmath1 resonance . our model obtains a good description of the wa3 data both for the @xmath12 and @xmath13 final state channels . in our model , the peak in the @xmath70 mass distribution around the @xmath84 mev region is a superposition of the two poles , but in the @xmath12 channel the lower pole dominates and in the @xmath13 channel the higher pole gives the biggest contribution . this work is partly supported by dgicyt contract no . bfm2003 - 00856 , fpa2004 - 03470 , the generalitat valenciana , and the e.u . flavianet network contract no . hprn - ct-2002 - 00311 . this research is part of the eu integrated infrastructure initiative hadron physics project under contract no . rii3-ct-2004 - 506078 . m.  c.  birse , z.  phys . a * 355 * , 231 ( 1996 ) . s.  m.  flatte , phys . b * 63 * , 224 ( 1976 ) . r.  k.  carnegie _ et al . _ , b * 127 * , 509 ( 1977 ) . g.  w.  brandenburg _ et al . _ , phys .  rev .  lett .   * 36 * , 703 ( 1976 ) .
recent studies based on unitary chiral perturbation theory ( u@xmath0pt ) found that the low - lying axial vector mesons can be dynamically generated due to the interaction of the pseudoscalar octet of the pion and the vector nonet of the rho . 
 in particular , two poles in the second riemann sheet have been associated to the nominal @xmath1 resonance . in this talk , we present a recent analysis of the wa3 data on @xmath2 at 63 gev using the u@xmath0pt amplitudes , and show that it is in favor of the existence of two @xmath1 s [ phys . rev . 
 d 75 , 014017 ( 2007 ) ] . 
 = 11.6pt