ASSOCIATIVE PHOTOPRODUCTION OF ROPER RESONANCE WITH
VECTOR MESONS
T. BA BA CAN. H. BABACAN, A. G O K A L P a n d O .Y IL M A Z Physics Department, M iddle East Technical University, 06531 Ankara, Turkey
A B STR A C T
W e examine the associative photoproduction o f Roper resonance with vector mesons, y+N—>N ( 1440)+V. in the near threshold region by using the analogy with the processes o f “elastic” vector meson photoproduction, y+N—>N+V. Different production mechanisms are suggested. In addition to standart c - and ^-exchanges in the t-channel, the baryonic exchanges in the s- and u-channel are taken into account as well. We calculate the differential cross-section and beam asymmetry on proton and neutron targets for the reaction y+N—>N*(1440)+co. All calculations are performed at E.,= 2.5 GeV.
The recent study [1] on the associative photoproduction o f Roper resonance with vector mesons, y fN —>N*(1440)+V, in the near threshold region indicates the importance o f the pseudoscalar (tt) exchange for t-channel in associative co-meson photoproduction but it does not give a clear information about the relative role o f different possible threshold mechanisms on the production o f N*(1440). Therefore, in additon to standart G - and ^-exchanges in the t-channel, we consider the baryonic resonances in the s- and u-channel .
For t-channel, the pseudoscalar (n) and scalar exchanges (c), Fig. 1(a), are considered The pseudoscalar exchange amplitude can be obtained from the Lagrangian,
where Au is the photon field. Following Ref. [1,2] , we use the coupling constants as g2^ = 3.315 and g2K\ \ /4tt= 3 .4. The form factors that we have used are as follows:
(1)
2 2
A?r Mi
2 2
(2)
where Art = 0.7 and A ^ = 0.77 in GeV unit. [3]
The scalar (c) exchange amplitude can be obtained from the Lagrangian,
(3) M ®
The form factors for this exchange are given as,
2 2 _Ag Mg F oNN* _ 2 Ag- t F 2 2 A m ay M g may A 2 may (4)
where Ag = 1.0 GeV, A m0Y = 0.9 GeV, M g = 0.5 GeV and the coupling constant, 8 2onn/4n=0.34 [1,2] .
Finally, the baryonic resonaces in the s- and u-channel , Figure 1 (b,c), are considered. The Lagrangian for the (s+u) nucleonic resonances can be given by
Ls= N * g r g V Y y " ---^ mNN * g dv V" S mNN , . V ...U "V ° y " M N + M "V N - eN* Qn Y " A" " - 2 ^ -2 MN g " v d v A" N (5) Lu= N * g r K Y V " ----gmNN * g dV V "
g mNN *Y" V M n + M * g "V V N - eN* Qn Y" A " - ^ M * g"v dv A "2 M N
t
where k n is the anomalous magnetic moment o f the nucleon (kp = 1.79 Kn = -1.91) , g m N N* and g m NN * are the tensor and vector coupling constants, respectively. k n * is the anomalous magnetic moment o f the N*(1440), and, for the simplicity, in our calculations we take
= , g V =0 Kn * = Kn , g mNN*=0
The differential cross-section and beam asymmetry for the Roper resonce, y+N^N*(1440)+m, are calculated at EY = 2.5 GeV. The values o f the coupling constants, g^NN * and g mgY, are 3.41 and 0.13. The value o f the latter coupling constant is taken from reference [4,5]. The results for our model calculations are shown in Figure 2. From the curves, it can be concluded that the a - exchange contribution is so small that the behaviour o f the graphs for n and (n+g) terms does not change in both differential cross-section and beam asymmetry, and, the dominance o f the s+u nucleonic term contribution is obvious as the value o f |t| (momentum transfer) increases. A C K N O W L E D G E M E N T
We thank M .P.Rekalo for suggesting this problem to us and for his interest during the course o f our work.
R E FE R E N C E S:
1. M.Soyeur, Nucl.Phys.A 671, 532 (2000)
2. B.Friman and M.Soyeur, Nucl.Phys.A.600, 477 (1996) 3. A.I Titov et al., Phys.Rev C 60, 035205 (1999)
4. A.Gokalp, O.Yilmaz, Phys.Rev.C 62, 093018 (2000) 5. A.Gokalp, O.Yilmaz, Phys.Lett.B 464, 69 (2000)
(a) (b) (c)
Figure 1: Three mechanisms for vector meson (V= w) photoproduction involving ihe excitation of ihe N* (1440) resonance:
(a) one-boson exchange, (b) and (c) s- and u-channel intermediate nucleon diagrams.
Figure 2 : Differential cross-section and beam asymmetry for inelastic tt> meson
photoproduction on proton and neutron tagets at EY = 2.5 GeV
