arXiv:nucl-ex/0506018v1 14 Jun 2005 Dependence of the 12 C( γ ,pd) reaction on photon linear polarisation D. P. Watts, 1, ∗ J.R.M. Annand, 1 R. Beck, 2 D. Branford, 3 D. Glazier, 4 P. Grabmayr, 4 K. Livingston, 1 I. J. D. MacGregor, 1 J.C. McGeorge, 1 and R.O. Owens 1 1 Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK. 2 Institut f¨ ur Kernphysik, Universit¨at Mainz, D-55099 Mainz, Germany. 3 School of Physics, Edinburgh University, EH9 3JZ, UK. 4 Physikalisches Institut, Universit¨at T¨ ubingen, D-72076 T¨ ubingen,Germany. (Dated: February 8, 2008) The sensitivity of the 12 C(γ,pd) reaction to photon linear polarisation has been determined at MAMI, giving the first measurement of the reaction for a nucleus heavier than 3 He. Photon asym- metries and cross sections were measured for Eγ =170 to 350 MeV. For Eγ below the Δ resonance, reactions leaving the residual 9 Be near its ground state show a positive asymmetry of up to 0.3, similar to that observed for 3 He suggesting a similar reaction mechanism for the two nuclei. PACS numbers: PACS numbers: 25.20.Lj, 27.20.+n Three-body forces have consequences in many fields of physics. The study of photon induced proton-deuteron knockout from nuclei may give valuable information on the three-body interaction in the nucleus, since the direct mechanisms which contribute may be related to those thought to be involved in the three-nucleon force[1, 2, 3, 4]. However, as well as the direct 3-nucleon process (3N ) there will be contributions from initial photon absorption by a single nucleon (1N ), two-nucleons (2N ) and two- step 3N processes such as initial real pion production on one nucleon followed by reabsorption by a nucleon pair. Clearly, to extract reliable information from (γ,pd) measurements, the relative contributions from each of these mechanisms should be well understood. The (γ ,pd) reaction has received significant theoretical interest in recent years, mainly motivated by the possi- bility of obtaining information on the nature of the 3- nucleon force (3NF). Detailed 3 He calculations based on exact solutions of the three-particle scattering equations in the initial and final states have been carried out for photon energies up to 140 MeV[1, 3]. These show that the inclusion of a 3NF has a large effect on the magni- tude of the cross section, increasing the predictions by up to a factor of two at the top end of this E γ range. A microscopic theoretical treatment of the 3 He(γ ,pd) reac- tion, which includes contributions from 1N ,2N and 3N mechanisms, has been developed by Laget[4]. The 3N mechanisms include contributions from virtual and real pion exchange. Laget’s treatment relies on a factorisa- tion approximation to simplify the computation but is applicable up to higher photon energies. On the experimental side, most measurements of the (γ ,pd) reaction have been made using 3 He targets[5, 6, 7, 8, 9, 10]. An important feature of the measured exci- tation functions is that they show no evidence of struc- ture for photon energies in the Δ(1232) resonance region. Also, the centre-of-momentum (CM) proton angle dis- tributions are forward peaked and fall off rapidly with increasing angle up to ∼70 ◦ with a flatter distribution at more backward angles. The features are moderately well described by the Laget model when 1N ,2N and two-step 3N (including only real π exchange) mecha- nisms are included[4]. Laget notes that his model ac- counts less well for the 3 He(γ,pd) data than it does for π induced processes involving the A=3 nuclei and sug- gests two additional photon couplings, both involving two highly virtual mesons, which could be responsible. Above 100 MeV the 2N and two-step 3N mechanisms are pre- dicted to dominate with the 2N mechanisms only giving large contributions to the cross section at forward CM proton angles[4, 5]. Above ∼150 MeV the two-step 3N mechanism is predicted to provide most of the cross sec- tion for CM proton angles backwards of ∼70 ◦ . Studies of the (γ,pd) reaction for A>3 targets have been carried out only on 16 O[11] and 12 C[12]. Both mea- surements show a photon energy dependence similar to that observed in 3 He with no prominent enhancement for E γ around the Δ resonance. This behaviour is in con- trast to photon induced pp, pn, pπ and ppn[13, 14, 15, 16] knockout reactions where the Δ plays a prominent role. The 12 C(γ,pd) missing energy spectra obtained in Ref. [12] exhibit significant strength close to the reaction threshold, and the recoil momentum spectra of the (A–3) nucleus at low missing energies are consistent with those predicted if it were a spectator to the knockout of three 1p-shell nucleons. The photon asymmetry for the ( γ,pd) reaction has only been measured previously for 3 He[17, 18]. These measurements showed a positive asymmetry which ranged from around 0.2 to 0.5 over the sampled pho- ton energy region of 90-350 MeV and CM proton angle range of 60-135 ◦ . Comparison with a simple Faddeev calculation[17] which neglects meson exchange currents, Δ contributions and final state interactions gave limited agreement with the experimental data. The three nucleon photoabsorption mechanisms which operate in the (γ,pd) reaction also contribute in the (γ, ppn) reaction but with less restrictive spin and isospin