DOI 10.1140/epja/i2002-10289-y Eur. Phys. J. A 18, 141–145 (2003) T HE EUROPEAN P HYSICAL JOURNAL A Nucleon deformation A status report C.N. Papanicolas a University of Athens and Institute of Accelerating Systems and Applications, Athens, Greece Received: 30 September 2002 / Published online: 22 October 2003 – c  Societ` a Italiana di Fisica / Springer-Verlag 2003 Abstract. The conjectured deformation of hadrons and its experimental verification offer a particularly fer- tile ground for understanding the intricate dynamics of their constituents and QCD at the confiment scale. The detailed study of the N → Δ transition is viewed as the preferred method of experimental investigation of this central issue in hadronic physics. A brief overview of the field is presented, followed by a presentation of the most recent results from Bates N → Δ program. The new Bates/OOPS data at Q 2 =0.127(GeV/c) 2 yield RSM =(−6.27 ± 0.32stat+sys ± 0.10 model )% and REM =(−2.00 ± 0.40stat+sys ± 0.27 model )% and they exclude a spherical nucleon and/or Δ. The magnitude and the origin of the deformation is the focus of the ongoing and planned investigations. PACS. 13.60.Le Meson production – 13.88.+e Polarization in interactions and scattering – 13.40.Gp Electromagnetic form factors – 14.20.Gk Baryon resonances with S =0 1 Introduction QCD inspired models [1–3] and recent lattice calcula- tions [4,5] strongly suggest that the shapes of hadrons are expected to deviate from spherical symmetry. While the possibility of nucleon deformation was raised more than 20 years ago [6], it is only recently that results of ex- clusive experiments of high precision are able to confirm the deviation from spherical shape. The origin of defor- mation is attributed to different mechanisms in the var- ious nucleon models, suggesting that the deviation from spherical symmetry is the result of several mechanisms. In “QCD-inspired” constituent quark models, it arises from the intra-quark effective color-magnetic tensor forces [1], while in chiral bag models [2] most of the deformation can be attributed to the asymmetric coupling of the meson cloud to the spin of the nucleon. Our current understand- ing of the nucleon indicates that most of the deformation at long distances (low momenta) is driven by the pionic cloud, while at short distances (high momenta) it is gen- erated by intra-quark forces. The vanishing of the static quadrupole moment of the nucleon due to its J = 1 2 spin, precludes accesss to the most direct observable of deformation. As a result, the presence of resonant quadrupole amplitudes in the N → Δ transition has emerged as the definitive experimental sig- nature of deviation from the simplistic spherical models a Invited plenary talk; e-mail: cnp@cc.uoa.gr of the nucleon and/or the delta. This is easily understood in the spherical quark model of the nucleon, where the N → Δ excitation is a pure M 1(M 3/2 1+ ) transition. The resonant quadrupole multipoles E2(E 3/2 1+ ) and C2(S 3/2 1+ ) contain the pertinent information, as they arise from D state admixtures in the wave functions —a case reminis- cent of the deformation of the deuteron. In pion produc- tion the amplitudes are denoted by M I l± , E I l± , S I 1± and L I l± , indicating their character (magnetic, electric, scalar or longitudinal), their isospin I and their total angular momentum (J = l ± 1/2). Experimental and theoretical results are routinely quoted in terms of the Electric- and Scalar(Coulomb)- to-Magnetic-Ratios of amplitudes defined as R EM = ℜ(E 1+ /M 1+ ) and R SM = ℜ(S 1+ /M 1+ ), respectively. QCD-inspired models predict values of R SM in the range from −0.1% to −8%, at low momentum transfers, Q 2 ≤ 1.0 (GeV/c) 2 . However, the isolation of the resonant R EM and R SM is complicated by the presence of the nonreso- nant “background processes” which are coherent with the resonant excitation of the Δ(1232). These interfering pro- cesses (such as the pion pole, Born terms, tails of higher resonances) need to be constrained in order to isolate the resonant contributions to R EM and R SM which contain the physics of interest. As a result, R EM and R SM are in- variably extracted with model error which is often poorly known and rarely quoted.