γ*N→N*(1520) form factors in the timelike regime

Abstract

The covariant spectator quark model, tested before in a variety of electromagnetic baryon excitations, is applied here to the γ * N → N * (1520) reaction in the timelike regime. The transition form factors are first parametrized in the spacelike region in terms of a valence quark core model together with a parametrization of the meson cloud contribution. The form factor behavior in the timelike region is then predicted, as well as the N * (1520) → γN decay width and the N * (1520) Dalitz decay, N * (1520) → e + e − N . Our results may help in the interpretation of dielectron production from elementary pp collisions and from the new generation of HADES results using a pion beam. In the q 2 = 0-1 GeV 2 range we conclude that the QED approximation (a q 2 independent form factor model) underestimates the electromagnetic coupling of the N * (1520) from 1 up to 2 orders of magnitude. We conclude also that the N * (1520) and ∆(1232) Dalitz decay widths are comparable.

Semirelativistic approximation to the γ*N→N(1520) and γ*NRamalho1 2017Phys. Rev. DSelf Cite192120View full textAdd to dashboardCite

“…The functions G i (i = 1, .., 4) are form factor functions that depend on Q 2 , but only three of them are independent. From current conservation [9,28] we conclude thatAnother useful combination of the form factorsUsing the previous form factors we can express the γ * N → N (1520) helicity amplitudes defined by Eqs. (3.3)-(3.5) as [1, 9]…”

“…In a relativistic framework the imposition of the orthogonality condition is more complex, since the nucleon and the resonance R cannot be at rest in the same frame, and the boost changes the properties of the states. As a consequence, states that are orthogonal when the mass difference can be neglected may not be orthogonal when the mass difference is taken into account.The problem of how to define a wave function of a nucleon excitation that generalizes the nonrelativistic structure of the state and is also orthogonal to the nucleon was already discussed in the context of the covariant spectator quark model for the negative parity resonances 1 The nonrelativistic limit can also be defined as the equal mass limit (M R = M N ) or as the heavy baryon limit, when the terms 1520) and N (1535) [8][9][10]. The solution at the time was to define the radial wave functions for the N * states in order to ensure the orthogonality with the nucleon state.…”

“…[9]). Since the orthogonality condition between states is equivalent to I R (0) = 0 [9], one obtains G M (0) = 0, in contradiction with the experimental result G M (0) = −0.393 ± 0.044 [11].In the framework of the covariant spectator quark model, we can prove that the orthogonality condition (2.2) for the states R = N (1520), N (1535) is equivalent to [8,9] where ψ R and ψ N are radial wave functions from R and N , respectively and real functions of (P ± − k) 2 . The integral I R (0) is defined in Eq.…”

“…The general expression can be found in Refs. [8,9].In Sec. IV, we present the results for the γ * N → N (1520) and γ * N → N (1535) form factors and the connection with the helicity amplitudes within the covariant spectator quark model.…”

“…The wave functions of the nucleon, N (1520) and N (1535) are discussed in Refs. [8,9,22].The covariant spectator quark model was already applied to the nucleon [22,23,[36][37][38], several nucleon resonances [8-10, 25, 27], ∆ resonances [10,34,35,[39][40][41][42], and other transitions between baryon states [24,30,[43][44][45].When the baryon wave functions are represented in terms of the single quark and quark-pair states, one can write the transition current in a relativistic impulse approximation as [22][23][24] where j µ q is the quark current operator and Γ labels the scalar diquark and vectorial diquark (projections Λ = 0, ±) polarizations. The factor 3 takes account of the contributions of all the quark-pairs by symmetry.…”