Hyperon electromagnetic timelike elastic form factors at large q2

Abstract

We present estimates of the hyperon elastic form factors for the baryon octet and the Ω − baryon for large four-momentum transfer squared, q 2 , in the timelike region (q 2 > 0). Experimentally, those form factors can be extracted from the e + e − → BB and pp → BB processes, where B stands for a general baryon. Our results are based on calculations for the elastic electromagnetic form factors in the spacelike region (Q 2 = −q 2 > 0) within a covariant quark model. To connect the results in the spacelike to those in the timelike region, we use asymptotic relations between the two regions which are constraints derived from analyticity and unitarity. We calculate the effective form factors |G(q 2 )|, and compare them with the integrated cross section data σBorn(q 2 ) from BaBar, BES III and CLEO. The available data are at the moment restricted to Λ, Σ 0 , Σ − , Ξ − , Ξ 0 , Ω − as well as to e + e − → ΛΣ 0 and e + e − → Σ 0Λ reactions. Our results provide useful reference for future experiments and seem to indicate that the present data are still in the non-perturbative QCD region, while the onset for the asymptotic constraints from analyticity and unitarity happens much before the region of the pQCD falloff of the form factors.

Electromagnetic transition form factors and Dalitz decays of hyperons

“…The remaining dependence onF + (0) cancels out in the ratio of the Dalitz decay width (22) and the radiative decay width (21). Thus the such normalized decay width dΓ 3 /(dq 2 d(cos θ ))/Γ 2 depends only on the (squared) transition radii r 2 + and r 2 0 .…”

Covariant model for Dalitz decays of decuplet baryons to octet baryons

“…The covariant spectator quark model was already applied to the study of the electromagnetic structure of several baryons in the spacelike region [9,11,72,73,[78][79][80][81][82][83][84][85], in the timelike region [7,[30][31][32]75], to the structure of baryons in the nuclear medium [10], and to the lattice QCD regime [74,77,86]. We discuss next the formalism associated with the octet and decuplet baryons.…”

“…Another timelike subregion, not discussed in the present work, is the region probed by e þ e − and pp collisions at BABAR, BES-III, CLEO, and PANDA/FAIR [36][37][38][39][40], which access the baryon (B) elastic form factors when q 2 ≥ 4M 2 B , where M B is the baryon mass [7,41]. HADES provides a unique opportunity to explore the electromagnetic structure of baryons based on the B 0 → γ Ã B transitions, where B 0 and B are generic baryons, through the dilepton decays (B 0 → e þ e − B) [14,15,18,20,[23][24][25].…”

“…In practice, however, the technique is almost exclusively limited to nucleon targets, since hyperons targets are difficult to produce due to their short lifetime, except in the limit q 2 ¼ 0. In that limit, there are measurements of magnetic moments of a few hyperons and some magnetic transition moments [6][7][8][9][10][11][12][13]. Another limitation of the electron scattering technique is that it is restricted to the q 2 ≤ 0 region.…”

“…The covariant spectator quark model provides an alternative to valence quark models which do not take into account meson cloud excitations of the bare cores, and simplified vector meson dominance (VMD) models [19,[49][50][51], which do not take into account the underlying quark substructure of the baryons. The formalism has been used in the study of the electromagnetic and the axial structure of the nucleon, several nucleon excitations, and hyperons [7][8][9][10][11][30][31][32][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86].…”