Observational constraints on scalar-nonmetricity cosmology: the role of the connection

Scalar-nonmetricity gravity provides a natural scalar-tensor extension of symmetric teleparallel gravity, with the gravitational interaction encoded in nonmetricity. A distinctive feature is that, even for a spatially flat Friedmann-Robertson-Walker geometry, three inequivalent symmetric-teleparallel connection branches are allowed. Whether this geometrical freedom has observable consequences is therefore an important question. In this work we perform a systematic observational comparison of all three connection branches, considering a steep scalar-field potential and a power-law nonminimal coupling. We constrain the models using combinations of Pantheon Plus and DES Year 5 Type Ia supernovae, DESI DR2 baryon acoustic oscillations, and compressed cosmic microwave background information. We find that all three branches can reproduce the transition from matter domination to late-time acceleration, but their observational behavior is connection dependent. Connection I exhibits the strongest sensitivity to the supernova compilation, Connection II yields remarkably stable reconstructed histories, while Connection III shows an intermediate behavior. Most notably, for DES Y5+CMB+DESI DR2, Connection I gives $\mathrm{\Delta }{\chi }_{\min }^{}=-13.99$ and $\mathrm{\Delta }\mathrm{A}\mathrm{I}\mathrm{C}=-5.99$ relative to $\mathrm{\Lambda }$CDM, and is therefore favored by the Akaike information criterion.