  We present accretion-disk structure measurements from UV-optical
reverberation mapping observations of a sample of eight quasars at 0.24<z<0.85.
Ultraviolet photometry comes from two cycles of Hubble Space Telescope
monitoring, accompanied by multi-band optical monitoring by the Las Cumbres
Observatory network and Liverpool Telescopes. The targets were selected from
the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project sample
with reliable black-hole mass measurements from Hbeta reverberation mapping
results. We measure significant lags between the UV and various optical griz
bands using JAVELIN and CREAM methods. We use the significant lag results from
both methods to fit the accretion-disk structure using a Markov chain Monte
Carlo approach. We study the accretion disk as a function of disk
normalization, temperature scaling, and efficiency. We find direct evidence for
diffuse nebular emission from Balmer and FeII lines over discrete wavelength
ranges. We also find that our best-fit disk color profile is broadly consistent
with the Shakura \& Sunyaev disk model. We compare our UV-optical lags to the
disk sizes inferred from optical-optical lags of the same quasars and find that
our results are consistent with these quasars being drawn from a limited
high-lag subset of the broader population. Our results are therefore broadly
consistent with models that suggest longer disk lags in a subset of quasars,
for example, due to a nonzero size of the ionizing corona and/or magnetic
heating contributing to the disk response.
