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The redshift of $z\sim1.5$ is the transition epoch of protoclusters (PCs)from the star-forming phase into the quenching phase, and hence an appropriateera to investigate the build up of the quenched population. We define a `core'as the most massive halo in a given PC, where environmental effects are likelyto work most effectively, and search for cores at $1<z<1.5$. We use aphotometric redshift catalogue of a wide (effective area of$\sim22.2\,\mathrm{deg}^{2}$) and deep ($i\sim26.8\,\mathrm{mag}$) opticalsurvey with Subaru Hyper-Suprime Cam. Regarding galaxies with$\log(M_{*}/M_{\odot})>11.3$ as the central galaxies of PC cores, we estimatetheir average halo mass by clustering analysis and find it to be$\log(M_\mathrm{h}/M_{\odot})\sim13.7$. An expected mass growth by theIllustrisTNG simulation and the observed overdensities around them suggest thatthe PC cores we find are progenitors of present-day clusters. Classifying ourgalaxy sample into red and blue galaxies, we calculate the stellar massfunction (SMF) and the red galaxy fraction. The SMFs in the PC cores aremore-top heavy than field, implying early high-mass galaxy formation anddisruption of low-mass galaxies. We also find that the red fraction increaseswith stellar mass, consistent with stellar-mass dependent environmentalquenching recently found at $z>1$. Interestingly, although the cores with redand blue centrals have similar halo masses, only those with red centrals show asignificant red fraction excess compared to the field, suggesting a conformityeffect. Some observational features of PC cores may imply that the conformityis caused by assembly bias.

A systematic search for galaxy protocluster cores at the transition epoch of their star formation activity