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We present deep high-resolution ($\sim$50 mas, 8 au) ALMA 0.88 and 1.3 mmcontinuum observations of the LkCa 15 disk. The emission morphology shows aninner cavity and three dust rings at both wavelengths, but with slightlynarrower rings at the longer wavelength. Along a faint ring at 42 au, weidentify two excess emission features at $\sim$10$\sigma$ significance at bothwavelengths: one as an unresolved clump and the other as an extended arc,separated by roughly 120 degrees in azimuth. The clump is unlikely to be acircumplanetary disk (CPD) as the emission peak shifts between the twowavelengths even after accounting for orbital motion. Instead, the morphologyof the 42 au ring strongly resembles the characteristic horseshoe orbitproduced in planet--disk interaction models, where the clump and the arc tracedust accumulation around Lagrangian points $L_{4}$ and $L_{5}$, respectively.The shape of the 42 au ring, dust trapping in the outer adjacent ring, and thecoincidence of the horseshoe ring location with a gap in near-IR scatteredlight, are all consistent with the scenario of planet sculpting, with theplanet likely having a mass between those of Neptune and Saturn. We do notdetect point-like emission associated with a CPD around the putative planetlocation ($0.''27$ in projected separation from the central star at a positionangle of $\sim$60\degr), with upper limits of 70 and 33 $\mu$Jy at 0.88 and 1.3mm, respectively, corresponding to dust mass upper limits of 0.02--0.03$M_{\oplus}$.

ALMA Detection of Dust Trapping around Lagrangian Points in the LkCa 15 Disk