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We present results from a high-contrast adaptive optics imaging search forgiant planets and brown dwarfs (>1 MJup) around 122 newly identified nearby(<40 pc) young M dwarfs. Half of our targets are younger than 135 Myr and 90%are younger than the Hyades (620 Myr). Our H- and K-band coronagraphicobservations with Keck/NIRC2 and Subaru/HiCIAO achieve typical contrasts of12-14 mag and 9-13 mag at 1", respectively, which corresponds to limitingplanet masses of 0.5-10 MJup at 5-33 AU for 85% of our sample. We discoveredfour young brown dwarf companions: 1RXS J235133.3+312720 B (32 $\pm$ 6 MJup;L0$^{+2}_{-1}$; 120 $\pm$ 20 AU), GJ 3629 B (64$^{+30}_{-23}$ MJup; M7.5 $\pm$0.5; 6.5 $\pm$ 0.5 AU), 1RXS J034231.8+121622 B (35 $\pm$ 8 MJup; L0 $\pm$ 1;19.8 $\pm$ 0.9 AU), and 2MASS J15594729+4403595 B (43 $\pm$ 9 MJup; M8.0 $\pm$0.5; 190 $\pm$ 20 AU). Over 150 candidate planets were identified; we obtainedfollow-up imaging for 56% of these but all are consistent with backgroundstars. Our null detection of planets enables strong statistical constraints onthe occurrence rate of long-period giant planets around single M dwarfs. Weinfer an upper limit (at the 95% confidence level) of 10.3% and 16.0% for 1-13MJup planets between 10-100 AU for hot-start and cold-start (Fortney)evolutionary models, respectively. Fewer than 6.0% (9.9%) of M dwarfs harbormassive gas giants in the 5-13 MJup range like those orbiting HR 8799 and$\beta$ Pictoris between 10-100 AU for a hot-start (cold-start) formationscenario. Although the first directly imaged planets were found around massivestars, there is currently no statistical evidence for a trend of giant planetfrequency with stellar host mass at large separations as predicted by the diskinstability model of giant planet formation.

Planets Around Low-Mass Stars (PALMS). IV. The Outer Architecture of M Dwarf Planetary Systems