Detection of a cosmic microwave background decrement toward the z=3.8 quasar pair PC 1643+4631A, B

In a 15 GHz Ryle Telescope observation of PC 1643 + 4631A, B, a pair of quasars at redshifts z = 3.79 and 3.83 separated by 198 '' on the sky, we find a decrement in the cosmic microwave background (CMB) of -380 +/- 64 mu Jy in a 110 '' x 175 '' beam. Assuming this to be a Sunyaev-Zeldovich effect due to an intervening cluster, the minimum magnitude of the central temperature decrement is 560 mu K. A serendipitous ROSAT observation shows that there is no X-ray-luminous cluster in the direction of the decrement at z < 1. The implied gas mass is greater than or similar to 2 x 10(14) M. (assuming a temperature of similar to 5 keV), indicating a total mass of more than 10(15) M.. This result demonstrates the existence of a massive system too distant to be detected by its emission, but which can be found via its imprint on the CMB.