DETECTION OF A COSMIC MICROWAVE BACKGROUND DECREMENT TOWARD THE Z=3.8QUASAR PAIR PC 1643+4631A,B

In a 15 GHz Ryle Telescope observation of PC 1643 + 4631A, B, a pair o f quasars at redshifts z = 3.79 and 3.83 separated by 198 '' on the sk y, we find a decrement in the cosmic microwave background (CMB) of -38 0 +/- 64 mu Jy in a 110 '' x 175 '' beam. Assuming this to be a Sunyae v-Zeldovich effect due to an intervening cluster, the minimum magnitud e of the central temperature decrement is 560 mu K. A serendipitous RO SAT observation shows that there is no X-ray-luminous cluster in the d irection of the decrement at z < 1. The implied gas mass is greater th an 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 dem onstrates 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.