We present first results from a deep ( J = 18.7 ) , wide-field ( 6.5 square degrees ) infrared ( ZYJHK ) survey in the Upper Sco association conducted within the science verification phase of the UKIRT Infrared Deep Sky Survey Galactic Cluster Survey ( GCS ) .
Cluster members define a sequence well separated from field stars in the ( Z - J , Z ) colour-magnitude diagram .
We have selected a total of 164 candidates with J = 10.5–18.7 mag from the ( Z - J , Z ) and ( Y - J , Y ) diagrams .
We further investigated the location of those candidates in the other colour–magnitude and colour–colour diagrams to weed out contaminants .
The cross–correlation of the GCS catalogue with the 2MASS database confirms the membership of 116 photometric candidates down to 20 Jupiter masses as they lie within a 2 \sigma circle centred on the association mean motion .
The final list of cluster members contains 129 sources with masses between 0.3 and 0.007 M _ { \odot } .
We extracted a dozen new low-mass brown dwarfs below 20 M _ { Jup } , the limit of previous surveys in the region .
Finally , we have derived the mass function in Upper Sco over the 0.3–0.01 M _ { \odot } mass range , best fit by a single segment with a slope of index \alpha = 0.6 \pm 0.1 , in agreement with previous determination in open clusters .