Measurement of the W-pair cross-section and of the W mass in e(+)e(-) interactions at 172 GeV

From a data sample of 9.98 pb(-1) integrated luminosity, collected by DELPHI at a centre-of-mass energy of 172 GeV, 118 events were selected as W-pair candidates. From these, the branching fraction Br(W --> q (q) over bar) was measured to be 0.660(-0.037)(+0.036)(stat.) +/- 0.009(syst.) and the cross-section for the doubly resonant process e(+)e(-) --> W+W- to be 11.58(-1.35)(+1.44)(stat.) +/- 0.32(syst.) pb. The mass of the W boson, obtained from direct reconstruction of the invariant mass of the fermion pairs in the decays WW --> lvq (q) over bar and WW --> q (q) over bar q (q) over bar, was determined to be m(w) = 80.22 +/- 0.41(stat.) +/- 0.04(syst.) +/- 0.05(int.) +/- 0.03(LEP) GeV/c(2), where "int." denotes the uncertainty from interconnection effects like colour reconnection and Bose-Einstein interference. Combined with the W mass obtained from the cross-sections measured by DELPHI at threshold, a value of m(w) = 80.33 +/- 0.30(stat.) +/- 0.05(syst.) +/- 0.03(int.) +/- 0.03(LEP) GeV/c(2) was found.