Language:
English

In:
The European Physical Journal C, 2014, Vol.74(9), pp.1-26

Description:
A measurement of the cross section for the production of top quark–antiquark pairs ( $${\mathrm {t}}\overline{{\mathrm {t}}}$$ t t ¯ ) in association with a vector boson V (W or Z) in proton-proton collisions at $$\sqrt{s} = 8$$ s = 8 $$\,\text {TeV}$$ TeV is presented. The results are based on a dataset corresponding to an integrated luminosity of 19.5 fb $$^{-1}$$ - 1 recorded with the CMS detector at the LHC. The measurement is performed in three leptonic (e and $$\upmu $$ μ ) channels: a same-sign dilepton analysis targeting $${\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W} $$ t t ¯ W events, and trilepton and four-lepton analyses designed for $${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}} $$ t t ¯ Z events. In the same-sign dilepton channel, the $${\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W} $$ t t ¯ W cross section is measured as $$\sigma _{{\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W}} = 170 ^{+90}_{-80}\,\text {(stat)} \pm 70\,\text {(syst)} \, \text {fb} $$ σ t t ¯ W = 170 - 80 + 90 (stat) ± 70 (syst) fb , corresponding to a significance of 1.6 standard deviations over the background-only hypothesis. Combining the trilepton and four-lepton channels, a direct measurement of the $${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}} $$ t t ¯ Z cross section, $$\sigma _{{\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}}} = 200 ^{+80}_{-70}\,\text {(stat)} ^{+40}_{-30}\,\text {(syst)} \mathrm{fb}^{-1} $$ σ t t ¯ Z = 200 - 70 + 80 (stat) - 30 + 40 (syst) fb - 1 , is obtained with a significance of 3.1 standard deviations. The measured cross sections are compatible with standard model predictions within their experimental uncertainties. The inclusive $${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {V}} $$ t t ¯ V process is observed with a significance of 3.7 standard deviations from the combination of all three leptonic channels.

Keywords:
Regular Article - Experimental Physics;

ISSN:
1434-6044

E-ISSN:
1434-6052

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