Measurement of the Ƴ(1S) production cross-section in pp collisions at √s = 7 TeV in ATLAS

Abstract

A measurement of the cross-section for Υ (1S) → μ+μ− production in proton–proton collisions at centre of mass energy of 7 TeV is presented. The cross-section is measured as a function of the Υ (1S) transverse momentum in two bins of rapidity, <math><mo stretchy="false" is="true">|</mo><msup is="true"><mi is="true">y</mi><mrow is="true"><mi is="true">ϒ</mi><mo stretchy="false" is="true">(</mo><mn is="true">1</mn><mi is="true">S</mi><mo stretchy="false" is="true">)</mo></mrow></msup><mo stretchy="false" is="true">|</mo><mo is="true"><</mo><mn is="true">1.2</mn></math> and <math><mn is="true">1.2</mn><mo is="true"><</mo><mo stretchy="false" is="true">|</mo><msup is="true"><mi is="true">y</mi><mrow is="true"><mi is="true">ϒ</mi><mo stretchy="false" is="true">(</mo><mn is="true">1</mn><mi is="true">S</mi><mo stretchy="false" is="true">)</mo></mrow></msup><mo stretchy="false" is="true">|</mo><mo is="true"><</mo><mn is="true">2.4</mn></math>. The measurement requires that both muons have transverse momentum <math><msubsup is="true"><mi is="true">p</mi><mi is="true">T</mi><mi is="true">μ</mi></msubsup><mo is="true">></mo><mn is="true">4</mn><mtext is="true"> GeV</mtext></math> and pseudorapidity <math><mo stretchy="false" is="true">|</mo><msup is="true"><mi is="true">η</mi><mi is="true">μ</mi></msup><mo stretchy="false" is="true">|</mo><mo is="true"><</mo><mn is="true">2.5</mn></math> in order to reduce theoretical uncertainties on the acceptance, which depend on the poorly known polarisation. The results are based on an integrated luminosity of 1.13 pb−1, collected with the ATLAS detector at the Large Hadron Collider. The cross-section measurement is compared to theoretical predictions: it agrees to within a factor of two with a prediction based on the NRQCD model including colour-singlet and colour-octet matrix elements as implemented in Pythia while it disagrees by up to a factor of ten with the next-to-leading order prediction based on the colour-singlet model.