First results at 7 TeV from ATLAS

First results at 7 TeV from ATLAS

(See more in ATLAS Public Results)

The transverse momentum spectrum of charged particles:

The first question when entering a new energy regime is whether the average collision can be understood from extrapolation from lower energies (in this case proton-antiproton collisions at Fermilab in the US).

The answer is no, not exactly. There are more low pT tracks in the data than expected. In particlular we do not understand well the "diffractive events". These events constitute a significant fraction of the inelastic proton collisions. Here, energy and momentum is exchanged between the two protons, but no colour charges.

For events with more than 6 charged particles, the diffractive component is suppressed, and in these events we find reasonable agreement with model extrapolations from lower energy.

The jet cross-section

The observed jet cross-section is found in good agreement with QCD out to pT=500 GeV. But remember that at high pT, we are not yet exploring energies beyond those visited by Fermilab. This will first happen when we have collected more statistics over the summer.

The first measurement of W production at 7 TeV

The distribution of missing pT recoiling from a high pT electron or muon shows a clear signal of W production. The spectrum is found in good agreement with predictions from QCD.