Jet Missing Transverse Momenta for Quark-gluon Plasma Thermometry

At the LHC, where hard multi-jet events are common, large imbalances in the total jet-pT can arise due to the interaction of jets with the plasma formed in heavy-ion collisions. We find that this missing-pT spectrum yields a measurement of the initial temperature of the plasma. Using commonly accepted guesses for this temperature, and ISAJET estimates for jet cross sections, we predict an order of thousand events at LHC with unbalanced pT > 50 GeV. TIFR/TH/94-29 February 2008 For the heavy-ion collider mode of the LHC, with √ S = 5.5 TeV, the cross section for multi-jet events in which none of the jets is less energetic than, say, 50 GeV is of the order of several μb. In a year’s run one expects to see about 10–10 jet events. Of these about 10 events contain four or more jets. The expected distribution in the number of jets, N , is shown in Fig. 1. Since the underlying event comes with a multiplicity density, dNπ/dy, of 2000–3000 pions per unit of rapidity, the transverse energy within a cone of ∆R = 0.5 is about 20–30 GeV (assuming 〈pT 〉 ≈ 0.5 GeV per pion). Thus 50 GeV jets are easily observed above this background.