(some Of) Recent Γγ Measurements from Lep

Inclusive measurement of J/Ψ (DELPHI), search for η b (ALEPH) and measurement of the cross-section of the double-tagged γγ events (ALEPH) are briefly overviewed. 1 Measurement of inclusive J/Ψ production in γγ events (DELPHI) The measurement of J/Ψ production in γγ interactions provides an insight into heavy quark production and also a possibility to study the gluon parton density of a resolved photon. Selection of the signal is done in two steps. First, preselection of hadronic γγ events is performed, defining the " visible " region (visible inv.mass W vis below 35 GeV/c 2 , charged multiplicity 4 ≤ N ch ≤ 30, transverse energy above 3 GeV, cuts ensuring high trigger efficiency). The preselected sample contains about 1.2% of Z 0 /γ * background. Please note that requirement of at least 4 reconstructed tracks suppress the J/Ψ + γ final state. Next, the J/Ψ signal is obtained requiring a pair of identified muons. The mass spectrum of the muon pair in the region of interest is shown in Fig.1a, with a clear J/Ψ signal (36 ± 7 events) on top of combinatorial γγ background. The fit of the mass spectrum gives M = 3.119 ± 0.008 GeV/c 2 , Γ(obs) = 0.035 ± 0.007 GeV. In order to learn more about the production mechanism (the absolute predictions of Pythia 6.156 1 fall well below (∼ 20%) the observed production rate), the relative fraction of resolved and diffractive processes was fitted to the shape of the transverse momentum squared of the reconstructed J/Ψ. For this study, the shapes of Pythia distributions, with full detector simulation and event selection, were used. Fig.1b shows the best fit dN dp 2 t = f · dN dp 2 t | Resolved +(1 − f) · dN dp 2 t | Dif f ractive , f = (74.0 ± 22.0)%, suggesting the dominant contribution of resolved processes, in agreement with the expectation (the fit is obviously driven by the tail of the transverse momen