Single- and multi-photon events with missing energy in e+e− collisions at LEP

Abstract

Single- and multi-photon events with missing energy are selected in 619 pb−1 of data collected by the L3 detector at LEP at centre-of-mass energies between 189 GeV and 209 GeV. The cross sections of the process e+e− → vv-bar gamma (gamma) are found to be in agreement with the Standard Model expectations, and the number of light neutrino species is determined, including lower energy data, to be N = 2.98 +/- 0.05 +/- 0.04.\ud Selection results are given in the form of tables which can be used to test future models involving single- and multi-photon signatures at LEP. These final states are also predicted by models with large extra dimensions and by several supersymmetric\ud models. No evidence for such models is found. Among others, lower limits between 1.5 TeV and 0.65 TeV are set, at 95% confidence level, on the new scale of gravity for the number of extra dimensions between 2 and 8

Search for new phenomena in events with a photon and missing transverse momentum inppcollisions ats=8 TeVwith the ATLAS detecto

“…Events that contain a high-momentum photon and large missing transverse momentum (referred to as γ þ E miss T ) constitute a low-background sample that provides powerful sensitivity to some models of new phenomena [1][2][3][4][5][6][7]. Theories with large extra spatial dimensions (LED), presence of dark matter (DM), or supersymmetric partners of the quarks (squarks) in a compressed mass spectrum scenario predict the production of γ þ E miss T events in pp collisions beyond Standard Model (SM) expectations.…”

First look at the physics case of TLEP

“…Above the Z peak, the e + e − → Zγ process provides a very clean photon-tagged sample of on-shell Z bosons, with which the Z properties can be measured. From the WW threshold scan alone, the cross section of about 5 pb [59][60][61][62] ensures that 10 million Zγ events will be produced in each TLEP experiment with a Z → νν decay and a high-energy photon in the detector acceptance. The three million Zγ events with leptonic Z decays will in turn provide a direct measurement of the ratio Γ inv Z /Γ lept Z , in which uncertainties associated with absolute luminosity and photon detection efficiency cancel.…”

ALPs at colliders

“…The lighter masses are conducive to a stable ALP and are best searched for in mono-γ + / E T searches. These have been performed at the 7 and 8 TeV runs of the LHC [39][40][41][42] as well as previously at LEP [43][44][45][46]. The limit from electron-positron colliders, having already been determined, was taken from [38] and reproduced in figure 2 (solid purple area) while we reinterpret the LHC analyses in order to constrain the ALP scenario.…”