A method of top tagging is introduced. Using the anti-kt algorithm to define jets, events with nj = 2 fat jets of cone size R = 1.5 are decomposed into R = 0.6 sub-jets and retained if nj (R= 0.6) ≥ 4 . One pair of sub-jets reconstructs the W-mass and another jet is tagged as a b-jet, as necessary for hadronic or semileptonic events of tt origin. This 'hollow cone' method distinguishes the tt events from the light parton QCD dijet events.The top quark is important in the Standard Model (SM) because of its the large coupling to the Higgs field. Also, top is of great interest in new physics extensions of the SM because one or more new particles commonly decay into a single top quark or into top-quark pairs. Examples include SUSY [1], where the stop companion of top may decay to top and the gluino companion of the gluon may decay to top pairs, the Little Higgs Theory [2] where a heavy top may decay to top, the Randall-Sundrum model [3] where the decays of Kaluza-Klein gluons may lead to an enhancement of top signals. Thus, the study of top at the Tevatron and at the Large Hadron Collider (LHC) should be an excellent probe of new physics. We focus on the semileptonic channel (one t→ b l ν decay and one t→b q q decay) and the hadronic channel (both tops decay to quarks); the leptonic channel with both tops decaying via t →b l ν has a clean signal but a reduced rate. The expected tt event ratios in the SM are hadronic : semileptonic : leptonic = 0.322 : 1: 3.13 . Silicon vertex detectors have enabled the tagging of b-events with high efficiency (∼ 50%) using the secodary vertes algorithm [4] at the Tevatron and b-tagging efficiencies of 60% [5], 65% [6] are estimated for the ATLAS and CMS detectors.At the Tevatron collider, b-tagging and kinematical selection has been used to identify ttbar events in the fully hadronic channel [7]. For the semi-leptonic channel, the CDF collaboration employs b-tagging, jet multiplicity and an isolated electron or muon to do the event selection, and uses a matrix element method to tag a top [8]. The D0 collaboration selects events with exactly 4 jets and assigns the jets -1 jet associated with the lepton and the other 3 jets with the hadronic top decay in a way that minimizes the reconstructed mass difference between the two top quarks [9].