| 1 | //FJSTARTHEADER
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| 2 | // $Id: TopTaggerBase.cc 3433 2014-07-23 08:17:03Z salam $
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| 3 | //
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| 4 | // Copyright (c) 2005-2014, Matteo Cacciari, Gavin P. Salam and Gregory Soyez
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| 5 | //
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| 6 | //----------------------------------------------------------------------
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| 7 | // This file is part of FastJet.
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| 8 | //
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| 9 | // FastJet is free software; you can redistribute it and/or modify
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| 10 | // it under the terms of the GNU General Public License as published by
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| 11 | // the Free Software Foundation; either version 2 of the License, or
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| 12 | // (at your option) any later version.
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| 13 | //
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| 14 | // The algorithms that underlie FastJet have required considerable
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| 15 | // development. They are described in the original FastJet paper,
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| 16 | // hep-ph/0512210 and in the manual, arXiv:1111.6097. If you use
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| 17 | // FastJet as part of work towards a scientific publication, please
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| 18 | // quote the version you use and include a citation to the manual and
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| 19 | // optionally also to hep-ph/0512210.
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| 20 | //
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| 21 | // FastJet is distributed in the hope that it will be useful,
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| 22 | // but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 23 | // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 24 | // GNU General Public License for more details.
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| 25 | //
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| 26 | // You should have received a copy of the GNU General Public License
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| 27 | // along with FastJet. If not, see <http://www.gnu.org/licenses/>.
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| 28 | //----------------------------------------------------------------------
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| 29 | //FJENDHEADER
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| 30 |
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| 31 | #include <fastjet/tools/TopTaggerBase.hh>
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| 32 |
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| 33 | FASTJET_BEGIN_NAMESPACE
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| 34 |
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| 35 | using namespace std;
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| 36 |
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| 37 | // compute the W helicity angle
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| 38 | //
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| 39 | // The helicity angle is a standard observable in top decays, used to
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| 40 | // determine the Lorentz structure of the top- W coupling [13]. It is
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| 41 | // defined as the angle, measured in the rest frame of the
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| 42 | // reconstructed W, between the reconstructed top's flight direction
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| 43 | // and one of the W decay products. Normally, it is studied in
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| 44 | // semi-leptonic top decays, where the charge of the lepton uniquely
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| 45 | // identifies these decay products. In hadronic top decays there is an
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| 46 | // ambiguity which we resolve by choosing the lower pT subjet, as
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| 47 | // measured in the lab frame.
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| 48 | //
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| 49 | // The jet passed to this function is expected to already have
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| 50 | // the structure of a top, including a functional "W()" call;
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| 51 | // the W must be made of two pieces.
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| 52 | double TopTaggerBase::_cos_theta_W(const PseudoJet & res) const{
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| 53 | // the two jets of interest: top and lower-pt prong of W
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| 54 | const PseudoJet & W = res.structure_of<TopTaggerBase>().W();
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| 55 | vector<PseudoJet> W_pieces = W.pieces();
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| 56 | assert(W_pieces.size() == 2);
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| 57 | //assert(W_pieces[0].perp2() >= W_pieces[1].perp2());
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| 58 | //PseudoJet W2 = W_pieces[1];
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| 59 | // extract the softer of the two W pieces.
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| 60 | PseudoJet W2 = (W_pieces[0].perp2() < W_pieces[1].perp2())
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| 61 | ? W_pieces[0]
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| 62 | : W_pieces[1];
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| 63 | PseudoJet top = res;
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| 64 |
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| 65 | // transform these jets into jets in the rest frame of the W
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| 66 | W2.unboost(W);
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| 67 | top.unboost(W);
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| 68 |
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| 69 | return (W2.px()*top.px() + W2.py()*top.py() + W2.pz()*top.pz())/
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| 70 | sqrt(W2.modp2() * top.modp2());
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| 71 | }
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| 72 |
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| 73 |
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| 74 | FASTJET_END_NAMESPACE
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