1 | //STARTHEADER
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2 | // $Id$
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3 | //
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4 | // Copyright (c) 2005-2011, 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 and are described in hep-ph/0512210. If you use
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16 | // FastJet as part of work towards a scientific publication, please
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17 | // include a citation to the FastJet paper.
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18 | //
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19 | // FastJet is distributed in the hope that it will be useful,
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20 | // but WITHOUT ANY WARRANTY; without even the implied warranty of
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21 | // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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22 | // GNU General Public License for more details.
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23 | //
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24 | // You should have received a copy of the GNU General Public License
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25 | // along with FastJet. If not, see <http://www.gnu.org/licenses/>.
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26 | //----------------------------------------------------------------------
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27 | //ENDHEADER
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28 |
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29 | #include <fastjet/tools/TopTaggerBase.hh>
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30 |
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31 | FASTJET_BEGIN_NAMESPACE
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32 |
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33 | using namespace std;
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34 |
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35 | // compute the W helicity angle
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36 | //
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37 | // The helicity angle is a standard observable in top decays, used to
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38 | // determine the Lorentz structure of the top- W coupling [13]. It is
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39 | // defined as the angle, measured in the rest frame of the
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40 | // reconstructed W, between the reconstructed top's flight direction
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41 | // and one of the W decay products. Normally, it is studied in
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42 | // semi-leptonic top decays, where the charge of the lepton uniquely
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43 | // identifies these decay products. In hadronic top decays there is an
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44 | // ambiguity which we resolve by choosing the lower pT subjet, as
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45 | // measured in the lab frame.
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46 | //
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47 | // The jet passed to this function is expected to already have
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48 | // the structure of a top, including a functional "W()" call;
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49 | // the W must be made of two pieces.
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50 | double TopTaggerBase::_cos_theta_W(const PseudoJet & res) const{
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51 | // the two jets of interest: top and lower-pt prong of W
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52 | const PseudoJet & W = res.structure_of<TopTaggerBase>().W();
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53 | vector<PseudoJet> W_pieces = W.pieces();
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54 | assert(W_pieces.size() == 2);
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55 | //assert(W_pieces[0].perp2() >= W_pieces[1].perp2());
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56 | //PseudoJet W2 = W_pieces[1];
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57 | // extract the softer of the two W pieces.
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58 | PseudoJet W2 = (W_pieces[0].perp2() < W_pieces[1].perp2())
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59 | ? W_pieces[0]
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60 | : W_pieces[1];
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61 | PseudoJet top = res;
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62 |
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63 | // transform these jets into jets in the rest frame of the W
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64 | W2.unboost(W);
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65 | top.unboost(W);
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66 |
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67 | return (W2.px()*top.px() + W2.py()*top.py() + W2.pz()*top.pz())/
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68 | sqrt(W2.modp2() * top.modp2());
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69 | }
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70 |
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71 |
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72 | FASTJET_END_NAMESPACE
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