[Rivet] Bug in jet definition

[Andy Buckley]

Hi Roman,

That's very worrying. I'll look into it asap. Annoying that things like 
this always seem to appear just after we make a release, but thanks 
*very* much for investigating.

Cheers,
Andy


On 14/11/11 14:12, Roman Kogler wrote:
> Dear Rivet developers,
>
> I believe I found a bug in the jet definition of Rivet. It's a quite
> annoying feature and it took me quite some time to figure it out.
>
> First, I couldn't reproduce the results that come out of my jet analysis
> in H1. To find the difference I put some Sherpa events into the format
> we use in H1 for our analyses. Then I started to compare the output
> even-by-event, using the same events in my H1 analysis and the Rivet
> analysis I wrote. The boost of the HFS to the Breit frame is identical,
> as well as the number of jets found. Even the jet constituents of each
> jet are the same in both analyses. So finally I tracked the difference
> to the jet four-vector. It seems that Rivet overwrites the jet
> four-vector as calculated by FastJet. To clarify that I looked into the
> FastJets projection, Projections/FastJets.C. I put some output
> statements into the method _pseudojetsToJets, the output is labelled
> "from FastJet" below (all code can be found at the end of this email).
> Also, I put some output into my analysis using the standard
> jet::momentum() method ("from Rivet"). I then recalculate the jet
> four-vector with the pt-recombination scheme ("my calculation"). Here
> are the results for one event:
>
> jet 0: from FastJet, Pt = 14.6163 phi = 6.27012 eta = 0.406647
> jet 1: from FastJet, Pt = 14.3426 phi = 3.08307 eta = 0.240164
>
> jet 0: from Rivet, Pt = 14.0764 phi = 6.27134 eta = 0.443595, pt
> weighted phi = -0.0106254 pt weighted eta = 0.406647
> jet 1: from Rivet, Pt = 13.5777 phi = 3.08787 eta = 0.259143, pt
> weighted phi = 3.09267 pt weighted eta = 0.240164
>
> jet 0: my calculation: pt = 14.6163 phi = 6.27012 eta = 0.406647
> jet 1: my calculation: pt = 14.3426 phi = 3.08307 eta = 0.240164
>
> The jet four-vectors from FastJet agree with the ones using my
> implementation of the pt-recombination scheme. Also, these numbers are
> identical with the numbers I get using the H1 analysis framework (with
> its own interface to FastJet). But the jets stored by Rivet have
> different pt! How could this happen? It is really quite a dangerous bug,
> I believe. Please have a look at it soon.
>
> Cheers,
> Roman
>
>
> P.S. Here is the code I used for the output above:
>
>
> -----------------------------------------------
> Projections/FastJets.C:
>
> Jets FastJets::_pseudojetsToJets(const PseudoJets& pjets) const {
> Jets rtn;
> int i=0;
> foreach (const fastjet::PseudoJet& pj, pjets) {
> cout << "jet " << i++ << ": ";
> cout << "from FastJet, Pt = " << pj.perp() << " phi = " << pj.phi() << "
> eta = " << pj.rap() << endl;
> ...
> }
>
>
> -----------------------------------------------
> H1 jet analysis:
>
> void init() {
> ...
> addProjection(FastJets(dfs, fastjet::kt_algorithm, fastjet::pt_scheme,
> 1.), "FastJet");
> ...
> }
>
> void analyze(const Event& event) {
>
> const FastJets & fj = applyProjection<FastJets>(event, "FastJet");
> Jets jets = fj.jetsByPt(3., 80.);
>
> vector<FourMomentum> jet_vecs;
> int njet = 0;
>
> foreach (const Jet& j, jets){
>
> cout << "jet " << njet++ << ": ";
> cout << "from Rivet, Pt = " << j.momentum().pT() << " phi = " <<
> j.momentum().phi() << " eta = " << j.momentum().eta()
> << " pt weighted phi = " << j.ptWeightedPhi() << " pt weighted eta = "
> << j.ptWeightedEta() << endl;
>
> // recalculate the jet four-vector with my own implementation of the
> pt-recombination scheme
> const vector<Particle> parts = j.particles();
> FourMomentum jet_vec(0, 0, 0, 0);
>
> foreach(const Particle& jp, parts){
>
> // scale particle momtenta to make them massless
> double pp = sqrt(jp.momentum().px()*jp.momentum().px() +
> jp.momentum().py()*jp.momentum().py() +
> jp.momentum().pz()*jp.momentum().pz() );
> FourMomentum pv(pp, jp.momentum().px(), jp.momentum().py(),
> jp.momentum().pz());
> if (ij==0){
> jet_vec = pv;
> } else {
> jet_vec = MergeParts(pv, jet_vec);
> }
> ++ij;
> }
>
> cout << "my calculation: pt = " << jet_vec.pT() << " phi = " <<
> jet_vec.phi() << " eta = " << jet_vec.eta() << endl;
> }
>
>
> FourMomentum MergeParts(FourMomentum a, FourMomentum b)
> {
> // merge momenta a and b with pt-recombination scheme
>
> double pt = a.pT() + b.pT();
> double phi_a = a.phi();
> double phi_b = b.phi();
> if (phi_a - phi_b > fastjet::pi) phi_b += fastjet::twopi;
> if (phi_a - phi_b < -fastjet::pi) phi_b -= fastjet::twopi;
> double phi_sum = a.pT()*phi_a + b.pT()*phi_b;
> double phi = phi_sum / pt;
> double eta_sum = a.pT()*a.pseudorapidity() + b.pT()*b.pseudorapidity();
> double eta = eta_sum / pt;
>
> FourMomentum v(pt*cosh(eta), pt*cos(phi), pt*sin(phi), pt*sinh(eta));
> return v;
>
> }
>
>


-- 
Dr Andy Buckley
SUPA Advanced Research Fellow
Particle Physics Experiment Group, University of Edinburgh

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.