[Rivet] Bug in jet definition

[Andy Buckley]

Hi Roman,

I just had a bit of a look at this -- to check that I've understood this 
correctly, is this mismatch issue only seen when using a jet 
recombination scheme other than E (i.e. pure 4-vector addition)?

Cross-checking FastJet's manual re. the pT scheme and Rivet's 
pT-weighted jet properties, there seems to be a difference in that the 
FastJet one rescales momenta to be massless before combining. Rivet 
doesn't do that: we just do this

       foreach (const FourMomentum& p, momenta()) {
         double pt = p.pT();
         ptsum += pt;
         ptwetasum += pt * p.pseudorapidity();
         ptwdphisum += pt * mapAngleMPiToPi(phi0 - p.azimuthalAngle());
       }
       _ptWeightedEta = ptwetasum/ptsum;
       [...]

Maybe that's definitely wrong -- if so we can fix it. Or just remove it 
/ base it on the accumulated 4-vector. There is probably work that can 
be done to improve the Rivet::Jet interface, which was not very 
carefully designed and doesn't provide options for recombination other 
than the E scheme (but does have other reasons to exist).

Anyway, if I've understood correctly this is an area where I'd like to 
hear suggestions about how we can do better, but I'm breathing a sign of 
relief because I think all the "official" analyses that I know are using 
E-recombination and should be unaffected.

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

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