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Why CP in gtt?. ?. Standard model contribution is not detectable -> If you see something it must indicate new physics! Sizeable effects possible from popular models due to the large top mass, for example: SUSY and multiple higgs doublets.

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why cp in gtt

Why CP in gtt?


Standard model contribution is not detectable -> If you see something it must indicate new physics!

Sizeable effects possible from popular models due to the large top mass, for example: SUSY and multiple higgs doublets.

Huge sample of tt will be available at LHC which enables precision measurements.

model independent approach

Model independent approach


Standard model symmetries

Lorentz invariance

Topology of: pp -> tt+X

Keep lowest order operator

Use the lowest order operator to construct an effective term that can be added to the SM Lagrangian:


Chromo-Electric Dipole Moment.

event generator

Event generator

Extract the Feynman rules from the effective Lagrangian and implement them as a MC generator.

Only gluon-gluon production included since it dominates at LHC (90%).

Four diagrams needed. Full tree-level matrix elements (Madgraph) used in order to maintain the spin correlation needed to see the CP violation (controlled by the parameter D5). Fragmentation by an interface to PYTHIA 6.1.

200 more SM diagrams exist to this final state, they are all sub dominant.



Theoretically, the optimal observable for D5 is given by:

That is, the cross-section introduced by the perturbation controlled by D5 divided by the unperturbed cross-section.

It is a very complicated function but serves as a benchmark for other less complicated observables. Many simple ones are found in the literature. The best one found for a real D5 is:

(One of the leptons can be replaced by a d-type quark)



The most robust and efficient way to extract the asymmetry turns out to be by pure counting:

This point was missed earlier in the investigation but does not affect the end result. The mean or a fit to the asymmetry distribution gives at most the same efficiency as pure counting.

event reconstruction

Event reconstruction

Fast parameterised simulation of the ATLAS detector.

Complete kinematics reconstructed, both for the dilepton (analytic) and lepton+jet decay mode (3C fit).

  • Dilepton selection:
  • Valid trigger and two leptons, oppositely signed, pt > 15 (6) GeV.
  • Two b-jets, pt > 30 GeV, Only the four most energetic considered.
  • Analytical solutions, require top & W within 10 GeV.
  • Lepton+jet selection:
  • Valid trigger and at least four jets and one lepton, pt > 20 GeV.
  • Two b-jets, pt > 30 GeV, two jets with pt > 15 GeV.
  • Consider the six most energetic jets. Keep events with top(W) within 60(40) GeV.
  • 3C fit, keep prob(2) > 10%.

Typical event, s-channel, including initial and final state radiation.

details of the analysis

Details of the analysis

60% of the b-jets identified within a 0.3 cone.

Critical to identify the b-jet charge, otherwise you are blind! Rely on the kinematics to do the job. Turns out to work well.

Some improvement to the sensitivity by tagging the d-type quark using the least energetic jet (however, this is not critical).

The simple histogram mean is a good estimator of the asymmetry.

The asymmetry after 2 fb-1 of jet+lepton data.

S/N=7 for D5=6*10-18 *gs.



For one experiment at LHC after one year of low luminosity, both the dilepton and the lepton+jet channel has the potential to a 5 discovery for a chromo-electric dipole moment

  • One requirement is that C5 is small. A good handle to certify this is the lepton pt spectrum.
  • f2 is very robust against systematic effects. Virtually no fake asymmetries expected.
  • Most model predictions are an order of magnitude smaller. However, this is a model independent estimation.


The huge sample of pp -> tt+X at LHC will enable precision measurements of anomalous couplings in the top sector. The anomalous couplings may provide an unbiased and model independent window to new physics. For example new sources of CP violation.

Lorentz invariant asymmetries, like f2, are extremely robust against systematic effects, both to the instrumental and the SM contributions. For example high loop orders are needed to fake CP violating asymmetries and asymmetric detector efficiencies do not contribute.

Analysis almost complete. Remains to check the unitarity constraints and the efficiency of the optimal observable. I expect to be finished within the next two weeks.