After discovery of the narrow mass peak of the ZH boson, the next step is model selection. Determination of the parameters of the ZH may allow/help to distinguish between the many models of New Physics that give rise to a narrow TeV resonance. The couplings of the ZH to fermions take on the following form:
gH bar{f} &gamma&mu (vf - af &gamma5) f (ZH)&mu
where gH is the coupling constant and vf and af represent the vector and axial parts of the coupling. In the LR Twin Higgs model the couplings of the heavy partner of the Z boson to fermions (except for t, tH and the right-handed neutrino &nuR) take the following form:
- gH = g/(2 &sdot cw &sdot &radic(cw2-sw2))
- vf = +T3 -2 Q sw 2
- af = -T 3*(cw2-sw2)
, whereas the Standard Model coupling for the Z boson looks like:
- gZ=g/(2 &sdot cw)
- vf =+T3-2 Q sw2
- af =+T 3
In case sufficient statistics can be accumulated, the determination of the forward-backward assymetry of the produced electron positron pair may allow to distinguish between different models.
Samples
As part of the di-lepton session of the Les Houches work-shop (2007) some event samples were generated with Pythia 6. To this the couplings of the generic LR Z' prime are set to the values expected in the left-right Twin Higgs model. For all benchmark points listed above, events are generated for the signal only process (Z H , process 141 with MSTP(44) = 3), background only (Z / &gamma *, process 1) and the process where the full interference between Z H / Z / &gamma * (process 141 with MSTP(44) = 7). The center-of-mass energy cut-off was set to 500 GeV, in order to achieve sufficient statistics in the relevant mass range. The three gauge bosons Z H, Z and &gamma are forced to decay into an electron-positron pair.
As a first intent events are generated for the 1196 GeV point. The cross-sections (including the branching ratio) for the different processes are 152.4 fb (signal), 108.5 fb (bkg) and 217.4 (signal+bkg). Note that the Pythia cross-sections differ slightly (less than 5 %) from the results quoted in the table above.
The momenta of electron and positron are combined to reconstruct the parent invariant mass. These ROOT files for 1196 GeV , 1495 GeV , 2407 GeV, and 3587 GeV contain the three resulting histograms. The same histograms are shown in the figure above (the histogram with red fill colour represents the signal, the one in green fill colour the background and the histogram with the blue hash corresponds to the interference of both).
For each case, the number of events corresponding to 10 fb -1 is generated. The numbers of events are listed in the table below. The total histogram content (remember the cut-off at 500 GeV) is given in the first three columns. The last three columns list the number of events in a mass window of twice the natural width of the ZH (50 GeV, 62 GeV, 104 GeV and 153 GeV, respectively).The significances one would calculate with these numbers are valid in the limit of a perfect lepton energy measurement (which may be OK for electrons). As soon as I have a reliable estimate of the energy resolution I can re-evaluate the numbers in a more realistic mass window.
| mass point | signal (total) | bkg. (total) | signal+bkg (total) | signal (window) | bkg (window) | signal+bkg (window) |
| 1196 | 1524 | 1085 | 2127 | 1301 | 6 | 1123 |
| 1495 | 558 | 1085 | 1493 | 466 | 7 | 464 |
| 2407 | 52.6 | 1085 | 1036 | 44 | 1 | 43 |
| 3587 | 4.5 | 1085 | 1000 | 5 | 0 | 2 |
Further pages:
