-F

Cheers,
T.

sorry for the ignorance, but do you have a quick reference to HDV’s work on W mass predictions ?

Cheers,
T.

no, the Monte Carlo is really a precise simulation if the physics of the collision – the interaction, the emitted particles, the hadronization, the decays in flight. Then there is a simulation of the interaction of these bodies with the detector material, and simulated readouts of the detector components. Some parts are parametrized, but most is simulated to the best of our knowledge. A Monte Carlo event looks exactly like a real one after reconstruction, except for the presence of an additional bank containing the particle level information.

Cheers,
T.

let’s see if I can answer your questions.
Using the Z to calibrate the calorimeter response to 40 GeV electrons is a good way but not the preferred one. The E/P method is more precise.

The recoil modeling is very important in the W mass measurement. In fact, it is one of the most important parts of the study. The recoil is modeled on Z->ee and Z->mm decays, and then the Monte Carlo is used to extrapolate to the W.

The Z->ll BR can be measured only in the ratio (it is about 10). Through it one can get a direct measurement of the W width, by the way.

Using the transverse mass is better than using the lepton Pt because of the recoil: the transverse mass is less sensitive on the recoil modeling – although the neutrino missing Et is then to be measured accurately.

Yes, the E/p relate to the electron measurement.

No, the bremsstrahlung occurs whenever the electron passes close to heavy-Z atoms. This occurs throughout its path. The soft photons get collected in a cluster of calorimeter energy, and are measured together with the electron shower.

The tracker measures the five parameters of a track helix. There is a 1.4 Tesla axial magnetic field, and there are stereo layers in the tracker that allow z-coordinate measurements. Track momenta are tuned precisely with J/psi mesons, Y mesons, Z bosons, to a very good accuracy (see the recent mass measurement of the X meson, I discussed it a week ago here).

When you reconstruct a transverse mass, you lose the z component of momenta. So the Mt is strictly smaller than the true M. That explains the skewed jacobian peak. The precise distribution depends on how central W production is, and that depends on PDF, on detector acceptance, etcetera…

Hope that helps.
Cheers,
T.

Do you have a physical model of the interaction region and the detectors and are running events like actual collisions, where you get outputs that look like collision pictures? Or do you have an interaction model for a particular detector and run off of a distribution for that? (e.g. a distribution of voltage values or temperature readings) Or both? I could see where putting several of the second kinds of run together might give something different than the first.

1. If I understand correctly, then you use to calibrate the electromagnetic calorimeter? Then this calibration is used to identify the energy of the electron in ?

What about hadronic recoil? E.g. ? This process is not so suppressed, especially when the gluon is close to collinear with the beam and one gets a factor of the strong coupling and a Sudakov logarithm. One would thus have a with nonzero transverse momentum and some hadronic jet. Isn’t it then important to somehow model the jet so that an accurate determination of the mass/width can be done?

I naively thought (reading one of the CDF papers, arXiv:0707.0085) that measuring the branching ratio had to do with some kind of modeling of the hadronic transverse momentum?

[My understanding is that plotting instead of helps in this case, but it's not obvious to me how this works.]

2. A very naive question: when you say that you look at the E/p distribution, which E and p are you using? Is this the electron E/p? Does this mean we’re comparing the energy deposited in the calorimeter compared to the momentum extracted from tracking?

When you explained the tail on the E/p distribution, you said this was because of bremsstrahlung of high-energy electrons. So the sot photons count towards the energy in the calorimeter, but not the curving in the tracking used to fit the momentum. Since the tracking system is closer to the beam pipe than the calorimeter, are you saying that the bremsstrahlung occurs nearly instantaneously so that the electron radiates away photons before drifting through the tracking chamber?

Also, how do you know what the momentum is in the expression ? Presumably energy of an electron can be measured in the EM calorimeter, but we can’t determine since we don’t know the longitudinal component of momentum? (Or do we just use the information from the pseudorapidity of the deposited energy in the calorimeter?)

3. In your last plot of the distribution with respect to transverse mass, can you explain why the shape takes this form? My understanding is that there’s a Jacobian peak that is smoothed out by various effects. Why is it that the `smoothing’ out causes the lower values of to be enhanced relative to the Jacobian peak?

Thanks very much!
Flip