Transverse momentum of Z

bosons in Zee and Zmm decays

Daniel Beecher12 December 2005

Outline

● Why measure W width?● Problems measuring W and Z pT● Ways to predict W pT● Fast detector simulation in fits –

smearing matrix● Z pT fits● Next work...

Why measure the W width?

● UCL currently working toward measuring the W width from CDF Run II data.

● An anomalous W width would indicate new physics.

● Useful because any new physics in the electroweak sector should be seen at CDF.

● Sets the foundations if you wish to measure the mass of the W boson.

W boson pT● W width is measured from

the Mt of the W – accurate W pT needed.

● Main process behind W pT is emission of a gluon by one or both of the colliding quarks.

● One way to measure the boson pT is from combining the pT of the decay products.

● Alternatively, measure the pT of the jet from the radiated gluons.

● Neither of these are accurate ways to measure W pT.

Z boson pT● Z boson decay is easier

to detect.● Not only leptonic decay

routes for W and Z but Z/W to quarks can be lost in QCD background.

Predict W pT from QCD?● Why not calculate the pT

of the gluon from QCD?

Predict W pT from QCD?● Why not calculate the pT

of the gluon from QCD?

Predict W pT from QCD?● Why not calculate the pT

of the gluon from QCD?● Can use QCD to describe

to the higher pT gluons.

?

Predict W pT from QCD?● Why not calculate the pT

of the gluon from QCD?● Can use QCD to describe

to the higher pT gluons.

✔

Predict W pT from QCD?● Why not calculate the pT

of the gluon from QCD?● Can use QCD to describe

to the higher pT gluons.● Low pT gluons are in the

region where perturbative QCD breaks down.

● Low pT gluons can be modelled using resummed calculations or non-perturbative physics.

✔?

Predict W pT from QCD?● Why not calculate the pT

of the gluon from QCD?● Can use QCD to describe

to the higher pT gluons.● Low pT gluons are in the

region where perturbative QCD breaks down.

● Low pT gluons can be modelled using resummed calculations or non-perturbative physics.

● The techniques can involve fitting to data.

✔✘

W pT from Z pT?● The ratio for the crossections of W and Z with respect

to pT is stable.● Fit an ad hoc functional form to Z pT and then it can be

used to predict the W pT.

True pT and measured pT● The functional form only

describes the true pT.● The effect of the

detector resolution is taken into account by 'smearing' the true pT and fitting the measured pT to data.

● Did not deconvolute the data and fit for a true pT.

pT smearing● Use group's fast simulation to generate MC Z decay events to

plot true and measured pT distributions for a given pT profile.● Functional form for an arbitrary choice of parameters can be

smeared using bin-by-bin reweighting.

● Does not reveal how each pT bin is affected by the simulation.

pT smearing matrix● Each element corresponds

to the likelihood of how a true pT will be measured.

● Matrix is generated by creating millions of MC Z decays and binning the weight of the event into the correct matrix element.

● Normalise the matrix by insisting that every pT column (true pT) adds up to unity; an identity matrix would correspond to a perfect detector.

True pT

Meas. pT

pT smearing matrix● Measured pT histogram created by multiplying a vector

of true pT bins by the matrix.

● Method allows greater insight into how various pT bins are affected by the detector.

● Only need to generate the matrix once and can then be used to simulate the detector resolution quickly on any supposed true pT.

Zee fit

Zmm fit

Next work...● Is the measured Et scale

linear in Et?

Next work...● Is the measured Et scale

linear in Et?● Low energy photons

from bremsstrahlung can be absorbed by the solenoid.

Next work...● Is the measured Et scale

linear in Et?● Low energy photons

from bremsstrahlung can be absorbed by the solenoid.

● At high Et energetic electron showers can punch through the EM calorimeter.

Next work...● Is the measured Et scale

linear in Et?● Low energy photons from

bremsstrahlung can be absorbed by the solenoid.

● At high Et energetic electron showers can punch through the EM calorimeter.

● Over time the back of the calorimeter's relative response increases as a result of radiation damage.

● Build into the simulation.

less responsive

Summary● An accurate W width measurement needs an accurate

W pT.● W pT is difficult to measure directly but Z pT is

relatively straightforward.● Can partially predict W pT from theory but it is

simpler to predict the W pT by modelling the Z pT● Can carry the effect of a detector into a fitting

process by using a matrix.● Choice of functional form did fit to data after taking

detector smearing into account. Fit parameters can be used to generate W pT.

● Et response to be explored next.

The End

● Thank you for listening.

Fitting pTM to data● A smearing matrix for Zee was generated by simulating 10

million MC Z decays and binning the pTT and pTM of electrons passing the cuts built into the simulation – 0.9 million passed cuts.

● For a given choice of input parameters a new true pT is generated from the functional form, the bin weights are put into a vector which is then multiplied by the smearing matrix to create a pT distribution as seen by the detector.

● TMinuit used to minimise the chi square fit of the smeared pT profile to the Run-II data.

● Process repeated for Zmm with 0.7 million events passing cuts (10 million initial Z decays).

Matrix check