1. Transverse momentum of Z bosons in Zee and Zmm decays Daniel Beecher 12 December 2005

2. 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...

3. 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.

4. 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.

5. 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.

6. Predict W pT from QCD? ● Why not calculate the pT of the gluon from QCD?

7. Predict W pT from QCD? ● Why not calculate the pT of the gluon from QCD?

8. 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. ?

9. 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. ✔

10. 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. ✔ ?

11. 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. ✔ ✘

12. 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.

13. 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.

14. 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.

15. 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

16. 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.

17. Zee fit

18. Z mm fit

19. Next work... ● Is the measured Et scale linear in Et?

20. Next work... ● Is the measured Et scale linear in Et? ● Low energy photons from bremsstrahlung can be absorbed by the solenoid.

21. 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.

22. 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

23. 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.

24. The End ● Thank you for listening.

26. 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).

27. Matrix check