# Presentation: 547729

## W/Z+ CSC Note Discussion

- ATLAS Physics Coordinator required NLO cross-sections /analysis for the CSC note.
- ATLAS doesn’t have official NLO MC generator for W and Z event generation. So far, we used PYTHIA event generator and performed full simulations for CSC note studies.
- The BHO generator includes both the LO & NLO calculations, also the non-SM couplings.  But the parton shower is incomplete and vector bosons have zero width.
- We noticed that the production cross-sections highly depend on Et() cut, and  isolation cut, e.g. R(lepton, g) > 0.7 to reject final state lepton bremstrahlung background
- The k-factor = (NLO)/(Born) highly depend on Et() cut and R cut.

## A proposal

- How to ‘normalize’ the LO spectra to the NLO spectra – with what initial cuts, and with running k-factor (or differential k-factors)?
- How to produce the non-SM spectra to study the anomalous couplings?  e.g. based on Born calculations or NLO calculations?  -- if it is NLO, we need to produce the NLO SM spectra – to do it, we need to have the selection efficiency as a function of Photon Pt or Mt(Wgamma)
**Proposal:**
**Obtain the event selection efficiencies as a func of ET(********) (based on fully simulated pythia events)**
- -- Apply the efficiency (bin-by-bin) to the BHO LO curve to compare that to PYTHIA full simulation spectra
- -- Then, apply the efficiency to the NLO curve to obtain the NLO ‘expectations’ and mock data.

## Proposal (continued)

- LO After cuts
- (obtain eff. Vs pt)

**NLO (no cuts)**

**NLO (w/ cuts)**

## W and Z Production

**Major theoretical ref:**

## Born Term Diagrams

## Next-to-Leading-Order diagrams (1)

## Next-to-Leading-Order diagrams (2)

## Additional ‘Leading-Order’ term in BHO generator for W and Z calulations

- Process: q + g  V + q (q->q )
-    photons can be produced from the bremsstrahlung of final state quark.  At the leading-logarithm level, the photon bremsstrahlung process is of the same order as the Born process. It is included in the generator as leading order x-section:
- ******(****LL****) = ******** (Born) + ********(Bremstrahlung)**
- Note: in PYTHIA, only the W Born term is in the LO calculations; q+g  V + q are W+jet and Z+jet events

## Some Difference between BHO and PYTHIA for W, Z @LO (Born) Calculations

- BHO: only includes on-mass-shell W & Z
- PYTHIA: includes real & virtue Z, and W
- BHO cut the photon energy after ‘boost’ – in LHC lab frame
- PYTHIA cut the photon energy before ‘boost’ – in CM frame.
**Consequences**: the cross-section directly obtained from generators are not the same even at the Born term level.

## Our Definition of k-factor

- K =  (NLO) /  (Born)
- We noticed that k-factor will depend on the photon energy cut and photon-lepton isolation cut (R (l, ) > 0.7).
- We should use the Born cross-section with defined photon cut (after boost) and photon isolation cut (delta R > 0.7)
- Using PYTHIA  (Born) with corrected cross-section* (see next page)

## *How to obtain  (Born)

- Run Pythia with ‘default’ photon energy cut (1 GeV – this cut applied at CM frame)
- Determine the  (Born) with photon cut at higher level after ‘boost’ (event filter) – need only cut on ‘prompt’ photon Pt
- Above should automatically include the virtual W, Z, and  contributions

## How to separate brem. Terms?

- Experimentally, there are events with photon come from final state quark and lepton bremstrahlung. Those events are our background come from the processes
- ‘Single’ W and Z production with final state radiation
- Z+jet and W+jet with photon radiating from quarks (jets)
**Reject those background:**
- Photon isolation (with 0.7 corn size)
- Mass (lepton, lepton, photon)>100 GeV
-    or MT(lepton, MET,photon) > 60 (?) GeV)

## CSC Data-Sets and K-factor

**MC signal samples**
-    Using W/Z + gamma10 (PYTHIA Born term production)
- In analysis program cut Photon Et > 25 GeV.
- W (Born) = 1.73 pb (for W e)
-  (Born)  = 1.28 pb (for Z ee)  (including virtue)
- (here Z Born cross-section is higher than BHO cross-section since virtue Z, gamma decay to dilepton is included)
**Background samples (with PYTHIA generator):**
- W+jet, Z+jet
- Single W and single Z with final state gamma brem.
**K-factors** will be provided by Tiesheng for the CSC note following this message discussions.

## W  e cross-sections

- The next two pages list the corss-sections for W eas function of photon Et cut
- Not lepton-photon separation (R) cut
- With R(lepton, ) > 0.7 cut
- Listed cross-sections are obtained from BHO MC generator. Cross-checked with PYTHIA generator Born term cross-sections. Both cross-sections for born and NLO are given in the tables

## Wgamma Cross-sections as a function of photon Et cut

| Et() >1 GeV | 74.82pb | 115.28pb | 98.79pb | 158.26pb |
| --- | --- | --- | --- | --- |
| Et() >10 GeV | 8.21pb | 16.93pb | 11.75pb | 22.63pb |
| Et() >20 GeV | 2.67pb | 6.67pb | 3.93pb | 8.80pb |
| Et() >25 GeV | 1.73pb | 4.65pb | 2.57pb | 6.18pb |

**W****-******

**W****+******

******(Born)**

**(Born****)**

**(NLO)**

**(NLO)**

****

****

****

**(W ******** e ********)**

**(W ******** e ********)**

## W Cross-sections as a function of photon Et cut (with R(l, ) > 0.7)

| Et() >10 GeV | 8.15pb | 13.63pb | 11.67pb | 19.25pb |
| --- | --- | --- | --- | --- |
| Et() >20 GeV | 2.65pb | 5.35pb | 3.90pb | 7.51pb |
| Et() >25 GeV | 1.72pb | 3.75pb | 2.56pb | 5.25pb |
| Et() >30 GeV | 1.18pb | 2.75pb | 1.76pb | 3.83pb |

**W****-******

**W****+******

******(Born)**

**(Born****)**

**(NLO)**

**(NLO)**

****

****

****

**(W ******** e ********)**

**(W ******** e ********)**