# Wbb in the high pT HW region

### From Wiki Les Houches 09

Current revision (00:17, 5 August 2009) (view source) |
|||

(8 intermediate revisions not shown.) | |||

Line 5: | Line 5: | ||

* [http://xxx.soton.ac.uk/abs/0906.1923 Recent preprint on] Zbb and Wbb at NLO. | * [http://xxx.soton.ac.uk/abs/0906.1923 Recent preprint on] Zbb and Wbb at NLO. | ||

* [http://www.slac.stanford.edu/spires/find/hep/www?irn=7662955 HV high pT paper] | * [http://www.slac.stanford.edu/spires/find/hep/www?irn=7662955 HV high pT paper] | ||

- | * [Jet Substructure] | + | * [[Jet Substructure]] |

+ | * [http://wwwhep.physik.uni-freiburg.de/~giacinto/LesHouches/LesHouchesJetsWbbSession.pdf WH high pT analysis: actual estimation of Wbb background + future plans] | ||

+ | * Might also be interesting to contact Michael Deak, who did a short-term MCnet project on Zbb in Herwig context. | ||

Link slides here if you have them please! | Link slides here if you have them please! | ||

Line 17: | Line 19: | ||

General point is that because the qg -> Wqbbar channel opens up only at order alpha_s^2, this process is only LO and has a large scale dependence. Also, because the gluon PDF is larger than the qbar, this process has a large cross section. However, it also usually gives an extra jet, so being more exclusive (i.e. applying jet vetoes) reduces the K-factor from around 3 to probably below 1.5. | General point is that because the qg -> Wqbbar channel opens up only at order alpha_s^2, this process is only LO and has a large scale dependence. Also, because the gluon PDF is larger than the qbar, this process has a large cross section. However, it also usually gives an extra jet, so being more exclusive (i.e. applying jet vetoes) reduces the K-factor from around 3 to probably below 1.5. | ||

- | Other point is the existence of LO matched calculations (do they exist?) and possibility of doing a NLO matched calculation. | + | While the extra parton veto can be easily implemented in the NLO calculation, the problem in the NLO calculation at the moment is having to deal with large corrections in the region of phase space of high m(bb)/pT(bb). So even if in the WH analysis we are interested mostly at high m(bb) (around the Higgs mass), since pT(bb) is constrained to be above 200 GeV, we still land in a region where these logarithms are large. However a re-summation of these large logarithms could be implemented analytically in the calculation, which in principle permits to have up to NLL accuracy. |

+ | |||

+ | Other point is the existence of LO matched calculations (do they exist?) and possibility of doing a NLO matched calculation. Infact in a second step the NLO calculation | ||

+ | could be in principle interfaced for example to POWHEG, matching the exact NLO calculation with a LL parton shower. | ||

== Plan == | == Plan == | ||

GP and JMB to pass exact cuts and distributions of interest to LR and SD, who will do the calculation in the regions of interest. | GP and JMB to pass exact cuts and distributions of interest to LR and SD, who will do the calculation in the regions of interest. | ||

+ | |||

+ | Cuts implemented by GP in the MCFM calculation: | ||

+ | |||

+ | * use kT algorithm with DR=0.3 | ||

+ | * impose DR(b-jet,antib-jet)<1.2 | ||

+ | * pT(W)>200 GeV | ||

+ | * pT(bbbar)>200 GeV | ||

+ | * pT(each b-jet)>30 GeV |eta|<2.5 | ||

+ | * for a jet to exist pT(jet)>15 GeV |eta|<5 | ||

+ | Lepton cuts are not essential, but can be implemented as well: | ||

+ | * pT(lepton)>30 GeV |eta(lepton)|<2.5 | ||

+ | * ptMiss>30 GeV | ||

+ | |||

+ | |||

+ | |||

+ | Quantities of experimental interest: | ||

+ | * m(bbbar) | ||

+ | * pT(bbbar) | ||

+ | * pT additional jet in the event (especially in the region 0-150 GeV) | ||

+ | * DeltaEta(W,bbbar) | ||

+ | * K factor as a function of m(bbbar), pT of additional jet in the event and pT(bbbar) | ||

+ | |||

+ | |||

+ | Quantities of interest for the calculation (to look at the region of large logarithms): | ||

+ | * m(bbbar)/pT(bbbar) | ||

+ | * Delta_pT(b-jet,antib-jet)/pT(bbbar) |

## Current revision

**Interested parties**

*Laura Reina, Giacinto Piacquadio, Sally Dawson, Jon Butterworth, Ketavi Assamagan, Steve Mrenna, Matthew Schwartz, Rohini Godbole, +...*

- Recent preprint on Zbb and Wbb at NLO.
- HV high pT paper
- Jet Substructure
- WH high pT analysis: actual estimation of Wbb background + future plans
- Might also be interesting to contact Michael Deak, who did a short-term MCnet project on Zbb in Herwig context.

Link slides here if you have them please!

## Discussion

Getting reliable predictions, especially for the shape of the mass and pT of bbar pairs and for extra jets, in the Wbb (and Zbb) process, will be important for the eventual HV analysis. Having a better idea of the rate is also interesting now, to estimate the sensitivity.

The region of interest is where parton showers/LL are likely to do a good job, but fixed-HO and mass effects can also have significant impact.

General point is that because the qg -> Wqbbar channel opens up only at order alpha_s^2, this process is only LO and has a large scale dependence. Also, because the gluon PDF is larger than the qbar, this process has a large cross section. However, it also usually gives an extra jet, so being more exclusive (i.e. applying jet vetoes) reduces the K-factor from around 3 to probably below 1.5.

While the extra parton veto can be easily implemented in the NLO calculation, the problem in the NLO calculation at the moment is having to deal with large corrections in the region of phase space of high m(bb)/pT(bb). So even if in the WH analysis we are interested mostly at high m(bb) (around the Higgs mass), since pT(bb) is constrained to be above 200 GeV, we still land in a region where these logarithms are large. However a re-summation of these large logarithms could be implemented analytically in the calculation, which in principle permits to have up to NLL accuracy.

Other point is the existence of LO matched calculations (do they exist?) and possibility of doing a NLO matched calculation. Infact in a second step the NLO calculation could be in principle interfaced for example to POWHEG, matching the exact NLO calculation with a LL parton shower.

## Plan

GP and JMB to pass exact cuts and distributions of interest to LR and SD, who will do the calculation in the regions of interest.

Cuts implemented by GP in the MCFM calculation:

- use kT algorithm with DR=0.3
- impose DR(b-jet,antib-jet)<1.2
- pT(W)>200 GeV
- pT(bbbar)>200 GeV
- pT(each b-jet)>30 GeV |eta|<2.5
- for a jet to exist pT(jet)>15 GeV |eta|<5

Lepton cuts are not essential, but can be implemented as well:

- pT(lepton)>30 GeV |eta(lepton)|<2.5
- ptMiss>30 GeV

Quantities of experimental interest:

- m(bbbar)
- pT(bbbar)
- pT additional jet in the event (especially in the region 0-150 GeV)
- DeltaEta(W,bbbar)
- K factor as a function of m(bbbar), pT of additional jet in the event and pT(bbbar)

Quantities of interest for the calculation (to look at the region of large logarithms):

- m(bbbar)/pT(bbbar)
- Delta_pT(b-jet,antib-jet)/pT(bbbar)