# Presentation: 044052

## WG1+WG2: Progress Report

**ME Calculation Validation**
**&**
**ME-MC Interfaces**

## Concentration on interfacing ME predictions to parton showers

**LHA establishes a “universal” framework for passing ME events to PS generators**
**Implemented in:**
**ACERMC**
**AlpGen**
**CompHEP**
**Grappa**
**Herwig**
**Madgraph**
**Pythia**

- Accord does NOT specify how to pick the scales in the PS
- Possible that PS generates emissions that are harder than the “hard” process
- Also possible to underestimate additional radiation below hard scale

## Q2 choice for shower evolution

- The choice is almost unambiguous for final states with 1 or 2 partons:

**Ex:**

- q

- Z, W   Q2 = s

- 

**Ex:**

**Q****2**** ******** p****T****2 ******** – t**

-  the factorization theorem is easily implemented, due to the existence of a single scale

## The choice is more difficult in more complex cases

- g3

- q

- g1 (from shower evolution)

- g4 (from matrix element)

- g2

**versus**

- g3

- q

- g4 (from shower evolution)

- g1 (from matrix element)

- g2

**with ****p****T1**** << p****T4 ****<< p****T2****,**** ****p****T3**** **

- For Winter Conferences, CDF/D0 are relying on QCD predictions of W+Jets backgrounds to top production

## Leading vs subleading double countingExample: corrections to 3-parton final states

**Double counting is sub-leading provided ********R and**
** ****are not too large**

## MLMs prescription for estimating PS uncertainties in W+Jet events

**Generate W+N parton events with loose cuts**
**Feed events to PS**
**Match each hard parton to a unique reconstructed jet (dR matching)**
**N partons ******** N Jets**
**Throw events away with fewer matchings**
**Also, throw away events with additional hard jet solely from PS**
**Do as little harm as possible**

## The prescription Nmatch=3 gives a much smaller uncertainty and, even more important, leads to a saturation of the rate at small Rpart :

| Rpart | 0.7 | 0.5 | 0.3 | 0.1 | 0.7 | 0.7 | 0.4 |
| --- | --- | --- | --- | --- | --- | --- | --- |
| pT min | 20 | 20 | 20 | 20 | 15 | 10 | 15 |
| (partons) | 0.58 | 0.79 | 1.17 | 2.10 | 1.34 | 3.78 | 2.21 |
| (all Nmatch) | 0.35 | 0.40 | 0.46 | 0.58 | 0.42 | 0.48 | 0.51 |
| (Nmatch=3) | 0.29 | 0.31 | 0.33 | 0.36 | 0.31 | 0.32 | 0.36 |

## Presentations

**Comparisons of ME predictions (G. G.)**
**General agreement in TOTAL RATES for standard tools**
**Parton level kinematics in good agreement, but less rigorous (eyeball)**
**Need tests of unweighting (color combinations....)**
**AMEGIC++ and APACIC++ (F. K.)**
**Another ME generator written in C++**
**Will interface smoothly to HERWIG++**
**Parton shower generator (Pythia-like) well-suited for adding showers to AMEGIC++ events**
**Promises update on April 1!!!**

## (cont)

**W+3Jet Events and MLM Prescription (A.M.)**
**Stable results “matched-exclusive” jet events**
**Must understand factor of 2-3 loss of events relative to parton level that would pass same cuts**
**Heavy Flavor Fraction in W+nJet Events (J.H.)**
**Prescription of “matched-exclusive” jet events yields a HFF that is stable and agrees with Run I measurement**
**Maybe it doesn’t matter too much what you do as long as you look at ratios (?)**

## Lively Discussions

**MLM method not a prescription in general**
**Even within its scope, one could do better**
**Catani/Krauss/etc Veto-ed Showers**
**Clustering ME events and interepreting as a parton shower with appropriate Sudakov reweighting**
**Generate regular shower (cleverly) below clustering scale**
**Kludging Vetoed-Showers with present generators**
**HERWIG (ala P.R.) and PYTHIA (ala S.M.) are being modified to carry out a vetoed shower on ME events**
**Beta versions in < 1 month (?)**
**HERWIG already vetos showering scales that are larger than the Factorization Scale**

## Website/writeup

**The talks given in Working Groups 1+2 will be collected at **_[www.pa.msu.edu/~huston/durham/](http://www.pa.msu.edu/~huston/durham/)_
**A short writeup will summarize the prescription adopted by the WG’s for dealing with the ME/MC interfacing once a proof of principle is available. **

## Followups

**At the Tevatron, we have had a series of one-day ME/MC workshops to deal with these types of issues**
**see **_[http://www-cpd.fnal.gov/MCTuning/](http://www-cpd.fnal.gov/MCTuning/)_** for copies of the talks**
**The next workshop in the series will be sometime in April at Fermilab**
**details will be sent to this workshop mailing list**
**it will also be carried by live streaming**
**this would be a good place to follow up on how well the interfacing works**

## Les Houches 2003

**May 26 - June 5**
**The wine is not as good as here but the view is better**

## Monte Carlo Workshop at CERN 2003

**A few words from the sponsor ...**

## Next Year 2004

**There will be a Santa Barbara workshop on collider physics from January thru March concentrating on the data from the Tevatron, prospects for the  LHC and the tools needed to exploit them both**
**Organizers: Zvi Bern, Joey Huston, Zoltan Kunzst, Kirill Melnikov**
**Contact Joey if interested**