# Presentation: 500990

## B Production in Multijet Events:  Data and Pythia, Herwig Simulations 

- Alberto Annovi
*alberto.annovi@pi.infn.it*
**Matrix Element/Monte Carlo Tuning Workshop**
**Fermilab, October 4, 2002**** **

- A. Annovi     P. Giannetti

## QCD Multijet background with 

**QCD Multijet background with **
**b-quarks inside: is it well**
**reproduced by Herwig & Pythia?**
-  Important background for a light 	Higgs (< 140 GeV) in hadronic events
-  Pythia problems in the CDF Run I 	experience
-  Compare Pythia with Herwig: is a 		Pythia or a shower-montecarlo 		problem?
-  Test independently different b        	production mechanisms

## SM Higgs Xsec 

- For Higgs mass < 140 GeV,
- H  bb is important
- HV  bbqq is known @ CDF
- what about Hqq?

- With 2 fb-1

- 2000 eve.

- 200 eve.

- 20 eve.

- SM Higgs Xsec
- @ Fermilab Tevatron

## Pythia vs CDF

**H/A bb**** ******** ****bbbb**

- CDF run I           PYTHIA
- 50.611.0     1856.2 703.5

- VH bbqq
- PRL 81, 5748
- QCD  Data
- how much?

- Fermilab-Conf-99/164-E

- 1 fb-1

## All-hadronic decay of top quark

**All-hadronic decay of top quark**
**PRL 79, 1992**
******QCD ******** ********Data**
**how much?**

## The World doesn’t know about this!!!

** ****Physic ****	****	****background ****	****Pythia Xsec**
** ****study****	****	****sample****	****Data Xsec**
** ****pp   bb********bbbb****	****4bjet+X****	****    ****30**
** ****pp   VH    bbqq****	****2bjet+2jet   how much?**
** ****tt   bbqqqq****	****	****2bjet+4jet   how much?**

**Multijet QCD background**

- To   summarize

## BUT!!

- The diff. Xsec as a function
- of the two b-jet mass is well
- reproduced by Pythia!!!

- hep-ex/9809022

## Pythia & Herwig  Simulation

-  Pythia v6.136
-  Herwig v6.201
-  CMSJET easy package to run
-  ECM = 1800 GeV
-  Pt hat > 40 GeV
-  |parton| < 4
-  |b jet| < 2
-  Rcone = 0.6
-  seed ET = 5 GeV
-  min Tower ET = 1 GeV

## 2b-jet :  Pythia & Herwig

**Direct production** is dominant

## 2b-jet :  Pythia & Herwig

- Pythia

- Herwig

- Direct production

- Gluon splitting

- Flavor excitation

## 4jet of which 2b-jet PT>250 GeV:

-     Pythia & Herwig
**Direct production** is less important

## 6jet of which 2b-jet PT>250 GeV:  

-     Pythia & Herwig
**Direct production** is negligible

## Independent Test of the Production Channels 

- Direct production
- Flavor excitation
- Gluon splitting

- CDF             ok
- LEP              ok
- CDF         ? not ok ?
- LEP               ---
- CDF         ? not ok ?
- LEP          not tested

- Run II
- CDF?		D0?

- Init. State Rad.	75%
- Fin. State Rad.		25%
- Phys. Rev. D 50, 5562 (1994)

## QCD multijet without bs:

- only Herwig is studied and looks OK.

- CDF Note 2443
- Jan 1994

- relative Xsec vs Jet Multiplicity

## Conclusions 

- Shower Montecarlos have some
- problems reproducing multijet b Xsec.
- We need a specific paper.
- Low jet multiplicity: well reproduced.
- Direct production is the tested
- Mechanism.
- High jet multiplicity: badly reproduced.
- Flavor excitation and Gluon
- splitting are the main mechanisms.
- Flavor excitation and Gluon splitting
- are not tested @ LEP.
- The Tevatron should test them!