# Presentation: 752129

## Low-pT Multijet Cross Sections

- John Krane
- Iowa State University

- MC Workshop Oct. 4 2002, Fermilab

- 	Part I: Data vs MC, interpreted as physics
- 	Part II:
- Data vs MC, interpretedas a tuning problem

## Motivation

- High-pT inclusive jet spectra appear to be well described by NLO QCD
- Possible exceptions include kT algorithm analysis, possibly also the ratio of cross sections at 630/1800 GeV, large- dijets (BFKL).  But only at 1 or 2 ,not actual *disagreement*
- Originally, this was a search for BFKL effects, which could produce extra jets in low-Q2 events

## The Analysis

- Jets with ET > 20 GeV, usual jet and event cuts,efficiencies applied but *no unsmearing*
- Study inclusive samples of events having *at least:* 1-jet, 2-jets, 3-jets, 4-jets
- Compare to normalized Pythia + GEANT and Herwig + GEANT, tune if necessary

**The usual sample**

## Data and Pythia

- Inclusive xsec looks fine
- Multijet xsecs exhibit deviations from Pythia
- Let’s pretend it’s physics

- points=data,   histo=Pythia

## (D-T)/T

- Solid lines: energy scale lum uncertainty
- Dash: smearing uncertainty
- Dotted: total error in ratio

## Data and Herwig

**Data and ****Herwig**

- Started generating jets at 0.5 GeV
- Multijet xsecs exhibit similar deviations

- points=data,   histo=Herwig

## Vector sum pT 

- Define
- The more jets in the event, the more imbalance in energy
- Could this be
- ISR, with pT
- lost down the
- Beampipe?

- Events > 250 are the
- excess in 3+ jet events

- (Data and Pythia)

- >150 in 4+

## Angles in 3-jet events

- Find which jet is “the third one” by isolating the two jets with minimal pT

- Many back-to-backin the data

- Usually, third jet is near one of the first two, but more so in Pythia

- (Data and Pythia)

## Angles in 3-jet events

**Angles in 3-jet events**

- Find which jet is “the third one” by isolating the two jets with minimal pT

- Min pT not bad...

- Third jet is often at 90 degrees, often composed of underlying event E

- (Data and Herwig)

## Signs point to initial state radiation effects in data
DGLAP style?

BFKL style?

...or a need to tune the MC

- DGLAP style?
- BFKL style?
- ...or a need to tune the MC

**Early impressions of these results**

## “Try tuning Pythia, also compare to Herwig;see what works...” 

- Herwig defaults also did poorly
- Many iterations required
- Only compared to distributions shown today (and a few other very similar ones...)

- Pythia works if
- PARP(83) = 0.32 (from 0.5)
- Fraction of matterin the proton “core”

- Herwig works if
- PTMIN = 3.7 GeV
- pT generation threshold

- A Multiplepartonscattering
- parameter

- Does this changeunderlying eventin some way?
- Both higher and lowervalues do worse!

- Didn’t try Jimmy...

## Data and Tuned MC

- Points = Data
- Solid = Pythia
- Dash = Herwig

## (D-T)/T

- No remaining deviations from data
- Is this because there were no ISR effects?
- Answer lies in the validity of our tuning

## Vector sum pT 

- Tuned MC reproduced the small “shoulder”in addition to the 3+ and 4+

## Angles with Pythia (and Jetrad)

- Points = Data
- Solid = Pythia
-    (error bands and...)
- Dash-dot = Jetrad

## Angles with Herwig

- Points = Data
- Solid = Herwig

- Dot = Herwig
-   with cut on   merged jets

- Peak (from “underlying
- event jets”) becomes enormous if ptmin>3.7 GeV

- Pythia’s CKIN(3) showsno such sensitivity...

- Cross section shapes verystrange if ptmin<3.7 GeV

## Conclusions

- Results not entirely satisfying
- Would like to make definitive statements about ISR
- ...or provide solid tuning suggestions
- Instead, we found sensitivity to several params.
- Think the multiple parton scattering is constrainedby other data, we provide a new handle
- We don’t understand the Herwig tuning at all
- Our decision: publish the data, leave tuning to experts with a more global view
- Tuning isn’t really our forté
- If we do it, we probably want a second paper out of it!

## Backup Slides

## Cone Algorithm Details (Run I)

- Draw a cone around a “seed”
- Calc sum ET, and ET-weighted position
- Draw new cone here and recalculate sum ET, position
- Reiterate until stable

## Energy Scale

- Correction back to “the particle level”
- Remove noise, underlying event,extra pp interactions
- Correct for detector response
- Undo misassignment of particle energies to jets

***calorimeter jet***

***Time***

***parton jet***

***particle jet***

- 

- K