Low-p T Multijet Cross Sections John Krane Iowa State University MC Workshop Oct. 4 2002, Fermilab Part I: Data vs MC, interpreted as physics Part II:

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

John Krane -- Iowa State University 2

Motivation

High-pT inclusive jet spectra appear to be well described by NLO QCDPossible 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

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?

2TnT2T1

2T )ppp(Q

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

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

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 Multiplepartonscatteringparameter

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 constrained

by 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

q

calorimeter jet

Tim

e

q g

parton jet

particle jet

hadrons

CH

FH

EM

p p

K

Documents

Low-p T Multijet Cross Sections John Krane Iowa State University MC Workshop Oct. 4 2002, Fermilab Part I: Data vs MC, interpreted as physics Part II: