Quantum Chromodynamics (QCD)

Andrew BrandtUT-Arlington/DØ Experiment

QuarknetJune 6, 2001

Structure of Matter

cm 10-10m 10-14m 10-15m

u

<10-18m

10-9m

Matter Molecule Atom Nucleus QuarkBaryon

Electron

<10-19mprotons, neutrons,

mesons, etc.

top, bottom,charm, strange,

up, down

ChemistryAtomic Physics

NuclearPhysics

High Energy Physics

Massproton ~ 1 GeV/c2

(Hadron)

(Lepton)

Forces

Forces work by the exchange of Boson’s

Electromagnetic: Photon Exchange

Weak Nuclear Force:Causes Nuclear Decays

e pphoton

neutronproton

W boson

electron

Forces: Strong Nuclear or Color

Strong Nuclear Force:Quantum ChromodynamicsGluon Exchange, also holds the nucleus together.All quarks carry a color chargeGluons carry two color charges

Different from other Forces:Gluons can interact with other gluons.

Quarks and gluons are free at small distances (asymptotic freedom),

but not at large distances (confinement)

cannot observe bare color

Always observe quarks in multiplets: Baryons qqq (Proton neutron) and Mesons (quark antiquark pair )

Proton: uudAlso contains gluons and quark-antiquark pairs in a sea.

Neutron: udd

Pion: ud

Proton Antiproton Collisions

A word about units:HEP uses “natural units”

Collide protons and antiprotons each with 900 GeV of kinetic energy.

1c The mass of a proton is then given by

kg 10 9.109

GeV 1MeV 93831-

pm

900 GeV Protons 900 GeV Antiprotons

Life at Fermilab

Particle Colliders as Microscopes

m10TeV 1

MeV/c23.197 18

ph

How we see different-sized objects:

QM: large momenta= small distances

Rutherford Scattering

The actual result was very different.

“It was almost as incredible as if you fired a 15 inch shell at a piece of tissue paper and it came back at you”

Implied the existence of the nucleus.

We perform a similar experiment at Fermilab to look for fundamental structure

Proton Structure

Proton contains three valance quarks: uud

Also contains sea of virtual quark anti-quark pairs.

All held together by gluons

Quarks and gluons are called partons.

Proton with momentum P. Individual parton carries momentum xP

uv

uv

dv

u

u

u

u

d

d

s

s

Parton-Parton Scattering

Described by QCD.

Anti-Proton900 GeV

Proton900 GeV

Scattered Parton

Scattered Parton

1x

2x

sxxss

21energy c.o.m.parton ˆTeV 8.1energy c.o.m.proton

Perturbative QCD and Jet Production

p

~ 2s (LO)^

Includes radiative correctionsand gluon emission - much

of current QCD is a study ofthis additional radiation

jetqq (x2)

qq (x1) jetg

~ 3s (NLO)^

p

p

p

Partondistribution(PDF)

Hard scatter (pQCD)

Observable jet of particles in detector

Fragmentationinto hadrons

Jets

Jets are formed by the scattered partons.

QCD requires that colourless objects are produced (hadrons) e.g..:, K, , etc.

At DØ a jet is defined to be the energy deposited in a cone of radius:

7.022 R

Measured Event Variables

In a Two Jet event the following is measured:

massless coscosh2

2/tanlndity pseudorapi ,, :Jet

21212,1,2

TT

T

EEM

E

Jet 2: ET

2, 2, 2

Jet 1: ET

1, 1, 1

= 0

ET = Energy x sin

The DØ Detector

x

y

z

E TE

Detection

EM hadronicB

InteractionPoint

Scintillating FiberSilicon Tracking Calorimeter (dense)

Wire Chambers

Abs

orbe

r Mat

eria

l

electron

photon

jet

muon

neutrino -- or any non-interacting particle missing transverse momentum

Charged Particle Tracks Energy Muon Tracks

We know x,y starting momenta is zero, butalong the z axis it is not, so many of our measurements are in the xy plane, or transverse

Inclusive Jet Cross Section as aTest of the Standard Model (pQCD)

1P

2P

11Px

22Px

1jet

2jet

s

1/ xf Aa

2/ xf Bb )(ˆ

)2(

2/1/21

21

cdabxfxfdxdx jetspp

abcdBbAa

TT

jet

TT

T

ELdtE

NddE

dEddE

vs. 1 2

binthe in jets of NLuminosity inst.Lsize bin

efficiency selectionsize binEE

jet

TT

#

Single Inclusive Jets: X jetpp

q

Time

p p

q g

K

“par

ton

jet”

“par

ticle

jet”

“cal

orim

eter

jet”

hadrons

CH

FH

EM

Highest ET dijet event at DØ

0.69 GeV, 472E

0.69 GeV, 475E21

T

11T

0.7R

),(η 00 ),( Fixed cone-size

jetsAdd up towers

Iterative algorithmJet quantities:

0.7R

towerT

jetT

i

EE

,,ET

Jet Production and Reconstruction

“Typical DØ Dijet Event”

ET,1 = 475 GeV, 1 = -0.69, x1=0.66ET,2 = 472 GeV, 2 = 0.69, x2=0.66

MJJ = 1.18 TeVQ2 = ET,1×ET,2=2.2x105 GeV2

High Energy Art

Phys. Rev. Lett. 82, 2451 (1999)

The DØ Central Inclusive Jet Cross Section

1

pb 92 LdtDØ Run 1B

0.0 0.5 JETRAD

PDF, substructure, … ?

d2 /d

E T d

ET

How well do we know proton structure (PDF)?

Is NLO ( ) QCD “sufficient”?

Are quarks composite?

3sα

E0

DØ

Unfold effects of finite jet energy resolutions from very steeply falling inclusive jet cross

sections

“observed”“true”

“smearing”

“unsmearing” or “unfolding”

Data Selection and Corrections

ET (GeV)

Smearing Correction

0.86

0.90

0.94

0.98

50 100 150 200 250 300 350 400 450 500

Jet energy scale correction:“calorimeter” “particle” jet

Cut on central p-pbar vertex position Eliminate events with large missing ET Apply jet quality cuts

q

p p

q g

K

“par

ton

jet”

“par

ticle

jet”

“cal

orim

eter

jet”

hadrons

CH

FH

EM

Data Selection and Corrections

E = (EObs-Offset)*Det.Uniformity RH * Out of Cone Showering

CTEQ5

Tevatron jet data serves as stronger constraint in medium x region for CTEQ. MRST uses does not use these data.

x-Q region spanned byexperimental data in modern fitsTevatron jets in blue

Jets in PDFs

1/ xQ

(GeV

)100 101 102 103 104

100

101

101

Inclusive Jets- CDF

PRL82, 2451 (1999)

Inclusive Jet Cross Section at 1.8TeV DD

D0 and CDF data in good agreement. NLO QCD describes the data well.

Preliminary

ET (GeV)

d2 d

ET d (

fb/G

eV) 0.0 0.5

0.5 1.0 1.0 1.5 1.5 2.0 2.0 3.0

DØ Preliminary

Run 1B

Nominal cross sections & statistical errors only

Rapidity Dependence of the Inclusive Jet Cross Section

Compositeness

Continuing Search for fundamental building blockAtom Nucleus Nucleons Quarks

Three quark and lepton generations suggests that quark and leptons are composites.

QuestionAre Quarks composite

particles? Search for compositeness in

Proton Anti-proton collisions

Atom

Nucleus

Nucleon

Quark

Search for Compositeness

The presence of three quark and lepton generations suggests that they could be composite particles

Composed of “preons”

Define the preons interaction scale as

Existence of substructure at energies below indicated by presence of four-fermion contact interactions.

Strength of interactions related to

ProtonQuark

Preons? 2ˆs

s

M cos

Predictions

If quarks are made up of smaller particles then expect more events at high mass, and at smaller scattering angles

Prediction for fundamental

quarksM

Num

ber o

f Eve

nts

Prediction for composite

quarks

Num

ber o

f Eve

nts

cos *

Dijet Production

1P

2P

1xfi11Px

22Px

1jet

2jet

sij

2xf j

,,,ˆ

,,

2

2

2

22

2211

22

2121

RFRsij

ijFjFi

QQPxPx

xfxfdxdx

To search for compositeness we need a good prediction for Standard Model dijet production NLO QCD.

NLO event generator JETRAD (Giele, Glover, Kosower Nucl. Phys. B403, 633)

Need to choose pdf Choose Renormalization and

Factorization scales (set equal) Rsep: maximum separation allowed

between two partons to form a jet (mimic exp. algorithm)Rsep=1.3R(Snowmass: Rsep=2.0R)

2R

1.3R

Dijet Cross Section

Phys. Rev. Lett. 82, 2457 (1999)

Cross Section Ratio

Submitted to PRL: hep-ex/9807014

Model with LL coupling

Calculate Ratio of Cross Sections.

Two different angular regions

Quark-Quark Compositeness Limits

Model + -

LL 2.7 TeV 2.4 TeVVVAA 3.2 TeV 3.1 TeV

V8V8 2.0 TeV 2.3 TeVA8A8 2.1 TeV 2.1 TeV

fm105

TeV 2.5MeV/c 23.197

4

phLimit on size of preons is

fempto-meters4105

Conclusions

No evidence for Compositeness found at the Tevatron

Standard Model (QCD) in excellent agreement with the data

Quark-Quark Compositeness > 2 to 3 TeV depending on models

p

J p1 1 1( ),

J p2 2 2( ),

p

X pi i i( ),

W/Z PT,W/Z+Jets

+ +...W, Z

q(x)

Title: WJET_FLOW.DVICreator: dvips 5.55 Copyright 1986, 1994 Radical Eye SoftwareCreationDate:

Numerous other QCD studies to probe

scattering dynamics

Color Flow

Diffraction

Jets inHigh E Limit Photons

etc...

Measurement of S from Inclusive Jet Production

)(),()()( 3,

22

TFRSTFRST

EBEAddE

d

NLO x-section can be parametrized as

Measuredby CDF

Obtained from JETRAD

2TE

FR

• Fitting the NLO prediction to the data determines S(ET)• S(ET) is evolved to S(MZ) using 2-loop renormalization group equation

• Systematic uncertainties (~8%) from understanding of calorimeter response• Measured value consistent with world average of S(MZ)=0.119

0089.00078.0

0001.01129.0)( ZS M

New measurement of S by a single experiment & from a single observable over a wide range of Q2.

Conclusions

Standard Model (QCD) in excellent agreement with the data

No evidence for Compositeness of quarks found at the Tevatron

Studies continue improving theory, detectors, and using better microscopes