Quantum Chromodynamics (QCD)
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Quantum Chromodynamics (QCD)
Andrew BrandtUT-Arlington/DØ Experiment
QuarknetJune 6, 2001
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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)
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Forces
Forces work by the exchange of Boson’s
Electromagnetic: Photon Exchange
Weak Nuclear Force:Causes Nuclear Decays
e pphoton
neutronproton
W boson
electron
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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
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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
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Life at Fermilab
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Particle Colliders as Microscopes
m10TeV 1
MeV/c23.197 18
ph
How we see different-sized objects:
QM: large momenta= small distances
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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
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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
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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
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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
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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
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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
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The DØ Detector
x
y
z
E TE
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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
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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
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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
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“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
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High Energy Art
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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
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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
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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
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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
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Inclusive Jets- CDF
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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
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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
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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
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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
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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 *
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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
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Dijet Cross Section
Phys. Rev. Lett. 82, 2457 (1999)
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Cross Section Ratio
Submitted to PRL: hep-ex/9807014
Model with LL coupling
Calculate Ratio of Cross Sections.
Two different angular regions
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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
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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
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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...
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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.
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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