Jets in ep collisions at HERA

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Jets in ep collisions at HERA
Alexander A. Savin
on behalf of the H1 and ZEUS collaborations
Hadron Structure and QCD 2004, 18-22 May
2004, St.Petersburg, Russia
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Kinematic variables
e(-)
p
(820)
Photon virtuality
?P center-of-mass energy squared
inelasticity
Bjorken scaling variable
ep enter-of-mass energy squared
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Jets at HERA
1 jet
QCD Compton
Boson-gluon fusion
2 jets
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Three jets
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Events simulation
Parton Level
Hadron Level
LO ARIADNE LEPTO PYTHIA RAPGAP
CDMLUND etc
GEANT
Detector Simulation
NLO DISENT DISASTER MEPJET NLOJETS
Uncertainties
pdf
hadronisation
detector
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Breit Frame
Jet Finding
Longitudinally invariant kT cluster algorithm
DIS Event
Lab Frame
Breit Frame
Inclusive mode, combine if
PT
Exclusive mode
PL
Breit Frame
combine until
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DGLAP vs BFKL
DGLAP
BFKL
no ordering in k
T
Unordered parton evolution, e.g. CCFM (Cascade)
Different parton-shower models BFKL/DGLAP-like
Ariadne/LEPTO
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Real and virtual photon structure
High Pt jets at Pt gt Q probe the photon structure.
direct
resolved
In the BFKL both are taken in account due to
arbitrary ordering of the kT
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Real photon structure inclusive jet PHP at high
Et
Non-perturbative effects multiple interactions
and hadronisation are small
Comparison with NLO
H1
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Real photon structure dijet PHP with charm
ZEUS
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Virtual photon structure dijet DIS at hight Et
Comparison with NLO
ZEUS
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Virtual photon structure dijet DIS at hight Et
Comparison with Herwig and Cascade
H1
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Virtual photon structure dijet DIS with charm
ZEUS
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Structure of the proton

• Uncertainties
• -NLO QCD due to terms beyong NLO
• NLO QCD due to
• -statistical and correlated systematic
• uncertainties(fits)
• -hadronisation etc.

PDF
New results on PDFs using jets in global PDF fits!
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Structure of the proton inclusive jets in DIS
ZEUS
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Structure of the proton jets in the PDF fit
Use jet data to tie down the high-x region in
PDF analysis
The improvement in the determination of the
gluon distribution at moderate to high-x is
striking
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Structure of the proton inclusive jets in DIS
H1
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Forward jets
DGLAP
The probability for
is low
BFKL
Low xBj,high Q2 , high pt,jet2 in the
p-direction
H1
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Forward jets
Comparison with NLO
ZEUS

to suppress
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Interjet energy flow
Dijet events with Rapidity Gap
H1
All Dijet events
Color singlet added
g-remnant
P- remnant
2p
Jet
R
Gap
Jet
R
0
-2.4
2.4
h
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Multijet production
Ratios R2/1 and R3/2
ZEUS
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Multijet production ratio 3jets/2jets
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Jets substructure
kT inclusive - for jets kT exclusive - for subjets
At high ET,jet and ycut not too
low, fragmentation effects are small subjet
multiplicity in pQCD
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Jets substructure
ZEUS
PHP and DIS
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measurements
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at HERA
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Conclusions
Considerable progress in jets studying in ep
precise measurements, NLO etc
In many cases the theoretical uncertainties are
still dominant
One may expect more new information about
photon and proton pdfs by including more jet
data in global PDF fits.