Jets in ep collisions at HERA

1
Jets in ep collisions at HERA and the measurement
of
a
s
Alexander A. Savin
on behalf of the H1 and ZEUS collaborations
40th Rencontres De Moriond on QCD and
High Energy Hadronic Interactions, 12-19
March 2005, La Thuile, Italy
2
Kinematic Variables
e(-)
p
(820)
Photon virtuality
?P center-of-mass energy squared
inelasticity
Bjorken scaling variable
ep center-of-mass energy squared
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Jets in ep collision
1 jet
QCD Compton
Boson-gluon fusion
2 jets
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3 jets at ZEUS
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Event simulation
Parton Level
Hadron Level
ARIADNE LEPTO PYTHIA RAPGAP
CDMLUND etc
GEANT
Detector Simulation
NLO DISENT DISASTER MEPJET NLOJETS
Uncertainties
pdf
hadronisation
detector
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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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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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Do we need the resolved virtual photon?
Di-jet DIS at high-Et and comparison with NLO
ZEUS
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NLO can describe the inclusive jets !?
Inclusive jets in DIS
ZEUS
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2
Forward jets at low Q are not well described
Inclusive jets in DIS
H1
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Forward jets
DGLAP
The probability for
is low
BFKL phase space!
low x !
H1
Forward jets can be better described by adding
the resolved contribution
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Forward jets
comparison with NLO adding one more aS order
ZEUS

to suppress
NNLO is needed ?
Is the difference with the NLO smaller now?
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Inclusive dijets at low x Azimuthal Asymmetry
H1
NLO still can not describe the data at low x and
low Q2
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Extraction of aS
Multijet studies jet productiuon ratio
ZEUS
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Extraction of aS
Jets substructure
The integrated jet shape is the the fraction of
the jet transverse energy that lies in a cone of
radius r in the ?-f plane
ZEUS
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Running of aS
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at HERA
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Conclusions

• Considerable progress in jets studying in ep
• precise measurements
• good agreement with NLO
• extraction of as

In many cases the theoretical uncertainties are
still dominant
Existing calculations based on DGLAP fail to
reproduce HERA data at low x BFKL and/or
high-order calculations are needed