pQCD%20?%20JETS

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LISHEP 2006 Workshop Rio de Janeiro, April 3-7,
2006
pQCD ? JETS
(AND)
at HERA
Luca Stanco INFN Padova
on behalf of H1 and ZEUS collaborations
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more or less
Single particles production Charged
multiplicities Bose-Einstein correlation Neutral,
strange, Baryons Charm fragmentation Remnant/Multi
ple interactions Prompt Photon Resolved-Direct
g-production Parton Evolutions Color dynamics,
Energy flow Multiscale Strong coupling Dijets,
Trijets Inclusive Jets production Forward Jets
production Heavy Quarks Jets Diffractive Jets
QCD items
JETS items
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QCD

• WHAT ?

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WHY ?
at least 3 answers

1. NOT TRUE it is so well understood ..
2. Strong coupling, aS, measurement
3. Parton Density Functions measurements (see
   previous talk)

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Z.Czyczula et al., pT of Higgs (120 GeV) from
different Parton-Showers (PS) models and QCD
scale choices

• Uncertainties
• up to 170 due to PS (which evolution type ?
  which ordering ?)
• up to 100 due to the choice of QCD scale (Q2,
  m2, s etc.)

? low-x and forward jets
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HOW ?
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before going into physics, let us look at
environment...
Results on pQCD?JETS only from HERA-I (up till
now)
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...if nobody else has already shown ...
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Kinematics of Neutral Current (NC) Deep
Inelastic Scattering (DIS)
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Processes at HERA
(defined by Initial States of HARD Scattering)
Electron beam
Proton beam
quark/gluon PARTONS
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Jets Production in NC Deep Inelastic Scattering
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Inclusive JETS in DIS
The BREIT frame !

• Frame where virtual boson
• is enterely spacelike
• At lowest order, quark is
• back scattered

In pratice, high Et jet events must be at least
of order aS
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JETS observables in the BREIT Frame
more technically
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NLO pQCD calculations
and

• interfaced to various PDFs (like CTEQ, MRST)
• Parton Level cross sections need to rely on LO
  MC for hadronisation
• corrections

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Excellent agreement between DATA and NLO over
many orders of magnitude !
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...and the experimental error is smaller than the
theoretical one !
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ZEUS Inclusive Jet Production in DIS
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measurements at different Q2 also for ZEUS
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Running and Extraction of aS from Inclusive jets
aS vs Et
aS vs Q2
Best result for Q2 bins with Q2gt500 GeV2 ?
uncertainties minimised
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and H1
before showing the whole sets of aS measurements,
look at the MULTI-JET data sets ?
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Di-Jet and Tri-Jet PRODUCTION in DIS
larger errors, but ...
The aS dependency of their ratio is a sensitive
test of pQCD
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3-jet / 2-jet cross section ratio
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aS measurements from 3-jet / 2-jet cross section
ratio
H1
room for improvements (HERA-II)
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aS running
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JETS in g-production
again, another testing ground for QCD
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Well observed from the first days
ZEUS
xg0.8 cut
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H1
Proton structure
More QCD fits from H1 will come !
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Similar situation for ET distributions
ZEUS high Et di-jets in g-p

• Consider the enriched sample of
• direct-g
• Good agreement with NLO at high ET
• small experimental uncertainties
• ? jet energy scale (?1)
• ? 5 on the cross section
• small theoretical uncertainties

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Gluon Parton Distribution

• Disadvantages of ortodox method (inclusive ep
  scattering)
• gluon distribution contributes indirectly
• Fixed-target DIS higher twists, heavy target
  corrections,
• isospin symmetry assumption

• With HERA, sufficient sensitivity to determine
  the proton PDF within a single (ep) experiment.
• Use of measurements of jet cross section
• Directly sensitive to the gluon density with
  small
• experimental theoretical uncertainties

Recent results from ZEUS on old data are very
good, improving the determination of the proton
PDF
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Color and sub-jet dynamics
Photo-production of three-jet events
Direct-g processes provide a clean way to study
the effects of the different color configuration
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Angular observables for angular correlations
Comparisons to fixed order calculations based on
different symmetry group
The prediction of SU(3) describes reasonably well
the data
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ZEUS Sub-jet distributions
DIS
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H1 g-p data
The jets in the region xglt0.75 are broader than
in the region xggt0.75
Consistent with a larger fraction of gluon
(broad) jets in resolved-photon events than in
direct-photon events
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QCD dynamics
HERA DIS at low-xBj down to 10-5 ? large
gp center of mass energy (W)
Inclusive F2 measurement not able to
discriminate between different QCD approaches ?
Study of Hadronic Final States (JETS)
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HERA extends xBj until 10-5

• FORWARD JETS
• (proton direction)

possibility to study parton dynamics, by looking
for effects beyond DGLAP
Caveat different Monte Carlo models not clear
where applicable
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H1 data in the kinematic region

• LO-DGLAP fails completely !
• NLO-DGLAP well below data at low-x
• Monte Carlo models with extra parton
• radiation provide an improved description
• of the data
• inclusion of a resolved-photon component
• (RG-DIRES)
• parton emissions not ordered in kT (CDM)

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ZEUS data less striking (NLO(aS2)-DISENT MC
below the data but within theoretical errors) ?
needs of next NLO ?
( but ZEUS uses mR2Q2, while H1 takes
mR2ltpT2gt )
Is it a HERA business ? or HERA phase space too
limited to distinguish between different
evolution models ?
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...if nobody else is going to show ...
Definitively, physics has to be clarified.
Also consider Structure Functions results at low-x
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Conclusions

• QCD is a lively field at HERA !
• Up till now studies on 1996-2000 DATA
• Statistical Theoretical errors
• HERA-II will lower systematics errors
• Need THEORY works !

• Most precise aS measurement !
• Use of Jets for PDF
• Forward Jet discrepancy
• pQCD evolution(s) not clear yet
• low-xBj regime not clear yet

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Backup Slides
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PENTAQUARKS at HERA
q(1530) still compatibility ZEUS/H1, need
HERA-II DATA
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Not seen by ZEUS
qC(3100) still compatibility H1/ZEUS, need
HERA-II DATA
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