Lowx AND Low Q2

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Small-x and Diffraction 2007 Workshop FermiLab,
March 28-30, 2007
Low-x AND Low Q2
at HERA with JETS
Luca Stanco INFN Padova
on behalf of H1 and ZEUS collaborations
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Kinematics of Neutral Current (NC) Deep
Inelastic Scattering (DIS)
W gP c.o.m. energy
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Processes at HERA
(defined by Initial States of HARD Scattering)
Electron beam
low-x regime
Proton beam
gluons quarks PARTONS
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HERA exploits many (correlated) processes over
large kinematic regimes !
Q2
XBj
LHC
Tevatron

• Allow to take care of
• low-x effects
• saturation
• multiple interactions

HERA
perturbative
QCD DYNAMICS (DGLAP vs BFKL evolutions)
Non-pert
Low-x regime
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...if nobody else have already shown ...
Gluons and QCD dynamics
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WHY studying pQCD ?
at least 4 answers

• NOT TRUE it is so well understood ..
• Strong coupling, aS, measurement (see next talk)
• Parton Density Functions measurements
• Parton evolutions

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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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Hard FORWARD Jets (near proton direction where
effects of gluon radiation should be largest -
A. Mueller 1990)
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before going into physics, let us look at the
environment...

• HeraI H1 and ZEUS took
• about 130 pb-1
• 2001 luminosity upgrade
• 2002/03 background problems
• 2004 positrons (50 pb-1)
• 2005 electrons
• 2006 positrons
• 2007 from March HERA for
• Low Energy Run (FL measure)
• END oh HERA PETRAIII

H1 and ZEUS will end up with 0.5 fp-1, each
Results on JETS mainly from HERA-I (up till now)
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QCD dynamics Evolution Resummation
HERA DIS at low-xBj down to 10-5 ? large
gp center of mass energy (W 1/x)
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two different parton ladders.
?Expect more energetic jets in forward region
(LEPTO, DISENT)
(ColorDipM)
(CASCADE)
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saturation
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 (with jets until 10-4)

• FORWARD JETS
• (proton direction)

possibility to study parton dynamics, by looking
for effects beyond DGLAP

• Where ? in the BREIT frame
• where virtual boson is enterely spacelike
• at lowest order, quark is back scattered

Caveat different Monte Carlo models not fully
clear where applicable
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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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FORWARD JETS
H1 data in the kinematic region
L13.7 pb-1
10-4ltxlt4.10-3 and 5ltQ2lt85 GeV2

• 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
• (RAPGAP DIRES)
• parton emissions not ordered in kT (CDM)

0.5 lt (pT,jet)2/Q2 lt 2 to suppress DGLAP and
enhance BFKL
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FORWARD JETS
ZEUS data less striking
L81.8 pb-1
4.10-4ltxlt5.10-3 and 20ltQ2lt100 GeV2
E?jetgt5 GeV, 2lthjetlt3.5
0.5 lt (E?jet)2/Q2 lt 2
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FORWARD JETS
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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Very detailed investigation from H1 to enhance
BFKL phase space
Select dijets plus forward-jet

• different Dh1 and Dh2
• regions affect
• Kt-ordering
• Gluon radiation
• Evolution chain

Compute differential cross-sections of Dh1 Dh2
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FORWARD JETS
Data towards NLO
too little Phase Space for NLO
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FORWARD JETS
Data towards CCFM
NO description !
Data towards CDM and gres
gres not sufficient
CDM may mimic BFKL evolution
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MULTIJETS (dijets, trijets) ANALYSIS
Kinematic selection to enhance (unordered) gluon
radiation and to study parton dynamics (at low-x)
H1 ?Ldt 44.2 pb-1
10-4ltxBjlt10-2, 5ltQ2lt80 GeV2
ZEUS ?Ldt 82.2 pb-1
10-4ltxBjlt10-2, 10ltQ2lt100 GeV2
Hadronic Center-of-Mass (like Breit-frame boosted)
Comparisons with NLO at order O(aS2) and O(aS3)
NLOJET, DISENT
Conparisons with Monte Carlos LO-PS with/out
gres (CDM-DJANGOH, RAPGAP)
H1 compares to parton level, ZEUS compares to
hadron level
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MULTIJETS JETS
H1
All the data
Low-x
E?gt 4 GeV, E?1E?2gt 9 GeV
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MULTIJETS JETS
H1
Select TWO subsamples with Forward JETS
(qjetlt200, xjetEjet/Ep gt 0.035)
NLO comparison
NLO at O(aS3) GREAT !
One central jets plus two forward
Two central jets plus one forward
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MULTIJETS JETS
H1
LO Monte Carlos comparison
RAPGAP directresolved contributions
Resolved contribution NOT working !
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MULTIJETS JETS
ZEUS
NLO not bad
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MULTIJETS JETS
ZEUS
Dijets comparison
NLO consistent.
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MULTIJETS JETS
ZEUS
Trijets comparison
but theor. errors (mF, mR) too large
Need next order
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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

• low-xBj regime very messy at HERA
• Statistical Theoretical errors
• need more data and theory computations
• unordered gluon emission significant at low-x ?

A VERY EXCITING FIELD WHERE TO LOOK DEEPLY
INSIDE QCD PARTON EVOLUTION DYNAMICS
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Backup Slides
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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 Production in NC Deep Inelastic Scattering
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5 jets event at low-x (H1)