at low-x - INFN - Sezione di Padova

Small-x and Diffraction 2007 Workshop
FermiLab, March 28-30, 2007
Low-x
AND
Low
2
Q
at HERA with JETS
Luca Stanco – INFN Padova
on behalf of
H1 and ZEUS collaborations
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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Processes at HERA
(defined by Initial States of HARD Scattering)
DIRECT g-PRODUCTION
Electron
beam
xg < 1
xg = 1
Q2 = 0
RESOLVED g-PRODUCTION
xg = 1
Q2 > 0
DIS
low-x regime
Proton
beam
DIFFRACTION
xBj << 1
xBj < 1
gluons… quarks PARTONS
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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HERA exploits many (correlated) processes
over large kinematic regimes !
Q2
XBj
LHC
perturbative
To extrapolate to other
regions need to take care
of low-x effects:
• saturation
• multiple interactions
QCD DYNAMICS
(DGLAP vs BFKL evolutions)
Luca Stanco - Padova
HERA
Non-pert
study at HERA:
Tevatron
Low-x “regime”
Low-x at HERA, Small-x 2007
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...if nobody else have already shown ...
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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Outline of the talk:
• a little bit of theory, modelling, Monte Carlo’s…
• ”Forward” Jets in DIS
H1 (1997 data, final analysis)
ZEUS (1998/00 data, preliminary analysis)
• Multi Jets in DIS
H1 (1999/00 data, preliminary analysis)
ZEUS (1998/00 data, preliminary analysis)
Hard FORWARD Jets (near proton direction where
effects of gluon radiation should be largest - A. Mueller 1990)
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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before going into physics, let us look at the environment...
Luminosity growth at HERA (I: 92-00, II: 03-07)
• 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 of HERA  next PETRAIII
H1 and ZEUS will end up
with ~ 0.5 fb-1, each
Results on JETS mainly from HERA-I (up till now)
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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QCD dynamics: Evolution & Resummation
HERA: DIS at low-xBj down to 10-5
 large g*p center of mass energy (W ~ 1/x)
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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Inclusive F2 measurement not able to
discriminate between different QCD
dynamics  Study of
Hadronic Final States (JETS)
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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...if nobody else is going to show ...
even if we consider Structure Functions results at low-x
Definitively, physics has to be clarified.
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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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
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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JETS observables in the BREIT Frame
… more technically
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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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
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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FORWARD JETS
H1 data in the kinematic region:
L=13.7 pb-1
10-4<x<4.10-3 and 5<Q2<85 GeV2
•
0.5 < (pT,jet)2/Q2 < 2
to suppress DGLAP
and enhance BFKL
Luca Stanco - Padova
•
•
•
•
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: DI+RES)
parton emissions not ordered in kT (CDM)
Low-x at HERA, Small-x 2007
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FORWARD JETS
ZEUS data less striking…
L=81.8 pb-1
4.10-4<x<5.10-3 and
20<Q2<100 GeV2
Ejet>5 GeV, 2<hjet<3.5
0.5 < (Ejet)2/Q2 < 2
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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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 mR2=Q2,
while H1 takes mR2=<pT2> )
Is it a HERA business ?
or HERA phase space
too limited to distinguish
between different
evolution models ?
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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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
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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FORWARD JETS
Data towards NLO
too little Phase Space for NLO
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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Data towards CCFM
FORWARD JETS
NO description !
Data towards CDM and gres
gres not sufficient …
CDM may mimic
BFKL evolution …
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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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
ZEUS: Ldt = 82.2 pb-1
10-4<xBj<10-2, 5<Q2<80 GeV2
10-4<xBj<10-2, 10<Q2<100 GeV2
Hadronic Center-of-Mass (like Breit-frame boosted)
Comparisons with NLO at order O(aS2) and O(aS3)
NLOJET, DISENT
Conparisons with Monte Carlo’s LO-PS with/out gres
(CDM-DJANGOH, RAPGAP)
H1 compares to parton level, ZEUS compares to hadron level
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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MULTIJETS
H1
All the sample:
E*> 4 GeV, E1*+E2*> 9 GeV
-1 < hjet < 2.5
LO discrepancy
Low-x region:
more DATA than NLO predicted
Overall systematics error of 19% not included on DATA
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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MULTIJETS
H1
Select TWO subsamples with Forward JETS
(qjet<200, xjet=E*jet/Ep > 0.035)
NLO++ comparison
Two central jets plus one forward
-1 < hjet < 1
One central jets plus two forward
NLO at O(aS3) describes well the TWO central jets,
expected to be produced from hard scattering
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
FORWARD jets are expected to
be produced from gluon radiation
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MULTIJETS
H1
LO Monte Carlo’s comparison
RAPGAP: direct+resolved contributions
Resolved contribution NOT working !
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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MULTIJETS
ZEUS
Jet energy scale uncertainty
Reasonable description by NLOJET
Hadron Level
Luca Stanco - Padova
E*1> 7 GeV, E*2,3> 5 GeV
-1 < hjet < 2.5
Low-x at HERA, Small-x 2007
mR2=mF2 = (<ET>2+Q2)/4
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ZEUS
MULTIJETS
NLOJET comparison with Dijets DATA
O(aS2) does not
describe DATA
10 < Q2 > 15 GeV2
20 < Q2 > 30 GeV2
NLO consistent within errors
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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ZEUS
MULTIJETS
NLOJET comparison with Trijets DATA
… but theor. errors (mF, mR) too large
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
Need next order
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Conclusions
• low-xBj regime unsettled at HERA
• Multijets (Forward) production measured in
xBJ>10-4 and Q2<100 GeV2
• Statistical ~ Theoretical errors (NLO)
• 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
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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Backup Slides
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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Kinematics of Neutral Current (NC)
Deep Inelastic Scattering (DIS)
W: g*P c.o.m. energy
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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WHY studying pQCD ?
at least 4 answers:
1. NOT TRUE it is so well understood ..
2. Strong coupling,
aS, measurement (see next talk)
3. Parton Density Functions measurements
4. Parton evolutions
From HERA-LHC workshops, examples of uncertainties,
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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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, ŝ etc.)
 low-x and forward jets…
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
30
two different parton ladders….
Expect more energetic
jets in forward region
(LEPTO, DISENT)
(~ColorDipM)
(CASCADE)
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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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
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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Jets Production in NC Deep Inelastic Scattering
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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5 jets event at low-x (H1)
Luca Stanco - Padova
Low-x at HERA, Small-x 2007
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