Low-x AND Low Q 2
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Transcript of Low-x AND Low Q 2
Luca Stanco - Padova Low-x at HERA, Small-x 2007 1
Low-x AND Low Q2
Luca Stanco – INFN Padova
Small-x and Diffraction 2007 WorkshopFermiLab, March 28-30, 2007
on behalf ofH1 and ZEUS collaborations
at HERA with JETS
Luca Stanco - Padova Low-x at HERA, Small-x 2007 2
Processes at HERA
Electron beam
Protonbeam xBj < 1
DIFFRACTION
x = 1Q2 > 0
Photon*
DIS
x < 1
photon
RESOLVED -PRODUCTION
x = 1Q2 = 0
photon
DIRECT -PRODUCTION
gluons… quarks PARTONS
(defined by Initial States of HARD Scattering)
xBj << 1
low-x regime
Luca Stanco - Padova Low-x at HERA, Small-x 2007 3
TevatronLHC
HERA
HERA exploits many (correlated) processes over large kinematic regimes !
XBj
Q2
Low-x “regime”
pert
urb
ati
ve
Non
-pert
To extrapolate to other regions need to take careof low-x effects:• saturation• multiple interactions
QCD DYNAMICS(DGLAP vs BFKL evolutions)
study at HERA:
Luca Stanco - Padova Low-x at HERA, Small-x 2007 4
...if nobody else have already shown ...
Gluon
s an
d
QCD
dyn
amics
Luca Stanco - Padova Low-x at HERA, Small-x 2007 5
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, final 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 6
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 of HERA next PETRAIII
Results on JETS mainly from HERA-I (up till now)
H1 and ZEUS will end up with ~ 0.5 fb-1, each
Luminosity growth at HERA (I: 92-00, II: 03-07)
Luca Stanco - Padova Low-x at HERA, Small-x 2007 7
QCD dynamics: Evolution & Resummation
HERA: DIS at low-xBj down to 10-5
large *p center of mass energy (W ~ 1/x)
Luca Stanco - Padova Low-x at HERA, Small-x 2007 8
Inclusive F2 measurement not able to discriminate between different QCD dynamics Study of Hadronic Final States (JETS)
satu
ratio
n
Luca Stanco - Padova Low-x at HERA, Small-x 2007 9
...if nobody else is going to show ...
Definitively, physics has to be clarified.
even if we consider Structure Functions results at low-x
Luca Stanco - Padova Low-x at HERA, Small-x 2007 10
FORWARD JETS (proton direction)
possibility to study parton dynamics, by looking for effects beyond DGLAP
Caveat: different Monte Carlo models not clear where applicable
HERA extends xBj until 10-5
(with jets until 10-4)
Where ? in the BREIT frame:• where virtual boson is enterely spacelike• at lowest order, quark is back scattered
Luca Stanco - Padova Low-x at HERA, Small-x 2007 11
JETS observables in the BREIT Frame
… more technically
Luca Stanco - Padova Low-x at HERA, Small-x 2007 12
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 13
• 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)
H1 data in the kinematic region: FORWARD JETS
L=13.7 pb-110-4<x<4.10-3 and 5<Q2<85 GeV2
0.5 < (pT,jet)2/Q2 < 2to suppress DGLAP and enhance BFKL
Luca Stanco - Padova Low-x at HERA, Small-x 2007 14
ZEUS data less striking… NEW FINAL RESULTS !!
ARIADNE (CDM) tuned for parameters attaining tothe -resolved part
FORWARD JETS
0.001
L=81.8 pb-1
Ejet>5 GeV, 2<jet<3.5
4.10-4<x<5.10-3 and 20<Q2<100 GeV2
0.5 < (Ejet)2/Q2 < 2
Luca Stanco - Padova Low-x at HERA, Small-x 2007 15
Comparison with CCFM Monte Carlo’s
not really bad…
CASCADE with twodifferent sets of unintegrated PDF’s
FORWARD JETS
Luca Stanco - Padova Low-x at HERA, Small-x 2007 16
Is it a HERA business ?or HERA phase space too limited to distinguishbetween different evolution models ?
ZEUS data less striking also for NLO comparison… (NLO(S
2)-DISENT MC below the data but within theoretical errors
( but ZEUS uses R2=Q2,
while H1 takes R
2=<pT2> )
FORWARD JETS
Luca Stanco - Padova Low-x at HERA, Small-x 2007 17
Very detailed investigation from H1 to enhance BFKL phase space:
Compute differential cross-sections of 1 & 2
Select dijets plus forward-jet
different 1 and 2 regions affect:• Kt-ordering• Gluon radiation• Evolution chain
Luca Stanco - Padova Low-x at HERA, Small-x 2007 18
Data towards NLO
too little Phase Space for NLO
FORWARD JETS
Luca Stanco - Padova Low-x at HERA, Small-x 2007 19
Data towards CCFM
Data towards CDM and res
res not sufficient …
CDM may mimic BFKL evolution …
NO description !
FORWARD JETS
Luca Stanco - Padova Low-x at HERA, Small-x 2007 20
Similar Analysis from ZEUS. NEW !! Here there are “different” discrepancies between data and models…
FORWARD JETS
Luca Stanco - Padova Low-x at HERA, Small-x 2007 21
MULTIJETS (dijets, trijets) ANALYSIS
Kinematic selection to enhance (unordered) gluon radiationand to study parton dynamics (at low-x)
H1: Ldt = 44.2 pb-1 10-4<xBj<10-2, 5<Q2<80 GeV2
ZEUS: Ldt = 82.2 pb-1 10-4<xBj<10-2, 10<Q2<100 GeV2
Hadronic Center-of-Mass (like Breit-frame boosted)
Comparisons with NLO at order O(S2) and O(S
3) NLOJET, DISENT
Conparisons with Monte Carlo’s LO-PS with/out res (CDM-DJANGOH, RAPGAP)
H1 compares to parton level, ZEUS compares to hadron level
Luca Stanco - Padova Low-x at HERA, Small-x 2007 22
All the sample:H1
Low-x region: more DATA than NLO predicted
E*> 4 GeV, E1*+E2*> 9 GeV
MULTIJETS
Overall systematics error of 19% not included on DATA
LO discrepancy
-1 < jet < 2.5
Luca Stanco - Padova Low-x at HERA, Small-x 2007 23
H1Select TWO subsamples with Forward JETS(jet<200, xjet=E*jet/Ep > 0.035)
Two central jets plus one forwardOne central jets plus two forward
NLO++ comparison
NLO at O(S3) describes well the TWO central jets,
expected to be produced from hard scattering
MULTIJETS
-1 < jet < 1
FORWARD jets are expected to be produced from gluon radiation
Luca Stanco - Padova Low-x at HERA, Small-x 2007 24
LO Monte Carlo’s comparisonH1
Resolved contribution NOT working !
RAPGAP: direct+resolved contributions
MULTIJETS
Luca Stanco - Padova Low-x at HERA, Small-x 2007 25
ZEUSReasonable description by NLOJET
MULTIJETS
Hadron Level
Jet
en
erg
y s
cale
un
cert
ain
ty
E*1> 7 GeV, E*2,3> 5 GeV
-1 < jet < 2.5R
2=F2 = (<ET>2+Q2)/4
Luca Stanco - Padova Low-x at HERA, Small-x 2007 26
ZEUSMULTIJETS
NLO consistent within errors
10 < Q2 > 15 GeV2 20 < Q2 > 30 GeV2
O(S2) does not
describe DATA
NLOJET comparison with Dijets DATA
Luca Stanco - Padova Low-x at HERA, Small-x 2007 27
ZEUS
… but theor. errors (F, R) too large Need next order
MULTIJETS
NLOJET comparison with Trijets DATA
Luca Stanco - Padova Low-x at HERA, Small-x 2007 28
Conclusions
• low-xBj regime unsettled at HERA
• Multijets (Forward) production measured in xBJ>10-4 and Q2<100 GeV2
• Statistical ~ Theoretical errors (NLO)• H1 and ZEUS not so in agreement (FORWARD JETS)• 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 30
Kinematics of Neutral Current (NC) Deep Inelastic Scattering (DIS)
W: *P c.o.m. energy
Luca Stanco - Padova Low-x at HERA, Small-x 2007 31
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 32
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 S