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


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


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:


...if nobody else have already shown ...

Gluon

s an

d

QCD

dyn

amics


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)


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)


QCD dynamics: Evolution & Resummation

HERA: DIS at low-xBj down to 10-5

large *p center of mass energy (W ~ 1/x)


satu

ratio

n

Inclusive F2 measurement not able to discriminate between different QCD dynamics Study of Hadronic Final States (JETS)


...if nobody else is going to show ...

Definitively, physics has to be clarified.

even if we consider Structure Functions results at low-x


FORWARD JETS (proton direction)

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

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

Caveat: different Monte Carlo models not fully clear where applicable


JETS observables in the BREIT Frame

… more technically


NLO pQCD calculations

and

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


• 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


ZEUS data less striking… FORWARD JETS

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


Is it a HERA business ?or HERA phase space too limited to distinguishbetween different evolution models ?

ZEUS data less striking… (NLO(S

2)-DISENT MC below the data but within theoretical errors)

needs of next NLO ?

( but ZEUS uses R2=Q2,

while H1 takes R

2=<pT2> )

FORWARD JETS


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


Data towards NLO

too little Phase Space for NLO

FORWARD JETS


Data towards CCFM

Data towards CDM and res

res not sufficient …

CDM may mimic BFKL evolution …

NO description !

FORWARD JETS


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


All the sample:H1

Low-x region: more DATA than NLO predicted

MULTIJETS

LO discrepancy

E*> 4 GeV, E1*+E2*> 9 GeV

Overall systematics error of 19% not included on DATA

-1 < jet < 2.5


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


LO Monte Carlo’s comparisonH1

Resolved contribution NOT working !

RAPGAP: direct+resolved contributions

MULTIJETS


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


ZEUSMULTIJETS

NLOJET comparison with Dijets DATA

NLO consistent within errors

10 < Q2 > 15 GeV2 20 < Q2 > 30 GeV2

O(S2) does not

describe DATA


ZEUSNLOJET comparison with Trijets DATA

… but theor. errors (F, R) too large Need next order

MULTIJETS


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


Backup Slides


Kinematics of Neutral Current (NC) Deep Inelastic Scattering (DIS)

W: *P c.o.m. energy


WHY studying pQCD ?at least 4 answers:

1. NOT TRUE it is so well understood ..

2. Strong coupling, S, measurement (see next talk)

3. Parton Density Functions measurements

4. Parton evolutions

From HERA-LHC workshops, examples of uncertainties,


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…


two different parton ladders….

Expect more energetic jets in forward region

(LEPTO, DISENT)

(~ColorDipM)

(CASCADE)


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


Jets Production in NC Deep Inelastic Scattering


5 jets event at low-x (H1)

Documents

Low-x AND Low Q 2