Lev Khein (Moscow State University) on behalf of the ZEUS collaboration

18.04.2007 Munich Lev Khein Forward jets in DIS 1

Lev Khein (Moscow State University)

on behalf of the ZEUS collaboration DIS 2007 Munich

Parton dynamics at low x

QCD Calculations

MC Models

Forward Jets

Forward Jet + Dijet

Summary & conclusions

Forward jet production in DIS


Parton dynamics at low xParton dynamics at low x

DGLAP: ∑ (s ln Q2 )n

BFKL : ∑ (s ln (1/x) )n

CCFM : ln Q2 and ln (1/x)

High

Density

Region

Non-perturbative region

BFKL

DGLAP

CCFM

ln x

ln Q

2

e

*(Q2)

e´

q

q

Xn, kT,n

Xn-1, kT,n-1

X2, kT,2

.

.

.

X1, kT,1

p

Perturbative expansion of parton evolution equations

DGLAP successful at high Q2 but expected to break down at low Q2 and low x

BFKL should be applicable at very low x

CCFM expected to be valid in whole x, Q2 range

Cannot be explicitly calculated to all orders

Approximations summing subsets of terms

Strong ordering in x, no kno kTT orderingordering

x1 >> x2 >> ... >> xn-1 >> xn = xBj

θn >> θn-1 >> ... >> θ2 >> θ1 Angular orderingAngular ordering

x1 > x2 > ... > xn-1 > xn = xBj

-

kT,12 << kT,2

2 << ... kT,n-12 << kT,n

2 ≈ Q 2

Ordering in x, strong ordering in kstrong ordering in kTT


QCD CalculationsQCD Calculations

DISENT/NLOJET++: Fixed order QCD partonic cross section, on mass shell ME + DGLAP , (collinear factorization)

NLOJET++

+ Virtual Corr. to LO

DISENT


MCMC ModelsModels

CASCADE: LO off mass shell ME + PS based on kT factorized CCFM evolution

transverse momentum of emitted gluon k> k

cut

uPDF set1 : k

cut = 1.33 GeV

uPDF set2 : k

cut = 1.18 GeV

Non-singular term in splitting function

LEPTO: LO ME on mass shell + PS in DGLAP

Strong ordering in kT

ARIADNE: implementation of Color Dipole Model (CDM)

Independently radiating dipoles formed by emitted gluons

Random walk in kT

Two versions, with defalt tuning (default) and retuned by H1 (tuned)

At small xBj no ordering in kT


Kinematic range

2< ηjet<4.3

Inclusive KT algorithm

Forward Jet selection

0.04 < y < 0.7

0.0004 < xBj < 0.005

20 < Q2 < 100 GeV2

xjet = Ejet/Eproton >> xBj enhances BFKL evolution xjet > 0.036

0.5 < (Etjet)2/Q2 < 2 Et

jet Q2 suppresses DGLAP evolution

ee’

p

Event & Jet selection

Etjet > 5 GeV

(Etjet)2 1.4 unit more forward than before

Forward JetsForward Jets

98-00 Data, L 82 pb-1


NLO below data, especially at small xBj

but theoretical uncertainty is large



• Lepto doesn’t suffice

• Ariadne default underestimates high Et

jet, overestimates high ηjet (proton remnant)

• Ariadne tuned is good



Non singular term (in set 2) reduces cross section, but not improves agreement

Shape of all distributionsdisagrees with data

Shape is a problem



NLO is below data

Large theoretical uncertaintyprevents decisive conclusion

Triple differential cross sections



Pronounced excess of high Etjet

in Ariadne default

Ariadne tuned is fine

Triple differential cross sections



el < jet 1 < jet-2 < forward-jet

0.5 < (Etjet)2/Q2 < 2

Event & Jet selection

Kinematic range the same as for inclusive forward jets

Forward jet the same,

constraint excluded

Two additional jets with Etjet > 5 GeV

Forward Jet + DijetForward Jet + Dijet


Small and jetsare most forward. At small xBj space is left for additional partons closer to the photon.NLOJET++ underpredicts many partons below data

21

NLOJET++ vs data



Lepto below data

Ariadne default pronouncedly above data –> too high multigluon emission rate

Ariadne tuned is fine



ZEUS measured jets at small xBj in highly extended forward region NLO is significantly below the data for inclusive forward jets NLO for forward jet + dijet undershoots data in the region of pseudorapidities where multigluon emission is favoured LO DGLAP-based MC, Lepto, is twice below the data CDM (Ariadne) is capable of successfully describing the whole volume of the data Cascade MC fails to successfully describe the measurements, other sets of uPDF are to be tried. The data obtained could be highly useful for further uPDF adjusting

Summary & ConclusionsSummary & Conclusions


Forward Jets

Neither set accommodatesfor all data features


Forward Jet + Dijet

• Set 1 is above data

• Set 2 is below data


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Lev Khein (Moscow State University) on behalf of the ZEUS collaboration