Structure functions atlow x and Q2 at ZEUSStructure functions atlow x and Q2 at ZEUS

Jonathan ScottZEUS Collaboration

University of Bristol

8th International Workshop onDeep Inelastic Scattering and QCD

Liverpool, April 2000

Introduction

HERA provides a unique opportunity to explorethe transition region between perturbative and

non perturbative QCD.

The kinematic region at low Q2 and x can be reached using small angle detectors and

Initial State radiative events.

This presentation• Kinematics • BPC/BPT Detectors• Results• Possibilities from ISR• Summary

Negative 4-momentum squared of virtual photon

 Bjorken scaling

 

Fractional energy loss of electron in proton rest frame

Mass2 of Hadronic system

Kinematics 1

2sinE4E)k(kqQ 2

ee222

sy

Q

2p.q

Qx

22

)cos(12E

E-1y

e

e

27.5GeV positrons

820GeV protons

x

xQqPW

)1()(

222

Kinematics 2

Differential cross section for * + p X is

 )(

23

224

2

2

2

xFYFyFYyQdxdQ

dL

2)1(1 yY

FL Longitudinal structure function - relevant at high y

xF3 - Parity violating Term

Low x approximation

),(4

),( 222

2**2* QxF

QQw p

Lp

Tp

tot

)(22 qqxeF

Quarksq

Kinematic Region

Lowest Q2 bin 0.045GeV2 Highest y bin 0.84Lowest y bin 0.005

BPC/BPT

BPC (Beam Pipe Calorimeter)Tungsten-scintillator sampling calorimterScattering angle range 18-32mradEnergy resolution 17%/EExtend Q2 coverage down to 0.1GeV2

BPT (Beam Pipe Tracker)2 Silicon Microstrip detectors

Improve Q2 resolutionVertex resolution 3cm

Control systematicsBgd rejection

BPC/BPT Bins

High y E´e > 4.4GeV Electron Method

Medium y E´e > 7GeV Electron Method

Low y(+prescale)E´e > 20GeV Sigma Method

BPC/BPT event selection

Electromagnetic shower in BPC

Track in BPT1. Matching with BPCReject fake positrons (Php +radiative)2. Originates from valid

interaction vertexReject beam related background

Background cuts on = (E-pZ)TotalyJB = (E-pZ)Hadronic/2Ee

3.9pb-1 data 5.3pb-1 non-diffractive MC 0.9pb-1diffractive MC

Control Plots

Control Plots

Results

Results

Results

ISRFL sensitive to QCD properties which

cannot be explored frommeasurements of F2.

FL measurement requiresmeasurement of differential cross-section at different centre of massenergies i.e different y for same

(x,Q2). Very difficult measurement.

Use Initial State Radiation toEffectively vary the beam energy.

FL

D i f f e r e n t i a l c r o s s s e c t i o n f o r * + p Xi n t e r m s o f R . .

24

2

2

2

1

12F

R

RY

xQdxdQ

d

W h e r e

L

LL

T

L

FF

F

QxxF

QxFQxR

22

1

22

),(2

),(),(

2)1(1

)1(2

y

y

2)1(1 yY

M e a s u r e I S R p h o t o n i n L U M I - c a l o r i m e t e r .

U n r e s o l v e d i s s u e sL U M I - a c c e p t a n c eL a r g e B a c k g r o u n d .

ISR Kinematic range

Bridges Q2 range between BPC/BPT and nominal F2 analyses as well

as overlap in y with Fixed Target experiments

Summary

T h e B P T w a s i n s t a l l e d i n 1 9 9 7 t o i m p r o v e t h e d e t e c t i o n o f p o s i t r o n s ,

i m p r o v i n g t h e q u a l i t y o f l o w Q 2 d a t a .

T h i s a l l o w e d t h e m e a s u r e m e n t o f F 2

a n d p

tot* i n t h e r e g i o n

0 . 0 4 5 G e V 2 < Q 2 < 0 . 6 5 G e V 2

6 . 1 0 - 7 < x < 1 . 1 0 - 3

I n i t i a l s t a t e r a d i a t i v e e v e n t s s h o u l d

p r o v i d e m e a s u r e m e n t o f F 2 i n t h e r e g i o n s p a n n i n g B P T d a t a a n d

n o m i n a l F 2 r e s u l t s , a s w e l l a s g i v i n g a f i r s t d i r e c t m e a s u r e m e n t o f F L .