On behalf of the ZEUS and H1 Collaborations

Enrico Tassi Univ. Autonoma de Madrid enrico.tassi@uam.es

Measurements of FMeasurements of F22 and F and FLL at low-Q at low-Q22 in e-p in e-p interactions interactions at HERA at HERA

Outline

• Motivation • Zeus and H1 low-Q2 analyses• New measurements of F2 and FL

• Summary and Outlook

Motivation

The soft and hard regimes of QCD:-Regge Phenomenology: successful in describing soft peripheral processes at high energy - no QCD picture of underlying dynamics yet-pQCD: extremely successful in describing hard-scattering processes (down to surprisingly small scales: Q2~1 GeV2)

Can we merge them into a unified QCD picture ?

- HERA has played/is playing an important role towards this goal → new F2 and FL measurements in the transition region… (→ this talk)

Photon virtuality allows to smoothlyinterpolate between non pert. andpert. regions

e

e’

p

ZEUS ISR analysis H1 SVTX 2000 H1 99 (min. bias) H1 Compton analysis (covers high-x region)

Precision H1/Zeus inclusive data cover the transition region between perturbative and non-perturbative domains (Q2 ~ 1GeV2), in a wide range of x and y

Data sets and kinematic coverage

Focus on new low-Q2 analyses:

Zeus BPT

Nominal DIS

ZEUS F2 BPT – (reminder)

Phase space:0.045 < Q2 < 0.65 GeV2

6x10-7 < x < 10-3

Regge fit gives good description:

F2:- Typical uncert. < 4%- Rise persists down to the smallest Q2

ZEUS F2 BPT – (reminder)

?

~F2/Q2

ZEUS ISR analysis

Emission of ISR γ => reduction of Ee

-Lower Ee -> lower Q2

Measure F2

Emission of ISR γ => reduction of √s-Access range of y values (for fixed x,Q2)Needed for measuring FL (see later)

-ISR sample well understood -Higher-x reach in some Q2 regions-Statistics at low-x still rather low (but 96 only)

F2 from Compton analysis (H1)

e + p → e + γ + X

kinematics reconstructed with hadrons → detailed final state simulation at low W2

Luminosity: 9.25 pb-1

F2 at very high x accessed; domain of fixed target experiments

Complementary measurement to inclusive DIS

Backward Silicon Tracker

Shifted vertex

SpaghettiCalorimeter

e+ pNominal vertex

~70cm

Access to very low Q2 by shifting vertex(H1)

Shifting vertex opens detector acceptance at low Q2

Precise reconstruction of the scattered positron: ( Δθ = 0.3 mrad, ΔE = 0.3% @ beam energy) Measure very low positron energy (~3GeV) -> high-y

New low-Q2 F2 (H1)

H1 / ZEUS / NMC data used to fit Q2 dependence for x < 0.01 :

λ(Q2) ~ ln[Q2/Λ2] and c(Q2) ~ const. for Q2 > 3.5

GeV2

Behaviour is changing at around Q2 ~ 1 GeV2

From soft hadronic

interactions it is expected that λ → ~0.08 for Q2 → 0

)(-22

2

.)( QλxQcF =

Rise of F2 towards low x

FL measurements: Glue at low-Q2

For Q2 ≥ 5 GeV2 gluon density muchlarger than the sea

At lower Q2, xS continues to gentlyrise at low x whereas the gluon densitybecomes valence like and tends to benegative

From the Zeus NLO-QCD fit

Measuring FL around Q2 ~ 1 GeV2 couldtell us a lot…

Zeus and H1:Two different approaches

First direct determination of FL from ISR (ZEUS)

96+97 sample: ~36 pb-1

1 < Q2 < 30 GeV2

0.11 < y < 0.23

1. Measure F2

2. Determine FL via a 2 param. fit of:

Fitting function: N δFL

where

LFIT FY

yF

2

2

λxcF -2 =

New FL determination with ‘shape method’

Idea: Shape of σr at high y is driven by y2/Y+ factor rather than by FL behaviour

Neglects FL variation in a narrow x-range

The whole x-range of measured data is used to fit F2 and FL,

Fit:

with:

FL determination from low Q2,x H1 data

FL was for the first time extracted at very low Q2 ~ 1 GeV2

It is clearly non-zero

FL determination from low Q2,x H1 data

Starts to descriminate different predictions Need to measure the x-dependence of FL

FL determination from H1 data

H1 NLO QCD fit is consistent with data in the DIS region MRST NLO QCD fit too low at low Q2

GBW saturation dipole model describes the whole range BKS model is steeper but still consistent with the data

New H1/Zeus F2 measurements in the important transition region

from the non-perturbative to perturbative domain Rise of F2 : rate is changing at Q2 ~ 1GeV2 (-> gluon at low-Q2)

FL was extracted for the first time with ISR events by ZEUS

H1 FL now cover range 0.75 < Q2 < 800 GeV2

New low-Q2 FL measurem. show that FL>0 and discriminate models

Precision FL measurements (x-dependence!) require low proton energy ep runs

Summary and Outlook