Enrico Tassi Univ Autonoma de Madrid enrico tassiuam

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Enrico Tassi Univ. Autonoma de Madrid enrico. tassi@uam. es Measurements of F 2 and

Enrico Tassi Univ. Autonoma de Madrid enrico. tassi@uam. es Measurements of F 2 and FL at low-Q 2 in e-p interactions at HERA On behalf of the ZEUS and H 1 Collaborations Outline • Motivation • Zeus and H 1 low-Q 2 analyses • New measurements of F 2 and FL • Summary and Outlook

Motivation The soft and hard regimes of QCD: -Regge Phenomenology: successful in describing soft

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 -p. QCD: extremely successful in describing hard-scattering processes (down to surprisingly small scales: Q 2~1 Ge. V 2) Can we merge them into a unified QCD picture ? - HERA has played/is playing an important role towards this goal → new F 2 and FL measurements in the transition region… (→ this talk) e’ e Photon virtuality allows to smoothly interpolate between non pert. and pert. regions p

Data sets and kinematic coverage Nominal DIS Focus on new low-Q 2 analyses: §

Data sets and kinematic coverage Nominal DIS Focus on new low-Q 2 analyses: § § ZEUS ISR analysis H 1 SVTX 2000 H 1 99 (min. bias) H 1 Compton analysis (covers high-x region) Zeus BPT § Precision H 1/Zeus inclusive data cover the transition region between perturbative and non-perturbative domains (Q 2 ~ 1 Ge. V 2), in a wide range of x and y

ZEUS F 2 BPT – (reminder) Phase space: 0. 045 < Q 2 <

ZEUS F 2 BPT – (reminder) Phase space: 0. 045 < Q 2 < 0. 65 Ge. V 2 6 x 10 -7 < x < 10 -3 F 2: - Typical uncert. < 4% - Rise persists down to the smallest Q 2 Regge fit gives good description:

ZEUS F 2 BPT – (reminder) ~F 2/Q 2 ?

ZEUS F 2 BPT – (reminder) ~F 2/Q 2 ?

ZEUS ISR analysis Emission of ISR γ => reduction of Ee -Lower Ee ->

ZEUS ISR analysis Emission of ISR γ => reduction of Ee -Lower Ee -> lower Q 2 Measure F 2 Emission of ISR γ => reduction of √s -Access range of y values (for fixed x, Q 2) Needed for measuring FL (see later) -ISR sample well understood -Higher-x reach in some Q 2 regions -Statistics at low-x still rather low (but 96 only)

F 2 from Compton analysis (H 1) § e+p→e+γ+X § kinematics reconstructed with §

F 2 from Compton analysis (H 1) § e+p→e+γ+X § kinematics reconstructed with § hadrons → detailed final state simulation at low W 2 Luminosity: 9. 25 pb-1 § F 2 at very high x accessed; domain of fixed target experiments § Complementary measurement to inclusive DIS

Access to very low Q 2 by shifting vertex(H 1) Spaghetti Calorimeter Shifted vertex

Access to very low Q 2 by shifting vertex(H 1) Spaghetti Calorimeter Shifted vertex Nominal vertex e+ ~70 cm Backward Silicon Tracker § Shifting vertex opens detector acceptance at low Q 2 § Precise reconstruction of the scattered positron: § ( Δθ = 0. 3 mrad, ΔE = 0. 3% @ beam energy) Measure very low positron energy (~3 Ge. V) -> high-y p

New low-Q 2 F 2 (H 1)

New low-Q 2 F 2 (H 1)

Rise of F 2 towards low x § H 1 / ZEUS / NMC

Rise of F 2 towards low x § H 1 / ZEUS / NMC data used to fit Q 2 dependence for x < 0. 01 : § λ(Q 2) ~ ln[Q 2/Λ 2] and c(Q 2) ~ const. for Q 2 > 3. 5 Ge. V 2 § Behaviour is changing at around Q 2 ~ 1 Ge. V 2 § From soft hadronic interactions it is expected that λ → ~0. 08 for Q 2 → 0

FL measurements: Glue at low-Q 2 From the Zeus NLO-QCD fit For Q 2

FL measurements: Glue at low-Q 2 From the Zeus NLO-QCD fit For Q 2 ≥ 5 Ge. V 2 gluon density much larger than the sea At lower Q 2, x. S continues to gently rise at low x whereas the gluon density becomes valence like and tends to be negative Measuring FL around Q 2 ~ 1 Ge. V 2 could tell us a lot… Zeus and H 1: Two different approaches

First direct determination of FL from ISR (ZEUS) 96+97 sample: ~36 pb-1 1 <

First direct determination of FL from ISR (ZEUS) 96+97 sample: ~36 pb-1 1 < Q 2 < 30 Ge. V 2 0. 11 < y < 0. 23 1. Measure F 2 2. Determine FL via a 2 param. fit of: Fitting function: N δFL where

New FL determination with ‘shape method’ Idea: Shape of σr at high y is

New FL determination with ‘shape method’ Idea: Shape of σr at high y is driven by y 2/Y+ factor rather than by FL behaviour Fit: with: § Neglects FL variation in a narrow x-range § The whole x-range of measured data is used to fit F 2 and FL,

FL determination from low Q 2, x H 1 data § FL was for

FL determination from low Q 2, x H 1 data § FL was for the first time extracted at very low Q 2 ~ 1 Ge. V 2 § It is clearly non-zero

FL determination from low Q 2, x H 1 data § Starts to descriminate

FL determination from low Q 2, x H 1 data § Starts to descriminate different predictions § Need to measure the x-dependence of FL

FL determination from H 1 data § § H 1 NLO QCD fit is

FL determination from H 1 data § § H 1 NLO QCD fit is consistent with data in the DIS region MRST NLO QCD fit too low at low Q 2 GBW saturation dipole model describes the whole range BKS model is steeper but still consistent with the data

Summary and Outlook § New H 1/Zeus F 2 measurements in the important transition

Summary and Outlook § New H 1/Zeus F 2 measurements in the important transition region § § § from the non-perturbative to perturbative domain Rise of F 2 : rate is changing at Q 2 ~ 1 Ge. V 2 (-> gluon at low-Q 2) FL was extracted for the first time with ISR events by ZEUS H 1 FL now cover range 0. 75 < Q 2 < 800 Ge. V 2 New low-Q 2 FL measurem. show that FL>0 and discriminate models Precision FL measurements (x-dependence!) require low proton energy ep runs