On behalf of the ZEUS and H1 Collaborations Enrico Tassi Univ. Autonoma de Madrid Measurements of F 2 and F L at low-Q 2 in e-p interactions Measurements of F 2 and F L at low-Q 2 in e-p interactions at HERA at HERA Outline Motivation Zeus and H1 low-Q 2 analyses New measurements of F 2 and F L 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: Q 2 ~1 GeV 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 F L measurements in the transition region… ( → this talk) Photon virtuality allows to smoothly interpolate between non pert. and pert. 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 (Q 2 ~ 1GeV 2 ), in a wide range of x and y Data sets and kinematic coverage Focus on new low-Q 2 analyses: Zeus BPT Nominal DIS

ZEUS F 2 BPT – (reminder) Phase space: < Q2 < 0.65 GeV 2 6x10 -7 < x < Regge fit gives good description: F 2 : - Typical uncert. < 4% - Rise persists down to the smallest Q 2

ZEUS F 2 BPT – (reminder) ? ~F 2 /Q 2

ZEUS ISR analysis Emission of ISR γ => reduction of E e -Lower E e -> 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 F L (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 (H1)  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

Backward Silicon Tracker Shifted vertex Spaghetti Calorimeter e+e+ p Nominal vertex ~70cm Access to very low Q 2 by shifting vertex(H1)  Shifting vertex opens detector acceptance at low Q 2  Precise reconstruction of the scattered positron: ( Δθ = 0.3 mrad, ΔE = beam energy)  Measure very low positron energy (~3GeV) -> high-y

New low-Q 2 F 2 (H1)

 H1 / 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 GeV 2  Behaviour is changing at around Q 2 ~ 1 GeV 2  From soft hadronic interactions it is expected that λ → ~0.08 for Q 2 → 0 Rise of F 2 towards low x

F L measurements: Glue at low-Q 2 For Q 2 ≥ 5 GeV 2 gluon density much larger than the sea At lower Q 2, xS continues to gently rise at low x whereas the gluon density becomes valence like and tends to be negative From the Zeus NLO-QCD fit Measuring F L around Q 2 ~ 1 GeV 2 could tell us a lot… Zeus and H1: Two different approaches

First direct determination of F L from ISR (ZEUS ) sample: ~36 pb -1 1 < Q 2 < 30 GeV < y < Measure F 2 2. Determine F L via a 2 param. fit of: Fitting function: N δ FL where

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

F L determination from low Q 2,x H1 data  F L was for the first time extracted at very low Q 2 ~ 1 GeV 2  It is clearly non-zero

F L determination from low Q 2,x H1 data  Starts to descriminate different predictions  Need to measure the x-dependence of F L

F L determination from H1 data  H1 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

 New H1/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 ~ 1GeV 2 (-> gluon at low-Q 2 )  F L was extracted for the first time with ISR events by ZEUS  H1 F L now cover range 0.75 < Q 2 < 800 GeV 2  New low-Q 2 F L measurem. show that F L >0 and discriminate models  Precision F L measurements (x-dependence!) require low proton energy ep runs Summary and Outlook