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Measurement of F 2 and F L at low Q 2 in ep Interactions at HERA H1 and ZEUS analyses at low Q 2 Extraction of F L Summary and Outlook Tomáš Laštovička DESY Zeuthen, Charles University Prague EPS 2003, Aachen, 17.-23.7.2003

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 2/12 Kinematic plane coverage Precision H1 inclusive data cover the transition region between perturbative and non-perturbative domains (Q 2 ~ 1GeV 2 ), in wide range of x and y Focus on new low Q 2 data in this presentation (high Q 2 results presented by M. Moritz) H1 Compton analysis covers region of high x ZEUS ISR analysis is focused on F L at low Q 2 and y HERA ~ 0.1-0.2 ! NEW !

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 3/12 F 2 from Compton analysis (H1) F 2 at very high x accessed; domain of fixed target experiments Complemenary measurement to inclusive DIS e + p → e + p + X kinematics reconstructed with hadrons → detailed final state simulation at low W 2 Luminosity: 9.25 pb -1 (Sophia MC)

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 4/12 Access to very low Q 2 by shifting vertex High precision reconstruction: Δθ = 0.3 mrad, ΔE = 0.3% at beam energy Shifting vertex opens detector acceptance at low Q 2 Backward Silicon Tracker Shifted vertex Spaghetti Calorimeter e+e+ p Nominal vertex ~70cm

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 5/12 Inclusive low Q 2 cross-sections (H1) High precision cross section measurements (errors 2-3% in bulk region) Wide range of inelasticity y: 0.002 < y < 0.89 Backward silicon detector allows access to both high y and low y regions Q 2 = 12 GeV 2

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 6/12 Rise of F 2 towards low x H1 / ZEUS / NMC data used to fit Q 2 dependencies 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 = 1GeV 2 From soft hadronic interactions it is expected that λ → ~0.08 for Q 2 → 0

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 7/12 Neglects F L variation in narrow x range The whole x-range of measured data is used to fit F 2 and F L, i.e. no extrapolation of F 2, use of full information → smaller errors New F L determination with ‘shape method’ Shape of σ r at high y is driven by kinematic factor y 2 /Y + rather than by F L behaviour Fit in Q 2 bins:

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 8/12 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

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 9/12 F L determination from low Q 2,x H1 data Requires to measure the x-dependence of F L

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 10/12 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

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 11/12 F L determination from ISR (ZEUS) ISR (Initial State Radiation) changes s → shifts high y (F L sensitive) region to higher x. Ratio N data /N MC(F L =0) analysed → F L First direct determination of F L from ISR; result is consistent with QCD y ISR = 0.20 - 0.45 Q 2 = 1 - 30GeV 2

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 12/12 Summary and Outlook F 2 was measured at high x, low Q 2 with Compton events H1 has obtained precision cross section data at low Q 2 in the transition region from non-perturbative to deep inelastic domain Rise of F 2 was studied; rate is changing at about Q 2 ~ 1GeV 2 H1 F L data cover range 0.75 < Q 2 < 800 GeV 2 New low Q 2 F L data show F L >0 and discriminate models F L was extracted for the first time with ISR events by ZEUS Precision data on F L are crucial and require low proton energy ep runs in the future to measure the x-dependence

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Tomáš LaštovičkaEPS 2003, Aachen 17.7.-23.7 13/12 New F L determination with ‘shape method’ Shape of σ r at high y is driven by kinematic factor y 2 /Y + rather than by F L behaviour itself One F L point per one Q 2 bin extracted Neglects F L variation in narrow x range Fit in Q 2 bins: F L extracted at, weight = y 2 /(Y + σ i 2 ) Method makes use of the whole x-range of measured data to fit F 2 and F L, i.e. no extrapolation of F 2, use of full information → smaller errors λ is in good agreement with previous measurements Excellent description of σ r by ‘shape’ fits in the whole kinematic region

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