1. HERA Physics and Gustav Kramer Alice Valkárová, Charles University, Prague 2nd April 2013Festkolloquium - G.Kramer1

2. 2nd April 2013Festkolloquium - G.Kramer2 Gustav Kramer has more than 5000 citations to his papers….(h-ind. 38). A lot of citations to G.Kramer and co-authors in H1 and ZEUS publications! Photoproduction dijets, inclusive.. Fragmentation functions, charm Isolated photons in DIS Diffraction, dijets, factorization

3. Dijet photoproduction 2nd April 2013Festkolloquium - G.Kramer3 direct photoproduction photon directly involved in hard scattering x γ =1 (at parton level) hadron –like component (resolved) photon fluctuates into hadronic system, which takes part in hadronic scattering x γ <0.2 (at parton level) point –like component of resolved photon dominates in the region of 0.2 <x γ <1 In LO! Q 2 ~ 0 GeV 2 x γ - fraction of photon’s momentum in hard subprocess

4. Photoproduction, NLO QCD calculations 2nd April 2013Festkolloquium - G.Kramer4 Measurement of the Inclusive Di-Jet Cross Section in Photoproduction and Determination of an Effective Parton Distribution in the Photon, H1 Collab., C. Adloff et al., Eur. Phys. J. C1 (1998) 97-107, 09/97 Q 2 <4 GeV 2 H1 – untagged photoproduction Hadronization correction not taken into account…… H1 Compared to KK NLO calculations

5. Low Q 2 - dijets 2nd April 2013Festkolloquium - G.Kramer5 Program for NLO QCD calculations JETviP (Kramer,Poetter) (the only program with resolved contribution included). G. Kramer and B. Poetter, Eur. Phys. J. C 5 (1998) 665 used for comparison with dijet photoproduction. H1 Collab., A. Aktas et al., Eur. Phys. J. C37 (2004) 141-159 Measurement of Dijet Production at Low Q 2 at HERA M. Klasen and G. Kramer, Phys. Lett. B 366 (1996) 385 The asymmetric E T cuts avoid regions of instability in the NLO calculations! Incorporation of resolved component to NLO brings predictions closer to the data!

6. Inclusive photoproduction of jets 2nd April 2013Festkolloquium - G.Kramer6 ZEUS Coll., Inclusive –jet photoproduction at HERA and determination of α s, Nucl.Phys.B 864,1 (2012) (reference to:M.Klasen,T.Kleinwort,G.Kramer, Eur.Phys.J C1,(1998),1.) Untagged photoproduction

7. 2nd April 2013Festkolloquium - G.Kramer7 Charged Particle Cross Sections in Photoproduction and Extraction of the Gluon Density in the Photon H1 Collab., C. Adloff et al., Eur. Phys. J. C10 (1999) 363 B.A.Kniehl,G.Kramer, Z.Physik C62 (1994),53. Inclusive particle production at HERA: Resolved and direct quasireal photon contributions in NLO QCD Inclusive particle photoproduction Testing the universality of fragmentation functions – B.A.Kniehl,G.Kramer,B.Poetter Nucl.Phys. B597 (2001), 337. 150 citations Tagged photoproduction

8. 2nd April 2013Festkolloquium - G.Kramer8 Three different parameterisations of the FF are used which are obtained from NLO fits to e+e- data: KKP,KRETZER and AKK Charged Particle Production in High Q 2 Deep- Inelastic Scattering at HERA H1 Collab., Phys.Lett.B654:148-159,2007. B.A.Kniehl,G.Kramer,B.Potter,Nucl.Phys.B582(2000)514 with 314 citations…. S.Albino,B.A.Kniehl,G.Kramer,Nucl.Phys.B725(2005) 181 KKP AKK Fragmentation functions In Breit frame,the measured normalized distribution of the scaled momentum 1/N dn/dx p, N is the total number of events, dn is the total number of charged tracks with scaled momentum x p in the interval dx p, x p =p h /(Q/2)

9. Charm production 2nd April 2013Festkolloquium - G.Kramer9 Measurement of Inclusive and Dijet D*-Meson Cross Sections in Photoproduction at HERA H1 Collab., F.D. Aaron et al.,Eur. Phys. J. C72 (2012) 1995 B.A. Kniehl, G. Kramer, I. Schienbein and H.Spiesberger, Eur. Phys. J.C62 (2009), 365 Single differential D* cross sections compared to NLO calculations Difficult, due to the presence of two scales, mass of quark and p T

10. 2nd April 2013Festkolloquium - G.Kramer10 Observation of isolated high-E T photons in deep inelastic scattering ZEUS Collab. Phys.Lett. B596 (2004), 86 Isolated photons and jets in DIS Photon plus jet cross-sections in DIS ep Collisions A. Gehrmann-De Ridder, G. Kramer, H.Spiesberger, Nucl. Phys. B 578 (2000) 326 Photons with 5 GeV < E T γ < 10 GeV accompanied by jet Quark radiative diagrams Lepton radiative diagrams LO

11. 2nd April 2013Festkolloquium - G.Kramer11 H1 Collab., F.D. Aaron et al., Eur.Phys.J.C54:371,(2008). A. Gehrmann-De Ridder, G. Kramer and H.Spiesberger, Nucl. Phys. B 578 (2000) 326 The production of isolated photons in DIS with at least one jet compared to NLO calculations Measurement of isolated photons accompanied by jets in deep inelastic ep scattering, ZEUS collab.,Phys.Lett.B 715 (2012),88. The GKS predictions describe shapes quite well… Isolated photons and jets in DIS

12. Diffraction 2nd April 2013Festkolloquium - G.Kramer12 Diffraction was not expected to play so important role at HERA experiments! W Two classes of diffractive events: Q 2 ~0 → photoproduction Q 2 >>0 → deep inelastic scattering (DIS) Large rapidity gap HERA: ~ 10% of low-x DIS events are diffractive

13. Diffraction and questions of factorization 2nd April 2013Festkolloquium - G.Kramer13 QCD factorization theoretically proven for DIS (Collins, 1998) It allows the extraction of DPDFs from the (DIS) data p epep S2=S2= σ (data) σ (theory) Exporting DPDFs from HERA toTevatron does not work ! Factorisation broken -> S 2 ~ 0.1. → DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm) → universal hard scattering cross section (same as in inclusive DIS ) Factorisation in dijet photoproduction (resolved component similar to hadron-hadron collisions) - interactions of spectators filling rapidity gap? Factorization in photoproduction checked by both H1 and ZEUS….

14. Factorization in diffractive photoproduction 2nd April 2013Festkolloquium - G.Kramer14 H1 -diffractive dijet photoproduction results compared to Frixione and Kramer-Klasen NLO QCD calculations. Data suppressed by factor ~ 0.58 The same conclusion also for ZEUS, however without suppression! H1 Collab., Eur.Phys.J. C70 (2010) 15

15. Factorization in diffraction 2nd April 2013Festkolloquium - G.Kramer15 Eur.Phys.J. C70 (2010) 91-106 M.Klasen, G.Kramer, Suppression factors in diffractive photoproduction of dijets Suppression factor R An attempt to explain the difference in conclusions of both experiments. The hint that suppression depends on E T of jets…. (e.g. H1 Collab. Eur.Phys.J. C70 (2010) 15) This puzzle not solved yet, new H1 experimental results will come soon… (DIS 2013, Marseille).

16. Summary 2nd April 2013Festkolloquium - G.Kramer16 both HERA experiments profited a lot from the close relations with G.Kramer and his group H1 and ZEUS used frequently the predictions of G.Kramer and co-authors for the interpretation of the experimental results In the name of both experiments - best wishes for the next fruitful years and good health!