0 degree and DIS

1. International workshop for High-Energy Scattering at Zero Degree 2-4 March 2013 at Kobayashi-Maskawa Institute, Nagoya University Yuji Yamazaki (Kobe University) 0 degree and DIS

2. HERA 1992-2007 • The only collider,with the energy comparableto other and heavy ion colliders • 27.5 GeV920 GeV • Super microscope for the proton • 0.5on tape each

3. Parton densities at low- • Rapid increase intowards low- rest of the proton Higher Q2 scattered quark Quark density decreasing at high-x with Q2

4. e' e g* Higher Q2 evolution p p Scaling violation in DIS • behaviour described by pQCD • Sea quarks dynamically produced • DGLAP evolution equation fits data well • Gluon density obtainedthrough the DGLAP fit You cansee gluonsindirectly

5. Extracting parton densities • Precise determination • ~ 5% for gluon, sea ~ 2% • In low- regime: • completely dominated bysea or gluon • gluon twice more than sea A lot of partons at low-, especially gluons

6. Today’s menu • This talk covers the forward physics and related subjectsat the HERA collider • Introduction for 𝑒𝑝 scattering • Proton: many partons in low- regime • Forward scattering and low- physics • Diffraction • Leading baryons • Forward energy flow

7. Low- forward • Protons almost unperturbed after small- partons taken out • Small- partons are pretty “backward” • Large rapidity interval between the small- parton and most forward particle remnant small large backward forward = 0 degree

8. at low- is scattering • low- low-, photon regarded as hadron • centre-of-mass energy is higherfor lower- events • low- = larger rapidity interval • Fast rise in cross section: due to increase in partonphoton is not completely a hadron, rather a “hard” object

9. Soft or hard? • Colourless particles appear more in forward (perhaps) • where is the transition, in which rapidity? small = hard very large = soft backward forward = 0 degree

10. partons phase space = various phenomena particle exchange(one-pion exch. etc.) multi-partoninteraction • where colourless,where colourful? diffraction by “Pomeron” increasing forward E diffraction destroyed by multi-parton interaction Large RapidityGap (LRG) diffraction by 2-gluon

11. Diffraction in high-energy DIS

12. Diffraction in collisions: issues • Diffraction at HERA: • photon dissociates into small mass (X) • proton stays intact or proton dissociates into small mass (Y) • Standard view: • Pomeron emitted from proton,which is scattered off by a photonDIS of the Pomeron • Pomeron or 2-gluon ? • Is the diffraction peripheral ? diffraction by “Pomeron” Large RapidityGap (LRG) X Y diffraction by 2-gluon

13. Diffractive DIS and diffractive PDFs • partonic structure of diffractive exchange (Pomeron) by • what to measure: • Structure function of diffractive process • Extracting diffractive PDFs (DPDFs)through scaling violation, using jets …(assuming factorisation theoremwould work for diffractive DIS) b: long. momentum fraction of the parton in the exchange xP : long. momentum fraction of the exchange in the proton : negative of momentum transfer squared

14. Is Pomeron a “particle” ? • Check if the cross section can befactorised into: • the Pomeron flux and • the upper part • This holds pretty well: cross section shape in is independent of and • If 2-glu: depends on dependence steeper with Fit by slope parameterof the figure below

15. Scaling violation analysis for in DPDF • Positive scaling violationin almost all values • Quarks dynamically produced through gluons • The exchanged object is gluon-richconsistent with naïve 2-gluon picture • some excess at low-(higher twist!):I will come to this point later

16. Extracted diffractive parton densities • Gluons are not strongly constrained in diffractive DIS • Jet cross sections are used to constrain gluons • 63% is gluon at ZEUS dijet cross section and DPDF SJ Longitudinal fraction of momentum carreid by the dijet system, wrtPomeron

17. Vector meson production in • Vector meson is higher twist (vector-meson dominance model) • Observing rapid rise of cross section in if a hard scale exists i.e. • if the VM is produced from a very virtual photon • or if the VM is heavy (, ) • Well explained by the 2-gluon picuture: • corresponds to vector meson

18. Forward part: t-dependence of VMs • Measuring proton recoil • If steep, the process is peripheral • parameterised by • Observation: approaches to ~ 4 • the interaction becomes point-like • supporting the “hard” picuture of VM production

19. Forward proton detectors at HERA • ZEUS LPS / H1 FPS: • rather complicated acceptancebut still covers: • at • some acceptance at the diffractivepeak • high acceptance for • H1 VFPS • very high acceptance for

20. Is inclusive diffraction peripheral? • Not completely peripheral,not completely point-like • Pomeronis perhaps not completely a particle? … but the slope is independent of (unlike vector meson)

21. Leading Baryon Production

22. Issues on leading baryons • Is baryons produced by • fragmentation? • or particle exchange? • Is the “first vertex” peripheral? • is the leading baryonrescattered by additional partons? • What are the difference betweenproton and neutron production?

23. Forward neutron detectors at HERA • Big space available there • proton beam is bent upward • the calorimeters are large and deep • Limited aperture due to Q magnets • Scintillator “tracker” • to detect precisely the positionof the first part of hadronic shower

24. Leading neutron: fragmentation or pion? • Data show fragmentationnot sufficient, need OPE • One Pion Exchange:charge exch. • Fragmentation can take care ofonly component like OPE fragmentation extrapolation assuming exponential

25. Leading proton? • very flat dependence • … which can be explained byan overlay of many trajectoriesin Regge-based framework … then, how does the productiondepend on the property of“the other side” of the ladder?

26. Does the photon and proton talk each other? • Both proton and neutron production rates arefairly independent of and neutron proton supporting limiting fragmentation

27. Is the leading baryon production “peripheral”? • Proton: (amazingly) flatat • Neutron: richer structure • sensitive to the pion flux neutron proton neutron

28. Proton and neutron yield in DIS • Neutron yield is 20-30% fewer than naïve prediction of expected from isovector exchange • Protons are more than neutron • .. at least in very forward regionGeV2 • Not consistent with isovector exch. proton neutron neutron This is puzzling.

29. Is neutron lost? • Neutron with PHP dijet • Resolved = “larger photon” is suppressed w.r.t. direct Ratio PHP/DIS Ratio resolved/direct • Photoproduction: “large photon” • more rescattering(more multi-parton scattering) • Phoproductionis suppressed

30. Multi-parton phenomena

31. Multi-parton interaction in photoproduction • Resolved photon: like a hadronmany partons can be exchanged • Excess observed in events with low- () jets • “Jet pedestal”: increasing energy flowoutside the jet core • particle flow direct → ←resolved Multi-parton interaction seems to exist in photoproduction resolved direct

32. Radiation pattern in the middle of ladder • Many attempts to find“BFKL”-like dynamics in DIS • -disordering in radiation • Excess in forward energy flow • but no clear evidence of particularmodel: we always just learn thatfixed order calculation is not enough ordering in DGLAP ordering in BFKL

33. Forward energy vs the length of the ladder • Y: rapidity interval measured from the scattered electron CCFM: angular ordering CDM (colour-dipole): some amount of disordering Forward energy is more than the prediction, but somewhat at the upper edge of the scale uncertainty

34. Summary • Particle exchange model explains many features offorward production in DIS • diffraction (Pomeron-like) • leading baryon (OPE etc.) • Multi-parton phenomena becomes important as vertices get closer to the photon • low- rise of (1-glu) • quasi-elastic vector meson production (2-glu) • forward jets (multi-radiation, multi-parton interaction) • We did not have large-acceptance detector to see the interplay • at around