Heavy Quark Physics @ HERA II

1. Brian Foster Oxford University What’s new for HERA II and what does it mean for HQ physics – a reminder.. Summary HQs in DIS, diffraction, photoproduction Spectroscopy – is there a role for HERA II? Production mechanisms Single t production? Cornell 05/13/05 Heavy Quark Physics@ HERA II Brian Foster - HQ physics @ HERA II

2. HERA @DESY in Hamburg HERA & experiments Brian Foster - HQ physics @ HERA II

3. Both ZEUS & H1 have made major upgrades in order to utilise the increase in HERA luminosity to the full. HERA II Physics Brian Foster - HQ physics @ HERA II

4. The ZEUS MVD mostly 3 layers in barrel and 4 forward wheels; > 200K readout channels. Vertex Region Brian Foster - HQ physics @ HERA II

5. H1 Si – 2 very thin CST layers; 5 disks covering 8 < q < 17o Vertex Region Brian Foster - HQ physics @ HERA II

6. ZEUS major upgrade in forward direction replacement of TRD’s with two stations of straw-tube chambers, each with 3 stereo layers. H1 have made improvements to various parts of their tracking systems. Forward Physics Brian Foster - HQ physics @ HERA II

7. So optimistically HERA II promised factor 5 increase in luminosity, with lepton polarisation, greatly improved tracking, DAQ and triggering. What does all this mean for the physics reach of HERA II? This implies generally order of magnitude improvement in statistics and increased kinematic range and coverage. The reality has been somewhat different. Major problems with backgrounds delayed then achievement of the design goals by almost 2 years - only now are we reaching, sometimes, 1pb-1 per day. HERA II prospects Brian Foster - HQ physics @ HERA II

8. Kinematics e(k) e'(k') 2 Q Q2 = xys g *(q) 2 W xP p(P) Low-x structure function data  s = k+P=energy in the ep c.m.s. Q2 = -(k-k')2 = -q2 =virtuality of the exchanged  x = Q2/(2P•Q)=fraction of proton momentum carried by the struck quark y = (P•q)/(P•k) =fraction of beam lepton energy transferred to the photon W 2 = ys ~ Q2/xenergy in the *p c.m.s. Brian Foster - HQ physics @ HERA II

9. e e q e e p p remnant p remnant e p p q Inside the proton Brian Foster - HQ physics @ HERA II

10. Heisenberg’s UP allows gluons, and qq pairs to be produced for a very short time. Low Q2 (large l) Medium Q2 (medium l) Large Q2 (short l) Inside the proton At higher and higher resolutions, the quarks emit gluons, which also emit gluons, which emit quarks, which……. At highest Q2, l ~ 1/Q ~ 10-18 m Brian Foster - HQ physics @ HERA II

11. Inside the proton Brian Foster - HQ physics @ HERA II

12. ZEUS has very precise F2 data over 6 orders of magnitudein (x, Q2). F2 data Brian Foster - HQ physics @ HERA II

13. Inside the proton – the movie Brian Foster - HQ physics @ HERA II

14. Factorization - hard processes can be regarded as convolution of “sub-process” cross section with probability to find participating partons in target & probe - subsequent hadronisation ~ independent process For DIS can (normally) consider virtual photon as d-function => s ~ f  swhere s is sub-process cross section f is parton dist. function, satisfying f/ m ~ f P (m = renormalisation scale, Pis a splitting function) Parton evolution Brian Foster - HQ physics @ HERA II

15. In general Ps are perturbative expansions to particular orders, keeping terms most important for particular regions: where are AP splitting functions Leading lnQ2 terms come, in axial gauge, from evolution along parton chain strongly ordered in transverse momenta, LO DGLAP sums up terms - NLO sums terms which arise when two adjacent kts become comparable, losing factorlnQ2. Parton evolution Brian Foster - HQ physics @ HERA II

16. In small x region, leading terms in ln 1/x must be summed independent of Q2. This is done by the BFKL equation. LO (s ln 1/x)n terms arise from strong x ordering: xn << x(n-1) << x(n-2) … Generally, however, QCD coherence  angular ordering - work in unintegrated f(x,kt2,m2) - 2 hard scales  more complicated CCFM evolution equation. DGLAP/BFKL two limits of angular ordering. DGLAP, q  kt/kl, q grows since kt grows; in BFKL, q grows because kl  x falls. QCD evolution Brian Foster - HQ physics @ HERA II

17. Much interest in dipole models & saturation. L.T. Breit,  mom. prest In principle offers unification of inclusive DIS, diffraction + 1 g exchange 2 g octet exchange 2 g singlet exchange Diffraction Dipole Models Inclusive F2 Brian Foster - HQ physics @ HERA II

18. Specialise to HQ structure functions. There are many theoretical issues with how to include heavy quarks, in particular the mass thresholds; variations on the MVFN scheme a la Thorne&Roberts are currently favoured and have been examined at NNLO. HQ in DIS Brian Foster - HQ physics @ HERA II

19. We will need substantial part of the HERA II statistics to resolve many of the interesting issues in charm in DIS, even with substantial improvement in tagging efficiency. Thorne, Roberts 1998 The mechanism to take charm mass into account can give substantial mods. to theoretical expectation as function of Q2 at higher x. Intrinsic charm can also modify these predictions but unlikely we can get useful info. on this at HERA II. HQ in DIS Brian Foster - HQ physics @ HERA II

20. The D* tagging method in D* → Kpps will still be useful although in general multiple scattering increases. However this will be more than offset by ability to use separated vertex or large impact parameter tag, as already demonstrated at HERA I by H1. HQ in DIS Brian Foster - HQ physics @ HERA II

21. HERA I has already told us a lot about the charm quark density inside the proton; very nice new H1 results at DIS05. HQ in DIS Brian Foster - HQ physics @ HERA II

22. H1 now using the power of their silicon tracker which was present in a modified form for the last few years of HERA I running - uses impact parameter tagging in 2-D to tag long- lived quarks. HQ in DIS Brian Foster - HQ physics @ HERA II

23. H1 analysed 57 pb-1 of HERA I data using impact parameter. HQ in DIS Brian Foster - HQ physics @ HERA II

24. The fraction of c is more or less independent of Q2, whereas that of b is strongly rising. Both agree well with the the predictions from PDFs. HQ in DIS ~0.2% ~20% ~1% ~30% ~20% ~4% Brian Foster - HQ physics @ HERA II

25. Some of the open questions are obvious. HQ in DIS Brian Foster - HQ physics @ HERA II

26. Some were (much) more obscure. HQ in DIS Brian Foster - HQ physics @ HERA II

27. Fortunately, this seems to have gone away at HERA II. HQ in DIS Brian Foster - HQ physics @ HERA II

28. Saturation model of Kowalski & Teaney. The promise of HERA II is great, since charm production in DIS is sensitive probe to all sorts of dynamical models. HQ in DIS Brian Foster - HQ physics @ HERA II

29. First HERA II results starting to come through. HQ in DIS - HERA II Brian Foster - HQ physics @ HERA II

30. First HERA II results starting to come through. HQ in DIS - HERA II Brian Foster - HQ physics @ HERA II

31. 500 pb-1 500 pb-1 Very precise measurements of F2C will be possible for HERA II; also gives accurate gluon determination and cross-check with the more global QCD fits. Accurate b contribution to F2 will become possible – cross-check of VFNS and clean test of photon-gluon fusion process. HQ in DIS Brian Foster - HQ physics @ HERA II

32. HERA II should allow both collaborations to make a full flavour decomposition of the inclusive F2 structure function. For example, charm signal in charged currents (expect ~ 50K events) in principle measures the s-quark density (+ leading particles in NC etc.; but also competing non-s diagrams from gluon splitting in CC). Constrain fit with s from leading f? At HERA II, both singlet & non-singlet pdfs-u,d,s (CC DIS) & c,b,g (NC DIS) - can be determined with good accuracy. HQ in DIS Brian Foster - HQ physics @ HERA II

33. Polarisation in DIS will be a unique tool @ HERA II for exploring EW couplings. In principle the spin orientation of the struck quark can carry through a memory into the final-state hadron. Using weak decay of Lc, can analyse for spin orientation of struck quark and hence disentangle spin-dependent ss. In c and b production, one can be sure that the struck quark is in one of the c or b hadrons and there is a correlation between the detected heavy-quark hadron and the struck quark. HQ & Polarisation Brian Foster - HQ physics @ HERA II

34. But first signal reported for inclusiveLcproduction from a substantial fraction of HERA I data makes it difficult to believe that statistically significant results can be obtained via this technique. HQ & Polarisation Brian Foster - HQ physics @ HERA II

35. The structure of diffraction and its mechanism and explanation in the framework of pQCD is one of the most fruitful areas of HERA I physics. In the charm sector, very strongly limited by statistics. However, also very good discrimination amongst models – one of the areas where we will most benefit from the statistics of HERA II. HQ in Diffraction Brian Foster - HQ physics @ HERA II

36. Charm production cf inclusive F2 will also be additional constraint on gluon pdf of diffractive exchange. In exclusive processes, the heavy c quark gives clearly different behaviour to light quarks – opening the relationship of QCD models of diffraction. One of the areas where one clearly sees the effect of mc as a hard scale. Again, in all of these studies, the statistics of HERA II will be decisive. HQ in Diffraction Brian Foster - HQ physics @ HERA II

37. In general statistics not a problem with photoproduction results, except where extra requirements on tagging – e.g. double jet tags. Can use impact parameter tagging to isolate HQ sample in dijet events. HQ in Photoproduction Visible range: pt jet1(2) >11(8) GeV Measured quark fractions: fuds~58%, fc~35% , fb~7% Reach almost values expected for massless u,d,s,c,b ! Brian Foster - HQ physics @ HERA II

38. Differential cross sections can be compared to NLO QCD predictions. HQ in Photoproduction Excess Data/NLO in more forward direction Higher pT reached than for D* measurements Brian Foster - HQ physics @ HERA II

39. Study of angular distributions in dijet events where one of the jets contains a D* can disentangle the dynamics of the c-production process. In principle, this method, with greatly increased statistics at HERA II, can lead to determination of c (and g) density in g. HQ in Photoproduction Brian Foster - HQ physics @ HERA II

40. Since charm tagging works over a wide range of momenta, has rather high efficiency and high purity, charm is a useful method to look at fragmentation. The study of all these charm particles has allowed the determination of fragmentation probs. to u,d, ratios of P/V, strangeness suppression etc. to ~5 – 10%. This can certainly be improved at HERA II. HQ in Photoproduction Brian Foster - HQ physics @ HERA II

41. H1 data agrees well with that from e+e- - not surprisingly so do the fragmentation parameters deduced. HQ in Photoproduction Brian Foster - HQ physics @ HERA II

42. ZEUS published very similar results some time ago. More evidence that fragmentation factorises from the hard production process. HQ in Photoproduction Brian Foster - HQ physics @ HERA II

43. By looking at events containing leading neutrons and a charm tag, we can get a handle on the c (& g) density in the p. This is a classic “double-tag” experiment which needs the increased statistics of HERA II (and more!). HQ in Photoproduction Brian Foster - HQ physics @ HERA II

44. B production both in photoproduction and DIS is really in its infancy at HERA I and will be a major study at HERA II. Whether there is really a problem with the QCD prediction of s(b) will surely have to wait for HERA II. B production Brian Foster - HQ physics @ HERA II

45. Rich spectrum - D1 ,D2* established; D*' seen by DELPHI, not OPAL/CLEO. The large HERA I data sample gave the opportunity to make useful contributions to charm spectroscopy. Charm spectroscopy in gp Brian Foster - HQ physics @ HERA II

46. Not obvious why such states should want to decay to charm; but then we didn’t expect to see them in light-quark decays either. However HERA II can reach states that other machines cannot reach – e.g. states coupling strongly to gluons can be copiously produced at HERA. HERA II can do charm spectroscopy – we should look in those places where we are competitive. Charm spectroscopy Brian Foster - HQ physics @ HERA II

47. H1 have signal for a charmed pentaquark. • Many experiments have seen the strange pentaquark • (1530)K0s p; many more haven’t. Some who did now aren’t so sure. Unambiguously ruled out by ZEUS. Needs HERA II. Charm pentaquark? Acceptance corr. event yields Brian Foster - HQ physics @ HERA II

48. As an electron-proton collider, HERA II has access to unique production mechanisms that can provide strong tests of QCD. Production mechanisms Brian Foster - HQ physics @ HERA II

49. Interplay with Tevatron can give extra constraints on relative importance of CO & CS; large uncertainties in MEs mean not so obvious that HERA II statistics very helpful without significantly more theoretical work. Inelastic J/y photoproduction Production mechanisms Brian Foster - HQ physics @ HERA II

50. One process at least where we know HERA II will be vital, and which may well be a help in unraveling the J/y problems, is Υ production – at HERA I we barely (?) saw elastic production, let alone inelastic. Production mechanisms Brian Foster - HQ physics @ HERA II