1. John Womersley QCD at the Tevatron Current results and future prospects John Womersley Fermilab Fifth International Symposium on Radiative Corrections (RADCOR 2000) Carmel, CA, September 2000 http://d0server1.fnal.gov/users/womersley/radcor2000.ppt

2. John Womersley Outline It is over four years since we completed data taking in Run I, so there are rather few new results This presentation will therefore be more of a review of the current state of knowledge, highlighting unresolved issues and prospects for Run 2: –jets –vector bosons –photons –heavy flavor Since this is a review, what you will hear are generally my personal opinions and not necessarily the party line of the experiments

3. John Womersley The Fermilab Tevatron Collider 1992-95 Run 1: 100 pb -1, 1.8TeV Major detector upgrades  now 2001-03 Run 2a: 2 fb -1, 1.96 TeV Short shutdown to install new silicon 2003-07(?) Run 2b: ~ 15 fb -1 Main Injector (new) Tevatron DØCDF Chicago   p source Booster CDF DØ

4. John Womersley Hadron-hadron collisions Complicated by –parton distributions — a hadron collider is really a broad-band quark and gluon collider –both the initial and final states can be colored and can radiate gluons –underlying event from proton remnants fragmentation parton distribution parton distribution Jet Underlying event Photon, W, Z etc. Hard scattering ISR FSR

5. John Womersley A high-E T event at CDF

6. John Womersley 0.1   jet   0.7 Jet cross sections at  s = 1.8 TeV R = 0.7 cone jets Cross section falls by seven orders of magnitude from 50 to 450 GeV Pretty good agreement with NLO QCD over the whole range DØ  jet   0.5

7. John Womersley What’s happening at high E T ? NB Systematic errors not plotted CDF 0.1<|  |<0.7DØ |  |<0.5 So much has been said about the high-E T behaviour of the cross section that it is hard to know what can usefully be added: Figure 1: “The Horse is Dead” Figure 1: “The horse is dead”

8. John Womersley The DØ and CDF data agree DØ analyzed 0.1 <|  |< 0.7 to compare with CDF One can (e.g. CTEQ4HJ distributions shown above) boost the gluon distribution at high-x without violating experimental constraints*; results are more compatible with CDF data points *except maybe fixed-target photons, which require big k T corrections before they can be made to agree with QCD (see later)

9. John Womersley Jet data with latest CTEQ5 PDF’s CDF data DØ data

10. John Womersley What have we learned from all this? Do the CDF data show a real or just a “visual” excess at high E T ? –depends critically on understanding the systematic errors and their correlations as a function of E T Whether nature has actually exploited the “freedom” to enhance gluon distributions at large x will only be clear with the addition of more data –with 2fb -1 in Run II, the reach will extend a further 50-100 GeV in E T which should make the asymptotic behavior clearer whatever the Run II data show, this has been a useful lesson: –parton distributions have uncertainties, whether made explicit or not –we should aim for a full understanding of experimental systematics and their correlations It’s a good thing

11. John Womersley Forward Jets DØ inclusive cross sections up to |  | = 3.0 Comparison with JETRAD using CTEQ3M,  = E T max /2 l 0.5    1.0 l 0.0    0.5 l 1.0    1.5 E T (GeV) DØ Preliminary l 1.5    2.0 l 2.0    3.0 DØ Preliminary E T (GeV) Data - Theory / Theory E T (GeV)  d 2  (dE T d  )  (fb/GeV) l 0.0    0.5 H 0.5    1.0 s 1.0    1.5 n 1.5    2.0 t 2.0    3.0 DØ Preliminary

12. John Womersley Triple differential dijet cross section Can be used to extract or constrain PDF’s At high E T, the same behaviour as the inclusive cross section, presumably because largely the same events Beam line Trigger Jet 0.1<|  |<0.7 Probe Jet E T >10 GeV 0.1<|  |<0.7, 0.7<|  |<1.4, 1.4<|  |<2.1, 2.1<|  |<3.0 1 2

13. John Womersley DØ: same side (  1 ~  2 ) and opposite side (  1 ~ –  2 ) topologies measured up to |  | = 2.0 ( Data - Theory ) / Theory SS, 0.0    0.5 OS, 0.0    0.5 SS, 0.5    1.0OS, 0.5    1.0 SS, 1.0    1.5OS, 1.0    1.5 SS, 1.5    2.0OS, 1.5    2.0 Beam line

14. John Womersley Tevatron jet data can constrain PDF’s Tevatron HERA Fixed Target

15. John Womersley Highest E T jet event in DØ E T1 = 475 GeV,  1 = -0.69, x 1 =0.66 E T2 = 472 GeV,  2 = 0.69, x 2 =0.66 M JJ = 1.2 TeV Q 2 = 2.2x10 5 GeV 2

16. John Womersley Extracting  s from the jet cross section CDF parametrize the NLO cross-section: Nice demonstration of the evolution of  s But rather large sensitivity to choice of PDF’s and to input  s : more of a consistency check than a measurement Obtained from JETRAD Measured by CDF

17. John Womersley Jet cross section ratio 630/1800 GeV DØ and CDF both measure the ratio of scale invariant cross sections E T 3 /2  d 2  /dE T d  vs. x T =E T /  s/2 (  1 in pure parton model) Not obviously consistent with each other (at low x T )... or with NLO QCD (at any x T ) various PDF’s various scales

18. John Womersley Suggested explanations Different renormalization scales at the two energies –OK, so it’s allowed, but... Mangano proposes an O(3 GeV) non-perturbative shift in jet energy –losses out of cone? –underlying event? –intrinsic k T ? –could be under or overcorrecting the data (or even different between the experiments — DØ?)

19. John Womersley Ratio of 3-jet/2-jet events at DØ Plot ratio for various third jet thresholds as a function of H T =  E T jets Note how large the ratio is: 70% of high E T jet events have a third jet above 20 GeV, 50% have a third jet above 40 GeV Insensitive to PDF’s DØ

20. John Womersley Ratio of 3-jet/2-jet events at DØ Can this ratio be predicted by QCD? –Yes, reasonably well even by JETRAD (a leading order prediction of R 32 ) Can any information be extracted on the best renormalization scale for the emission of the third jet? –Same scale as the first two jets seems better than a scale tied to E T3 –  = 0.6 E T max is pretty good –  = 0.3 H T is best as E T3  DØ   ’’ or

21. John Womersley Quark jets and gluon jets Probability to radiate proportional to color factors: We might then naively expect Instead of counting tracks, look at energy flow: use k T algorithm to find subjets inside jets –subjets separated by y = 0.001 Compare jets of same (E T,  ) produced at different  s –assume relative q/g content is as given by MC (= 33% g at 630 GeV, 59% g at 1800 GeV) and q/g jet multiplicities do not depend on  s ~ C F = 4/3 q q g 2 g g g ~ C A = 3 2

22. John Womersley Quark and Gluon Subjet Multiplicities DØ Data HERWIG 5.9 Quark Jets Gluon Jets Subjet Multiplicity k T algorithm D=0.5, y cut = 10 -3 55 < E T (jet) < 100 GeV |  jet | < 0.5 DØ Preliminary 12345 0.1 0.2 0.3 0.4 0.5 Dominant uncertainties come from g jet fraction and jet E T scale measure M 630 = f g 630 M g + (1 – f g 630 ) M q M 1800 = f g 1800 M g + (1 – f g 1800 ) M q Have we glimpsed the holy grail (quark/gluon jet separation)? –The real test will be to use subjet multiplicity in (for example) the top  all jets analysis, but unfortunately this will probably have to wait for Run II

23. John Womersley Weak Boson Production O(  2 ) QCD predictions for W/Z production –  (pp  W + X) B(W  ) –  (pp  Z + X) B(Z  ) QCD in excellent agreement with data –so much so that it has been seriously suggested to use  W as the absolute luminosity normalization in future Note: CDF luminosity normalization is 6.2% higher than DØ (divide CDF cross sections by 1.062 to compare with DØ)

24. John Womersley DØ p T Z measurement Phys. Rev. D61, 032004 (2000) Data–Theory/Theory Fixed Order NLO QCD Data–Theory/Theory Resummed Ladinsky & Yuan Ellis & Veseli and Davies, Webber & Stirling (Resummed) not quite as good a description of the data Low p T (< 10 GeV) resum large logarithms of m W 2 /p T 2 and include nonperturbative parameters extracted from the data Large p T (> 30 GeV) perturbative calculation

25. John Womersley W + jet measurements DØ used to show a W+1jet/W+0jet ratio badly in disagreement with QCD. This is no longer shown (the data were basically correct, but there was a bug in the DØ version of the DYRAD theory program). CDF measurements of W+jets cross sections agree well with QCD: alas, no sensitivity to  s W+  1 jet/W vs. NLO QCD W+n jets vs. LO QCD (various scales)

26. John Womersley Isolated photon cross sections New DØ PRL 84 (2000) 2786 QCD prediction is NLO by Owens et al. ±14% normalization statistical errors only New CDF preliminary

27. John Womersley What’s happening at low E T ? Gaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low E T (hep-ph/9808467) “k T ” from soft gluon emission k T = 3.5 GeV  PYTHIA 3.5 GeV Even larger deviations from QCD observed in fixed target (E706) Again, Gaussian smearing (~1.2 GeV here) can account for the data

28. John Womersley Resummation Predictive power of Gaussian smearing is small –e.g. what happens at LHC? At forward rapidities? The “right way” to do this should be resummation of soft gluons –as we have seen, this works nicely for W/Z p T Catani et al. hep-ph/9903436 Threshold resummation: does not model E706 data very well Threshold + recoil resummation: looks promising Threshold resummation Fixed Order Laenen, Sterman, Vogelsang, hep-ph/0002078

29. John Womersley Contrary viewpoints Aurenche et al., hep-ph/9811382: NLO QCD (sans k T ) can fit all the data with the sole exception of E706 “It does not appear very instructive to hide this problem by introducing an extra parameter fitted to the data at each energy” E706 Ouch! Aurenche et al. vs. E706

30. John Womersley Is it just the PDF? New PDF’s from Walter Giele can describe the observed photon cross section at the Tevatron without any k T : CDF (central) DØ (forward) New Blue = Giele/Keller set Green = MRS99 set Orange = CTEQ5M and L

31. John Womersley Photons: final remarks For many years it was hoped that direct photon production could be used to pin down the gluon distribution through the dominant process: Theorist’s viewpoint (Giele): –“... discrepancies between data and theory for a wide range of experiments have cast a dark spell on this once promising cross section … now drowning in a swamp of non-perturbative fixes” Experimenter’s viewpoint: an interesting puzzle –k T remains a controversial topic –experiments may not all be consistent –resummation has proved disappointing so far (though the latest results look better) –new results only increase the mystery is it all just the PDF’s? 

32. John Womersley b production at the Tevatron b cross section at CDF and at DØ Data continue to lie ~ 2  central band of theory b B centralforward Cross section vs. |y| p T > 5 GeV/c p T > 8 GeV/c

33. John Womersley bb correlations CDF rapidity correlations DØ angular correlations NLO QCD does a good job of predicting the shapes of inclusive distributions and correlations, hence it’s unlikely that any exotic new production mechanism is responsible for the higher than expected cross section

34. John Womersley DØ b-jet cross section at higher p T Differential cross section Integrated p T > p Tmin varying the scale from 2μ O to μ O /2, where μ O = (p T 2 + m b 2 ) 1/2 New

35. John Womersley b-jet and photon production compared 0 - 0.5 0.5 1.0 1.5 DØ b-jets (using highest QCD prediction) CDF photons  1.33 DØ photons Data – Theory/Theory Photon or b-jet p T (GeV/c) DØ b-jets

36. John Womersley b production summary Experimental measurements at Tevatron are all consistent and are all several times higher than the QCD prediction –factor of ~ 2 at low rapidity –factor of ~ 4 at high rapidity Note that the same magnitude of excess is now seen in b-production at HERA and in  collisions at LEP2 Modifications to theory improve agreement but do not fix New measurement at higher p T : jets from DØ –better agreement above about 50 GeV –shape of data–theory/theory is similar to photons The same story? (whatever that is)

37. John Womersley t production at the Tevatron CDF 1999 result:  (175 GeV) = 6.5 +1.7 –1.4 pb DØ PRD 60 (1999) 012001:  (172 GeV) = 5.9 ± 1.7 pb Excellent agreement between data and theory –let no one say that we can’t calculate heavy quark production (provided the quark is heavy enough!) In Run II, top could become a nice laboratory for QCD

38. John Womersley Things we can look forward to More data — the next decade belongs to the hadron colliders Improved calculations (NNLO calculations, resummations...) PDF’s with uncertainties (or a technique for the propagation of PDF uncertainties) as implemented by Giele, Keller, and Kosower –see pdf.fnal.gov and Walter’s presentation –we won’t get excited unnecessarily by things like the high E T jet excess (if there is one) –but imposes significant work on the experiments understand and publish all the errors and their correlations Better jet algorithms –CDF and DØ accord for Run II from recent workshop –k T will be used from the start

39. John Womersley Jet Algorithms Experimental desires –high efficiency, low biases –minimize sensitivity to noise, pileup, negative energies –computationally efficient (may be an issue for k T ) Theoretical desires –“infrared safety is not a joke!” – avoid ad hoc parameters like R sep Can the cone algorithm be made acceptable? –e.g. by modification of seed choices –or with a seedless algorithm? Many variations of k T exist — choose one and fully define it “Midpoint cone” Additional seed DØ MC Effect of pileup on Thrust k T algorithm jets, E T > 30 GeV

40. John Womersley Some other things I would like Theoretical and experimental effort to understand the underlying event –don’t subtract it out from jet energies it’s an inconsistent treatment of the event the 1800/630 GeV jet data may indicate problems with our usual assumption that the underlying event is ~ a minbias event –would also allow a consistent treatment of double parton scattering (where more than one pair of partons in the same two colliding nucleons undergoes a hard interaction) –There are very nice new results from CDF on the underlying event A consistent approach to hard diffraction –a high E T jet production process: should be amenable to perturbative calculation –we need to break down the walls of the “pomeron ghetto”

41. John Womersley Conclusions Tevatron QCD measurements have become precision measurements –no longer testing QCD, now testing our ability to make precise predictions within the framework of QCD –the state of the art is NNLO calculations, NLL resummations … but this level of precision demands considerable care both from the experimentalists and the phenomenologists, in understanding — –jet algorithms –jet calibrations –all the experimental errors and their correlations –the level of uncertainty in PDF’s In general our calculational tools are working very well; the open issues generally relate to –pushing calculations closer to the few GeV scale (b’s? low-E T photons?) –PDF uncertainties (high E T jets, photons?)