Wu-Ki Tung
Talk given at DIS97, Chicago; 8 pages; Latex, uses aipproc.sty. Report-no: MSU-HEP-70624; CTEQ-724
Conventional perturbative QCD calculations on the production of a heavy quark ``H'' consist of two contrasting approaches: the usual QCD parton formalism uses the zero-mass approximation (m_{H}=0) once above threshold, and treats H just like the other light partons; on the other hand, most recent ``NLO'' heavy quark calculations treat m_{H} as a large parameter and always consider H as a heavy particle, never as a parton, irrespective of the energy scale of the physical process. By their very nature, both these approaches are limited in their regions of applicability. This dichotomy can be resolved in a unified general-mass variable-flavor-number scheme, which retains the m_{H} dependence at all energies, and which naturally reduces to the two conventional approaches in their respective region of validity. Recent applications to lepto- and hadro-production of heavy quarks are briefly summarized.
Fredrick I. Olness and Randall J. Scalise (SMU)
Contribution to DIS97 Workshop, 14-18 April, Chicago, IL, USA. Report-no: CTEQ-708, SMU-HEP-9709 6 pages, LaTeX, 4 PostScript figures.
In a consistently formulated pQCD framework incorporating non-zero mass heavy quark partons, there is still the freedom to define parton distributions obeying either mass-independent or mass-dependent evolution equations, contrary to statements made in a recent paper by MRRS. With properly matched hard cross-sections, different choices merely correspond to different factorization schemes, and they yield the same physical cross-sections. We demonstrate this principle in a concrete order calculation of the DIS charm structure function. We also examine the proper matching between parton definitions and subtractions in the hard cross-section near threshold where the calculation is particularly sensitive to mass effects of the heavy quark. The results obtained from the general-mass formalism are quite stable against different choices of scale and exhibit a smooth transition in the threshold region (using either mass-independent or mass-dependent evolution), in contrast to results of another recently proposed scheme.
C. Balazs and C.-P. Yuan
5 pages, REVTeX, 1 figure, Report-no: MSUHEP-70428, CTEQ-706
To offer a possible resolution to the apparent discrepancy between the experimental and the theoretical values of the W + 1 jet to W + 0 jets ratio reported by the D0 group, we examine the effects of the multiple soft gluon radiation on the W boson production at the Tevatron. Based on the calculation of the W boson transverse momentum (Q_{T}) distribution in the Collins-Soper-Sterman resummation formalism, we conclude that the effect of the soft gluon radiation is important in the region of Q_{T} < 50 GeV, and it can be better tested by a more inclusive observable R_{CSS} = (Q_{T}>Q_{T}^{min})/ _{Total}
S. Kuhlmann, H.L. Lai, W.K. Tung
13 page Latex, 8 ps figures Report-no: Michigan State Report # MSU-HEP-70316, CTEQ-705
The recently reported excess of events at HERA compared to QCD calculations impels us to examine all possible Standard Model explanations before invoking ``new physics''. We explore the possibility of adding an unusual, but small, component of additional quarks at large x (beyond x>0.75) as a way to increase the predicted SM cross-section in the HERA kinematic region by the QCD evolution feed-down effect. We find this can be achieved while maintaining good global fits to all established data sets. In addition, the modified parton distributions provide another possible mechanism to account for the CDF high-p_{t} jet excess which occurs at similar x and Q^{2} values.
B. W. Harris, J. F. Owens
16 pages, Latex, uses Revtex and psfig macros, FSU-HEP-970411, submitted to Phys. Rev. D
A new next-to-leading order Monte Carlo program for the calculation of jet cross sections in photoproduction is described. The contributions from both resolved and direct components are included to O( ^{2}). Properties of the predictions for various inclusive jet and dijet observables are discussed and comparisons with HERA data are presented.
C. Balazs and C.-P. Yuan
50 pages, REVTeX, 16 figures (27 eps files) Report-no: MSUHEP-70402, CTEQ-704
With a large integrated luminosity expected at the Tevatron, a next-to-leading order (NLO) calculation is no longer sufficient to describe the data which yield the precision measurement of M_{W}, etc. Thus, we extend the Collins-Soper-Sterman resummation formalism, for on-shell vector boson production, to correctly include the effects of the polarization and the width of the vector boson to the distributions of the decay leptons. We show how to test the rich dynamics of the QCD multiple soft gluon radiation, for example, by measuring the ratio R_{CSS} = (Q_{T}>Q_{T}^{min})/ _{Total}. (Q_{T} is the transverse momentum of the vector boson.) We conclude that both the total rates and the distributions of the lepton charge asymmetry predicted by the resummed and the NLO calculations are different when kinematic cuts are applied.
C. Balazs and C.-P. Yuan
9 pages, LaTeX, REVTeX, one EPS figure and epsf.tex included Report-no: MSUHEP-70317, CTEQ-703
We propose the measurement of the ratio R_{CSS}(Q_{T}^{min}) = (Q_{T}>Q_{T}^{min})/ _{Total} to study the effects of the multiple soft gluon radiation, predicted by QCD, on the transverse momentum (Q_{T}) distribution of the weak gauge bosons W^{+-} and Z^{o} produced at the Tevatron. We compare the prediction of the extended Collins-Soper-Sterman resummation formalism with the next-to-leading and next-to-next-to-leading order calculations. We show that both the rich dynamics of the QCD multiple soft gluon radiation and the non-perturbative sector of QCD can be tested by measuring R_{CSS}.
Lyndon Alvero, John C. Collins, Juan Terron, Jim Whitmore
23 pages, LaTeX with psfig and amsfonts, 6 postscript figures Report-no: CTEQ-701, PSU/TH/177, CERN-TH/96-340
We extract diffractive parton densities from diffractive, deep inelastic (DIS) ep data from the ZEUS experiment. Then we use these fits to predict the diffractive production of jets and of W's and Z's in pp-bar collisions at the Tevatron. Although the DIS data require a hard quark density in the pomeron, we find fairly low rates for the Tevatron processes (a few percent of the inclusive cross section). This results from the combined effects of Q^{2} evolution and of a normalization of the parton densities to the data. The calculated rates for W production are generally consistent with the preliminary data from the Tevatron. However, the jet data from CDF with a "Roman pot" trigger are substantially lower than the results of our calculations; if confirmed, this would signal a breakdown of hard-scattering factorization.
H. L. Lai, W. K. Tung (Michigan State Univ.)
11 pages, 4 figures, plain Latex, Report-no: MSU-HEP-61222, CTEQ-622
Recent accurate data on F_{2}(x,Q) and on F_{2}^{c}(x,Q) from HERA at small-x require a more precise treatment of charm production in the global QCD analysis of parton distributions. We improve on existing global analyses by implementing the leptoproduction formalism of Aivazis et al. which represents a natural generalization of the conventional zero mass QCD parton framework to include heavy quark mass effects. We also perform analyses based on the fixed-flavor-number scheme, which is widely used in the literature, and demonstrate their uses and limitations. We discuss the implications of the improved treatment of heavy quark mass effect in practical applications of PQCD and compare our results with recent related works.
Wu-Ki Tung (Michigan State University)
9 pages, procl.sty, in-line eps figures included Report-no: MSUHEP-60701
We report on an extensive global QCD analysis of new DIS and hadronic inclusive jet production data emphasizing the impact of these recent data on the determination of the gluon distribution, and on the interpretation of the high E_{t} jets highlighted by the CDF collaboration. This analysis results in (i) a better handle on the range of uncertainty of the gluon distribution, (ii) a new generation of CTEQ parton distributions which incorporates this uncertainty, (iii) a viable scenario for accommodating the high E_{t} jets in the conventional pQCD framework, and (iv) a systematic study of the sensitivity of the various hard processes to and the consistency of determination in global analysis.
S. Mrenna (Argonne), C.-P. Yuan (Michigan State)
25 pages, including 11 Postscript figures, uses epsf.tex, also available at this http URL: http://www.hep.anl.gov/theory/mrenna/, Report-no: ANL-HEP-PR-96-49, CTEQ-606, MSUHEP-60528
Journal-ref: Phys.Rev. D55 (1997) 120
We study the kinematic distributions of top--antitop quark pairs produced at the Tevatron, including the effects of initial state and final state multiple soft gluon emission, using the Collins--Soper--Sterman resummation formalism. The resummed results are compared with those predicted by the showering event generator PYTHIA for various distributions involving the top--antitop quark pair and the individual top quark or antiquark. The comparison between the experimental and predicted distributions will be a strong test of our understanding and application of perturbative QCD. Our results indicate that the showering event generators do not produce enough radiation. We reweight the PYTHIA distributions to agree with our resummed calculation, then use the reweighted events to better estimate the true hadronic activity in top--antitop quark pair production at hadron colliders.
H. L. Lai and W. K. Tung
2.5 pages, 4 figures. Report-no: MSU-HEP-60508, CTEQ-605
The recently reported CDF and D0 inclusive jet cross-sections are compared, using a uniform theoretical NLO QCD calculation to account for the different kinematic coverages of the pseudo-rapidity variable in the two experiments. The two data sets are found to be in good agreement. With a 2-3% relative overall normalization adjustment, the data sets appear to agree over the entire E_{t} range, even without taking into account the other systematic errors.
H.L. Lai, J. Huston, S. Kuhlmann, F. Olness, J. Owens, D. Soper, W. K. Tung, and H. Weerts
37 pages, embedded figures, Report-no: MSUHEP-60426, CTEQ-604
The impact of recent precision measurements of DIS structure functions and inclusive jet production at the Tevatron on the global QCD analysis of parton distribution functions is studied in detail. Particular emphasis is placed on exploring the range of variation of the gluon distribution G(x,Q) allowed by these new data. The strong coupling of G(x,Q) with is fully taken into account. A new generation of CTEQ parton distributions, CTEQ4, is presented. It consists of the three standard sets (MS-bar, DIS, and leading order), a series that gives a range of parton distributions with corresponding 's, and a set with a low starting value of Q. Previously obtained gluon distributions that are consistent with the high E_{t} jet cross-section are also discussed in the context of this new global analysis.
J. Pumplin
35 pages, REVTeX, 14 figures (epsf) Report-no: MSU-HEP-60605
Journal-ref: Phys.Rev. D55 (1997) 173-182
Mass measurements of objects that decay into hadronic jets, such as the top quark, are shown to be improved by using a variant of the k_{t} jet algorithm in place of standard cone algorithms. The possibility and importance of better estimating the neutrino component in tagged b jets is demonstrated. These techniques will also be useful in the search for Higgs boson to b-bar b.
J. Huston, E. Kovacs, S. Kuhlmann, H. L. Lai, J. F. Owens, D. Soper, W. K. Tung
6 page Latex file using epsf.sty for figures. 5 ps figures submitted separately in uuencoded file. Report-no: Michigan State Report # MSU-HEP-50812, FSU-HEP-951031, CTEQ-512
Journal-ref: Phys. Rev. Lett. 77 (1996) 444
The CDF experiment has reported an excess of high-p_{t} jets compared to next-to-leading order QCD expectations based on existing parton distributions. Before attributing this to new physics effects, we investigate whether these high-p_{t} jets can be explained by a modified gluon distribution inside the proton. We find enough flexibility in a global QCD analysis including the CDF inclusive jet data to provide a 25-35% increase in the jet cross sections at the highest p_{t} of the experiment. Two possible sets of parton distributions are presented, and the effects of these on the comparison with deep-inelastic scattering, fixed-target direct photon, and UA2 jet data are discussed. Further theoretical and experimental work needed to clarify unresolved issues is outlined.
Ruibin Meng (Kansas U.), Fredrick I. Olness (Southern Methodist U.), Davison E. Soper (Oregon U.). CTEQ-409, Nov 1995. 32pp.
RevTeX format, 32 pages, 15 uuencoded figures with epsf.tex; Report-no: CTEQ-409
Journal-ref: Phys.Rev. D54 (1996) 1919
Measurement of the distribution of hadronic energy in the final state in deeply inelastic electron scattering at HERA can provide a good test of our understanding of perturbative QCD. For this purpose, we consider the energy distribution function, which can be computed without needing final state parton fragmentation functions. We compute this distribution function for finite transverse momentum q_{T} at order , and use the results to sum the perturbation series to obtain a result valid for both large and small values of transverse momentum.
J. Huston (Michigan State U.) , Jeff Owens (Florida State U.)
10 pages text, REVTEX, 9 figures, Report-no: FSUHEP-950802, MSUHEP-50801, CTEQ-95/501
The event structure associated with the production of massive photon pairs is studied to O( ^{2} ). The cross sections for two photon and two photon plus one jet events are given. The Dalitz plot structure of events with two photons and a jet is also presented. These distributions will provide checks on our understanding of the production mechanisms for massive photon pairs, a signal thought to be important for searches for the Higgs boson in the intermediate mass region.
J. Pumplin
17 pages, RevTeX, 4 uuencoded figures Report-no: MSU-HEP/50808
Journal-ref: Phys. Rev. D53 (1996) 1282
Two possible ways to improve the mass resolution for observing hadronic top quark decay t \to bW \to 3 jets are studied: (1) using fixed cones in the rest frames of the t and W to define the decay jets, instead of the traditional cones in the rest frame of the detector; and (2) using the jet angles in the top rest frame to measure m_{t}/m_{W}. By Monte Carlo simulation, the second method is found to give a useful improvement in the mass resolution. It can be combined with the usual invariant mass method to get an even better mass measurement. The improved resolution can be used to make a more accurate determination of the top quark mass, and to improve the discrimination between t-bar t events and background for studies of the production mechanism.
Csaba Balazs , Jianwei Qiu , C.-P. Yuan
12 pages, 3 figures uuencoded, LaTeX, uses epsf.tex for figures; Report-no: CTEQ/408, MSUHEP-50319
Journal-ref: Phys. Lett. B355 (1995) 548-554
We study the distributions of leptons from the decay of electroweak vector bosons produced in hadron collisions. The effects of the initial state multiple soft-gluon emission, using the Collins--Soper resummation formalism, are included. The resummed results are compared with the next-to-leading-order results for the distributions of the transverse momentum, rapidity asymmetry, and azimuthal angle of the decay leptons.
Journal-ref: Rev. Mod. Phys. 67 (1995) 157-248
George Sterman, John Smith, John C. Collins, James Whitmore, Raymond Brock, Joey Huston, Jon Pumplin, Wu-Ki Tung, Hendrik Weerts, Chien-Peng Yuan, Stephen Kuhlmann, Sanjib Mishra, Jorge G. Morfín, Fredrick Olness, Joseph Owens, Jianwei Qiu, Davison E. Soper
J. Huston, E. Kovacs, S. Kuhlmann, H. L. Lai, J. F. Owens, W. K. Tung
8 page Latex file using epsf.sty for figures. 6 eps figures submitted separately in uuencoded file; Report-no: Michigan State Report # MSU-HEP-41027 and CTEQ-407
Journal-ref: Phys. Rev. D51 (1995) 6139-6145
A global QCD analysis of the direct photon production process from both fixed target and collider experiments is presented. These data sets now completely cover the parton x range from 0.01 to 0.6, thereby providing a stringent test of perturbative QCD and parton distributions. Previous detailed studies of direct photons emphasized fixed target data. We find most data sets have a steeper p_{t} distribution than the QCD prediction. Neither global fits with new parton distributions nor improved photon fragmentation functions can resolve this problem since the deviation occurs at different x values for experiments at different energies. A more likely explanation is the need for additional broadening of the k_{t} of the initial state partons. The magnitude and the possible physical origin of this effect are investigated and discussed.
John C. Collins , Davison E. Soper
15 pages, latex, no figures; Report-no: CTEQ NOTE 94/01
The CTEQ and MRS parton distributions involve a substantial number (~30) of parameters that are fit to a large number (~900) of data. Typically, these groups produce fits that represent a good fit to the data, but there is no substantial attempt to determine the errors associated with the fits. Determination of errors would involve consideration of the experimental statistical and systematic errors and also the errors in the theoretical formulas that relate the measured cross sections to parton distributions. We discuss the principles that would be needed in such an error analysis. These principles are standard. However, certain aspects of the principles appear counter-intuitive in the case of a large number of data. Accordingly, we strive to devote careful attention to the logic behind the methods.
H.L. Lai, J. Botts, J. Huston, J.G. Morfín, J.F. Owen, J.W. Qiu, W.K. Tung and H. Weerts
35 pages, 23 figures separately submitted as uuencoded compressed ps-file; Michigan State Report # MSU-HEP/41024 and CTEQ 404.
Journal-ref: Phys. Rev. D51 (1995) 4763-4782
The CTEQ program for the determination of parton distributions through a global QCD analysis of data for various hard scattering processes is fully described. A new set of distributions, CTEQ3, incorporating several new types of data is reported and compared to the two previous sets of CTEQ distributions. Comparison with current data is discussed in some detail. The remaining uncertainties in the parton distributions and methods to further reduce them are assessed. Comparisons with the results of other global analyses are also presented.
Fredrick I. Olness and Stephan T. Riemersma
LaTeX format, 19 pages, 20 figures in uuencoded format. SMU-HEP/94-21, CTEQ 94-07
Journal-ref: Phys. Rev. D51 (1995) 4746-4755
We compare the results of the fixed-flavor scheme calculation of Laenen, Riemersma, Smith and van Neerven with the variable-flavor scheme calculation of Aivazis, Collins, Olness and Tung for the case of neutral-current (photon-mediated) heavy-flavor (charm and bottom) production. Specifically, we examine the features of both calculations throughout phase space and compare the structure function F_{2}(x,Q^{2} ). We also analyze the dependence of F_{2} on the mass factorization scale . We find that the former is most applicable near threshold, while the latter works well for asymptotic Q^{2}. The validity of each calculation in the intermediate region is dependent upon the x and Q^{2} values chosen.
John Collins, Joey Huston, Jon Pumplin, Harry Weerts, Jim Whitmore
Contains revisions based on Phys. Rev. D referee comments. RevTeX version 3, epsf, 31 pages. Uuencoded compressed postscript figures appended. Uncompressed postscript files available at this ftp URL: ftp://ftp.phys.psu.edu/pub/preprint/psuth136/; Report-no: CTEQ/PUB/02, psu/th/136
Journal-ref: Phys. Rev. D51 (1995) 3182-3196
We present a program to measure the parton densities in the pomeron using diffractive deep inelastic scattering and diffractive photoproduction, and to test the resulting parton densities by applying them to other processes such as the diffractive production of jets in hadron-hadron collisions. Since QCD factorization has been predicted NOT to apply to hard diffractive scattering, this program of fitting and using parton densities might be expected to fail. Its success or failure will provide useful information on the space-time structure of the pomeron.
M.A.G. Aivazis, J.C. Collins, F.I. Olness and W.-K. Tung
LaTeX format, 28 pages, no figures. Postscript file available from anonymous ftp at smuphy.physics.smu.edu. MSU-HEP 93/17, PSU/TH/138, SMU-HEP/93-17
Journal-ref: Phys. Rev. D50 (1994) 3102-3118
A unified QCD formulation of leptoproduction of massive quarks in charged current and neutral current processes is described. This involves adopting consistent factorization and renormalization schemes which encompass both vector-boson-gluon-fusion ("flavor creation") and vector-boson-massive-quark-scattering ("flavor excitation") production mechanisms. It provides a framework which is valid from the threshold for producing the massive quark (where gluon-fusion is dominant) to the very high energy regime when the typical energy scale is much larger than the quark mass m_{Q} (where the quark-scattering should be prevalent). This approach effectively resums all large logarithms of the type ( () log(^{2} /m_{Q}^{2})^{n} which limit the validity of existing fixed-order calculations to the region ~ O(m_{Q}). We show that the (massive) quark-scattering contribution (after subtraction of overlaps) is important in most parts of the (x, Q) plane except near the threshold region. We demonstrate that the factorization scale dependence of the structure functions calculated in this approach is substantially less than those obtained in the fixed-order calculations, as one would expect from a more consistent formulation.