Abstracts Hard Interaction Working Group

New resummed results for event shapes in DIS

 
This talk presents first results for the resummation of the
thrust, C-parameter and jet mass in DIS. The resummation includes a new
class of all-order effect, neglected so far in the literature. The results
are compared to H1 data, including power corrections and matching to
fixed order calculations, and show considerable improvement over
calculations without a resummation.

Testing the Universality of Fragmentation Functions

 
In the last year three different groups have obtained NLO
parametrizations of fragmentation functions (FF) from fits to e+e-
annihilation data at the Z mass. We review the status of the
calculations, show the results of our own fit, including a determination
of the strong coupling constant, and compare the three different
parametrizations. We apply our FFs to eP- and pp-scattering and to
e+e- scattering at higher and lower energies than M_z as a test of
the universality of the FFs.

Heavy quark production in the semihard QCD approach at HERA and beyond

Hadronic final state predictions from CCFM Generators

We have obatined a good description of F2 and also the forward jet
measurements at HERA as well as measurements on charm. The bb_bar
x-section is in nice agreement with recent measurements, and also
J/psi production is much better described. Furthermore, with the same
unintegrated gluon density, we obtained from a CCFM fit to F2, we are
able to nicely describe also bb_bar production at the TEVATRON.

Some results are, however, very sensitive to effects of non-leading
terms in the gluon splitting function, and more studies are needed
before reliable predictions can be made with CCFM.  

Heavy-flavour production at HERA

I discuss the problem of a consistent matching between the
results relevant to the small and large transverse momenta of charm
and bottom quarks. I illustrate a formalism which includes the complete
fixed-order result, accurate to next-to-leading order, and the resummation
at the next-to-leading-log level of the large transverse momentum
logarithms. Phenomenological results, relevant to HERA, will also be given.

NNLO soft and virtual QCD corrections to Higgs boson production

Combining NLO Matrix Elements and Parton Showers

Two-Loop Jet Physics: Status and Prospects

Measurement of Pion Multiplicities and Hadron $P_{\bot}$ in DIS at HERMES

Hadron Formation in DIS in a Nuclear Environment

The influence of the nuclear medium on the production of charged hadrons
in semi-inclusive deep-inelastic scattering has been studied by the HERMES
experiment. The differential multiplicity of charged hadrons from nucleus
relative to that from deuterium has been measured as a function of the
virtual photon energy $\nu$ and the fraction z of this energy transferred
to the hadron. There are observed substantial reductions of the
multiplicity ratio at low $\nu$ and in the previously unexplored high z
region thus suggesting a possible influence of the nuclear environment on
the fragmentation functions. The results are discussed in the framework of
the space-time structure of the hadronization process.  Quantitative
information on the formation time of hadrons have been derived by the
HERMES measurements  suggesting  a difference between the formations times
of protons and pions.                    

Sleuth : A model independent search method

Prospects for Higgs at the Tevatron Run II

Beauty production at HERA

Three-Jet Production in DIS

Events with an isolated high energy lepton and missing transverse momentum and W production at HERA

Searches for Leptoquarks, squarks in R_P violating SUSY, and excited fermions at HERA

Isolated High Energy Lepton and Missing Transverse Momentum at HERA

A search for events containing both isolated electron or muon carrying
large transverse momentum and missing transverse momentum, has been done
using ZEUS detector at HERA. At large Pt^X (hadronic transverse momentum),
such events are expected mainly from on-shell W production and are of
particular interest due to the excess observed in the H1 experiment,
reported in last summer.  At that time ZEUS showed results from 1994 to
1999 data.  In this conference, ZEUS will show the updated results
including 2000 data, which increase the statistics of the sample from 82
pb-1 to 130 pb-1.  In the Pt^X > 25 GeV region, one muon event has been
found from the new data but, integrating over the total data sample, such
rate is consistent with that expected from Standard Model.  The result,
when interpreted in terms of single-top production with t->bW decay, yields
the most stringent constraint on the Flavor-Changing-Neutral-Current
anomalous magnetic coupling at the t-u-gamma vertex.  The coupling limit
presented improves by more than 20% compared to what shown in last summer
conference.  

Prompt photons in photoproduction at HERA

Recent results on inclusive prompt photon production in photoproduction
processes in the ZEUS detector are compared with new theoretical
calculations.  New measurements on prompt photons production accompanied
by a jet are used to evaluate the effective transverse momentum of the
quarks in the proton when it undergoes a hard interaction.

Determination of alpha_s using subjet multiplicities in NC-DIS

Measurements of the mean subjet multiplicity for jets in neutral
current DIS have been made in the laboratory frame for Q^2> 125 GeV^2.
The mean subjet multiplicity has been studied as a function of the
resolution parameter, the jet transverse energy and the jet
pseudorapidity. Next-to-leading order QCD calculations have been compared
to the measurements and used to make a new determination of the strong
coupling constant.

Inelastic J/psi and psi' production at HERA

Recent photoproduction measurements of inelastic J/psi
differential cross-sections are presented and compared to the available
theoretical models.
Psi(2S) photoproduction is also clearly identified and a measurement of
the Psi(2S) to J/psi inelastic cross section ratio is given and compared
to the LO QCD prediction.

b-quark mass measurements at high energy

Theoretical calculations at Next-to-leading order (NLO) in QCD
perturbation theory allow for improved experimental tests of QCD. Flavour
independence of $\alpha_s$ has been verified with high precision by the
DELPHI, SLD, ALEPH and OPAL collaborations, taking as an input the
$b$-quark mass extracted from independent measurements done at low scales.
Assuming $\alpha_s$ universality, three compatible measurements of the
running $b$-quark mass, $m_b(M_Z)$, defined in the $\overline{MS}$ scheme
at the $M_Z$ scale, have been performed with DELPHI, SLD and ALEPH data.

QCD studies at LEP2

The upgrade of the LEP centre-of-mass energy up to more than
200 GeV in 1999-2000 offered the opportunity to test the
Standard Model in this new energy regime.
A summary of some recent QCD measurements concerning the general
properties of multihadronic events produced in e+e- annihilations
will be given. Studies of event-shapes, determinations of alpha_s,
measurements of jet-rates, charged particle momentum spectra and
charged hadron multiplicity will be presented and discussed. 

Di-jet production in photon photon collisions at LEP

Recent results on high pT di-jet production at LEP are reported. The
measured cross sections are compared to NLO QCD calculations and in
general show good agreement.