Lepton and Photon Interactions at High Energies

The following sections are included:

• Preface

• Acknowledgments

• Editors' Acknowledgments

• List of Committees

• Contents

• Welcoming Remarks to Lepton-Photon 2003

The top quark, discovered at the Tevatron in 1995, is a very interesting particle. Precise measurement of the top properties using large data samples will allow stringent tests of the Standard Model and offer a unique window on new physics. This report contains a review of the status of the current knowledge of the top quark as provided by the Run I results of the CDF and DO experiments. A first look at various preliminary measurements obtained with data collected during Run II will also be presented.

The CDF and DØ detectors were fully commissioned for physics running in Run II at the Tevatron collider in early 2002. Since then both experiments have collected data samples corresponding to an integrated luminosity of around ∫ L = 200 pb⁻¹ at a centre-of-mass energy of . Datasets corresponding ∫ L = 120 pb⁻¹ have been analyzed for physics so far. Recent electroweak measurements from Run II are reviewed. Cross section times branching ratio measurements (σ · Br) are presented for the intermediate vector bosons (IVB's) in their leptonic decay modes: W → lν and Z → l⁺l⁻. For the first time, a combination of the σ · Br results from the CDF and DØ experiments is made; this includes using a consistent choice of the total inelastic cross section for the luminosity determinations of the two experiments. Quantities derived from these σ · Br values are also updated. These include: R_l the ratio of the σ · Br values for W and Z; Br(W → lν), the leptonic branching ratio of the W; and Γw, the total decay width of the W. Other measurements using events containing W and Z leptonic decays are presented, including studies that probe the QCD phenomenology of W/Z production and searches for events containing two intermediate vector bosons.

Overall, the Standard Model describes electroweak precision data rather well. There are however a few areas of tension (charged current universality, NuTeV, (g – 2)_μ, b quark asymmetries), which I review critically, emphasizing recent theoretical and experimental progress. I also summarize what precision data tell us about the Higgs boson and new physics scenarios. In this context, the role of a precise measurement of the top mass is crucial.

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An overview of recent experimental results on searches for new phenomena at the LEP, HERA and Tevatron high energy colliders is presented, including in particular new results obtained from the analysis of the Run II data at the Tevatron. No significant evidence for physics beyond the Standard Model has been found and limits at the 95% confidence level have been set on the mass and couplings of several new particles. The complementarity between the different experiments is discussed, as well as future prospects for ongoing and future experiments.

I review recent developments in extensions of the Standard Model that address the question of electroweak symmetry breaking and discuss how these theories can be tested at future colliders.

I review recent theoretical advances in quantum chromodynamics. Particular emphasis is put on developments related to the precise prediction and interpretation of experimental data from present and future high energy colliders.

The success of the theory of Quantum Chromodynamics (QCD) in describing processes controlled by the strong interaction is generally seen as a triumph of modern particle physics. This paper reviews recent QCD measurements using hadronic jet final states from the Fermilab Tevatron, DESY's HERA, and CERN's LEP colliders. Recent advancements in the measurements of jet production cross sections, events shapes, and energy flow, along with improved theoretical calculations, allow for new levels of precision in the study of the physics of strong interactions and point to areas in need of further refinement.

We review recent results in lattice QCD from numerical simulations that allow for a much more realistic QCD vacuum than has been possible before. Comparison with experiment for a variety of hadronic quantities gives agreement to within statistical and systematic errors of 3%. We discuss the implications of this for future calculations in lattice QCD, particularly those which will provide input for B-factory experiments.

Recent progress in the field of rare kaon decays will be described. A brief summary of future experimental activities will also be given. Among the highlights, the improved upper limit on K_L → π⁰e⁺e⁻, and the first observation of the K_S → π⁰e⁺e⁻ are presented. A precise measurement of K_S → γγ and high statistics studies of the Dalitz kaon decays have recently been reported and will be briefly described.

In the first part of the talk the flavor physics input to models beyond the Standard Model is described. One specific example of such a new physics model is given: a model with bulk fermions in one non-factorizable extra dimension. In the second part of the talk we discuss several observables that are sensitive to new physics. We explain what type of new physics can produce deviations from the Standard Model predictions in each of these observables.

Recent data from the rare decays of B mesons into hadronic final states is presented from BaBar, Belle, CDF and CLEO. Where possible the data are compared with theoretical calculations, with the twin aims of further testing the Standard Model and searching for evidence of new physics. A brief description is given of some theoretical approaches in order to indicate which decays are the most sensitive for further study.

This report summarizes the latest experimental results on radiative and electroweak rare B meson decays. These rare decay processes proceed through the Flavor-Changing-Neutral-Current processes, and thus are sensitive to the postulated new particles in the theories beyond the Standard Model. Experiments at e⁺e⁻ colliders, Belle, BaBar and CLEO, have been playing the dominant role, while the CDF and DO experiments have just started to provide new results from Tevatron Run-II. The most significant achievement is the first observation of the decay B → K*l⁺l⁻, which opens a new window to search for new physics in B meson decays.

Recent measurements in the charm sector are reviewed, concentrating on results which are sensitive to New Physics effects. The scope of the presentation includes mixing searches, a CPT / Lorentz invariance study, and a range of searches for rare and forbidden decays. Results from the BaBar, Belle, CDF, CLEO, and FOCUS collaborations are presented, including an important first observation.

I review results related to the CKM angle φ₁(β)- These results include recent measurements of CP-violation from the BaBar and Belle experiments in , and processes.

I report on the experimental studies of the CKM unitarity angles α(φ₂) and γ(φ₃) with emphasis on recent measurements by the Belle and BaBar experiments at the B-factory accelerators.

The Fermilab Tevatron offers unique opportunities to perform measurements of the heavier B hadrons that are not accessible at the γ(4S) resonance. In this summary, we describe some recent heavy flavor results from the DØ and CDF collaborations and discuss prospects for future measurements.

The present status of experimental results for the magnitudes of Cabibbo-Kobayashi-Maskawa matrix elements is reviewed and used for a unitarity test. The matrix is found to be unitary within ±1.8 standard deviations. The matrix violates CP-symmetry and the size of its CP-violation, as derived from only magnitude measurements and unitarity, is in perfect agreement with the observed CP-violations in K and B meson decays.

We review recent developments in QCD pertaining to its application to weak decays of heavy hadrons. We concentrate on exclusive rare and nonleptonic B-meson decays, discussing both the theoretical framework and phenomenological issues of current interest.

I review heavy quarkonium physics in view of recent experimental results. In particular, I discuss new results on spin singlet states, photon and hadronic transitions, D–states and discovery of the yet unexplained narrow X(3872) state.

Recent charm results are reviwed with special attention to spectroscopy and hadronic decays. Two new states decaying to and were recently discovered by BaBar and CLEO. In the baryon sector, the first observation of a doubly charmed baryon was announced by SELEX. Several amplitude analysis on charm hadronic decays are also discussed.

Data from the HERA collider experiments, HI and ZEUS, have been fundamental to the rapid recent development of our understanding of the partonic composition of the proton and of QCD. This report focuses on inclusive measurements of neutral and charged current cross sections at HERA, using the full available data taken to date. The present precision on the proton parton densities and the further requirements for future measurements at the Teva-tron and LHC are explored. Emphasis is also placed on the region of very low Bjorken-x and Q². In this region, the ‘confinement’transition takes place from partons to hadrons as the relevant degrees of freedom and novel or exotic QCD effects associated with large parton densities are most likely to be observed. Finally, prospects for the second phase of HERA running are discussed.

I discuss our current understanding of parton distributions. I begin with the underlying theoretical framework, and the way in which different data sets constrain different partons, highlighting recent developments. The methods of examining the uncertainties on the distributions and those physical quantities dependent on them is analyzed. Finally I look at the evidence that additional theoretical corrections beyond NLO perturbative QCD may be necessary, what type of corrections are indicated and the impact these may have on the uncertainties.

Recent progress in physics with polarized hadrons is described with the emphasis on the spin structure of the nucleon. The nucleon spin problem, which was discovered by EMC in 1988, is now being studied in various experiments. Flavor separation of the quark helicity distributions has been made. Recent observations of asymmetries in deeply virtual Compton scattering (DVCS) and exclusive meson productions provide possibilities to access the total angluar momentum carried by quarks in the framework of generalized parton distributions. Single spin azimuthal asymmetries observed in semi-inclusive measurements provide a new handle to determine the transverse quark distributions which are basic but have never been measured so far.

A review is given of the measurements of the diffractive process in recent years from two high-energy colliders, the HERA ep collider and the Tevatron collider. The energy dependence of the cross sections and the factorisation properties of diffractive processes are discussed.

High energy collisions of heavy nuclei at the Relativistic Heavy-Ion Collider permit the study of nuclear matter at extreme densities and temperatures. Selected experimental highlights from the early RHIC program are presented. Measurements of the total multiplicity in heavy-ion collisions show a surprising similarity to measurements in e⁺e⁻ collisions after nuclear geometry is taken into account. RHIC has sufficient center-of-mass energy to use large transverse momentum particles and jets as a probe of the nuclear medium. Signatures of “jet quenching” due to radiative gluon energy loss of fast partons in a dense medium are observed for the first time at RHIC. In order to account for this energy loss, initial energy densities of 30-100 times normal nuclear matter density are required.

The Wilkinson Microwave Anisotropy Probe (WMAP) team has recently analyzed and released the first-year data. We will review the implications for cosmology of these results. The highlight is that cosmology now has a standard cosmological model. With only 6 parameters the model fits not only WMAP data remarkably well, but also a host of other astronomical observations. We also present the results on neutrino mass limits and on dark energy properties from a joint likelihood analysis of WMAP data with small-scale CMB experiments and large-scale structure surveys. The data and supplementary information are publicly available and can be found on the experiment web site at http://lambda.gsfc.nasa.gov.

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Some new developments obtained in the last few years concerning the propagation of high energy cosmic rays are discussed. In particular, it is shown how the inclusion of drift effects in the transport diffusion equations leads naturally to an explanation for the knee, for the second knee and for the observed behavior of the composition and anisotropies between the knee and the ankle. It is shown that the trend towards a heavier composition above the knee has significant impact on the predicted neutrino fluxes above 10¹⁴ eV. The effects of magnetic lensing on the cosmic rays with energies above the ankle are also discussed, analyzing the main features of the different regimes that appear between the diffusive behavior that takes place at lower energies and the regime of small deflections present at the highest ones.

The most popular candidate for non-baryonic dark matter is the neutralino. More than twenty experiments are dedicated to its direct detection. This review describes the most competitive and promising experiments with different detection techniques. The most recent results are presented with some prospects for the near future.

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Previous searches for neutrino oscillations with reactor neutrinos have been done only with baselines less than 1 km. The observed neutrino flux was consistent with the expectation and only excluded regions were drawn on the neutrino-oscillation-parameter space. Thus, those experiments played important roles in understanding neutrinos from fission reactors. Based on the knowledge from those experiments, an experiment with about a 180 km baseline became possible. Results obtained from this baseline experiment showed evidence for reactor neutrino disappearance and finally provide a resolution for the long standing solar neutrino problem when combined with results from the solar neutrino experiments. Several possibilities to explore the last unmeasured mixing angle θ₁₃ with reactor neutrinos have recently been proposed. They will provide complementary information to long baseline accelerator experiments when one tries to solve the degeneracy of oscillation parameters. Reactor neutrinos are also useful to study the neutrino magnetic moment and the most stringent limits from terrestrial experiments are obtained by measuring the elastic scattering cross section of reactor neutrinos.

This paper reviews the constraints on the solar neutrino mixing parameters with data collected by the Homestake, SAGE, GALLEX, Kamiokande, SuperKamiokande, and SNO experiments. An emphasis will be given to the global solar neutrino analyses in terms of matter-enhanced oscillation of two active flavors. The results to-date, including both solar model dependent and independent measurements, indicate that electron neutrinos are changing to other active types on route to the Earth from the Sun. The total flux of solar neutrinos is found to be in very good agreement with solar model calculations. Future measurements will focus on greater accuracy for mixing parameters and on better sensitivity to low neutrino energies.

We know that neutrino mass and mixing provide a window to physics beyond the Standard Model. Now this window is open, at least partly. And the questions are: what do we see, which kind of new physics, and how far “beyond”? I summarize the present knowledge of neutrino mass and mixing, and then formulate the main open questions. Following the bottom-up approach, properties of the neutrino mass matrix are considered. Then different possible ways to uncover the underlying physics are discussed. Some results along the line of: seesaw, GUT and SUSY GUT are reviewed.

This report describes the goals of the next generations of accelerator-based neutrino experiments, and the various strategies that are being considered to achieve those goals. Because these next steps in the field are significantly different from the current or previous steps, novel techniques must be considered for both the detectors and the neutrino beams themselves. We consider not only conventional neutrino beams created by decays of pions, but also those which could be made by decays of beams of relativistic isotopes (so-called “β-beams”) and also by decays of beams of muons (neutrino factories).

The next big project in high energy physics should be a high energy e⁺e⁻ linear collider, operating at energies up to around 1 TeV. A vigorous R&D program has started to prepare the grounds for a detector at such a machine. The amounts of precision data expected at this machine make a novel approach to the reconstruction of events necessary; the particle flow ansatz. This in turn influences significantly the design of a detector for such an experiment. Apart from work ongoing for the linear collider detector, preparations are under way for an update of the LHC. This requires extremely radiation hard detectors. In this paper the state of the different detector development projects is reviewed.

The status of LHC construction, machine and detectors, is reviewed, with particular emphasis on the expected physics and on the industrial production of machine components.

This presentation intends to illustrate the specific capabilities of an e⁺e⁻ sub-TeV collider to provide answers on the basic issues in physics: the origin of mass, hierarchy of masses, and cosmological problems. Some foreseeable scenarios are discussed with a possible synergy with the LHC.

R&D for the linear collider, LC, is well along towards the point where a technology selection can be made. The status of the R&D is described briefly and the evolving procedure for making a selection is presented together with the current ideas on how to proceed.

In this talk, I survey the merits and demerits of Supersymmetry, as well as other approaches to the gauge hierarchy problem.

I present an outlook for the next twenty years in particle physics. I start with the big questions in our field, broken down into four categories: horizontal, vertical, heaven, and hell. Then I discuss how we attack the big questions in each category during the next twenty years. I argue for a synergy between many different approaches taken in our field.

The following sections are included:

• Symposium Program

• Physics Posters

• Laboratory Posters

• Contributed Papers

• List of Participants

• Symposium Photographs

• Author Index