Electron-positron Linear Collider Research Articles

Convocation at Snowmass Looks into the Future of US High-Energy Physics

A 500-GeV electron-positron linear collider, most particle physicists believe, should be the next big accelerator.

Pair production from vacuum at the focus of an X-ray free electron laser

There are definite plans for the construction of X-ray free electron lasers (FEL), both at DESY, where the so-called XFEL is part of the design of the electron-positron linear collider TESLA, as well as at SLAC, where the so-called Linac Coherent Light Source (LCLS) has been proposed. Such an X-ray laser would allow for high-field science applications: One could make use of not only the high energy and transverse coherence of the X-ray beam, but also of the possibility of focusing it to a spot with a small radius, hopefully in the range of the laser wavelength. Along this route one obtains very large electric fields, much larger than those obtainable with any optical laser of the same power. In this letter we discuss the possibility of obtaining an electric field so high that electron-positron pairs are spontaneously produced in vacuum (Schwinger pair production). We find that if X-ray optics can be improved to approach the diffraction limit of focusing, and if the power of the planned X-ray FELs can be increased to the terawatt region, then there is ample room for an investigation of the Schwinger pair production mechanism.

Superconducting TESLA cavities

The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of Eacc >= 25 MV/m at a quality factor Q0 > 5E+9. The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of Eacc >= 15 MV/m. In a first series of 27 industrially produced TTF cavities the average gradient at Q0 = 5E+9 was measured to be 20.1 +- 6.2 MV/m, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities additional quality control measures were introduced, in particular an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electron-beam welds. The average gradient of these cavities at Q0 = 5E+9 amounts to 25.0 +- 3.2 MV/m with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing R&D towards higher gradients is briefly addressed.

Logarithmic expansion of electroweak corrections to four-fermion processes in the TeV region

Starting from a theoretical representation of the electroweak component of four-fermion neutral current processes that uses as theoretical input the experimental measurements at the Z peak, we consider the asymptotic high energy behavior in the standard model at one loop of those gauge-invariant combinations of self-energies, vertices and boxes that contribute all the different observables. We find that the logarithmic contribution due to the renormalization group running of the various couplings is numerically overwhelmed by single and double logarithmic terms of purely electroweak (Sudakov-type) origin, whose separate relative effects grow with energy, reaching the 10% size at about 1 TeV. We then propose a simple ``effective'' parametrization that aims at describing the various observables in the TeV region, and discuss its validity both beyond and below 1 TeV, in particular in the expected energy range of future linear electron-positron (LC) and muon-muon colliders.

Can the trilinear Higgs self-coupling be measured at future linear colliders?

We assess the possibility of measuring the trilinear self-coupling of the Standard Model Higgs boson at future electron-positron linear colliders with centre-of-mass energies between 500 and 1500 GeV. We consider rather light Higgs scalars, with mass below the \(W^+W^-\) threshold, produced in the double Higgs-strahlung channel \(e^+e^-\to HHZ\) and decaying via the dominant mode \(H\rightarrow b\bar b\). Assuming ideal vertex tagging of the heavy-quark jets and mass reconstruction of the Z boson produced in association with the Higgs pair, we compare the yield of the above process to those of all irreducible electroweak and strong backgrounds proceeding through a \(b\bar bb\bar b Z\) intermediate state. Total cross sections and differential spectra of phenomenological relevance to the selection and analysis of the signal are given and discussed.

High-Order-Mode Absorbers–Couplers for Superconductive Accelerating Sections of a Linear Electron–Positron Collider

Rectangular-waveguide devices intended for the withdrawal and absorption of high-order modes (HOMs) from the superstructure of the TESLA linear electron–positron collider are investigated. These devices, known as couplers, are set in between the subcavities of the superstructure and at its ends. The HOM couplers investigated are connected to the drift tubes via slots oriented along the tube axis (longitudinal slots), at right angles to the axis (azimuthal slots), and at an angle to the axis. In order to damp the HOMs of both polarizations, two rectangular waveguides are used for each coupler. The results of calculation and experimental investigation of two copper subcavities of the superstructure, each one consisting of seven cells, are presented. The investigations were conducted at room temperature at a frequency of 3 GHz using models.

Final-focus systems in linear colliders

In colliding beam facilities, the ''final focus system'' must demagnify the beams to attain the very small spot sizes required at the interaction points. The first final focus system with local chromatic correction was developed for the Stanford Linear Collider where very large demagnifications were desired. This same conceptual design has been adopted by all the future linear collider designs as well as the SuperConducting Supercollider, the Stanford and KEK B-Factories, and the proposed Muon Collider. In this paper, the over-all layout, physics constraints, and optimization techniques relevant to the design of final focus systems for high-energy electron-positron linear colliders are reviewed. Finally, advanced concepts to avoid some of the limitations of these systems are discussed.

Electroweak precision measurements and collider probes of the standard model with large extra dimensions

The elementary particles of the standard model may reside in more than $3+1$ dimensions. We study the consequences of large compactified dimensions on scattering and decay observables at high-energy colliders. Our analysis includes global fits to electroweak precision data, indirect tests at high-energy electron-positron colliders (CERN LEP2 and NLC), and direct probes of the Kaluza-Klein resonances at hadron colliders (Fermilab Tevatron and CERN LHC). The present limits depend sensitively on the Higgs sector, both the mass of the Higgs boson and how many dimensions it feels. If the Higgs boson is trapped on a $(3+1)$-dimensional wall with the fermions, large Higgs boson masses (up to 500 GeV) and relatively light Kaluza-Klein mass scales (less than 4 Tev) can provide a good fit to precision data. That is, a light Higgs boson is not necessary to fit the electroweak precision data, as it is in the standard model. If the Higgs boson propagates in higher dimensions, precision data prefer a light Higgs boson (less than 260 GeV) and a higher compactification scale (greater than 3.8 TeV). Future colliders can probe much larger scales. For example, a 1.5 TeV electron-positron linear collider can indirectly discover Kaluza-Klein excitations up to 31 TeV if $500 {\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity is obtained.

The future of HERA and TESLA

With the advent of HERA a new phase of deep inelastic scattering experiments has begun, the results of which are the main topic of this workshop. In this contribution the status and future perspectives of the scientific program of DESY are presented. They include the luminosity upgrade of HERA and the plans for an electron-positron linear collider, TESLA, with an integrated X-ray free electron laser.

Can the trilinear Higgs self-coupling be measured at future linear colliders?

We assess the possibility of measuring the trilinear self-coupling of the Standard Model Higgs boson at future electron-positron linear colliders with centre-of-mass energies between 500 and 1500 GeV. We consider rather light Higgs scalars, with mass below the W+W- threshold, produced in the double Higgs-strahlung channel e+e- -> HHZ and decaying via the dominant mode H -> bbbar. Assuming ideal vertex tagging of the heavy-quark jets and mass reconstruction of the Z boson produced in association with the Higgs pair, we compare the yield of the above process to those of all irreducible electroweak and strong backgrounds proceeding through a bbbar bbbar Z intermediate state. Total cross sections and differential spectra of phenomenological relevance to the selection and analysis of the signal are given and discussed.

Production of high power microwaves for particle acceleration with an FEL bunched electron beam

Among the studies in the framework of high gradient linear electron–positron collider research, the Two-Beam Accelerator (TBA) is a very promising concept, and two projects are in progress, the Compact Linear Collider project at CERN (W. Schnell, Report no. CERN SL/92-51 and CLIC note 184; K. Hübner, CERN/PS 92-43, CLIC note 176; S. Van der Meer, CERN/PS 89-50, CLIC note 97.) and the Relativistic Klystron-TBA project at LBNL (Technical Review Committee, International Linear Collider Technical Review Committee Report 1995, SLAC-R-95-471, 1995). In a TBA an extremely intense low-energy electron beam, called the drive beam, is bunched at the desired operating frequency, and upon passing through resonant cavities generates radio-frequency power for accelerating the main beam. Among the different approaches to the production of a suitable drive beam, the use of an FEL has been proposed and is under active study at CEA/CESTA.

A method for producing polarized neutrons by the magnetic bremsstrahlung γ-radiation

Circularly polarized magnetic bremsstrahlung γ-radiation of ultrarelativistic electrons is suggested to be used to produce polarized neutrons by photonuclear reactions on light nuclei with a single valence neutron in the p-shell. The polarization, spectral and integral characteristics of photoneutrons produced have been determined by multigroup calculations for 9Be distinguished by an anomalously low neutron binding energy. It is shown that electron beam parameters of modern storage rings, such as LEP, PETRA or HERA, and linear electron-positron colliders, such as SLC, allows the pulsed neutron fluxes with unprecedented space-time, spectral and polarization properties to be produced.

Performance studies of a lead-scintillation fiber calorimeter prototype for the linear collider detector

We constructed calorimeter modules composed of lead plates and scintillating fibers as an R&D program for the experiment at a future electron-positron linear collider. We tested the performance of the modules using electron and pion beams at the KEK proton synchrotron. The results of the beam tests are presented in this paper.

Status of electron accelerators for linear colliders

The paper outlines the basic problems concerning creation of electron-positron linear colliders, as well as their present-day status. More details on the question can be found in the proceedings of recent workshops on linear colliders contained in the References.

Relativistic klystron two-beam accelerator

Relativistic klystrons (RK's) are being developed as an RF power source for high gradient accelerator applications which include large linear electron-positron colliders, compact accelerators, and FEL sources. In a relativistic klystron two-beam accelerator (RK-TBA), the drive beam passes through a large number of RF output structures. High conversion efficiency of electron beam energy to RF energy is achieved in this concept by reacceleration of the modulated drive beam between output structures. We have conducted experiments studying the RF power extracted from various RK structures driven by modulated induction accelerator current pulses; our studies include work on improving the transport dynamics of the drive beam. We have started a demonstration in which the modulated induction beam current is reaccelerated by passage through subsequent induction accelerator cells. >

Radiated energy loss due to beamstrahlung in high-energy electron-positron linear colliders

Using the recently discovered property of independent photon emissions at fixed impact distance, an integro-differential equation is set up to give the radiated energy due to beamstrahlung to all orders in the expansion parameter ;L b/ℓ c . HereL b is the bunch length and ℓ c is the coherence length. This integro-differential equation is solved for both the Klein-Gordon case and the physically important Dirac case.

Pair production of light and heavy W-bosons of the left-right model in e +e − collisions

We investigate the possible direct tests of the SU(2) L × SU(2) R × U(1) B- L electroweak model in future TeV-scale electron-positron linear colliders. We calculate to the lowest order the cross sections of the processes e +e − → W 1 + W 1 −, W 1 +W 2 −, W 2 +W 2 −, where W 1 is the ordinary charged gauge boson and W 2 is a new “right-handed” gauge boson with a mass in the range M W 2 ⋧ 500 GeV . It is shown that the total cross sections depend quite strongly on the mass of the “right-handed” neutrino offering a possibility to test the so-called see-saw mechanism of neutrino masses. The effects of the right-handed sector on the pair production of the light charged gauge bosons have also been investigated.

Electroproduction of pairs in beam-beam collisions

Charged particle pair production in beam-beam collisions in electron-positron linear colliders is discussed taking into account the finite size of the beams (both longitudinal and transverse) and end effects. Contributions of the main acting mechanisms are singled out which depend on the energy of the initial particles and the masses of created particles. A spectral distribution of produced particles is presented.

Lasertron: A Pulsed rf Source Using a Laser-Triggered Photocathode

A new pulsed rf source, the “lasertron”, is being developed as a possible rf power source for future electron-positron linear colliders. In a series of systematic studies, a prototype lasertron was fabricated and tested. A peak power of 80 kW was attained at a 2.856 GHz rf frequency with a 1 µs time duration. This paper describes the results of experimentation on the lasertron, including the development of the photocathode and the laser system. Test results are compared with the analysis of beam dynamics in the lasertron.

Beamstrahlung in high energy electron-positron linear colliders with non-uniform bunches

A general method for the calculation of beamstrahlung radiation in bunch-bunch collisions in the case of electron-positron linear colliders in the TeV range is presented. It applies to bunches with varying longitudinal densities and takes edge effects into account. An extensive use of the Mellin transform is made in a Feynman-graph approach.