Higgs Boson Theory and Phenomenology

Naturalness bounds on weak scale supersymmetry in the context of radiative breaking of the electroweak symmetry are analyzed. In the case of minimal supergravity it is found that for low $\mathrm{tan}\ensuremath{\beta}$ and for low values of fine-tuning $\ensuremath{\Phi},$ where $\ensuremath{\Phi}$ is defined essentially by the ratio ${\ensuremath{\mu}}^{2}{/M}_{Z}^{2}$ where $\ensuremath{\mu}$ is the Higgs mixing parameter and ${M}_{Z}$ is the $Z$ boson mass, the allowed values of the universal scalar parameter ${m}_{0},$ and the universal gaugino mass ${m}_{1/2}$ lie on the surface of an ellipsoid with radii fixed by $\ensuremath{\Phi}$ leading to tightly constrained upper bounds $\ensuremath{\sim}\sqrt{\ensuremath{\Phi}}.$ Thus for $\mathrm{tan}\ensuremath{\beta}l~2(l~5)$ it is found that the upper limits for the entire set of sparticle masses lie in the range $l700 \mathrm{GeV} (l1.5 \mathrm{TeV})$ for any reasonable range of fine-tuning $(\ensuremath{\Phi}l~20).$ However, it is found that there exist regions of the parameter space where the fine-tuning does not tightly constrain ${m}_{0}$ and ${m}_{1/2}.$ Effects of nonuniversalities in the Higgs boson sector and in the third generation sector on naturalness bounds are also analyzed and it is found that nonuniversalities can significantly affect the upper bounds. It is also found that achieving the maximum Higgs boson mass allowed in supergravity unified models requires a high degree of fine-tuning. Thus a heavy sparticle spectrum is indicated if the Higgs boson mass exceeds 120 GeV. The prospect for the discovery of supersymmetry at the Fermilab Tevatron and at the CERN LHC in view of these results is discussed.

https://repository.library.northeastern.edu/files/neu:331876/fulltext.pdf

Naturalness, weak scale supersymmetry, and the prospect for the observation of supersymmetry at the Fermilab Tevatron and at the CERN LHC

Characterizing femtosecond X-ray pulses that vary from shot to shot is important for data interpretation. Here, Behrens et al. measure time-resolved lasing effects on the electron beam and extract the temporal profile of X-ray pulses using an X-band radiofrequency transverse deflector.

/pdf/few-femtosecond-time-resolved-measurements-of-x-ray-free-41ai8lsc3h.pdf

Few-femtosecond time-resolved measurements of X-ray free-electron lasers

The purpose of this article is to serve as a tutorial review on the subject of field emission and rf breakdown in high-gradient room-temperature accelerator structures and associated devices. The need to understand and control these two phenomena has become increasingly important because of the prospect of using high-gradient structures in future linear colliders. Electron field emission creates so-called dark current which parasitically absorbs rf energy, causes radiation, backgrounds, and possibly wakefields; it seems to be the precursor of rf breakdown, possibly in combination with local outgassing and plasma formation. In turn, rf breakdown limits the operation of accelerators and can cause irreversible damage to their physical structures. Research on these topics is interesting and challenging because it involves a mixture of disciplines such as surface physics, metallurgy, fabrication technologies, microwaves, beam dynamics and plasmas. This review consists of four parts: (1) field emission under dc, enhanced and rf conditions; (2) experimental set-ups; (3) prebreakdown stage--dark current and radiation; (4) experimental observations and analysis of rf breakdown. The review ends with conclusions and an outline of work that remains to be done.

/pdf/field-emission-and-rf-breakdown-in-high-gradient-room-155is62616.pdf

Field emission and RF breakdown in high gradient room temperature linac structures

This paper describes a new method of pulse compression, the binary power multiplier (BPM), a device which multiplies RF power in binary steps. It comprises one or more stages, each of which doubles the input power and halves the input pulse length. Practical designs are described and expressions for their compression efficiency are derived. The usefulness of pulse compression for accelerator design is illustrated and compared with the pulse compression system currently in use at the Stanford Linear Accelerator Center.

/pdf/binary-peak-power-multiplier-and-its-application-to-linear-31uvbtjpak.pdf

Binary Peak Power Multiplier and its Application to Linear Accelerator Design

Over the past few years, several schemes for *. making significant increases in the energy of the SLAC beam have been proposed. Two of the proposals, namely the use of superconducting accelerating sections1 and recirculation 1 of the beam for a second pass through the existing acceler. &or,’ have been abandoned for technical and economic reasons after extensive investigation. An on-going method of gradually raising the beam energy is the development and installation of 30and 40-MW klystrons by the SLAC Klystron Group. It is clear, however, that the accelerator would have to be completely refitted with klystrons producing about 100 MW in order that the present machine energy be approximate1 doubled. While such an approach is not inconceivable, 3 the realization of such klystrons and the modulators needed to drive them would require further years of development and a high initial capital investment.

/pdf/sled-a-method-of-doubling-slac-s-energy-3nm26601o9.pdf

SLED: A Method of Doubling SLAC's Energy

In the SLED method of RF pulse compression, two high Q resonators store energy from an RF source for a relatively long time interval (typically 3 to 5 {mu}sec). Triggered by a reversal in RF phase, this stored energy is then released during a much shorter interval equal to the filling time of the accelerating structure. A peak power gain on the order of three and a compression efficiency on the order of 60% are typically attained. The shape of the output pulse is, however, a sharply decaying exponential. In SLED-II the two cavities are replaced by two lengths of resonant line, forming a Resonant Line SLED (RELS) and resulting in a flat output pulse. Therefore, RELS stages can be cascaded to give a greater peak power gain. Using two stages, a peak power gain greater than ten can be achieved with a reasonable compression efficiency. Unlike the BEC, the RELS compression factor per stage is not limited to two, albeit at the expense of intrinsic efficiency. Like the BEC, it uses long lines rather than short cavities.

SLED II: A new method of rf pulse compression

For more than ten years, we have been working on R&D for X-band accelerator structures for the JLC/NLC linear collider. Several types of Detuned (DS) and Damped Detuned Structures (DDS) have been successfully designed and fabricated. They have been experimentally tested at both low power and high power to characterize their mechanical and electrical properties. Recently we started developing a new type of damped detuned structure with optimized round-shaped cavities (RDDS). This paper discusses the special specifications, design methods, fabrication procedures, measurement technologies, and anticipated future improvements for all these structures.

Accelerator structure R&D for linear colliders

The Delay Line Distribution System (DLDS) is an alternative to conventional pulse compression, which enhances the peak power of rf sources while matching the long pulse of those sources to the shorter filling time of accelerator structures. We present an implementation of this scheme that combines pairs of parallel delay lines of the system into single lines. The power of several sources is combined into a single waveguide delay line using a multi-mode launcher. The output mode of the launcher is determined by the phase coding of the input signals. The combined power is extracted from the delay line using mode-selective extractors, each of which extracts a single mode. Hence, the phase coding of the sources controls the output port of the combined power. The power is then fed to the local accelerator structures. We present a detailed design of such a system, including several implementation methods for the launchers, extractors, and ancillary high power rf components. The system is designed so that it can handle the 600 MW peak power required by the NLC design while maintaining high efficiency.

/pdf/a-multi-moded-rf-delay-line-distribution-system-for-the-next-3tv5d5dgg5.pdf

A multi-moded rf delay line distribution system for the next linear collider

From the following description, it will be seen that SLED raises the accelerator peak energy by 40% if the present 2.7 1s RF pulse length is used. The energy increase ” is 80% if the pulse length is extended to 5 ps. TO do this, however, changes have to be made in the modulators and trigger system, and the maximum repetition rate has to be halved to maintain the present average power level. In addition, more extensive switchyard modifications are required for~handling the higher energy beams. For these reasons, the SLED system will be initially installed and run at the present 2.7 ns pulse length. Performance at both pulse lengths is discussed in this paper.

/pdf/recent-progress-on-sled-the-slac-energy-doubler-38tggpmmp5.pdf

Z.D. Farkas

Papers

SLED: A Method of Doubling SLAC's Energy

SLED II: A new method of rf pulse compression

Accelerator structure R&D for linear colliders

A multi-moded rf delay line distribution system for the next linear collider

Recent Progress on Sled, The SLAC Energy Doubler