A concept of the photon collider beam dump

Photon beams at photon colliders are very narrow, powerful (10--15 MW) and cannot be spread by fast magnets (because photons are neutral). No material can withstand such energy density. For the ILC-based photon collider, we suggest using a 150 m long, pressurized (P ~ 4 atm) argon gas target in front of a water absorber which solves the overheating and mechanical stress problems. The neutron background at the interaction point is estimated and additionally suppressed using a 20 m long hydrogen gas target in front of the argon.

Energy calibration at high-energy photon colliders

Calibration of the absolute energy scale at high-energy photon (gamma-gamma, gamma-electron) colliders is discussed. The luminosity spectrum at photon colliders is broad and has a rather sharp high-energy edge, which can be used, for example, to measure the mass of the Higgs boson in the process gamma-gamma to H or masses of charged scalars by observing the cross-section threshold. In addition to the precise knowledge of the edge energy of the luminosity spectrum, it is even more important to have a way to calibrate the absolute energy scale of the detector. At first sight, Compton scattering itself provides a unique way to determine the beam energies and produce particles of known energies that could be used for detector calibration. The energy scale is given by the electron mass m_e and laser photon energy omega_0. However, this does not work at realistic photon colliders due to large nonlinear effects in Compton scattering at the conversion region (xi^2 sim 0.3). It is argued that the process gamma-electron to eZ_0 provides the best way to calibrate the energy scale of the detector, where the energy scale is given by m_Z.

Photon collider Higgs factories

The discovery of the Higgs boson (and still nothing else) have triggered appearance of many proposals of Higgs factories for precision measurement of the Higgs properties. Among them there are several projects of photon colliders (PC) without e+e- in addition to PLC based on e+e- linear colliders ILC and CLIC. In this paper, following a brief discussion of Higgs factories physics program I give an overview of photon colliders based on linear colliders ILC and CLIC, and of the recently proposed photon-collider Higgs factories with no e+e- collision option based on recirculation linacs in ring tunnels.

Comments on photon colliders for Snowmass 2013

For more than 30 years [1], gamma-gamma and gamma-electron photon colliders have been considered a natural addition to e+e- linear-collider projects. Following the recent discovery of the Higgs boson, the physics community has been actively considering various approaches to building a Higgs factory, a photon collider (with or without e+e-) being one of them. In this note, following a brief discuss of photon colliders based on ILC and CLIC, I give a critical overview of the recently proposed photon-collider Higgs factories with no e+e- collision option.

Limitation on the luminosity of e+e- storage rings due to beamstrahlung

Particle loss due to the emission of single energetic beamstrahlung photons in beam collisions is shown to impose a fundamental limit on storage-ring luminosities at energies greater than 2E~140 GeV for head-on collisions and 2E~40 GeV for crab-waist collisions. Above these threshold energies, the suppression factor due to beamstrahlung scales as 1/E^{4/3}, and for a fixed power of synchrotron radiation, the luminosity L is proportional to R/E^{13/3}, where R is the collider radius. For 2E > 150 GeV, both collision schemes have similar luminosity limits. The luminosities attainable at storage-ring and linear-collider (LC) 2E=240 GeV Higgs factories are comparable; at higher energies, LCs are preferable. This conference paper is based on my recent PRL publication [1], supplemented with additional comments on linac-ring e+e- colliders and ring e+e- colliders with charge compensation (four-beam collisions).

Higgs factories

Over the past two decades, the high energy physics community has been actively discussing and developing a number of post-LHC collider projects; however, none of them have been approved due to high costs and the uncertainty in post-LHC physics scenarios. There have been great expectations of rich new physics in the 0.1-1 TeV mass region: the Higgs boson (one or several), supersymmetry, or perhaps new particles from the dark-matter family. It has been the general consensus that the best machine for the detailed study of new physics to be discovered at the LHC would be an energy-frontier linear e+e- collider. Physicists held their breath waiting for the results from the LHC. In summer 2012, two LHC detectors, ATLAS and CMS, announced the discovery of a Higgs boson with the mass of 126 GeV and (still) nothing else. The absence of new physics in the region below 1 TeV has changed the post-LHC collider R&D priorities and triggered a zoo of project proposals for the precision study of the 126 GeV Higgs boson, possibly with further upgrades to higher energies. This paper gives an overview of these projects; it is based largely on the reports presented at the first workshop on Higgs factories held at FNAL a few days prior to the present workshop in Protvino.

Restriction on the energy and luminosity of e+e- storage rings due to beamstrahlung

The role of beamstrahlung in high-energy e+e- storage-ring colliders (SRCs) is examined. Particle loss due to the emission of single energetic beamstrahlung photons is shown to impose a fundamental limit on SRC luminosities at energies 2E_0 >~ 140 GeV for head-on collisions and 2E_0 >~ 40 GeV for crab-waist collisions. With beamstrahlung taken into account, we explore the viability of SRCs in the E_0=240-500 GeV range, which is of interest in the precision study of the Higgs boson. At 2E_0=240 GeV, SRCs are found to be competitive with linear colliders; however, at 2E_0=400-500 GeV, the attainable SRC luminosity would be a factor 15-25 smaller than desired.

Recent results from the KEDR Detector

We report results of experiments performed with the KEDR detector at the VEPP-4M e+e- collider. They include precise measurement of the D0 and D+- meson masses, determination of the psi(3770) resonance parameters, and a search for narrow resonances in e+e- annihilation at center-of-mass energies between 1.85 and 3.1 GeV.