Graviton resonances in e+e-→μ+μ- at linear colliders with beamstrahlung and ISR effects

Abstract

Electromagnetic radiation emitted by the colliding beams is expected to play an important role at the next generation of high energy e + e − linear collider(s). Focusing on the simplest process e + e − → µ + µ − , we show that radiative effects like initial state radiation (ISR) and beamstrahlung can lead to greatly-enhanced signals for resonant graviton modes of the Randall-Sundrum model. IntroductionThe next generation of high-energy e + e − colliders [1, 2] will necessarily have linear design in order to avoid crippling energy losses from synchrotron radiation. Obviously any linear collider will have single-pass colliding beams, unlike a storage ring where multiple bunchcrossings are possible. High luminosities at linear machines can only be achieved, therefore, by using beams with bunches of high number density -which, in practice, means that the bunches must be focussed to very small sizes. Thus, as compared to a bunch length of around 100 µm at the now-decommissioned LEP Collider, the bunches at the planned TESLA are expected to be around 553 nm × 5 nm × 300 nm, while at the CLIC the planned size is 202 nm × 2.5 nm × 30 nm. In fact, narrow, intense beams of this kind constitute a basic and unavoidable part of the design of all the proposed high-energy e + e − machines including the International Linear Collider (ILC), which is now being planned and designed through a major international effort [3].Although high number densities per bunch do enable the machine(s) in question to achieve the required luminosity, this feature of the design is not unaccompanied by its own set of problems. These arise because very high densities of charged particles at the interaction point will naturally lead to the generation of strong electromagnetic fields in and around every colliding bunch. Interaction of beam constituents (e ± ) with the (strong) accelerating

Universal extra dimensions, radiative returns and the inverse problem at a lineare+e−collider

“…At the ILC-2, existing constraints on such models preclude pair-producion of the extra charged gauge bosons, but at the CLIC, this may be a real possibility. It is true that in most of the existing models the W ′ would be more-or-less as heavy as the Z ′ (if not heavier) and not about half as massive as is envisaged in this work 10 . However, if we confine ourselves to a signature space of 4ℓ+ E T signals versus dijet signals, any such model will certainly yield nonzero results for both.…”

“…Similarly, one could have nearly-degenerate sneutrinos, ν µ and ν τ coupling to both e + e − pairs as well as quarks with R-parity-violating couplings of similar strength. Of course, it may be possible to identify the spin of the exchanged particle [10] by reconstructing the angular distribution of the jets in the centre-of-mass frame, though this would be hampered by the missing longitudinal momentum due to the radiated photon(s). If this difficulty can be overcome and the angular distribution indicates an exchanged particle of spin 1, the sneutrino and graviton options would be eliminated, but we would still have the Z ′ option to reckon with.…”

“…To cut a long story short, therefore, radiative returns at the ILC will be a combined effect due to both ISR and beamstrahlung, and will create a substantial spread in the energy of the colliding beams 4 . Turning this fact to our advantage, we can claim that even in the case of a fixed-energy ILC (or any other highenergy e + e − machine), such radiative returns could well be the discovery mechanism of such resonances [10]. The rarity of a radiative return (the probability is suppressed by the QED coupling α) would be adequately offset by the large resonant cross-section at the pole of the propagator.…”