Generation Of Linear Colliders Research Articles

A laser based fiducial line for high precision multipoint alignment system

The next generation of linear colliders is very demanding concerning the alignment tolerances of their components. For the CLIC project, the reference axis of the components will have to be pre-aligned within 10 μm at 1 sigma with respect to a straight line in a sliding window of 200 m. A solution based on stretched wires with wire positioning sensors has been proposed in order to fulfill the alignment requirements in the Conceptual Design Report of the project. This solution has some drawbacks and laser based alternative solutions are under study in order to validate the wire solution and possibly replace it. A new proposal is introduced in this paper, using a laser beam over 150 m as a straight alignment reference, with the objective of having an uncertainty in the determination of its straightness within 10 μm. Sensors coupled to the components to be aligned, would provide after calibration the horizontal and vertical offsets with respect to the laser beam, within a few micrometers, in their coordinate system. The method is based on the laser beam space stabilization effect when a beam propagates in atmospheric air inside a pipe with standing acoustic wave. The principal schemes of corresponding optoelectronics devices and temperature stabilization solutions are also proposed, making probable the extension of the laser fiducial line up to a 500 m length.

Does experiment show that beamstrahlung theory – strong field QED – can be trusted?

We present a discussion of the experimental foundation for the strong field effects that are applied in beamstrahlung calculations for the planned next generation of linear colliders. In particular, quantum suppression of synchrotron radiation, coherent pairs and tridents are treated. It is shown, that many of these phenomena can be – and some have been – tested using present day technology.

High-Power Picosecond Pulse Recirculation for Inverse Compton Scattering

In the next generation of linear colliders, inverse Compton scattering (ICS) of intense laser pulses on relativistic electron bunches will enable a mode of operation based on energetic γe and γγ collisions, with a significant complementary scientific potential. The efficiency of γ -ray generation via ICS is constrained by the Thomson scattering cross section, resulting in typical laser photon-to- γ efficiencies of <10 −9 . Furthermore, repetition rates of the state-of-art high-energy short-pulse lasers are poorly matched with those available from electron accelerators. Laser recirculation has been proposed as a method to address those limitations, but has been limited to only small pulse energies and peak powers. We propose and experimentally demonstrate an alternative, non-interferometric method for laser pulse recirculation that is uniquely capable of recirculating short pulses with energies exceeding 1 J [ I. Jovanovic, M. Shverdin, D. Gibson, and C. Brown, Nucl. Instrum. Methods A 578 160 (2007)]. ICS of recirculated Joule-level laser pulses is compatible with the proposed pulse structure for ILC and has a potential to produce unprecedented peak and average γ -ray brightness in the next generation of sources.

Two-parton contribution to the heavy-quark AFB at NNLO QCD in collisions

At the next generation of linear colliders, forward-backward asymmetries will be involved again in the precise determination of the neutral current couplings of heavy quarks in inclusive heavy-quark production. The theoretical understanding of these observables at the level of NNLO radiative corrections is required. We review a recent calculation of the two-parton contribution to the heavy-quark A_FB at NNLO QCD in e+e- collisions. The results are valid for arbitrary values of the momentum transfer and non-vanishing heavy-quark mass m_Q.

Virtual bilepton effects in polarized Møller scattering

We investigate the indirect effects of heavy vector bileptons being exchanged in polarized Moeller scattering, at the next generation of linear colliders. Considering both longitudinal and transverse beam polarization, and accounting for initial-state radiation, beamstrahlung and beam energy spread, we discuss how angular distributions and asymmetries can be used to detect clear signals of virtual bileptons, and the possibility of distinguishing theoretical models that incorporate these exotic particles. We then estimate 95% C.L. bounds on the mass of these vector bileptons and on their couplings to electrons.

Corrections to the processe+e−→bb¯in the topcolor assisted technicolor model

In the framework of the topcolor assisted technicolor model we calculate the contributions from the pseudo Goldstone bosons and new gauge bosons to ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}b\overline{b}$. We find that, for reasonable ranges of the parameters, the top pions afford a dominate contribution, the corrections arising from technipions and new gauge bosons are negligibly small, the maximum of the total corrections is 43% with the c.m. energy $\sqrt{s}=500\text{ }\text{ }\mathrm{GeV}$; whereas in the case of $\sqrt{s}=1500\text{ }\text{ }\mathrm{GeV}$, the maximum of the relative corrections is only 3.1%. It might open a window to detect a topcolor assisted technicolor model in experiment of next generation linear colliders.

Chargino pair production at linear collider and split supersymmetry

Recently Arkani-Hamed and Dimopoulos proposed a supersymmetric model [ hep-th/0405159], dubbed “Split supersymmetry” in [ hep-ph/0406088], which can remove most of the unpleasant shortcomings of TeV Supersymmetry. In this model all scalars except one finely tuned Higgs boson are ultra heavy while the neutralino and chargino might remain light in order to achieve gauge coupling unification and accord with the dark matter density. In this Letter, we investigated the impact of this new model on chargino pair production at next generation linear colliders. Our numerical results show that this process can be used to probe sneutrino mass up to 10 TeV. Therefore, precise measurements of chargino pair production at the linear colliders could distinguish split supersymmetry from TeV supersymmetry.

Sudakov Electroweak effects in transversely polarized beams

We study Standard Model electroweak radiative corrections for fully inclusive observables with polarized fermionic beams. Our calculations are relevant in view of the possibility for Next Generation Linear colliders of having transversely and/or longitudinally polarized beams. The case of initial transverse polarization is particularly interesting because of the interplay of infrared/collinear logarithms of different origins, related both to the nonabelian SU(2) and abelian U(1) sectors. The Standard model effects turn out to be in the 10% range at the TeV scale, therefore particularly relevant in order to disentangle possible New Physics effects.

Ee4fγ—A program for e+e−→4f,4f γ with nonzero fermion masses

A computer program ee4f γ for calculating cross-sections of any four fermion final state of e + e −-annihilation at high energy and the corresponding bremsstrahlung reaction that is possible in the framework of the Standard Model is presented. As the fermion masses are arbitrary, the cross-sections for channels that do not contain e + and/or e − in the final state can be computed without any collinear cut, the on-shell top quark production can be studied and the Higgs boson exchange can be incorporated in a consistent way. The program can be used as a Monte Carlo generator of unweighted events as well. Program summary Title of program: ee4f γ Version: 1.0 (February 2004) Catalogue identifier: ADTQ Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADTQ Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Licensing provisions: none Computers: all Operating systems: Unix/Linux Programming language used: FORTRAN 90 CPC Program Library subprograms used: RANLUX, ACPR RANLUX 79 (1994) 111—a random number generator Memory required to execute with typical data: 4.0 Mb No. of bits in a word: 32 No. of bytes in distributed program, including test data, etc.: 364 490 No. of lines in distributed program, including test data, etc.: 45 278 Distribution format: tar gzip file Nature of physical problem: Description of all e + e −→4 fermions and corresponding bremsstrahlung reactions that are possible in the Standard Model (SM) to lowest order and with nonzero fermion masses at center of mass energies typical for next generation linear colliders. Such reactions are relevant, typically, for W-pair or intermediate mass Higgs boson production and decay. Method of solution: Matrix elements are calculated with the helicity amplitude method. The phase space integration is performed numerically utilizing a multi-channel Monte Carlo method. Restrictions on complexity of the problem: No higher order effects are taken into account, except for assuming the fine structure constant and the strong coupling at the appropriate scale and partial summation of those one particle irreducible loop corrections which are inducing the (fixed) finite widths of unstable particles. Typical running time: The running time depends strongly on a selected channel and desired precision of the result. The results of the appended test run have been obtained on a 800 MHz Pentium III processor with the use of Absoft FORTRAN 90 compiler in about 9 minutes. In order to obtain a precision level below one per mile a few million calls to the integrand are required. This results in less than one hour running time for the fastest channel e +e −→ν μ ν ̄ μν τ ν ̄ τ and more than 100 hours running time for the slowest channel e + e −→ e + e − e + e −. However, the typical running time for channels that are relevant for the W-pair or Higgs boson production is several hours.

Slepton Flavor Physics at Linear Colliders

If low energy supersymmetry is realized in nature it is possible that a first generation linear collider will only have access to some of the superpartners with electroweak quantum numbers. Among these, sleptons can provide sensitive probes for lepton flavor violation through potentially dramatic lepton violating signals. Theoretical proposals to understand the absence of low energy quark and lepton flavor changing neutral currents are surveyed and many are found to predict observable slepton flavor violating signals at linear colliders. The observation or absence of such sflavor violation will thus provide important indirect clues to very high energy physics. Previous analyses of slepton flavor oscillations are also extended to include the effects of finite width and mass differences.

Beam dynamics experiments to study the suppression of transverse instabilities

Two-beam accelerators based upon relativistic klystron's (RK's) have been proposed as power sources for future generation linear electron-positron colliders. These drivers are susceptible to several transverse beam breakup (BBU) instabilities. An experiment to study a particular technique (the ``betatron node scheme'') for ameliorating the high-frequency BBU has been performed at LBNL on a 1 MeV, 500 A induction accelerator beam. The results of this experiment are particularly important for RK, but apply to any system where the betatron phase advance between perturbing structures is an integral multiple of 180\ifmmode^\circ\else\textdegree\fi{}. This phase advance is beneficial in linear accelerators as the instability growth changes from exponential to linear. In the experiment described below, the beam is contained in a solenoidal focusing channel, rf cavities are spaced every 60 cm, and growth in the transverse motion was measured as a function of phase advance. Details of the experiment and results are presented.

SUSY-induced top quark FCNC processes at linear colliders

In the Minimal Supersymmetric Model (MSSM) the hitherto unconstrained flavor mixing between top-squark and charm-squark will induce the flavor-changing neutral-current (FCNC) interaction between top quark and charm quark, which then lead to various processes at the next generation linear collider (NLC), i.e., the top-charm associated productions via e + e −, e − γ and γγ collisions as well as the top quark rare decays t→ cV ( V= g, γ or Z). All these processes involve the same part of the parameter space of the MSSM. Through a comparative analysis for all these processes at the NLC, we found that the best channel to probe such SUSY-induced top quark FCNC is the top-charm associated production in γγ collision, which occurs at a much higher rate than e + e − or e − γ collision and may reach the detectable level for some part of the parameter space.

Higher-order modes in periodic accelerating structures analyzed using the cavity mode method

The cavity mode method is a new method for computing the dispersion curves and fields of accelerating structures. The method assumes that the field of interest is expanded with open and short modes. These modes are constructed on the basis of the resonant fields in a unit cell. With the wave equation and the Floquet boundary conditions, the method is mathematically simplified into a general eigenvalue problem to calculate the dispersion curve and fields for any phase shift in the structures. This method has been used in analyzing the higher-order modes in the periodic structures. In further study, it is also expected to be useful for analyzing non-periodic structures in the next generation linear collider.

Sneutrino-induced like sign dilepton signal with conservedRparity

Lepton number violation could be manifest in the sneutrino sector of supersymmetric extensions of the standard model with conserved R parity. Then sneutrinos decay partly into the ``wrong sign charged lepton'' final state, if kinematically accessible. In sneutrino pair production or associated single sneutrino production, the signal then is a like sign dilepton final state. Under favorable circumstances, such a signal could be visible at the CERN LHC or a next generation linear collider for a relative sneutrino mass splitting of order $\mathcal{O}(0.001)$ and sneutrino width of order $\mathcal{O}(1\mathrm{GeV}).$ On the other hand, the like sign dilepton event rate at the Fermilab Tevatron is probably too small to be observable.

Single charged Higgs boson production at next generation linear colliders

We study the single production of charged Higgs bosons in e+e- collisions, chiefly in the Minimal Supersymmetric Standard Model. Our analysis complements foregoing studies of the pair production channel especially in regions where the kinematic constraint suppresses pair production. We present cross sections relevant to experiments at the next generation linear colliders and some brief discussions of their phenomenology. Our analysis shows that the single production is a useful alternative channel for studying the phenomenology of charged Higgs bosons, and there are regions of parameter space where it is accessible beyond the kinematic limit for pair production.

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

The experimental reconstruction of the Higgs self-energy potential is essential to a verification of the Higgs boson's rǒle in spontaneous electroweak symmetry breaking. The first step towards this goal, the measurement of the triple Higgs self-coupling, can possibly be accomplished at the next generation of linear colliders. Here we discuss the most promising channels at future hadron and e + e − colliders and present background studies to evaluate the feasibility of its measurement.

Linear collider signal of a Wino LSP in anomaly mediated scenarios

Selectron (smuon) pair-production in a next generation Linear Collider, yielding a fast electron (muon) trigger, a visible heavily ionizing track and/or a resolved soft pion impact parameter and overall $\mET$, is shown to provide a smoking gun signature for Anomaly Mediated Supersymmetry Breaking models with a neutral Wino as the Lightest Supersymmetric Particle, nearly mass-degenerate with the lighter chargino.

Electroweak Sudakov form factors and nonfactorizable soft QED effects at NLC energies

We study the leading log infrared behavior of electroweak corrections at TeV scale energies, that will be reached by next generation of linear colliders (NLC). We show that, contrary to what happens at typical LEP energies, it is not anymore possible to disentangle “pure electroweak” from “photonic” corrections. This means that soft QED effects do not factorize and therefore cannot be treated in the usual “naive” way they were accounted for in the LEP-era. The nonfactorizable effects come up first at the two loop LL level, that we calculate explicitly for a fermion source that is neutral under the SU(2)⊗U(1) gauge group (explicitly, a Z' decay into two fermions). The basic formalism we set up can be used to calculate LL effects at any order of perturbation theory. The results of this paper might be important for future calculations of electroweak corrections at NLC energies.

SUSY QCD effect on top-charm associated production at linear colliders

We evaluate the contribution of SUSY-QCD to top-charm associated production at next generation linear colliders. Our results show that the production cross section of the process $e^+e^-\to t\bar c{or}\bar t c$ could be as large as 0.1 fb, which is larger than the prediction of the SM by a factor of $10^8$.

Using scalars to probe theories of low scale quantum gravity

Arkani-Hamed, Dimopoulos and Dvali have recently suggested that gravity may become strong at energies near 1 TeV which would remove the hierarchy problem. Such a scenario can be tested at present and future colliders since the exchange of towers of Kaluza-Klein gravitons leads to a set of new dimension-8 operators that can play important phenomenological roles. In this paper we examine how the production of pairs of scalars at $e^+e^-$, $\gamma \gamma$ and hadron colliders can be used to further probe the effects of graviton tower exchange. In particular we examine the tree-level production of pairs of identical Higgs fields which occurs only at the loop level in both the Standard Model and its extension to the Minimal Supersymmetric Standard Model. Cross sections for such processes are found to be potentially large at the LHC and the next generation of linear colliders. For the $\gamma\gamma$ case the role of polarization in improving sensitivity to graviton exchange is emphasized.