Longitudinal Beam Polarization Research Articles

Polarized Z cross sections in Higgsstrahlung for the determination of anomalous ZZH couplings

The production of a Higgs boson in association with a Z at an electron-positron collider is one of the cleanest methods for the measurement of the couplings of the Higgs boson. In view of the large production cross section at energies a little above the threshold, it seems feasible to make a more detailed study of the process by measuring the cross sections for polarized Z in order to measure possible anomalous ZZH couplings. We show that certain combinations of cross sections in e+e−→ZH with different Z polarizations help to enhance or isolate the effect of one of the two kinds of anomalous ZZH couplings possible on general grounds of CP and Lorentz invariance. These combinations can be useful to get information on the ZZH coupling in the specific contexts of an effective field theory, two-Higgs-doublet models, and composite Higgs models, in a relatively model-independent fashion. We find in particular that the longitudinal helicity fraction of the Z is expected to be insensitive to anomalous couplings, and would be close to its value in the standard model in the scenarios we consider. We also discuss the sensitivity of the proposed measurements to the anomalous couplings, including longitudinal beam polarizations, which suppress backgrounds, and can improve the sensitivity if appropriately chosen.

Beam focus and longitudinal polarization influence on spin dynamics in the Kapitza-Dirac effect

We theoretically investigate the influence of a longitudinal laser polarization component from beam focusing on spin dynamics in Kapitza-Dirac scattering by solving the relativistic Dirac equation with time-dependent perturbation theory. The transverse spacial dependence of the longitudinal beam polarization component is accounted for, by approximating a Gaussian beam with plane-wave components. We find that corrections from a longitudinal laser beam polarization component approximately scale with the second power of the diffraction angle $\epsilon$, from which we conclude that a related influence from beam focusing can be made negligibly small for sufficiently low beam foci.

Indirect measurement of triple-Higgs coupling at an electron–positron collider with polarized beams

We examine the possibility of using single-Higgs production at an [Formula: see text] collider with polarized beams to measure, or constrain, indirectly a possible anomalous triple-Higgs coupling, which can contribute to the process via one-loop diagrams. In the dominant process [Formula: see text], longitudinally polarized beams can lead to an improvement in the cross-section by 50% for [Formula: see text] and [Formula: see text] polarizations of [Formula: see text] and [Formula: see text], respectively. This corresponds to an improvement in the sensitivity to the triple-Higgs coupling of about 18% for a center-of-mass energy of 250 GeV and an integrated luminosity of 2 ab[Formula: see text], making a strong case of beam polarization. This also implies that with polarized beams, the luminosity needed to get a particular sensitivity is less by about 33% as compared to that needed with unpolarized beams. Even when only the [Formula: see text] beam is polarized [Formula: see text], the improvement in the sensitivity is about 8%. We also study the effect of longitudinal beam polarization on the sensitivity to the triple-Higgs coupling of Higgs production through the subdominant process [Formula: see text] occurring through [Formula: see text] fusion.

On the choice of beam polarization in e^+e^-rightarrow ZZ/Zgamma and anomalous triple gauge-boson couplings

The anomalous trilinear gauge couplings of Z and gamma are studied in e^+e^-rightarrow ZZ/Zgamma with longitudinal beam polarizations using a complete set of polarization asymmetries for the Z boson. We quantify the goodness of the beam polarization in terms of the likelihood and find the best choice of e^- and e^+ polarizations to be (+0.16, -0.16), (+0.09, -0.10) and (+0.12, -0.12) for ZZ, Zgamma and combined processes, respectively. Simultaneous limits on anomalous couplings are obtained for these choices of beam polarizations using Markov-Chain–Monte-Carlo (MCMC) for an e^+e^- collider running at sqrt{s}=500 GeV and mathscr {L}=100 fb^{-1}. We find the simultaneous limits for these beam polarizations to be comparable with each other and also comparable with the unpolarized beam case.

Probing left-right seesaw models using beam polarization at an e+e− collider

We show that the longitudinal beam polarization option at a future electron-positron collider provides an unambiguous distinction between low-scale seesaw models of neutrino mass. This is possible due to the fact that the pair production cross section of the heavy neutrinos in seesaw models is sensitive to the polarization of the initial lepton beams, and for a suitable choice of the polarization, shows a clear enhancement over the unpolarized cross section. More interestingly, the choice of the beam polarization for which the enhancement is maximum is governed by the size of the light-heavy neutrino mixing parameter. We also find that using this effect, one can probe a previously uncharted parameter space of the left-right seesaw model, which is complementary to the existing searches at both energy and intensity frontiers.

Ratios of helicity amplitudes for exclusive rho ^0 electroproduction on transversely polarized protons

Exclusive rho ^0-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV^2<Q^2< 7.0 GeV^2, 3.0 GeV <W< 6.3 GeV, and -t^prime < 0.4 GeV^2. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing rho ^{0}-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude F_{0frac{1}{2}0frac{1}{2}}, which describes the production of a longitudinal rho ^{0}-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to F_{0frac{1}{2}0frac{1}{2}}. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the pi -rho transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 rho ^0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.

Investigation of beam self-polarization in the futuree+e−circular collider

The use of resonant depolarization has been suggested for precise beam energy measurements (better than 100 keV) in the $e^{+}e^{-}$ Future Circular Collider (FCC-$e^{+}e^{-}$) for Z and WW physics at 45 and 80 GeV beam energy respectively. Longitudinal beam polarization would benefit the Z peak physics program; however it is not essential and therefore it will be not investigated here. In this paper the possibility of self-polarized leptons is considered. Preliminary results of simulations in presence of quadrupole misalignments and beam position monitors (BPMs) errors for a simplified FCC-$e^{+}e^{-}$ ring are presented.

Generalized top-spin analysis and new physics ine+e−collisions with beam polarization

A generalized top-spin analysis proposed some time ago in the context of the standard model and subsequently studied in varying contexts is now applied primarily to the case of ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}t\overline{t}$ with transversely polarized beams. This extends our recent work with new physics couplings of scalar ($S$) and tensor ($T$) types. We carry out a comprehensive analysis assuming only the electron beam to be transversely polarized, which is sufficient to probe these interactions, and also eliminates any azimuthal angular dependence due to the standard model or new physics of the vector ($V$) and axial-vector ($A$) type interactions. We then consider new physics of the general four-Fermi type of $V$ and $A$ type with both beams transversely polarized and discuss implications with longitudinal polarization as well. The generalized spin bases are all investigated in the presence of either longitudinal or transverse beam polarization to look for appreciable deviation from the SM prediction in case of the new physics. 90% confidence level limits are obtained on the interactions for the generalized spin bases with realistic integrated luminosity. In order to achieve this we present a general discussion based on helicity amplitudes and derive a general transformation matrix that enables us to treat the spin basis. We find that beamline basis combined with transverse polarization provides an excellent window of opportunity both for $S$, $T$ and $V$, $A$ new physics, followed by the off-diagonal basis. The helicity basis is shown to be the best in case of longitudinal polarization to look for new physics effects due to $V$ and $A$.

Decay-lepton correlations as probes of anomalous ZZH and γZH interactions in [formula omitted] with polarized beams

We examine the contributions of various couplings in general ZZH and γZH interactions arising from new physics to the Higgs production process e+e−→HZ, followed by the decay of the Z into a charged-lepton pair. We take into account possible longitudinal or transverse beam polarization likely to be available at a linear collider. We show how expectation values of certain simple observables in suitable combinations with appropriate longitudinal beam polarizations can be used to disentangle various couplings from one another. Longitudinal polarization can also improve the sensitivity for measurement of several couplings. A striking result is that using transverse polarization, one of the γZH couplings, not otherwise accessible, can be determined independently of all other couplings.

Erratum: Charged lepton distributions as a probe of contacte+e−HZinteractions at a linear collider with polarized beams [Phys. Rev. D77, 015009 (2008)

We examine very general four-point interactions arising from new physics and contributing to the Higgs production process e+e- -> HZ. We write all possible forms for these interactions consistent with Lorentz invariance. We allow the possibility of CP violation. Contributions to the process from anomalous ZZH and gamma ZH interactions studied earlier arise as a special case of our four-point amplitude. We consider the decay of Z into a charged lepton pair and obtain expressions for angular distributions of charged leptons arising from the interference of the four-point contribution with the standard-model contribution. We take into account possible longitudinal or transverse beam polarization likely to be available at a linear collider. We examine several correlations which can be used to study the various form factors present in the e+e-HZ contact interactions. We also obtain the sensitivity of these correlations in constraining the new-physics interactions at a linear collider operating at a centre-of-mass energy of 500 GeV with longitudinal or transverse polarization.

Angular distributions as a probe of anomalousZZHandγZHinteractions at a linear collider with polarized beams

We examine the contribution of general Z*ZH and gamma*ZH three-point interactions arising from new physics to the Higgs production process e+e- -> HZ. From Lorentz covariance, each of these vertices may be written in terms of three (complex) form factors, whose real and imaginary parts together make six independent couplings. We take into account possible longitudinal or transverse beam polarization likely to be available at a linear collider. We show how partial cross sections and angular asymmetries in suitable combinations with appropriate beam polarizations, can be used to disentangle various couplings from one another. A striking result is that using transverse polarization, one of the gammaZH couplings, not otherwise accessible, can be determined independently of all other couplings. Transverse polarization also helps in the independent determination of a combination of two other couplings, in contrast to a combination of four accessible with unpolarized or longitudinally polarized beams. We also obtain the sensitivity of the various observables in constraining the new-physics interactions at a linear collider operating at a centre-of-mass energy of 500 GeV with longitudinal or transverse polarization.

Charged lepton distributions as a probe of contacte+e−HZinteractions at a linear collider with polarized beams

We examine very general four-point interactions arising from new physics and contributing to the Higgs production process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}HZ$. We write all possible forms for these interactions consistent with Lorentz invariance. We allow the possibility of $CP$ violation. Contributions to the process from anomalous $ZZH$ and $\ensuremath{\gamma}ZH$ interactions studied earlier arise as a special case of our four-point amplitude. We consider the decay of $Z$ into a charged-lepton pair and obtain expressions for angular distributions of charged leptons arising from the interference of the four-point contribution with the standard-model contribution. We take into account possible longitudinal or transverse beam polarization likely to be available at a linear collider. We examine several correlations which can be used to study the various form factors present in the ${e}^{+}{e}^{\ensuremath{-}}HZ$ contact interactions. We also obtain the sensitivity of these correlations in constraining the new-physics interactions at a linear collider operating at a center-of-mass energy of 500 GeV with longitudinal or transverse polarization.

CP asymmetries with longitudinal and transverse beam polarizations in neutralino production and decay into theZ0boson at the ILC

We study neutralino production at the linear collider with the subsequent two-body decays 0i → 0nZ0 and Z0 → , with l = e, μ, τ, or Z0 → q with q = c, b. We show that transverse electron and positron beam polarizations allow the definition of unique CP observables. These are azimuthal asymmetries in the distributions of the final leptons or quarks. We calculate these CP asymmetries and the cross sections in the Minimal Supersymmetric Standard Model with complex higgsino and gaugino parameters μ and M1. For final quark pairs, we find CP asymmetries as large as 30%. We discuss the significances for observing the CP asymmetries at the International Linear Collider (ILC). Finally we compare the CP asymmetries with those asymmetries which require unpolarized and/or longitudinally polarized beams only.

Studying possible CP-violating Higgs couplings through top-quark pair productions at muon colliders

We study possible anomalous CP-violating Higgs couplings to mu mu-bar and t t-bar fully model-independent way through top-quark pair productions at muon colliders. Assuming additional non-standard neutral Higgs bosons, whose couplings with top-quark and muon are expressed in the most general covariant form, we carry out analyses of effects which they are expected to produce via CP-violating asymmetries and also the optimal-observable (OO) procedure under longitudinal and transverse muon polarizations. We find the measurement of the asymmetry for longitudinal beam polarization could be useful to catch some signal of CP violation, and an OO analysis might also be useful if we could reduce the number of unknown parameters with a help of other experiments and if the size of the parameters is at least O(1) ~ O(10).

Neutralino production and decay at ane+e−linear collider with transversely polarized beams

Once supersymmetric neutralinos $\tilde{\chi}^0$ are produced copiously at $e^+e^-$ linear colliders, their characteristics can be measured with high precision. In particular, the fundamental parameters in the gaugino/higgsino sector of the minimal supersymmetric extension of the standard model (MSSM) can be analyzed. Here we focus on the determination of possible CP--odd phases of these parameters. To that end, we exploit the electron/positron beam polarization, including transverse polarization, as well as the spin/angular correlations of the neutralino production $e^+ e^- \to \tilde{\chi}^0_i \tilde{\chi}^0_j$ and subsequent 2--body decays $\tilde{\chi}^0_i \to \tilde{\chi}^0_k h, \tilde{\chi}^0_k Z, \tilde \ell^\pm_R \ell^\mp$, using (partly) optimized CP--odd observables. If no final--state polarizations are measured, the $Z$ and $h$ modes are independent of the $\tilde{\chi}^0_i$ polarization, but CP--odd observables constructed from the leptonic decay mode can help in reconstructing the neutralino sector of the CP--noninvariant MSSM. In this situation, transverse beam polarization does not seem to be particularly useful in probing explicit CP violation in the neutralino sector of the MSSM. This can most easily be accomplished using longitudinal beam polarization.

Probing CP-violating contact interactions in e+e−→HZ with polarized beams

Abstract We examine very general four-point interactions arising due to new physics contributing to the Higgs production process e + e − → H Z . We write all possible forms for these interactions consistent with Lorentz invariance. We allow the possibility of CP violation. Contributions to the process from anomalous ZZH and γZH interactions studied earlier arise as a special case of our four-point amplitude. Expressions for polar and azimuthal angular distributions of Z arising from the interference of the four-point contribution with the standard-model contribution in the presence of longitudinal and transverse beam polarization are obtained. An interesting CP-odd and T-odd contribution is found to be present only when both electron and positron beams are transversely polarized. Such a contribution is absent when only anomalous ZZH and γZH interactions are considered. We show how angular asymmetries can be used to constrain CP-odd interactions at a linear collider operating at a centre-of-mass energy of 500 GeV with transverse beam polarization.

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.

The transmission Compton polarimeter of the A4 experiment

A transmission Compton polarimeter as a simultaneous, relative monitor for the longitudinal polarisation of a stabilised, polarised electron beam (polarisation degree ∼ 80 %) has been constructed in a new design. It is located in the vacuum between the target and the beam dump of the A4 parity violation experiment at MAMI. The analysing power is ∼ 80 ppm at 854 MeV and ∼ 115 ppm at 570 MeV. The measurement precision for the polarimeter asymmetry (which is proportional to the longitudinal beam polarisation degree) is ∼ 3 ppm within 5 min. at 854 MeV and ∼ 7 ppm within 5 min. at 570 MeV and the systematical error is about 0.5 ppm + 1 – 2 %. This simple polarimeter consists mainly of two graphite scatterers, a water-cooled Samarium-Cobalt permanent magnet, an aluminum secondary electron emission monitor, an understructure and electronics.

Interference of Higgs boson resonances in $\mu^ + \mu^- \to\tilde{\chi}^0_i\tilde{\chi}^0_j$ with longitudinal beam polarization

We study the interference of resonant Higgs boson exchange in neutralino production in $\mu^ + \mu^-$ annihilation with longitudinally polarized beams. We use the energy distribution of the decay lepton in the process $\tilde{\chi}^0_j \to \ell^{\pm} \tilde{\ell}^\mp$ to determine the polarization of the neutralinos. In the CP-conserving minimal supersymmetric standard model a non-vanishing asymmetry in the lepton energy spectrum is caused by the interference of Higgs boson exchange channels with different CP-eigenvalues. The contribution of this interference is large if the heavy neutral bosons H and A are nearly degenerate. We show that the asymmetry can be used to determine the couplings of the neutral Higgs bosons to the neutralinos. In particular, the asymmetry allows one to determine the relative phase of the couplings. We find large asymmetries and cross sections for a set of reference scenarios with nearly degenerate neutral Higgs bosons.

Probing triple gauge couplings with transverse beam polarisation in $e^ + e^- \rightarrow W^ + W^-$

The prospects of measuring triple gauge couplings in W pair production at future linear colliders with transverse beam polarisation are studied. We consider CP conserving and CP violating couplings with both real and imaginary parts. The maximum achievable sensitivity in a simultaneous measurement of all couplings is determined using optimal observables, extending an earlier analysis for longitudinal beam polarisation. We find good sensitivity to the coupling Im(g^R_1 + kappa_R), which is not measurable with longitudinal polarisation. In contrast, for the real parts (including the CP violating couplings) the sensitivity is better if both beams are longitudinally polarised. We conclude that a comprehensive measurement of all triple gauge couplings requires both transverse and longitudinal beam polarisation.