Initial-state Photon Radiation Research Articles

Sensitivity of Future \(e^+e^-\) Colliders to Processes of Dark Matter Production with Light Mediator Exchange

One of the primary goals of the proposed future collider experiments is to search for dark matter (DM) particles using different experimental approaches. High energy $e^+e^-$ colliders offer unique possibility for the most general search based on the mono-photon signature. As any $e^+e^-$ collision process may include hard initial-state photon radiation, analysis of the energy spectrum and angular distributions of observed photons can be used to search for hard processes with an invisible final state. Dedicated procedure of merging the matrix element calculations with the lepton ISR structure function was developed to model the Standard Model background processes contributing to mono-photon signature with WHIZARD. In this work, we consider production of DM particles at the International Linear Collider (ILC) and Compact Linear Collider (CLIC) experiments via a mediator exchange. Detector effects are taken into account within the DELPHES fast simulation framework. Limits on the light DM production in a simplified model are set as a function of the mediator mass and width based on the expected two-dimensional distributions of the reconstructed mono-photon events. The experimental sensitivity is extracted in terms of the DM production cross section. Limits on the mediator couplings are then presented for a wide range of mediator masses and widths. For light mediators, for masses up to the centre-of-mass energy of the collider, coupling limits derived from the mono-photon analysis are more stringent than those expected from direct resonance searches in decay channels to SM particles.

\(\mathcal{O}(\alpha^3)\) ISR Corrections to Single Higgs Productions at Future Lepton Colliders

In this paper, we present \(\mathcal{O}(\alpha^3)\) QED corrections due to initial-state photon radiation (ISR) to processes \(e^{−} e^{+} \to f \bar{f} H\) with \(f = e, \mu_e, \nu_{\mu}, \nu_{\tau}\) at future lepton colliders. The impact of the ISR corrections on the total cross sections are studied. We find that the corrections vary from ∼ −30% to −10% in the range of center-of-mass energies from 200 GeV to 700 GeV. The ISR corrections to some realistic distributions of experimental interest for searching the properties of the Higgs boson are also evaluated and they are order of \(\sim −10\)% contributions at \(s = 250\) GeV. The corrections are massive contributions and they must be taken into account for matching high precision data at future lepton colliders.

Measurement of the e+e−→Ks0K±π∓π0 and Ks0K±π∓η cross sections using initial-state radiation

The processes $e^+e^-\to K^0_{\scriptscriptstyle S}K^\pm\pi^{\mp}\pi^0$ and $e^+e^-\to K^0_{\scriptscriptstyle S}K^\pm\pi^{\mp}\eta$ are studied over a continuum of energies from threshold to 4 GeV with the initial-state photon radiation method. Using 454 fb$^{-1}$ data collected with the BABAR detector at the SLAC PEP-II storage ring, the first measurements of the cross sections for these processes are obtained. The intermediate resonance structures from $K^{*0}(K\pi)^0$, $K^{*}(892)^\pm(K\pi)^\mp$ and $K^0_{\scriptscriptstyle S}K^\pm\rho^{\mp}$ are studied. The $J/\psi$ is observed in all of these channels, and corresponding branching fractions are measured.

Recent results on hadronic final states from Babar

Two recent studies from the Babar Collaboration at SLAC are presented on the production of hadrons at low energies. The first is a study of exclusive K+K− production in e+e− annihilation events with initial-state photon radiation. The second is a study of ηc production in two-photon interactions and a three-body Dalitz-plot analysis searching for intermediate scalar meson production in ηc decays.

Tackling light higgsinos at the ILC

In supersymmetric extensions of the Standard Model, higgsino-like charginos and neutralinos are preferred to have masses of the order of the electroweak scale by naturalness arguments. Such light $\widetilde{\chi}^{0}_{1}$ , $\widetilde{\chi}^{0}_{2}$ and $\widetilde{\chi}^{\pm}_{1}$ states can be almost mass degenerate, and their decays are then difficult to observe at colliders. In addition to the generic naturalness argument, light higgsinos are well motivated from a top-down perspective. For instance, they arise naturally in certain models of hybrid gauge-gravity mediation. In the present analysis, we study two benchmark points which have been derived in the framework of such a model, which exhibit mass differences of $\mathcal {O}(\mathrm {GeV})$ in the higgsino sector. For chargino-pair and neutralino associated production with initial-state photon radiation, we simulate the detector response and determine how accurately the small mass differences, the absolute masses and the cross sections can be measured at the International Linear Collider. Assuming that 500 fb−1 has been collected at each of two beam-polarisations P(e +,e −)=(±30 %,∓80 %), we find that the mass differences can be measured to 40–300 MeV, the cross sections to 2–5 %, and the absolute masses to 1.5–3.3 GeV, where the range of values correspond to the different scenarios and channels. Based on these observables we perform a parameter fit in the MSSM, from which we infer that the higgsino mass parameter μ can be measured to a precision of about Δμ=2–7 GeV. For the electroweak gaugino mass parameters M 1, M 2, which are chosen in the multi-TeV range, a narrow region is compatible with the measurements. For both parameters independently, we can determine a lower bound.

RacoonWW1.3: A Monte Carlo program for four-fermion production at e +e − colliders

We present the Monte Carlo generator RacoonWW that computes cross sections to all processes e +e −→4 f and e +e −→4 fγ and calculates the complete O(α) electroweak radiative corrections to e +e −→WW→4 f in the electroweak Standard Model in double-pole approximation. The calculation of the tree-level processes e +e −→4 f and e +e −→4 fγ is based on the full matrix elements for massless (polarized) fermions. When calculating radiative corrections to e +e −→WW→4 f, the complete virtual doubly-resonant electroweak corrections are included, i.e. the factorizable and non-factorizable virtual corrections in double-pole approximation, and the real corrections are based on the full matrix elements for e +e −→4 fγ. The matching of soft and collinear singularities between virtual and real corrections is done alternatively in two different ways, namely by using a subtraction method or by applying phase-space slicing. Higher-order initial-state photon radiation and naive QCD corrections are taken into account. RacoonWW also provides anomalous triple gauge-boson couplings for all processes e +e −→4 f and anomalous quartic gauge-boson couplings for all processes e +e −→4 fγ.

Angular analysis of the muon pair asymmetry at LEP 1

Data on muon pair production obtained by the OPAL collaboration at centre of mass energies near the Z peak are analysed. Small angular mismatches between the directions of the two muons are used to assess the effects of initial state photon radiation and initial-final-state radiation interference on the forward-backward asymmetry of muon pairs. The dependence of the asymmetry on the invariant mass of the pair is measured in a model-independent way. Effective vector and axial-vector couplings of the Z boson are determined and compared to the Standard Model expectations.

Electroweak radiative corrections to e+e−→WW→4 fermions in double-pole approximation — the RacoonWW approach

Abstract We calculate the complete O (α) electroweak radiative corrections to e + e − →WW→4 f in the electroweak Standard Model in the double-pole approximation. We give analytical results for the non-factorizable virtual corrections and express the factorizable virtual corrections in terms of the known corrections to on-shell W-pair production and W decay. The calculation of the bremsstrahlung corrections, i.e., the processes e + e − →4 fγ in lowest order, is based on the full matrix elements. The matching of soft and collinear singularities between virtual and real corrections is done alternatively in two different ways, namely by using a subtraction method and by applying phase-space slicing. The O (α) corrections as well as higher-order initial-state photon radiation are implemented in the Monte Carlo generator RacoonWW . Numerical results of this program are presented for the W-pair-production cross section, angular and W-invariant-mass distributions at LEP2. We also discuss the intrinsic theoretical uncertainty of our approach.

Electroweak radiative corrections toWboson production in hadronic collisions

The O(\alpha) electroweak radiative corrections to the process pp, ppbar\to W \to\ell^\pm\nu (\ell=e,\mu) are calculated. The O(\alpha) corrections can be decomposed into separately gauge invariant contributions to the W boson production and decay processes. Factorizing the collinear singularity associated with initial state photon radiation into the parton distribution functions, we find that initial state corrections have a significantly smaller effect than final state radiative corrections. We study in detail the effect of electroweak radiative corrections on a number of interesting observables: the W transverse mass distribution, the W to Z transverse mass ratio, the charge asymmetry of leptons in W\to\ell\nu decays, as well as the W production cross section and the W to Z cross section ratio. We also investigate how experimental lepton identification requirements change the effect of the electroweak corrections.

A measurement and QCD analysis of the proton structure function F2 ( x, Q2) at HERA

A new measurement of the proton structure function F2 ( x, Q 2) is reported for momentum transfers squared Q 2 between ].5 GeV 2 and 5000 GeV 2 and for Bjorken x between 3 · 10 −5 and 0.32 using data collected by the HERA experiment H1 in 1994. The data represent an increase in statistics by a factor of ten with respect to the analysis of the 1993 data. Substantial extension of the kinematic range towards low Q 2 and x has been achieved using dedicated data samples and events with initial state photon radiation. The structure function is found to increase significantly with decreasing x, even in the lowest accessible Q 2 region. The data are well described by a Next to Leading Order QCD fit and the gluon density is extracted.

Measurement of muon-pair production at [formula omitted] at LEP

Using the data recorded with the L3 detector at LEP, we study the process e +e − → μ + μ −( γ) for events with hard initial-state photon radiation. The effective centre-of-mass energies of the muons range from 50 GeV to 86 GeV. The data sample corresponds to an integrated luminosity of 103.5 pb −1 and yields 293 muon-pair events with a hard photon along the beam direction. The events are used to determine the cross sections and the forward-backward charge asymmetries at centre-of-mass energies below the Z resonance.

Multiple-photon effects in fermion-(anti)fermion scattering at Superconducting Super Collider energies.

We use the theory of Yennie, Frautschi and Suura to realize, via Monte Carlo methods, the process $f\,\bbbarf\to f'\,\bbbarf'+n\gamma$ at SSC and LHC energies, where $f$ and $f'$ are quarks or leptons. QED infrared divergences are canceled to all orders in perturbation theory. The resulting Monte Carlo event generator, SSC-YFS2, is used to study the effects of initial-state photon radiation on these processes in the SSC environment. Sample Monte Carlo data are presented and discussed. We find that the respective multiple-photon effects must be taken into account in discussing precise predictions for SSC physics processes.

A measurement of photon radiation in lepton pair events from Z 0 decays

We have measured the photon yield in lepton pair events recorded by the OPAL detector in a data sample corresponding to an integrated luminosity of 7.1 pb −1 at centre-of-mass energies between 88 GeV and 94 GeV. The results are compared to QED expectations for initial and final state photon radiation. No anomalous photon yield has been found, and stringent limits on the branching ratio for exotic radiative three body Z 0 decays into a photon and a pair of leptons are obtained. We also place limits on possible Z 0 decays into a photon and a resonance X with subsequent decays of X into a pair of leptons. Acollinear μ + μ − events with missing momentum along the beam direction are identified as events with hard initial state photon radiation and used to measure an average cross section of 15 ± 8 6 pb for e +e − annihilation into μ + μ −, in the so far untested range of centre-of-mass energies between 60 GeV and 84 GeV. This value is consistent with a cross section of 24 pb, expected from Z 0 and photon exchange.

Experimental studies on the heavy quark fragmentation functions

The influence of perturbative QCD gluon radiation and initial state photon radiation on the experimental determination of the heavy quark fragmentation functions is studied in order to extract <z c>, the mean of the charm fragmentation function, from the recent measurements of inclusiveD * production ine + e − annihilation processes. The result is <z c>=0.71±0.014±0.03, which is scale invariant in the c.m. energy range of 10GeV to 34GeV. This result is interpreted in terms of kinematical calculations on heavy quark fragmentation functions and also compared with results fromv-N-reactions and from investigations of inclusive lepton production ine + e − annihilation. Results of a QCD shower model are in good agreement with the data and offer an alternative description of phenomenological fragmentation functions.