Initial State Radiation Research Articles

Investigation of charged Higgs bosons production from vectorlike T quark decays at eγ collider

Within the extended framework of the two-Higgs-doublet model type II (2HDM-II), enhanced by a vectorlike quark (VLQ) doublet TB, we present a comprehensive analysis of the process e−γ→bνeT¯ at future high-energy eγ colliders, focusing on the decays T¯→H−b¯ and H−→t¯b. Using current theoretical and experimental constraints, we calculate production cross sections for both unpolarized and polarized beams at center-of-mass energies of s=2 and 3 TeV, demonstrating that polarized beams significantly enhance detection prospects by increasing production rates. By analyzing kinematic distributions, we establish optimized selection criteria to effectively separate signal events from background. At s=2 TeV with an integrated luminosity of 1500 fb−1, we find exclusion regions within sRd∈[0.085,0.16] for mT∈[1000,1260] GeV and a discovery potential within sRd∈[0.14,0.17] for mT∈[1000,1100] GeV, with these regions expanding to sRd∈[0.05,0.15] for mT∈[1000,1340] GeV and sRd∈[0.11,0.17] for mT∈[1000,1160] GeV at 3000 fb−1. At s=3 TeV and 1500 fb−1, we identify exclusion regions of sRd∈[0.055,0.135] for mT∈[1000,1640] GeV and discovery regions of sRd∈[0.09,0.15] for mT∈[1000,1400] GeV, which further expand to sRd∈[0.028,0.12] for mT∈[1000,1970] GeV and sRd∈[0.04,0.122] for mT∈[1000,1760] GeV at 3000 fb−1. Our findings emphasize the increased detection potential at higher center-of-mass energies, particularly at 3 TeV compared to 2 TeV, with notable improvements when polarized beams are utilized. We also account for the effects of initial state radiation, beamstrahlung, and systematic uncertainties, which influence both exclusion and discovery prospects. Published by the American Physical Society 2025

Simulation of the process e+e−→W+W− with the heavy right-handed neutrino exchange at 1 TeV future lepton colliders

We study potential contribution of the heavy right-handed neutrino exchange in the process e+e−→W+W−. This process is sensitive to heavy neutrinos with masses larger than s. The Monte Carlo simulation of the studied process is performed assuming the seesaw type-I model, where heavy right-handed neutrinos (heavy neutral leptons, HNLs) are introduced in the leptonic sector. Within the Standard Model (SM), the process has a large cross section described by diagrams with s-channel Z/γ exchange and t-channel active neutrino exchange. Respectively, the t-channel right-handed neutrino exchange amplitude will interfere with these SM amplitudes. However, the angular distributions of the W boson production and decay are different for the right-handed neutrino and SM amplitudes. That can be used to evaluate potential HNL contribution using the extended likelihood method. The simulation of the e+e−→W+W− process is performed at the 1 TeV center-of-mass energy and polarization Pe+e− of (20%, −80%), which is a standard option for the future linear e+e− International Linear Collider. Both W bosons are reconstructed from two hadronic jets. Simulation of the SM background processes is also done. The beam-induced backgrounds and the initial state radiation effects are taken into account. The majority of background processes are effectively suppressed by the cuts on the invariant masses of two and four jets. Finally, we obtain upper limits on the mixing parameter |VeN|2 as a function of M(N). Published by the American Physical Society 2025

Semi-visible jets + X: illuminating dark showers with radiation

We investigate the potential to search for semi-visible jets (SVJs) at the Large Hadron Collider (LHC) using initial-state radiation (ISR). Both photon ISR and jet ISR channels are considered, using a benchmark signal model with the decay of a leptophobic Z′ mediator forming two SVJs. We compare and extend several techniques to decompose the missing transverse momentum into per-jet contributions, in order to reconstruct the mediator mass and to define a new observable measuring the fraction of invisible dark hadrons. The presence of ISR facilitates the identification of the SVJs, and the resulting boost improves the resolution of the observables, especially for models with high invisible fractions. We combine the two observables to propose a complete search strategy and discuss an extension of the strategy to probe the whole model parameter space.

Measurement of the e+e−→π+π−π0 cross section in the energy range 0.62–3.50 GeV at Belle II

We report a measurement of the e+e−→π+π−π0 cross section in the energy range from 0.62 to 3.50 GeV using an initial-state radiation technique. We use an e+e− data sample corresponding to 191 fb−1 of integrated luminosity, collected at a center-of-mass energy at or near the ϒ(4S) resonance with the Belle II detector at the SuperKEKB collider. Signal yields are extracted by fitting the two-photon mass distribution in e+e−→π+π−π0γ events, which involve a π0→γγ decay and an energetic photon radiated from the initial state. Signal efficiency corrections with an accuracy of 1.6% are obtained from several control data samples. The uncertainty on the cross section at the ω and ϕ resonances is dominated by the systematic uncertainty of 2.2%. The resulting cross sections in the 0.62–1.80 GeV energy range yield aμ3π=[48.91±0.23(stat)±1.07(syst)]×10−10 for the leading-order hadronic vacuum polarization contribution to the muon anomalous magnetic moment. This result differs by 2.5 standard deviations from the most precise current determination. Published by the American Physical Society 2024

Herwig 7.3 release note

A new release of the Monte Carlo event generator Herwig (version 7.3) has been launched. This iteration encompasses several enhancements over its predecessor, version 7.2. Noteworthy upgrades include: the implementation of a process-independent electroweak angular-ordered parton shower integrated with QCD and QED radiation; a new recoil scheme for initial-state radiation improving the behaviour of the angular-ordered parton shower; the incorporation of the heavy quark effective theory to refine the hadronization and decay of excited heavy mesons and heavy baryons; a dynamic strategy to regulate the kinematic threshold of cluster splittings within the cluster hadronization model; several improvements to the structure of the cluster hadronization model allowing for refined models; the possibility to extract event-by-event hadronization corrections in a well-defined way; the possibility of using the string model, with a dedicated tune. Additionally, a new tuning of the parton shower and hadronization parameters has been executed. This article discusses the novel features introduced in version 7.3.0.

Analytic forms for the e+e− annihilation cross sections around a resonance including initial state radiation

For the first time, the exact analytic integration of the product between the Kuraev-Fadin radiative function and the Born-level cross section has been achieved for e+e− annihilation near a narrow resonance. The analytic result is in a perfect agreement with the one obtained through direct numerical integration. The extraction of resonance parameters is demonstrated through the unfolding of the initial state radiation effect. The analytic integration significantly accelerates the regression process by more than 160 times compared to numerical integration. Moreover, it facilitates the determination of experimental measurements for physical parameters, such as the two-dimensional confidence region of the branching fraction and the relative phase between the strong and electromagnetic amplitudes, within just one hour instead of one week typically required for numerical integration. Published by the American Physical Society 2024

Analytical formula for the cross section of hadron production from e + e − collisions around the narrow charmouinum resonances

The paper reports an analytical formula for the production cross section of e + e − annihilation to hadrons in the vicinity of a narrow resonance, particularly in the τ-charm region, while considering initial state radiation. Despite some approximations in its derivation, comparison between the analytical formula and direct integration of ISR shows good accuracy, indicating that the analytical formula meets current experimental requirements. Furthermore, this paper presents a comparison of the cross section between the analytical formula and calculations using the ConExc Monte Carlo generator. The efficiency of the analytical formula in significantly reducing computing time makes it a favorable choice for the regression procedure to extract the parameters of narrow charmonium resonances in experiments.

Study of e+e−→π+π−π0 at s from 2.00 to 3.08 GeV at BESIII

With the data samples taken at center-of-mass energies from 2.00 to 3.08 GeV with the BESIII detector at the BEPCII collider, a partial wave analysis on the e+e−→π+π−π0 process is performed. The Born cross sections for e+e−→π+π−π0 and its intermediate processes e+e−→ρπ and ρ(1450)π are measured as functions of s. The results for e+e−→π+π−π0 are consistent with previous results measured with the initial state radiation method within one standard deviation, and improve the uncertainty by a factor of ten. By fitting the line shapes of the Born cross sections for the e+e−→ρπ and e+e−→ρ(1450)π, a structure with mass M=2119±11±15 MeV/c2 and width Γ=69±30±5 MeV is observed with a significance of 5.9σ, where the first uncertainties are statistical and the second ones are systematic. This structure can be interpreted as an excited ω state. Published by the American Physical Society 2024

Search for low-mass resonances decaying into two jets and produced in association with a photon or a jet at s=13 TeV with the ATLAS detector

A search is performed for localized excesses in the low-mass dijet invariant mass distribution, targeting a hypothetical new particle decaying into two jets and produced in association with either a high transverse momentum photon or a jet. The search uses the full Run 2 data sample from LHC proton-proton collisions collected by the ATLAS experiment at a center-of-mass energy of 13 TeV during 2015–2018. Two variants of the search are presented for each type of initial-state radiation: one that makes no jet flavor requirements and one that requires both of the jets to have been identified as containing b-hadrons. No excess is observed relative to the Standard Model prediction, and the data are used to set upper limits on the production cross section for a benchmark Z′ model and, separately, for generic, beyond the Standard Model scenarios which might produce a Gaussian-shaped contribution to dijet invariant mass distributions. The results extend the current constraints on dijet resonances to the mass range between 200 and 650 GeV. © 2024 CERN, for the ATLAS Collaboration 2024 CERN

Revisiting azimuthal angular asymmetries in diffractive di-jet production

We explore the impact of initial state soft gluon radiations on the azimuthal angle asymmetries in photo-production of hard di-jet via coherent diffraction in ultraperipheral heavy ion collisions, as well as in electron-proton (ep) and electron-nucleus (eA) collisions. The primary production mechanism is identified as the diffractive production of two hard jets, accompanied by a collinear gluon emission along the beam direction. In contrast, the diffractive exclusive di-jet production, where the initial state radiation is absent, is suppressed due to color transparency. Our analysis shows that azimuthal asymmetries, traditionally attributed to final state gluon emissions, are reduced by the presence of initial state radiations. The sensitivity of azimuthal asymmetries to both initial and final state radiations suggests that they could provide novel insights into the mechanisms of di-jet production in diffractive processes.

Higher-order NLO initial state QED radiative corrections to e+e− annihilation revisited

Radiative corrections due to initial state radiation in electron-positron annihilation are calculated within the QED structure function approach. Results are shown in the next-to-leading logarithmic approximation up to O(α4L3) order, where L=ln(s/me2) is the large logarithm. Several mistakes in previous calculations are corrected. The results are relevant for future high-precision experiments at e+e− colliders. Published by the American Physical Society 2024

Dark Matter searches with photons at the LHC

We unveil blind spot regions in dark matter (DM) direct detection (DMDD), for weakly interacting massive particles with a mass around a few hundred GeV that may reveal interesting photon signals at the LHC. We explore a scenario where the DM primarily originates from the singlet sector within the Z3-symmetric Next-to-Minimal Supersymmetric Standard Model (NMSSM). A novel DMDD spin-independent blind spot condition is revealed for singlino-dominated DM, in cases where the mass parameters of the higgsino and the singlino-dominated lightest supersymmetric particle (LSP) exhibit opposite relative signs (i.e., κ < 0), emphasizing the role of nearby bino and higgsino-like states in tempering the singlino-dominated LSP. Additionally, proximate bino and/or higgsino states can act as co-annihilation partner(s) for singlino-dominated DM, ensuring agreement with the observed relic abundance of DM. Remarkably, in scenarios involving singlino-higgsino co-annihilation, higgsino-like neutralinos can distinctly favor radiative decay modes into the singlino-dominated LSP and a photon, as opposed to decays into leptons/hadrons. In exploring this region of parameter space within the singlino-higgsino compressed scenario, we study the signal associated with at least one relatively soft photon alongside a lepton, accompanied by substantial missing transverse energy (ɆT) and a hard initial state radiation jet at the LHC. In the context of singlino-bino co-annihilation, the bino state, as the next-to-LSP, exhibits significant radiative decay into a soft photon and the LSP, enabling the possible exploration at the LHC through the triggering of this soft photon alongside large ɆT and relatively hard leptons/jets resulting from the decay of heavier higgsino-like states.

Search for an exotic vectorlike X-quark in same-sign dilepton final states at the future 3 TeV CLIC*

Abstract Vectorlike quarks (VLQs) with masses at the TeV-scale have been predicted in many new physics scenarios beyond the Standard Model (SM). Based on a simplified doublet model including the exotic vectorlike X quark (VLQ-X) with an electric charge , we study the production of the VLQ-X decaying into at the future Compact Linear Collider (CLIC) with TeV including the initial state radiation and beamstrahlung effects. We focus on the final signals, including same-sign dileptons (electrons or muons), at least one b-tagged jet, and large missing transverse momentum. By performing detailed signal-to-background analyses and detector simulations, we obtain the exclusion capabilities and discovery reaches, respectively, on the mass of the VLQ-X as well as the relevant parameters ( for GeV and the coupling strength for GeV) for some typical luminosities at the 3 TeV CLIC.

Measurements of Σ electromagnetic form factors in the timelike region using the untagged initial-state radiation technique

The process e+e−→Σ+Σ¯− is studied from threshold up to 3.04 GeV/c2 via the initial-state radiation technique using data with an integrated luminosity of 12.0 fb−1, collected at center-of-mass energies between 3.773 and 4.258 GeV with the BESIII detector at the BEPCII collider. The pair production cross sections and the effective form factors of Σ are measured in eleven Σ+Σ¯− invariant mass intervals from threshold to 3.04 GeV/c2. The results are consistent with the previous results from Belle and BESIII. Furthermore, the branching fractions of the decays J/ψ→Σ+Σ¯− and ψ(3686)→Σ+Σ¯− are determined and the obtained results are consistent with the previous results of BESIII. Published by the American Physical Society 2024

Forward production of a Drell-Yan pair and a jet at small x at next-to-leading order

We perform the analytical next-to-leading order calculation of the process p + A γ∗ + jet + X, at forward rapidities and low x. These kinematics justify a hybrid approach, where a quark from the ‘projectile’ proton scatters off the gluon distribution of the ‘target’, which can be a nucleus or a highly boosted proton. By using the Color Glass Condensate effective theory approach, this gluon distribution is allowed to be so dense that the quark undergoes multiple scattering. Moreover, large high-energy logarithms in the ratio of the hard scale and the center-of-mass energy are resummed by the Balitsky, Kovchegov, Jalilian-Marian, Iancu, McLerran, Weigert, Leonidov, Kovner or BK-JIMWLK evolution equations. We demonstrate that all ultraviolet divergences encountered in the calculation cancel, while the high-energy divergences are absorbed into BK-JIMWLK. The remaining singularities are collinear in nature and can be either absorbed into the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution of the incoming quark, when they stem from initial-state radiation, or else can be treated by a jet function in case they are caused by final-state emissions. The resulting cross section is completely finite and expressed in function of only a small set of color operators.

Search for the Y(2175) in photoproduction at GlueX

Abstract. The Y(2175), recently renamed to ϕ(2170), is one of the rare exotic candidates connected to strangeonium as compared to the heavier charmonium-like and bottomonium-like exotic states. Originally observed in initial state radiation events by the BaBar experiment in 2006, it is suggested to be a strange partner state of the charmonium-like exotic vector state Y(4230) that was one of the first of the so-called XYZ states reported in the charmonium mass region. The Y(2175) has been seen in various experiments and decay channels,but it has not yet been searched for in a photo-production experiment. In this talk we report on a measurement of the production cross section of the reaction γ + p → ϕπ+ π− + p based on data recorded by the GlueX experiment in order to search for Y(2175) → ϕπ+π−.&gt;

Small Angle Initial State Radiation Analysis of the Pion Form Factor at BESIII

The time-like pion form factor (FF), i.e. the cross section of the process e+e− → π+π−, is presently a matter of intense discussion within the particle physics community. The e+e− → π+π− process is the largest contributor to the hadronic vacuum polarization (HVP) correction in the Standard Model prediction of the muon anomalous magnetic moment (aμ). Recent lattice QCD evaluations of the HVP, as well as a new measurement of the pion FF by the CMD-3 collaboration, have hinted at possible shortcomings in previous measurements, thus preventing an unambiguous interpretation of the latest experimental value of aμ. An intense effort is ongoing to provide new experimental insights into the pion FF, and the BESIII collaboration aims to contribute with a new measurement of world-leading precision. In this work, preliminary studies towards such a measurement are presented.

R Measurement at BESIII

The R value is measured with a total of 14 data points with the corresponding center-of-mass energy going from 2.2324 to 3.6710 GeV. The statistical uncertainty of the measured R is less than 0.6%. Two different simulation models, the LUARLW and a new Hybrid generator, are used and give consistent detection and initial-state radiation corrections. An accuracy of better than 2.6% below 3.1 GeV and 3.0% above is achieved for the R value. The precise measurement will be an important constraint for precision tests of the Standard Model, such as calculations of the anomalous magnetic moment of the moun and the running of the QED coupling.

Prospects for constraining light-quark electroweak couplings at Higgs factories

Electroweak Precision Measurements are stringent tests of the Standard Model and sensitive probes to New Physics. Accurate studies of the Z-boson couplings to the first-generation quarks could reveal potential discrepancies between the fundamental theory and experimental data. Future e+e− colliders running at the Z-pole and around the ZH threshold would be an excellent tool to perform such a measurement, unlike the LHC where hadronic Z decays are only available in boosted topologies. The measurement is based on comparison of radiative and non-radiative hadronic decays. Due to the difference in quark charge, the relative contribution of the events with final-state radiation (FSR) directly reflects the ratio of decays involving up-and down-type quarks. Such an analysis requires proper modeling and statistical discrimination between photons coming from different sources, including initial-state radiation (ISR), FSR, parton showers and hadronisation. In our contribution, we show how to extract the values of the Z couplings to light quarks and present the estimated uncertainties of the measurement.

A simulation study of the soft and hard radiations using jets at the LHC

In this work, different aspects of the high-energy radiation are looked at considering the LHC scenario. An event-shape variable and several jet substructure observables are studied with the Mote Carlo event simulators at the 13 TeV center of mass energy scale to mimic the current LHC environment. The event-shape and the jet substructure observables are chosen such that they are not only sensitive to the different aspects of the high energy radiation measurement but also exhibit promising features to distinguish the possible existence of new physics that considers a dark matter candidate decaying into semi-visible jet. It is verified that the observables exhibit significant sensitivities to disentangle two jets to multi-jet radiations, presence of a final state and initial state radiations, presence of a large amount of missing transverse energy as a strong indication of the possible existence of a dark matter as well as couple of promising features of a semi-visible jet are explored.