Initial State Radiation Research Articles

Systematic studies of exact O(α2L) CEEX EW corrections in a hadronic MC for precision Z/γ* physics at LHC energies

With an eye toward the precision physics of the LHC, such as the recent measurement of $M_W$ by the ATLAS Collaboration, we present here systematic studies relevant to the assessment of the expected size of multiple photon radiative effects in heavy gauge boson production with decay to charged lepton pairs. We use the new version 4.22 of ${\cal KK}$MC-hh so that we have CEEX EW exact ${\cal O}(\alpha^2 L)$ corrections in a hadronic MC and control over the corresponding EW initial-final interference (IFI) effects as well. In this way, we illustrate the interplay between cuts of the type used in the measurement of $M_W$ at the LHC and the sizes of the expected responses of the attendant higher order corrections. We find that there are per cent to per mille level effects in the initial-state radiation, fractional per mille level effects in the IFI and per mille level effects in the over-all ${\cal O}(\alpha^2 L)$ corrections that any treatment of EW corrections at the per mille level should consider. Our results have direct applicability to current LHC experimental data analyses.

Enhancing Long-Lived Particles Searches at the LHC with Precision Timing Information.

We explore the physics potential of using precision timing information at the LHC in searches for long-lived particles (LLPs). In comparison with the light standard model particles, the decay products of massive LLPs arrive at detectors with time delays around the nanosecond scale. We propose new strategies to take advantage of this time delay feature by using initial state radiation to time stamp the collision event and require at least one LLP to decay within the detector. This search strategy is effective for a broad range of models. In addition to outlining this general approach, we demonstrate its effectiveness with the projected reach for two benchmark scenarios: a Higgs boson decaying into a pair of LLPs, and pair production of long-lived neutralinos in the gauge mediated supersymmetry breaking models. Our strategy increases the sensitivity to the lifetime of the LLP by two orders of magnitude or more and particularly exhibits a better behavior with a linear dependence on the lifetime in the large lifetime region compared to traditional LLP searches. The timing information significantly reduces the standard model background and provides a powerful new dimension for LLP searches.

Angular Analysis of the e+e− → D(*)±D*∓ Process Near the Open Charm Threshold with Initial-State Radiation

The results of measurements of the exclusive cross section for the e+e− → D(*)±D*∓ process near the open charm threshold with initial-state radiation are presented. The analysis is based on the experimental data collected with the Belle detector at energies $$\sqrt s $$ equal to ϒ(4S), ϒ(5S) production energies, and energies of the nearby continuum corresponding to an integrated luminosity of 951 fb−1. The accuracy of the measurements of the cross sections is twice higher in comparison with the previous results of the Belle collaboration. An angular analysis is performed for the final state D*+D*− for the first time with the result that the cross section is decomposed into three components corresponding to the possible combinations of D*-meson polarizations in the final state.

Performance study of the full hadronic WW and ZZ events\u2019 separation at the CEPC

The full hadronic WW and ZZ events’ separation is an important benchmark for the Circular Electron Positron Collider (CEPC) detector design and reconstruction algorithm development. This separation performance is determined by the intrinsic boson mass distributions, the detector performance, and the jet confusion. The latter refers to the uncertainties induced by the jet clustering and pairing algorithms. Using the CEPC baseline simulation, we demonstrate that the full hadronic WW and ZZ events can be efficiently separated. We develop an analytic method that quantifies the impact of each component and conclude that the jet confusion dominates the separation performance. The impacts of the Initial State Radiation (ISR) and the heavy flavor jets are also analyzed and confirmed to be critical for the separation performance.

Joint two-dimensional resummation in qT and 0-jettiness at NNLL

We consider Drell-Yan production pp → Z/γ∗ → ℓ+ℓ− with the simultaneous measurement of the Z-boson transverse momentum qT and 0-jettiness {mathcal{T}}_0 . Since both observables resolve the initial-state QCD radiation, the double-differential cross section in qT and {mathcal{T}}_0 contains Sudakov double logarithms of both qT/Q and {mathcal{T}}_0/Q , where Q ∼ mZ is the dilepton invariant mass. We simultaneously resum the logarithms in qT and {mathcal{T}}_0 to next-to-next-to-leading logarithmic order (NNLL) matched to next-to-leading fixed order (NLO). Our results provide the first genuinely two-dimensional analytic Sudakov resummation for initial-state radiation. Integrating the resummed double-differential spectrum with an appropriate scale choice over either {mathcal{T}}_0 or qT recovers the corresponding single-differential resummation for the remaining variable. We discuss in detail the required effective field theory setups and their combination using two-dimensional resummation profile scales. We also introduce a new method to perform the qT resummation where the underlying resummation is carried out in impact-parameter space, but is consistently turned off depending on the momentum-space target value for qT. Our methods apply at any order and for any color-singlet production process, such that our results can be systematically extended when the relevant perturbative ingredients become available.

The O(α2) initial state QED corrections to e+e− annihilation to a neutral vector boson revisited

We calculate the non-singlet, the pure singlet contribution, and their interference term, at O(α2) due to electron-pair initial state radiation to e+e− annihilation into a neutral vector boson in a direct analytic computation without any approximation. The correction is represented in terms of iterated incomplete elliptic integrals. Performing the limit s≫me2 we find discrepancies with the earlier results of Ref. [1] and confirm results obtained in Ref. [2] where the effective method of massive operator matrix elements has been used, which works for all but the power corrections in m2/s. In this way, we also confirm the validity of the factorization of massive partons in the Drell–Yan process. We also add non-logarithmic terms at O(α2) which have not been considered in [1]. The corrections are of central importance for precision analyses in e+e− annihilation into γ⁎/Z⁎ at high luminosity.

Probing underlying event in Z-boson events using event shape observables

Experimental measurements of observables sensitive to the underlying event (UE) in Z-boson events have been performed by both ATLAS and CMS experiments at the LHC. However, in the busy LHC environment, these observables receive substantial contribution from jets originating from initial state radiation (ISR). We probe if using event shape observables in conjunction with the UE observables can help us to disentangle the effect of the UE from jets originating in ISR.

Compressed stop searches with two leptons and two b-jets

In top squark (stop) searches with a compressed spectrum, it is very helpful to consider the stop production recoiling against a hard jet from the initial state radiation to obtain a significant amount of missing transverse energy. In particular, the kinematic variable RM which measures the ratio of the lightest neutralino mass and the stop mass proved to be crucial in separating the signals from the backgrounds in both the all-hadronic decay and the semileptonic decay of the stops. Here we generalize the search method to the dileptonic stop decays. In this case, due to the two missing neutrinos, there are not enough kinematic constraint equations to solve for the RM variable exactly, but only render an allowed interval consistent with the event. However, we show that the minimum and the maximum values of this allowed interval still provide useful variables in discriminating signals from the backgrounds. Although in the traditional stop decay to a top quark and the lightest neutralino, the dileptonic mode is not as competitive due to its small branching ratio, it becomes the main search mode if the stops decay through the charginos and sleptons with a compressed spectrum. We show that with the new variables, the dileptonic search of the stop can cover regions of the parameter space which have not been constrained before.

Constraining R-axion models through dijet searches at the LHC

The search at hadron colliders for new massive resonances of a few 100 GeVs that couple effectively to colored states is an extremely challenging issue, due principally to the presence of large QCD multijet backgrounds at this energy, rendering the searches at the LHC particularly difficult. Recently, it was realized that these large backgrounds could be overcome by demanding one high-pT jet from initial-state radiation (ISR) and by means of novel jet-reconstruction techniques through which the resulting hadronized products of the massive resonances are reconstructed as a fat-jet, a unique large-radius jet. The ATLAS and CMS Collaborations have recently reported searches for the experimental signature of a single fat-jet in association with an ISR jet. Models of dynamical supersymmetry breaking with an spontaneously broken R-symmetry give rise to the appearance of a pseudo-Nambu–Goldstone boson called the R-axion, which naturally tends to be light. In the parameter space regions where the anomalous R-axion coupling to gluons is boosted, these models can be tested against these new LHC dijet searches. Taking into account the CMS search, we apply the qμ statistical method to the signal events against the background-only expectation and obtain the 95% C.L. exclusion limits on the most relevant model parameters for a particular messenger sector, namely, the R-axion mass ma, the decay constant fa, and the number of color messengers N, being these limits suitable to be applied to more general models with axion-like particles.

Angular analysis of the e+e- → D(*)D* process near the open-charm threshold using initial-state radiation

We report new measurement of the exclusive e+e- → D(*)±D*∓ cross sections as a function of center-of-mass energy from the D(∗)±D∗∓ threshold up to $ \sqrt s $ = 6.0 GeV with initial state radiation. The analysis is based on a data sample collected by the Belle detector with an integrated luminosity of 951 fb−1. The accuracy of the cross section measurement is increased by a factor of 2 in comparison with the first Belle study. We have performed the first angular analysis of the e+e- → D∗±D∗∓ process and decomposed this exclusive cross section into three components corresponding to the different D∗ helicities.

New results on charmonium like states at Belle

New results on charmonium-like states measured by the Belle experiment at the KEKB asymmetric e+e- collider are presented, in particular (a) Search for the Zc pair production in $\Upsilon (1{\rm{S}}),\,\Upsilon (2S)$ decays and in e+e- $\sqrt s $ of 10.52, 10.58, and 10.867 GeV. (b) Measurement of the absolute branching fractions of ${B^ + } \to {X_{c\overline c }}{K^ + }$ decays. (c) Observation of Ξc(2930)0 in the updated measurement of ${B^ - } \to {K^ - }{\Lambda _c}{\overline \Lambda _{\overline c }}.$ (d) Angular analysis of ${e^ + }{e^ - } \to {D^{(*) \pm }}{D^{* \mp }}$ process near the open charm threshold using initial state radiation.

An extension of BDK event generator

An extension of the well known ee → 4l event generator of F. A. Berends, P. H. Daverveldt, and R. Kleiss is presented. This extension accounts for production of narrow resonances, as well as the initial state radiation. In addition, it allows one to simulate the production of l+ l−π+π− with the lepton pair and the pion pair in 1−− states.

Initial State Radiation Measurements at BESIII

Precision measurements of the hadronic cross sections e+ e−→ π+ π− , π+π−π0, π+π−2π0, ωπ0 and π+π−3π0 are performed at the BESIII experiment located in Beijing, China. These cross section measurements are needed as input for the Standard Model prediction of the anomalous magnetic moment of the muon, (g − 2)µ.The initial state radiation method is used to access the energy regions of interest for (g − 2)µ. Our results will improve the precision of the Standard Model prediction of (g − 2)µ .

Measurements of Hadronic and Transition Form Factors at BESIII

Motivated by the need of experimental input to improve the Standard Model prediction of the anomalous magnetic moment of the muon aµ, the BESIII Collaboration started a dedicated program to measure hadronic cross sections as well as transition form factors (TFF) with high accuracy. The large data sets acquired by the BESIII Collaboration allow to exploit initial state radiation in order to study hadron production over a wide energy range, as well as two-photon collisions to study the momentum dependence of TFFs in the space like regime. The current status and ongoing investigations in both endeavors are discussed.

Event Generators for Multi Hadronic Production in e+e− Collisions

An event generator for multi hadron production is presented for measuring the R value in the τ-charm energy region with e+e− collisions. The initial state radiation effects are considered up to second order accuracy, and the radiative correction factor is calculated with hadronic Born cross sections. The established exclusive processes are generated according to their measured cross sections, while the missing processes are generated using the LUND Area Law model, and its parameters are tuned with data collected at $ \sqrt s $ = 3.08 GeV. The optimized values are validated with data in the range $ \sqrt s $= 2.2324 ∼ 3.671 GeV. These optimized parameters are universally valid for event generation below the DD¯threshold.

BESIII results on time-like baryon form factors

This contribution reports on the recent measurements of baryon electromagnetic form factors at the BESIII experiment in Beijing. The Beijing e+e − collider BEPCII is a double-ring symmetric collider running at $ \sqrt s $ between 2.0 and 4.6 GeV. Baryon electromagnetic form factors can be measured at BESIII in direct e+e−-annihilation and in initial state radiation processes. Based on the data collected by the BESIII detector at 12 center of mass energies between 2.23 and 3.67 GeV, the measured $ \mathop e\nolimits^ + \mathop e\nolimits^ - \to \mathop p\limits^ - p $ cross section and the time-like proton form factor are presented. Preliminary results from the analysis of the initial state radiation process $ \mathop e\nolimits^ + \mathop e\nolimits^ - \to \mathop p\limits^ - p\gamma $ using a data set of 7.408 fb−1 collected at center of mass energies between 3.773 and 4.6 GeV, are also shown. The cross sections for $ \mathop e\nolimits^ + \mathop e\nolimits^ - \to \mathop {\mathop \Lambda \nolimits_c^ - }\limits^ - \mathop \Lambda \nolimits_c^ + $ and $ \mathop e\nolimits^ + \mathop e\nolimits^ - \to \mathop \Lambda \limits^ - \Lambda $ are measured near threshold with unprecedented precision. Preliminary results on the first measurement of the ^c form factor ratio |GE|/|GM| are also given. Ongoing analysis based on the high luminosity energy scan from 2015 and from radiative return at different $ \sqrt s $ are also described.

Open charm studies at Belle

We present the new results of measurements of the exlusive cross sections of the e+ e− annihilation to charmed meson pairs as a function of √ ceneter-of-mass energy from the open charm threashold up to $ \sqrt s = {\rm{6 GeV}} $ using initial state radiation technique. The analysis is based on a data sample collected by the Belle detector with an integrated luminosity of 951 fb−1. The accuracy of the cross section measurement is increased by a factor of 2 in comparison with the first Belle study. We have performed the first angular analysis + −→ D*± D*∓ of the $ {e^{\rm{ + }}}{e^ - } \to {D^{* \pm }}{D^{* \mp }} $ process and decomposed this exclusive cross section into three components corresponding to the different D* helicities.

Actual Physics behind Mono-X

Mono-X searches are standard dark matter search strategies at the LHC. First, we show how in the case of initial state radiation they essentially collapse to mono-jet searches. Second, we systematically study mono-X signatures from decays of heavier dark matter states. Direct detection constraints strongly limit our MSSM expectations, but largely vanish for mono-Z and mono-Higgs signals once we include light NMSSM mediators. Finally, the decay topology motivates mono-W-pair and mono-Higgs-pair searches, strengthening and complementing their mono-X counterparts.

A forward branching phase space generator for hadron colliders

In this paper we develop a projective phase space generator appropriate for hadron collider geometry. The generator integrates over bremsstrahlung events which project back to a single, fixed Born event. The projection is dictated by the experimental jet algorithm allowing for the forward branching phase space generator to integrate out the jet masses and initial state radiation. When integrating over the virtual and bremsstrahlung amplitudes this results in a single K-factor, assigning an event probability to each Born event. This K-factor is calculable as a perturbative expansion in the strong coupling constant.One can build observables from the Born kinematics, giving identical results to traditional observables as long as the observable does not depend on the infrared sensitive jet mass or initial state radiation.

From underlying event sensitive to insensitive: factorization and resummation

In this paper we study the transverse energy spectrum for the Drell-Yan process. The transverse energy is measured within the central region defined by a (pseudo-) rapidity cutoff. Soft-collinear effective theory (SCET) is used to factorize the cross section and resum large logarithms of the rapidity cutoff and ratios of widely separated scales that appear in the fixed order result. We develop a framework which can smoothly interpolate between various regions of the spectrum and eventually match onto the fixed order result. This way a reliable calculation is obtained for the contribution of the initial state radiation to the measurement. By comparing our result for Drell-Yan against Pythia we obtain a simple model that describes the contribution from multiparton interactions (MPI). A model with little or no dependence on the primary process gives results in agreement with the simulation. Based on this observation we propose MPI insensitive measurements. These observables are insensitive to the MPI contributions as implemented in Pythia and we compare against the purely perturbative result obtained with the standard collinear factorization.