Hadronic Events Research Articles

Novel Method to Reliably Determine the Photon Helicity in B→K_{1}γ.

A sizable right-handed photon polarization in b→sγ is a clear signal for new physics. In this Letter, we point out that the photon helicity in b→sγ can be unambiguously extracted by combining the measurements in B→K_{1}γ and the Cabibbo-favored D→K_{1}e^{+}ν decay. We propose a ratio of up-down asymmetries in D→K_{1}e^{+}ν to quantify the hadronic effects. A method for measuring, in experiment, the involved partial decay widths in the ratio is discussed, and experimental facilities like BESIII, Belle-II and LHCb are likely to measure this ratio. We also give the angular distribution that is useful for extracting the photon polarization in the presence of different kaon resonances.

Signal-background interference for digluon resonances at the Large Hadron Collider

We study the interference between the amplitudes for $gg \rightarrow X \rightarrow gg$, where $X$ is a new heavy digluon resonance, and the QCD background $gg \rightarrow gg$, at the Large Hadron Collider. The interference produces a large low-mass tail and a deficit of events above the resonance mass, compared to the naive pure resonance peak. For a variety of different resonance quantum numbers and masses, we evaluate the signal-background interference contribution at leading order, including showering, hadronization, and detector effects. The resulting new physics dijet mass distribution may have a shape that appears, after QCD background fitting and subtraction, to resemble an enhanced peak, a shelf, a peak/dip, or even a pure dip. We argue that the true limits on new digluon resonances are likely to differ significantly from the limits obtained when interference is neglected, especially if the branching ratio to $gg$ is less than 1.

The {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil in HQET

In this paper we discuss the Standard Model and new physics sensitivity of the {Lambda}_bto {Lambda}^{ast }(1520)left(to Noverline{K}right){mathrm{ell}}^{+}{mathrm{ell}}^{-} decay at low-recoil, where ℓ± are massive leptons and Noverline{K}=left{{pK}^{-},n{overline{K}}^0right} . We provide a full angular distribution with a set of operators that includes the Standard Model operators and their chirality flipped counterparts, and new scalar and pseudo-scalar operators. The resulting angular distribution allows us to construct observables that we study in the Standard Model and in model-independent new physics scenarios. To reduce the hadronic effects emanating from the Λb→ Λ* (1520) transition form factors, we exploit the Heavy Quark Effective theory framework valid at low Λ∗(1520) recoil, i.e., large dilepton invariant mass squared {q}^2sim mathcal{O}left({m}_b^2right) . Working to the leading order in 1/mb and including mathcal{O}left({alpha}_sright) corrections, we compute the ‘improved Isgur-Wise relations’ between the form factors. The relations correlate the form factors and thereby allow the description of this decay at the low-recoil region with a smaller number of independent form factors.

Higgs production at NNLL\u2032+NNLO using rapidity dependent jet vetoes

The rapidity-dependent jet veto observables {mathcal{T}}_{Bj} and {mathcal{T}}_{Cj} provide a tight jet veto at central rapidity, gradually transitioning to a loose veto at forward rapidities. They divide the phase space into exclusive jet bins in a different way to the traditional jet veto observable pT j, and are advantageous to use under harsh pile-up conditions. We obtain predictions for the 0-jet gluon-fusion (ggF) Higgs cross section using both of these veto observables at NNLL′+NNLO, and compare these predictions to the prior state-of-the-art of NLL′+NLO. A significant reduction in perturbative uncertainty is observed going from NLL′+NLO to NNLL′+NNLO, with the NNLL′+NNLO predictions lying inside the uncertainty band of the NLL′+NLO predictions. We also investigate the relative sensitivities of ggF Higgs cross sections with {mathcal{T}}_{Bj} , {mathcal{T}}_{Cj} and pT j jet vetoes to underlying event and hadronisation effects using an NLO+parton shower calculation. We find that the cross sections with {mathcal{T}}_{Bj} and {mathcal{T}}_{Cj} vetoes have a reduced sensitivity to underlying event and hadronisation effects compared to that with a pT j veto.

Technique for Separating Electrons in a Wide PAMELA Calorimeter

In this study, to separate electron- and proton-induced cascades in the calorimeter at energies above 300 GeV, it is proposed to use the difference in the cascade absorption lengths of these particles. The applicability of the proposed criterion to reject hadron events in selecting electron-induced cascades in the “PAMELA” calorimeter is studied. The efficiency and applicability limits of this separation method are estimated.

Dispersion relations for hadronic light-by-light and the muon g − 2

The largest uncertainties in the Standard Model calculation of the anomalous magnetic moment of the muon (g−2)µ come from hadronic effects, namely hadronic vacuum polarization (HVP) and hadronic lightby-light (HLbL) contributions. Especially the latter is emerging as a potential roadblock for a more accurate determination of (g−2)µ. The main focus here is on a novel dispersive description of the HLbL tensor, which is based on unitarity, analyticity, crossing symmetry, and gauge invariance. This opens up the possibility of a data-driven determination of the HLbL contribution to (g−2)µ with the aim of reducing model dependence and achieving a reliable error estimate. Our dispersive approach defines unambiguously the pion-pole and the pion-box contribution to the HLbL tensor. Using Mandelstam double-spectral representation, we have proven that the pion-box contribution coincides exactly with the one-loop scalar-QED amplitude, multiplied by the appropriate pion vector form factors. Using dispersive fits to high-statistics data for the pion vector form factor, we obtain $ \alpha _\mu ^{\pi {\rm{ - box}}} = - 15.9(2) \times {10^{ - 11}} $. A first model-independent calculation of effects of ππ intermediate states that go beyond the scalar-QED pion loop is also presented. We combine our dispersive description of the HLbL tensor with a partial-wave expansion and demonstrate that the known scalar-QED result is recovered after partial-wave resummation. After constructing suitable input for the γ*γ* → ππ helicity partial waves based on a pion-pole left-hand cut (LHC), we find that for the dominant charged-pion contribution this representation is consistent with the two-loop chiral prediction and the COMPASS measurement for the pion polarizability. This allows us to reliably estimate S-wave rescattering effects to the full pion box and leads to $ \alpha _\mu ^{\pi {\rm{ - box}}} + \alpha _{\mu ,J = 0}^{\pi \pi ,\pi {\rm{ - pole}}\,{\rm{LHC}}} = - 24(1) \times {10^{ - 11}} $.

Nonperturbative corrections to soft drop jet mass

We provide a quantum field theory based description of the nonperturbative effects from hadronization for soft drop groomed jet mass distributions using the soft- collinear effective theory and the coherent branching formalism. There are two distinct regions of jet mass mJ where grooming modifies hadronization effects. In a region with intermediate mJ an operator expansion can be used, and the leading power corrections are given by three universal nonperturbative parameters that are independent of all kinematic variables and grooming parameters, and only depend on whether the parton initiating the jet is a quark or gluon. The leading power corrections in this region cannot be described by a standard normalized shape function. These power corrections depend on the kinematics of the subjet that stops soft drop through short distance coefficients, which encode a perturbatively calculable dependence on the jet transverse momentum, jet rapidity, and on the soft drop grooming parameters zcut and β. Determining this dependence requires a resummation of large logarithms, which we carry out at LL order. For smaller mJ there is a nonperturbative region described by a one-dimensional shape function that is unusual because it is not normalized to unity, and has a non-trivial dependence on β.

New factorization theory for heavy quarkonium production and decay

The widely used nonrelativistic QCD (NRQCD) factorization theory now encounters some notable difficulties in describing quarkonium production. This may be due to the inadequate treatment of soft hadrons emitted in the hadronization process, which causes bad convergence of velocity expansion in NRQCD. In this paper, starting from QCD we propose a rigorously defined factorization approach, soft gluon factorization (SGF), to better deal with the effects of soft hadrons. After a careful velocity expansion, the SGF can be as simple as the NRQCD factorization in phenomenological studies, but has a much better convergence. The SGF may provide a new insight to understand the mechanisms of quarkonium production and decay.

Fitting the strong coupling constant with soft-drop thrust

Soft drop has been shown to reduce hadronisation effects at e+e− colliders for the thrust event shape. In this context, we perform fits of the strong coupling constant for the soft-drop thrust distribution at NLO+NLL accuracy to pseudo data generated by the Sherpa event generator. In particular, we focus on the impact of hadronisation corrections, which we estimate both with an analytical model and a Monte-Carlo based one, on the fitted value of αs(mZ). We find that grooming can reduce the size of the shift in the fitted value of αs due to hadronisation. In addition, we also explore the possibility of extending the fitting range down to significantly lower values of (one minus) thrust. Here, soft drop is shown to play a crucial role, allowing us to maintain good fit qualities and stable values of the fitted strong coupling. The results of these studies show that soft-drop thrust is a promising candidate for fitting αs at e+e− colliders with reduced impact of hadronisation effects.

Prospects for disentangling long- and short-distance effects in the decays B \u2192 K\u2217\u03bc+\u03bc\u2212

Theory uncertainties on non-local hadronic effects limit the New Physics discovery potential of the rare decays B → K∗μ+μ−. We investigate prospects to disentangle New Physics effects in the short-distance coefficients from these effects. Our approach makes use of an event-by-event amplitude analysis, and relies on a particular parametrisation of the non-local contributions. We find that non-standard effects in the short-distance coefficients can be successfully disentangled from non-local hadronic effects. The impact of the truncation on the parametrisation of non-local contributions to the Wilson coefficients are for the first time systematically examined and prospects for their precise determination are discussed. Theoretical inputs on the non-local matrix elements beyond the physically- accessible phase space are crucial to stabilise the determination of Wilson coefficients, while we find that physical observables are unaffected by these uncertainties. Compared to other methods, our approach provides for a more precise extraction of the angular observables from data.

Dreaming awake: disentangling the underlying physics in case of a SUSY-like discovery at the LHC

The purpose of this review is to investigate what kind of physics can be extracted at the Large Hadron Collider (LHC), assuming a discovery is made in events with missing transverse momentum, as generically expected in supersymmetry (SUSY) with R-parity conservation. To set the scene, we first discuss the collider phenomenology of the six possible electroweakino benchmark scenarios, as they provide valuable insight into what one might be facing at the LHC. We review the existing methods for mass reconstruction from measured kinematic endpoints in the distributions of suitable variables, e.g. the invariant masses of various sets of visible decay products, as well as the MT2 and the M2 types of variables. We propose to extend the application of these methods to the various topologies of fully hadronic final states, possibly with hadronically reconstructed massive bosons (W, Z or h). We test the idea with a simplified simulation of events in the main electroweakino benchmark scenarios. We find that the fully hadronic events allow the complete determination of the relevant mass spectrum. For comparison, we also review the potential of the standard kinematic endpoint methods for final states involving leptons from the decays of (on-shell or off-shell) sleptons. We find that with 300 fb−1, the statistics for the leptonic events is very marginal and they look less promising than the fully hadronic channels. This corresponds to a complete reversal of the usual paradigm, where leptonic events comprised the gold-plated SUSY channels. Finally, we put together all available information and summarize what level of understanding of the underlying physics can be achieved. We show that, as a by-product of the mass reconstruction, it is also possible to determine the production cross sections and decay branching ratios, which in turn enable us to pinpoint the underlying model.

New Physics in {varvec{b rightarrow s ell ^+ ell ^-}} confronts new data on lepton universality

In light of the very recent updates on the R_K and R_{K^*} measurements from the LHCb and Belle collaborations, we systematically explore here imprints of New Physics in b rightarrow s ell ^+ ell ^- transitions using the language of effective field theories. We focus on effects that violate Lepton Flavour Universality both in the Weak Effective Theory and in the Standard Model Effective Field Theory. In the Weak Effective Theory we find a preference for scenarios with the simultaneous presence of two operators, a left-handed quark current with vector muon coupling and a right-handed quark current with axial muon coupling, irrespective of the treatment of hadronic uncertainties. In the Standard Model Effective Field Theory we select different scenarios according to the treatment of hadronic effects: while an aggressive estimate of hadronic uncertainties points to the simultaneous presence of two operators, one with left-handed quark and muon couplings and one with left-handed quark and right-handed muon couplings, a more conservative treatment of hadronic matrix elements leaves room for a broader set of scenarios, including the one involving only the purely left-handed operator with muon coupling.

Probing transverse-momentum distributions with groomed jets

We present the transverse momentum spectrum of groomed jets in di-jet events for e+e− collisions and semi-inclusive deep inelastic scattering (SIDIS). The jets are groomed using a soft-drop grooming algorithm which helps in mitigating effects of non-global logarithms and underlying event. At the same time, by reducing the final state hadronization effects, it provides a clean access to the non-perturbative part of the evolution of transverse momentum dependent (TMD) distributions. In SIDIS experiments we look at the transverse momentum of the groomed jet measured w.r.t. the incoming hadron in the Breit frame. Because the final state hadronization effects are significantly reduced, the SIDIS case allows to probe the TMD parton distribution functions. We discuss the sources of non-perturbative effects in the low transverse momentum region including novel (but small) effects that arise due to grooming. We derive a factorization theorem within SCET and resum any large logarithm in the measured transverse momentum up to NNLL accuracy using the ζ-prescription as implemented in the artemide package and provide a comparison with simulations.

Metric Space of Collider Events.

When are two collider events similar? Despite the simplicity and generality of this question, there is no established notion of the distance between two events. To address this question, we develop a metric for the space of collider events based on the earth mover's distance: the "work" required to rearrange the radiation pattern of one event into another. We expose interesting connections between this metric and the structure of infrared- and collinear-safe observables, providing a novel technique to quantify event modifications due to hadronization, pileup, and detector effects. We showcase how this metrization unlocks powerful new tools for analyzing and visualizing collider data without relying upon a choice of observables. More broadly, this framework paves the way for data-driven collider phenomenology without specialized observables or machine learning models.

Studies on General Kinematic Effects of Hadrons in Electron-Positron Annihilation at High Energy

Studies on General Kinematic Effects of Hadrons in Electron-Positron Annihilation at High Energy

Searching for the dead cone effects with iterative declustering of heavy-flavor jets

We present a new method to expose the dead cone effect at colliders using iterative declustering techniques. Iterative declustering allows to unwind the jet clustering and to access the subjets or branches at different depths of the jet tree. Our method consists on declustering the heavy flavour-tagged jet using Cambridge-Achen algorithm following the branch containing the heavy flavour at each step and registering the kinematics of the complementary untagged prong. The kinematics of the complementary untagged prong fill a Lund map representing the gluon radiation off the heavy flavour quark at each step of the vacuum shower. Using Pythia8 MC, we show that a simple cut on the Lund plane introduced by log($k_{T}$) $> 0$, suppresses hadronisation effects and the angular separation between the jet prongs becomes very sensitive to flavour effects. A clear suppression for heavy flavour jets relative to inclusive jets in the region of splitting angles delimited by the relation $\theta < m_{Q}/E$ is observed, where $m_{Q}$ is the mass of the heavy quark and $E$ is the energy of the radiator or splitting prong.

Thermodynamics of strongly interacting matter in a hybrid model

The equation of state and fluctuations of conserved charges in a strongly interacting medium under equilibrium conditions form the baseline upon which various possible scenarios in relativistic heavy-ion collision experiments are built. Many of these quantities have been obtained in the lattice QCD framework with reliable continuum extrapolations. Recently the Polyakov$-$Nambu$-$Jona-Lasinio model has been reparametrized to some extent to reproduce quantitatively the lattice QCD equation of state at vanishing chemical potentials. The agreement was precise except at low temperatures, possibly due to inadequate representation of the hadronic degrees of freedom in the model. This disagreement was also observed for some of the fluctuation and correlations considered. Here we address this issue by introducing the effects of hadrons through the Hadron Resonance Gas model. The total thermodynamic potential is now a weighted sum of the thermodynamic potential of the Polyakov$-$Nambu$-$Jona-Lasinio model and that of the Hadron Resonance Gas model. We find that the equation of state and the fluctuations and correlations obtained in this hybrid model agrees satisfactorily with the lattice QCD data in the low temperature regime.

Modification of Jet Structure in High-Multiplicity pp Collisions due to Multiple-Parton Interactions and Observing a Multiplicity-Independent Characteristic Jet Size

We study the multiplicity dependence of jet structures in pp collisions using Monte Carlo event generators. We give predictions for multiplicity-differential jet structures and present evidence for a nontrivial jet shape dependence on charged hadron event multiplicity that can be used as a sensitive tool to experimentally differentiate between equally well-preforming simulation tunes. We also propose a way to validate the presence and extent of effects such as multiple-parton interactions (MPI) or color reconnection (CR), based on the detection of nontrivial jet shape modification in high-multiplicity events at high pT. Using multiplicity-dependent jet structure observables in various pT windows might also help understanding the interplay between jet particles and the underlying event (UE). We introduce a multiplicity-independent characteristic jet size measure and use a simplistic model to aid its physical interpretation.

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.

Hadronic light-by-light scattering in the anomalous magnetic moment of the muon

Hadronic light-by-light scattering in the anomalous magnetic moment of the muon a_\muaμ is one of two hadronic effects limiting the precision of the Standard Model prediction for this precision observable, and hence the new-physics discovery potential of direct experimental determinations of a_\muaμ. In this contribution, I report on recent progress in the calculation of this effect achieved both via dispersive and lattice QCD methods.