Final Decay Products Research Articles

PASCL: supervised contrastive learning with perturbative augmentation for particle decay reconstruction

Abstract In high-energy physics, particles produced in collision events decay in a format of a hierarchical tree structure, where only the final decay products can be observed using detectors. However, the large combinatorial space of possible tree structures makes it challenging to recover the actual decay process given a set of final particles. To better analyse the hierarchical tree structure, we propose a graph-based deep learning model to infer the tree structure to reconstruct collision events. In particular, we use a compact matrix representation termed as lowest common ancestor generations matrix, to encode the particle decay tree structure. Then, we introduce a perturbative augmentation technique applied to node features, aiming to mimic experimental uncertainties and increase data diversity. We further propose a supervised graph contrastive learning algorithm to utilize the information of inter-particle relations from multiple decay processes. Extensive experiments show that our proposed supervised graph contrastive learning with perturbative augmentation method outperforms state-of-the-art baseline models on an existing physics-based dataset, significantly improving the reconstruction accuracy. This method provides a more effective training strategy for models with the same parameters and makes way for more accurate and efficient high-energy particle physics data analysis.

Dark matter bound-state formation in the Sun

The Sun may capture asymmetric dark matter (DM), which can subsequently form bound-states through the radiative emission of a sub-GeV scalar. This process enables generation of scalars without requiring DM annihilation. In addition to DM capture on nucleons, the DM-scalar coupling responsible for bound-state formation also induces capture from self-scatterings of ambient DM particles with DM particles already captured, as well as with DM bound-states formed in-situ within the Sun. This scenario is studied in detail by solving Boltzmann equations numerically and analytically. In particular, we take into consideration that the DM self-capture rates require a treatment beyond the conventional Born approximation. We show that, thanks to DM scatterings on bound-states, the number of DM particles captured increases exponentially, leading to enhanced emission of relativistic scalars through bound-state formation, whose final decay products could be observable. We explore phenomenological signatures with the example that the scalar mediator decays to neutrinos. We find that the neutrino flux emitted can be comparable to atmospheric neutrino fluxes within the range of energies below one hundred MeV. Future facilities like Hyper-K, and direct DM detection experiments can further test such scenario.

Inflaton oscillations and post-inflationary reheating

We analyze in detail the perturbative decay of the inflaton oscillating about a generic form of its potential V(ϕ) = ϕ k , taking into account the effects of non-instantaneous reheating. We show that evolution of the temperature as a function of the cosmological scale factor depends on the spin statistics of the final state decay products when k > 2. We also include the inflaton-induced mass of the final states leading to either kinematic suppression or enhancement if the final states are fermionic or bosonic respectively. We compute the maximum temperature reached after inflation, the subsequent evolution of the temperature and the final reheat temperature. We apply our results to the computation of the dark matter abundance through thermal scattering during reheating. We also provide an example based on supersymmetry for the coupling of the inflaton to matter.

Fast resonance decays in nuclear collisions

We present a new method to calculate the final decay spectrum of direct resonance decays directly from hydrodynamic fields on a freeze-out surface. The method is based on identifying components of the final particle spectrum that transform in an irreducible way under rotations in the fluid-restframe. Corresponding distribution functions can be pre-computed including all resonance decays. Just a few of easily tabulated scalar functions then determine the Lorentz invariant decay spectrum from each space-time point, and simple integrals of these scalar functions over the freeze-out surface determine the final decay products. This by-passes numerically costly event-by-event calculations of the intermediate resonances. The method is of considerable practical use for making realistic data to model comparisons of the identified particle yields and flow harmonics, and for studying the viscous corrections to the freeze-out distribution function.

Relaxing cosmological neutrino mass bounds with unstable neutrinos

At present, cosmological observations set the most stringent bound on the neutrino mass scale. Within the standard cosmological model (ΛCDM), the Planck collaboration reports ∑mv< 0.12 eV at 95 % CL. This bound, taken at face value, excludes many neutrino mass models. However, unstable neutrinos, with lifetimes shorter than the age of the universe τν ≲ tU, represent a particle physics avenue to relax this constraint. Motivated by this fact, we present a taxonomy of neutrino decay modes, categorizing them in terms of particle content and final decay products. Taking into account the relevant phenomenological bounds, our analysis shows that 2-body decaying neutrinos into BSM particles are a promising option to relax cosmological neutrino mass bounds. We then build a simple extension of the type I seesaw scenario by adding one sterile state ν4 and a Goldstone boson ϕ, in which νi→ ν4ϕ decays can loosen the neutrino mass bounds up to ∑mv ∼ 1 eV, without spoiling the light neutrino mass generation mechanism. Remarkably, this is possible for a large range of the right-handed neutrino masses, from the electroweak up to the GUT scale. We successfully implement this idea in the context of minimal neutrino mass models based on a U(1)μ−τ flavor symmetry, which are otherwise in tension with the current bound on ∑mv.

Stochastic gravitational waves from inflaton decays

Due to the universality of gravitational interactions, it is generally expected that a stochastic gravitational wave (GW) background could form during the reheating period when the inflaton perturbatively decays with the emission of gravitons. Previously, only models in which the inflaton dominantly decays into a pair of light scalar and/or fermion particles were considered in the literature. In the present paper, we focus on the cases with a vector particle pair in the final decay product. The differential decay rates for the three-body gravitational inflaton decays are presented for two typical couplings between the inflaton and vector fields, from which we predict their respective GW frequency spectra. It turns out that, similar to the scalar and fermion cases, the obtained GW spectra is too high in frequency to be observed by the current and near-future GW detection experiments and calls for a new design of high-frequency GW detectors.

Fast resonance decays in nuclear collisions

In the context of ultra-relativistic nuclear collisions, we present a fast method for calculating the final particle spectra after the direct decay of resonances from a Cooper–Frye integral over the freeze-out surface. The method is based on identifying components of the final particle spectrum that transform in an irreducible way under rotations in the fluid-restframe. Corresponding distribution functions can be pre-computed including all resonance decays. Just a few of easily tabulated scalar functions then determine the Lorentz invariant decay spectrum from each space-time point, and simple integrals of these scalar functions over the freeze-out surface determine the final decay products. This by-passes numerically costly event-by-event calculations of the intermediate resonances. The method is of considerable practical use for making realistic data to model comparisons of the identified particle yields and flow harmonics, and for studying the viscous corrections to the freeze-out distribution function.

Precise calculation of differential cross sections for the Z boson plus jets production in proton-proton collisions at a center-of-mass energy of 13 TeV

The differential cross section predictions for the associated production of a Z boson and jets are presented in proton-proton collisions at a center-of-mass energy of 13 TeV. The differential cross sections are calculated by using a computational framework for the dielectron decay mode of the Z boson. Higher-order differential cross section predictions are obtained in the fiducial phase space and are compared at next-to-leading order (NLO) and next-to-NLO (NNLO) in perturbative quantum chromodynamics (pQCD). The predicted results are reported as functions of the variables including the jet multiplicity, the invariant mass of the Z boson, and transverse momenta of the Z boson and of the decay products. The predicted results are also reported for the angular correlation variables for the final state decay products. The differential results are generally improved in precision upon inclusion of the NNLO corrections. The total cross sections are calculated in the fiducial phase space at different orders in pQCD up to NNLO. The total production rates are predicted more accurately by the NNLO calculation.

Probing the type-II seesaw mechanism through the production of Higgs bosons at a lepton collider

We investigate the production and decays of doubly-charged Higgs bosons for the Type-II seesaw mechanism at an $e^{+} e^{-}$ collider with two center of mass energies, $\sqrt{s}=380$ GeV and 3 TeV, and analyze the fully hadronic final states in detail. Lower mass ranges can be probed during the 380 GeV run of the collider, while high mass ranges, which are beyond the 13 TeV Large Hadron Collider discovery reach, can be probed with $\sqrt{s}=3$ TeV. For such a heavy Higgs boson, the final decay products are collimated, resulting in fat-jets. We perform a substructure analysis to reduce the background and find that a doubly-charged Higgs boson in the mass range 800-1120 GeV can be discovered during the 3 TeV run, with integrated luminosity $\mathcal{L} \sim 95\, \rm{fb}^{-1}$ of data. For 380 GeV center of mass energy, we find that for the doubly-charged Higgs boson in the range 160-172 GeV, a $5\sigma$ significance can be achieved with only integrated luminosity $\mathcal{L} \sim 24 \, \rm{fb}^{-1}$. Therefore, a light Higgs boson can be discovered immediately during the run of a future $e^{+} e^{-}$ collider.

Probing the littlest Higgs model withTparity using di-Higgs events throughZH-pair production at the LHC in NLO QCD

We investigate the di-Higgs events through $Z_H$-pair production at the CERN Large Hadron Collider including the pure next-to-leading order (NLO) QCD correction and the $gg$-fusion contribution in the framework of the littlest Higgs model with $T$ parity. We employ the diagram subtraction scheme in the QCD NLO calculations to avoid double counting and keep the convergence of the perturbative QCD description for the $Z_H$-pair production. We investigate the dependence of the leading order and QCD corrected integrated cross sections on the renormalization/factorization scale, and find that the total QCD corrections slightly reduce the scale uncertainty in the plotted range. By considering the subsequent decays of the intermediately produced $Z_H$ bosons and adopting the exclusive four-$b$-jet event selection criterion, the QCD correction provides considerable enhancement of the kinematic distributions for final decay products. We find that it is possible to select the signature of the $Z_H$-pair production from possible standard model background by taking proper kinematic cuts.

Precise calculation for heavy gauge boson production in the LHT model

In the framework of the littlest Higgs model with T parity, we study the WH/Zh+q_ and WH-pair productions at the CERN Large Hadron Collider up to the QCD next-to-leading order (NLO). The kinematic distributions of final decay products and the theoretical dependence of the cross section on the factorization/renormalization scale are analyzed. We adopt the PROSPINO scheme in the QCD NLO calculations to avoid double counting and keep the convergence of the perturbative QCD description. By using the subtraction scheme, the QCD NLO corrections enhance the leading order cross section with a K-factor in the range of 1.00 ~ 1.43 for WH (ZH)q- production process, and in the range of 1.09 ~ 1.22 for the WH pair production process.

Precise predictions forAHq−-associated production in the littlest Higgs model withTparity at the LHC

In the framework of the littlest Higgs model with $T$ parity, we present complete calculations for the $A_H q_-$ $(q_-=u_-, \bar{u}_-,d_-, \bar{d}_-,c_-, \bar{c}_-,s_-, \bar{s}_-)$ associated production up to the QCD next-to-leading order (NLO) at the CERN Large Hadron Collider with subsequent pure weak decay of $T$-odd mirror quark. We apply the PROSPINO scheme to avoid the double counting problem and to keep the convergence of the perturbative QCD description. The theoretical correlations between the integrated cross section and the factorization and renormalization scale, the global symmetry-breaking scale and the Yukawa coupling parameter are studied separately. We also provide the kinematic distributions of the final decay products. Our numerical results show that the NLO QCD correction reduces the scale uncertainty and enhances the leading-order integrated cross section remarkably, with the $K$ factor varying in the range of $1.41 \sim 1.68$ ($1.58 \sim 1.89$) as the increment of the global symmetry-breaking scale $f$ from $500~{\rm GeV}$ to $1.5~{\rm TeV}$ ($1.1~{\rm TeV}$) at the $\sqrt{s} = 14~{\rm TeV}$ ($8~{\rm TeV}$) LHC. We find that it is possible to select the signal events of the $A_Hq_-$ production from its background by putting proper cuts on the final leading jet and missing energy.

Decoherence freeBdandBsmeson systems

We study the impact of decoherence on $B$ meson systems with specific emphasis on ${B}_{s}$. For consistency we also study the ${B}_{d}$ mesons based on the most recent data. Even though the study of decoherence made here depends upon the basis chosen and the final decay products, our analysis suggests that the coherence property of $B$ meson systems is robust and could be a good candidate for making a study of foundational aspects of quantum mechanics.

Precise QCD predictions onWHZHproduction in the littlest Higgs model withTparity at the LHC

We investigate the effects of the littlest Higgs model with $T$ parity up to the QCD next-to-leading order (NLO) on the ${W}_{H}^{\ifmmode\pm\else\textpm\fi{}}{Z}_{H}$ productions at the CERN Large Hadron Collider (LHC), and discuss the kinematic distributions of final decay products and the theoretical dependence of the cross section on the factorization/renormalization scale. We find that the QCD NLO corrections reduce the scale uncertainty of the leading-order cross section in the case of ${\ensuremath{\mu}}_{F}={\ensuremath{\mu}}_{R}$. By adopting the prospino subtraction scheme [scheme (II)] in analysing the QCD NLO contributions, we obtain the numerical results which keep the convergence of the perturbative QCD description. Our results by adopting scheme (II) at the 14 TeV (8 TeV) LHC show that the $K$-factor for the ${W}_{H}^{+}{Z}_{H}$ production varies in the range of 1.01--1.10 (1.00--1.08), while the $K$-factor for the ${W}_{H}^{\ensuremath{-}}{Z}_{H}$ production varies in the range of 1.11--1.13 (1.11--1.12), when the global symmetry-breaking scale $f$ goes from 400 GeV to 1.5 TeV (1 TeV).

WH-pair production in the littlest Higgs model withTparity in next-to-leading order QCD at the LHC

In the framework of the littlest Higgs model with $T$ parity, we study the $W_H$-pair production at the CERN Large Hadron Collider up to the QCD next-to-leading order (NLO). The kinematic distributions of final decay products and the theoretical dependence of the cross section on the factorization/renormalization scale are analyzed. We adopt the PROSPINO scheme in the QCD NLO calculations to avoid double counting and keep the convergence of the perturbative QCD description. Our numerical results show that the QCD NLO corrections significantly reduce the scale uncertainty, and enhance the leading order integrated cross section with a $K$-factor in the range of $1.10-1.22$ ($1.09-1.17$) with the symmetry breaking scale $f$ varying from $400 GeV$ ($400 GeV$) to $1.5 TeV$ ($1.0 TeV$) at the $14 TeV$ $(8 TeV)$ LHC. We find that it is possible to select the signal events of the $W_H$-pair production from the $pp\to W^+ W^- \to e^+ \mu^-\nu_{e}\bar{\nu}_{\mu}+X$ background with high ratio of signature over background by taking proper lower limits on transverse momenta, invariant mass of the final charged leptons and the missing transverse momentum.

AssociatedZ0H0production with leptonic decays at the LHC in next-to-leading order QCD

In this work we investigate the effects of the littlest Higgs model (LHM) up to the QCD next-to-leading order (NLO) on the $Z^0H^0$ associated production at the CERN Large Hadron Collider (LHC). We study the dependences of the leading order and NLO QCD corrected integrated cross sections for this process on the factorization/renormalization scale and the LHM parameters. We also provide the distributions of the transverse momenta of final decay products $\mu^-$ and $\tau^-$. Our results show that the heavy neutral gauge bosons $Z_H$ and $A_H$ could induce significant discrepancies from the standard model predictions. It is found that when the LHM parameters are taken as $c=0.5$, $c^{\prime}=0.22$, $f=4 TeV$ and $\mu=(M_H+M_Z)/2$, the effects at the $\sqrt{s}=14 TeV$ LHC from the heavy neutral gauge boson are about 12.83% and 10.37% to the leading order and NLO QCD corrected integrated cross sections, respectively. We also conclude that the NLO QCD corrections at the $\sqrt{s}=14 TeV$ LHC can obviously reduce the scale uncertainty of the integrated cross section, and significantly enhance the differential cross sections of $p_T^{\mu^-}$ and $p_T^{\tau^-}$. It demonstrates that the precision measurement of the $Z^0H^0$ associated production process at the LHC could provide the clue of the LHM physics.

WH/ZHproduction associated with aT-odd (anti)quark at the LHC in next-to-leading order QCD

In the framework of the littlest Higgs model with T parity, we study the ${W}_{H}/{Z}_{H}$ production in association with a T-odd (anti)quark of the first two generations at the CERN Large Hadron Collider up to the QCD next-to-leading order. The kinematic distributions of final decay products and the theoretical dependence of the cross section on the factorization/renormalization scale are discussed. We apply three schemes in considering the QCD NLO contributions and find that the QCD NLO corrections by adopting the (II) and (III) subtraction schemes can keep the convergence of the perturbative QCD description and reduce the scale uncertainty of the leading order cross section. By using these two subtraction schemes, the QCD NLO corrections to the ${W}_{H}({Z}_{H}){q}_{\ensuremath{-}}$ production process enhance the leading order cross section with a K-factor in the range of $1.00\ensuremath{\sim}1.43$.

Monitoring Light-induced Structural Changes of Channelrhodopsin-2 by UV-visible and Fourier Transform Infrared Spectroscopy

Channelrhodopsin-2 (ChR2) is a microbial type rhodopsin and a light-gated cation channel that controls phototaxis in Chlamydomonas. We expressed ChR2 in COS-cells, purified it, and subsequently investigated this unusual photoreceptor by flash photolysis and UV-visible and Fourier transform infrared difference spectroscopy. Several transient photoproducts of the wild type ChR2 were identified, and their kinetics and molecular properties were compared with those of the ChR2 mutant E90Q. Based on the spectroscopic data we developed a model of the photocycle comprising six distinguishable intermediates. This photocycle shows similarities to the photocycle of the ChR2-related Channelrhodopsin of Volvox but also displays significant differences. We show that molecular changes include retinal isomerization, changes in hydrogen bonding of carboxylic acids, and large alterations of the protein backbone structure. These alterations are stronger than those observed in the photocycle of other microbial rhodopsins like bacteriorhodopsin and are related to those occurring in animal rhodopsins. UV-visible and Fourier transform infrared difference spectroscopy revealed two late intermediates with different time constants of tau = 6 and 40 s that exist during the recovery of the dark state. The carboxylic side chain of Glu(90) is involved in the slow transition. The molecular changes during the ChR2 photocycle are discussed with respect to other members of the rhodopsin family.

Wall crossing formula for 𝒩 = 4 dyons: a macroscopic derivation

We derive the wall crossing formula for the decay of a quarter BPS dyon into a pair of half-BPS dyons by analyzing the quantum dynamics of multi-centered black holes in = 4 supersymmetric string theories. Our analysis encompasses the cases where the final decay products are non-primitive dyons. The results are in agreement with the microscopic formula for the dyon spectrum in the special case of heterotic string theory on T6.

Decay of super-heavy particles: user guide of the SHdecay program

I give here a detailed user guide for the C++ program SHdecay, which has been developed for computing the final spectra of stable particles (protons, photons, LSPs, electrons, neutrinos of the three species and their antiparticles) arising from the decay of a super-heavy X particle. It allows to compute in great detail the complete decay cascade for any given decay mode into particles of the Minimal Supersymmetric Standard Model (MSSM). In particular, it takes into account all interactions of the MSSM during the perturbative cascade (including not only QCD, but also the electroweak and 3rd generation Yukawa interactions), and includes a detailed treatment of the SUSY decay cascade (for a given set of parameters) and of the non-perturbative hadronization process. All these features allow us to ensure energy conservation over the whole cascade up to a numerical accuracy of a few per mille. Yet, this program also allows to restrict the computation to QCD or SUSY-QCD frameworks. I detail the input and output files, describe the role of each part of the program, and include some advice for using it best. Program summary Title of program: SHdecay Catalogue identifier:ADSL Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADSL Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Computer and operating system: Program tested on PC running Linux KDE and Suse 8.1 Programming language used: C with STL C++ library and using the standard gnu g++ compiler No. lines in distributed program: 14 955 No. of bytes in distributed program, including test data, etc.: 624 487 Distribution format: tar gzip file Keywords: Super-heavy particles, fragmentation functions, DGLAP equations, supersymmetry, MSSM, UHECR Nature of physical problem: Obtaining the energy spectra of the final stable decay products (protons, photons, electrons, the three species of neutrinos and the LSPs) of a decaying super-heavy X particle, within the framework of the Minimal Supersymmetric Standard Model (MSSM). It can be done numerically by solving the full set of DGLAP equations in the MSSM for the perturbative evolution of the fragmentation functions D p 2 p 1 ( x, Q) of any particle p 1 into any other p 2 ( x is the energy fraction carried by the particle p 2 and Q its virtuality), and by treating properly the different decay cascades of all unstable particles and the final hadronization of quarks and gluons. In order to obtain proper results at very low values of x (up to x∼10 −13), NLO color coherence effects have been included by using the Modified Leading Log Approximation (MLLA). Method of solution: the DGLAP equations are solved by a four order Runge–Kutta method with a fixed step. Typical running time: Around 35 hours for the first run, but the most time consuming sub-programs can be run only once for most applications.