Leading-order Process Research Articles

Higgs Production at µ+µ+ Colliders. Based on hep-ph/2408.01068 [1

Motivated by recent advancements in antimuon cooling, we consider Higgs boson production at µ+µ+ colliders. The leading-order W+W− fusion process present at, e.g., µ+µ− colliders does not occur since both intial-state particles carry the same charge. Nevertheless, Higgs production is possible via a higher-order process mediated by a photon or Z boson. We find that at high energies, the associated cross secction grows as (log s)3 with the center-of-mass energy √s, as opposed to the log s growth of the leading-order process at µ+µ− colliders. Thus, the higher-order cross section with polarized beams can become about half as large as the leading-order process at µ+µ− colliders at O(10) TeV energies, despite its naive suppression. When calculating the higher-order cross section, collinear photon emissions make direct computations using event generators difficult. We therefore treat these emissions by splitting the phase-space into the sum of a non-collinear region calculable using event generators, and a collinear region approximated by a parton distribution function for the photon. Furthermore, since the same process can occur at any high-energy lepton collider, this large cross section needs to be considered also for Higgs production at µ+µ− and e+e− colliders.

Acceleration beyond lowest order event generation. An outlook on further parallelism within MadGraph5_aMC@NLO

An important area of high energy physics studies at the Large Hadron Collider (LHC) currently concerns the need for more extensive and precise comparison data. Important tools in this realm are event reweighing and evaluation of more precise next-to-leading order (NLO) processes via Monte Carlo event generators, especially in the context of the upcoming High Luminosity LHC. Current event generators need to improve throughputs for these studies. MadGraph5_aMC@NLO (MG5aMC) is an event generator being used by LHC experiments which has been accelerated considerably with a port to GPU and vector CPU architectures, but as of yet only for leading order processes. In this contribution a prototype for event reweighing using the accelerated MG5aMC software, as well as plans for an NLO implementation, are presented.

Equilibration of weakly coupled QCD plasmas

We employ a non-equilibrium Quantum Chromodynamics (QCD) kinetic description to study the kinetic and chemical equilibration of the Quark-Gluon Plasma (QGP) at weak coupling. Based on our numerical framework, which explicitly includes all leading order processes involving light flavor degrees of freedom, we investigate the thermalization process of homogeneous and isotropic plasmas far-from equilibrium and determine the relevant time scales for kinetic and chemical equilibration. We further simulate the longitudinally expanding pre-equilibrium plasma created in ultrarelativistic heavy-ion collisions at zero and non-zero density of the conserved charges and study its microscopic and macroscopic evolution towards equilibrium.

The Impact of Ocean‐Wave Coupling on the Upper Ocean Circulation During Storm Events

AbstractMany human activities rely on accurate knowledge of the sea surface dynamics. This is especially true during storm events, when wave‐current interactions might represent a leading order process of the upper ocean. In this study, we assess and analyze the impact of including three wave‐dependent processes in the ocean momentum equation of the Met Office North West European Shelf ocean‐wave forecasting system on the accuracy of the simulated surface circulation. The analysis is conducted using ocean currents and Stokes drift data produced by different implementations of the coupled forecasting systems to simulate the trajectories of surface (iSphere) and 15 m drogued (SVP) drifters affected by four storms selected from winter 2016. Ocean and wave simulations differ only in the degree of coupling and the skills of the Lagrangian simulations are evaluated by comparing model results against the observed drifter tracks. Results show that, during extreme events, ocean‐wave coupling improves the accuracy of the surface dynamics by 4%. Improvements are larger for ocean currents on the shelf (8%) than in the open ocean (4%): this is thought to be due to the synergy between strong tidal currents and more mature decaying waves. We found that the Coriolis‐Stokes forcing is the dominant wave‐current interaction for both type of drifters; for iSpheres the secondary wave effect is the wave‐dependent sea surface roughness while for SVPs the wave‐modulated water‐side stress is more important. Our results indicate that coupled ocean‐wave systems may play a key role for improving the accuracy of particle transport simulations.

Speeding up MadGraph5_aMC@NLO

In this paper we will describe two new optimisations implemented in MadGraph5_aMC@NLO, both of which are designed to speed-up the computation of leading-order processes (for any model). First we implement a new method to evaluate the squared matrix element, dubbed helicity recycling, which results in factor of two speed-up. Second, we have modified the multi-channel handling of the phase-space integrator providing tremendous speed-up for VBF-like processes (up to thousands times faster).

Prompt photon-jet angular correlations at central rapidities in p+A collisions

Photon-jet azimuthal correlations in proton-nucleus collisions are a promising tool for gaining information on the gluon distribution of the nucleus in the regime of non-linear color fields. We compute such correlations from the process $g+A\to q \bar{q} \gamma$ in the rapidity regime where both the projectile and target light-cone momentum fractions are small. By integrating over the phase space of the quark which emits the photon, subject to the restriction that the photon picks up most of the transverse momentum (to pass an isolation cut), we effectively obtain a $g+A\to q \gamma$ process. For nearly back-to-back photon-jet configurations we find that it dominates over the leading order process $q+A\to q \gamma$ by two less powers of $Q_\perp / Q_S$, where $\boldsymbol{Q}_\perp$ and $Q_S$ denote the net photon-jet pair momentum and the saturation scale of the nucleus, respectively. We determine the transverse momentum dependent gluon distributions involved in $g+A\to q \gamma$ and the scale where they are evaluated. Finally, we provide analytic expressions for $\langle\cos n\phi\rangle$ moments, where $\phi$ is the angle between $\boldsymbol{Q}_\perp$ and the average photon-jet transverse momentum $\tilde{\boldsymbol{P}}_\perp$, and first qualitative estimates of their transverse momentum dependence.

Photon emission from quark-gluon plasma out of equilibrium

The photon emission from a non-equilibrium quark-gluon plasma (QGP) is analyzed. We derive an integral equation that describes photon production through quark-antiquark annihilation and quark bremsstrahlung. It includes coherence between different scattering sites, also known as the Landau-Pomeranchuk-Migdal effect. These leading-order processes are studied for the first time together in an out-of-equilibrium field theoretical treatment that enables the inclusion of viscous corrections to the calculation of electromagnetic emission rates. In the special case of an isotropic, viscous, plasma the integral equation only depends on three constants which capture the non-equilibrium nature of the medium.

Universality of next-to-leading power threshold effects for colourless final states in hadronic collisions

We consider the production of an arbitrary number of colour-singlet particles near partonic threshold, and show that next-to-leading order cross sections for this class of processes have a simple universal form at next-to-leading power (NLP) in the energy of the emitted gluon radiation. Our analysis relies on a recently derived factorisation formula for NLP threshold effects at amplitude level, and therefore applies both if the leading-order process is tree-level and if it is loop-induced. It holds for differential distributions as well. The results can furthermore be seen as applications of recently derived next-to-soft theorems for gauge theory amplitudes. We use our universal expression to re-derive known results for the production of up to three Higgs bosons at NLO in the large top mass limit, and for the hadro-production of a pair of electroweak gauge bosons. Finally, we present new analytic results for Higgs boson pair production at NLO and NLP, with exact top-mass dependence.

Photofragmentation of Tetranitromethane: Spin-Unrestricted Time-Dependent Excited-State Molecular Dynamics.

In this study, the photofragmentation dynamics of tetranitromethane (TNM) is explored by a spin-unrestricted time-dependent excited-state molecular dynamics (u-TDESMD) algorithm based on Rabi oscillations and principles similar to trajectory surface hopping, with a midintensity field approximation. The leading order process is represented by the molecule undergoing cyclic excitations and de-excitations. During excitation cycles, the nuclear kinetic energy is accumulated to overcome the dissociation barriers in the reactant and a sequence of intermediates. The dissociation pathway includes the ejection of NO2 groups followed by the formation of NO and CO. The simulated mass spectra at the ab initio level, based on the bond length in possible fragments, are extracted from simulation trajectories. The recently developed methodology has the potential to model and monitor photoreactions with open-shell intermediates and radicals.

Digamma, what next?

If the 750 GeV resonance in the diphoton channel is confirmed, what are the measurements necessary to infer the properties of the new particle and understand its nature? We address this question in the framework of a single new scalar particle, called digamma ($\digamma$). We describe it by an effective field theory, which allows us to obtain general and model-independent results, and to identify the most useful observables, whose relevance will remain also in model-by-model analyses. We derive full expressions for the leading-order processes and compute rates for higher-order decays, digamma production in association with jets, gauge or Higgs bosons, and digamma pair production. We illustrate how measurements of these higher-order processes can be used to extract couplings, quantum numbers, and properties of the new particle.

Phenomenological modeling of the photon production rate from the QGP at finite quark chemical potential

We show the extended calculation of leading order process for photon production from Quark–Gluon Plasma (QGP) using a finite quark mass incorporating phenomenological parameter of quarks and gluons with the effect of quark chemical potential. The photon emission rate is observed in the range of low and intermediate transverse momentum. Our modified results of photon production give significant contribution in the range of quark phenomenological parameter, i.e. [Formula: see text] of high-energy heavy-ion collisions. The results obtained are compared with other results.

Photon radiation from hot and dense matter of quark gluon plasma

We work on photon radiation from hot and dense matter of quark gluon plasma (QGP). The total photon spectra of leading order (LO) process is produced in relativistic massive collisions of two nuclei using various value of quark phenomenological flow parameter dependent on quark chemical potential in quark mass. The total photon measurement is strongly increasing function of quark flow parameter ranging 2γg ≤ γq ≤ 8γg and also find appreciable enhancement at the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN as a result of high temperature and chemical potential using quark phenomenological flow parameter. The results obtained are presented on the study of photon production of leading order process. Thus, the study of photon radiation from hot and dense matter provides a valuable information in the formation and evolution of QGP and in the subsequent investigation of its properties. The results are compared with other theoretical work.

Measurement of the muon charge asymmetry in inclusivepp→W+Xproduction ats=7 TeVand an improved determination of light parton distribution functions

Measurements of the muon charge asymmetry in inclusive pp to WX production at sqrt(s) = 7 TeV are presented. The data sample corresponds to an integrated luminosity of 4.7 inverse femtobarns recorded with the CMS detector at the LHC. With a sample of more than twenty million W to mu nu events, the statistical precision is greatly improved in comparison to previous measurements. These new results provide additional constraints on the parton distribution functions of the proton in the range of the Bjorken scaling variable x from 10E-3 to 10E-1. These measurements and the recent CMS measurement of associated W + charm production are used together with the cross sections for inclusive deep inelastic ep scattering at HERA in a next-to-leading-order QCD analysis. The determination of the valence quark distributions is improved, and the strange-quark distribution is probed directly through the leading-order process g + s to W + c in proton-proton collisions at the LHC.

Effective three-body interactions via photon-assisted tunneling in an optical lattice

We present a simple, experimentally realizable method to make coherent three-body interactions dominate the physics of an ultracold lattice gas. Our scheme employs either lattice modulation or laser-induced tunneling to reduce or turn off two-body interactions in a rotating frame, promoting three-body interactions arising from multi-orbital physics to leading-order processes. This approach provides a route to strongly-correlated phases of lattice gases that are beyond the reach of previously proposed dissipative three-body interactions. In particular, we study the mean-field phase diagram for spinless bosons with three- and two- body interactions, and provide a roadmap to dimer states of varying character in 1D. This new toolset should be immediately applicable in state-of-the-art cold atom experiments.

Emission of Thermal Photon in Heavy-Ion Collisions

We study the leading order processes for photon production using a phenomenological model of quark-gluon plasma (QGP) in relativistic heavy-ion collisions incorporating the parametrization factors in thermal dependent quark mass. The measurement of photon emission provides valuable insights into the early conditions of QGP. The production rate is observed in the low and intermediate range of energy and transverse momentum. The photon yield is found to increase marginally with the effect of thermal dependent quark mass. The results are little enhanced and in good agreement with other work.

Tt¯+largemissing energy from top-quark partners: A comprehensive study at next-to-leading order QCD

We perform a detailed study of top-quark partner production in the $t\overline{t}$ plus large missing energy final state at the LHC, presenting results for both scalar and fermionic top-quark partners in the semileptonic and dileptonic decay modes of the top quarks. We compare the results of several simulation tools: leading order matrix elements, next-to-leading order (NLO) matrix elements, leading order plus parton shower simulations, and merged samples that contain the signal process with an additional hard jet radiated. We find that predictions from leading order plus parton shower simulations can significantly deviate from NLO QCD or LO merged samples and do not correctly model the kinematics of the $t\overline{t}+{E}_{T,\mathrm{miss}}$ signature. They are therefore not a good framework for modeling this new physics signature. On the other hand, the acceptances obtained with a merged sample of the leading-order process together with the radiation of an additional hard jet are in agreement with the NLO predictions. We also demonstrate that the scale variation of the inclusive cross section, plus that of the acceptance, does not accurately reflect the uncertainty of the cross section after cuts, which is typically larger. We show the importance of including higher-order QCD corrections when using kinematic distributions to determine the spin of the top-quark partner.

Direct photon measurements in pp collisions at = 7 TeV with ALICE

The production of direct photons in pp collisions via Compton scattering and qq̄ annihilation leading order processes is of great interest to test pQCD predictions and also to study parton fragmentation. The fragmentation of the recoil scattered parton can be studied using correlation between the photon and the charged particles emitted in the opposite direction. We present the study of the parton fragmentation in pp collisions at = 7 TeV measuring the imbalance parameter . Furthermore, a first uncorrected direct photon spectrum measured with the ALICE electromagnetic calorimeter (EMCal) is shown. These measurements open the way to a straightforward comparison between pp and Pb-Pb collisions concerning parton fragmentation and direct photon production.

Isolated photon–hadron correlations in proton–proton collisions at s=7 TeV with the ALICE experiment

At high pT, direct photons produced in Compton and annihilation QCD leading order processes are associated to a jet in the opposite direction. Such processes are tagged experimentally by identifying leading isolated photons and their correlated associated hadrons in the opposite azimuthal direction. The jet fragmentation can be estimated from the hadrons and the photon via the imbalance parameter xE. We present the results extracted from gamma-hadron correlations measured by the ALICE experiment in pp collisions at {\sqrt(s)} = 7 TeV. Direct photons are first identified using isolated criteria. Then, the remaining contamination from neutral mesons decay photons is subtracted statistically to extract the xE distributions of isolated photon-hadron and isolated {\pi}^0-hadron correlations.

Inclusive charmonium production in the PANDA experiment

The production of the charmonium states in proton-antiproton experiments is considered at the energy rates near threshold in NLO. Such a consideration allows one to obtain non zero distributions over the transverse momentum of the final charmonium and gives a natural explanation to the existence of $\chi_{c1}$-meson in final state, that is observed experimentally and cannot be produced in leading order processes. The crucial role of scale parameter choice in $\alpha_s (Q^2)$ and partonic distributions $f(x,Q^2)$ shown, and the correct choice offered.

The annual cycle in lower stratospheric temperatures revisited

Abstract. Observed lower stratospheric temperatures show a prominent annual cycle. The cycles in the tropics and Northern Hemisphere are in phase and the cycle in the Southern Hemisphere has the opposite phase. In an elegant and influential paper, Yulaeva, Holton and Wallace (1994) explained the observed pattern as a direct consequence of hemispheric asymmetries in the dynamical forcing of the stratospheric circulation. They showed that in Microwave Sounding Unit channel 4 (weighting centered in the lower stratosphere) data the combined extratropical and the tropical temperature cycle nearly compensate and interpreted the out-of-phase temperature variations between tropics and extratropics as the temperature response to an annual cycle in the wave driven residual circulation. We show that the near-compensation of temperature variations observed by Yulaeva et al. (1994) is artefact of the weighting function of the MSU-4 channel and does not hold on individual pressure levels. We discuss in detail the conditions required that temperature variations compensate, and what insights can be obtained from analysis of tropical, extratropical and global mean temperature variations. Dynamically induced seasonal variations of lower stratospheric ozone lead to an amplification of the seasonal temperature cycle particularly in the tropics. The latitudinal structure of static stability also induces a significant deviation from compensation of tropical and combined extratropical temperature variations. In line with Yulaeva et al. (1994) we affirm that the see-saw pattern in the annual cycles of tropical and combined extratropical temperatures provides an important pointer to mechanistic models for interannual variability and trends, but additionally conclude that the feedback of dynamically induced ozone variations on temperatures and the latitudinal structure of static stability should be included as leading order processes in such models.