Pair Production Of Particles Research Articles

The effect of the scalar unparticle on the production of Higgs–radion at high energy colliders

An attempt is made to present the influence of the scalar unparticle on some scattering processes in the Randall–Sundum model. The contribution of the scalar unparticle on the production of Higgs–radion at high energy colliders is studied in detail. We evaluate the production cross-sections in the electron–positron (e+e−), photon–photon (γγ) and gluon–gluon (gg) collisions, which depend strongly on the collision energy s, the scaling dimension dU of the unparticle operator OU and the energy scale ΛU. Numerical evaluation shows that the cross-sections for the pair production of scalar particles are much larger than that of the associated production of the scalar particle with unparticle under the same conditions.

Magnetic field effect on charged scalar pair creation at finite temperature

In this work we explore the effects of a weak magnetic field and a thermal bath on the decay process of a neutral scalar boson into two charged scalar bosons. Our findings indicate that magnetic field inhibits while temperature enhances the pair production. The employed formalism allows us to isolate the contribution of magnetic fields in vacuum, leading to a separate analysis of the effects of different ingredients. This is essential since the analytical computation of the decay width necessarily needs of some approximation and the results that can be found in the literature are not always coincident. We perform the calculation in vacuum by two different weak field approximations. The particle pair production in vacuum was found to coincide with finite temperature behavior, which is opposite to results obtained by other authors in scenarios that involve neutral particles decaying into a pair of charged fermions. Among other differences between these scenarios, we traced that the analytical structure of the self-energy imposed by the spin of particles involved in the process is determinant in the behavior of the decay rate with the magnetic field.

Search for dark matter and supersymmetry in the vector boson fusion topology in proton-proton collisions at CMS

A search for pair production of dark matter candidates and supersymmetry (SUSY) production with two jets in vector-boson fusion (VBF) topology is presented using data collected by the Compact Muon Solenoid (CMS) detector in proton-proton collisions at the Large Hadron Collider (LHC). Final states with no leptons are expected in pair production of dark matter particles or scalar quarks in SUSY compressed mass-spectra scenarios. Final states with low-energy leptons are expected in the production of charginos and neutralinos in SUSY compressed mass-spectra scenarios. Results for both zero and two lepton final states at 8 TeV are presented with brief prospects at 13 TeV.

Model independent constraints on charges of new particles

Any particle that is charged under $SU(3)_C$ and $U(1)_{EM}$ can mediate the $gg \rightarrow {\gamma}{\gamma}$ process through loops. Near the threshold for the new particle pair production, gauge boson exchanges necessitate the resummation of ladder diagrams. We discuss the leading log order matching of the one-loop result with non-relativistic effective theory resummed result. We show how the diphoton invariant mass spectrum varies depending on decay width, color representation and electric charge of the new particle. The exclusion limits on the product of $SU(3)_C$ and $U(1)_{EM}$ charges of the new scalar or fermion particle are obtained from current LHC data.

Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in varvec{sqrt{s}=13}\xa0text {TeV}varvec{pp} collisions with the ATLAS detector

Two searches for new phenomena in final states containing a same-flavour opposite-sign lepton (electron or muon) pair, jets, and large missing transverse momentum are presented. These searches make use of proton–proton collision data, collected during 2015 and 2016 at a centre-of-mass energy sqrt{s}=13 text {TeV} by the ATLAS detector at the large hadron collider, which correspond to an integrated luminosity of 14.7~mathrm {fb}^{-1}. Both searches target the pair production of supersymmetric particles, squarks or gluinos, which decay to final states containing a same-flavour opposite-sign lepton pair via one of two mechanisms: a leptonically decaying Z boson in the final state, leading to a peak in the dilepton invariant-mass distribution around the Z boson mass; and decays of neutralinos (e.g. tilde{chi }_2^0 rightarrow ell ^+ell ^- tilde{chi }_1^0), yielding a kinematic endpoint in the dilepton invariant-mass spectrum. The data are found to be consistent with the Standard Model expectation. Results are interpreted in simplified models of gluino-pair (squark-pair) production, and provide sensitivity to gluinos (squarks) with masses as large as 1.70 text {TeV} (980 text {GeV}).

Pair production in strong SU(2) background fields

Fermion particle pair production in strong SU(2)-gauge chromoelectric fields is studied using the Boltzmann–Vlasov equation in a classical way. The existence of a preproduction process in a classical description is shown using the distribution evolution of non-Abelian particle production. It is interesting to find that the distribution of the particle number density is centered on two islands and shows a split on the color charge sphere as it evolves, ultimately reaching a steady state that is related to the amplitude and variation of the field.

String pair production in non homogeneous backgrounds

We consider string pair production in non homogeneous electric backgrounds. We study several particular configurations which can be addressed with the Euclidean world-sheet instanton technique, the analogue of the world-line instanton for particles. In the first case the string is suspended between two D-branes in flat space-time, in the second case the string lives in AdS and terminates on one D-brane (this realizes the holographic Schwinger effect). In some regions of parameter space the result is well approximated by the known analytical formulas, either the particle pair production in non-homogeneous background or the string pair production in homogeneous background. In other cases we see effects which are intrinsically stringy and related to the non-homogeneity of the background. The pair production is enhanced already for particles in time dependent electric field backgrounds. The string nature enhances this even further. For spacial varying electrical background fields the string pair production is less suppressed than the rate of particle pair production. We discuss in some detail how the critical field is affected by the non-homogeneity, for both time and space dependent electric field backgrouds. We also comment on what could be an interesting new prediction for the small field limit. The third case we consider is pair production in holographic confining backgrounds with homogeneous and non-homogeneous fields.

The Schwinger mechanism in (Anti) de Sitter spacetimes

We present a short and novel derivation of the Schwinger mechanism for particle pair production in $1+1$ dimensional de Sitter and Anti de Sitter spacetimes. We work directly in the flat embedding space and derive the pair production rates in these spacetimes via instanton methods. The derivation is manifestly coordinate independent, and also lends support to the possible deep connection between two conceptually disparate quantum phenomena - Schwinger effect and the Davies-Unruh effect.

High-energy behavior of photon, neutrino, and proton cross sections

By combining the color dipole model of the nucleon with the assumption that cross sections behave asymptotically as $\ln^2(s)$, we are able to describe the data for photon, neutrino and hadron interactions with protons at all energies, $s$ is the center-of-mass energy of the interacting particles. Specifically, we extrapolate the perturbative QCD calculations into the regime of small fractional parton momenta $x$ using a color dipole description of the proton target that guarantees an asymptotic $\ln^2(s)$ behavior of all cross sections. The ambiguity of introducing a parametrization associated with the dipole approximation is mitigated by the requirement that the saturation of the small-$x$ structure functions produces $\ln^2(s)$-behaved asymptotic cross sections, in agreement with the data. The same formalism allows us to calculate the cross section for the hadronic pair production of charm particles. The results, in particular those for the high-energy neutrino and charm cross sections, are relevant for evaluating the sensitivity as well as the background in neutrino telescopes.

Spinor particle creation in near extremal Reissner–Nordström black holes

The pair production of spinor particles, which can be captured by the solution of the Dirac equation with an appropriate boundary condition, in charged black holes is investigated. We obtain the closed form of the production rate in the near extremal limit of Reissner–Nordström black holes. The cosmic censorship conjecture is seemingly guaranteed by the pair production process. Moreover, the absorption cross section ratio and retarded Green’s functions of the spinor fields calculated from the gravity side match well with those of spinor operators in the dual conformal field theory (CFT) side.

Partículas e antipartículas no cone de luz

Na teoria quântica de campos usual, descrita no espaço-tempo de Minkowski, os conceitos de partícula e antipartícula surgem associados aos estados de energia positiva e negativa respectivamente. Nesse artigo focando como público alvo alunos de pós-graduação em Física, discutimos de maneira pedagógica como esses conceitos podem ser ou não transferidos quando fazemos uma mudança de coordenadas, do espaço-tempo de Minkowski para as coordenadas do cone de luz. Concluímos que nessas novas coordenadas temos uma arbitrariedade na escolha, uma vez que o sinal da energia fica atrelado ao sinal da componente longitudinal do momento no cone de luz. Embora haja essa arbitrariedade, a correlação de sinal que existe entre a energia e o momento longitudinal implica profundas consequencias físicas. Momentos positivos implicam energias positivas e momentos negativos implicam energias negativas. Isso significa, por exemplo, que no vácuo quântico da frente de luz não pode haver produção de pares partículas e antipartículas, ao contrário do que ocorre no espaço de Minkowski usual.

Search for neutral color-octet weak-triplet scalar particles in proton-proton collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

A search for pair production of neutral color-octet weak-triplet scalar particles (Theta) is performed in processes where one Theta decays to a pair of b quark jets and the other to a Z boson plus a jet, with the Z boson decaying to a pair of electrons or muons. The search is performed with data collected by the CMS experiment at the CERN LHC corresponding to an integrated luminosity of 19.7 inverse femtobarns of proton-proton collisions at sqrt(s) = 8 TeV. The number of observed events is found to be in agreement with the standard model predictions. The 95% confidence level upper limit on the product of the cross section and branching fraction is obtained as a function of the Theta mass. The 95% confidence level lower bounds on the Theta mass are found to be 623 and 426 GeV, for two different octo-triplet theoretical scenarios. These are the first direct experimental bounds on particles predicted by the octo-triplet model.

Non-relativistic high-energy physics: top production and dark matter annihilation

Non-relativistic physics is often associated with atomic physics and low-energy phenomena of the strong interactions between nuclei and quarks. In this review we cover three topics in contemporary high-energy physics at or close to the TeV scale, where non-relativistic dynamics plays an important if not defining role. We first discuss in detail the third-order corrections to top-quark pair production in electron-positron collisions in the threshold region, which plays a major role at a future high-energy e+e− collider. Threshold effects are also relevant in the production of heavy particles in hadronic collisions, where in addition to the Coulomb force soft gluon radiation contributes to enhanced quantum corrections. We review the joint resummation of non-relativistic and soft gluon effects for pair production of top quarks and supersymmetric particles to next-to-next-to-leading logarithmic accuracy. The third topic deals with pair annihilation of dark matter particles within the framework of the Minimal Supersymmetric Standard Model. Here the electroweak Yukawa force generated by the exchange of gauge and Higgs bosons can cause large “Sommerfeld” enhancements of the annihilation cross section in some parameter regions.

World-line instantons and the Schwinger effect as a Wentzel–Kramers–Brillouin exact path integral

A detailed study of the semiclassical expansion of the world line path integral for a charged relativistic particle in a constant external electric field is presented. We show that the Schwinger formula for charged particle pair production is reproduced exactly by the semiclassical expansion around classical instanton solutions when the leading order of fluctuations is taken into account. We prove that all corrections to this leading approximation vanish and that the WKB approximation to the world line path integral is exact.

Search for nonpointing and delayed photons in the diphoton and missing transverse momentum final state in 8 TeVppcollisions at the LHC using the ATLAS detector

A search has been performed, using the full 20.3 fb$^{-1}$ data sample of 8 TeV proton--proton collisions collected in 2012 with the ATLAS detector at the LHC, for photons originating from a displaced vertex due to the decay of a neutral long-lived particle into a photon and an invisible particle. The analysis investigates the diphoton plus missing transverse momentum final state, and is therefore most sensitive to pair-production of long-lived particles. The analysis technique exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction, as well as the time of flight, of photons. No excess is observed over the Standard Model predictions for background. Exclusion limits are set within the context of Gauge Mediated Supersymmetry Breaking models, with the lightest neutralino being the next-to-lightest supersymmetric particle and decaying into a photon and gravitino with a lifetime in the range from 250 ps to about 100 ns.

Pair production of Dirac particles in a $$d+1$$ d + 1 -dimensional noncommutative space–time

This work addresses the computation of the probability of fermionic particle pair production in \((d+1)\)-dimensional noncommutative Moyal space. Using Seiberg–Witten maps, which establish relations between noncommutative and commutative field variables, up to the first order in the noncommutative parameter \(\theta \), we derive the probability density of vacuum–vacuum pair production of Dirac particles. The cases of constant electromagnetic, alternating time-dependent, and space-dependent electric fields are considered and discussed.

Super-razor and searches for sleptons and charginos at the LHC

Direct searches for electroweak pair production of new particles at the LHC are a difficult proposition, due to the large background and low signal cross sections. We demonstrate how these searches can be improved by a combination of new razor variables and shape analysis of signal and background kinematics. We assume that the pair-produced particles decay to charged leptons and missing energy, either directly or through a W boson. In both cases the final state is a pair of opposite sign leptons plus missing transverse energy. We estimate exclusion reach in terms of sleptons and charginos as realized in minimal supersymmetry. We compare this super-razor approach in detail to analyses based on other kinematic variables, showing how the super-razor uses more of the relevant kinematic information while achieving higher selection efficiency on signals, including cases with compressed spectra.

Pitfalls of dark matter crossing symmetries

We explore the connection between pair production of dark matter particles at collider experiments and annihilation of dark matter in the early and late universe, with a focus on the correlation between the two time-reversed processes. We consider both a model-independent effective theory framework, where the initial and final states are assumed to not change under time reversal and concrete UV-complete models within the framework of supersymmetric extensions to the Standard Model. Even within the effective theory framework (where crossing symmetry is in some sense assumed), we find that the predictions of that symmetry can vary by orders of magnitude depending on the details of the selected effective interaction. Within the supersymmetric models we consider, we find that there is an even wilder variation in the expectations one can derive for collider observables based on cross-symmetric processes such as having a thermal relic or given indirect dark matter detection rates. We also explore additional ``pitfalls'' where na\{\i}ve crossing symmetry badly fails, including models with very light mediators leading to Sommerfeld enhancements and/or dark matter bound states.

Scale-invariant resonance tagging in multijet events and new physics in Higgs pair production

Abstract We study resonant pair production of heavy particles in fully hadronic final states by means of jet substructure techniques. We propose a new resonance tagging strategy that smoothly interpolates between the highly boosted and fully resolved regimes, leading to uniform signal efficiencies and background rejection rates across a broad range of masses. Our method makes it possible to efficiently replace independent experimental searches, based on different final state topologies, with a single common analysis. As a case study, we apply our technique to pair production of Higgs bosons decaying into $ b\overline{b} $ pairs in generic New Physics scenarios. We adopt as benchmark models radion and massive KK graviton production in warped extra dimensions. We find that despite the overwhelming QCD background, the 4b final state has enough sensitivity to provide a complementary handle in searches for enhanced Higgs pair production at the LHC.

Contact interactions probe effective dark-matter models at the LHC

Effective field theories provide a simple framework for probing possible dark-matter (DM) models by re-parametrising full interactions into a reduced number of operators with smaller dimensionality in parameter space. In many cases these models have four particle vertices, e.g., , leading to the pair production of dark-matter particles, χ, at a hadron collider from initial state quarks, q. In this analysis we show that for many fundamental DM models with s-channel DM couplings to pairs, these effective vertices must also produce quark contact interactions (CI) of the form . The respective effective couplings are related by the common underlying theory which allows one to translate the upper limits from one coupling to the other. We show that at the LHC, the experimental limits on quark contact interactions give stronger translated limits on the DM coupling than the experimental searches for dark-matter pair production.