Gauge Boson Production Research Articles

Dynamics of the cosmological relaxation after reheating

We examine if the cosmological relaxation mechanism, which was proposed recently as a new solution to the hierarchy problem, can be compatible with high reheating temperature well above the weak scale. As the barrier potential disappears at high temperature, the relaxion rolls down further after the reheating, which may ruin the successful implementation of the relaxation mechanism. It is noted that if the relaxion is coupled to a dark gauge boson, the new frictional force arising from dark gauge boson production can efficiently slow down the relaxion motion, which allows the relaxion to be stabilized after the electroweak phase transition for a wide range of model parameters, while satisfying the known observational constraints.

Double-real corrections at {{mathcal {O}}(alpha alpha _s)},to single gauge boson production

We consider the {{mathcal {O}}(alpha alpha _s)},corrections to single on-shell gauge boson production at hadron colliders. We concentrate on the contribution of all the subprocesses where the gauge boson is accompanied by the emission of two additional real partons and we evaluate the corresponding total cross sections. The latter are divergent quantities, because of soft and collinear emissions, and are expressed as Laurent series in the dimensional regularization parameter. The total cross sections are evaluated by means of reverse unitarity, i.e. expressing the phase-space integrals in terms of two-loop forward box integrals with cuts on the final-state particles. The results are reduced to a combination of master integrals, which eventually are evaluated in terms of generalized polylogarithms. The presence of internal massive lines in the Feynman diagrams, due to the exchange of electroweak gauge bosons, causes the appearance of 14 master integrals which were not previously known in the literature and have been evaluated via differential equations.

Measurements of properties of strong and electroweak forces with the ATLAS detector at the LHC

These proceedings summarise ATLAS measurements of properties of the strong and electroweak forces in proton-proton collisions at 7 and 8 TeV during Run-1 at the LHC. The specific measurements presented here are concerned with the gauge symmetries in the Standard Model and related parameters, starting with the production cross section of a W boson in association with jets, the forward-backward asymmetry in the production of Z/γ∗ decaying to electrons or muons and extraction of the effective weak mixing angle. Then a number of measurements specifically probing electroweak processes at very high energies are presented, including the first measurement of triple gauge boson production pprightarrow W(lnu )gamma gamma , the production of a leptonically decaying W boson in association with W or Z boson decaying to jets, electroweak production of two jets in association with a Z boson, and same-electric-charge diboson production in association with two jets providing the first evidence of electroweak vector boson scattering.

Violent preheating in inflation with nonminimal coupling

We study particle production at the preheating era in inflation models with nonminimal coupling ξϕ2R and quartic potential λϕ4/4 for several cases: real scalar inflaton, complex scalar inflaton and Abelian Higgs inflaton. We point out that the preheating proceeds much more violently than previously thought. If the inflaton is a complex scalar, the phase degree of freedom is violently produced at the first stage of preheating. If the inflaton is a Higgs field, the longitudinal gauge boson production is similarly violent. This is caused by a spike-like feature in the time dependence of the inflaton field, which may be understood as a consequence of the short time scale during which the effective potential or kinetic term changes suddenly. The produced particles typically have very high momenta k ≲ √λMP. The production might be so strong that almost all the energy of the inflaton is carried away within one oscillation for ξ2λ ≳ \U0001d4aa(100). This may partly change the conventional understandings of the (p)reheating after inflation with the nonminimal coupling to gravity such as Higgs inflation. We also discuss the possibility of unitarity violation at the preheating stage.

Measurements of the production of prompt photons, jets and vector bosons+jets in pp collisions with the ATLAS detector

The production of jets, photons and gauge bosons in association with jets at hadron colliders provides a stringent test of perturbative QCD and can be used to probe the gluon density function of the proton. In this letter we present recent measurements of the inclusive jet cross section in data collected at center-of-mass energies of 8 TeV and 13 TeV. The measurements have been performed differentially in various kinematic and angular observables and are compared with state-of-the-art predictions at NLO in pQCD. ATLAS has performed precise measurements of the inclusive production of isolated prompt photons in data collected at center-of-mass energies of 8 and 13 TeV, differential in both rapidity and the photon transverse momentum. The measurements are compared with NLO pQCD calculations and with several MC generators. The cross section of photons, W and Z bosons in association with one or more jets have been measured differentially in various observables at center-of-mass energies of 8 and 13 TeV and compared with state-of-the-art pQCD calculations up to NNLO QCD and NLO EWK as well as LO and NLO ME+PS generators. Dedicated phase spaces sensitive to emission of massive bosons collinear with jets or the electroweak production of vector bosons are tested.

Isospin asymmetry of quark distributions and implications for single top-quark production at the LHC

We present an improved determination of the up- and down-quark distributions in the proton using recent data on charged lepton asymmetries from $W^\pm$ gauge-boson production at the LHC and Tevatron. The analysis is performed in the framework of a global fit of parton distribution functions. The fit results are consistent with a non-zero iso-spin asymmetry of the sea, $x(\bar d - \bar u)$, at small values of Bjorken $x\sim 10^{-4}$ indicating a delayed onset of the Regge asymptotics of a vanishing $(\bar d - \bar u)$-asymmetry at small-$x$. We compare with up- and down-quark distributions available in the literature and provide accurate predictions for the production of single top-quarks at the LHC, a process which can serve as a standard candle for the light quark flavor content of the proton.

Probing diffractive production of gauge bosons at forward rapidities

The gauge boson production at forward rapidities in single diffractive events at the LHC is investigated considering $pp$ collisions at $\sqrt{s} =$ 8 and 13 TeV. The impact of gap survival effects is analysed using two different models for the soft rescattering contributions. We demonstrate that using the Forward Shower Counter Project at LHCb -- HERSCHEL, together with the Vertex Locator -- VELO, it is possible to discriminate diffractive production of the gauge bosons $W$ and $Z$ from the non-diffractive processes and studies of the Pomeron structure and diffraction phenomenology are feasible. Moreover, we show that the analysis of this process can be useful to constrain the modelling of the gap survival effects.

Associated production of electroweak bosons and heavy mesons at LHCb and the prospects to observe double parton interactions

The production of weak gauge bosons in association with heavy flavored mesons at the LHCb conditions is considered, and a detailed study of the different contributing processes is presented including single (SPS) and double (DPS) parton scattering mechanisms. We find that the usual DPS factorization formula needs to be corrected for the limited partonic phase space, and that including the relevant corrections reduces discrepancies in the associated $ZD$ production. We conclude finally that double parton scattering dominates the production of same-sign $W^\pm D^\pm$ states, as well as the production of $W^-$ bosons associated with $B$-mesons. The latter processes can thus be regarded as new useful DPS indicators.

An analysis of the impact of LHC Run I proton-lead data on nuclear parton densities.

We report on an analysis of the impact of available experimental data on hard processes in proton–lead collisions during Run I at the large hadron collider on nuclear modifications of parton distribution functions. Our analysis is restricted to the EPS09 and DSSZ global fits. The measurements that we consider comprise production of massive gauge bosons, jets, charged hadrons and pions. This is the first time a study of nuclear PDFs includes this number of different observables. The goal of the paper is twofold: (i) checking the description of the data by nPDFs, as well as the relevance of these nuclear effects, in a quantitative manner; (ii) testing the constraining power of these data in eventual global fits, for which we use the Bayesian reweighting technique. We find an overall good, even too good, description of the data, indicating that more constraining power would require a better control over the systematic uncertainties and/or the proper proton–proton reference from LHC Run II. Some of the observables, however, show sizeable tension with specific choices of proton and nuclear PDFs. We also comment on the corresponding improvements as regards the theoretical treatment.

IR-improved DGLAP-CS QCD parton showers in Pythia8

We introduce the recently developed IR-improved DGLAP-CS theory into the showers in Pythia8, as this Monte Carlo event generator is in wide use at LHC. We show that, just as it was true in the IR-improved shower Monte Carlo Herwiri, which realizes the IR-improved DGLAP-CS theory in the Herwig6.5 environment, the soft limit in processes such as single heavy gauge boson production is now more physical in the IR-improved DGLAP-CS theory version of Pythia8. This opens the way to one’s getting a comparison between the actual detector simulations for some of the LHC experiments between IR-improved and unimproved showers as Pythia8 is used in detector simulations at LHC whereas Herwig6.5, the environment of the only other IR-improved DGLAP-CS QCD MC in the literature, Herwiri1.031, is not any longer so used. Our achieving the availability of the IR-improved DGLAP-CS Pythia8 then is an important step in the further development of the LHC precision theory program under development by the author and his collaborators.

Similarity and differences between the radion and Higgs boson production and decay processes involving off-shell fermions

The radion is a scalar particle that occurs in brane world models and interacts with the trace of the energy–momentum tensor of the Standard Model (SM). The radion–SM fermion interaction Lagrangian differs from the Higgs boson–fermion interaction Lagrangian for off-shell fermions. It is shown that all additional, as compared to the Higgs boson, contributions to the amplitudes of radion production and decay processes involving off-shell fermions are canceled out for both massless and massive fermions. Thus, additional terms in the interaction Lagrangian do not change properties of these processes for the radion and the Higgs boson, except for the general normalization factors. This similarity is a consequence of gauge invariance for the processes with production of gauge bosons. When an additional scalar particle is produced, there are no apparent reasons for the above cancellation, as confirmed, for example, by the process with production of two scalar particles, which features an additional contribution of the radion in comparison with the Higgs boson.

Neutral triple vector boson production in Randall-Sundrum model at the LHC

In this paper, triple neutral electroweak gauge boson production processes, viz. \gamma\gamma\gamma, \gamma\gamma Z, \gamma ZZ and ZZZ productions merged to 1-jet have been studied at the leading order in QCD in the context of Randall-Sundrum model at the LHC with center of mass energy \sqrt{S}=13 TeV. Decay of Z bosons into lepton-pairs has been considered. We present a selection of kinematical distributions matched to parton shower and show their deviation from the SM results as a result of the RS model. The uncertainties as a result of the factorization and renormalization scales are also presented.

Addendum to “Electroweak Sudakov effects inW,Zandγproduction at large transverse momentum”

In this addendum to Phys. Rev. D 88, no. 1, 013009 (2013), we give results for the electroweak Sudakov corrections in gauge-boson production at large transverse momentum p_T at proton colliders. In order for the results to be easily usable, we provide a simple and accurate parameterization of the corrections as a function of p_T and the center-of-mass energy \sqrt{s}. Additionally, we also discuss the dependence of the electroweak corrections on the rapidity of the produced boson, and comment on the complications that arise in the photon-production case due to isolation requirements

Family gauge boson production at the LHC

Family gauge boson production at the LHC is investigated according to a U(3) family gauge model with twisted family number assignment. In the model we study, a family gauge boson with the lowest mass, A11, interacts only with the first generation leptons and the third generation quarks. (The family numbers are assigned, for example, as (e1,e2,e3)=(e−,μ−,τ−) and (d1,d2,d3)=(b,d,s) [or (d1,d2,d3)=(b,s,d)].) In the model, the family gauge coupling constant is fixed by relating to the electroweak gauge coupling constant. Thus measurements of production cross sections and branching ratios of A11 clearly confirm or rule out the model. We calculate the cross sections of inclusive A11 production and bb¯(tt¯) associated A11 production at s=14 TeV and 100 TeV. With the dielectron production cross section, we discuss the determination of diagonalizing matrix of quark mass matrix, Uu and Ud, respectively.

Evidence of Wγγ Production in pp Collisions at sqrt[s]=8 TeV and Limits on Anomalous Quartic Gauge Couplings with the ATLAS Detector.

This Letter reports evidence of triple gauge boson production pp→W(ℓν)γγ+X, which is accessible for the first time with the 8 TeV LHC data set. The fiducial cross section for this process is measured in a data sample corresponding to an integrated luminosity of 20.3 fb^{-1}, collected by the ATLAS detector in 2012. Events are selected using the W boson decay to eν or μν as well as requiring two isolated photons. The measured cross section is used to set limits on anomalous quartic gauge couplings in the high diphoton mass region.

Production of Z0 and in relativistic heavy-ion collisions at the LHC

The production of massive gauge bosons, Z0 and , in heavy-ion reactions at the Large Hadron Collider (LHC) provides an excellent tool for studying cold nuclear matter effects in high-energy nuclear collisions. In this paper we investigate Z0 and production in p+Pb and Pb+Pb at the LHC, at next-to-leading order and next-to-next-to-leading order, using the DYNNLO program and incorporating the nuclear parton distribution function (nPDFs) parametrization sets EPS09 and DSSZ, within the framework of perturbative quantum chromodynamics. Numerical simulations of the transverse momentum spectra, rapidity dependence and related nuclear modification factors for Z0 and W particles, as well as the charge asymmetry for the W boson, are provided and tested against the latest experimental data. It is found that the theoretical results with EPS09 and DSSZ nPDFs can provide good descriptions of the recent data on Z0 and particles in p+Pb and Pb+Pb within the experimental error bars, although some differences between the results with EPS09 and DSSZ can be observed, especially in the rapidity dependence of the Z0 yield. Theoretical predictions for future measurements on Z0 and W in p+Pb and Pb+Pb collisions at the LHC are also provided.

Electroweak gauge-boson production in association with b jets at Hadron Colliders

The production of both charged and neutral electroweak gauge bosons in association with b jets has attracted a lot of experimental and theoretical attention in recent years because of its central role in the physics programs of both the Fermilab Tevatron and the CERN Large Hadron Collider. The improved level of accuracy achieved both in the theoretical predictions and experimental measurements of these processes can promote crucial developments in modeling b-quark jets and b-quark parton distribution functions, and can provide a more accurate description of some of the most important backgrounds to the measurement of Higgs-boson couplings and several new physics searches. In this paper, we review the status of theoretical predictions for cross sections and kinematic distributions of processes in which an electroweak gauge boson is produced in association with up to two b jets in hadronic collisions, namely [Formula: see text], pp → V + 1b jet and [Formula: see text], pp → V + 2b jets with V = W±, Z/γ*, γ. Available experimental measurements at both the Fermilab Tevatron and the CERN Large Hadron Collider are also reviewed and their comparison with theoretical predictions is discussed.

Upper limit on the scale of new physics phenomena from rising cross sections in high multiplicity Higgs and vector boson events

In very high energy scattering events production of multiple Higgs and electroweak gauge bosons becomes possible. Indeed the perturbative cross section for these processes grows with increasing energy, eventually violating perturbative unitarity. In addition to perturbative unitarity we also examine constraints on high multiplicity processes arising from experimentally measured quantities. These include the shape of the Z-peak and upper limits on scattering cross sections of cosmic rays. We find that the rate of high multiplicity electroweak processes will exceed these upper limits at energies not significantly above what can be currently tested experimentally. This leaves two options: 1) The electroweak sector becomes truly non-perturbative in this regime or 2) Additional physics beyond the Standard Model is needed. In both cases novel physics phenomena must set in before these energies are reached. Based on the measured Higgs mass we estimate the critical energy to be in the range of $10^3$ TeV but we also point out that it can potentially be significantly less than that.

Associated [formula omitted] production at the LHC and prospects to observe double parton interactions

Associated production of charged gauge bosons and charged charmed mesons at the LHC is considered in the framework of kt-factorization approach. Theoretical predictions are compared with ATLAS data, and reasonably good agreement is found. Predictions on the same-sign W±D± configurations are presented including single parton scattering and double parton scattering contributions. The latter are shown to dominate over the former, thus giving evidence that the proposed process can serve as another indicator of double parton interactions.

Probing triple-W production and anomalous WWWW coupling at the CERN LHC and future O 100 $$ \mathcal{O}(100) $$ TeV proton-proton collider

Triple gauge boson production at the LHC can be used to test the robustness of the Standard Model and provide useful information for VBF di-boson scattering measurement. Especially, any derivations from SM prediction will indicate possible new physics. In this paper we present a detailed Monte Carlo study on measuring WWW production in pure leptonic and semileptonic decays, and probing anomalous quartic gauge WWWW couplings at the CERN LHC and future hadron collider, with parton shower and detector simulation effects taken into account. Apart from cut-based method, multivariate boosted decision tree method has been exploited for possible improvement. For the leptonic decay channel, our results show that at the sqrt{s}=8(14)[100] TeV pp collider with integrated luminosity of 20(100)[3000] fb-1, one can reach a significance of 0.4(1.2)[10]sigma to observe the SM WWW production. For the semileptonic decay channel, one can have 0.5(2)[14]sigma to observe the SM WWW production. We also give constraints on relevant Dim-8 anomalous WWWW coupling parameters.