Electroweak Boson Production Research Articles

Measurement of electroweak WZ boson production and search for new physics in WZ + two jets events in pp collisions at [formula omitted

A measurement of WZ electroweak (EW) vector boson scattering is presented. The measurement is performed in the leptonic decay modes WZ→ℓνℓ′ℓ′, where ℓ,ℓ′=e,μ. The analysis is based on a data sample of proton-proton collisions at s=13 TeV at the LHC collected with the CMS detector and corresponding to an integrated luminosity of 35.9 fb−1. The WZ plus two jet production cross section is measured in fiducial regions with enhanced contributions from EW production and found to be consistent with standard model predictions. The EW WZ production in association with two jets is measured with an observed (expected) significance of 2.2 (2.5) standard deviations. Constraints on charged Higgs boson production and on anomalous quartic gauge couplings in terms of dimension-eight effective field theory operators are also presented.

Electroweak probes of small and large systems with the ATLAS detector

Measurements of isolated prompt photon and massive electroweak (W and Z) boson production in different collision systems are of great interest to understand the partonic structure of heavy nuclei, and serve as a constraint on the initial state in larger collision systems. These channels are sensitive to a variety of effects such as the modification of the parton densities in nuclei in certain kinematic regions, and the energy loss of partons as they undergo multiple interactions in the nucleus before the hard parton-parton scattering. High-statistics samples of lead–lead and proton–lead collision data at sNN=5.02TeV and8.16TeV, respectively, taken by the ATLAS experiment at the LHC, as well as proton–proton comparison data at analogous collision energies, allow for a detailed study of these phenomena in data and comprehensive comparisons to the predictions of a variety of theoretical approaches. This paper presents the latest ATLAS results in these topics, including updated results on inclusive prompt photon production in proton–lead collisions over a broad kinematic range and high-precision W boson results in lead–lead collisions.

Electroweak boson measurements in p-Pb and Pb-Pb collisions at [formula omitted] with ALICE at the LHC

The ALICE collaboration has measured the electroweak boson production at forward rapidity in Pb-Pb and p-Pb collisions at sNN=5.02TeV. Final results on the Z0 boson production in Pb-Pb collisions are presented as a function of centrality and rapidity. Calculations based on free parton distribution functions overestimate the measurement up to 3σ. In addition, final results on W± and Z0 bosons production in p-Pb collisions at the same energy will be presented and discussed.

Recent results on hard processes in p+Pb, Pb+Pb, and pp collisions from the ATLAS Experiment at the LHC

In relativistic heavy-ion collisions, a hot medium with a high density of unscreened colour charges is produced. Hard processes from parton-parton scattering in the early stages of the collision can be used as probes of the medium since the initial state is mostly understood such that differences from pp collisions in the final state can be attributed to interactions with the medium. The initial and final state effects are separated by comparing p+Pb and pp collisions. Recent results from the ATLAS experiment at the LHC on the production of electroweak bosons, jets, and quarkonia are discussed in p+Pb and Pb+Pb collisions, compared to pp. Recent results on ultra-peripheral collisions are also shown.

NLO QCD corrections to SM-EFT dilepton and electroweak Higgs boson production, matched to parton shower in POWHEG

We discuss the Standard Model — Effective Field Theory (SM-EFT) contributions to neutral- and charge-current Drell-Yan production, associated production of the Higgs and a vector boson, and Higgs boson production via vector boson fusion. We consider all the dimension-six SM-EFT operators that contribute to these processes at leading order, include next-to-leading order QCD corrections, and interface them with parton showering and hadronization in Pythia8 according to the POWHEG method. We discuss existing constraints on the coefficients of dimension-six operators and identify differential and angular distributions that can differentiate between different effective operators, pointing to specific features of Beyond-the-Standard-Model physics.

Measurements of event properties and multi-differential jet cross sections and impact of CMS measurements on Proton Structure and QCD parameters

We present results on the measurements of characteristics of events with jets including jet-charge, investigations of shapes and jet mass distributions. The measurements are compared to theoretical predictions including those matched to parton shower and hadronization. Multi-differential jet cross sections are also presented over a wide range in transverse momenta from inclusive jets to multi-jet final states. These measurements have an impact on the determination of the strong coupling constant as well as on parton distribution functions (PDFs) and are helpful in the treatment of heavy flavours in QCD analyses. We also show angular correlations in multi-jet events at highest center-of-mass energies and compare the measurements to theoretical predictions including higher order parton radiation and coherence effects. Measurements of cross sections of jet and top-quark pair production are in particular sensitive to the gluon distribution in the proton, while the electroweak boson production - inclusive or associated with charm or beauty quarks - gives insight into the flavour separation of the proton sea and to the treatment of heavy quarks in PDF-related studies.

Heavy-ion Physics (ATLAS)

The ATLAS experiment at the Large Hadron Collider has undertaken a broad physics program to probe and characterize the hot nuclear matter created in relativistic heavy-ion collisions. This talk presents recent results on production of electroweak bosons and quarkonium, charged particles and jets, bulk particle collectivity and electromagnetic processes in ultra-peripheral collisions, from Pb+Pb and p+Pb systems.

Parton distributions from high-precision collider data

We present a new set of parton distributions, NNPDF3.1, which updates NNPDF3.0, the first global set of PDFs determined using a methodology validated by a closure test. The update is motivated by recent progress in methodology and available data, and involves both. On the methodological side, we now parametrize and determine the charm PDF alongside the light-quark and gluon ones, thereby increasing from seven to eight the number of independent PDFs. On the data side, we now include the D0 electron and muon W asymmetries from the final Tevatron dataset, the complete LHCb measurements of W and Z production in the forward region at 7 and 8 TeV, and new ATLAS and CMS measurements of inclusive jet and electroweak boson production. We also include for the first time top-quark pair differential distributions and the transverse momentum of the Z bosons from ATLAS and CMS. We investigate the impact of parametrizing charm and provide evidence that the accuracy and stability of the PDFs are thereby improved. We study the impact of the new data by producing a variety of determinations based on reduced datasets. We find that both improvements have a significant impact on the PDFs, with some substantial reductions in uncertainties, but with the new PDFs generally in agreement with the previous set at the one-sigma level. The most significant changes are seen in the light-quark flavor separation, and in increased precision in the determination of the gluon. We explore the implications of NNPDF3.1 for LHC phenomenology at Run II, compare with recent LHC measurements at 13 TeV, provide updated predictions for Higgs production cross-sections and discuss the strangeness and charm content of the proton in light of our improved dataset and methodology. The NNPDF3.1 PDFs are delivered for the first time both as Hessian sets, and as optimized Monte Carlo sets with a compressed number of replicas.

Electroweak boson production in p–Pb and Pb–Pb collisions at [formula omitted] with ALICE

W and Z bosons are massive weakly-interacting particles, insensitive to the strong interaction. They provide therefore a medium-blind probe of the initial state of the heavy-ion collisions. The final results for the W and Z-boson production in p–Pb collisions at sNN=5.02 TeV are presented. The production cross-section is well described by perturbative calculations but the data cannot further constrain the nuclear modification of the parton distribution functions. The W-boson production is also studied as a function of the collision centrality, and found to be constant within uncertainties.The Z-boson production was also measured in Pb–Pb collisions at sNN=5.02 TeV with ALICE. This is the first measurement at forward rapidity at such energies. The preliminary results for the invariant yields divided by the nuclear overlap functions are described by perturbative calculations including nuclear modifications of the PDFs.

Multiboson interactions at the LHC

This review covers results on the production of all possible electroweak boson pairs and 2-to-1 vector boson fusion at the CERN Large Hadron Collider (LHC) in proton-proton collisions at a center of mass energy of 7 and 8 TeV. The data were taken between 2010 and 2012. Limits on anomalous triple gauge couplings (aTGCs) then follow. In addition, data on electroweak triple gauge boson production and 2-to-2 vector boson scattering yield limits on anomalous quartic gauge boson couplings (aQGCs). The LHC hosts two general purpose experiments, ATLAS and CMS, which have both reported limits on aTGCs and aQGCs which are herein summarized. The interpretation of these limits in terms of an effective field theory is reviewed, and recommendations are made for testing other types of new physics using multi-gauge boson production.

W+W−+3 -jet production at the Large Hadron Collider in next-to-leading-order QCD

We present next-to-leading order (NLO) QCD predictions to $W^+W^-$ production in association with up to three jets at hadron colliders. We include contributions from couplings of the $W$ bosons to light quarks as well as trilinear vector couplings. These processes are used in vector-boson coupling measurements, are background to Higgs signals and are needed to constrain many new physics scenarios. For the first time NLO QCD predictions are shown for electroweak di-vector boson production with three jets at a hadron collider. We show total and differential cross sections for the LHC with proton center-of-mass energies of 8 and 13 TeV. To perform the calculation we employ on-shell and unitarity methods implemented in the BlackHat library along with the SHERPA package. We have produced event files that can be accessed for future dedicated studies.

Recent heavy ion results from the ATLAS experiment

The ATLAS experiment at the LHC has undertaken a broad physics program to probe and characterize the hot nuclear matter created in relativistic lead-lead collisions. This overview presents recent results on production of electroweak bosons, charmonia, charged-particles, jets, and bulk particle collectivity from Pb+Pb and p +Pb collisions at 2.76 TeV and 5.02 TeV, respectively, as well as on electromagnetic processes in ultra-peripheral Pb+Pb collisions at 5.02 TeV.

Top physics at the LHC ?

The status of top physics at the Large Hadron Collider is discussed, with particular attention on measurements of top production cross sections, including associate production of top quarks and electroweak bosons, and on measurements of the top mass.

Consistent simulation of nonresonant diphoton production in hadron collisions including associated jet production up to two jets

An event generator for diphoton ($\gamma\gamma$) production in hadron collisions that includes associated jet production up to two jets has been developed using a subtraction method based on the LLL subtraction. The parton shower (PS) simulation to restore the subtracted divergent components involves both QED and QCD radiation, and QED radiation at very small $Q^{2}$ are simulated by referring to a fragmentation function (FF). The PS/FF simulation has the ability to enforce the radiation of a given number of energetic photons. The generated events can be fed to PYTHIA to obtain particle (hadron)-level event information, which enables us to perform realistic simulations of photon isolation and hadron-jet reconstruction. The simulated events, in which the loop-mediated $gg \rightarrow \gamma\gamma$ process is involved, reasonably reproduce the diphoton kinematics measured at the LHC. Using the developed simulation, we found that the 2-jet processes significantly contribute to diphoton production. A large 2-jet contribution can be considered as a common feature in electroweak-boson production in hadron collisions although the reason is yet to be understood. Discussion concerning the treatment of the underlying events in photon isolation is necessary for future higher precision measurements.

W and Z bosons with CMS in pp, pPb and PbPb collisions

Electroweak boson production is an important benchmark process in high-energy heavy-ion collisions at the LHC. W and Z bosons do not participate in the strong interaction and their leptonic decays provide medium-blind probes of the initial state of the collisions. The final results on the W and Z production in pPb collisions at 5.02 TeV, combining both the muon and electron channels, will be presented. When compared to theory calculations that include nuclear modifications to the parton distributions, data show a clear sensitivity to this type of effects. The final results in PbPb collisions at 2.76 TeV, compared to pp collisions at the same centre of mass energy, will also be presented. The centrality dependence confirms the binary scaling of hard probes in heavy-ion collisions, while the differential cross sections points to initial state effects small compared to the statistical precision of the available data.

Vector-boson production in p + Pb collisions measured with ATLAS at the LHC

Electroweak–boson production processes (W, Z and photon) provide access to the earliest moments of heavy–ion collisions. Furthermore, because they do not undergo strong interactions, they are sensitive to the initial–state geometry of the collision and potentially the details of the nuclear parton distribution functions. ATLAS results on vector–boson yields have demonstrated binary collision scaling in Pb+Pb collisions. In p + Pb collisions, the measurement of vector bosons provides possible constraints on the nuclear parton distribution functions and insights into the details of the initial collision geometry. We report on the latest results of vector–boson production in p + Pb collisions at sNN=5.02 TeV. Production yields of Z and W bosons and lepton charge asymmetry of W bosons are presented as a function of pseudo-rapidity and centrality. The vector–boson yields are compared to calculations incorporating different parton distribution functions, as well as different centrality calculations.

CMS results on electroweak probes from LHC Run 1

Electroweak boson production is an important benchmark process in high-energy heavy-ion collisions at the LHC. The Z and W bosons do not participate in the strong interaction and their leptonic decays provide medium-blind probes of the initial state of the collisions. Final results on Z boson production in PbPb collisions compared to pp collisions are presented. The centrality dependence confirms the binary scaling of hard probes in heavy-ion collisions and the differential cross sections show that initial state effects are small compared to the statistical precision of the available data. Measurements of the W and Z boson production in pPb collisions, combining both the muon and electron decay channels are also presented. The data are compared to theory predictions for nuclear modifications of the parton distributions and show a clear sensitivity to these effects.

Hard probes of small systems

This proceedings discusses the experimental status of high-pT jet, hadron, and electroweak boson production measurements in proton- and deuteron–nucleus (p/d+A) collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), with an emphasis on results presented for the first time at the Hard and Electromagnetic Probes conference at McGill University in June 2015.

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.

Electroweak Physics at LHCb

The LHCb forward acceptance covers a range of rapidities not accessible by the other LHC experiments, allowing for complementary measurements. We report recent measurements of electroweak boson production, either inclusive, or in association with a jet or a D meson.