Vector Boson Research Articles

Peculiarities of pancreatic dysfunction in patients with chronic obstructive pulmonary disease

We consider the production of a Standard Model Higgs boson decaying to bottom quarks in association with a vector boson W±/Z in hadron collisions. We present a fully exclusive calculation of QCD radiative corrections both for the production cross section and for the Higgs boson decay rate up to next-to-next-to-leading order (NNLO) accuracy. Our calculation also includes the leptonic decay of the vector boson with finite-width effects and spin correlations. We consider typical kinematical cuts applied in the experimental analyses at the Large Hadron Collider (LHC) and we find that the full NNLO QCD corrections significantly decrease the accepted cross section and have a substantial impact on the shape of distributions. We point out that these additional effects are essential to obtain precise theoretical predictions to be compared with the LHC data.

Measurement of the Higgs quartic coupling c 2 v from di-Higgs Vector Boson Fusion in the channel

The Brout Englert Higgs (BEH) mechanism of electroweak symmetry breaking and mass generation was experimentally confirmed after the discovery of the Higgs boson at the Large Hadron Collider in 2012. The BEH mechanism not only predicts the existence of a massive scalar particle, but also requires this scalar particle to couple to itself. Double Higgs production provides a unique handle, since it allows the extraction of the trilinear Higgs self-coupling. VBF di-Higgs production also probes the quartic Higgs bosons to vector bosons coupling (c 2 v). In this topic the effort on setting constraints on c 2 v will be discussed. Event selection and reconstruction will be illustrated as well as a Neural Network designed to identify VBF events.

Radiatively generated fermion mass hierarchy from flavor nonuniversal gauge symmetries

A framework based on a class of abelian gauge symmetries is proposed in which the masses of only the third generation quarks and leptons arise at the tree level. The fermions of the first and second families receive their masses through radiative corrections induced by the new gauge bosons in the loops. It is shown that the class of abelian symmetries which can viably implement this mechanism are flavour non-universal in nature. Taking the all-fermion generalization of the well-known leptonic $L_\mu-L_\tau$ and $L_e - L_\mu$ symmetries, we construct an explicit renormalizable model based on two $U(1)$ which is shown to reproduce the observed fermion mass spectrum of the Standard Model. The first and second generation fermion masses are loop suppressed while the hierarchy between these two generations results from a gap between the masses of two vector bosons of the extended gauge symmetries. Several phenomenological aspects of the flavourful new physics are discussed and lower limits on the masses of the vector bosons are derived.

Ghost Instabilities in Self-Interacting Vector Fields: The Problem with Proca Fields.

Massive vector fields feature in several areas of particle physics, e.g., as carriers of weak interactions, dark matter candidates, or an effective description of photons in a plasma. Here, we investigate vector fields with self-interactions by replacing the mass term in the Proca equation with a general potential. We show that this seemingly benign modification inevitably introduces ghost instabilities of the same kind as those recently identified for vector-tensor theories of modified gravity (but in this simpler, minimally coupled theory). It has been suggested that nonperturbative dynamics may drive systems away from such instabilities. We demonstrate that this is not the case by evolving a self-interacting Proca field on a Kerr background, where it grows due to the superradiant instability. The system initially evolves as in the massive case, but instabilities are triggered in a finite time once the self-interaction becomes significant. These instabilities have implications for the formation of condensates of massive, self-interacting vector bosons, the possibility of spin-one bosenovae, vector dark matter models, and effective models for interacting photons in a plasma.

Prospects for Heavy Neutral SUSY HIGGS Scalars in the hMSSM and Natural SUSY at LHC Upgrades

We examine production and decay of heavy neutral SUSY Higgs bosons pp→H,A→ττ¯ within the hMSSM and compare against a perhaps more plausible natural supersymmetry scenario dubbed mh125(nat) which allows for a natural explanation for mweak≃mW,Z,h∼100 GeV while maintaining mh≃125 GeV. We evaluate signal against various Standard Model backgrounds from γ,Z→ττ¯, tt¯ and vector boson pair production VV. We combine the transverse mass method for back-to-back (BtB) taus along with the ditau mass peak mττ method for acollinear taus as our signal channels. This technique ultimately gives a boost to the signal significance over the standard technique of using just the BtB signal channel. We evaluate both the 95% CL exclusion and 5σ discovery reach in the mA vs. tanβ plane for present LHC with 139 fb−1, Run 3 with 300 fb−1 and high luminosity LHC (HL-LHC) with 3000 fb−1 of integrated luminosity. For tanβ=10, the exclusion limits range up to mA∼1, 1.1 and 1.4 TeV, respectively. These may be compared to the range of mA values gleaned from a statistical analysis of the string landscape wherein mA can range up to ∼8 TeV.

Resolving an ambiguity of Higgs couplings in the FSM, greatly improving thereby the model’s predictive range and prospects

We show that, after resolving what was thought to be an ambiguity in the Higgs coupling, the FSM gives, apart from two extra terms (i) and (ii) to be specified below, an effective action in the standard sector which has the same form as the SM action, the two differing only in the values of the mass and mixing parameters of quarks and leptons which the SM takes as Γinputs from experiment while the FSM obtains as a result of a fit with a few parameters. Hence, to the accuracy that these two sets of parameters agree in value, and they do to a good extent as shown in earlier work, 1 the FSM should give the same result as the SM in all the circumstances where the latter has been successfully applied, except for the noted modifications due to (i) and (ii). If so, it would be a big step forward for the FSM. The correction terms are: (i) a mixing between the SM’s [Formula: see text] with a new vector boson in the hidden sector, (ii) a mixing between the standard Higgs with a new scalar boson also in the hidden sector. And these have been shown a few years back to lead to (i[Formula: see text]) an enhancement of the W mass over the SM value, 2 and (ii[Formula: see text]) effects consistent with the [Formula: see text] and some other anomalies, 3 precisely the two deviations from the SM reported by experiments 4 , 5 recently much in the news.

Vortex String Formation in Black Hole Superradiance of a Dark Photon with the Higgs Mechanism.

Black hole superradiance, which only relies on gravitational interactions, can provide a powerful probe of the existence of ultralight bosons that are weakly coupled to ordinary matter. However, as a boson cloud grows through superradiance, nonlinear effects from interactions with itself or other fields may become important. As a representative example of this, we use nonlinear evolutions to study black hole superradiance of a vector boson that attains a mass, via a coupling to a complex scalar, through the Higgs mechanism. For the cases considered, we find that the superradiant instability can lead to a transient period where the scalar field reaches its symmetry restoration value, leading to the formation of closed vortex strings, the temporary disruption of the exponential growth of the cloud, and an explosive outburst of energy. After the cloud loses sufficient mass, the superradiant growth resumes, and the cycle repeats. Thus, the black hole will be spun down but, potentially, at a much lower rate compared to when nonlinear effects are unimportant and with the liberated energy going primarily into bosonic radiation instead of gravitational waves.

Prospects for a flavour violating Z^prime explanation of Delta a_{mu ,e}

The apparent tensions emerging from the comparison of experimental data of the anomalous magnetic moments of the muon and electron to the Standard Model predictions (Delta a_{mu ,e}) could be interpreted as a potential signal of New Physics. Models encompassing a light vector boson have been known to offer a satisfactory explanation to Delta a_{mu }, albeit subject to stringent experimental constraints. Here we explore a minimal extension of the Standard Model via a leptophilic vector boson Z^prime , under the hypothesis of strictly flavour-violating couplings of the latter to leptons. The most constraining observables to this ad-hoc construction emerge from lepton flavour universality violation (in Z and tau decays) and from rare charged lepton flavour violating transitions. Once these are accommodated, one can saturate the tensions in Delta a_{mu }, but Delta a_{e} is predicted to be Standard Model-like. We infer prospects for several observables, including leptonic Z decays and several charged lepton flavour violating processes. We also discuss potential signatures of the considered Z^prime at a future muon collider, emphasising the role of the mu ^+mu ^- rightarrow tau ^+tau ^- forward-backward asymmetry as a key probe of the model.

Effective field theory of Stückelberg vector bosons

We explore the effective field theory of a vector field ${X}^{\ensuremath{\mu}}$ that has a St\"uckelberg mass. The absence of a gauge symmetry for ${X}^{\ensuremath{\mu}}$ implies Lorentz-invariant operators are constructed directly from ${X}^{\ensuremath{\mu}}$. Beyond the kinetic and mass terms, allowed interactions at the renormalizable level include ${X}_{\ensuremath{\mu}}{X}^{\ensuremath{\mu}}{H}^{\ifmmode\dagger\else\textdagger\fi{}}H$, $({X}_{\ensuremath{\mu}}{X}^{\ensuremath{\mu}}{)}^{2}$, and ${X}_{\ensuremath{\mu}}{j}^{\ensuremath{\mu}}$, where ${j}^{\ensuremath{\mu}}$ is a global current of the SM or of a hidden sector. We show that all of these interactions lead to scattering amplitudes that grow with powers of $\sqrt{s}/{m}_{X}$, except for the case of ${X}_{\ensuremath{\mu}}{j}^{\ensuremath{\mu}}$ where ${j}^{\ensuremath{\mu}}$ is a nonanomalous global current. The latter is well known when $X$ is identified as a dark photon coupled to the electromagnetic current, often written equivalently as kinetic mixing between $X$ and the photon. The power counting for the energy growth of the scattering amplitudes is facilitated by isolating the longitudinal enhancement. We examine in detail the interaction with an anomalous global vector current ${X}_{\ensuremath{\mu}}{j}_{\mathrm{anom}}^{\ensuremath{\mu}}$, carefully isolating the finite contribution to the fermion triangle diagram. We calculate the longitudinally-enhanced observables $Z\ensuremath{\rightarrow}X\ensuremath{\gamma}$ (when ${m}_{X}<{m}_{Z}$), $f\overline{f}\ensuremath{\rightarrow}X\ensuremath{\gamma}$, and $Z\ensuremath{\gamma}\ensuremath{\rightarrow}Z\ensuremath{\gamma}$ when $X$ couples to the baryon number current. Introducing a ``fake'' gauge-invariance by writing ${X}^{\ensuremath{\mu}}={A}^{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{\partial}}^{\ensuremath{\mu}}\ensuremath{\pi}/{m}_{X}$, the would-be gauge anomaly associated with ${A}_{\ensuremath{\mu}}{j}_{\mathrm{anom}}^{\ensuremath{\mu}}$ is canceled by ${j}_{\mathrm{anom}}^{\ensuremath{\mu}}{\ensuremath{\partial}}_{\ensuremath{\mu}}\ensuremath{\pi}/{m}_{X}$; this is the four-dimensional Green-Schwarz anomaly-cancellation mechanism at work. Our analysis demonstrates there is a much larger set of possible interactions that an EFT with a St\"uckelberg vector field can have, revealing scattering amplitudes that grow with energy. The growth of these amplitudes can be tamed by a dark Higgs sector, but this requires dark Higgs boson interactions (and reintroduces fine-tuning in the dark Higgs sector) that can be separated from $X$ interactions only in the limit $g\ensuremath{\ll}1$.

Probing the proton structure with associated vector boson and heavy flavor jet production at the LHC

We consider the production of $Z$ bosons associated with heavy (charm and beauty) jets at the LHC energies using two scenarios based on the transverse momentum dependent (TMD) parton densities in a proton. The first of them employs the Catani-Ciafaloni-Fiorani-Marchesini gluon evolution and is implemented in the Monte-Carlo event generator PEGASUS. Here, the heavy quarks are always produced in the hard partonic scattering. The second scheme is based on the parton branching approach, currently implemented into the Monte-Carlo event generator CASCADE. In this scenario, the $Z$ + jets sample is generated and then events containing the heavy flavor jet in a final state are selected. We compare the predictions obtained within these two TMD-based approaches to each other, investigate their sensitivity to the TMD gluon densities in a proton and estimate the effects coming from parton showers and double parton scattering mechanism. Additionally, we compare our predictions with the results of traditional (collinear) pQCD calculations performed at NLO accuracy. It is shown that the TMD-based results agree with the LHC experimental data collected at $\sqrt s = 8$ and $13$ TeV. We discuss the sensitivity of observables to the quark distributions in a proton and present predictions to search for the intrinsic charm signal in forthcoming analyses of the LHC experimental data.

Evidence for WW/WZ vector boson scattering in the decay channel ℓνqq produced in association with two jets in proton-proton collisions at [formula omitted] TeV

Evidence is reported for electroweak (EW) vector boson scattering in the decay channel ℓνqq of two weak vector bosons WV (V=W or Z), produced in association with two parton jets. The search uses a data set of proton-proton collisions at 13 TeV collected with the CMS detector during 2016–2018 with an integrated luminosity of 138fb−1. Events are selected requiring one lepton (electron or muon), moderate missing transverse momentum, two jets with a large pseudorapidity separation and a large dijet invariant mass, and a signature consistent with the hadronic decay of a W/Z boson. The cross section is computed in a fiducial phase space defined at parton level requiring all parton transverse momenta pT>10GeV and at least one pair of outgoing partons with invariant mass mqq>100GeV. The measured and expected EW WV production cross sections are 1.90−0.46+0.53pb and 2.23−0.11+0.08(scale)±0.05(PDF)pb, respectively, where PDF is the parton distribution function. The observed EW signal strength is μEW=0.85±0.12(stat)−0.17+0.19(syst), corresponding to a signal significance of 4.4 standard deviations with 5.1 expected, and it is measured keeping the quantum chromodynamics (QCD) associated diboson production fixed to the standard model prediction. This is the first evidence of vector boson scattering in the ℓνqq decay channel at LHC. The simultaneous measurement of the EW and QCD associated diboson production agrees with the standard model prediction.

Measuring the anomalous quartic gauge couplings in the W+W−→ W+W− process at muon collider using artificial neural networks

The muon collider provides a unique opportunity to study the vector boson scattering processes and dimension-8 operators contributing to anomalous quartic gauge couplings (aQGCs). Because of the cleaner final state, it is easier to decode subprocess and certain operator couplings at a muon collider. We attempt to identify the anomalous WWWW coupling in the exclusive WW → WW scattering in this paper. Since one aQGC can be induced by multiple dimension-8 operators, the study of one coupling can help to confine different operators. Meanwhile, singling out the WW → WW process can help to study the unitarity bounds. The vector boson scattering process corresponding to the anomalous WWWW coupling is μ+μ− → nu nu overline{nu}overline{nu } ℓ+ℓ−, with four (anti-)neutrinos in the final state, which brings troubles in phenomenological studies. In this paper, the machine learning method is used to tackle this problem. We find that, the artificial neural network is helpful to extract the W+W− → W+W− contribution, and reconstruct the center of mass energy of the subprocess which is important in the study of the Standard Model effective field theory. The sensitivities and the expected constraints on the dimension-8 operators at the muon collider with sqrt{s} = 30 TeV are presented. We demonstrate that the artificial neural networks exhibit great potential in the phenomenological study of processes with multiple neutrinos in the final state.

Multi-jet merging with TMD parton branching

One of the main theoretical systematic uncertainties in studies of final states with large jet multiplicities at high-energy hadron colliders is associated with the merging of QCD parton showers and hard-scattering matrix elements. We present a method to incorporate the physics of transverse momentum recoils due to initial-state shower evolution into multi-jet merging algorithms by using the concept of transverse momentum dependent (TMD) distributions and the associated parton branching. We investigate the dependence on the merging scale and illustrate the impact of the new method at the level of both exclusive and inclusive final-state observables by studying differential jet rates, transverse momentum spectra and multiplicity distributions, using vector boson + jets events at the LHC as a case study.

Soft QCD effects in VBS/VBF topologies

We consider the impact of multi-parton interactions and colour reconnection on the modeling of vector boson fusion and vector boson scattering (VBS) final states at the Large Hadron Collider (LHC). We investigate how the variation of the model parameters, compatible with a reasonable spread of predictions around typical tuning observables, extrapolates into the VBS phase space. We study the implications of this variation on the total uncertainty budget attached to realistic simulations of the final states in current event generator predictions. We find that the variations have a non-trivial phase space dependence and become comparable in size to the perturbative uncertainties once next-to-leading order predictions are combined with parton shower evolution.

Mass Generation and Gravity

Despite the success of the Higgs mechanism to account for the generation of masses of the Standard Model (SM) elementary particles, the ultimate nature and origin of “mass” remain open questions in contemporary physics. From a foundational perspective, mass should be fundamentally related to the gravitational interaction and, according to Mach, with the structure of the Universe. In the present letter, a fully dynamical mass generation mechanism induced by higher-order corrections in the GR Lagrangian is discussed. Notably, the vacuum energy plays a key role in this process, which applies to both vector bosons and fermions. We show that, at the classical level, the Higgs mechanism can be though of as a particular case of the present mechanism.

SMEFT analysis of mW

We use the Fitmaker tool to incorporate the recent CDF measurement of mW in a global fit to electroweak, Higgs, and diboson data in the Standard Model Effective Field Theory (SMEFT) including dimension-6 operators at linear order. We find that including any one of the SMEFT operators mathcal{O} HWB, mathcal{O} HD, mathcal{O} ℓℓ or {mathcal{O}}_{Hell}^{(3)} with a non-zero coefficient could provide a better fit than the Standard Model, with the strongest pull for mathcal{O} HD and no tension with other electroweak precision data. We then analyse which tree-level single-field extensions of the Standard Model could generate such operator coefficients with the appropriate sign, and discuss the masses and couplings of these fields that best fit the CDF measurement and other data. In particular, the global fit favours either a singlet Z′ vector boson, a scalar electroweak triplet with zero hypercharge, or a vector electroweak triplet with unit hypercharge, followed by a singlet heavy neutral lepton, all with masses in the multi-TeV range for unit coupling.

Amplitude bases in generic EFTs

We present for the first time an efficient algorithm to find a basis of kinematically independent structures built of (massless and massive) spinor helicity variables in four dimensions. This method provides a classification of independent contact terms for the scattering amplitudes with generic masses, spins, and multiplicity in any effective field theory (EFT). These contact terms are in one-to-one correspondence with a complete set of irrelevant operators in the EFT. As basic applications of our method, we classify the D2nF4 contact terms in SU(N) Yang-Mills theory for n ≤ 8, dimension-six operators involving five W±, Z and γ vector bosons, and spin-tidal effective interactions for spin-1 massive particles in gravitational theories.

Electroweak ALP searches at a muon collider

A high-energy muon collider with center-of-mass energy around and above 10 TeV is also a vector boson fusion (VBF) machine, due to the significant virtual electroweak (EW) gauge boson content of high-energy muon beams. This feature, together with the clean environment, makes it an ideal collider to search for TeV-scale axion-like particles (ALP) coupling to Standard Model EW gauge bosons, which current and other future colliders have limited sensitivities to. We present detailed analyses of heavy ALP searches in both the VBF and associated production channels at a muon collider with different running benchmarks. We also show projected constraints on the ALP couplings in the effective field theory, including an operator with its coefficient not determined by the mixed Peccei-Quinn anomaly. We demonstrate that a muon collider could probe new ALP parameter space and push the sensitivities of the couplings between the ALP and EW gauge bosons by one order of magnitude compared to HL-LHC. The projected limits and search strategies for ALPs could also be applied to other types of resonances coupling to EW gauge bosons.

Search for a light Z′ in the Lμ−Lτ scenario with the NA64- e experiment at CERN

The extension of Standard Model made by inclusion of additional $U(1)$ gauge $L_\mu-L_\tau$ symmetry can explain the difference between the measured and the predicted value of the muon magnetic moment and solve the tension in $B$ meson decays. This model predicts the existence of a new, light $Z^\prime$ vector boson, predominantly coupled to second and third generation leptons, whose interaction with electrons is due to a loop mechanism involving muons and taus. In this work, we present a rigorous evaluation of the upper limits in the $Z^\prime$ parameter space, obtained from the analysis of the data collected by the NA64-$e$ experiment at CERN SPS, that performed a search for light dark matter with $2.84\times10^{11}$ electrons impinging with 100 GeV on an active thick target. The resulting limits, despite being included in a region already investigated by neutrino experiments,touch the muon $g-2$ preferred band for values of the $Z^\prime$ mass of order of 1 MeV. The sensitivity projections for the future high-statistics NA64-$e$ runs demonstrate the power of the electrons/positron beam approach in this theoretical scenario.

FIMP dark matter mediated by a massive gauge boson around the phase transition period and the gravitational waves production

We study the feebly-interacting massive particle dark matter whose production processes are significantly affected by the phase evolution and the complicated thermal corrections to the vector boson. We calculate the freeze-in processes to obtain the correct dark matter relic density by enumerating all the possible $1\ensuremath{\leftrightarrow}2$ and $2\ensuremath{\leftrightarrow}2$ processes. The predicted gravitational waves emitted by the first-order phase transitions and the cosmic strings are evaluated.