CP-odd Couplings Research Articles

Determination of CP-violating HZZ interaction with polarised beams at the ILC

We study possible CP-violation effects of the 125 GeV Higgs to Z boson coupling at the 250 GeV ILC with transverse and longitudinal beam polarisation via the process e+e-→HZ→Hμ-μ+. We explore the azimuthal angular distribution of the muon pair from the Z boson decay, and constructe CP-odd observables sensitive to CP-violation effects, where we derived this observable both by analytical calculations and by Whizard simulations. Particularly, we can construct two CP-odd observables with the help of transversely-polarised initial beams and improve the statistical significance of CP-violation effects by combining two measurements. We defined the asymmetries between the signal regions with different signs of the CP-odd observables, and determine the CP-violation effect by comparing with the SM 95% CL upper bound. In this paper, we setup a scenario which assumes that the total cross-section is always fixed while CP-violation is varying, and such a scenario helps us to determine the intrinsic CP-mixing angle limit around |ξCP|∼0.03rad with (90%, 40%) polarised electron-positron beams and 5 ab-1 integrated luminosity. In addition, we determine the CP-odd coupling limit |c~HZZ|∼0.01 as well, where we suppose that the SM tree-level cross-section is fixed and the CP-violation is the varying additional contribution. Comparing with the analysis with unpolarised beams, the sensitivity to the CP-violation effect can be improved by transverse or longitudinal polarisation.

Prospect study for the measurement of the Hbb¯ coupling at the LHC and FCC-hh

This paper employs the H+b+jets signature in proton-proton collisions to explore the structure of the Hbb¯ couplings. The focus of the analysis lies in the decay of the Higgs boson into a photon pair, taking into account both reducible and irreducible backgrounds and a realistic simulation of the detector effects. To enhance the discrimination between signal and background, a multivariate analysis is employed to analyze the kinematic variables and optimise the signal-to-background ratio. The results indicate that the H+b+jets process can significantly contribute to the precise measurement of CP-even and CP-odd couplings between the bottom quark and the Higgs boson at the LHC and FCC-hh. Finally, a novel asymmetry is introduced for the purpose of probing CP violation within the Hbb¯ coupling, formulated exclusively based on lab-frame momenta. Published by the American Physical Society 2024

Test of CP-invariance of the Higgs boson in vector-boson fusion production and in its decay into four leptons

A search for CP violation in the decay kinematics and vector-boson fusion production of the Higgs boson is performed in the H → ZZ* → 4ℓ (ℓ = e, μ) decay channel. The results are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb−1. Matrix element-based optimal observables are used to constrain CP-odd couplings beyond the Standard Model in the framework of Standard Model effective field theory expressed in the Warsaw and Higgs bases. Differential fiducial cross-section measurements of the optimal observables are also performed, and a new fiducial cross-section measurement for vector-boson-fusion production is provided. All measurements are in agreement with the Standard Model prediction of a CP-even Higgs boson.

Impact of longitudinally and transversely polarized beams on probing CP-violating HZZ interactions at the ILC

We study possible CP-violation effects of the Higgs to Z-boson coupling at a future e+e− collider, e.g. the International Linear Collider (ILC). We find that the azimuthal angular distribution of the muon pair, produced by e+e− → HZ → Hµ+µ−, can be sensitive to such a CP-violation effect when the initial beams are transversely polarized. Based on this angular distribution, we construct a CP sensitive asymmetry and obtain this asymmetry by Whizard simulation. By comparing the SM prediction with 2σ range of this asymmetry, we estimate the limit of the CP-odd coupling in the HZZ interaction.

Constraining new physics in entangled two-qubit systems: top-quark, tau-lepton and photon pairs

The measurement of quantum entanglement can provide a new and most sensitive probe to physics beyond the Standard Model. We use the concurrence of the top-quark pair spin states produced at colliders to constrain the magnetic dipole term in the coupling between top quark and gluons, that of tau -lepton pair spin states to bound contact interactions and that of tau -lepton pairs or two-photons spin states from the decay of the Higgs boson in trying to distinguish between CP-even and odd couplings. These four examples show the power of the new approach as well as its limitations. We show that differences in the entanglement in the top-quark and tau -lepton pair production cross sections can provide constraints better than those previously estimated from total cross sections or classical correlations. Instead, the final states in the decays of the Higgs boson remain maximally entangled even in the presence of CP-odd couplings and cannot be used to set bounds on new physics. We discuss the violation of Bell inequalities featured in all four processes.

Global constraints on Yukawa operators in the standard model effective theory

CP-violating contributions to Higgs-fermion couplings are absent in the standard model of particle physics (SM), but are motivated by models of electroweak baryogenesis. Here, we employ the framework of the SM effective theory (SMEFT) to parameterise deviations from SM Yukawa couplings. We present the leading contributions of the relevant operators to the fermionic electric dipole moments (EDMs). We obtain constraints on the SMEFT Wilson coefficients from the combination of LHC data and experimental bounds on the electron, neutron, and mercury EDMs. We perform, for the first time, a combined fit to LHC and EDM data allowing the presence of CP-violating contributions from several fermion species simultaneously. Among other results, we find non-trivial correlations between EDM and LHC constraints even in the multi-parameter scans, for instance, when floating the CP-even and CP-odd couplings to all third-generation fermions.

Measuring single-top-associated Higgs production at the HL-LHC

Precision measurements of top-associated Higgs production are an important ingredient to unravel the CP nature of the Higgs boson. In this work, we constraint the CP nature of the top-Yukawa coupling taking into account all relevant inclusive and differential Higgs boson measurements. Based upon this fit, we show that it is crucial to disentangle single- and di-top-associated Higgs production for tightening indirect constraints on a CP-odd top-Yukawa coupling in the future. In this context, we propose an analysis strategy for measuring tH production at the HL-LHC without relying on assumptions about the Higgs CP character.

CP-violating axion interactions in effective field theory

Axions are introduced to explain the observed smallness of the overline{theta} term of QCD. Standard Model extensions typically contain new sources of CP violation, for instance to account for the baryon asymmetry of the universe. In the presence of additional CP-violating sources a Peccei-Quinn mechanism does not remove all CP violation, leading to CP-odd interactions among axions and Standard Model fields. In this work, we use effective field theory to parametrize generic sources of beyond-the-Standard-Model CP violation. We systematically compute the resulting CP-odd couplings of axions to leptons and hadrons by using chiral perturbation theory. We discuss in detail the phenomenology of the CP-odd axion couplings and compare limits from axion searches, such as fifth force and monopole-dipole searches and astrophysics, to direct limits on the CP-violating operators from electric dipole moment experiments. While limits from electric dipole moment searches are tight, the proposed ARIADNE experiment can potentially improve the existing constraints in a window of axion masses.

Non-derivative axionic couplings to nucleons at large and small N

Among the possible CP-odd couplings of the axion to ordinary matter, the most relevant ones for phenomenology are the Yukawa couplings to nucleons. We analyze such non-derivative couplings within three different approaches: standard effective field theory, the Skyrme model and holographic QCD. In all the cases, the couplings can be related to the CP-odd non-derivative couplings to nucleons of the low-lying mesons and the η′. Using the effective field theory approach we discuss how to derive the expressions for the CP-odd interaction terms as functions of the parameters of the effective Lagrangian at generic number of colors Nc and flavors Nf. Then, we compute the CP-odd couplings to nucleons of the axion, the η′ and the pseudo-Goldstone mesons in both the Skyrme and the holographic QCD model with Nf = 2, 3. We present model-independent expressions for the coefficients of the non-derivative axion-nucleon couplings. This allows us to provide quantitative estimates of these couplings.

Anomalous triple gauge boson couplings in ZZ production at the LHC and the role of Z boson polarizations

We study anomalous couplings among neutral gauge bosons in ZZ production at the LHC for s=13 TeV in 4-lepton final state. We use the cross section and polarization asymmetries of the Z boson to estimate simultaneous limits on anomalous coupling using Markov-chain–Monte-Carlo (MCMC) method for luminosities 35.9 fb−1, 150 fb−1, 300 fb−1 and 1000 fb−1. The CP-even polarization asymmetry Ax2−y2 is sensitive mainly to the CP-odd couplings f4Z/γ (quadratically) providing a probe to identify CP-odd nature of interaction at the LHC. We find that the polarization asymmetries significantly improve the estimation of anomalous couplings should a deviation from the Standard Model (SM) be observed.

Direct signals from electroweak singlets through the Higgs portal

We review predictions and constraints for nuclear recoil signals from Higgs portal dark matter under the assumption of standard thermal creation from freeze-out. Thermally created scalar and vector Higgs portal dark matter masses are constrained to be in the resonance region near half the Higgs mass, [Formula: see text], or above several TeV. The resonance region for these models will be tested by XENONnT and LZ. The full mass range up to the unitarity limit can be tested by DarkSide-20k and DARWIN. Fermionic Higgs portal dark matter with a pure CP odd coupling is constrained by the Higgs decay width, but has strongly suppressed recoil cross sections which cannot be tested with upcoming experiments. Fermionic Higgs portal dark matter with a combination of CP even and odd Higgs couplings can be constrained by the direct search experiments.

Measuring properties of a Heavy Higgs boson in the Hto toverline{t}to b{W}^{+}overline{b}{W}^{-} decay

Suppose a heavy neutral Higgs or scalar boson H is discovered at the LHC, it is important to investigate its couplings to the standard model particles as much as possible. Here in this work we attempt to probe the CP-even and CP-odd couplings of the heavy Higgs boson to a pair of top quarks, through the decay Hto toverline{t}to b{W}^{+}overline{b}{W}^{-} . We use the helicity-amplitude method to write down the most general form for the angular distributions of the final-state b quarks and W bosons. We figure out that there are 6 types of angular observables and, under mathrm{C}mathrm{P}tilde{mathrm{T}} conservation, one-dimensional angular distributions can only reveal two of them. Nevertheless, the H couplings to the toverline{t} pair can be fully determined by exploiting the one-dimensional angular distributions. A Higgs-boson mass of 380 GeV not too far above the toverline{t} threshold is illustrated with full details. With a total of 104 events of Hto toverline{t}to b{W}^{+}overline{b}{W}^{+} , one can determine the couplings up to 10-20% uncertainties.

Measurement of the Higgs boson coupling properties in the H \u2192 ZZ\u2217 \u2192 4\u2113 decay channel at sqrt{s}=13 TeV with the ATLAS detector

The coupling properties of the Higgs boson are studied in the four-lepton (e, μ) decay channel using 36.1 fb−1 of pp collision data from the LHC at a centre-of-mass energy of 13 TeV collected by the ATLAS detector. Cross sections are measured for the main production modes in several exclusive regions of the Higgs boson production phase space and are interpreted in terms of coupling modifiers. The inclusive cross section times branching ratio for H → ZZ∗ decay and for a Higgs boson absolute rapidity below 2.5 is measured to be 1. 73− 0.23+ 0.24(stat.)− 0.08+ 0.10(exp.) ± 0.04(th.) pb compared to the Standard Model prediction of 1.34±0.09 pb. In addition, the tensor structure of the Higgs boson couplings is studied using an effective Lagrangian approach for the description of interactions beyond the Standard Model. Constraints are placed on the non-Standard-Model CP-even and CP-odd couplings to Z bosons and on the CP-odd coupling to gluons.

The C2HDM revisited

The complex two-Higgs doublet model is one of the simplest ways to extend the scalar sector of the Standard Model to include a new source of CP-violation. The model has been used as a benchmark model to search for CP-violation at the LHC and as a possible explanation for the matter-antimatter asymmetry of the Universe. In this work, we re-analyse in full detail the softly broken ℤ2 symmetric complex two-Higgs doublet model (C2HDM). We provide the code C2HDM_HDECAY implementing the C2HDM in the well-known HDECAY program which calculates the decay widths including the state-of-the-art higher order QCD corrections and the relevant off-shell decays. Using C2HDM_HDECAY together with the most relevant theoretical and experimental constraints, including electric dipole moments (EDMs), we review the parameter space of the model and discuss its phenomenology. In particular, we find cases where large CP-odd couplings to fermions are still allowed and provide benchmark points for these scenarios. We examine the prospects of discovering CP-violation at the LHC and show how theoretically motivated measures of CP-violation correlate with observables.

Sigma decomposition: the CP-odd Lagrangian

In Alonso et al., JHEP 12 (2014) 034, the CP-even sector of the effective chiral Lagrangian for a generic composite Higgs model with a symmetric coset has been constructed, up to four momenta. In this paper, the CP-odd couplings are studied within the same context. If only the Standard Model bosonic sources of custodial symmetry breaking are considered, then at most six independent operators form a basis. One of them is the weak-$\theta$ term linked to non-perturbative sources of CP viola- tion, while the others describe CP-odd perturbative couplings between the Standard Model gauge bosons and an Higgs-like scalar belonging to the Goldstone boson sector. The procedure is then applied to three distinct exemplifying frameworks: the original $SU(5)/SO(5)$ Georgi-Kaplan model, the minimal custodial-preserving $SO(5)/SO(4)$ model and the minimal $SU(3)/(SU(2)\times U(1))$ model, which intrinsically breaks cus- todial symmetry. Moreover, the projection of the high-energy electroweak effective theory to the low-energy chiral effective Lagrangian for a dynamical Higgs is per- formed, uncovering strong relations between the operator coefficients and pinpointing the differences with the elementary Higgs scenario.

Baryon mass splittings and strongCPviolation in SU(3) chiral perturbation theory

We study $SU(3)$ flavor-breaking corrections to the relation between the octet baryon masses and the nucleon-meson CP-violating interactions induced by the QCD $\bar\theta$ term. We work within the framework of $SU(3)$ chiral perturbation theory and work through next-to-next-to-leading order in the $SU(3)$ chiral expansion, which is $\mathcal{O}(m_q^2)$. At lowest order, the CP-odd couplings induced by the QCD $\bar\theta$ term are determined by mass splittings of the baryon octet, the classic result of Crewther et al. We show that for each isospin-invariant CP-violating nucleon-meson interaction there exists one relation which is respected by loop corrections up to the order we work, while other leading-order relations are violated. With these relations we extract a precise value of the pion-nucleon coupling $\bar g_0$ by using recent lattice QCD evaluations of the proton-neutron mass splitting. In addition, we derive semi-precise values for CP-violating coupling constants between heavier mesons and nucleons with $\sim 30\%$ uncertainty and discuss their phenomenological impact on electric dipole moments of nucleons and nuclei.

Single Top and Higgs Production in e−p collisions

In this proceedings some studies on the prospects of single top production at the Large Hadron Electron Collider (LHeC) and double Higgs production at the Future Circular Hadron Electron Collider (FCC-he) shall be presented. In particular, we investigated the tbW couplings via single top quark production with the introduction of possible anomalous Lorentz structures, and measured the sensitivity of the Higgs self coupling (λ) through double Higgs production. The studies are performed with 60 GeV electrons colliding with 7 (50) TeV protons for the LHeC (FCC-he).For the single top studies a parton level study has been performed, and we find the sensitivity of the anomalous coupling at a 95% C.L, considering 10-1% systematic errors. The double Higgs production has been studied with speculated detector parameters and the sensitivity of λ estimated via the cross section study around the Standard Model Higgs self coupling strength (λSM) considering 5% systematic error in signal and backgrounds. Effects of non-standard CP-even and CP-odd couplings for hhh, hWW and hhWW vertices have been studied and constrained at 95% C.L.

Double Higgs production at FCC-he and prospects for measurements of self-coupling

The measurement of the triple Higgs boson coupling is one of the most important goals of the Higgs physics program in present and future collider experiments. This would provide the first direct information on the Higgs potential, which is responsible for electroweak symmetry breaking mechanism. We present a double Higgs production scenario at the Large Hadron-Electron Collider (LHeC) and Future Circular Hadron-Electron Collider (FCC- he) through e-p collisions, which will provide information about trilinear coupling and the possibility of probing new physics, if there is any. The LHeC will provide e- beams to collide head-on with proton beams of 7 TeV from the Large Hadron Collider (LHC). The prospect of replacing the LHC with the high energy FCC, with proton beams of 50 TeV, is used for FCC-he studies. Energy of the e- is taken to be 60 GeV for both LHeC and FCC-he. Effects of non-standard CP-even and CP-odd couplings for hhh, hWW and hhWW have been studied and constrained at a 95% C.L.

Anomalous Higgs couplings in angular asymmetries of H →Zℓ + ℓ − and e + e − →HZ

We study in detail the impact of anomalous Higgs couplings in angular asymmetries of the crossing-symmetric processes H --> Zl+l- and e+e- --> HZ. Beyond Standard Model physics is parametrized in terms of the SU(3)xSU(2)_LxU(1)_Y dimension-six effective Lagrangian. In the light of present bounds on d = 6 interactions we study how angular asymmetries can reveal non-standard CP-even and CP-odd couplings. We provide approximate expressions to all observables of interest making transparent their dominant dependence on anomalous couplings. We show that some asymmetries may reveal BSM effects that are hidden in other observables. In particular, CP-even and CP-odd d = 6 HZgamma couplings as well as (to a lesser extent) HZll contact interactions can generate asymmetries at the several percent level, while having small or no effects on the di-lepton invariant mass spectrum of H --> Zl+l-. Finally, the higher di-lepton invariant mass probed in e+e- --> HZ leads to interesting differences in the asymmetries with respect to those of H --> Zl+l- that may lead to complementary anomalous coupling searches at the LHC and e+e- colliders.

ConstrainingCP-violating Higgs sectors at the LHC using gluon fusion

We investigate the constraints that the LHC can set on a 126 GeV Higgs boson that is an admixture of CP eigenstates. Traditional analyses rely on Higgs couplings to massive vector bosons, which are suppressed for CP-odd couplings, so that these analyses have limited sensitivity. Instead we focus on Higgs production in gluon fusion, which occurs at the same order in the strong coupling for both CP-even and -odd couplings. We study the Higgs plus two jet final state followed by Higgs decay into a pair of tau leptons. We show that using the 8 TeV dataset it is possible to rule out the pure CP-odd hypothesis in this channel alone at nearly 95\% C.L, assuming that the Higgs is CP-even. We also provide projected limits for the 14 TeV LHC run.