Dilepton Production Research Articles

Missing energy plus jet in the SMEFT

We study the production of dineutrinos in proton-proton collisions, with large missing transverse energy and an energetic jet as the experimental signature. Recasting a search from the ATLAS collaboration we work out constraints on semileptonic four-fermion operators, gluon and electroweak dipole operators and Z-penguins in the SMEFT. All but the Z-penguin operators experience energy-enhancement. Constraints on gluon dipole operators are the strongest, probing new physics up to 14 TeV, and improve over existing ones from collider studies. Limits on FCNC four-fermion operators are competitive with Drell-Yan production of dileptons, and improve on those for tau final states. For left-handed |∆s| = |∆d| = 1 and right-handed |∆c| = |∆u| = 1 transitions these are the best available limits, also considering rare kaon and charm decays. We estimate improvements for the 3000 fb−1 High Luminosity Large Hadron Collider.

Dilepton Polarization as a Signature of Plasma Anisotropy.

We propose the angular distribution of lepton pairs produced in ultrarelativistic heavy-ion collisions as a probe of thermalization of the quark-gluon plasma. We focus on dileptons with invariant masses large enough that they are produced through quark-antiquark annihilation in the early stages of the collision. The angular distribution of the lepton in the rest frame of the pair then reflects the angular distribution of quark momenta. At early times, the transverse pressure of the quark-gluon plasma is larger than its longitudinal pressure as a result of the fast longitudinal expansion, which results in an oblate lepton distribution. By contrast, direct (Drell-Yan) production by quarks and antiquarks from incoming nuclei, whose momenta are essentially longitudinal, results in a prolate distribution. As the invariant mass increases, Drell-Yan gradually becomes the dominant source of dilepton production, and the lepton distribution evolves from oblate to prolate. The invariant mass at which the transition occurs is highly sensitive to the equilibration time of the quark-gluon plasma or, equivalently, the shear viscosity over entropy ratio η/s in the early stages of the collision.

Dilepton production at next-to-leading order and intermediate invariant-mass observables

Dilepton production at next-to-leading order and intermediate invariant-mass observables

Enhanced Dilepton production near the color superconducting phase and the QCD critical point

Enhanced Dilepton production near the color superconducting phase and the QCD critical point

Photon-photon fusion and tau g-2 measurement in ATLAS

Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons. New measurements of exclusive dilepton production (electron, muon, and tau pairs) performed by the ATLAS experiment are discussed. We present the photon-induced production of tau pairs and constraints on the tau lepton’s anomalous magnetic dipole moment. In addition, measurements of photon-induced electron and muon pair production are presented, which provide strong constraints on the nuclear photon flux and its dependence on the impact parameter and photon energy. Forward neutrons are utilised to provide an experimental handle on the impact parameter range sampled in the events.

Electroweak Corrections to Forward–Backward Asymmetry in the Dilepton Production Via Photon Fusion at the LHC

Electroweak Corrections to Forward–Backward Asymmetry in the Dilepton Production Via Photon Fusion at the LHC

Fate of the ρ – a1 mixing in dilepton production

We investigate the effect of chiral mixing on dilepton production by combining the in-medium spectral function in the chiral effective field theory with the state-of-the-art fluid dynamical simulations. We compare the spectral functions with different chiral symmetry restoration scenarios: without chiral symmetry restoration, with chiral symmetry restoration, and the lowtemperature mixing theory extrapolated to high-temperature. We find that the scenario with proper chiral symmetry restoration that takes into account the degenerate ρ and a1 mesons leads to an increase of the yield in the window of 1.1 < M < 1.4 GeV. Whereas, the extrapolated low-temperature theorem leads to a substantial overestimate at M = 1.2 GeV.

Pre-equilibrium Photon and Dilepton Production

We use QCD kinetic theory to compute photon and dilepton production in the chemically equilibrating out-of-equilibrium quark-gluon plasma created in the early stages of high-energy heavy-ion collisions. We derive universal scaling functions for the pre-equilibrium spectra of photons and dileptons. These scaling functions can be used to make realistic predictions for the pre-equilibrium emission and consequently establish the significance of the preequilibrium phase for the production of electromagnetic probes in heavy-ion collisions.

Dilepton production in proton-proton reaction at 4.5 GeV with the HADES spectrometer

Spectra of dileptons emitted in hadron collisions are important tool to study electromagnetic decays of resonances and the validity of the Vector Meson Dominance model. Furthermore, they provide reference spectra for the hot and dense (heavy-ion A+A collisions) and cold nuclear matter (p+A collisions). In February 2022, proton-proton reactions at 4.5 GeV beam kinetic energy were measured by HADES experiment. The aim of the analysis is to study the specific channels that produced dileptons, such as baryonic resonance Dalitz decays (Δ/N* → Ne+e−) and vector meson decays (ρ/ω/φ→ e+e−). In addition, the investigations of dielectron production above the φ meson mass will be enabled. In this contribution, different steps of the e+e− pair reconstruc tion are discussed.

Thermal dilepton production in heavy-ion collisions at beam-energy-scan (BES) energies

The lepton pair production rate at finite temperature and at nextto-leading-order (NLO) is calculated for quark-gluon plasma at non-zero net baryon density. Yields are obtained using a (3+1)D multicomponent simulation capable of reproducing hadronic observables measured in the RHIC Beam Energy Scan (BES). Spectra of intermediate invariant mass dileptons are compared with measurements from the STAR collaboration. The result of a study where the temperature information obtained from the dilepton spectrum from heavyion collisions performed at different energies and centralities is presented.

Electromagnetic probes for critical fluctuations of phase transitions in dense QCD

We study how the dilepton production rates and electric conductivity are affected by the phase transition to color superconductivity and the QCD critical point. Effects of the soft modes associated with these phase transitions are incorporated through the photon self-energy called the Aslamazov- Larkin, Maki-Thompson, and density-of-states terms, which are responsible for the paraconductivity in metallic superconductors. We show that anomalous enhancements of the production rate in the low energy/momentum region and the conductivity occur around the respective critical points.

Ellipticity of dilepton production from a hot and magnetized hadronic medium

Ellipticity of dilepton production from a hot and magnetized hadronic medium

Two-photon production of dileptons at the LHC with electroweak corrections taken into account

Two-photon production of dileptons at the LHC with electroweak corrections taken into account

Anomalous triple gauge couplings in electroweak dilepton tails at the LHC and interference resurrection

We study the electroweak dilepton production with two forward jets at the LHC, aiming to measure the anomalous triple gauge couplings in the Effective Field Theory (EFT) approach. This process exhibits a distinctive feature, namely, the interference between Standard Model (SM) and beyond the SM is resurrected in the inclusive cross section of the full amplitude, including two forward jets. As a concrete illustration, we perform the detailed analytic and numerical study of the interference using a simpler toy process, and discuss the subtlety of the effective W approximation. We propose a new kinematic variable, VBFhardness, that controls the amount of energy flowing into the dilepton subprocess. We show that an appropriate cut on VBFhardness makes the interference resurrection manifest. Finally, we use the invariant mass of the dilepton system as well as the transverse momentum, as done in the literature, to derive the sensitivity to anomalous triple gauge couplings at the LHC and the high luminosity LHC. Our result is compared with the existing limits from the experiments.

Exclusive dilepton production in photon-induced interactions at RHIC

The exclusive dilepton photoproduction in pp and pAu collisions at RHIC is investigated considering the contribution of the Bethe–Heitler (BH) and the timelike Compton scattering (TCS) processes. The TCS amplitude is estimated at next-to-leading-order of the strong running coupling constant alpha _s and assuming the Goloskokov–Kroll model for the quark and gluon generalized parton distributions. The TCS differential cross section is estimated as well the contribution associated to the interference between the TCS and BH amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range covered by the RHIC detectors.

Enhancement of dilepton production rate and electric conductivity around the QCD critical point

Abstract We investigate whether the soft mode that becomes massless at the QCD critical point (CP) causes an enhancement of the dilepton production rate (DPR) and electric conductivity around the CP through modification of the photon self-energy using the two-flavor Nambu–Jona-Lasinio model. The modification is described by the so-called Aslamazov–Larkin, Maki–Thompson, and density of states terms, which have been taken into account in our previous study on the DPR near the color-superconducting phase transition, with a replacement of the diquark modes with the soft mode of the QCD CP. We show that the coupling of photons with the soft modes brings about an enhancement of the DPR in the low-invariant-mass region and the conductivity near the CP, which would be observable in relativistic heavy-ion collisions.

Drell-Yan tails beyond the Standard Model

We investigate the high-pT tails of the pp → ℓν and pp → ℓℓ Drell-Yan processes as probes of New Physics in semileptonic interactions with an arbitrary flavor structure. For this purpose, we provide a general decomposition of the 2 → 2 scattering amplitudes in terms of form-factors that we match to specific scenarios, such as the Standard Model Effective Field Theory (SMEFT), including all relevant operators up to dimension-8, as well as ultraviolet scenarios giving rise to tree-level exchange of new bosonic mediators with masses at the TeV scale. By using the latest LHC run-II data in the monolepton (eν, μν, τν) and dilepton (ee, μμ, ττ, eμ, eτ, μτ) production channels, we derive constraints on the SMEFT Wilson coefficients for semileptonic four-fermion and dipole operators with the most general flavor structure, as well as on all possible leptoquark models. For the SMEFT, we discuss the range of validity of the EFT description, the relevance of mathcal{O} (1/Λ2) and mathcal{O} (1/Λ4) truncations, the impact of d = 8 operators and the effects of different quark-flavor alignments. Finally, as a highlight, we extract for several New Physics scenarios the combined limits from high-pT processes, electroweak pole measurements and low-energy flavor data for the b → cτν transition, showing the complementarity between these different observables. Our results are compiled in HighPT (https://highpt.github.io), a package in Mathematica which provides a simple way for users to extract the Drell-Yan tails likelihoods for semileptonic effective operators and for leptoquark models.

Dilepton production from hot and magnetized hadronic matter

The rate of dilepton emission from a magnetized hot hadronic medium is calculated in the framework of real time formalism of finite temperature field theory. We evaluate the one loop self-energy of neutral rho-meson containing thermo-magnetic propagators for the charged pions in the loop. The in-medium thermo-magnetic spectral function of rho obtained by solving the Dyson-Schwinger equation is shown to be proportional to the dilepton production rate. The study of the analytic structure of the neutral rho-meson spectral function in such a medium shows that in addition to the usual contribution coming from the Unitary cut beyond the two-pion threshold there is a non-trivial yield in the low invariant mass region originating due to the fact that the charged pions occupy different Landau levels before and after scattering with the neutral rho-meson and is purely a finite magnetic field effect.

Timelike Compton scattering in ultraperipheral pPb collisions

The study of exclusive processes in photon-induced interactions allows us to probe the generalized parton distributions (GPDs), which provide information about the 3-dimensional description of the quark and gluon content of hadrons. In this paper we investigate the timelike Compton scattering (TCS) in ultraperipheral pPb collisions at the LHC. Such process is characterized by the exclusive dilepton production through the subprocess γp→γ⁎p→l+l−p, with the real photon in the initial state being emitted by the nucleus. Assuming a given model for the GPDs, the TCS differential cross section is estimated as well the contribution associated to the interference between the TCS and Bethe–Heitler (BH) amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range covered by the LHC detectors.

Electroweak Corrections to Dilepton Production in Photon Fusion at the LHC

Electroweak Corrections to Dilepton Production in Photon Fusion at the LHC