Dilepton Spectra Research Articles

Photon and dilepton production at the Facility for Proton and Anti-Proton Research and beam-energy scan at the Relativistic Heavy-Ion Collider using coarse-grained microscopic transport simulations

We present calculations of dilepton and photon spectra for the energy range $E_{\text{lab}}=2-35$ $A$GeV which will be available for the Compressed Baryonic Matter (CBM) experiment at the future Facility for Anti-Proton and Ion Research (FAIR). The same energy regime will also be covered by phase II of the Beam Energy Scan at the Relativistic Heavy-Ion Collider (RHIC-BES). Coarse-grained dynamics from microscopic transport calculations of the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model is used to determine temperature and chemical potentials, which allows for the use of dilepton and photon-emission rates from equilibrium quantum-field theory calculations. The results indicate that non-equilibrium effects, the presence of baryonic matter and the creation of a deconfined phase might show up in specific manners in the measurable dilepton invariant mass spectra and in the photon transverse momentum spectra. However, as the many influences are difficult to disentangle, we argue that the challenge for future measurements of electromagnetic probes will be to provide a high precision with uncertainties much lower than in previous experiments. Furthermore, a systematic study of the whole energy range covered by CBM at FAIR and RHIC-BES is necessary to discriminate between different effects, which influence the spectra, and to identify possible signatures of a phase transition.

Examinations of the early degrees of freedom in ultrarelativistic nucleus-nucleus collisions

The Parton-Hadron-String-Dynamics (PHSD) transport model is used to study the impact on the choice of initial degrees of freedom on the final hadronic and electromagnetic observables in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We find that a non-perturbative system of massive gluons (scenario I) and a system dominated by quarks and antiquarks (scenario II) lead to different hadronic observables when imposing the same initial energy-momentum tensor $T_{\mu \nu}(x)$ just after the passage of the impinging nuclei. In case of the gluonic initial condition the formation of $s,{\bar s}$ pairs in the QGP proceeds rather slow such that the anti-strange quarks and accordingly the $K^+$ mesons do not achieve chemical equilibrium even in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Accordingly, the $K^+$ rapidity distribution is suppressed in the gluonic scenario and in conflict with the data from the BRAHMS Collaboration. The proton and antiproton rapidity distributions also disfavor the scenario I. Furthermore, a clear suppression of direct photon and dilepton production is found for the pure gluonic initial conditions which is not so clearly seen in the present photon and dilepton spectra from Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV due to a large contribution from other channels. It is argued that dilepton spectra in the invariant mass range 1.2 GeV $< M <$ 3 GeV will provide a definitive answer once the background from correlated $D$-meson decays is subtracted experimentally.

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

Parametrizing beyond Standard Model physics by the SU(3)×SU(2)L×U(1)Y dimension-six effective lagrangian, we study the impact of anomalous Higgs couplings in angular asymmetries of the crossing symmetric processes H→Zℓ+ℓ− and e+e−→HZ. In the light of present bounds on d = 6 couplings, we show that some asymmetries can reveal BSM effects that would otherwise be hidden in other observables. The d=6 HZγ couplings as well as (to a lesser extent) HZℓℓ contact interactions can generate asymmetries at the several percent level, albeit having less significant effects on the di-lepton invariant mass spectrum of the decay H→Zℓ+ℓ−. The higher di-lepton invariant mass probed in e+e−→HZ can lead to complementary anomalous coupling searches at e+e− colliders.

Role of the pion electromagnetic form factor in theΔ(1232)→γ*Ntimelike transition

The $\Delta(1232) \to \gamma^\ast N$ magnetic dipole form factor ($G_M^\ast$) is described here within a new covariant model that combines the valence quark core together with the pion cloud contributions. The pion cloud term is parameterized by two terms: one connected to the pion electromagnetic form factor, the other to the photon interaction with intermediate baryon states. The model can be used in studies of pp and heavy ion collisions. In the timelike region this new model improves the results obtained with a constant form factor model fixed at its value at zero momentum transfer. At the same time, and in contrast to the Iachello model, this new model predicts a peak for the transition form factor at the expected position, i.e. at the $\rho$ mass pole. We calculate the decay of the $\Delta \to \gamma N$ transition, the Dalitz decay ($\Delta \to e^+ e^- N$), and the $\Delta$ mass distribution function. The impact of the model on dilepton spectra in pp collisions is also discussed.

Dileptons in a coarse-grained transport approach

We calculate dilepton spectra in heavy-ion collisions using a coarse-graining approach to the simulation of the created medium with the UrQMD transport model. This enables the use of dilepton-production rates evaluated in equilibrium quantum-field theory at finite temperatures and chemical potentials.

Thermal dileptons as fireball thermometer and chronometer

Thermal dilepton radiation from the hot fireballs created in high-energy heavy-ion collisions provides unique insights into the properties of the produced medium. We first show how the predictions of hadronic many-body theory for a melting ρ meson, coupled with quark–gluon plasma emission utilizing a modern lattice-QCD based equation of state, yield a quantitative description of dilepton spectra in heavy-ion collisions at the SPS and the RHIC beam energy scan program. We utilize these results to systematically extract the excess yields and their invariant-mass spectral slopes to predict the excitation function of fireball lifetimes and (early) temperatures, respectively. We thereby demonstrate that future measurements of these quantities can yield unprecedented information on basic fireball properties. Specifically, our predictions quantify the relation between the measured and maximal fireball temperature, and the proportionality of excess yield and total lifetime. This information can serve as a “caloric” curve to search for a first-order QCD phase transition, and to detect non-monotonous lifetime variations possibly related to critical phenomena.

Parton-hadron matter at finite temperature and chemical potential

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities or the electric conductivity of the hot QCD medium. The PHSD model reproduces a large variety of observables from SPS to LHC energies, e.g. the quark-number scaling of elliptic flow, transverse mass and rapidity spectra of charged hadrons, dilepton spectra, open and hidden charm production, collective flow coefficients etc., which are associated with the observation of a strongly interacting QGP (sQGP). The ’highlights’ of the latest results from LHC energies are presented with a focus on the correlation between the average transverse momentum < pT > and the number of charged particles Nch in p-p, p-Pb and Pb-Pb collisions at midrapidity as observed by the ALICE Collaboration.

Electroweak bounds on Higgs pseudo-observables and $$h\rightarrow 4\ell $$ h → 4 ℓ decays

We analyze the bounds on the Higgs pseudo-observables following from electroweak constraints, under the assumption that the Higgs particle is the massive excitation of an \(\text {SU}(2)_L\) doublet. Using such bounds, detailed predictions for \(h\rightarrow 4\ell \) decay rates, dilepton spectra, and lepton-universality ratios are presented.

Coarse-graining approach for dilepton production at energies available at the CERN Super Proton Synchrotron

Coarse-grained output from transport calculations is used to determine thermal dilepton emission rates by applying medium-modified spectral functions from thermal quantum field theoretical models. By averaging over an ensemble of events generated with the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model, we extract the local thermodynamic properties at each time step of the calculation. With an equation of state the temperature $T$ and chemical potential ${\ensuremath{\mu}}_{\mathrm{B}}$ can be determined. The approach goes beyond simplified fireball models of the bulk-medium evolution by treating the full (3+1)-dimensional expansion of the system with realistic time and density profiles. For the calculation of thermal dilepton rates we use the in-medium spectral function of the $\ensuremath{\rho}$ meson developed by Rapp and Wambach and consider thermal quark-gluon plasma (QGP) and multipion contributions as well. The approach is applied to the evaluation of dimuon production in In+In collisions at top CERN Super Proton Synchrotron (SPS) energy. Comparison to the experimental results of the NA60 experiment shows good agreement of this ansatz. We find that the experimentally observed low-mass dilepton excess in the mass region from 0.2 to 0.6 GeV can be explained by a broadening of the $\ensuremath{\rho}$ spectral function with a small mass shift. In contrast, the intermediate-mass region $(M&gt;1.5$ GeV) is dominated by a contribution from the quark-gluon plasma. These findings agree with previous calculations with fireball parametrizations. This agreement, in spite of differences in the reaction dynamics between both approaches, indicates that the time-integrated dilepton spectra are not very sensitive to details of the space-time evolution of the collision.

The QGP dynamics in relativistic collisions

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities or the electric conductivity of the hot QCD medium. The PHSD model reproduces a large variety of observables from SPS to LHC energies, e.g. the quark-number scaling of elliptic flow, transverse mass and rapidity spectra of charged hadrons, dilepton spectra, open and hidden charm production, collective flow coefficients etc., which are associated with the observation of a strongly interacting QGP (sQGP). The ‘highlights’ of the latest results from LHC energies are presented with a focus on observable effects from the non-equilibrium initial dynamics.

Model-independent analysis of B → πℓ + ℓ − decays

The results obtained by calculating, on the basis of the Standard Model, the branching fractions for the decays B → πℓ + ℓ − (ℓ ± = e ±, µ±) and the invariant-mass spectrum of dileptons for the process B + → π + ℓ + ℓ − are presented. The resulting theoretical estimates depend on the choice of model for the B → π transition form factors f +(q 2), f 0(q 2), and f T (q 2). However, one can get rid of this arbitrariness by employing experimental data or the results of QCD lattice calculations to determine the parameter dependence of the form factors. Data from B-meson factories were used to determine the dependence of the form factor f +(q 2) on the transfer momentum squared. Yet another required form factor, f T (q 2), was extracted from available lattice data on B → π and B → K transitions with allowance for nearly identical effects of the breakdown of flavor SU(3) F symmetry in the respective form factors. The resulting value of the decay branching fraction, B(B + → π +µ+µ−) = (1.88 +0.32−0.21 ) × 10−8, is in good agreement with the results of measurements of the LHCb Collaboration. The branching fraction for the isospin analog of the decay under consideration is B(B + → π +µ+µ−) = (0.94 +0.16−0.11 ) × 10−8. Therefore, this decay can be measured at the Super-B factory under construction.

Electromagnetic probes of strongly interacting matter

The nuclear matter under extreme conditions of temperatures (T) and baryonic densities ( n B) undergoes a phase transition to quark gluon plasma (QGP). It is expected that such extreme conditions can be achieved by colliding nuclei at ultrarelativistic energies. In the present review, the suitability of photons and dileptons as diagnostic tools of QGP has been discussed. The photon and dilepton spectra originating from heavy-ion collisions at LHC energies have been explicitly displayed in this article. Results from SPS and RHIC have been discussed adequately with appropriate references. The role of single electron spectra originating from the decays of heavy flavoured mesons on QGP detection has also been discussed briefly.

Dilepton Production in Transport-based Approaches

We investigate dilepton production in transport-based approaches and show that the baryon couplings of the ρ meson represent the most important ingredient for understanding the measured dilepton spectra. At SIS energies, the baryon resonances naturally play a major role and affect already the vacuum spectra via Dalitz-like contributions, which can be captured well in transport simulations. Recent pion-beam measurements at GSI will help to constrain the properties of the involved resonances further.

Dilepton production from RHIC to the LHC

The goal of ultrarelativistic heavy-ion collisions at RHIC and the LHC is to study the properties of the quark-gluon plasma (QGP), a phase of matter with partonic degrees of freedom. Electromagnetic radiation, in form of photons or lepton pairs, is a penetrating probe that allows the investigation of the full time evolution and dynamics of the produced matter as it does not undergo strong interaction in the final state. The dilepton spectrum is extremely rich in physics sources: Thermal black-body radiation is of particular interest as it carries information about the QGP temperature. Modifications of the spectral functions of light vector mesons are linked to the potential restoration of chiral symmetry in the QGP phase. Correlated lepton pairs from semi-leptonic charm and beauty decays provide additional information about the heavy-quark energy loss. Finally, the suppression of quarkonia in the QGP give access to an independent temperature measurement. In this proceedings, dilepton results from RHIC are reviewed and the status as well as prospects of low-mass dilepton measurements at the LHC are given.

Electromagnetic probes of the QGP

We investigate the properties of the QCD matter across the deconfinement phase transition in the scope of the parton-hadron string dynamics (PHSD) transport approach. We present here in particular the results on the electromagnetic radiation, i.e. photon and dilepton production, in relativistic heavy-ion collisions. By comparing our calculations for the heavy-ion collisions to the available data, we determine the relative importance of the various production sources and address the possible origin of the observed strong elliptic flow $v_2$ of direct photons. We argue that the different centrality dependence of the hadronic and partonic sources for direct photon production in nucleus-nucleus collisions can be employed to shed some more light on the origin of the photon $v_2$ puzzle. While the dilepton spectra at low invariant mass show in-medium effects like an enhancement from multiple baryonic resonance formation or a collisional broadening of the vector meson spectral functions, the dilepton yield at high invariant masses (above 1.1 GeV) is dominated by QGP contributions for central heavy-ion collisions at ultra-relativistic energies. This allows to have an independent view on the parton dynamics via their electromagnetic massive radiation.

Electromagnetic probes of the QGP

We investigate the properties of the QCD matter across the deconfinement phase transition in the scope of the parton-hadron string dynamics (PHSD) transport approach. We present here in particular the results on the electromagnetic radiation, i.e. photon and dilepton production, in relativistic heavy-ion collisions. By comparing our calculations for the heavy-ion collisions to the available data, we determine the relative importance of the various production sources and address the possible origin of the observed strong elliptic flow v2 of direct photons. We argue that the different centrality dependence of the hadronic and partonic sources for direct photon production in nucleusnucleus collisions can be employed to shed some more light on the origin of the photon v2 “puzzle”. While the dilepton spectra at low invariant mass show in-medium effects like an enhancement from multiple baryonic resonance formation or a collisional broadening of the vector meson spectral functions, the dilepton yield at high invariant masses (above 1.1 GeV) is dominated by QGP contributions for central heavy-ion collisions at ultra-relativistic energies. This allows to have an independent view on the parton dynamics via their electromagnetic massive radiation.

Resonance dynamics in the PHSD approach

We present an overview on the resonance dynamics within the microscopic parton-hadron-string dynamics (PHSD) approach which incorporates explicit partonic degrees-of-freedom in terms of strongly interacting quasiparticles (quarks and gluons) in line with an equation-of-state from lattice QCD as well as the dynamical hadronization and hadronic collision dynamics in the final reaction phase. We discuss how the vector meson resonances can be used as a probe of the in-medium effects and demostrate that the low mass dilepton spectra show visible in-medium effects from dynamical vector-meson spectral functions from SIS to SPS energies whereas at RHIC and LHC energies such medium effects become more moderate. We show also that the intermediate mass spectra are dominated by the radiation from the partonic degrees of freedom at RHIC and LHC energies.

Dilepton Production from the Quark--Gluon Plasma Using Leading-order $(3+1)$D Anisotropic Hydrodynamics

Dilepton production from the quark-gluon plasma (QGP) phase of ultra-relativistic heavy-ion collisions is computed using the leading-order (3+1)-dimensional anisotropic hydrodynamics. It is shown that high-energy dilepton spectrum is sensitive to the initial local-rest-frame momentum-space anisotropy of the QGP. Our findings suggest that it may be possible to constrain the early-time momentum-space anisotropy in relativistic heavy-ion collisions using high-energy dilepton yields.

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.

Searching for dilepton resonances below theZmass at the LHC

We consider LHC searches for dilepton resonances in an intermediate mass range, $\sim 10 -80$ GeV. We adopt a kinetically mixed $Z'$ as an example of weakly coupled new physics that might have evaded detection at previous experiments but which could still be probed by LHC dilepton spectrum measurements in this mass range. Based on Monte Carlo simulations, we estimate that existing data from the 7 and 8 TeV LHC could be used to test values of the kinetic mixing parameter $\epsilon$ several times smaller than precision electroweak upper bounds, were an appropriate analysis to be carried out by one of the experimental collaborations.