Low Mass Dilepton Production Research Articles

Bulk viscous effects on flow and dilepton radiation in a hybrid approach

Starting from IP-Glasma initial conditions, we investigate the effects of bulk pressure on low mass dilepton production at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) energies. Though thermal dilepton v2 is affected by the presence of both bulk and shear viscosity, whether or not these effects can be measured depends on the dilepton “cocktail” contribution to the the low mass dilepton v2. Combining the thermal and “cocktail” dileptons, the effects of bulk viscosity on total dilepton v2 is investigated.

Low mass Drell-Yan production of lepton pairs at forward directions at the LHC: A hybrid approach

We discuss Drell-Yan production of dileptons at high energies in forward rapidity region in a hybrid high-energy approach. This approach uses unintegrated gluon distributions in one proton and collinear quark/antiquark distributions in the second proton. Corresponding momentum-space formula for the differential cross sections in high-energy approximation has been derived and will be presented. The relation to the commonly used dipole approach is discussed. We conclude and illustrate that some results of the dipole approaches are too approximate, as far as kinematics is considered, and in fact cannot be used when comparing with real experimental data. We find that the dipole formula is valid only in very forward/backward rapidity regions ($|y| >$ 5) that cannot be studied experimentally in the moment. We performed calculations of some differential cross sections for low-mass dilepton production by the LHCb and ATLAS collaborations. In distinction to most of dipole approaches, we include all of the four Drell-Yan structure functions, although the impact of interference structure functions is rather small for the relevant experimental cuts. We find that both side contributions ($g + q/\bar q$ and $q/\bar q + g$) have to be included even for the LHCb rapidity coverage which is in contradiction with what is usually done in the dipole approach. We present results for different unintegrated gluon distributions from the literature (some of them include saturation effects). We see no clear hints of saturation even at small $M_{ll}$ when comparing with the LHCb data.

Low mass dilepton production in pp, p–Pb and Pb–Pb collisions with ALICE

Low mass vector meson ( ρ, ω, ϕ ) production provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions (called Quark-Gluon Plasma, QGP). Strangeness enhancement is one of the possible signatures of the Quark-Gluon Plasma formation and can be accessed through the measurement of ϕ meson production with respect to ρ and ω mesons. The production dynamics and hadronization process in relativistic heavy-ion collisions can be probed through the measurement of the ϕ nuclear modification factor.We present results on the low mass dimuon analysis in pp, p–Pb and Pb–Pb collisions. In pp collisions at √ s = 2.76 TeV the ϕ differential cross section as a function of the transverse momentum has been measured, serving as a baseline for Pb–Pb data.The ϕ yield and the nuclear modification factor R pPb at forward and backward rapidity have been measured for p–Pb collisions at √ s = 5.02 TeV. At forward rapidity, R pPb increases as a function of p T , saturating for p T > 3 GeV/ c at R pPb ~ 1. At backward rapidity, R pPb shows an increase as a function of the transverse momentum up to a factor of 1.6 for p T ~ 3-4 GeV/ c , followed by a decrease at higher p T .The ϕ yield and nuclear modification factor R AA have been measured for Pb–Pb collisions at √ s NN = 2.76 TeV in the intermediate p T region 2 p T c , as a function of the number of participating nucleons. Remarkable differences are observed in the comparison between the results at forward rapidity in the dimuon channel and the ones measured in the same p T range at midrapidity in the hadronic channel ϕ → KK .In the dielectron channel at midrapidity, the invariant mass distributions in the range 0 m ee c 2 are compared with the expected hadronic sources for pp collisions at √ s = 7 TeV, and for p–Pb collisions at √ s NN = 5.02 TeV. Latest results of the analysis of Pb–Pb collisions at √ s NN = 2.76 TeV are presented.

Low-mass dilepton production with ALICE at the LHC

The production of low-mass dileptons, including those from ρ, ω and ϕ light vector meson decays, provides key information on the hot and dense state of strongly interacting matter produced in high-energy heavy-ion collisions. In particular, strangeness production can be studied through ϕ meson measurements, while the detailed description of the full dilepton mass spectra down to the kinematic threshold can be used to reveal in-medium modifications of hadron properties and the thermal emission arising from the medium. Measurements in pp and p–A, respectively, provide a reference for the observations in heavy-ion collisions and give insight on soft particle production in cold nuclear matter. Dilepton production is studied with the ALICE apparatus at the LHC both at central (|y|<0.9) and forward (2.5<y<4) rapidity, respectively in the dielectron and dimuon channel. In this contribution, we present recent results on low-mass dimuon production at forward rapidity in Pb–Pb and p–Pb collisions.

Low-mass dilepton production through transport processes in a quark-gluon plasma

The dynamical development of collective flow is studied in a (3+1)D fluid dynamical model, with globally symmetric, peripheral initial conditions, which take into account the shear flow caused by the forward motion on the projectile side and the backward motion on the target side. While at $\sqrt{s_{NN}} = 2.76A$\,TeV semi-peripheral Pb+Pb collisions the earlier predicted rotation effect is visible, at more peripheral collisions, with high resolution and low numerical viscosity the initial development of a Kelvin-Helmholtz instability is observed, which alters the flow pattern considerably. This effect provides a precision tool for studying the low viscosity of Quark-gluon Plasma.

Low-mass dilepton production in a relativistic Hagedorn-resonance gas model

Dilepton production in the low-invariant-mass region in $p+p$ and Au + Au collisions at $\sqrt{{s}_{NN}}=200$ GeV and in Pb + Au collisions at 158 $A$GeV is studied in the relativistic hadron resonance gas model with Hagedorn mass spectrum that grows exponentially. The model provides good agreement with the observed yield and spectra of stable hadrons and resonances produced in both $p+p$ and nucleus-nucleus collisions. While the measured dilepton yields in $p+p$ collisions are well understood in the model with Dalitz and direct decays of conventional neutral meson sources, a sizable amount of the mesons produced from the Hagedorn states still cannot explain the large enhancement observed in the invariant mass region 200--700 MeV/c${}^{2}$ in Pb + Au and especially in Au + Au collisions. In a multiphase transport (AMPT) model, the dilepton produced from the standard hadronic sources in absence of Hagedorn states underpredicts the data to a larger extent. The study suggests that Hagedorn states in conjugation with in-medium modification of the properties of $\ensuremath{\rho}$ mesons may be essential for the enhanced dilepton production.

Low-mass dilepton production in pp and AA collisions

We adopt a factorized QCD formalism to describe the transverse momentum distribution of lowmass lepton pairs produced in pp collisions, when the pair transverse momentum Q T 4 ≫ Q , with the pair's invariant mass Q as low as Q ∼ Λ Q C D . We extend this formalism to dilepton production in AA collisions by including the nuclear-dependent power correction due to parton multiple scattering.

Low mass dilepton production in relativistic heavy ion collisions

Mechanisms of formation for low mass dilepton excess observed in the relativistic heavy ion collisions are considered. The experimental data are reviewed. In addition to discussing the standard mechanisms of dilepton production specific to the collisions of relativistic nuclei (the pion annihilation in the hadron-gas stage and the quark-antiquark annihilation in the quark-gluon phase), the mechanism of dilepton production in the mixed phase of nuclear matter is proposed, and its contribution to the low mass dilepton spectrum is estimated. In addition, the first-order corrections in the strong-interaction coupling constant to the dilepton production in the parton medium and the nonperturbative approaches are considered.

Low mass dilepton production at ultrarelativistic energies

Dilepton production in pp and Au+Au nucleus–nucleus collisions at s=200GeV as well as in In+In and Pb+Au at 158AGeV is studied within the microscopic HSD transport approach. A comparison to the data from the PHENIX Collaboration at RHIC shows that standard in-medium effects of the ρ,ω vector mesons—compatible with the NA60 data for In+In at 158AGeV and the CERES data for Pb+Au at 158AGeV—do not explain the large enhancement observed in the invariant mass regime from 0.2 to 0.5 GeV in Au+Au collisions at s=200 GeV relative to pp collisions.

Low mass dilepton production at RHIC energies

Recent results on low mass dilepton measurements from the PHENIX experiment are reported. Invariant mass spectra of \(\phi\rightarrow e^ + e^-\) are measured for the first time in Au-Au collisions at \(\sqrt{s_{NN}} = 200\) GeV in Run2. In d-Au collisions, the yields and M T slopes of both \(\phi\rightarrow e^ + e^-\) and \(\phi \rightarrow K^ + K^-\) are measured. Both results are consistent with each other within errors. In the future, a Hadron Blind Detector will be installed in PHENIX which will enhance our capabilities of rejecting external photon conversions and Dalitz pairs, that will result in a significant reduction of the large combinatorial background.

ρpropagation and dilepton production at finite pion density and temperature

We study the propagation properties of the $\ensuremath{\rho}$ vector in a dense and hot pion medium. We introduce a finite value of the chemical potential associated with a conserved pion number and argue that such description is valid during the hadronic phase of a relativistic heavy-ion collision, between chemical and thermal freeze-out, where the strong interaction drives pion number to a fixed value. By invoking vector dominance and $\ensuremath{\rho}$ saturation, we also study the finite pion density effects into the low mass dilepton production rate. We find that the distribution moderately widens and the position of the peak shifts toward larger values of the pair invariant mass, at the same time that the height of the peak decreases when the value of the chemical potential grows. We conclude by arguing that for the description of the dilepton spectra at ultrarelativistic energies, such as those of Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC), the proper treatment of the large pion density might be a more important effect to consider than the influence of a finite baryon density.

In-medium vector meson properties and low-mass dilepton production from hot hadronic matter

The in-medium properties of the vector mesons are known to be modified significantly in hot and dense hadronic matter due to vacuum polarisation effects from the baryon sector in the Walecka model. The vector meson mass drops significantly in the medium due to the effects of the Dirac sea. In the variational approach adopted in the present paper, these effects are taken into account through a realignment of the ground state with baryon condensates. Such a realignment of the ground state becomes equivalent to summing of the baryonic tadpole diagrams in the relativistic Hartree approximation (RHA). The approximation scheme adopted here goes beyond RHA to include quantum effects from the scalar meson and is nonperturbative and self--consistent. It includes multiloop effects, thus corresponding to a different approximation as compared to the one loop approximation of including scalar field quantum corrections. In the present work, we study the properties of the vector mesons in the hot and dense matter as modified due to such quantum correction effects from the baryon as well as scalar meson sectors. These medium modifications of the properties of the vector mesons are reflected, through the shifting and broadening of the respective peaks, in the low mass dilepton spectra. There is broadening of the peaks due to corrections from scalar meson quantum effects as compared to the relativistic Hartree approximation. It is seen to be rather prominent for the $\omega$ meson in the invariant mass plot.

Quark dispersion relation and dilepton production in the quark-gluon plasma

Under very general assumptions we show that the quark dispersion relation in the quark-gluon plasma is given by two collective branches, of which one has a minimum at a nonvanishing momentum. This general feature of the quark dispersion relation leads to structures (van Hove singularities, gaps) in the low mass dilepton production rate, which might provide a unique signature for the quark-gluon plasma formation in relativistic heavy ion collisions.

Towards the quark-gluon plasma

We discuss recent experimental results in the field of ultra-relativistic nuclear collisions. The emerging ``picture'' is a collectively expanding, initially hot and dense fireball in which strangeness- and low-mass di-lepton pair production are enhanced and J/$\Psi$ production is suppressed compared to expectations from nucleon-nucleon collisions. It is argued that, taken together, these data provide circumstantial evidence that a (at least partly) partonic phase was produced in such collisions.

Duality and chiral restoration from low-mass dileptons at the CERN-SpS

We review recent theoretical progress in low-mass dilepton production at CERN-SpS energies. Various hadronic approaches to calculate the vector correlator in hot/dense hadronic matter are discussed and confronted with each other. Possible consequences for the nature of chiral restoration are indicated.

Soft electromagnetic radiations from an equilibrating quark-gluon plasma

We evaluate the bremsstrahlung production of low mass dileptons and soft photons from equilibrating and transversely expanding quark gluon plasma which may be created in the wake of relativistic heavy ion collisions. We use initial conditions obtained from the self-screened parton cascade model. We consider a boost invariant longitudinal and cylindrically symmetric transverse expansion of the parton plasma and find that for low mass dileptons $(M&lt;~0.3 \mathrm{GeV})$ and soft photons ${(p}_{T}&lt;~0.5 \mathrm{GeV}),$ the bremsstrahlung contribution is rather large compared to annihilation process at both the Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies. We also find an increase by a factor of 15--20 in the low mass dileptons and soft photons yield as one goes from RHIC to LHC energies.

Unstable particles in matter at a finite temperature: The ρ and ω mesons

Unstable particles (such as the vector mesons) have an important role to play in low mass dilepton production resulting from heavy ion collisions and this has been a subject of several investigations. Yet subtleties, such as the implications of the generalization of the Breit-Wigner formula for nonzero temperature and density, e.g. the question of collisional broadening, the role of Bose enhancement, etc., the possibility of the kinematic opening (or closing) of decay channels due to environmental effects, the problem of double counting through resonant and direct contributions, are often given insufficient emphasis. The present study attempts to point out these features using the rho and omega mesons as illustrative examples. The difference between the two versions of the Vector Meson Dominance Model in the present context is also presented. Effects of non-zero temperature and density, through vector meson masses and decay widths, on dilepton spectra are studied, for concreteness within the framework of a Walecka-type model, though most of the basic issues highlighted apply to other scenarios as well.

Low-mass dileptons and dropping rho meson mass

Using the transport model, we have studied dilepton production from heavy-ion collisions at Bevalac energies. It is found that the enhanced production of low-mass dileptons observed in the experiment by the DLS collaboration cannot be explained by the dropping of hadron masses, in particular the ρ-meson mass, in dense matter.

Scalar correlations in a quark plasma and low mass dilepton production

We investigate possible consequences of resonant scalar interactions for dilepton production from a quark plasma at the chiral phase transition. It is found that this production mechanism is strongly suppressed compared to the Born process and has no significance for present experiments.

Thermal dileptons fromπ-ρinteractions in a hot pion gas

A systematic study of low mass dilepton production from $\pi-\rho$ interactions in a hot medium is presented. Applying finite temperature perturbation theory the dilepton rate, respectively the virtual photon rate, is computed up to order $g_\rho^2$. For dilepton masses below the $\rho$ the two-body reactions $\pi\pi\to \rho \gamma^*$, $\pi\rho\to \pi \gamma^*$, and the decay process $\rho \to \pi \pi \gamma^*$ give significant contributions. Non-equilibrium contributions to the thermal rate are estimated, including the modification of the particle distribution function with non-zero pion chemical potential. The comparison of the dilepton rate with the recent data measured in nucleus-nucleus collisions at SPS energy by the CERES Collaboration is also performed. It is shown that the additional thermal dileptons from $\pi-\rho$ interactions can partially account for the access of the soft dilepton yield seen experimentally.