Lepton Pair Production Research Articles

Coherent lepton pair production in hadronic heavy ion collisions

Recently, significant enhancements of e+e− pair production at very low transverse momentum (pT<0.15 GeV/c) were observed by the STAR collaboration in peripheral hadronic A+A collisions. This excesses can not be described by the QGP thermal radiation and ρ in-medium broadening calculations. This is a sign of coherent photon–photon interactions, which were conventionally studied only in ultra-peripheral collisions. In this article, we present calculations of lepton pair (e+e− and μ+μ−) production from coherent photon–photon interactions in hadronic A+A collisions at RHIC and LHC energies within the STAR and ALICE acceptance.

Matching factorization theorems with an inverse-error weighting

We propose a new fast method to match factorization theorems applicable in different kinematical regions, such as the transverse-momentum-dependent and the collinear factorization theorems in Quantum Chromodynamics. At variance with well-known approaches relying on their simple addition and subsequent subtraction of double-counted contributions, ours simply builds on their weighting using the theory uncertainties deduced from the factorization theorems themselves. This allows us to estimate the unknown complete matched cross section from an inverse-error-weighted average. The method is simple and provides an evaluation of the theoretical uncertainty of the matched cross section associated with the uncertainties from the power corrections to the factorization theorems (additional uncertainties, such as the nonperturbative ones, should be added for a proper comparison with experimental data). Its usage is illustrated with several basic examples, such as Z boson, W boson, H0 boson and Drell–Yan lepton-pair production in hadronic collisions, and compared to the state-of-the-art Collins–Soper–Sterman subtraction scheme. It is also not limited to the transverse-momentum spectrum, and can straightforwardly be extended to match any (un)polarized cross section differential in other variables, including multi-differential measurements.

Quantum tomography for collider physics: illustrations with lepton-pair production

Quantum tomography is a method to experimentally extract all that is observable about a quantum mechanical system. We introduce quantum tomography to collider physics with the illustration of the angular distribution of lepton pairs. The tomographic method bypasses much of the field-theoretic formalism to concentrate on what can be observed with experimental data. We provide a practical, experimentally driven guide to model-independent analysis using density matrices at every step. Comparison with traditional methods of analyzing angular correlations of inclusive reactions finds many advantages in the tomographic method, which include manifest Lorentz covariance, direct incorporation of positivity constraints, exhaustively complete polarization information, and new invariants free from frame conventions. For example, experimental data can determine the entanglement entropy of the production process. We give reproducible numerical examples and provide a supplemental standalone computer code that implements the procedure. We also highlight a property of complex positivity that guarantees in a least-squares type fit that a local minimum of a chi ^{2} statistic will be a global minimum: There are no isolated local minima. This property with an automated implementation of positivity promises to mitigate issues relating to multiple minima and convention dependence that have been problematic in previous work on angular distributions.

Spin and model determination of Z′ - boson in lepton pair production at CERN LHC

Many new physics models predict production of heavy resonances in Drell-Yan channel and can be observed at the CERN LHC. If a new resonance is discovered as a peak in the dilepton invariant mass distribution at the LHC, the identification of its spin and couplings can be done by measuring production rates and angular distributions of the decay products. Here we discuss the spin-1 identification of Z′-boson for a set of representative models (SSM, E6, LR, and ALR) against the spin-2 RS graviton resonance and a spin-0 sneutrino resonance with the same mass and producing the same number of events under the resonance peak. We use the center-edge asymmetry for spin identification, as well as the total dilepton production cross section for the distinguishing the considered Z′-boson models from one another.

Searching for the doubly-charged Higgs bosons in the Georgi-Machacek model at the electron-proton colliders

The Georgi-Machacek model is one of many beyond Standard Model scenarios with an extended scalar sector which can group under the custodial $\rm SU(2)_C$ symmetry. There are 5-plet, 3-plet and singlet Higgs bosons under the classification of such symmetry in addition to the Standard Model Higgs boson. In this paper, we study the prospects for detecting the doubly-charged Higgs boson ($\rm H_5^{\pm\pm}$) through the vector boson fusion production at the electron-proton (ep) colliders. We concentrate on our analysis through $\mu$-lepton pair production via pair of same-sign W bosons decay. The $\rm H_5^{\pm\pm}$ discovery potentials at the ep colliders are presented.

LePaProGen—lepton pair production generator

A Monte Carlo generator “LePaProGen” for simulation of lepton pair production at hadron colliders is presented. Higher order electroweak radiative effects are implemented. A new algorithm for selection of the optimal phase space parameterization is applied.

Higgs-Boson Decay to Lepton Pair and Photon and Possible Non-Hermiticity of the Yukawa Interaction

The production of lepton pairs in the Higgs boson decay $h \to \ell^+ \ell^- \gamma$ is studied. The emphasis is put on the structure of the Higgs boson interaction with the fermions. This interaction is chosen as a mixture of the scalar and pseudo-scalar couplings, and, in addition, it is supposed to be non-Hermitian. We study prediction of this model for the observables in the $h \to \ell^+ \ell^- \gamma$ decay for the $e^+ e^-$, $\mu^+ \mu^-$ and $\tau^+ \tau^-$ pairs. The differential decay width and lepton forward-backward asymmetry are calculated as functions of the dilepton invariant mass for several sets of $h f \bar{f}$ coupling constants. The influence of non-Hermitian $h f \bar{f}$ interaction on the forward-backward asymmetry is studied, and large influence of a possible non-Hermiticity of the Higgs interaction with the top quarks on forward-backward asymmetry for $e^+ e^-$ and $\mu^+ \mu^-$ pairs is stressed.

Calibration of the KEDR detector Tagging System with two-photon lepton pair production

The tagging system of the KEDR detector for the two-photon studies is briefly described. The procedure of the energy calibration of the system is considered. Two-photon events e+e− → e+e− +μ+μ− selected with the central detector were employed to check the energy calibration and to measure the tagging efficiencies. The results obtained demonstrate that the achieved accuracy of the energy calibration is about 1%. To improve the accuracy to the level of 0.3% the upgrade of the calibration system is ongoing. The tagging efficiencies obtained with the simulation agree with the measured one within the error of the measurement.

Azimuthal asymmetries in QCD hard scattering: infrared safe but divergent

We consider high-mass systems of two or more particles that are produced by QCD hard scattering in hadronic collisions. We examine the azimuthal correlations between the system and one of its particles. We point out that the perturbative QCD computation of such azimuthal correlations and asymmetries can lead to divergent results at fixed perturbative orders. The fixed-order divergences affect basic (and infrared safe) quantities such as the total cross section at fixed (and arbitrary) values of the azimuthal-correlation angle φ. Examples of processes with fixed-order divergences are heavy-quark pair production, associated production of vector bosons and jets, dijet and diboson production. A noticeable exception is the production of high-mass lepton pairs through the Drell-Yan mechanism of quark-antiquark annihilation. However, even in the Drell-Yan process, fixed-order divergences arise in the computation of QED radiative corrections. We specify general conditions that produce the divergences by discussing their physical origin in fixed-order computations. We show lowest-order illustrative results for cos(nφ) asymmetries (with n = 1, 2, 4, 6) in top-quark pair production and associated production of a vector boson and a jet at the LHC. The divergences are removed by a proper all-order resummation procedure of the perturbative contributions. Resummation leads to azimuthal asymmetries that are finite and computable. We present first quantitative results of such a resummed computation for the cos(2φ) asymmetry in top-quark pair production at the LHC.

The Sivers asymmetry in Drell–Yan production at the J/Ψ peak at COMPASS

The abundant production of lepton pairs via J/Ψ creation at COMPASS, π±p↑→J/ΨX→ℓ+ℓ−X, allows a measurement of the transverse single spin asymmetry, ANJ/Ψ, generated by the Sivers effect. The crucial issue of the sign change of the Sivers function in Drell–Yan lepton pair production, with respect to Semi Inclusive Deep Inelastic Scattering processes, can be addressed in a different context. Assuming that the Sivers asymmetry is related to a universal and intrinsic property of the proton, predictions for the expected magnitude of ANJ/Ψ, which turns out to be large, are given. A comparison with the suggested measurement of this single spin asymmetry – an important quantity by itself – should give valuable information.

Jet-dilepton conversion from an anisotropic quark-gluon plasma

We calculate the yield of lepton pair production from jet-plasma interaction where the plasma is anisotropic in momentum space. We compare both the $M$ and $p_T$ distributions from such process with the Drell-Yan contribution. It is observed that the invariant mass distribution of lepton pair from such process dominate over the Drell-Yan up to $3$ GeV at RHIC and up to $10$ GeV at LHC. Moreover, it is found that the contribution from anistropic quark gluon plasma (AQGP) increases marginally compared to the isotropic QGP. In case of $p_T$-distribution we observe an increase by a factor of $3-4$ in the entire $p_T$-range at RHIC for AQGP. However, at LHC the change in the $p_T$-distribution is marginal as compared to the isotropic case.

The effect of real and virtual photons in the di-lepton channel at the LHC

We present a study of di-lepton production at the CERN Large Hadron Collider with a particular focus on the contribution resulting from both real and virtual photons in the initial state. We discuss the region of phase space in which the invariant mass of the lepton pair is of the order of several TeV, where searches for new physics phenomena yielding a di-lepton signature are presently carried out. We study both the yield and associated uncertainties for all possible topologies in photon-induced di-lepton production and compare these with what is expected in the standard Drell–Yan channel, where quark–antiquark pairs are responsible for the production of lepton pairs. We analyse the impact of these QED contributions on the expected Standard Model background and on searches for new physics. In this latter case, we use the production of an extra heavy Z′-boson predicted by the Sequential Standard Model (SSM) as a benchmark process.

Neutrino trident production at the intensity frontier

We have calculated cross sections for the production of lepton pairs by a neutrino incident on a nucleus using both the equivalent photon approximation, and deep inelastic formalism. We find that production of mixed flavour lepton pairs can have production cross sections as high as 35 times those of the traditional muon pair-production process. Rates are estimated for the SHiP and DUNE intensity frontier experiments. We find that multiple trident production modes, some of which have never been observed, represent observable signals over the lifetime of the detectors. Our estimates indicate that the SHiP collaboration should be able to observe on the order of 300 trident events given $2\cdot 10^{20}$ POT, and that the DUNE collaboration can expect approximately 250 trident events in their near detector given 3X10^{22} POT. We also discuss possible applications of the neutrino trident data to be collected at SHiP and DUNE for SM and BSM physics.

Enhanced threshold resummation formalism for lepton pair production and its effects in the determination of parton distribution functions

We demonstrate that theoretical predictions using current resummation techniques for the lepton pair production (LPP) rapidity and $x_F$ distributions can be inconsistent with data in high rapidity and $x_F$ kinematic regions by observing their effect in global fits of parton distribution functions (PDFs). We present an enhanced resummation technique for the LPP rapidity and $x_F$ distributions that agrees with LPP data. The enhanced resummation method is used in conjunction with threshold resummation in deep inelastic scattering (DIS) to perform two global fits of PDFs using the minimal and Borel prescriptions. The results are analyzed to determine the effect of threshold resummation on global fits of PDFs.

Spin Physics Experiments at NICA-SPD

Nuclotron based Ion Collider fAcility (NICA) is a flagship project of the Joint Institute for Nuclear Research which is expected to be operational by 2021. Main tasks of “NICA Facility” are study of hot and dense baryonic matter, investigation the polarisation phenomena and the nucleon spin structure. The material presented here based on the Letter of Intent (LoI) dedicated to nucleon spin structure studies at NICA. Measurements of asymmetries in the lepton pair (Drell-Yan) production in collisions of non-polarised, longitudinally and transversely polarised proton and deuteron beams to be performed using the specialized Spin Physics Detector (SPD). These measurements can provide an access to all leading twist collinear and Transverse Momentum Dependent Parton Distribution Functions (TMD PDFs) in nucleons. The measurements of asymmetries in production of J/ψ and direct photons, which supply complimentary information on the nucleon structure, will be performed simultaneously. The set of these measurements permits to tests the quark-parton model of nucleons at the QCD twist-2 level with minimal systematic errors.

Nuclear parton distributions

We review a microscopic model of the nuclear parton distribution functions, which accounts for a number of nuclear effects including Fermi motion and nuclear binding, nuclear meson-exchange currents, off-shell corrections to bound nucleon distributions and nuclear shadowing. We also discuss applications of this model to a number of processes including lepton-nucleus deep inelastic scattering, proton-nucleus Drell-Yan lepton pair production at Fermilab, as well as W+- and Z0 boson production in proton-lead collisions at the LHC.

Testable radiative neutrino mass model without additional symmetries and explanation for the b→sℓ+ℓ− anomaly

We propose a one-loop radiative Majorana-type neutrino-mass matrix without any kind of additional symmetries by introducing two leptoquark-like bosons only. In this scenario, we show that the anomaly appearing in the process $b\ensuremath{\rightarrow}s\ensuremath{\ell}\overline{\ensuremath{\ell}}$ can be explained without any conflicts against various constraints such as lepton-flavor violations, flavor-changing neutral currents, oblique parameters $\mathrm{\ensuremath{\Delta}}S$, $\mathrm{\ensuremath{\Delta}}T$, and the Drell-Yan process. We make the predictions for the flavor-violating lepton-pair production ($e\ensuremath{\mu}$, $e\ensuremath{\tau}$, and $\ensuremath{\mu}\ensuremath{\tau}$) at the LHC, as well as the cross sections for pair production of these leptoquark-like bosons.

NNLO QCD corrections for Drell-Yan p ZT and $$ {\phi}_{{}^{\eta}}^{\ast } $$ observables at the LHC

AbstractDrell-Yan lepton pairs with finite transverse momentum are produced when the vector boson recoils against (multiple) parton emission(s), and is determined by QCD dynamics. At small transverse momentum, the fixed order predictions break down due to the emergence of large logarithmic contributions. This region can be studied via thepTZdistribution constructed from the energies of the leptons, or through the$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗distribution that relies on the directions of the leptons. For sufficiently small transverse momentum, the$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗observable can be measured experimentally with better resolution. We study the smallpTZand$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗distributions up to next-to-next-to-leading order (NNLO) in perturbative QCD. We compute the$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗distributions for the fully inclusive production of lepton pairs via Z/γ∗to NNLO and normalise them to the NNLO cross sections for inclusive Z/γ∗production. We compare our predictions with the$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗distribution measured by the ATLAS collaboration during LHC operation at 8 TeV. We find that at moderate to large values of$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗, the NNLO effects are positive and lead to a substantial improvement in the theory-data comparison compared to next-to-leading order (NLO). At small values ofpTZand$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗, the known large logarithmic enhancements emerge through and we identify the region where resummation is needed. We find an approximate relationship between the values ofpTZand$$ {\phi}_{{}^{\eta}}^{\ast } $$ϕη∗where the large logarithms emerge and find perturbative consistency between the two observables.

Next-to-leading order transverse momentum broadening for Drell-Yan production in p+A collisions

We present the nuclear transverse momentum broadening for Drell-Yan lepton pair production in p+A collisions at next-to-leading order (NLO) in powers of strong coupling constant $\alpha_s$. We verify that the transverse momentum weighted differential cross section in NLO perturbative QCD (pQCD) at twist-4 can be factorized into the convolution of parton distribution function of an active parton in the projectile proton, a twist-4 multiparton correlation function of the target nucleus, and perturbatively calculable hard coefficient function. We identify a QCD evolution equation for such a twist-4 nuclear gluon-quark correlation function, and verify its universality -- its process independence. This finding demonstrates the prediction power of pQCD factorization approach for studying parton multiple scattering in nuclear medium.

Recent developments in the theory of electromagnetic probes in relativistic heavy-ion collisions

The theoretical developments in the study of electromagnetic radiation in relativistic heavy-ion collisions are reviewed. The recent progress in the rates for photon and lepton pair production is discussed. Together with the improvements in the hydrodynamic descriptions of the bulk medium, the combined effort is discussed to resolve the “direct photon flow puzzle” in the RHIC and the LHC experiments. Further prediction of the direct photon production in high multiplicity proton-nucleus collisions at the LHC energy can serve as a signature of the quark gluon plasma formation in these small systems. Phenomenological study of dilepton production at finite net baryon density is highlighted at the collision energies available for the RHIC beam energy scan program.