Dilepton Invariant Mass Research Articles

Virtual Photons Shed Light on the Early Temperature of Dense QCD Matter.

Dileptons produced during heavy-ion collisions represent a unique probe of the QCD phase diagram, and convey information about the state of the strongly interacting system at the moment their preceding off-shell photon is created. In this study, we compute thermal dilepton yields from Au+Au collisions performed at different beam energies, employing a (3+1)-dimensional dynamic framework combined with emission rates accurate at next-to-leading order in perturbation theory and which include baryon chemical potential dependencies. By comparing the effective temperature extracted from the thermal dilepton invariant mass spectrum with the average temperature of the fluid, we offer a robust quantitative validation of dileptons as an effective probe of the early quark-gluon plasma stage.

Probing new physics with polarization components of the tau lepton in quasielastic e−p→Λcτ− scattering process

Kinematics restricts the ability of rare charm decays to explore the charged lepton-flavor-violation processes mediated by the quark-level c→uℓτ transition. To fill the gap, we propose exploring new physics (NP) through the quasielastic scattering process e−p→τ−Λc and the polarization of the τ lepton. As analyzing modes for the τ polarization, we consider the decays τ−→π−ντ, τ−→ρ−ντ, and τ−→ℓ−ν¯ℓντ, and show that the τ polarization components can be extracted from analyzing the kinematics of the τ visible decay products. In the framework of a general low-energy effective Lagrangian, we then perform a detailed analysis of the polarization components in various aspects and scrutinize possible NP signals. With one upcoming experimental setup, we finally demonstrate promising event rate can be expected for the cascade process and, even in the worst-case scenario—no signals are observed at all—it can still provide a competitive potential for constraining the NP, compared with those from the high-pT dilepton invariant mass tails at high-energy colliders. Published by the American Physical Society 2024

Analytic third-order QCD corrections to top-quark and semileptonic b→u decays

We present the first analytic results of next-to-next-to-next-to-leading-order (N3LO) QCD corrections to the top-quark decay width. We focus on the dominant leading color contribution, which includes light-quark loops. At next-to-next-to-leading order (NNLO), this dominant contribution accounts for 95% of the total correction. By utilizing the optical theorem, the N3LO corrections are related to the imaginary parts of the four-loop self-energy Feynman diagrams, which are calculated with differential equations. The results are expressed in terms of harmonic polylogarithms, enabling fast and accurate evaluation. The third-order QCD corrections decrease the leading-order decay width by 0.667%, and the scale uncertainty is reduced by half compared to the NNLO result. The most precise prediction for the top-quark width is now 1.321 GeV for mt=172.69 GeV. Additionally, we obtain the third-order QCD corrections to the dilepton invariant mass spectrum and decay width in the semileptonic b→u transition. The perturbative series in the on-shell mass scheme exhibits poor convergence behavior. In the MS¯ mass scheme, the scale dependence is greatly improved. A more precise determination of the CKM matrix element Vub could be obtained with such higher-order corrections. Published by the American Physical Society 2024

Inclusive semileptonic b→cℓν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ b\ o c\\ell \\overline{\ u} $$\\end{document} decays to order 1/mb5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ 1/{m}_b^5 $$\\end{document}

Inclusive semileptonic B→Xcℓν¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ B\ o {X}_c\\ell \\overline{\ u} $$\\end{document} decays can be described in the Heavy Quark Expansion (HQE) and allow for a precision determination of the CKM element |Vcb|. We calculate the terms of 1/mb5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ 1/{m}_b^5 $$\\end{document} and derive a “trace formula” which allows for the computation of the decay rate and kinematic moments of the spectrum up to this order in the HQE. We focus specifically on the reparametrization invariant (RPI) dilepton invariant mass q2 moments of the spectrum, which depend on a reduced set of HQE parameters. At this order, “intrinsic charm” (IC) contributions proportional to 1/mb3mc2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ 1/\\left({m}_b^3{m}_c^2\\right) $$\\end{document} enter, which are numerically expected to be sizeable. Using the “lowest-lying state saturation ansatz” (LLSA), we estimate the size of these contributions. Within this approximation, we observe a partial cancellation between the IC and the “genuine” 1/mb5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ 1/{m}_b^5 $$\\end{document} contributions, resulting in a small overall contribution.

Complementary constraints on Zbb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ Zb\\overline{b} $$\\end{document} couplings at the LHC

We propose a new strategy to probe the Z boson couplings to bottom and charm quarks at the LHC. In this work we mainly focus on the case of bottom quarks. Here, the Z boson is produced in association with two b-jets and decays to electrons or muons. In this final state, tagging the charge of the b-jets allows us to measure the charge asymmetry and thus to directly probe the Zbb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ Zb\\overline{b} $$\\end{document} couplings. The leptonic final state not only allows us to cleanly reconstruct the Z boson but also to mitigate the otherwise overwhelming backgrounds. Furthermore, while LEP could only scan a limited range of dilepton invariant masses, there is no such limitation at the LHC. Consequently, this allows us to make full use of the interference between the amplitudes mediated by a Z boson and a photon. Using the full high-luminosity LHC dataset of 3 ab−1 and with the current flavor and charge-tagging capabilities would allow us to reject the wrong-sign right-handed coupling solution by 4σ. Further improving the charge-tagging efficiency would disfavor it by 6σ.

Searches for new phenomena in events with two leptons, jets, and missing transverse momentum in 139 fb^{-1} of varvec{sqrt{s}}=13 TeV varvec{pp} collisions with the ATLAS detector

Searches for new phenomena inspired by supersymmetry in final states containing an e^+e^- or mu ^+mu ^- pair, jets, and missing transverse momentum are presented. These searches make use of proton–proton collision data with an integrated luminosity of 139~text {fb}^{-1}, collected during 2015–2018 at a centre-of-mass energy sqrt{s}=13~TeV by the ATLAS detector at the Large Hadron Collider. Two searches target the pair production of charginos and neutralinos. One uses the recursive-jigsaw reconstruction technique to follow up on excesses observed in 36.1~text {fb}^{-1} of data, and the other uses conventional event variables. The third search targets pair production of coloured supersymmetric particles (squarks or gluinos) decaying through the next-to-lightest neutralino (tilde{chi }_2^0) via a slepton (tilde{ell }) or Z boson into ell ^+ell ^-tilde{chi }_1^0, resulting in a kinematic endpoint or peak in the dilepton invariant mass spectrum. The data are found to be consistent with the Standard Model expectations. Results are interpreted using simplified models and exclude masses up to 900 GeV for electroweakinos, 1550 GeV for squarks, and 2250 GeV for gluinos.

Laboratory-frame tests of quantum entanglement in H→WW

Quantum entanglement between the two $W$ bosons resulting from the decay of a Higgs boson may be investigated in the dilepton channel $H\ensuremath{\rightarrow}WW\ensuremath{\rightarrow}\ensuremath{\ell}\ensuremath{\nu}\ensuremath{\ell}\ensuremath{\nu}$ using laboratory-frame observables that only involve the charged leptons $\ensuremath{\ell}=e$, $\ensuremath{\mu}$. The dilepton invariant mass distribution, already measured by the ATLAS and CMS Collaborations at the LHC, can be used to observe the quantum entanglement of the $WW$ pair with a statistical sensitivity of $7\ensuremath{\sigma}$ with run 2 data, and of $6\ensuremath{\sigma}$ when including theoretical systematics. As a by-product, the relation between $W$ rest frame (four-dimensional) angular distributions, $H\ensuremath{\rightarrow}WW$ decay amplitudes, and spin correlation coefficients, is written down.

Parton distributions and new physics searches: the Drell–Yan forward–backward asymmetry as a case study

We discuss the sensitivity of theoretical predictions of observables used in searches for new physics to parton distributions (PDFs) at large momentum fraction x. Specifically, we consider the neutral-current Drell–Yan production of gauge bosons with invariant masses in the TeV range, for which the forward–backward asymmetry of charged leptons from the decay of the gauge boson in its rest frame is a traditional probe of new physics. We show that the qualitative behaviour of the asymmetry depends strongly on the assumptions made in determining the underlying PDFs. We discuss and compare the large-x behaviour of various different PDF sets, and find that they differ significantly. Consequently, the shape of the asymmetry observed at lower dilepton invariant masses, where all PDF sets are in reasonable agreement because of the presence of experimental constraints, is not necessarily reproduced at large masses where the PDFs are mostly unconstrained by data. It follows that the shape of the asymmetry at high masses may depend on assumptions made in the PDF parametrization, and thus deviations from the traditionally expected behaviour cannot be taken as a reliable indication of new physics. We demonstrate that forward–backward asymmetry measurements could help in constraining PDFs at large x and discuss the accuracy that would be required to disentangle the effects of new physics from uncertainties in the PDFs in this region.

QED in overline{B} → overline{K} ℓ+ℓ− LFU ratios: theory versus experiment, a Monte Carlo study

Using analytic results obtained in a meson effective theory, that includes all infrared sensitive logs, we build a dedicated Monte Carlo framework to describe QED corrections in overline{B} → overline{K} ℓ+ℓ− for a generic form factor. For the neutral mode {overline{B}}^0 → {overline{K}}^0 ℓ+ℓ−, we perform a detailed numerical comparison versus those obtained with the general-purpose photon-shower tool PHOTOS. The comparison indicates a good agreement, at the few per-mil level, when focusing on the rare mode only. In addition, our framework allows us to investigate the impact of the charmonium resonances. Interference effects, not described by PHOTOS in the experimental analysis, are found to be small in the dilepton invariant mass region q2< 6GeV2, which is used to determine {R}_{K^{left(ast right)}} . Using a semi-analytic framework we assess the full, rare and resonant, mode. Based thereupon, we discuss strategies to check the subtraction of the resonant mode, which has a sizeable impact at q2 ≈ 6GeV2 in the electron mode.

First extraction of inclusive Vcb from q2 moments

We present the first determination of Vcb from inclusive Bto {X}_c ell {overline{v}}_{ell} using moments of the dilepton invariant mass, q2. These moments are reparametrization invariant quantities and depend on a reduced set of non-perturbative parameters. This reduced set of parameters opens a new path to extract these parameters up to 1/{m}_b^4 purely from data and can potentially reduce the uncertainty on Vcb. In this paper, we present our first determination of Vcb using this method. Combining the recent measurements of q2 moments by Belle and Belle II, our default fit gives |Vcb| = (41.69±0.63)·10−3. This result presents an important independent cross check of, and is consistent with, the previous state-of-the-art inclusive determinations using lepton energy and hadronic invariant mass moments.

Investigating the color-suppressed decays Λb→Λψ in the perturbative QCD approach

The nonleptonic two-body ${\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}\ensuremath{\psi}$ decays with $\ensuremath{\psi}=J/\ensuremath{\psi}$ or $\ensuremath{\psi}(2S)$ are investigated based on the perturbative QCD approach. These are color-suppressed processes in which the nonfactorizable contributions are confirmed to be dominant. Angular momentum conservation allows us to describe the concerned decays by four independent complex helicity amplitudes. It is observed that the negative-helicity states for the $\mathrm{\ensuremath{\Lambda}}$ baryon are preferred as expected in the left-handed nature of the charged-current interaction. The obtained results for the helicity amplitudes are used to compute the branching ratios and various observable parameters, which are then compared to the existing theoretical predictions and experimental data. In particular, we predict the ratio $\mathcal{R}=\frac{\mathcal{B}({\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}\ensuremath{\psi}(2S))}{\mathcal{B}({\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}J/\ensuremath{\psi})}=0.4{7}_{\ensuremath{-}0.00}^{+0.02}$ in comparison with $0.508\ifmmode\pm\else\textpm\fi{}0.023$ from the Particle Data Group at the level of 1 standard deviation. We also briefly explore the long-distance contributions to the semileptonic ${\mathrm{\ensuremath{\Lambda}}}_{b}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}{l}^{+}{l}^{\ensuremath{-}}$ decays in the kinematic regions where the dilepton invariant masses are around the $J/\ensuremath{\psi}$ and $\ensuremath{\psi}(2S)$ resonances.

Rate and ellipticity of dilepton production in a magnetized quark-gluon plasma

Using the Landau-level representation for the imaginary part of the photon polarization tensor, we derive an explicit expression for the dilepton production rate from a hot quark-gluon plasma in a quantizing background magnetic field. We study in detail the dependence of the production rate on the dilepton invariant mass and the transverse momentum at midrapidity. We also investigate in detail the angular dependence and ellipticity of dilepton emission. By comparing the result with the zero-field Born approximation, we find that the magnetic field leads to a strong enhancement of the dilepton rate at small values of the invariant mass ($M\ensuremath{\lesssim}\sqrt{|eB|}$). In the same kinematic region, the dilepton production is characterized by a sizable ellipticity. At large values of the dilepton invariant mass ($M\ensuremath{\gtrsim}\sqrt{|eB|}$), the role of the magnetic field decreases and the result approaches the isotropic zero-field Born rate. By investigating the dependence of ellipticity on the transverse momentum, we argue that the future measurements of dilepton rate in the region of small invariant masses can constrain the magnetic field produced in heavy-ion collisions.

Searching and identifying leptoquarks through low-energy polarized scattering processes e−p→e−Λc

We investigate the potential for searching and identifying the leptoquark (LQ) effects in the charm sector through the low-energy polarized scattering processes $\vec{e}^{\,-}p\to e^-\Lambda_c$, $e^-\vec{p}\to e^-\Lambda_c$, and $\vec{e}^{\,-}\vec{p}\to e^-\Lambda_c$. Considering only the longitudinally polarized processes, we show that the different LQ models can be disentangled from each other by measuring the four spin asymmetries, $A_{L}^e$, $A_{L}^p$, $A_{L3}^{ep}$, and $A_{L6}^{ep}$, constructed in terms of the polarized cross sections. Although it is challenging to accomplish the same goal with transversely polarized processes, we find that investing them in future experiments is especially beneficial, since they can directly probe into the imaginary part of the Wilson coefficients in the general low-energy effective Lagrangian. With our properly designed experimental setups, it is also demonstrated that promising event rates can be expected for all these processes and, even in the worst-case scenario -- no LQ signals are observed at all, they can still provide a competitive potential for constraining the new physics, compared with those from the conventional charmed-hadron weak decays and the high-$p_T$ dilepton invariant mass tails at high-energy colliders.

Mixed QCD-electroweak corrections to dilepton production at the LHC in the high invariant mass region

We compute mixed QCD-electroweak corrections to the neutral-current Drell-Yan production of a pair of massless leptons in the high invariant mass region. Our computation is fully differential with respect to the final state particles. At relatively low values of the dilepton invariant mass, mℓℓ ∼ 200 GeV, we find unexpectedly large mixed QCD-electroweak corrections at the level of −1%. At higher invariant masses, mℓℓ ∼ 1 TeV, we observe that these corrections can be well approximated by the product of QCD and electroweak corrections. Hence, thanks to the well-known Sudakov enhancement of the latter, they increase at large invariant mass and reach e.g. −3% at mℓℓ = 3 TeV. Finally, we note that the inclusion of mixed corrections reduces the theoretical uncertainty related to the choice of electroweak input parameters to below the percent level.

Tests of Lepton Universality Using B^{0}→K_{S}^{0}ℓ^{+}ℓ^{-} and B^{+}→K^{*+}ℓ^{+}ℓ^{-} Decays.

Tests of lepton universality in B^{0}→K_{S}^{0}ℓ^{+}ℓ^{-} and B^{+}→K^{*+}ℓ^{+}ℓ^{-} decays where ℓ is either an electron or a muon are presented. The differential branching fractions of B^{0}→K_{S}^{0}e^{+}e^{-} and B^{+}→K^{*+}e^{+}e^{-} decays are measured in intervals of the dilepton invariant mass squared. The measurements are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 fb^{-1} . The results are consistent with the standard model and previous tests of lepton universality in related decay modes. The first observation of B^{0}→K_{S}^{0}e^{+}e^{-} and B^{+}→K^{*+}e^{+}e^{-} decays is reported.

Electromagnetic Dalitz decays of D(s)∗ mesons

Rare electromagnetic (EM) decays of charmed mesons are useful laboratories to explore the structure of hadronic states and the interactions between photons and charmed mesons, to test the chiral perturbation theory in flavor sector and to search for new physics including dark photons. In this paper, we calculate the relative branching ratios of EM Dalitz decays [Formula: see text] to their corresponding radiative decays [Formula: see text] with transition form factor in vector-meson dominance model, where [Formula: see text] represents [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text]. The corresponding dileptonic invariant mass spectra and the leptonic angular distributions are also presented.

X17 Discovery Potential in the γN→e^{+}e^{-}N Process at Electron Scattering Facilities.

We propose a direct search for the X17 particle, which was conjectured to explain the ATOMKI ^{8}Be and ^{4}He anomalies, through the dilepton photoproduction process on a nucleon in the photon energy range below or around the pion production threshold. For the scenarios of either pseudoscalar, vector, or axial-vector quantum numbers of the conjectured X17, we use existing constraints to estimate the X17 signal process. For dilepton invariant mass resolutions which have been achieved in previous experiments, a signal-to-background ratio of up to an order of magnitude is found for a neutron target, and, in particular, for the pseudoscalar and vector X17 scenarios.

Inclusive and differential cross section measurements of single top quark production in association with a Z boson in proton-proton collisions at sqrt{s} = 13 TeV

Inclusive and differential cross sections of single top quark production in association with a Z boson are measured in proton-proton collisions at a center-of-mass energy of 13 TeV with a data sample corresponding to an integrated luminosity of 138 fb−1 recorded by the CMS experiment. Events are selected based on the presence of three leptons, electrons or muons, associated with leptonic Z boson and top quark decays. The measurement yields an inclusive cross section of {87.9}_{-7.3}^{+7.5}{left(mathrm{stat}right)}_{-6.0}^{+7.3}left(mathrm{syst}right) fb for a dilepton invariant mass greater than 30 GeV, in agreement with standard model (SM) calculations and represents the most precise determination to date. The ratio between the cross sections for the top quark and the top antiquark production in association with a Z boson is measured as {2.37}_{-0.42}^{+0.56}{left(mathrm{stat}right)}_{-0.13}^{+0.27}left(mathrm{syst}right) . Differential measurements at parton and particle levels are performed for the first time. Several kinematic observables are considered to study the modeling of the process. Results are compared to theoretical predictions with different assumptions on the source of the initial-state b quark and found to be in agreement, within the uncertainties. Additionally, the spin asymmetry, which is sensitive to the top quark polarization, is determined from the differential distribution of the polarization angle at parton level to be 0.54 ± 0.16 (stat) ± 0.06 (syst), in agreement with SM predictions.

Mixed Strong-Electroweak Corrections to the Drell-Yan Process.

We report on the first complete computation of the mixed QCD-electroweak (EW) corrections to the neutral-current Drell-Yan process. Superseding previously applied approximations, our calculation provides the first result at this order that is valid in the entire range of dilepton invariant masses. The two-loop virtual contribution is computed by using semianalytical techniques, overcoming the technical problems in the evaluation of the relevant master integrals. The cancellation of soft and collinear singularities is achieved by a formulation of the q_{T} subtraction formalism valid in the presence of charged massive particles in the final state. We present numerical results for the fiducial cross section and selected kinematical distributions. At large values of the lepton p_{T} the mixed QCD-EW corrections are negative and increase in size, to about -15% with respect to the next-to-leading-order QCD result at p_{T}=500 GeV. Up to dilepton invariant masses of 1TeV the computed corrections amount to about -1.5% with respect to the next-to-leading-order QCD result.

Leptons lurking in semi-visible jets at the LHC

This Letter proposes a new search for confining dark sectors at the Large Hadron Collider. As a result of the strong dynamics in the hidden sector, dark matter could manifest in proton-proton collisions at the Large Hadron Collider in form of hadronic jets containing stable invisible bound states. These semi-visible jets have been studied theoretically and experimentally in the fully hadronic signature where the unstable composite dark matter can only decay promptly back to Standard Model quarks. We present a simplified model based on two messenger fields separated by a large mass gap allowing dark bound states to decay into pairs of oppositely charged leptons. The resulting experimental signature is characterized by non-isolated lepton pairs inside semi-visible jets. We propose a search strategy independent from the underlying model assumptions targeting this new signature, and discuss the orthogonality with respect to the existing searches. Remaining agnostic on the shape of the di-lepton spectrum, we determine the sensitivity of a dedicated analysis to the target signal. The proposed search can claim the 3 sigma evidence (exclusion) of the heavier mediator up to masses of 3.5 TeV (4.5 TeV) with the full Run 2 data of the LHC. Exploiting the resonant feature of the lepton pairs can enhance the sensitivity reach on a specific model. We estimate that an analysis using the di-lepton invariant mass information can reach 5 sigma discovery up to masses of 3.5 TeV and improve the exclusion up to more than 5 TeV.