Charged Lepton Pairs Research Articles

Final state Coulomb interaction and asymmetry of pair production close to threshold in e+e− annihilation

We investigate a contribution of the $d$ wave to the cross section of $e^+e^-$ annihilation to the pair of charged leptons or nucleons close to threshold of the process. In contrast to the point of view accepted in literature, due to the Coulomb final state interaction this contribution does not vanish even at zero relative velocity of produced particles. This results in the nonzero asymmetry in angular distribution at threshold. Though value of the asymmetry is small, observation of this effect is not hopeless.

Lepton number violation at the LHC with leptoquarks and diquarks

We investigate a model in which tiny neutrino masses are generated at the two-loop level by using scalar leptoquark and diquark multiplets. The diquark can be singly produced at the LHC, and it can decay into a pair of leptoquarks through the lepton number violating interaction. Subsequent decays of the two leptoquarks can provide a clear signature of the lepton number violation, namely two QCD jets and a pair of same-signed charged leptons without missing energy. We show that the signal process is not suppressed while neutrino masses are appropriately suppressed.

Lepton Number Violation and Scalar Searches at the LHC

We review the SM extensions with scalar multiplets including doubly-charged components eventually observable as di-leptonic resonances at the LHC. Special emphasis is payed to the limits on LNV implied by doubly-charged scalar searches at the LHC, and to the characterization of the multiplet doubly-charged scalars belong to if they are observed to decay into same-sign charged lepton pairs.

Lepton number violation at the LHC with leptoquark and diquark

We investigate a model in which tiny neutrino masses are generated at the two-loop level by using scalar leptoquark and diquark multiplets. The diquark can be singly produced at the LHC, and it can decay into a pair of leptoquarks through the lepton number violating interaction. Subsequent decays of the two leptoquarks can provide a clear signature of the lepton number violation, namely two QCD jets and a pair of same-signed charged leptons without missing energy. We show that the signal process is not suppressed while neutrino masses are appropriately suppressed.

Production of an arbitrary number of lepton and pion pairs at peripheral hadron collisions

The known results of unitarity violation in QED are revised. Taking into account the peripheral processes in high energy ion collisions with production of an arbitrary number of pairs of charged leptons and pions, we confirm the known result of violation of unitarity in the frame of QED. Namely, the total cross section of production of an arbitrary number of pairs of charged particles grows more rapidly than any logarithmic power of the center-of-mass energy. The contribution from the production of a single neutral meson (scalar and pseudo-scalar) is discussed. An interpretation of this fact is expected to have a similar base to the known QED problem—the absence of the total cross section in the scattering of a charged particle on the Coulomb center. The relevant conditions cannot be realized in nature. The novelty of our calculations is in obtaining the explicit expression for differential cross sections of (in principle) any finite number of lepton and π+ π− pairs at peripheral hadron collisions. It can be measured at such facilities as RHIC or LHC.

Precise predictions forZ-boson+4jet production at hadron colliders

We present the cross section for production of a $Z$ boson in association with four jets at the Large Hadron Collider, at next-to-leading order in the QCD coupling. When the $Z$ decays to neutrinos, this process is a key irreducible background to many searches for new physics. Its computation has been made feasible through the development of the on-shell approach to perturbative quantum field theory. We present the total cross section for $pp$ collisions at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$, after folding in the decay of the $Z$ boson, or virtual photon, to a charged-lepton pair. We also provide distributions of the transverse momenta of the four jets, and we compare cross sections and distributions to the corresponding ones for the production of a $W$ boson with accompanying jets.

Zγγproduction with leptonic decays and triple photon production at next-to-leading order QCD

We present a calculation of the O(alpha_s) QCD corrections to the production of a Z boson in association with two photons and to triple photon production at hadron colliders. All final-state photons are taken as real. For the Z boson, we consider the decays both into charged leptons and into neutrinos including all off-shell effects. Numerical results are obtained via a Monte Carlo program based on the structure of the VBFNLO program package. This allows us to implement general cuts and distributions of the final-state particles. We find that the NLO QCD corrections are sizable and significantly exceed the expectations from a scale variation of the leading-order result. In addition, differential distributions of important observables change considerably. The prediction of two-photon-associated Z production with Z decays into neutrinos from the charged-lepton rate works well, once we use an additional cut on the invariant mass of the charged-lepton pair.

Implications of high-resolution simulations on indirect dark matter searches

We study the prospects for detecting the annihilation products of dark matter (DM) in the framework of the two highest-resolution numerical simulations currently available, i.e., Via Lactea II and Aquarius. We propose a strategy to determine the shape and size of the region around the Galactic center that maximizes the probability of observing a DM signal, and we show that, although the predicted flux can differ by a factor of 10 for a given DM candidate in the two simulation setups, the search strategy remains actually unchanged, since it relies on the angular profile of the annihilation flux, not on its normalization. We present mock $\ensuremath{\gamma}$-ray maps that keep into account the diffuse emissions produced by unresolved halos in the Galaxy, and we show that, in an optimistic DM scenario, a few individual clumps can be resolved above the background with the Fermi-LAT. Finally, we calculate the energy-dependent boost factors for positrons and antiprotons and show that they are always of $\mathcal{O}(1)$, and, therefore, they cannot lead to the large enhancements of the antimatter fluxes required to explain the recent PAMELA, ATIC, Fermi, and HESS data. Still, we show that the annihilation of 100 GeV weakly interacting massive particles into charged lepton pairs may contribute significantly to the positron budget.

Gamma-ray lines from radiative dark matter decay

The decay of dark matter particles which are coupled predominantly to charged leptons has been proposed as a possible origin of excess high-energy positrons and electrons observed by cosmic-ray telescopes PAMELA and Fermi LAT. Even though the dark matter itself is electrically neutral, the tree-level decay of dark matter into charged lepton pairs will generically induce radiative two-body decays of dark matter at the quantum level. Using an effective theory of leptophilic dark matter decay, we calculate the rates of radiative two-body decays for scalar and fermionic dark matter particles. Due to the absence of astrophysical sources of monochromatic gamma rays, the observation of a line in the diffuse gamma-ray spectrum would constitute a strong indication of a particle physics origin of these photons. We estimate the intensity of the gamma-ray line that may be present in the energy range of a few TeV if the dark matter decay interpretation of the leptonic cosmic-ray anomalies is correct and comment on observational prospects of present and future Imaging Cherenkov Telescopes, in particular the CTA.

Decay-lepton correlations as probes of anomalous ZZH and γZH interactions in [formula omitted] with polarized beams

We examine the contributions of various couplings in general ZZH and γZH interactions arising from new physics to the Higgs production process e+e−→HZ, followed by the decay of the Z into a charged-lepton pair. We take into account possible longitudinal or transverse beam polarization likely to be available at a linear collider. We show how expectation values of certain simple observables in suitable combinations with appropriate longitudinal beam polarizations can be used to disentangle various couplings from one another. Longitudinal polarization can also improve the sensitivity for measurement of several couplings. A striking result is that using transverse polarization, one of the γZH couplings, not otherwise accessible, can be determined independently of all other couplings.

Spontaneous R-parity violating type III seesaw

We present a model where neutrino masses are generated by a combination of spontaneous R-parity violation and Type III seesaw. In addition to the usual MSSM particle content, our model consists of one extra triplet matter chiral superfield containing heavy SU(2) triplet fermions and its superpartners. R-parity is broken spontaneously when the sneutrinos associated with the one heavy neutrino as well as the three light neutrinos get vacuum expectation values, giving rise to the mixed $8\times 8$ neutralino-neutrino mass matrix. We show that our model can comfortably explain all the existing neutrino oscillation data. Due to the presence of the triplet fermion, we have a pair of additional heavy charged leptons which mix with the standard model charged leptons and the charginos. This gives rise to a $6\times 6$ chargino-charged lepton mass matrix, with 6 massive eigenstates. Finally we discuss about the different R-parity violating possible decay modes and the distinctive collider signatures which our model offers.

Erratum: Charged lepton distributions as a probe of contacte+e−HZinteractions at a linear collider with polarized beams [Phys. Rev. D77, 015009 (2008)

We examine very general four-point interactions arising from new physics and contributing to the Higgs production process e+e- -> HZ. We write all possible forms for these interactions consistent with Lorentz invariance. We allow the possibility of CP violation. Contributions to the process from anomalous ZZH and gamma ZH interactions studied earlier arise as a special case of our four-point amplitude. We consider the decay of Z into a charged lepton pair and obtain expressions for angular distributions of charged leptons arising from the interference of the four-point contribution with the standard-model contribution. We take into account possible longitudinal or transverse beam polarization likely to be available at a linear collider. We examine several correlations which can be used to study the various form factors present in the e+e-HZ contact interactions. We also obtain the sensitivity of these correlations in constraining the new-physics interactions at a linear collider operating at a centre-of-mass energy of 500 GeV with longitudinal or transverse polarization.

Detecting light leptophilic gauge boson at BESIII detector

The O(GeV) extra U(1) gauge boson named U-boson, has been proposed to mediate the interaction among leptons and dark matter (DM), in order to account for the observations by PAMELA and ATIC. In such kind of models, the extra U(1) gauge group can be chosen as U(1)Li−Lj with Li the i-th generation lepton number. This anomaly-free model provides appropriate dark matter relic density and boost factor required by experiments. In this work the observability of such kind of U-boson at BESIII detector is investigated through the processes e+e−→Uγ, followed by U→e+e−, U→μ+μ− and U→νν¯. In the invisible channel where U-boson decays into neutrinos, BESIII can measure the coupling of the extra U(1) down to O(10−4)∼O(10−5) because of the low Standard Model backgrounds. In the visible channel where U-boson decays into charged lepton pair, BESIII can only measure the coupling down to O(10−3)∼O(10−4) due to the large irreducible QED backgrounds.

LHC discovery potential of a leptophilic Higgs boson

In this work, we examine a two-Higgs-doublet extension of the standard model in which one Higgs doublet is responsible for giving mass to both up- and down-type quarks, while a separate doublet is responsible for giving mass to leptons. We examine both the theoretical and experimental constraints on the model and show that large regions of parameter space are allowed by these constraints in which the effective couplings between the lightest neutral Higgs scalar and the standard-model leptons are substantially enhanced. We investigate the collider phenomenology of such a ``leptophilic'' two-Higgs-doublet model and show that in cases where the low-energy spectrum contains only one light, $CP$-even scalar, a variety of collider processes essentially irrelevant for the discovery of a standard model Higgs boson (specifically those in which the Higgs boson decays directly into a charged-lepton pair) can contribute significantly to the discovery potential of a light-to-intermediate-mass (${m}_{h}\ensuremath{\lesssim}140\text{ }\text{ }\mathrm{GeV}$) Higgs boson at the LHC.

Charged lepton distributions as a probe of contacte+e−HZinteractions at a linear collider with polarized beams

We examine very general four-point interactions arising from new physics and contributing to the Higgs production process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}HZ$. We write all possible forms for these interactions consistent with Lorentz invariance. We allow the possibility of $CP$ violation. Contributions to the process from anomalous $ZZH$ and $\ensuremath{\gamma}ZH$ interactions studied earlier arise as a special case of our four-point amplitude. We consider the decay of $Z$ into a charged-lepton pair and obtain expressions for angular distributions of charged leptons arising from the interference of the four-point contribution with the standard-model contribution. We take into account possible longitudinal or transverse beam polarization likely to be available at a linear collider. We examine several correlations which can be used to study the various form factors present in the ${e}^{+}{e}^{\ensuremath{-}}HZ$ contact interactions. We also obtain the sensitivity of these correlations in constraining the new-physics interactions at a linear collider operating at a center-of-mass energy of 500 GeV with longitudinal or transverse polarization.

Boson fusion and Higgs production at the CERN LHC in six fermion final states with one charged lepton pair

Boson boson scattering and Higgs production in boson boson fusion will be actively investigated at the LHC. We have performed a parton level study of all processes of the type $q_1 q_2 \to q_3 q_4 q_5 q_6 l^+l^-$ using for the first time a full fledged six fermion Monte Carlo event generator which employs exact matrix elements at $\O(\alpha_{em}^6)$. We have examined Higgs production in vector boson fusion followed by the decay chain $H\to ZZ\to l^+l^-jj$, including exactly all electroweak irreducible backgrounds. In the high mass region we have compared the case of a relatively light Higgs with the no-Higgs case. The integrated cross section for the latter case is more than twice that in the former for a minimum invariant mass of the $ZV$ pair of about 800 \GeV. We find, in a preliminary anlysis at parton level that, summing up the muon and the electron channels, about 30 events are expected in the light Higgs case for L=100 $fb^{-1}$.

Associated production of a light pseudoscalar Higgs boson with a chargino pair in the NMSSM

In the next-to-minimal supersymmetric standard model (NMSSM), the unique $\lambda S H_u H_d$ in the superpotential gives rise to a coupling involving the lighter pseudoscalar Higgs boson and a pair of charged or neutral Higgsinos, even in the limit of zero mixing between the two pseudoscalar Higgs bosons. We study the associated production of a very light pseudoscalar Higgs boson with a pair of charginos. The novel signature involves a pair of charged leptons from chargino decays and a pair of photons from the pseudoscalar Higgs boson decay, plus large missing energy at the LHC and ILC. The signal may help us to distinguish the NMSSM from MSSM, provided that the experiment can resolve the two photons from the decay of the pseudoscalar Higgs boson.

NLO QCD CORRECTIONS TO THE PRODUCTION OF HEAVY CHARGED LEPTONS BY GLUON FUSION AT THE LHC

The search for heavy charged leptons predicted by generalizations of the standard model is an objective of the experiments at future colliders, in particular at the LHC. We compute the Next-to-Leading Order (NLO) QCD virtual corrections to the production of pairs of heavy charged leptons by gluon fusion. The radiative corrections increase the production rate by about 30% and reduce its dependence on the renormalization and the factorization scales. The effect of a fourth family of quarks on the production cross-section is briefly discussed.

Rare t-quark decays t → cl +j l −k and t → cṽ j v k induced by doublets of scalar leptoquarks within the minimal four-color-symmetry model

The rare t-quark decays t → clj+lk− and t → cṽjkk induced by scalar-leptoquark doublets are considered within the minimal model involving four-color quark-lepton symmetry and the Higgs mechanism of quark and lepton mass splitting. The partial widths with respect to the decays being considered and the total widths Γ(t → cl+′l−) = Σj,k Γ(t → clj+lk−) and Γ(t → cl+′l−) = Σj,kΓ(t → cṽjvk) with respect to, respectively, the charged leptonic and neutrino modes are calculated. It is shown that, at scalar-leptoquark masses higher than the t-quark mass (mS > mt), the branching ratios for these modes are Br(t → cl+′l−) ≈ (3.5−0.4) × 10−5 and Br(t → cṽ′v) ≈ (7.1−0.8) × 10−5 at ms = 180–250 GeV and an appropriate value of the leptoquark-mixing angle (sin β ≈ 0.2) and can increase for mS < mt to Br(t → cl+′l−) ≈ 0.03−0.002 and Br(t → cl+′l−) ≈ 0.46−0.05 for the charged mode at mS = 150–170 GeV for sin β ≈ 1 and sin β ≈ 0.2, respectively. In the cases being considered, t-quark decays to pairs of charged leptons can be accessible to detection at LHC. In the last case, these decays could manifest themselves (for example, in dilepton events) at the Tevatron as well.

Event shape discrimination of supersymmetry from large extra dimensions at a linear collider

The production of a charged lepton (ℓ=e,μ) pair with a large missing energy at a linear collider is discussed as a means of distinguishing the minimal supersymmetry (MSSM) scenario from that with large extra dimensions (ADD) for parameter ranges where the total cross-sections are comparable for both. Analyses in terms of event shape variables, specifically sphericity and thrust, are shown to enable a clear discrimination in this regard.