Are scalar leptoquarks testable at the present CERN collider?

We study the single production of all possible scalar leptoquarks at the present Spp̄S collider. Results on cross sections as a function of the leptoquark masses and couplings are presented. Clear evidence for leptoquarks would be narrow peaks in the M_1-jet-distribution of jet + ℓ⁺+ℓ^- events. The production mechanism here studied provides an efficient way of searching for leptoquarks and allows to probe the couplings and the masses together.     

Search for pair-produced leptoquarks in e + e- interactions at $\sqrt{s} \simeq 189\text{--}209$ GeV

A search for pair-produced leptoquarks is performed using collision events collected by the OPAL detector at LEP at centre-of-mass energies between 189 and 209 GeV. The data sample corresponds to a total integrated luminosity of 596 pb^-1. The leptoquarks are assumed to be produced via couplings to the photon and the Z⁰. For a given search channel only leptoquark decays involving a single lepton generation are considered. No evidence for leptoquark pair production is observed. Lower limits on masses for scalar and vector leptoquarks are calculated. The results improve most of the LEP limits derived from previous searches for the pair production process by 10-25 GeV, depending on the leptoquark quantum numbers.Received: 6 May 2003, Published online: 24 October 2003     

Search for lepton flavor violation in ep collisions at 300 GeV center of mass energy

Using the ZEUS detector at the HERA electronproton collider, we have searched for lepton flavor violation in ep collisions at a center—of—mass energy ($sqrt S$) of 300 GeV. Events of the type e + p → ? + X with a final—state lepton of high transverse momentum, ? = μ or τ, were sought. No evidence was found for lepton flavor violation in the combined 1993 and 1994 data samples, for which the integrated luminosities were 0.84 pb{?1}for e{～p collisions and 2.94 pb{?1} for e⁺p collisions. Limits on coupling vs. mass are provided for leptoquarks and R—parity violating squarks. For flavor violating couplings of electromagnetic strength, we set 95% confidence level lower limits on leptoquark masses between 207 GeV and 272 GeV, depending on the leptoquark species and final—state lepton. For leptoquark masses larger than 300 GeV, limits on flavor—changing couplings are determined, many of which supersede prior limits from rare decay processes.     

Search for single leptoquark and squark production in electron-photon scattering at $\sqrt{s_{\rm ee}}=189$ GeV at LEP

A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks () in R-parity violating SUSY models with the direct decay of the into Standard Model particles has been performed using ee collisions collected with the OPAL detector at LEP at an ee centre-of-mass energy of 189 GeV. The data correspond to an integrated luminosity of about 160 pb. The dominant process for this search is , where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95% confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings to fermions larger than , the mass limits range from 121 GeV to GeV ( GeV to GeV) depending on the branching ratio of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings for leptoquarks and for squarks, and on as a function of the scalar leptoquark/squark mass. Received: 22 May 2001 / Published online: 25 January 2002     

Leptoquark signal from global analysis

Data from HERA, LEP and the Tevatron as well as from low energy experiments are used to constrain the Yukawa couplings for scalar and vector leptoquarks in the Buchmüller–Rückl–Wyler effective model. In the limit of very high leptoquark masses constraints on the coupling to the mass ratio are derived using the contact-interaction approximation. For finite masses the coupling limits are studied as a function of the leptoquark mass. Some leptoquark models are found to describe the existing experimental data much better than the standard model. The increase in the global probability observed for models including or leptoquark production/exchange corresponds to an effect of more than . Assuming that a real leptoquark signal is observed, the allowed region in the –M plane is calculated. The leptoquark signal mostly results from the new data on the atomic parity violation in cesium, but is also supported by recent LEP2 measurements, unitarity violation in the CKM matrix and HERA high- results. Received: 21 June 2000 / Revised version: 31 August 2000 / Published online: 23 October 2000     

Search for first generation leptoquarks in ep collisions at HERA

A search for first generation scalar and vector leptoquarks produced in ep collisions is performed by the H1 experiment at HERA. The full H1 data sample is used in the analysis, corresponding to an integrated luminosity of 446 pb⁻¹. No evidence for the production of leptoquarks is observed in final states with a large transverse momentum electron or with large missing transverse momentum, and constraints on leptoquark models are derived. For leptoquark couplings of electromagnetic strength λ=0.3, first generation leptoquarks with masses up to 800 GeV are excluded at 95% confidence level.     

Search for third-generation scalar leptoquarks in pp at square root s=1.96 TeV

We report on a search for charge-1/3 third-generation leptoquarks (LQ) produced in pp collisions at square root s =1.96 TeV using the D0 detector at Fermilab. Third-generation leptoquarks are assumed to be produced in pairs and to decay to a tau neutrino and a b quark with branching fraction B. We place upper limits on sigma(pp --> LQLQ)B2 as a function of the leptoquark mass M(LQ). Assuming B=1, we exclude at the 95% confidence level third-generation scalar leptoquarks with M(LQ)<229 GeV.     

Search for pair production of second-generation scalar leptoquarks in pp collisions at √s = 7 TeV

A search for pair production of second-generation scalar leptoquarks in the final state with two muons and two jets is performed using proton-proton collision data at √s = 7 TeV collected by the CMS detector at the LHC. The data sample used corresponds to an integrated luminosity of 34 pb?1. The number of observed events is in good agreement with the predictions from the standard model processes. An upper limit is set on the second-generation leptoquark cross section times β2 as a function of the leptoquark mass, and leptoquarks with masses below 394 GeV are excluded at a 95% confidence level for β = 1, where β is the leptoquark branching fraction into a muon and a quark. These limits are the most stringent to date.     

On the expectations for leptoquarks in the mass range of O(200 GeV)

A summary is given on the experimental bounds for the couplings and masses of scalar and vector leptoquarks associated to the first fermion generation. We investigate to which extent an interpretation of the recently reported excess of events in the large x and Q ² range at HERA in terms of single leptoquark production is compatible with other experimental results.     

Study of Top Quark FCNC

We study the one-loop contribution of the effective flavor changing neutral couplings (FCNC) tcZ on the charm quark electric dipole moment. Using the known limits on the top and charm quarks electric dipole moments, we place limits on these FCNC anomalous couplings.     

Implications of BNL measurement of \delta a_{\mu} on a class of scalar leptoquark interactions

Recently BNL have measured the muon magnetic moment anomaly with increased precision. The world average experimental value at present shows a discrepancy of from the standard model (SM) value. In this paper we investigate the implications of this difference on a class of scalar leptoquark interactions to SM quark–lepton pair. We find that for a leptoquark mass of a few hundred GeV the BNL muon anomaly can arise from couplings much smaller than even the electromagnetic coupling. We also find that the BNL value for the muon anomaly leads to an unambiguous prediction for the electric dipole moment of the muon and a bound on the flavor changing leptoquark coupling relevant for the rare decay . Received: 15 April 2001 / Published online: 6 July 2001     

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.     

Scalar leptoquark production at TESLA and CLIC based $e\gamma$ colliders

We study scalar leptoquark production at TESLA and CLIC based colliders. Both direct and resolved contributions to the cross section are examined. We find that the masses of scalar leptoquarks can be probed up to about 0.9 TeV at TESLA and 2.6 TeV at CLIC. Received: 10 December 2002 / Revised version: 12 February 2003 / Published online: 31 March 2003     

CP violation in radiative Z decays

We propose to test the CP symmetry in the reactions Z → μ⁺μ^?γ and Z → τ⁺τ^?γ. The experimental analysis of angular correlations allows to determine a set of effective couplings: the electric and weak dipole moments of the muon and the tau lepton, and in particular chirality conserving 4-particle couplings, all of which can be induced by CP-violation in renormalizable theories of electroweak interactions beyond the Standard Model. We update an indirect bound on the weak dipole moment of the muon.     

Leptoquarks in lepton-quark collisions

Low-energy experiments permit the existence of leptoquarks with masses of order 100 GeV and couplings to quark-lepton pairs as large as gauge couplings. We study systematically the signatures of all possible scalar and vector leptoquarks in electron (positron)-proton collisions. Clear evidence for leptoquarks would be narrow peaks in the x-distributions of inclusive neutral and charged current processes. At HERA one will be able to explore the mass range up to 300 GeV through direct production, and even somewhat beyond the CM energy of 314 GeV through virtual effects. Conversely, leptoquarks with masses of 200 GeV can be discovered for couplings as small as 10⁻³ α_em.     

Leptonic CP violation in leptoquark models

We study the leptonic CP violation effects in leptoquark models. We show that in general the leptoquark contribution to the longitudinal muon polarization effect inη→μ ⁺ μ ^? decay is very small and in a model which has no couplings induced by the leptoquark between different generations, both the electric dipole moment of electron or muon and longitudinal polarization of electron in π⁰,η→e ⁺ e ^? decays can be large.     

Strong unification

We investigate the possibility that unification occurs at strong coupling. We show that, despite the fact the couplings pass through a strong coupling regime, accurate predictions for their low energy values are possible because the couplings of the theory flow to infrared fixed points. We determine the low-energy QCD coupling in a favoured class of strong coupling models and find it is reduced from the weak coupling predictions, lying close to the experimentally measured value. We extend the analysis to the determination of quark and lepton masses and show that (even without Grand Unification) the infra-red fixed point structure may lead to good predictions for the top mass, the bottom to tau mass ratio and tan β. Finally we discuss the implications for the unification scale finding it to be increased from the MSSM value and closer to the heterotic string prediction.     

Search for leptoquark pairs decaying into nunu+jets in pp collisions at square root[s] = 1.8 TeV

We present the results of a search for leptoquark (LQ) pairs in (85.2+/-3.7) pb(-1) of pp* collider data collected by the D0 experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on sigma(pp*-->LQLQ-->nunu+jets) as a function of the mass of the leptoquark (m(LQ)). Assuming the decay LQ-->nuq, we exclude scalar leptoquarks for m(LQ) < 98 GeV/c(2), and vector leptoquarks for m(LQ) < 200 GeV/c(2) and coupling which produces the minimum cross section, at a 95% confidence level.     

Search for second and third generation leptoquarks including production via technicolor interactions in p&pmacr; collisions at radicals = 1.8 TeV

We report the results of a search for second and third generation leptoquarks using 88 pb(-1) of data recorded by the Collider Detector at Fermilab. Color triplet technipions, which play the role of scalar leptoquarks, are investigated due to their potential production in decays of strongly coupled color octet technirhos. Events with a signature of two heavy flavor jets and missing energy may indicate the decay of a second (third) generation leptoquark to a charm (bottom) quark and a neutrino. As the data are found to be consistent with standard model expectations, mass limits are determined.     

Scalar-leptoquark contributions to the neutrino magnetic moment

On the basis of astrophysical data on the neutrino magnetic moment, μ _ν < 3 × 10⁻¹² μ _B , constraints on the scalar-leptoquark masses are found within the minimal model involving four-color symmetry. It is shown that data on the neutrino magnetic moment are compatible with the mixing-parameter range that admits the existence of scalar leptoquarks whose masses are below 1 TeV, reaching experimental limits obtained from direct searches. In the case of mass degeneracy for the scalar leptoquarks S _m of electric charge Q = 2/3, the constraint m _{S m} > 330 GeV is obtained, which is independent of the mixing parameters of the model. The results are compared with the predictions of other leptoquark models. Original Russian Text ? A.V. Povarov, 2007, published in Yadernaya Fizika, 2007, Vol. 70, No. 5, pp. 905–911.