Four-fermion Contact Interactions Research Articles

Search for excited leptons in pp collisions at $\sqrt{s}= 7 $ TeV

A search for excited leptons, carried out with the CMS detector in pp collision at the LHC with $\sqrt{s} = 7$ TeV, is presented. The search has been performed for an associated production of a lepton and an oppositely charged excited lepton pp →ll *, followed by the decay l *→lγ, resulting in llγ final state, where l = e,μ. No excess above the Standard Model expectation is observed in the data. Interpreting the findings in the context of l * production through four-fermion contact interactions and l * decay via electroweak processes, upper limits are reported for l * production at this collision energy and the exclusion region in the Λ − M(l *) parameter space is extended beyond the previously established limits.

Fermions and Goldstone bosons in an asymptotically safe model

We consider a model in which Goldstone bosons, described by a SU(N) chiral nonlinear σ model, are coupled to an N-plet of colored fermions by means of a Yukawa interaction. We study the one-loop renormalization group flow and show that the non-Gaussian UV fixed point, which is present in the purely bosonic model, is lost because of fermion loop effects unless N is sufficiently large. We then add four-fermion contact interactions to the Lagrangian and show that in this case there exist several non-Gaussian fixed points. The strength of the contact interactions, predicted by the requirement that the theory flows towards a fixed point in the UV, is compared to the current experimental bounds. This toy model could provide an important building block of an asymptotically safe model of the weak interactions.

Search for contact interactions in [formula omitted] collisions at HERA

A search for physics beyond the Standard Model in neutral current deep inelastic scattering at high negative four-momentum transfer squared Q2 is performed in e±p collisions at HERA. The differential cross section dσ/dQ2, measured using the full H1 data sample corresponding to an integrated luminosity of 446 pb−1, is compared to the Standard Model prediction. No significant deviation is observed. Limits on various models predicting new phenomena at high Q2 are derived. For general four-fermion eeqq contact interaction models, lower limits on the compositeness scale Λ are set in the range 3.6 TeV to 7.2 TeV. Leptoquarks with masses MLQ and couplings λ are constrained to MLQ/λ>0.41–1.86 TeV and limits on squarks in R-parity violating supersymmetric models are derived. A lower limit on the gravitational scale in 4+n dimensions of MS>0.9 TeV is established for low-scale quantum gravity effects in models with large extra dimensions. For the light quark radius an upper bound of Rq<0.65⋅10−18 m is determined.

On the cosmic-ray spectra of three-body lepton-flavor-violating dark matter decays

We consider possible leptonic three-body decays of spin-1/2, charge-asymmetric dark matter. Assuming a general Dirac structure for the four-fermion contact interactions of interest, we study the cosmic-ray electron and positron spectra and show that good fits to the current data can be obtained for both charged-lepton-flavor-conserving and flavor-violating decay channels. We find that different choices for the Dirac structure of the underlying decay operator can be significantly compensated by different choices for the dark matter mass and lifetime. The decay modes we consider provide differing predictions for the cosmic-ray positron fraction at energies higher than those currently probed at the PAMELA experiment; these predictions might be tested at cosmic-ray detectors like AMS-02.

A search for excited leptons in pp collisions at [formula omitted

A search for excited leptons is carried out with the CMS detector at the LHC, using 36 pb−1 of pp collision data recorded at s=7 TeV. The search is performed for associated production of a lepton and an oppositely charged excited lepton pp→ℓℓ⁎, followed by the decay ℓ⁎→ℓγ, resulting in the ℓℓγ final state, where ℓ=e,μ. No excess of events above the standard model expectation is observed. Interpreting the findings in the context of ℓ⁎ production through four-fermion contact interactions and subsequent decay via electroweak processes, first upper limits are reported for ℓ⁎ production at this collision energy. The exclusion region in the compositeness scale Λ and excited lepton mass Mℓ⁎ parameter space is extended beyond previously established limits. For Λ=Mℓ⁎, excited lepton masses are excluded below 1070 GeV/c2 for e⁎ and 1090 GeV/c2 for μ⁎ at the 95% confidence level.

Search for single top quark production via contact interactions at LEP2

Single top quark production via four-fermion contact interactions associated to flavour-changing neutral currents was searched for in data taken by the DELPHI detector at LEP2. The data were accumulated at centre-of-mass energies ranging from 189 to 209 GeV, with an integrated luminosity of 598.1 pb^-1. No evidence for a signal was found. Limits on the energy scale Lambda, were set for scalar-, vector- and tensor-like coupling scenarios.

Search for excited electrons inpp¯collisions ats=1.96 TeV

We present the results of a search for the production of an excited state of the electron, e(*), in proton-antiproton collisions at root s = 1.96 TeV. The data were collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 1 fb(-1). We search for e(*) in the process p (p) over bar -> e(*)e, with the e(*) subsequently decaying to an electron plus photon. No excess above the standard model background is observed. Interpreting our data in the context of a model that describes e(*) production by four-fermion contact interactions and e(*) decay via electroweak processes, we set 95% C.L. upper limits on the production cross section ranging from 8.9 to 27 fb, depending on the mass of the excited electron. Choosing the scale for contact interactions to be Lambda = 1 TeV, excited electron masses below 756 GeV are excluded at the 95% C.L.

Revealing the large extra dimension effective interaction at ane+e−collider with polarized beams

Several types of new physics scenarios are represented by contactlike effective interactions. An example is the exchange of nonstandard quanta of very large mass scales, beyond the kinematical limit for direct production set by the available collider energy. This kind of interactions can be revealed only through deviations of observables from the standard model predictions. If such deviations were observed, the relevant source should be identified among the possible models that could explain them. Here, we assess the expected ``identification reach'' on the ADD model of gravity in large compactified extra dimensions, against the compositeness-inspired four-fermion contact interaction. As basic observables we take the differential cross sections for fermion-pair production at a 0.5\char21{}1 TeV electron-positron linear collider with both beams longitudinally polarized. For the four-fermion contact interaction, we assume a general linear combination of the individual models with definite chiralities, with arbitrary coupling constants. In this sense, the estimated identification reach on the ADD model can be considered as ``model independent.'' In the analysis, we give estimates also for the expected ``discovery reaches'' on the various scenarios. We emphasize the substantial role of beams polarization in enhancing the sensitivity to the contactlike interactions under consideration.

Fermion pair production in e+e- collisions at 189–209 GeV and constraints on physics beyond the standard model

Cross sections, angular distributions and forward-backward asymmetries are presented, of two-fermion events produced in e+e- collisions at centre-of-mass energies from 189 to 209 GeV at LEP, measured with the ALEPH detector. Results for e+e-, μ+μ-, τ+τ-, qq̄, bb̄ and cc̄ production are in agreement with the standard model predictions. Constraints are set on scenarios of new physics such as four-fermion contact interactions, leptoquarks, Z′ bosons, TeV-scale quantum gravity and R-parity violating squarks and sneutrinos.

Distinguishing new physics scenarios at a linear collider with polarized beams

Numerous nonstandard dynamics dominated by very high mass exchanges are described at current and future accelerator energies by appropriate contactlike effective interactions among the standard model particles. Correspondingly, they can manifest themselves only through deviations of the cross sections from the standard model predictions. If one such deviation were observed, it would be important to definitely identify, to a given confidence level, the actual source among the various possible nonstandard interactions that, in principle, can explain it. Here we estimate the identification reach on different new physics effective interactions, obtainable from angular distributions of lepton pair production processes at the planned electron-positron International Linear Collider with polarized beams. For each nonstandard model, such an identification reach defines the range in the relevant heavy mass scale parameter where it can be unambiguously distinguished from the other nonstandard models as the source of corrections to the standard model cross sections, in case these are observed. The effective interactions for which we estimate the expected identification reach are the interactions based on gravity in large extra dimensions, in TeV{sup -1} extra dimensions and the compositeness-inspired four-fermion contact interactions. The availability of both beams polarized at the International Linear Collider turns out, in manymore » cases, to dramatically enhance the identification sensitivity.« less

Search for excited muons inpp¯collisions ats=1.96 TeV

We present the results of a search for the production of an excited state of the muon, mu(*), in proton antiproton collisions at root s =1.96 TeV. The data have been collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 380 pb(-1). We search for mu(*) in the process p (p) over bar ->mu(*)mu, with the mu(*) subsequently decaying to a muon plus photon. No excess above the standard model expectation is observed in data. Interpreting our data in the context of a model that describes mu(*) production by four-fermion contact interactions and mu(*) decay via electroweak processes, we set a 95% confidence level production cross section upper limit ranging from 0.057 to 0.112 pb, depending on the mass of the excited muon. Choosing the scale for contact interactions to be Lambda=1 TeV, excited muon masses below 618 GeV are excluded.

NLO corrections to lepton pair production beyond the Standard Model at hadron colliders

We consider lepton pair production at a hadron collider in a class of effective theories with the relevant operators being four-fermion contact interaction. Despite the nonrenormalizable nature of the interaction, we explicitly demonstrate that calculating QCD corrections is both possible and meaningful. Calculating the corrections for various differential distributions, we show that these can be substantial and significantly different from those within the SM. Furthermore, the corrections have a very distinctive flavour dependence. And finally, the scale dependence of the cross sections are greatly reduced once the NLO corrections are taken into account.

Search for Compositeness in Dilepton Final States at DØ

We report preliminary results of a search for substructure in quarks and leptons assuming a four-fermion contact interaction for dielectron pairs of large invariant mass. The search is based on dielectron data collected by the DØ detector in [Formula: see text] collisions at [Formula: see text]. The results agree well with predictions of the standard model and we obtain model-dependent lower limits at 95% confidence level on the compositeness scale of 3.6 to 9.1 TeV for different chiral models of the composite structure of quarks and electrons.

Bounds on four-fermion contact interactions induced by string resonances

Based on tree-level open-string scattering amplitudes in the low string-scale scenario, we derive the massless fermion scattering amplitudes. The amplitudes are required to reproduce those of the Standard Model at tree level in the low energy limit. We then obtain four-fermion contact interactions by expanding in inverse powers of the string scale and explore the constraints on the string scale from low energy data. The Chan-Paton factors and the string scale are treated as free parameters. We find that data from the neutral and charged current processes at HERA, Drell-Yan process at the Tevatron, and from LEP-II put lower bounds on the string scale M_S >= 0.9-1.3 TeV for typical values of the Chan-Paton factors, which are comparable to Tevatron bounds on Z' and W' masses.

Effects of four-fermion contact interactions in the Bhabha process at linear electron-positron colliders

For the case of longitudinally polarized primary beams, effects are investigated that are induced by four-fermion contact interactions in the process e+e− → e+e− occurring at linear electron-positron colliders of energy in the range \(\sqrt s = 0.5 - 1 TeV\). Model-independent constraints on the parameters of four-fermion contact interactions are obtained. It is shown that the sensitivity of the Bhabha process to the parameter ΛLR is considerably higher than the sensitivity of the leptonic process e+e− → μ+μ−; at the same time, the sensitivities of these processes to the parameters ΛRR and ΛLL are commensurate.

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 189-209 GeV at LEP

Production of events with hadronic and leptonic final states has been measured in \({\rm e^+e^-}\) collisions at centre-of-mass energies of 130–172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z\(\gamma\) events, and compared to Standard Model expectations. The ratio \(R_{\rm b}\) of the cross-section for \({\rm b\bar b}\) production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of \(\gamma\)-Z interference. The energy dependence of \(\alpha_{\mathrm{em}}\) has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for \(t\)-channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of the gaugino into a light gluino and a quark pair.

Fermion pair production at LEP2

Preliminary results on combined cross-sections and forward-backward asymmetries from e +e − → f f processes at LEP2 for center-of-mass energy √ s from 130 t0 207 GeV, are summarized. These results provide a test of the Standard Model to O(1%) and allow to constraint physics beyond the Standard Model, such as four-fermion contact interactions, gravitational interaction in extra dimensions and leptoquarks models.

Constraints on electron–quark contact interactions and implications to models of leptoquarks and extra Z bosons

Abstract We update the global constraint on four-fermion eeqq contact interactions. In this update, we included the published data of H1 and ZEUS for the 1994–1996 run in the e+p mode and the newly published data of H1 for the 1999 run in the e−p mode. Other major changes are the new LEPII data on hadronic cross sections above 189 GeV, and the atomic parity violation measurement on cesium because of a new and improved atomic calculation, which drives the data within 1σ of the standard model value. The global data do not show any evidence for contact interactions, and we obtain 95% C.L. limits on the compositeness scale. A limit of Λ eu LL+(−) >23 (12.5) TeV is obtained. Implications to models of leptoquarks and extra Z bosons are examined.

Model independent constraints on contact interactions from LEP2

We quantitatively discuss the possibility of deriving model-independent constraints on the general four-fermion contact interaction couplings, from the currently available data on the two-fermion production processes e + e −→ μ + μ −, b b ̄ and c c ̄ with unpolarized initial beams. The method is essentially based on particular, simple, combinations of the measured total cross section and forward–backward asymmetry that allow partial separation of the helicity cross sections, and the combination of experimental data obtained at the different energies of TRISTAN, LEP1 and LEP2.

Can four-fermion contact interactions at one-loop explain the new atomic parity violation results?

We investigate the possibility that four-fermion contact interactions give rise to the observed deviation from the Standard Model prediction for the weak charge of Cesium, through one-loop contributions. We show that the presence of loops involving the third generation quarks can explain such deviation.