arXiv subjects

A. A. Pankov

Publications and source records attributed to A. A. Pankov.

At least 55 records · Page 3Linked to original sources

arXiv2001

New physics signatures at a Linear Collider: model-independent analysis from `conventional' polarized observables

We discuss four-fermion contact-interaction searches in the processes e^+e^-\toμ^+μ^-, c{\bar c} and b{\bar b} at a future e^+e^- Linear Collider with c.m. energy \sqrt{s}=0.5 TeV and with both beams longitudinally polarized. Our analysis is based on the measurements of familiar polarized observables such as the total cross section and the forward-backward/left-right asymmetries, and accounts for the general set of contact interaction couplings as independent, non-zero, parameters thus avoiding simplifying, model-dependent, assumptions. We derive the corresponding model-independent constraints on the above-mentioned coupling constants, and evaluate the corresponding reach at the Linear Collider, emphasizing the role of beam polarization. We compare the results with a model-dependent procedure where only one coupling is varied at a time.

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arXiv2001

Contact interactions and polarized beams at a Linear Collider

We discuss contact-interaction searches in the processes e^+e^-\to μ^+μ^-, b\bar{b} and c\bar{c} at an e^+e^- Linear Collider with c.m. energy \sqrt{s}=0.5 TeV and with longitudinally polarized beams. The measurement of polarized cross sections allows to study the individual helicity cross sections, and consequently to derive separate, model-independent, constraints on the four-fermion contact interaction couplings. We evaluate the reach on those parameters foreseeable in the case of both electron and positron polarization fixed at some reference values, and compare it with the situation where only electron polarization is available. The analysis is based on polarized integrated cross sections with optimal kinematical cuts that can improve the sensitivity to the relevant couplings. While electron polarization would by itself allow such an analysis, the additional positron polarization (with no loss of beam intensity) and optimization can have a crucial role in improving the sensitivity to the new interactions.

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arXiv2000

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 $e^+e^-\toμ^+μ^-$, $b\bar{b}$ and $c\bar{c}$ 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.

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arXiv2000

Contact interaction probes at the Linear Collider with polarized electron and positron beams

For contact-interaction searches at the Linear Collider, we discuss the advantages of polarizing both the electron and the positron beams as compared with polarizing only the electron beam. In particular, for the processes e^+e^-\to μ^+μ^-, τ^+τ^-, b\bar{b} and c\bar{c} at a future e^+e^- collider with \sqrt{s}=0.5 TeV we derive model-independent bounds on the four-fermion contact interaction parameters from studies of the helicity cross sections.

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arXiv2000

Optimal polarized observables for model-independent new-physics search at e+e- colliders

For the processes e+e-\to μ+μ-, τ+τ-, b\bar{b} and c\bar{c} at a future e+e- collider with \sqrt{s}=0.5 TeV, we examine the sensitivity of the helicity cross sections to four-fermion contact interactions. If longitudinal polarization of the electron beam were available, two polarized integrated cross sections would offer the opportunity to separate the helicity cross sections and, in this way, to derive model-independent bounds on the relevant parameters. The measurement of these polarized cross sections with optimal kinematical cuts could significantly increase the sensitivity of helicity cross sections to contact interaction parameters and could give crucial information on the chiral structure of such new interactions.

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arXiv1999

Polarization to Probe an Extra Neutral Gauge Boson at e^+e^- Linear Collider

The sensitivity to the $Z^\prime$ couplings of the processes $e^+e^-\to l^+l^-,\bar{b}b$ and $\bar{c}c$ at the linear collider with $\sqrt{s}=500 GeV$ with initial beam polarization, for typical extended model examples are studied. To this aim, the suitable integrated, polarized, observables directly related to the helicity cross sections that carry information on the individual $Z^\prime$ chiral couplings to fermions are used. We discuss the derivation of separate, model-independent limits on the couplings in the case of no observed indirect $Z^\prime$ signal within the expected experimental accuracy. In the hypothesis that such signals were, indeed, observed we assess the expected accuracy on the numerical determination of such couplings and the consequent range of $Z^\prime$ masses where the individual models can be distinguished from each other as the source of the effect.

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arXiv1999

Prospects on Compositeness and New Vector Bosons at LC with Polarization

Fermion compositness, and other new physics which can be described by an exchange of very massive particles ($Z^\prime$ boson, leptoquarks, sparticles with R-parity violating couplings), can be manifest itself as the presence of a strong four-fermion contact interaction. For the processes $e^+e^-\to μ^+μ^-$, $τ^+τ^-$, $\bar{b}b$ and $\bar{c}c$ at a future $e^+e^-$ linear collider (LC) with $\sqrt{s}=0.5$ TeV, we examine the sensitivity of the helicity cross sections to four-fermion contact interactions. If longitudinal polarization of the electron beam were available, two polarized integrated cross sections would offer the opportunity to separate the helicity cross sections and, in this way, to derive model-independent bounds on the relevant parameters. The measurement of these polarized cross sections with optimal kinematical cuts could significantly increase the sensitivity of helicity cross sections to contact interaction parameters and could give crucial information on the chiral structure of such new interactions. In addition, we consider the application of the proposed approach to the search for manifestations of a $Z^\prime$ for typical extended model examples.

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arXiv1998

Polarized observables to probe Z' at the e+e- linear collider

We study the sensitivity to the Z' couplings of the processes $e^+e^-\to l^+l^-,\bar{b}b$ and $\bar{c}c$ at the linear collider with $\sqrt{s}=500 GeV$ with initial beam polarization, for typical extended model examples. To this aim, we use suitable integrated, polarized, observables directly related to the helicity cross sections that carry information on the individual Z' chiral couplings to fermions. We discuss the derivation of separate, model-independent limits on the couplings in the case of no observed indirect Z' signal within the expected experimental accuracy. In the hypothesis that such signals were, indeed, observed we assess the expected accuracy on the numerical determination of such couplings and the consequent range of Z' masses where the individual models can be distinguished from each other as the source of the effect.

hep-ph↗

arXiv1998

Model-independent limits on four-fermion contact interactions at LC with polarization

Fermion compositeness, and other types of new physics that can be described by the exchange of very massive particles, can manifest themselves as the result of an effective four-fermion contact interaction. In the case of the processes $e^+e^-\to μ^+μ^-,τ^+τ^-,\bar{b}b$ and $\bar{c}c$ at future $e^+e^-$ colliders with $\sqrt{s}=0.5-1$ TeV, we examine the sensitivity to four-fermion contact interactions of two new integrated observables, $σ_+$ and $σ_-$, conveniently defined for such kind of analysis. We find that, if longitudinal polarization of the electron beam were available, these observables would offer the opportunity to separate the helicity cross sections and, in this way, to derive model-independent bounds on the relevant parameters.

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arXiv1998

On the phenomenology of a Z' coupling only to third-family fermions

The phenomenology of an additional U(1) neutral gauge boson Z' coupled to the third family of fermions is discussed. One might expect such a particle to contribute to processes where taus, b and t quarks are produced. Precision data from LEP1 put severe constraints on the mixing and heavy-boson mass. We find that the effects of such a particle could not be observed at hadronic colliders, be it at the Tevatron or the LHC, because of the QCD background. At LEP2 and future e^+e^- linear colliders, one could instead hope to observe such effects, in particular for b\bar b final states.

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arXiv1998

Z' and anomalous gauge coupling effects at LEP2 and their separation

In the case of the process $e^+e^-\toμ^+μ^-$, we study the sensitivity to either $Z^\prime$ or anomalous gauge coupling effects of two new observables, $σ_+$ and $σ_-$, conveniently defined for such kind of analysis. We discuss general properties of the deviations of these observables from the Standard Model predictions, and derive the model-independent bounds on the relevant parameters allowed by present and foreseeable LEP2 data. We also discuss the possibility of separating the two kinds of effect.

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arXiv1997

Model independent analysis of the trilinear gauge boson couplings at LC: role of polarized cross sections

By means of a model-independent analysis, we discuss the constraints on anomalous trilinear gauge-boson couplings that can be obtained from the study of electron-positron annihilation into $W$ pairs at LC with \sqrt{s}=0.5 TeV and 1 TeV. We consider the general CP conserving anomalous effective Lagrangian, as well as some specific models with reduced number of independent couplings. The analysis is based on combinations of observables with initial and final state polarizations, that allow to separately constrain the different couplings and to improve the corresponding numerical bounds.

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arXiv1997

Z' effects and anomalous gauge couplings at LC with polarization

We show that the availability of longitudinally polarized electron beams at a 500 GeV Linear Collider would allow, from an analysis of the reaction e^+e^-\to W^+W^-, to set stringent bounds on the couplings of a Z' of the most general type. In addition, to some extent, it would be possible to disentangle observable effects of the Z' from analogous ones due to competitor models with anomalous trilinear gauge couplings.

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arXiv1997

Optimal observables for new-physics search at LEP2

New observables σ_\pm for the process e^+e^-\toμ^+μ^- allow one to get more direct information on additional Z' boson effects than what is obtained from the canonical ones, σand A_{FB}. Their deviations from the Standard Model predictions have very specific energy dependences, which are precisely determined by SM parameters. At energies varying from TRISTAN to LEP2, one can uniquely predict the signs of Δσ_{\pm} induced by a Z' as well as the locations of their extrema and zeros. This unambiguous energy correlation could be quite useful in distinguishing effects due to Z' exchange from those caused by other new physics sources. Furthermore, there are two energy points, \sqrt{s_{+}}\simeq 78 GeV and \sqrt{s_{-}}\simeq 113 GeV, where the SM quantities σ_\pm^{SM} as well as the deviations Δσ_{\pm} attain their minimum values or vanish. These points could be very favourable for a search for new physics beyond the SM and beyond Z' effects.

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arXiv1996

Z' interference effects from TRISTAN to LEP2

We discuss the potential of present e^+e^- colliders to probe for a heavy Z^' boson via its interference effects, in a model-independent framework. The leptonic channel is unambiguous, and thus favourable over the hadronic ones. The effect on the leptonic cross section $σ_{μμ}$, induced by a heavy Z^' with arbitrary couplings, is uniquely determined at LEP2 energies. It results in a negative deviation from the standard model prediction, due to destructive $γ-Z'$ and Z-Z' interferences. At TRISTAN energies the former dominates and is given by the vector coupling of the Z^'. Around the Z peak, the Z-Z' interference dominates and depends on the axial-vector coupling. The forward-backward asymmetry is complementary to $σ_{μμ}$ in the Z^' search. Comparison of the e^+e^- potentials indicates that already after one year of operation, LEP2 will achieve a sensitivity to Z^' that exceeds the current one at TRISTAN.

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arXiv1996

Constraints on Z' from W^+W^- production at the NLC with polarized beams

We discuss the potential of NLC500 and NLC1000 to probe Z-Z' mixing and mass by the reaction e^+e^- -> W^+W^- with longitudinally polarized electrons. We perform a model-independent analysis of the deviations from the Standard Model, and apply it to a specific class of extended weak gauge models. Results indicate that the corresponding bounds at the NLC500 complement the present ones obtained from LEP1, and rapidly become quite stringent at the higher energy of the NLC1000. Also, we emphasize the importance of initial beam polarization in improving the sensitivity to mixing.

hep-ph↗

arXiv1995

Role of beam polarization in the determination of $WWγ$ and $WWZ$ couplings from $e^+e^-\to W^+W^-$

We evaluate the constraints on anomalous trilinear gauge-boson couplings that can be obtained from the study of electron-positron annihilation into $W$ pairs at a facility with either the electron beam longitudinally polarized or both electron and positron beams transversely polarized. The energy ranges considered in the analysis are the ones relevant to the next-linear collider and to LEP~200. We discuss the possibilities of a model independent analysis of the general $CP$ conserving anomalous effective Lagrangian, as well as its restriction to some specific models with reduced number of independent couplings. The combination of observables with initial and final state polarizations allows to separately constrain the different couplings and to improve the corresponding numerical bounds.

hep-ph↗

arXiv1995

Search for new physics indirect effects in $e^+e^-\to W^+W^-$ at linear colliders with polarized beams

We discuss the potential of a $0.5\hskip 2pt TeV$ linear collider to explore manifestations of extended (or alternative) electroweak models of current interest, through measurements of the reaction $e^+e^-\to W^+W^-$ with both initial and final states polarization. Specifically, we consider the possibility to put stringent constraints on lepton mixing (or extended lepton couplings) and $Z-Z^\prime$ mixing, showing in particular the usefulness of polarization in order to disentangle these effects.

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