arXiv subjects

N. Paver

Publications and source records attributed to N. Paver.

At least 19 recordsLinked to original sources

arXiv2012

Discriminating Z' from anomalous trilinear gauge coupling signatures in e+e- \to W+W- at ILC with polarized beams

New heavy neutral gauge bosons Z' are predicted by many models of physics beyond the Standard Model. It is quite possible that Z's are heavy enough to lie beyond the discovery reach of the CERN Large Hadron Collider LHC, in which case only indirect signatures of Z' exchanges may emerge at future colliders, through deviations of the measured cross sections from the Standard Model predictions. We discuss in this context the foreseeable sensitivity to Z's of W^\pm-pair production cross sections at the e^+e^- International Linear Collider (ILC), especially as regards the potential of distinguishing observable effects of the Z' from analogous ones due to competitor models with anomalous trilinear gauge couplings (AGC) that can lead to the same or similar new physics experimental signatures at the ILC. The sensitivity of the ILC for probing the Z-Z' mixing and its capability to distinguish these two new physics scenarios is substantially enhanced when the polarization of the initial beams and the produced W^\pm bosons are considered. A model independent analysis of the Z' effects in the process e^+e^- \to W^+W^- allows to differentiate the full class of vector Z' models from those with anomalous trilinear gauge couplings, with one notable exception: the sequential SM (SSM)-like models can in this process not be distinguished from anomalous gauge couplings. Results of model dependent analysis of a specific Z' are expressed in terms of discovery and identification reaches on the Z-Z' mixing angle and the Z' mass.

hep-ph↗

arXiv2011

Spin-analysis of s-channel diphoton resonances at the LHC

The high mass neutral quantum states envisaged by theories of physics beyond the standard model can at the hadron colliders reveal themselves through their decay into a pair of photons. Once such a peak in the diphoton invariant mass distribution is discovered, the determination of its spin through the distinctive photon angular distributions is needed in order to identify the associated nonstandard dynamics. We here discuss the discrimination of the spin-2 Randall-Sundrum graviton excitation against the hypothesis of a spin-0 exchange giving the same number of events under the peak, by means of the angular analysis applied to resonant diphoton events expected to be observed at the LHC. The spin-0 hypothesis is modelled by an effective interaction of a high mass gauge singlet scalar particle interacting with the standard model fields. The basic observable of our analysis is the symmetrically integrated angular asymmetry $A_{CE}$, calculated for both graviton and scalar s-channel exchanges to next-to-leading order in QCD.

hep-ph↗

arXiv2011

On the branching ratio of the "second class" tau --> eta' pi nu_tau decay

We present an assessment of the standard model expectations for the branching ratio of the isotopic spin and G-parity violating decay tau --> eta' pi nu_tau. The estimate is based on a vector and scalar meson dominance parametrization of the relevant form factors, that explicitly accounts for pi^0-eta-eta' mixing. The numerical results obtained in this framework indicate a branching ratio one order of magnitude (or more) below the current experimental limit, and suggest the possibility of evidencing some novel interaction in high statistics studies of this decay.

hep-ph↗

arXiv2010

Spin identification of heavy nonstandard bosons in dilepton and diphoton events at the LHC

New Physics scenarios generally predict the existence of very heavy quantum states that can possibly manifest themselves as peaks in the cross sections at the LHC. For values of the parameters in certain domains, different nonstandard models can generate peaks with the same mass and same number of events. In this case, the spin determination of a peak, requiring the angular analysis of the events, becomes crucial in order to identify the relevant nonstandard source. We here discuss, using a particularly suitable symmetrically integrated angular asymmetry applied to Drell-Yan dilepton and diphoton events at LHC, the identification reach on the exchanges in these reactions of the following heavy bosons: spin-2 Randall-Sundrum graviton excitations; spin-1 heavy neutral gauge bosons Z^\prime; and spin-0 SUSY R-parity violating sneutrinos.

hep-ph↗

arXiv2010

Sneutrino identification in dilepton events at the LHC

Heavy neutral resonances appearing in the clean Drell-Yan channel may be the first new physics to be observed at the proton-proton CERN LHC. If a new resonance is discovered at the LHC as a (narrow) peak in the dilepton invariant mass distribution, the characterization of its spin and couplings will proceed via the measurement of production rates and angular distributions of the decay products. We discuss the discrimination of a spin-0 resonance (sneutrino) predicted by supersymmetric theories with R-parity breaking against the spin-1 of Z^\prime bosons and the Randall-Sundrum graviton resonance (spin-2) with the same mass and producing the same number of events under the observed peak. To assess the region of sneutrino parameters (couplings and masses) where the spin determination can be performed to a given confidence level, we focus on the event rate and the angular distributions of the Drell-Yan leptons, in particular using the center-edge asymmetry, A_{\rm CE}. We find that although the measured event rate permits solving the above problem partially, the center-edge asymmetry, on the contrary allows to differentiate the various spins entirely with a minimal number of events around 200.

hep-ph↗

arXiv2010

Signals of Warped Extra Dimensions at the LHC

We discuss the signatures of the spin-2 graviton excitations predicted by the Randall-Sundrum model with one warped extra dimension, in dilepton and diphoton production at LHC. By using a specific angular analysis, we assess the ranges in mass and coupling constant where such gravitons can be discriminated against competitor spin-1 and spin-0 objects, that potentially could manifest themselves in these processes with the same mass and rate of events. Depending on the value of the coupling constant to quarks and leptons, the numerical results indicate graviton identification mass ranges up to 1.1-2.4 TeV and 1.6-3.2 TeV for LHC nominal energy of 14 TeV and time-integrated luminosity of 10 and 100~${\rm fb}^{-1}$, respectively.

hep-ph↗

arXiv2010

On meson dominance in the `second class' tau \to eta pi nu_tau decay

Motivated by recent estimates of the isospin-violating process tau \to eta pi nu_tau, mostly relying on the rho and a_0 dominance of the relevant form factors near threshold, we present an assessment for the branching ratio that accounts for additional, potential, effects from the lowest radial excitations rho(1450) and a_0(1450), respectively, also lying in the decay phase space.

hep-ph↗

arXiv2009

Spin and model identification of Z' bosons at the LHC

Heavy resonances appearing in the clean Drell-Yan channel may be the first new physics to be observed at the proton-proton CERN LHC. If a new resonance is discovered at the LHC as a peak in the dilepton invariant mass distribution, the characterization of its spin and couplings will proceed via measuring production rates and angular distributions of the decay products. We discuss the discrimination of the spin-1 of Z' representative models (Z'_{SSM}, Z'_{psi}, Z'_{eta}, Z'_{chi}, Z'_{LR}, and Z'_{ALR}) against the Randall-Sundrum graviton resonance (spin-2) and a spin-0 resonance (sneutrino) with the same mass and producing the same number of events under the observed peak. To assess the range of the Z' mass where the spin determination can be performed to a given confidence level, we focus on the angular distributions of the Drell-Yan leptons, in particular we use as a basic observable an angular-integrated center-edge asymmetry, A_{CE}. The spin of a heavy Z' gauge boson can be established with A_{CE} up to M_{Z'} \simeq 3.0 TeV, for an integrated luminosity of 100 fb^{-1}, or minimal number of events around 110. We also examine the distinguishability of the considered Z' models from one another, once the spin-1 has been established, using the total dilepton production cross section. With some assumption, one might be able to distinguish among these Z' models at 95% C.L. up to M_{Z'} \simeq 2.1 TeV.

hep-ph↗

arXiv2009

Spin Identification of the Randall-Sundrum Graviton at the LHC

Using as basic observable an angular-integrated asymmetry to be measured in Drell-Yan lepton-pair production at the LHC, we discuss the identification reach on the spin-2 of the lowest-lying Randall-Sundrum resonance predicted by gravity in one warped extra dimension, against the spin-1 and spin-0 hypotheses. Numerical results indicate that, depending on the graviton coupling strength to the standard model particles, such a spin-2 identification can extend up to mass scales of 1.0-1.6 TeV and 2.4-3.2 TeV for LHC integrated luminosities of 10 and 100 fb^{-1}, respectively.

hep-ph↗

arXiv2008

Spin identification of the Randall-Sundrum resonance in lepton-pair production at the LHC

The determination of the spin of the quantum states exchanged in the various non-standard interactions is a relevant aspect in the identification of the corresponding scenarios. We discuss the identification reach at LHC on the spin-2 of the lowest-lying Randall-Sundrum resonance, predicted by gravity with one warped extra dimension, against spin-1 and spin-0 non-standard exchanges with the same mass and producing the same number of events in the cross section. We focus on the angular distributions of leptons produced in the Drell-Yan process at the LHC, in particular we use as basic observable a normalized integrated angular asymmetry A_{CE}. Our finding is that the 95% C.L. identification reach on the spin-2 of the RS resonance (equivalently, the exclusion reach on both the spin-1 and spin-0 hypotheses for the peak) is up to a resonance mass scale of the order of 1.0 or 1.6 TeV in the case of weak coupling between graviton excitations and SM particles (k/{\bar M}_{Pl}=0.01) and 2.4 or 3.2 TeV for larger coupling constant (k/{\bar M}_{Pl}=0.1) for a time-integrated LHC luminosity of 10 or 100 fb^{-1}, respectively. Also, some comments are given on the complementary rôles of the angular analysis and the eventual discovery of the predicted second graviton excitation in the identification of the RS scenario.

hep-ph↗

arXiv2007

Discovery and Identification of Contactlike Interactions in Fermion-pair Production at ILC

Non-standard scenarios described by effective contactlike interactions can be revealed only by searching for deviations of the measured observables from the Standard Model (SM) predictions. If deviations were indeed observed within the experimental uncertainty, the identification of their source among the different non-standard interactions should be needed. We here consider the example of the discrimination of gravity in compactified extra dimensions (ADD model) against the four-fermion contact interactions (CI). We present assessments of the identification reach on this scenario, that could be obtained from measurements of the differential cross sections for the fermionic processes $e^+e^-\to{\bar f}f$, with $f=e,μ,τ,c,b$, at the planned ILC.

hep-ph↗

arXiv2006

Revealing the large extra dimension effective interaction at an e^+e^- collider with polarized beams

Contactlike nonstandard effective interactions can be revealed 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. We estimate the expected discrimination 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 0.5-1 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, and arbitrary coupling constants. Accordingly, the estimated identification reach on the ADD scenario can be considered as `model-independent'. We emphasize the substantial role of beams polarization in enhancing the identification sensitivity.

hep-ph↗

arXiv2006

Distinguishing new physics scenarios with polarized electron and positron beams

Contactlike nonstandard interactions can be revealed only through deviations of observables from the standard model predictions. We consider a number of such nonstandard scenarios, and discuss their identification as sources of deviations in fermion-pair production processes at the International Linear Collider, if they were observed. We emphasize the role of e^- and e^+ polarization in enhancing the identification reaches.

hep-ph↗

arXiv2006

Discriminating large extra dimensions at the ILC with polarized beams

Non-standard scenarios described by effective interactions can manifest themselves indirectly, via corrections to the Standard Model cross sections. It should be desirable to identify at a given confidence level the source of such deviations among the different possible explanations. We here discuss the identification reach on gravity in extra dimensions from the four-fermion compositeness-inspired contact interactions and viceversa, using as basic observable the differential cross section of $e^+e^-\to\bar{f}f$ at the ILC, and emphasize the role of beams polarization in enhancing the identification sensitivity.

hep-ph↗

arXiv2005

Distinguishing New Physics Scenarios at a Linear Collider with Polarized Beams

Numerous non-standard dynamics are described by contact-like effective interactions that can manifest themselves through deviations of the cross sections from the Standard Model predictions. If one such deviation were observed, one should try to identify to a given confidence level the actual source among the possible non-standard interactions that in principle can explain it. We here estimate the potential identification reach on different New Physics effective interactions, obtainable from the angular distributions of lepton pair production processes at the planned International Linear Collider with polarized beams. The models discussed are the interactions based on gravity in large and in TeV^{-1} extra dimensions and the compositeness-inspired four-fermion contact interactions. The availability of both beams polarized in many cases plays an essential role in enhancing the identification sensitivity.

hep-ph↗

arXiv2005

Identification of large extra spatial dimensions at the LC

We discuss the possibility to cleanly distinguish, in electron-positron annihilation into fermion pairs at a high energy collider, the indirect manifestations of graviton exchange from those of four-fermion contact interactions. The method is based on cross section asymmetries emphasizing the spin-2 character of graviton exchange and is explicitly applied to the ADD scenario for gravity in extra dimensions. The availability of initial beams longitudinal polarization is also taken into account. For typical c.m. energies and time-integrated luminosities foreseen at the International Linear Collider [\sqrt s=0.5-1 TeV, L_{int}=50-1000 fb^{-1}], a 5σidentification reach as high as 3.5-5.8 TeV for the mass scale M_H relevant to the ADD model can be obtained, while the size of the reach on the mass scales Lambda characterizing four-fermion contact interactions can be of the order of 45-65 TeV.

hep-ph↗

arXiv2005

Positron Polarization at the International Linear Collider

We review some recent arguments supporting the upgrade of the International Linear Collider by a polarized positron beam, in addition to the polarized electron beam. The examples presented here mainly focus on the impact of positron polarization on items relevant to new physics searches, such as the identification of novel interactions in fermion-pair production and the formulation of new CP-sensitive observables. In particular, in addition to the benefits from positron and electron longitudinal polarizations, the advantages in this field of having transverse polarization of both beams are emphasized.

hep-ph↗

arXiv2005

The role of polarized positrons and electrons in revealing fundamental interactions at the Linear Collider

The proposed International Linear Collider (ILC) is well-suited for discovering physics beyond the Standard Model and for precisely unraveling the structure of the underlying physics. The physics return can be maximized by the use of polarized beams. This report shows the paramount role of polarized beams and summarizes the benefits obtained from polarizing the positron beam, as well as the electron beam. The physics case for this option is illustrated explicitly by analyzing reference reactions in different physics scenarios. The results show that positron polarization, combined with the clean experimental environment provided by the linear collider, allows to improve strongly the potential of searches for new particles and the identification of their dynamics, which opens the road to resolve shortcomings of the Standard Model. The report also presents an overview of possible designs for polarizing both beams at the ILC, as well as for measuring their polarization.

hep-ph↗