Regular Series


Vol. 27 (1996), No. 7, pp. 1469 – 1679


Search for New Neutral Bosons at Future Colliders

abstract

This is a short review of present and future limits on new neutral gauge bosons, in particular on hadrophilic or leptophobic \(Z\)’s recently proposed to interpret the observed fluctuations of \({\mit \Gamma }_{c,b}\) at LEP. Light gauge bosons coupled to lepton number differences or to baryon number are also examples of the model dependence of these bounds. The mixing between the U(1) factors plays an important role in the phenomenology of these extended electroweak models. Future improvements based on the analysis of precise electroweak data are emphasized.


Signals of Non-Minimal Higgs Sectors

abstract

It is possible that the Standard Model possesses a non-minimal Higgs sector. We consider various extended Higgs models and highlight two distinctive signatures which may be of relevance at present colliders: a light charged scalar \((H^{\pm })\) and a fermiophobic neutral scalar \((H_{F})\).


Neutrino Mixing and Oscillations in Quantum Field Theory

abstract

We show that the generator of field mixing transformations in Quantum Field Theory induces a non trivial structure in the vacuum which turns out to be a coherent state, both for bosons and for fermions, although with a different condensate structure. The Fock space for mixed fields is unitarily inequivalent to the Fock space of the massive (free) fields in the infinite volume limit. As a practical application we study neutrino mixing and oscillations. A new oscillation formula is found where the oscillation amplitude is depressed, with respect to the usual one, by a factor which is momentum and mass dependent. In the relativistic limit, the usual formula is recovered. We finally discuss in some detail phenomeno-logical features of the modified oscillation formula.


Astrophysical Implications of the Mirror World with Broken Mirror Parity

abstract

We discuss the physics of the mirror (shadow) world which is completely analogous to the visible one except that its ‘weak’ scale is larger by one or two orders of magnitude than the weak scale in the standard model. The mirror neutrinos can mix the ordinary ones through the Planck scale induced higher order operators, which can help to reconcile the present neutrino puzzles that are the solar and atmospheric neutrino deficits, the recent LSND anomaly and the need in the \(\sim \) eV mass neutrino as the hot dark matter. In particular, the oscillation of \(\nu _{e}\) into its mirror partner \(\nu ^\prime _{e}\) emerges with parameters naturally in the MSW range. The nucleosynthesis constraint on the extra light particle species can be fulfilled by assuming the asymmetric postinflationary reheating between the usual and mirror worlds. One implication of our proposal is that bulk of the dark matter in the universe may be a warm dark matter consisting of the keV range mirror neutrinos rather than the conventional cold dark matter, while the mirror baryons can also contribute as dissipative dark matter. Implications of the mirror Machos for microlensing experiments are also discussed.


Present Status of Precise Calculation for Small-Angle Bhabha Scattering

abstract

The problem of obtaining a theoretical description of Bhabha scattering at small angle, to monitor the luminosity at the precision 0.1% requested by LEP experiments, is discussed and recent results are presented.


Neutrino Oscillations and Dark Matter

abstract

It is increasingly likely that the universe is flat, having a critical density \(({\mit \Omega }=1)\), and probably made up of \(\lt 10\%\) baryonic matter, \(\gtrsim 70\%\) cold dark matter, and \(\sim 20\%\) hot dark matter. Some evidence for the last  component may be coming from the LSND experiment which has observed a beam-on minus beam-off excess of events which are consistent with oscillations of \(\bar \nu \to \bar \nu _{e}\). While the chance that this is a statistical fluctuation is \(\lt 10^{-3}\), evidence for neutrino mass cannot be claimed unless the effect appears also in the \(\nu _{\mu }\to \nu _e\) channel, currently being analyzed. If \(\nu _{\mu }\to \nu _e\) is observed also, then \(\Delta m^{2}_{\mu e}\) is most likely \(\sim \) 6 eV\(^2\). Should a neutrino \((\nu _{\mu }\) or \(\nu _e)\) have a mass \(\sim \) 2.4 eV, then current models of a low density universe \(({\mit \Omega }\lt 0.4)\) do not work, and a critical density universe is favored. If two nearly degenerate neutrinos exist \((\nu _e \to \nu _{\tau }\) to explain the solar neutrino deficit or \(\mu _{\mu } \to \nu _{\tau }\) to account for the atmospheric \(\nu _{\mu }/\nu _e\) ratio) to share the \(\sim 20\%\) hot clark matter, this model explains the structure of the universe on all scales and solves the age problem, because the Hubble constant has to be close to 50 km\(\cdot \)s\(^{-1}\cdot \)Mpc\({-1}\). Constraints from supernova nucleosynthesis can be avoided by making \(m_{\nu _{e}}\gt m_{\nu _{\mu }}\), or by having \(\nu _e\to \nu _s\) explain solar neutrinos, since the sterile neutrino may even aid nucleosynthesis.


Chiral Perturbation Theory

abstract

An introduction to the methods and ideas of Chiral Perturbation Theory is presented in this talk. The discussion is illustrated with some phenomenological predictions that can be compared with available experimental results.


Physics of Heavy Neutrinos

abstract

Theoretical and experintental situation in physics of heavy neutrinos \((M_N\gt M_z)\) is briefly presented. Various experimental bounds on heavy neutrino masses and mixings are shortly reviewed. Special attention is paid to possibility of detecting heavy neutrinos in future lepton linear colliders.


Structure of Electroweak Radiative Corrections

abstract

Looking into the inside of radiative corrections is an interesting subject as a deeper study of the standard electroweak theory after its remarkable success in the precision analyses. I will discuss here a test of “structure” of the EW radiative corrections to the weak-boson masses, and show that we can now analyze several different parts separately.


Signals from Supersymmetry in Electroweak Precision Data?

abstract

The predictions of the Standard Model and the minimal supersymmetric standard model (MSSM) for the electroweak precision parameters are discussed in the light of the recent precision data. The results from global fits yield lower \(\chi ^2\) values in the MSSM than in the Standard model. The fits prefer regions in the MSSM parameter space with \(M_2\simeq |\mu |\) and allow chargino masses higher than the present exclusion limits of LEP 1.5.


High-Energy Tests of the Left-Right Symmetric Electroweak Model

abstract

The left-right symmetric extension of the Standard Model, based on \({\rm SU}(2)_L\times {\rm SU}(2)_R\times {\rm U}(1)_{B-L}\) symmetry, has many predictions one can test at TeV-scale accelerators. These include the existence of new heavy weak bosons \(W_2\) and \(Z_2\) and right-handed neutrinos, as well as of triplet Higgses. We discuss various direct tests of the basic ingredients of the left-right symmetric model, as well as of its supersymmetric version, in \(e^{-}e^{-},e^{-}\gamma \) and \(\gamma \gamma \) collisions at high energies.


From LEP 1 Physics to LEP 2 Physics the Gauge Connection

abstract

The most relevant theoretical aspects associated with the experimental results obtained at LEP t and foreseen at LEP 2 are discussed. In particular the quest for a fully gauge invariant formulation of radiative corrections, both for two fermion and four fermion processes at LEP 2 energies is addressed. The outcome of the analysis clearly shows that such a formulation is indeed possible and some of the subtleties are discussed.


Virtual Effects of Heavy Chiral Fermions at \(e^+e^-\) Colliders

abstract

We derive the low-energy electroweak effective Lagrangian for the case of additional heavy, unmixed, sequential fermions. Present LEP1 data still allow the presence of new fermionic doublets (quarks and/or leptons), with masses greater than \(M_Z/2\), provided that these multiplets are sufficiently degenerate. Deviations of the effective Lagrangian predictions from full one-loop computation are sizeable only for fermion masses close to the threshold \(M_Z/2\). We analyse the contribution of new heavy sequential fermions to the \(e^+e^-\to W^+W^-\) cross-section at the energies of LEP2 and Next Linear Collider (NLC). The signals coming from an additional doublet will be out of the LEP2 observability level. More interesting is the case of NLC where the simultaneous presence of the kinematic enhancement factor \(p^2/m^{2}_{W}\) and of the unitarity delay effect gives deviations from the Standard Model of the order of 10–50 per cent, for a wide range of new fermion masses.


Exotic Smoothness and Particle Physics

abstract

Short introduction to exotic differential structures on manifolds is given. The possible physical context of this mathematical curiosity is discussed. The topic is very interesting although speculative.


Gluon Radiation Patterns in Hadronic Collisions

abstract

The radiation pattern of soft gluons in hard scattering events is sensitive to the underlying color structure. Two examples are discussed: the production of rapidity gaps in dijet events at the Tevatron and heavy Higgs production via weak boson fusion at the LHC. A minijet veto for weak boson scattering at the LHC makes use of the different angular distributions and momentum scales of gluon emission in signal and background processes. It is a promising Higgs search tool.


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