Search

Ultraperipheral collisions of high energy protons are a source of approximately real photons colliding with each other. Photon fusion can result in production of yet unknown charged particles in very clean events. The cleanliness of such an event is due to the requirement that the protons survive during the collision. Finite sizes of the protons reduce the probability of such outcome compared to point-like particles. We calculate the survival factors and cross sections for the production of heavy charged particles at the Large Hadron Collider., Comment: 15 pages, 7 figures; version accepted by JHEP

We propose a model-independent approach for the search of charged long-lived particles produced in ultraperipheral collisions at the LHC. The main idea is to improve event reconstruction at ATLAS and CMS with the help of their forward detectors. Detection of both scattered protons in forward detectors allows complete recovery of event kinematics. Though this requirement reduces the number of events, it greatly suppresses the background, including the large background from the pile-up., Comment: 23 pages, 12 figures. This is the version published in JHEP

An approach to the properties of the [Formula: see text] system developed to solve the famous [Formula: see text] problem is used to calculate the partial widths ratios to [Formula: see text] and [Formula: see text] in the [Formula: see text] and [Formula: see text] decays. We obtain the results in agreement with the experimental data.

The dependence of the Z-resonance shape on the location of the threshold of the N\bar{N} production (N is the 4th generation neutrino) is analyzed. The bounds on the existence of 4th generation are derived from the comparison of the theoretical expression for the Z-lineshape with the experimental data. The 4th generation is excluded at 95% C. L. for m_N

A calculation of the cross section for the reaction νγ → νμ+μ− which is induced by the neutrino magnetic moment, is described in detail. The expression obtained for this cross section is used to determine the asymptotic behavior of the cross section for the reaction νN → νNμ+μ− by the Weiszacker-Williams method. It is shown that, upon going over from the nucleus involved to a structureless particle (for example, an electron), the resulting expression is doubled owing to integration with respect to the virtual-photon momentum over the region mμ≪Q≪Eν, where the Weiszacker-Williams approximation is not applicable. The method developed here makes it possible to obtain straightforwardly the known asymptotic expressions for the cross sections describing the reactions eN → eNμ+μ− and e+e− → W+W−e+e−.

We suggest returning to a different presentation of the e+e- \to \bar{f} f data off the Z peak, with the hope of using zeroes of specific amplitudes to enhance the sensitivity to new physics.

The analysis of the newest data on the leptonic $Z$-decays and $m_W$ appears to reveal the first manifestations of electroweak radiative corrections. In fact, these data differ, at the level of $2\sigma$, from their electroweak Born values, while they agree, to within $1\sigma$, with the theoretical values which take the electroweak radiative corrections into account. Previous data were within $1\sigma$ in agreement with both sets of values., Comment: 10 pages, 2 figures, preprint CERN-TH.7217/94

A set of equations representing the $W/Z$ mass ratio and various observables of $Z$ decays in terms of $\bar\alpha \equiv\alpha (m_Z)$, $G_{\mu}$, $m_Z$, $m_t$, $m_H$, $\bar\alpha_{s} \equiv\alpha_{s} (m_Z)$, $m_b$ and $m_\tau$ (all other fermion masses being neglected) are compared with the latest data of the four LEP detectors, which at the level of one standard deviation coincide with their Born values. Our global fit gives: $m_t = 161^{+ 15 +16}_{-16 -22} \bar\alpha_{s} = 0.119 \pm 0.006 \pm 0.002$, where the central values correspond to $m_H = 300$ GeV, the first errors are statistical and the second ones represent shifts of the central values corresponding to $m_H = 1000$ GeV($+$) and 60 GeV($-$). The predicted mass of the top is smaller than in the recent fits by 4 GeV. The {\it predicted} values of $m_W/m_Z$ and the LEP observables, based on the fitted values of $m_t$ and $\bar{\alpha}_s$, show a weak dependence on $m_H$ and differ by several {\it predicted} standard deviations from the corresponding Born values. The uncertainties of the predicted values and their deviations from the corresponding Born values determine the experimental accuracy required to observe electroweak radiative corrections.

A simple graphical method is presented for extracting limits on the masses of the top quark and of the Higgs boson from precision electroweak data.

The electroweak corrections to the Z -boson hadronic width are presented in a simple analytical form for the width itself, Λ h , for its ratio to the leptonic width, R l = Λ h Λ l , for the Λ b Λ h ratio, and for the total width Λ Z . The rational parametrization, advocated in our recent papers on leptonic decays, simplifies the analysis of the hadronic decays too. There are two main results of this analysis: (1) All electroweak precision measurements agree within 1 σ with the electroweak Born approximation if the gluon coupling constant α s = 0.12 . Thus the electroweak radiative corrections have not been observed experimentally. The unexpected smallness of electroweak radiative corrections is caused by the mutual cancellation of the large positive contribution of a heavy top quark and the large negative contribution of all other virtual particles. (2) With electroweak radiative corrections being taken into account, the value of α s extracted from the experimental value of Λ Z differs by ⩾ 3σ from that obtained from R l , if m t ⩾ 200 GeV; they agree within 1σ if m t ⩾ 150 GeV. Thus a low upper limit on m t can be obtained from hadronic decays of the Z -boson alone, even without data on its leptonic decays.

The data of the CHARM II and of the ALEPH, DELPHI, L3 and OPAL Collaborations are compared in the framework of (α(mZ), mZ, Gμ, mt, mH)-parametrization of the electroweak radiative corrections. The comparison shows that the physical meaning of the CHARM II results is more significant than was stated in the original publication of the CHARM Collaboration. In particular, CHARM II is very sensitive to the Z boson coupling to the muon neutrinos.

It is shown that charm photoproduction can be consistently described within an asymptotically free field theory. Quantum chromodynamics is used to derive sum rules for the total cross section σcγ which includes both production of mesons with hidden charm (J/ψ, ψ′ and so on) and of charmed particles (pairs D D , F F and so on). An estimate of σcγ as a function of energy is given and fast growth is discovered up to energies ∼ 1000 GeV. In this energy range σcγ turns out to be equal to several μb. It is argued that measurements of charm photoproduction would give the most direct information on the gluon distribution within a nucleon. All the results are generalized to production of heavier particles containing new quarks. In particular, a simple rescaling law is derived connecting the cross sections for charm and beauty.

Supersymmetric gauge theories with Higgs mechanism are considered. After all heavy fields are integrated out we are left with the instanton-induced effective action for light fields. It is demonstrated that the one-loop instanton result is not modified by higher order perturbative corrections. The peculiarity of the case considered is that the background scalar fields do not possess definite chirality, and the bosonic and fermionic modes are not degenerate for this reason.

We investigate the possibility of dynamical symmetry breaking, that is the appearance of 〈ϕ〉 = 0 due to radiative corrections in scalar theory with the interaction λϕ 4 , scalar electrodynamics and non-Abelian gauge theories. We show that in the one-loop approximation at small coupling constant the dynamical symmetry breaking is absent and the only mechanism of spontaneous symmetry breaking remains the Goldstone-Higgs-Kibble effect.

The supersymmetric O(3) sigma model is discussed. Using instanton calculus we find the exact Gell-Mann-Low function. Various aspects of the method proposed earlier in the context of supersymmetric gluodynamics are illustrated.

We discuss the status of the operator expansion at short distances within the framework of non-perturbative QCD. The question of instanton effects is investigated in various aspects. Consistency between instanton calculations and the general operator expansion requires a very special relation between the expansion coefficients. We check, by considering particular examples, that this relation does exist.

Sum rules for the decays of the C-even charmonium levels (1S0, 3P0, 3P2, 1D2) are derived. These rules are based on the asymptotic freedom of the quantum chromodynamics at small distances and on the analyticity. They refer to the various vacuum amplitudes involving products of charmed quark currents: electromagnetic current as well as currents with quantum numbers JPC=0−+,0++,2++,2−+. The contribution of the continuum to some of the sum rules is small, and they are saturated by the contribution of the lowest charmonium levels. In this way we predict the widths of the two-photon decays of the charmonium states and estimate their total hadronic widths.

A supersymmetric generalization of the Adler-Bardeen theorem in SUSY gauge theories is given. We show that within the Adler-Bardeen procedure, both the conformal and axial anomalies are exhausted by one loop.

Instanton effects are considered for a sample of supersymmetric theories, namely, quantum mechanics, gluodynamics, Higgs model. The problem is how to reconcile the apparent lack of the boson-fermion symmetry in the effective instanton induced interactions with supersymmetry of the corresponding lagrangians. It is shown that in the case of quantum mechanics and the Higgs model there is actually no conflict between supersymmetry and the instanton calculus since the Ward identities, associated with the supersymmetry transformations, are satisfied. In quantum mechanics this is due to spontaneous symmetry breaking, or pole terms in matrix elements of supercharge, while in the case of the supersymmetric Higgs model the effective fermion interaction just reduces to a total derivative. In the case of supersymmetric gluodynamics, however, the standard instanton calculus explicitly violates naive Ward identities.

We give predictions for the electronic widths of neutral vector mesons made from heavy quarks. Relying principally on dispersion relations and asymptotic freedom of QCD, these results are quite model-independent. Photonic and total hadronic widths of C -even mesons are also discussed.

The renormalization of the amplitudes of the second order in weak interactions by strong interactions in calculated. The strong interactions are assumed to be connected with the colour SU(3)′ group while weak interactions are described by the Weinberg-Salam model. It is shown, in particular, that the bounds on the masses of charmed particles are affected only weakly by the account of strong interactions.

One-loop electroweak corrections are rederived and analysed in a simple and straightforward way by using a set of natural and convenient parameters. As examples we consider the ratio of masses of the W and Z bosons, mWmZ, and the axial and vector couplings of leptonic Z decays, gA and gV. In the framework of the minimal standard model, each of these quantities is presented as an explicit function of the masses of the top quark, mt, and Higgs boson, mH, and of the phenomenological parameter θ, which is expressed in terms of the three quantities: mZ, α(mZ) and Gμ, which are known at present with much higher accuaracy than other electroweak observables.