J.J. van der Bij

Physics potential and experimental challenges of the LHC luminosity upgrade

Abstract.We discuss the physics potential and the experimental challenges of an upgraded LHC running at an instantaneous luminosity of 1035 cm-2s-1. The detector R&D needed to operate ATLAS and CMS in a very high radiation environment and the expected detector performance are discussed. A few examples of the increased physics potential are given, ranging from precise measurements within the Standard Model (in particular in the Higgs sector) to the discovery reach for several New Physics processes.

Higgs boson pair production via gluon fusion

Abstract We study pair production of Higgs bosons at hadron colliders via the gluon fusion process gg → HH. This process is dependent on the three-Higgs self-coupling. We find that this coupling is an important effect in determining the absolute size of the cross section. However, the shapes of the transverse momentum and the rapidity distributions are almost unaffected by its presence. At SSC energies we find cross sections in the range 1pb − 1fb, for Higgs boson masses of 50–250 GeV and top masses larger than 40 GeV. At LHC energies the cross sections are about a factor of five smaller.

Quest for precision in hadronic cross sections at low energy: Monte Carlo tools vs. experimental data

We present the achievements of the last years of the experimental and theoretical groups working on hadronic cross section measurements at the low-energy e+e− colliders in Beijing, Frascati, Ithaca, Novosibirsk, Stanford and Tsukuba and on τ decays. We sketch the prospects in these fields for the years to come. We emphasise the status and the precision of the Monte Carlo generators used to analyse the hadronic cross section measurements obtained as well with energy scans as with radiative return, to determine luminosities and τ decays. The radiative corrections fully or approximately implemented in the various codes and the contribution of the vacuum polarisation are discussed.

Z-boson pair production via gluon fusion

We study the coupling of two gluons with two Z-bosons via a quark loop keeping the full dependence on the quark mass. We discuss the structure of the ggZZ polarisation tensor and give explicit formulas for the helicity amplitudes. The results are used to study the production of Z-boson pairs at hadron colliders both with and without the presence of a Higgs boson. Invariant mass, transverse momentum and rapidity distributions of the produced Z-bosons are shown. The Z-bosons from the gluon fusion process are produced somewhat more centrally than those from the quark-antiquark background. The continuum gluon fusion process, gg → ZZ, is important, (60–70%), but never dominant over the qq → ZZ process at the SSC. The Higgs boson signal and the continnum background interfere destructively at large s and this leads to a reduction in the cross section at large invariant mass and transverse momentum.

Vector boson pair production via gluon fusion

Abstract We study the production of Z boson pairs at hadron colliders, produced by gluon fusion via an internal quark loop of arbitrary mass, both with and without the presence of a Higgs boson. As a background to Higgs boson production we find that the gluon fusion process, gg→ZZ, is important, 60–70%, but never dominant over the q q→ZZ process at the SSC. The Higgs boson signal and the continuum background interfere destructively for an invariant mass of the Z boson pair larger than the Higgs boson mass. We use the results to give and estimate of the cross section gg→WW in the limit where up- and down-type quarks are degenerate and massless.

Massive two--loop diagrams : The Higgs propagator

Abstract Formulae are provided to express any two-loop scalar integral with arbitrary masses and arbitrary external momenta in terms of an integral of one fairly simple function or of its derivatives. Such integrals can be calculated numerically with high precision. Good agreement is found with known analytical expressions of specific two-loop diagrams. To prove the effectiveness of these techniques, the two-loop self-energy of the Higgs boson to order ( g 2 M 2 H / m 2 w ) 2 is calculated, and the correctness of the result is checked by using the unitarity of the S -matrix. This result allows one to disentangle the leading correction to the location of the Higgs resonance in fermion scattering, and to derive a perturbative bound of ∼1.2 TeV on the mass of the Higgs boson.

Influence of strongly coupled, hidden scalars on Higgs signals

To investigate the possible effects of a light hidden sector on Higgs boson detection, we discuss a model of scalar singlets coupled to the Standard Model. The model effectively makes the Higgs width a free parameter due to additional invisible decay modes. This width can become arbitrarily large. Theoretical and experimental bounds on model parameters are presented. It is shown, how Standard Model predictions change and that in the case of large coupling, Higgs signals will be diluted. We study, to which extent such a strongly coupled, hidden sector can be excluded by present and future Higgs search experiments.

Three Loop Leading Top Mass Contributions to the ρ Parameter

We present analytic results for the three loop heavy top quark contributions of the electroweak ρ parameter. The calculation is carried out using the standard model lagrangian with vanishing gauge couplings. Within this gaugeless limit the masses of the Higgs boson M H and of the top quark m t are the single relevant mass scales. In the present calculation [1] we have investigated the behaviour of the ρ parameter in the limit of a Higgs mass equal to zero.

Three-loop electroweak correction to the rho parameter in the large Higgs mass limit

Abstract We present an analytical calculation of the leading three-loop radiative correction to the ρ-parameter in the Standard Model in the large Higgs mass limit. This correction, of order g 6 m H 4 / M W 4 , is opposite in sign to the leading two-loop correction of order g 4 m H 2 / M W 2 . The two corrections cancel each other for a Higgs mass of approximately 480 GeV . The result shows that it is extremely unlikely that a strongly interacting Higgs sector could fit the data of electroweak precision measurements.

Charged boson stars

Abstract We study the static spherically symmetric gravitational equilibria of scalars coupled to a U(1) gauge field and with a possible 1 2 λ(φ ∗ φ) 2 self-interaction. We solve numerically the coupled Einstein-Maxwell-Klein-Gordon equations for non-singular and asymptotically flat solutions. Static solutions only exist for e 2 4π N m 2 =e 2 crit . The maximum mass of the boson star increases with the gauge coupling constant as ( e crit −e) −1 2 for a charge close to the critical charge. We study the behaviour of the solutions as a function of the node number of the scalar field and as a function of the self-coupling λ.