arXiv subjects
Publications and source records attributed to M. Carena.
A review of the status of searches for Supersymmetry at the Tevatron Collider as of the Summer of 1997. This is a new version of the previous review, substantially shortened to fit editorial guidelines, and significantly different to warrant a separate posting. To appear in "Perspectives on Supersymmetry", ed. G.L. Kane (World Scientific, Singapore).
The possible phenomenological consequences of R-parity violating interactions in the framework of low energy supersymmetry breaking are studied. It is pointed out that even very weak R-parity violation would completely overshadow one of the basic signatures of low energy supersymmetry breaking models, that is, the decay of the next to lightest supersymmetric particle into a photon (lepton) and missing energy. Thus, the observation of these decays would put very strong limits on R-parity violating couplings. Vice-versa, if R-parity violation is established experimentally, before a detailed knowledge of the spectrum is obtained, it will be very difficult to distinguish gravity mediated from low energy gauge mediated supersymmetry breaking. Those conclusions are very model independent. We also comment on the possibility of mixing between charged and neutral leptons with charginos and neutralinos, respectively, and its phenomenological consequences for the photon (lepton) signatures, in scenarios where this mixing is generated by the presence of bilinear or trilinear R-parity violating terms in the superpotential.
We review the status of searches for Supersymmetry at the Tevatron Collider. After discussing the theoretical aspects relevant to the production and decay of supersymmetric particles at the Tevatron, we present the current results for Runs Ia and Ib as of the summer of 1997. To appear in the book "Perspectives in Supersymmetry", edited by G.L. Kane, World Scientific.
It has been recently shown that the observed baryon number may originate at the electroweak phase transition, provided that the Higgs boson and the lightest stop are sufficiently light. In this work, we perform a detailed analysis, including all dominant two-loop finite temperature corrections to the Higgs effective potential, as well as the non-trivial effects proceeding from the mixing in the stop sector, to define the region of parameter space for which electroweak baryogenesis can happen. The limits on the stop and Higgs masses are obtained by taking into account the experimental bounds on these quantities, as well as those coming from the requirement of avoiding dangerous color breaking minima. We find for the Higgs mass $m_h \simlt 105$ GeV, while the stop mass may be close to the present experimental bound and must be smaller than, or of order of, the top quark mass. These results provide a very strong motivation for further non-perturbative analysis of the electroweak phase transition, as well as for the search for Higgs and stop particles at the LEP and Tevatron colliders.
A light stop, with an R-parity-violating coupling $λ'_{131}$, has been suggested as an explanation of the excess in high-$Q^2$ neutral current events observed at the HERA collider. We show that in this scheme a corresponding excess in charged current events --- such as that reported by the H1 Collaboration --- can appear naturally, without calling for the presence of light sleptons or additional R-parity-violating couplings, if there exists a chargino lighter than the stop. The predicted event shapes agree well with the data. The relevant region of parameter space is identified, taking into account constraints coming from precision electroweak measurements, atomic parity violation and recent searches for first-generation leptoquarks at the Tevatron collider.
Electroweak Baryogenesis is a particularly attractive theoretical scenario, since it relies on physics which can be tested at present high energy collider facilities. Within the Standard Model, it has been shown that the requirement of preserving the baryon number generated at the weak scale leads to strong bounds on the Higgs mass, which are already inconsistent with the present experimental limits. In the Minimal Supersymmetric extension of the Standard Model we demonstrate that light stop effects can render the electroweak phase transition sufficiently strongly first order, opening the possibility of electroweak baryogenesis for values of the Higgs mass at the LEP2 reach. The generation of the observed baryon asymmetry also requires small chargino masses and new CP-violating phases associated with the stop and Higgsino mass parameters. We discuss the direct experimental tests of this scenario and other relevant phenomenological issues related to it.
Electroweak baryogenesis is an interesting theoretical scenario, which demands physics beyond the Standard Model at energy scales of the order of the weak boson masses. It has been recently emphasized that, in the presence of light stops, the electroweak phase transition can be strongly first order, opening the window for electroweak baryogenesis in the MSSM. For the realization of this scenario, the Higgs boson must be light, at the reach of the LEP2 collider. In this article, we compute the baryon asymmetry assuming the presence of non-trivial CP violating phases in the parameters associated with the left-right stop mixing term and the Higgsino mass $μ$. We conclude that a phase $|\sin ϕ_μ| > 0.01$ and Higgsino and gaugino mass parameters $|μ| \simeq M_2$, and of the order of the electroweak scale, are necessary in order to generate the observed baryon asymmetry.
ALEPH has reported a significant excess of four-jet events in the LEP runs above the Z^0 resonance, which however has not been confirmed by the other LEP collaborations. We assume here that this excess corresponds to a physics signal and try to interpret it in the context of supersymmetric models with R-parity violation. Associated production of a left and right selectron can explain all the distinctive features of the ALEPH data: the value of the cross section, the di-jet mass difference, the absence of bottom quarks in the final state, and the di-jet charge content. Our proposed scenario makes definite predictions which can be tested at future LEP runs at higher energies.
We present a bottom-up approach to the question of supersymmetry breaking in the MSSM. Starting with the experimentally measurable low-energy supersymmetry breaking parameters, which can take any values consistent with present experimental constraints, we evolve them up to an arbitrary high energy scale. Approximate analytical expressions for such an evolution, valid for low and moderate values of $tanβ$, are presented. We then discuss qualitative properties of the high-energy parameter space and, in particular, identify the conditions on the low energy spectrum that are necessary for the parameters at the high energy scale to satisfy simple regular pattern such as universality or partial universality. As an illustrative example, we take low energy parameters for which light sparticles, within the reach of the LEP2 collider, appear in the spectrum, and which do not affect the Standard Model agreement with the precision measurement data. Comparison between supersymmetry breaking at the GUT scale and at a low energy scale is made.
Within the minimal supersymmetric extension of the Standard Model, the best fit to the most recent precision-measurement data requires charginos and neutralinos, with dominant Higgsino components and with masses within the reach of LEP1.5 ($\sqrt{s}=140$ GeV). In this work, we present a detailed analysis of the neutralino and chargino production processes for the favoured region of parameter space, that is low values of $|μ|$ and either low or large values of $\tanβ$. We find that chargino and neutralino searches can cover the Higgsino region in the ($μ,M_2$) plane for values of $M_2 \simlt 1$ TeV, at the next phases of the LEP collider. We also show that, due mainly to phase-space constraints, the lightest neutralinos should be more easily detectable than charginos in most of the parameter space preferred by precision-measurement data.
We study the impact of present and future $(g-2)_μ$ measurements on supersymmetric models. The corrections to $(g-2)_μ$ become particularly relevant in the presence of light sleptons, charginos and neutralinos, especially in the large $\tanβ$ regime. For moderate or large values of $\tanβ$, it is possible to rule out scenarios in which charginos and sneutrinos are both light, but nevertheless escape detection at the LEP2 collider. Furthermore, models in which supersymmetry breaking is transferred to the observable sector through gauge interactions can be efficiently constrained by the $(g-2)_μ$ measurement.
We perform an analysis of the behaviour of the electroweak phase transition in the Minimal Supersymmetric Standard Model, in the presence of light stops. We show that, in previously unexplored regions of parameter space, the order parameter $v(T_c)/T_c$ can become significantly larger than one, for values of the Higgs and supersymmetric particle masses consistent with the present experimental bounds. This implies that baryon number can be efficiently generated at the electroweak phase transition. As a by-product of this study, we present an analysis of the problem of colour breaking minima at zero and finite temperature, and we use it to investigate the region of parameter space preferred by the best fit to the present precision electroweak measurement data, in which the left-handed stops are much heavier than the right-handed ones.
I briefly summarize the main features related to the unification of gauge and Yukawa couplings in the MSSM, emphasizing the predictions derived in this framework, which are of special interest in the light of present and near--future experiments
In this report we review the prospects for Higgs physics at LEP2. The theoretical aspects and the phenomenology of Higgs particles are discussed within the Standard Model (SM) and the Minimal Supersymmetric Standard Model (MSSM). The experimental search techniques are described and the discovery limits for Higgs bosons in the LEP2 energy range are summarized. In addition, opportunities of detecting Higgs particles in non-minimal extensions of the SM and the MSSM are investigated.
Review of prospects for discovery of new physics signals at LEP2. The areas covered include SUSY, exotic fermions, BESS models, leptoquarks, virtual effects and CP violating observables.
A supersymmetric model with four generations is proposed, in which the top quark is approximately degenerate in mass with the $W^{\pm}$ gauge boson, $m_t\simeq m_W$, leading to values of $R_b$ in better agreement with the present experimental data than in the Standard Model. The model shares many of the good features of the minimal supersymmetric extension of the Standard Model (MSSM), such as the unification of gauge and Yukawa couplings at a common high-energy scale. The model differs from the MSSM by re-interpreting the Tevatron ``top-quark'' events as the production of the fourth generation quark $t\pri$, which decays dominantly to $bW^+$. The top quark decays primarily into supersymmetric particles, $t\rta\widetilde t\widetildeχ^0_1$, with $\widetilde t\rta c\widetildeχ^0_1$, thereby evading previous searches. Light supersymmetric particles are predicted to lie in the mass range between 25 and 70 GeV, which together with the fourth generation leptons provide a rich spectrum of new physics which can be probed at LEP-2 and the Tevatron.
We generalize the analytical expressions for the two-loop leading-log neutral Higgs boson masses and mixing angles to the case of general left- and right-handed soft supersymmetry breaking stop and sbottom masses and left--right mixing mass parameters ($m_Q, m_U, m_D, A_t, A_b$). This generalization is essential for the computation of Higgs masses and couplings in the presence of light stops. At high scales we use the minimal supersymmetric standard model effective potential, while at low scales we consider the two-Higgs doublet model (renormalization group improved) effective potential, with general matching conditions at the thresholds where the squarks decouple. We define physical (pole) masses for the top-quark, by including QCD self-energies, and for the neutral Higgs bosons, by including the leading one-loop electroweak self-energies where the top/stop and bottom/sbottom sectors propagate. For $m_Q=m_U=m_D$ and moderate left--right mixing mass parameters, for which the mass expansion in terms of renormalizable Higgs quartic couplings is reliable, we find excellent agreement with previously obtained results.
We propose, for the computation of the Higgs mass spectrum and couplings, a renormalization-group improved leading-log approximation, where the renormalization scale is fixed to the top-quark pole mass. For the case $m_A\sim M_{\rm SUSY}$, our leading-log approximation differs by less than 2 GeV from previous results on the Higgs mass computed using a nearly scale independent renormalization-group improved effective potential up to next-to-leading order. Moreover, for the general case $m_A\simlt M_{\rm SUSY}$, we provide analytical formulae (including two-loop leading-log corrections) for all the masses and couplings in the Higgs sector. For $M_{\rm SUSY}\simlt 1.5$ TeV and arbitrary values of $m_A$, $\tanβ$ and the stop mixing parameters, they reproduce the numerical renormalization-group improved leading-log result for the Higgs masses with an error of less than 3 GeV. For the Higgs couplings, our analytical formulae reproduce the numerical results equally well. Comparison with other methods is also performed.