arXiv subjects
Publications and source records attributed to M. Drees.
The results obtained by the Working Group on Supersymmetry at the 1999 Les Houches Workshop on Collider Physics are summarized. Separate chapters treat "general" supersymmetry, R-parity violation, gauge mediated supersymmetry breaking, and anomaly mediated supersymmetry breaking.
We study the potential of hadron colliders in the search for the pair production of neutral Higgs bosons in the framework of the Minimal Supersymmetric Standard Model. We perform a detailed signal and background analysis, working out efficient kinematical cuts for the extraction of the signal. The important role of squark loop contributions to the signal is re--emphasized. If the signal is sufficiently enhanced by these contributions, it could even be observable at the next run of the upgraded Tevatron collider in the near future. At the LHC the pair production of light and heavy Higgs bosons might be detectable simultaneously.
Report of the Higgs working group for the Workshop "Physics at TeV Colliders", Les Houches, France 8-18 June 1999. It contains 6 separate sections: 1. Measuring Higgs boson couplings at the LHC. 2. Higgs boson production at hadron colliders at NLO. 3. Signatures of Heavy Charged Higgs Bosons at the LHC. 4. Light stop effects and Higgs boson searches at the LHC. 5. Double Higgs production at TeV Colliders in the MSSM. 6. Programs and Tools for Higgs Bosons.
A stable neutralino $χ_1^0$, assumed to be the lightest supersymmetric particle, is a favored particle physics candidate for the cosmological Dark Matter. We study co-annihilation of the lightest neutralino with the lighter scalar top quark $\tilde{t}_1$. We show that for natural values of the neutralino mass, $\lsim 300$ GeV, the $χ_1^0 - \tilde{t}_1$ mass difference has to exceed $\sim 10$ to 30 GeV if $χ_1^0$, is to contribute significantly to the Dark Matter. Scenarios with smaller mass splitting, where $\tilde{t}_1$ is quite difficult to detect at collider experiments, are thus cosmologically disfavored. On the other hand, for small $\tilde{t}_1 - χ_1^0$ mass splitting, we show that co--annihilation allows very large neutralino masses, $m_{χ_1^0} \sim 5$ TeV, without ``overclosing'' the Universe.
We study the potential of hadron colliders in the search for the pair production of neutral Higgs bosons in the framework of the Minimal Supersymmetric Standard Model. Using analytical expressions for the relevant amplitudes, we perform a detailed signal and background analysis, working out efficient kinematical cuts for the extraction of the signal. The important role of squark loop contributions to the signal is emphasised. If the signal is sufficiently enhanced by these contributions, it could even be observable at the next run of the upgraded Tevatron collider in the near future. At the LHC the pair production of light and heavy Higgs bosons might be detectable simultaneously.
We discuss effects of fragmentation and hard gluon radiation on the signal for the pair production of the lighter scalar top eigenstate $\tilde{t}_1$ at e^+e^- colliders. The main emphasis is on scenarios with small stop--LSP mass splitting, where strong interaction effects can considerably modify kinematical properties of the final state.
We study the pair production of neutral Higgs bosons through gluon fusion at hadron colliders in the framework of the Minimal Supersymmetric Standard Model. We present analytical expressions for the relevant amplitudes, including both quark and squark loop contributions, and allowing for mixing between the superpartners of left- and right-handed quarks. Squark loop contributions can increase the cross section for the production of two CP-even Higgs bosons by more than two orders of magnitude, if the relevant trilinear soft breaking parameter is large and the mass of the lighter squark eigenstate is not too far above its current lower bound. In the region of large $\tan β$, neutral Higgs boson pair production might even be observable in the $4 b$ final state during the next run of the Tevatron collider.
We correct our treatment of decays of the lightest chargino to final states containing the lightest neutralino in the case where the chargino and neutralino masses differ by less than 1 GeV. A brief summary of the phenomenological implications is given.
We investigate the phenomenology of an orbifold string model in which supersymmetry breaking is dominated by the overall `size' modulus field and all matter fields are in the untwisted sector. The possibly close degeneracy of the lightest neutralino and chargino and the possibly small splitting between the gluino and chargino/LSP mass imply that discovery of supersymmetry at future colliders could be more challenging than anticipated. Specialized search strategies and particular detector features could play an important role. For preferred model parameter choices, the phenomenology of dark matter in the universe is significantly modified.
We explore the techniques, cross sections and expected signal significance for detecting invisible and almost invisible particles at LEP2 and the NLC by means of a hard photon tag. Examples from supersymmetry include the lightest chargino and second lightest neutralino when their masses are nearly the same as that of the lightest neutralino (the LSP), and invisibly decaying sneutrinos. The importance of particular features of the detectors is discussed, instrumentation for vetoing a fast e^+ or e^- in the beam hole being especially crucial.
We analyze several signals at HERA and the Tevatron of a light $U(1)_B$ gauge boson ($γ_B$) coupling to baryon number. We show that the study of the production of $b \bar{b}$ pairs at the (upgraded) Tevatron can exclude $γ_B$ with masses ($m_B$) in the range $40 \lesssim m_B \lesssim 300$ GeV for $γ_B$ couplings ($α_B$) greater than $2 \times 10^{-2}$ ($3 \times 10^{-3}$). We also show that the HERA experiments cannot improve the present bounds on $γ_B$. Moreover, we demonstrate that the production at HERA and the Tevatron of di--jet events with large rapidity gaps between the jets cannot be explained by the existence of a light $γ_B$.
Renormalizable lepton number violating interactions that break R-parity can induce a Majorana mass for neutrinos. Based on this, we show that it is possible to obtain a phenomenologically viable neutrino mass matrix that can accommodate atmospheric neutrino data via $ν_μ$--$ν_τ$ mixing and the solar neutrino data via either the large or small angle MSW effect. We argue that such a mass matrix could result from an approximate discrete symmetry of the superpotential that forbids renormalizable baryon number violating couplings.
We re-investigate the question whether a light higgsino-like neutralino is a viable Dark Matter candidate. To this end we compute the dominant one-loop corrections to the masses of the higgsino-like states in the minimal Supersymmetric Standard Model (MSSM), due to loops involving heavy quarks and their superpartners. We also calculate analogous corrections to the couplings of higgsino-like neutralinos to Z and Higgs bosons. In the region of parameter space leading to high higgsino purity of the lightest neutralino, these corrections can change the expected relic density by up to a factor of five in either direction. We conclude that for favorable choices of soft supersymmetry breaking parameters, a state with more than 99% higgsino purity could indeed form all cold Dark Matter in the Universe. In some cases these corrections can also increase the expected cross section for LSP scattering off spinless nuclei by up to two orders of magnitude, or reduce it to zero.
We provide a mini-guide to some of the possible manifestations of weak scale supersymmetry. For each of six scenarios we provide a brief description of the theoretical underpinnings, the adjustable parameters, a qualitative description of the associated phenomenology at future colliders, comments on how to simulate each scenario with existing event generators.
We calculate the contribution to inclusive high tranverse momentum ($p_T$) charm production at HERA from the excitation of charm in the photon. At large values of \pt\ the results of such a calculation, in the structure function language, will be more reliable as it sums the large logs, $\log(p_T^2/m_c^2)$, as opposed to calculating the contribution of the $2 \to 3 $ subprocess in fixed order of perturbation theory. We find that this contribution is large and comparable to the contribution from $γg$ fusion production of charm. Suitable cuts on the rapidity of the `away-side' large \pt\ jet allow a very neat separation between the contributions from the excitation process and from pair-production. We further find that including this excitation contribution we can reproduce the measured inclusive $D^*$ and $μ$ cross--sections measured by the ZEUS and H1 collaborations respectively, in a LO calculation.
We discuss the motivations and implications of models of low-energy supersymmetry. We present the case for the minimal supersymmetric standard model, which we define to include the minimal particle content and soft supersymmetry-breaking interactions which are universal at the GUT or Planck scale. This model is in agreement with all present experimental results, and yet depends on only a few unknown parameters and therefore maintains considerable predictive power. From the theoretical side, it arises naturally in the context of supergravity models. We discuss radiative electroweak symmetry breaking and the superpartner spectrum in this scenario, with some added emphasis on regions of parameter space leading to unusual or interesting experimental signals at future colliders. We then examine how these results may be affected by various modifications and extensions of the minimal model, including GUT effects, extended gauge, Higgs, and matter sectors, non-universal supersymmetry breaking, non-conservation of R-parity, and dynamical supersymmetry breaking at low energies.
Relatively light sneutrinos, which are experimentally allowed, may significantly affect the currently popular search strategies for supersymmetric particles by decaying dominantly into an invisible channel. In certain cases the second lightest neutralino may also decay invisibly leading to two extra carriers of missing energy -- in addition to the lightest supersymmetric particle (LSP) \zi\ -- the virtual LSPs (VLSPs). It is shown that these VLSPs are allowed in supergravity models with common scalar and gaugino masses at the unification scale for a sizable region of parameter space and are consistent with all constraints derived so far from SUSY searches. The pair production of right handed sleptons, which can very well be the lightest charged SUSY particles in this scenario, at LEP 200 and their decay signatures are discussed. The signal survives kinematical cuts required to remove the standard model background. Charginos are also pair produced copiously if kinematically accessible; they also decay dominantly into hadronically quiet di--lepton + \etmiss\ modes leading to interesting unlike sign dilepton events which are again easily separable from the Standard Model backgrounds at LEP 200 energies.
The upper bond on the branching ratio for $b\to sγ$ decays implies a stringent lower bound on the mass of the pseudoscalar Higgs boson of the MSSM if sparticles are heavy. This leads to an upper bound on the expected event rate in experiments searching for heavy supersymmetric dark matter. Scenarios with lighter sparticle spectrum and light pseudoscalar Higgs boson are still possible, but only if $μ< 0$, which again implies a small LSP counting rate.