arXiv subjects

M. Drees

Publications and source records attributed to M. Drees.

51 records · Page 3Linked to original sources

arXiv1994

Higgs and Z boson decays into light gluinos

We calculate the decay rate of scalar and pseudoscalar Higgs bosons into a pair of gluinos, within the Minimal Supersymmetric Standard Model. In the theoretically and experimentally allowed light gluino window, $\mg \sim $ 3--5 GeV, gluino pairs can completely dominate the decays of the light scalar Higgs boson and play a prominent role in the decay of the pseudoscalar Higgs boson. This would alter the limits obtained from $Z$ decays on the lightest CP--even and CP--odd Higgs bosons, and could jeopardize the search for these Higgs particles at future hadron colliders. In contrast, the branching ratio for the two--body decay of $Z$ bosons into pairs of light gluinos is less than 0.1\%.

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arXiv1994

Phenomenology of the Minimal Supergravity SU(5) Model

The minimal grand unified supergravity model is discussed. Requiring radiative breaking of the electroweak gauge symmetry, the unification of b and tau Yukawa couplings, a sufficiently stable nucleon, and not too large a relic density of neutralinos produced in the Big Bang constrains the parameter space significantly. In particular, the soft breaking parameter m_1/2 has to be less than about 130 GeV, and the top quark Yukawa coupling has to be near its (quasi) fixed point. The former condition implies m_gluino < 400 GeV and hence very large production rates for gluino pairs at the LHC, while the latter constraint implies that the lighter stop and sbottom eigenstates are significantly lighter than the other squarks, leading to characteristic signatures for gluino pair events.

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arXiv1994

$γγ$ Processes at High Energy pp Colliders

In this note we investigate the production of charged heavy particles via \gaga\ fusion at high energy pp colliders. We revise previous claims that the \gaga\ cross section is comparable to or larger than that for the corresponding Drell-Yan process at high energies. Indeed we find that the \gaga\ contribution to the total production cross section at pp is far below the Drell-Yan cross section. As far as the individual elastic, semi-elastic and inelastic contributions to the \gaga\ process are concerned we find that they are all of the same order of magnitude.

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arXiv1994

Supercollider Signatures of Supergravity Models with Yukawa Unification

We study the predictions of the simplest SU(5) grand unified model within the framework of minimal supergravity, including constraints from the radiative breaking of electroweak symmetry. As a consequence of the unification of the $b$-quark and $τ$-lepton Yukawa couplings, the top quark mass is predicted to be close to its fixed point value. We delineate the regions of the supergravity parameter space allowed by constraints from the non-observation of proton decay and from the requirement that the LSP does not overclose the universe. These constraints lead to a definite pattern of sparticle masses: the feature unique to Yukawa unified models is that some of the third generation squarks are much lighter than those of the first two generations. Despite the fact that all sparticle masses and mixings are determined by just four SUSY parameters at the GUT scale (in addition to $m_t$), we find that the signals for sparticle production can vary substantially over the allowed parameter space. We identify six representative scenarios and study the signals from sparticle production at the LHC. We find that by studying the signal in various channels, these scenarios may be distinguished from one another, and also from usually studied ``minimal models'' where squarks and sleptons are taken to be degenerate. In particular, our studies allow us to infer that some third generation squarks are lighter than other squarks---a feature that could provide the first direct evidence of supergravity grand unification.

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arXiv1994

Supersymmetric Dark Matter: Relic Density and Detection

The Lightest Supersymmetric Particle (LSP) makes a good Dark Matter (DM) candidate, since its relic density quite naturally comes out close to the cosmologically required value. This is true even in minimal Supergravity models with radiative symmetry breaking, which have a rather small number of free parameters. On the other hand, the experimental detection of SUSY DM might be quite difficult, necessitating km$^2$ size neutrino detectors or direct detection experiments with several tons of detector material.

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arXiv1993

Production and Decay of Scalar Stoponium Bound States

In this paper we discuss possible signatures for the production of scalar \stst\ (stoponium) bound states \sigst\ at hadron colliders, where \st\ is the lighter scalar top eigenstate. We first study the decay of \sigst; explicit expressions are given for all potentially important decay modes. If \st\ has unsuppressed two--body decays, they will always overwhelm the annihilation decays of \sigst. Among the latter, we find that usually either the $gg$ or $hh$ final state dominates, depending on the size of the off--diagonal entry of the stop mass matrix; $h$ is the lighter neutral scalar Higgs boson of the minimal supersymmetric model. If \msig\ happens to be close to the mass of one of the neutral scalar Higgs bosons, $Q \bar{Q}$ final states dominate ($Q=b$ or $t$). \ww\ and $ZZ$ final states are subdominant. We argue that $\sigst \rightarrow \gamgam$ decays offer the best signal for stoponium production at hadron colliders. The tevatron should be able to close the light stop window left open by LEP searches, but its mass reach is limited to $\msig \leq 90$ GeV. In contrast, at the LHC one should ultimately be able to probe the region $\msig \leq 700$ GeV, if the $hh$ partial width is not too large. We also comment on the feasibility of searching for \sigst\ production at hadron colliders in the $ZZ, \ Z γ$ and \fourtau\ final states, and briefly mention \sigst\ production at \gamgam\ colliders.

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arXiv1993

Initial State Radiation in Resolved Photon Interactions

After a brief review of recent data that confirm qualitative and quantitative predictions for resolved photon processes, a possible problem with the implementation of initial state showering in existing Monte Carlo event generators is pointed out. It is argued that this is responsible for the rather poor description of the jet rapidity distribution measured by the H1 collaboration at HERA.

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arXiv1993

A new signal for scalar top bound state production

We study the production and decay of a scalar \stst\ bound state \sigst\ at hadron supercolliders, where \st\ is the lighter stop eigenstate. If \st\ has no tree--level 2--body decays, the dominant decay modes of \sigst\ are $gg$ or, if $m_h < \mst \ll \mstt$, a pair of light scalar Higgs bosons $h$. Nevertheless the branching ratio into two photons is often large enough to yield a detectable signal.

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arXiv1993

Scalar Stoponium

We study the decays of a scalar \stst\ bound state \sigst, where \st\ is the lighter stop eigenstate. If \st\ has no tree--level 2--body decays, the dominant decay modes of \sigst\ are $gg$ or, if $m_h < \mst \ll \mstt$, a pair of light scalar Higgs bosons $h$. The best signal for \sigst\ production at hadron colliders is probably its decay into two photons.

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arXiv1993

Hard $γγ$ Collisions at Future $e^+ e^-$ LINACs

We update our previous calculation of di--jet production in two--photon collisions at future \ee\ colliders, including both beam-- and bremsstrahlung. We then discuss the production of heavy ($c,b,t$) quarks. Our study re--confirms that colliders with much beamstrahlung offer no physics advantages over designs with little beamstrahlung.

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arXiv1993

Neutralino-Nucleon Scattering Revisited

We present a detailed discussion of the elastic scattering of a supersymmetric neutralino off a nucleon or nucleus, with emphasis on the spin--independent interaction. We carefully treat QCD effects on the squark exchange contribution. In particular, we identify a class of terms that survive even in the absence of mixing in both the neutralino and squark sectors; the corresponding quark and gluon operators also appear in analyses of deep--inelastic lepton--nucleon scattering (``twist--2 operators''), so their hadronic matrix elements are well known. We also re--emphasize the importance of mixing between the superpartners of left-- and right--handed quarks, and of the contribution from the heavier scalar Higgs boson. We use our refined calculation of the scattering amplitude to update predictions of signal rates for cosmic relic neutralino searches with Germanium detectors. In general the counting rate varies strongly with the values (even the signs) of model parameters; typical results lie between a few times $10^{-4}$ and a few times $10^{-1}$ events/(kg$\cdot$day).

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arXiv1993

Neutralino Annihilation to Gluons

We present a complete calculation of the cross section for neutralino annihilation into the two-gluon final state. This channel can be quite important for the phenomenology of neutralino annihilation due to the well-known helicity suppression of neutralino annihilation into light quarks and leptons. In addition, we calculate the cross section for annihilation of neutralinos into a gluon and quark-antiquark pair, and discuss QCD corrections to the tree-level cross sections for neutralino annihilation into quarks. If the neutralino is lighter than the top quark, the effect of these results on high-energy neutrino signals from neutralino annihilation in the Sun and in the Earth can be significant, especially if the neutralino is primarily gaugino. On the other hand, our results should have little effect on calculations of the cosmological abundance of neutralinos. We also briefly discuss implications for cosmic-ray antiprotons from neutralino annihilation in the galactic halo.

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arXiv1993

Supersymmetric QCD corrections to quark pair production in e+ e- annihilation

We calculate supersymmetric QCD corrections (squark/gluino loops) to quark pair production in e+ e- annihilation, allowing for mixing between left- and right-handed squarks and taking into account the effects of nonzero quark masses. Corrections to the Z boson partial widths are generally small and positive, except in the case of large b-squark mixing, where they become negative. At high-energy e+ e- colliders, larger corrections to the total cross sections are possible. Corrections to forward-backward asymmetries are negligible except possibly for top quarks, where they are sensitive to t-squark mixing. We also comment on the possibility that the gluino mass is only a few GeV.

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arXiv1992

The Neutralino Relic Density in Minimal N=1 Supergravity

We compute the cosmic relic (dark matter) density of the lightest supersymmetric particle (LSP) in the framework of minimal $N=1$ Supergravity models with radiative breaking of the electroweak gauge symmetry. To this end, we re--calculate the cross sections for all possible annihilation processes for a general, mixed neutralino state with arbitrary mass. Our analysis includes effects of all Yukawa couplings of third generation fermions, and allows for a fairly general set of soft SUSY breaking parameters at the Planck scale. We find that a cosmologically interesting relic density emerges naturally over wide regions of parameter space. However, the requirement that relic neutralinos do not overclose the universe does not lead to upper bounds on SUSY breaking parameters that are strictly valid for all combinations of parameters and of interest for existing or planned collider experiments; in particular, gluino and squark masses in excess of 5 TeV cannot strictly be excluded. On the other hand, in the ``generic'' case of a gaugino--like neutralino whose annihilation cross sections are not ``accidentally'' enhanced by a nearby Higgs or $Z$ pole, all sparticles should lie within the reach of the proposed $pp$ and $e^+e^-$ supercolliders. We also find that requiring the LSP to provide all dark matter predicted by inflationary models imposes a strict lower bound of 40 GeV on the common scalar mass $m$ at the Planck scale, while the lightest sleptons would have to be heavier

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arXiv1992

Aspects of Two-Photon Physics at Linear e+e- Colliders

We discuss various reactions at future e+e- and gamma-gamma colliders involving real (beamstrahlung or backscattered laser) or quasi--real (bremsstrahlung) photons in the initial state and hadrons in the final state. The production of two central jets with large pT is described in some detail; we give distributions for the rapidity and pT of the jets as well as the di--jet invariant mass, and discuss the relative importance of various initial state configurations and the uncertainties in our predictions. We also present results for `mono--jet' production where one jet goes down a beam pipe, for the production of charm, bottom and top quarks, and for single production of W and Z bosons. Where appropriate, the two--photon processes are compared with annihilation reactions leading to similar final states. We also argue that the behaviour of the total inelastic gamma-gamma cross section at high energies will probably have little impact on the severity of background problems caused by soft and semi--hard (`minijet') two--photon reactions. We find very large differences in cross sections for all two--photon processes between existing designs for future e+e- colliders, due to the different beamstrahlung spectra; in particular, both designs with <<1 and >>1 events per bunch crossing exist.

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