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Manuel Drees

Publications and source records attributed to Manuel Drees.

At least 127 records · Page 7Linked to original sources

Particle Dark Matter Physics: An Update

This write--up gives a rather elementary introduction into particle physics aspects of the cosmological Dark Matter puzzle. A fairly comprehensive list of possible candidates is given; in each case the production mechanism and possible ways to detect them (if any) are described. I then describe detection of the in my view most promising candidates, weakly interacting massive particles or WIMPs, in slightly more detail. The main emphasis will be on recent developments.

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Supersymmetry Reach of Tevatron Upgrades: The Large $\tanβ$ Case

The Yukawa couplings of the tau lepton and the bottom quark become comparable to, or even exceed, electroweak gauge couplings for large values of the SUSY parameter $\tanβ$. As a result, the lightest tau slepton $\ttau_1$ and bottom squark $\tb_1$ can be significantly lighter than corresponding sleptons and squarks of the first two generations. Gluino, chargino and neutralino decays to third generation particles are significantly enhanced when $\tanβ$ is large. This affects projections for collider experiment reach for supersymmetric particles. In this paper, we evaluate the reach of the Fermilab Tevatron $p\bar p$ collider for supersymmetric signals in the framework of the mSUGRA model. We find that the reach via signatures with multiple isolated leptons ($e$ and $μ$) is considerably reduced. For very large $\tanβ$, the greatest reach is attained in the multi-jet$+\eslt$ signature. Some significant extra regions may be probed by requiring the presence of an identified $b$-jet in jets$+\eslt$ events, or by requiring one of the identified leptons in clean trilepton events to actually be a hadronic 1 or 3 charged prong tau. In an appendix, we present formulae for chargino, neutralino and gluino three body decays which are valid at large $\tanβ$.

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Collider Phenomenology for Supersymmetry at Large $tanβ$

If the parameter $tan β$ of the minimal supersymmetric model is large, then $b$ and $τ$ Yukawa interactions are important. These can significantly modify the masses and decays of sparticles. We describe new calculations which allow a reliable exploration of large $tan β$ values, and discuss implications for collider experiments. For large values of $tan β$, charginos and neutralinos may dominantly decay to $τ$-leptons or $b$-quarks. The usual cross sections for multilepton signatures may be greatly reduced, but SUSY may be detectable via new signals involving $τ$'s or $b$'s in the final state.

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Higgs Boson Decays into Light Gravitinos

We study decays of the neutral and charged Higgs bosons of the minimal supersymmetric standard model into a very light gravitino and a neutralino or chargino. Present experimental constraints already imply that the lighter scalar Higgs boson can only have a very small branching ratio into this mode. However, if the gravitino mass is below $\sim 0.5$ eV, the heavier neutral and charged Higgs boson decays into a gravitino can be important or even dominant. We discuss the resulting signature for associate production of the heavy CP-even and the CP-odd states at future $e^+ e^-$ colliders, and comment on Higgs boson production at hadron colliders.

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Are the H1 and ZEUS "High Q^2 Anomalies" Compatible with Each Other?

Both ZEUS and H1 have recently reported an excess of events at high $Q^2$ and high Bjorken--$x$. However, the $x$ distributions differ considerably; moreover, H1 sees more events with lower luminosity. Taken separately, the $x$ distributions and the number of observed events are consistent between the two experiments at the few percent level. However, when combined it becomes clear that the results of H1 and ZEUS are as (in)compatible with each other as each is with the Standard Model.

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Recent Developments in Dark Matter Physics

After a short review of the arguments for the existence of Particle Dark Matter in the Universe, I list the most plausible candidates provided by particle physics, i.e. neutrinos, axions, and WIMPs. In each case I briefly describe how to estimate the relic density, and discuss attempts at detecting these particles. At the end I discuss my personal favorite, the lightest supersymmetric particle, in a little more detail.

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Return of the Light Higgsino

It is pointed out that loop corrections involving heavy quarks and their superpartners can re-introduce a state with 99.5% higgsino purity as a viable cold Dark Matter candidate. Such corrections can increase the mass splitting between the three higgsino-like states of the MSSM by several GeV, which results in a suppression of the co-annihilation rate by a factor of five or more. Related corrections to the couplings of the LSP to $Z$ and Higgs bosons can change the predicted LSP detection rate by two orders of magnitude.

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An Introduction to Supersymmetry

A fairly elementary introduction to supersymmetric field theories in general and the minimal supersymmetric Standard Model (MSSM) in particular is given. Topics covered include the cancellation of quadratic divergencies, the construction of the supersymmetric Lagrangian using superfields, the field content of the MSSM, electroweak symmetry breaking in the MSSM, mixing between different superparticles (current eigenstates) to produce mass eigenstates, and the embedding of the MSSM in so--called minimal supergravity.

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Particle Dark Matter

There is plenty of evidence that most matter in the Universe is dark (non-luminous). Particle physics offers several possible explanations. In this talk I focus on cold dark matter; the most promising candidates are then axions and the lightest supersymetric particle. I briefly summarize estimates of the present relic density of these particles, and describe efforts to detect them

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SUSY Contributions to R_b and Top Quark Decay

Stop contributions to radiative corrections to R_b and the top quark decay are analysed over the relevant MSSM parameter space. One sees a 30 % increase in the former along with a similar drop in the latter in going from the higgsino dominated to the mixed region. Consequently one can get a viable SUSY contribution to R_b within the constraint of the top quark data only in the mixed region, corresponding to a photino dominated LSP. We discuss the phenomenological implications of this model for top quark decay and direct stop production, which can be tested with the Tevatron data.

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Signals for Double Parton Scattering at the Fermilab Tevatron

Four double-parton scattering processes are examined at the Fermilab Tevatron energy. With optimized kinematical cuts and realistic parton level simulation for both signals and backgrounds, we find large samples of four-jet and three-jet+one-photon events with signal to background ratio being 20\%-30\%, and much cleaner signals from two-jet+two-photon and two-jet+$e^+e^-$ final states. The last channel may provide the first unambiguous observation of multiple parton interactions, even with the existing data sample accumulated by the Tevatron collider experiments.

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Multiple Interactions in Two-Photon Collisions

We compute cross sections for events where two pairs of partons scatter off each other in the same $γγ$ reaction, giving rise to at least 3 high--{\mbox{$p_T^{}$}} jets. Unlike in {\mbox{$p \bar p$}}\ collisions we find the signal to lie well above the background from higher order QCD processes. If the usual ``eikonaliztion" assumption is correct, the signal should be readily observable at LEP2, and might already be detectable in data taken at TRISTAN.

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Testing superstring-inspired E(6) models at linear e+e- colliders

It is shown that in a large class of E(6) models, either an e+e- collider operating at $\rs \geq 1.5$ TeV must find a signal for the production of exotic leptons, or a collider operating at $\rs \geq 300$ GeV must find at least one light neutral Higgs boson with a large invisible branching ratio. The region of parameter space where neither of these signals is visible can be excluded because here the lightest neutral exotic lepton, which is absolutely stable, would overclose the Universe.

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A Decisive Test of Superstring-Inspired E(6) Models

We point out that in a large class of superstring--inspired $E(6)$ models, either an e+e- collider operating at a center--of--mass energy $\rs= 1.5$ TeV or higher must detect the pair production of charged or neutral exotic leptons, or an e+e- collider with $\rs \geq 300$ GeV must discover at least one light neutral Higgs boson with invisible branching ratio exceeding 50\%. If neither of these two signals is seen, the lightest neutral exotic lepton would overclose the universe, and the model could be completely excluded, independent of the values of the numerous free parameters. Future Higgs searches might lower the energy of the e+e- collider needed to test these models decisively. The only assumptions we have to make are that $R-$parity is exact, so that the lightest exotic lepton is stable if it is lighter than the lightest neutralino, and that no $SO(10)$ singlet scalar gets a vacuum expectation of order $10^{10}$ GeV or higher. If the second condition is violated, the model effectively reduces to an $SO(10)$ model as far as collider experiments are concerned.

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Resolved Photon Processes

We review the present level of knowledge of the hadronic structure of the photon, as revealed in interactions involving quarks and gluons ``in" the photon. The concept of photon structure functions is introduced in the description of deep--inelastic $e γ$ scattering, and existing parametrizations of the parton densities in the photon are reviewed. We then turn to hard \gamp\ and \gaga\ collisions, where we treat the production of jets, heavy quarks, hard (direct) photons, \jpsi\ mesons, and lepton pairs. We also comment on issues that go beyond perturbation theory, including recent attempts at a comprehensive description of both hard and soft \gamp\ and \gaga\ interactions. We conclude with a list of open problems.

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Multiple Interactions in gamma gamma Collisions

The cross--section for two--photon events with (at least) two independent partonic scatters is estimated, for LEP energies as well as a 500 GeV ``photon collider". This results in events with (at least) four central (mini--)jets. Such events might be found in existing data, and should be clearly seen at the second stage of LEP.

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Modification of the Equivalent Photon Approximation for Resolved Photon Processes

We propose a modification of the equivalent photon approximation (EPA) for processes which involve the parton content of the photon, to take into account the suppression of the photonic parton fluxes due to the virtuality of the photon. We present simple, physically motivated ansätze to model this suppression and show that even though the parton content of the electron no longer factorizes into an electron flux function and a photon structure function, it is still possible to express it as a single integral. We also show that for the TRISTAN experiments its effect can be numerically of the same size as that of the NLO corrections. Further, we discuss a possible measurement at HERA, which can provide an experimental handle on the effect we model through our ansätze.

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Inclusive Two--Photon Reactions at TRISTAN

After briefly reviewing past accomplishments of TRISTAN experiments in the field of inclusive two--photon reactions, I discuss open problems in the Monte Carlo simulation of such reactions. The main emphasis is on multiple scattering, i.e. events where at least two pairs of partons scatter within the same \gaga collision to form at least four (mini)jets. The cross section for such events might just be observable at TRISTAN. While theoretical arguments for the existence of such events are strong, they have not yet been directly observed experimentally, thereby potentially opening a new opportunity for TRISTAN experiments.

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