arXiv subjects

Manuel Drees

Publications and source records attributed to Manuel Drees.

At least 73 records · Page 4Linked to original sources

arXiv2008

CP-violating Higgs at Tevatron

We analyze the prospect for observing the intermediate neutral Higgs ($h_2$) boson in its decay to two lighter Higgs bosons ($h_1$) at the Tevatron in the framework of the CP violating MSSM using the PYTHIA event generator. We consider the lepton+ 4-jets+ $\met$ channel from $p \bar p \ra W h_2 \ra W h_1 h_1 \ra l ν_l b \bar b b\bar b$, with two or three tagged $b$ jets. We found that it is very hard to observe this signature in the LEP-allowed region of parameter space, due to the small signal efficiency.

hep-ph↗

arXiv2008

Improvement of the Determination of the WIMP Mass from Direct Dark Matter Detection Data

Weakly Interacting Massive Particles (WIMPs) are one of the leading candidates for Dark Matter. We developed a model-independent method for determining the WIMP mass by using data (i.e., measured recoil energies) of direct detection experiments. Our method is independent of the as yet unknown WIMP density near the Earth, of the form of the WIMP velocity distribution, as well as of the WIMP-nucleus cross section. It requires however positive signals from at least two detectors with different target nuclei. At the first phase of this work we found a systematic deviation of the reconstructed WIMP mass from the real one for heavy WIMPs. Now we improved this method so that this deviation can be strongly reduced for even very high WIMP mass. The statistical error of the reconstructed mass has also been reduced. In a background-free evironment, a WIMP mass of ~ 50 GeV could in principle be determined with an error of ~ 35% with only 2 times 50 events.

hep-ph↗

arXiv2008

Model-Independent Determination of the WIMP Mass from Direct Dark Matter Detection Data

Weakly Interacting Massive Particles (WIMPs) are one of the leading candidates for Dark Matter. We develop a model-independent method for determining the mass $m_χ$ of the WIMP by using data (i.e., measured recoil energies) of direct detection experiments. Our method is independent of the as yet unknown WIMP density near the Earth, of the form of the WIMP velocity distribution, as well as of the WIMP-nucleus cross section. However, it requires positive signals from at least two detectors with different target nuclei. In a background-free environment, $m_χ\sim 50$ GeV could in principle be determined with an error of $\sim 35%$ with only $2 \times 50$ events; in practice upper and lower limits on the recoil energy of signal events, imposed to reduce backgrounds, can increase the error. The method also loses precision if $m_χ$ significantly exceeds the mass of the heaviest target nucleus used.

hep-ph↗

arXiv2007

Constraints on the Very Early Universe from Thermal WIMP Dark Matter

We investigate the relic density n_χof non-relativistic long-lived or stable particles χin non-standard cosmological scenarios. We calculate the relic abundance starting from arbitrary initial temperatures of the radiation-dominated epoch, and derive the lower bound on the initial temperature T_0 \geq m_χ/23, assuming that thermally produced χparticles account for the dark matter energy density in the universe; this bound holds for all χannihilation cross sections. We also investigate cosmological scenarios with modified expansion rate. Even in this case an approximate formula similar to the standard one is capable of predicting the final relic abundance correctly. Choosing the χannihilation cross section such that the observed cold dark matter abundance is reproduced in standard cosmology, we constrain possible modifications of the expansion rate at T \sim m_χ/20, well before Big Bang nucleosynthesis.

hep-ph↗

arXiv2007

Determining the WIMP Mass from Direct Dark Matter Detection Data

Weakly interacting massive particles (WIMPs) are one of the leading candidates for Dark Matter. So far we can use direct Dark Matter detection to estimate the mass of halo WIMPs only by fitting predicted recoil spectra to future experimental data. Here we develop a model-independent method for determining the WIMP mass by using experimental data directly. This method is independent of the as yet unknown WIMP density near the Earth as well as of the WIMP-nuclear cross section and can be used to extract information about WIMP mass with O(50) events.

hep-ph↗

arXiv2007

Electroweak Contributions to Squark Pair Production

We compute electroweak contributions to the production of squark pairs at hadron colliders. These include the exchange of electroweak gauge bosons in the $s-$channel as well as electroweak gaugino exchange in the $t-$ and/or $u-$channel. In many cases these can interfere with the dominant QCD contributions. As a result, we find sizable contributions to the production of two SU(2) doublet squarks. At the LHC, they amount to 10 to 20% for typical mSUGRA (or CMSSM) scenarios, but in more general scenarios they can vary between -40 and $+55%$, depending on size and sign of the SU(2) gaugino mass. The electroweak contribution to the total squark pair production rate at the LHC is about 3.5 times smaller.

hep-ph↗

arXiv2007

Electroweak Contributions to Squark Pair Production at the LHC

In this paper we compute electroweak contributions to the production of squark pairs at hadron colliders. These include the exchange of electroweak gauge bosons in the s-channel as well as electroweak gaugino exchange in the t- and/or u-channel. In many cases these can interfere with the dominant QCD contributions. As a result, we find sizable contributions to the production of two SU(2) doublet squarks. At the LHC, they amount to 10 to 20% for typical mSUGRA (or CMSSM) scenarios, but in more general scenarios they can vary between -40 and +55%, depending on size and sign of the SU(2) gaugino mass. The electroweak contribution to the total squark pair production rate at the LHC is about 3.5 times smaller.

hep-ph↗

arXiv2007

Reconstructing the Velocity Distribution of WIMPs from Direct Dark Matter Detection Data

Weakly interacting massive particles (WIMPs) are one of the leading candidates for dark matter. Currently, the most promising method to detect many different WIMP candidates is the direct detection of the recoil energy deposited in a low-background laboratory detector due to elastic WIMP-nucleus scattering. So far the usual procedure has been to predict the event rate of direct detection of WIMPs based on some model(s) of the galactic halo. The aim of our work is to invert this process. That is, we study what future direct detection experiment can teach us about the WIMP halo. As the first step we consider a time-averaged recoil spectrum, assuming that no directional information exists. We develop a method to construct the (time-averaged) one-dimensional velocity distribution function from this spectrum. Moments of this function, such as the mean velocity and velocity dispersion of WIMPs, can also be obtained directly from the recoil spectrum. The only input needed in addition to a measured recoil spectrum is the mass of the WIMP; no information about the scattering cross section or WIMP density is required.

astro-ph↗

arXiv2007

Signals of Very High Energy Neutralinos in Future Cosmic Ray Detectors

``Top--down'' models explain the observation of ultra high energy cosmic rays (UHECR; $E \gsim 5 \cdot 10^{19}$ eV) through the decay of very massive, long--lived ``$X$ particles''. If superparticles with masses near a TeV exist, $X$ decays also lead to a significant flux of very energetic neutralinos, assumed to be the (stable or long--lived) lightest superparticles. There is a range of energies where neutrinos get absorbed in the Earth, but neutralinos can still traverse it. These neutralinos could in principle be detected. We calculate the detection rate in planned experiments such as OWL and EUSO. For bino--like neutralinos, which have been considered previously, we find detection rates below 1 event per Teraton of target and year in all cases; often the rates are much smaller. In contrast, if the neutralino is higgsino--like, more than ten events per year per Teraton might be observed, if the mass of the $X$ particle is near its lower bound of $\sim 10^{12}$ GeV.

hep-ph↗

arXiv2006

QCD Effects in the Decays of TeV Black Holes

In models with ``large'' and/or warped extra dimensions, the higher-dimensional Planck scale may be as low as a TeV. In that case black holes with masses of a few TeV are expected to be produced copiously in multi-TeV collisions, in particular at the LHC. These black holes decay through Hawking radiation into typically O(20) Standard Model particles. Most of these particles would be strongly interacting. Naively this would lead to a final state containing 10 or so hadronic jets. However, it has been argued that the density of strongly interacting particles would be so large that they thermalize, forming a ``chromosphere'' rather than well-defined jets. In order to investigate this, we perform a QCD simulation which includes parton-parton scattering in addition to parton showering. We find the effects of parton scattering to remain small for all cases we studied, leading to the conclusion that the decays of black holes with masses within the reach of the LHC will not lead to the formation of chromospheres.

hep-ph↗

arXiv2006

One-loop calculations of the decay of the next-to-lightest neutralino in the MSSM

We calculate one-loop corrections to the decays of the next-to-lightest neutralino $\tildeχ_2^0$ into the lightest neutralino $\tildeχ_1^0$ and two leptons; this includes diagrams where a real photon is emitted. In cases where two-body decays $\tildeχ_2^0 \to \tilde{l}^\pm_1 l^\mp \to \tildeχ_1^0 l^- l^+$ are kinematically allowed, we calculate these decays both with and without the single-pole approximation, and find consistent results. For example, for the minimal supergravity parameter set SPS1a, the integrated partial widths (the branching ratios) for $\tildeχ_2^0\to \tildeχ_1^0 l^- l^+ (l = e, μ)$ are enhanced by about 15.5 (13.6) percent by the one-loop corrections. We also study a scenario where $\tilde χ_2^0$ cannot undergo two-body decays, and find corrections to these branching ratios of about 13.6 percent. Moreover, we study the dilepton invariant mass ($M_{l^+ l^-}$) distribution, whose endpoint is often used in analyses that aim to reconstruct (differences of) supersymmetric particle masses at the LHC. The shape of this distribution is altered significantly by the emission of hard photons. For example, for the SPS1a parameter set the peak of the $M_{l^+ l^-}$ distribution is shifted by several GeV when these contributions are included.

hep-ph↗

arXiv2006

Abundance of Cosmological Relics in Low-Temperature Scenarios

We investigate the relic density n_χof non-relativistic long-lived or stable particles χin cosmological scenarios in which the temperature T is too low for χto achieve full chemical equilibrium. The case with a heavier particle decaying into χis also investigated. We derive approximate solutions for n_χ(T) which accurately reproduce numerical results when full thermal equilibrium is not achieved. If full equilibrium is reached, our ansatz no longer reproduces the correct temperature dependence of the χnumber density. However, it does give the correct final relic density, to an accuracy of about 3% or better, for all cross sections and initial temperatures.

hep-ph↗

arXiv2006

Probing MeV Dark Matter at Low--Energy $e^+e^-$ Colliders

It has been suggested that the annihilation of Dark Matter particles χwith mass between 0.5 and 20 MeV into e^+e^- pairs could be responsible for the excess flux of 511 keV photons coming from the central region of our galaxy that has been detected by the IINTEGRAL satellite. The simplest way to achieve the required cross section for χpair annihilation while respecting existing constraints is to introduce a new vector boson U with mass M_U below a few hundred MeV. In this Letter we point out that over most of the allowed parameter space, the process e^+e^- to U γ, followed by the decay of U into either an e^+e^- pair or into an invisible (ν\bar νor χ\bar χ) channel, should lead to signals that can be detected by the B-factory experiments BaBar and Belle. A smaller, but still substantial, region of parameter space can also be probed at the Φfactory DAFNE.

hep-ph↗

arXiv2006

The Passage of Ultrarelativistic Neutralinos through Matter

The origin of the most energetic cosmic ray events, with $E \gsim 10^{20}$ eV, remains mysterious. One possibility is that they are produced in the decay of very massive, long--lived particles. It has been suggested that these so--called ``top--down scenarios'' can be tested by searching for ultrarelativistic neutralinos, which would be produced copiously if superparticles exist at or near the TeV scale. In this paper we present a detailed analysis of the interactions of such neutralinos with ordinary matter. To this end we compute several new contributions to the total interaction cross section; in particular, the case of higgsino--like neutralinos is treated for the first time. We also carefully solve the transport equations. We show that a semi--analytical solution that has been used in the literature to treat the somewhat analogous propagation of $τ$ neutrinos leads to large errors; we therefore use a straightforward numerical method to solve these integro--differential equations.

hep-ph↗

arXiv2006

Updated Constraints on the Minimal Supergravity Model

We provide an up-to-date analysis of the parameter space of the minimal supergravity model (mSUGRA). Novel features include the new central value of the top quark mass, an improved calculation of the masses of the supersymmetric particles and the neutral Higgs bosons, constraints from b to s \ell^+ \ell^- decays, and a careful treatment of the most important experimental and theoretical uncertainties. In addition to the by now traditional plots of the allowed region in the (m_0, m_{1/2}) plane, we show allowed regions in the planes spanned by pairs of {\em physical} sparticle or Higgs boson masses. Moreover, we search for the minimal allowed masses of new particles for various sets of constraints. We find that in many cases the direct experimental limits from collider and Dark Matter searches can be saturated even in this minimal model, and even after including the by now quite restrictive constraint on the Dark Matter relic density.

hep-ph↗

arXiv2005

Looking for a Heavy Higgsino LSP in Collider and Dark Matter Experiments

A large part of the mSUGRA parameter space satisfying the WMAP constraint on the dark matter relic density corresponds to a higgsino LSP of mass $\simeq 1$ TeV. We find a promising signal for this LSP at CLIC, particularly with polarized electron and positron beams. One also expects a viable monochromatic $γ$-ray signal from its pair annihilation at the galactic center at least for cuspy DM halo profiles. All these results hold equally for the higgsino LSP of other SUSY models like the non-universal scalar or gaugino mass models and the so-called inverted hierarchy and more minimal supersymmetry models.

hep-ph↗

arXiv2005

Neutralino Dark Matter in 2005

I summarize some recent work on supersymmetric neutralinos as candidates for cold Dark Matter in the Universe. This includes a new scan of mSUGRA parameter space, with special emphasis on neutralinos annihilating predominantly through exchange of the light CP--even Higgs boson, and on bounds on sparticle masses. Next, prospects of testing models with TeV higgsino--like Dark Matter at colliders are discussed. Finally, I briefly comment on extensions of the mSUGRA model, and on scenarios with non--standard cosmology.

hep-ph↗

arXiv2005

A Supersymmetric Explanation of the Excess of Higgs-Like Events at LEP

Searches for the Standard Model Higgs boson by the four LEP experiments found excess events in two mass ranges: a 2.3 sigma excess around 98 GeV, and an 1.7 sigma excess around 115 GeV. The latter has been discussed widely in the literature, but the former has attracted little attention so far. In this paper I explore the possibility of explaining the excess near 98 GeV through production of the lighter CP--even Higgs boson in the Minimal Supersymmetric Standard Model (MSSM). It is shown that this allows to simultaneously explain the excess near 115 GeV through the production of the heavier CP--even MSSM Higgs boson. The resulting light Higgs sector offers opportunities for charged Higgs boson searches at the Tevatron and LHC. Neutral Higgs boson searches at the LHC in the di--muon channel are also promising. However, conclusive tests of this scenario may have to wait for the construction of a linear e+ e- collider.

hep-ph↗