Searcharxiv⌕ Search

arXiv subjects

M. Drees

Publications and source records attributed to M. Drees.

At least 19 recordsLinked to original sources
arXiv2015

Minimal Natural Supersymmetry after the LHC8

In this work, we present limits on natural supersymmetry scenarios based on searches in data taken during run 1 of the LHC. We consider a set of 22000 model points in a six dimensional parameter space. These scenarios are minimal in the sense of only keeping those superparticles relatively light that are required to cancel the leading quadratically divergent quantum corrections (from the top and QCD sector) to the Higgs mass in the Standard Model. The resulting mass spectra feature higgsinos as the lightest supersymmetric particle, as well as relatively light third generation $SU(2)$ doublet squarks and $SU(2)$ singlet stops and gluinos while assuming a Standard Model like Higgs boson. All remaining supersymmetric particles and Higgs bosons are assumed to be decoupled. We check each parameter set against a large number of LHC searches as implemented in the public code {\tt CheckMATE}. These searches show a considerable degree of complementarity, i.e. in general many searches have to be considered in order to check whether a given scenario is allowed. We delineate allowed and excluded regions in parameter space. For example, we find that all scenarios where either $m_{\tilde t_1} < 230$ GeV or $m_{\tilde g} < 440$ GeV are clearly excluded, while all model points where $m_{\tilde t_1} > 660$ GeV and $m_{\tilde g} > 1180$ GeV remain allowed.

hep-ph↗
arXiv2013

Light stop phenomenology

We consider the discovery potential of light stops in the MSSM at the LHC. Here, we assume that the lightest neutralino is the LSP and that the lighter stop is the NLSP. Direct stop pair production is difficult to probe in scenarios with a small mass splitting between the stop and a neutralino. We discuss two different search channels: the monojet and the two $b$--flavoured jets and large missing transverse energy signature. We present the discovery reach in the stop--neutralino mass plane for both channels. The latter process is sensitive to the stop--higgsino--$b$ quark coupling. This allows us to test a supersymmetry relation involving superpotential couplings. We briefly comment on the possible precision with which the coupling can be measured.

hep-ph↗
arXiv2011

CP asymmetries in the supersymmetric trilepton signal at the LHC

In the CP-violating Minimal Supersymmetric Standard Model, we study the production of a neutralino-chargino pair at the LHC. For their decays into three leptons, we analyze CP asymmetries which are sensitive to the CP phases of the neutralino and chargino sector. We present analytical formulas for the entire production and decay process, and identify the CP-violating contributions in the spin correlation terms. This allows us to define the optimal CP asymmetries. We present a detailed numerical analysis of the cross sections, branching ratios, and the CP observables. For light neutralinos, charginos, and squarks, the asymmetries can reach several 10%. We estimate the discovery potential for the LHC to observe CP violation in the trilepton channel.

hep-ph↗
arXiv2010

Light Stop Searches at the LHC in Events with two b-Jets and Missing Energy

We propose a new method to discover light top squarks (stops) in the co-annihilation region at the Large Hadron Collider (LHC). The bino-like neutralino is the lightest supersymmetric particle (LSP) and the lighter stop is the next-to-LSP. Such scenarios can be consistent with electroweak baryogenesis and also with dark matter constraints. We consider the production of two stops in association with two b-quarks, including pure QCD as well as mixed electroweak-QCD contributions. The stops decay into a charm quark and the LSP. For a higgsino-like light chargino the electroweak contributions can exceed the pure QCD prediction. We show the size of the electroweak contributions as a function of the stop mass and present the LHC discovery reach in the stop-neutralino mass plane.

hep-ph↗
arXiv2009

Potentially Large One-loop Corrections to WIMP Annihilation

We compute one-loop corrections to the annihilation of non--relativistic particles $\chi$ due to the exchange of a (gauge or Higgs) boson $\phi$ with mass $\mu$ in the initial state. In the limit $m_\chi \gg \mu$ this leads to the "Sommerfeld enhancement" of the annihilation cross section. However, here we are interested in the case $\mu \lsim m_\chi$, where the one--loop corrections are well--behaved, but can still be sizable. We find simple and accurate expressions for annihilation from both $S-$ and $P-$wave initial states; they differ from each other if $\mu \neq 0$. In order to apply our results to the calculation of the relic density of Weakly Interacting Massive Particles (WIMPs), we describe how to compute the thermal average of the corrected cross sections. We apply this formalism to scalar and Dirac fermion singlet WIMPs, and show that the corrections are always very small in the former case, but can be very large in the latter. Moreover, in the context of the Minimal Supersymmetric Standard Model, these corrections can decrease the relic density of neutralinos by more than 1%, if the lightest neutralino is a strongly mixed state.

hep-ph↗
arXiv2009

Color-octet scalars at the LHC

Elements of the phenomenology of color-octet scalars (sgluons), as predicted in the hybrid N=1/N=2 supersymmetric model, are discussed in the light of forthcoming experiments at the CERN Large Hadron Collider.

hep-ph↗
arXiv2009

Color-Octet Scalars of N=2 Supersymmetry at the LHC

The color gauge hyper-multiplet in N=2 supersymmetry consists of the usual N=1 gauge vector/gaugino super-multiplet, joined with a novel gaugino/scalar super-multiplet. Large cross sections are predicted for the production of pairs of the color-octet scalars $σ$ [sgluons] at the LHC: $gg, q\bar{q} \to σσ^{\ast}$. Single $σ$ production is possible at one-loop level, but the $g g\to σ$ amplitude vanishes in the limit of degenerate $L$ and $R$ squarks. When kinematically allowed, $σ$ decays predominantly into two gluinos, whose cascade decays give rise to a burst of eight or more jets together with four LSP's as signature for $σ$ pair events at the LHC. $σ$ can also decay into a squark-antisquark pair at tree level. At one-loop level $σ$ decays into gluons or a $t \bar t$ pair are predicted, generating exciting resonance signatures in the final states. The corresponding partial widths are very roughly comparable to that for three body final states mediated by one virtual squark at tree level.

hep-ph↗
arXiv2008

Testing the Majorana Nature of Gluinos and Neutralinos

Gluinos and neutralinos, supersymmetric partners of gluons and neutral electroweak gauge and Higgs bosons, are Majorana particles in the Minimal Supersymmetric Standard Model [MSSM]. Decays of such self-conjugate particles generate charge symmetric ensembles of final states. Moreover, production channels of supersymmetric particles at colliders are characteristically affected by the Majorana nature of particles exchanged in the production processes. The sensitivity to the Majorana character of the particles can be quantified by comparing the predictions with Dirac exchange mechanisms. A consistent framework for introducing gluino and neutralino Dirac fields can be designed by extending the N=1 supersymmetry of the MSSM to N=2 in the gauge sector. We examine to which extent like-sign dilepton production in the processes q q -> ~q ~q and e- e- -> ~e- ~e- is affected by the exchange of either Majorana or Dirac gluinos and neutralinos, respectively, at the Large Hadron Collider (LHC) and in the prospective e- e- mode of a lepton linear collider.

hep-ph↗
arXiv2008

Benchmark scenarios for the NMSSM

We discuss constrained and semi--constrained versions of the next--to--minimal supersymmetric extension of the Standard Model (NMSSM) in which a singlet Higgs superfield is added to the two doublet superfields that are present in the minimal extension (MSSM). This leads to a richer Higgs and neutralino spectrum and allows for many interesting phenomena that are not present in the MSSM. In particular, light Higgs particles are still allowed by current constraints and could appear as decay products of the heavier Higgs states, rendering their search rather difficult at the LHC. We propose benchmark scenarios which address the new phenomenological features, consistent with present constraints from colliders and with the dark matter relic density, and with (semi--)universal soft terms at the GUT scale. We present the corresponding spectra for the Higgs particles, their couplings to gauge bosons and fermions and their most important decay branching ratios. A brief survey of the search strategies for these states at the LHC is given.

hep-ph↗
arXiv2006

Neutralino Production and Decay at an e^+e^- Linear Collider with Transversely Polarized Beams

Once supersymmetric neutralinos $\tildeχ^0$ are produced copiously at $e^+e^-$ linear colliders, their characteristics can be measured with high precision. In particular, the fundamental parameters in the gaugino/higgsino sector of the minimal supersymmetric extension of the standard model (MSSM) can be analyzed. Here we focus on the determination of possible CP--odd phases of these parameters. To that end, we exploit the electron/positron beam polarization, including transverse polarization, as well as the spin/angular correlations of the neutralino production $e^+ e^- \to \tildeχ^0_i \tildeχ^0_j$ and subsequent 2--body decays $\tildeχ^0_i \to \tildeχ^0_k h, \tildeχ^0_k Z, \tilde \ell^\pm_R \ell^\mp$, using (partly) optimized CP--odd observables. If no final--state polarizations are measured, the $Z$ and $h$ modes are independent of the $\tildeχ^0_i$ polarization, but CP--odd observables constructed from the leptonic decay mode can help in reconstructing the neutralino sector of the CP--noninvariant MSSM. In this situation, transverse beam polarization does not seem to be particularly useful in probing explicit CP violation in the neutralino sector of the MSSM. This can most easily be accomplished using longitudinal beam polarization.

hep-ph↗
arXiv2003

Analysis of CP Violation in Neutralino Decays to Tau Sleptons

In the minimal supersymmetric standard model, tau sleptons $\tilde τ_{1,2}$ and neutralinos $\tilde χ^0_{1,2}$ are expected to be among the lightest supersymmetric particles that can be produced copiously at future $e^+e^-$ linear colliders. We analyze $\tilde τ$ pair and $\tilde χ_1^0 \tilde χ_2^0$ production under the assumption $m_{\tilde χ_1^0} < m_{\tilde τ_1} < m_{\tilde χ_2^0}$, allowing the relevant parameters of the SUSY Lagrangian to have complex phases. We show that the transverse and normal components of the polarization vector of the $τ$ lepton produced in $\tilde χ_2^0$ decays offer sensitive probes of these phases.

hep-ph↗
arXiv2003

CPsuperH: a Computational Tool for Higgs Phenomenology in the Minimal Supersymmetric Standard Model with Explicit CP Violation

We provide a detailed description of the Fortran code CPsuperH, a newly--developed computational package that calculates the mass spectrum and decay widths of the neutral and charged Higgs bosons in the Minimal Supersymmetric Standard Model with explicit CP violation. The program is based on recent renormalization-group-improved diagrammatic calculations that include dominant higher--order logarithmic and threshold corrections, b-quark Yukawa-coupling resummation effects and Higgs-boson pole-mass shifts. The code CPsuperH is self--contained (with all subroutines included), is easy and fast to run, and is organized to allow further theoretical developments to be easily implemented. The fact that the masses and couplings of the charged and neutral Higgs bosons are computed at a similar high-precision level makes it an attractive tool for Tevatron, LHC and LC studies, also in the CP-conserving case.

hep-ph↗
arXiv2003

SUSY In The Sky: Observing Ultra-High Energy Cosmic Neutralinos

In models where the ultra-high energy cosmic ray problem is solved by top-down scenarios, a significant flux of ultra-high energy neutralinos is predicted. We calculate the number of events expected from such particles in future experiments such as EUSO or OWL. We show that by using the Earth as a filter, showers generated by neutralinos can be separated from neutrino generated showers. We find that for many models, observable rates are expected.

hep-ph↗
arXiv2002

Neutrinos Associated With Cosmic Rays of Top-Down Origin

Top-down models of cosmic rays produce more neutrinos than photons and more photons than protons. In these models, we reevaluate the fluxes of neutrinos associated with the highest energy cosmic rays in light of mounting evidence that they are protons and not gamma rays. While proton dominance at EeV energies can possibly be achieved by efficient absorption of the dominant high-energy photon flux on universal and galactic photon and magnetic background fields, we show that the associated neutrino flux is inevitably increased to a level where it should be within reach of operating experiments such as AMANDA II, RICE and AGASA. In future neutrino telescopes, tens to a hundred, rather than a few neutrinos per kilometer squared per year, may be detected above 1 PeV.

hep-ph↗
arXiv2001

Loop induced Higgs and Z boson couplings to Neutralinos and implications for collider and Dark Matter searches

We calculate the one-loop induced couplings of two gaugino-like neutralinos to the $Z$ and Higgs bosons in the Minimal Supersymmetric Standard Model. These couplings, which vanish at the tree level, can be generated through loops involving fermions and sfermions. We show that, while the neutralino contribution to the invisible $Z$ boson decay width remains small, the loop induced couplings to the lightest Higgs boson might be sufficiently large to yield a rate of invisible decays of this Higgs boson that should be detectable at future $e^+e^-$ colliders. We also study the implications of these couplings for direct searches of Dark Matter and show that they can modify appreciably the neutralino-nucleon elastic cross section for some parameter range.

hep-ph↗
arXiv2001

Constraints on the Minimal Supergravity Model and Prospects for SUSY Particle Production at Future Linear e^+ e^- Colliders

We perform a complete analysis of the supersymmetric particle spectrum in the Minimal Supergravity (mSUGRA) model where the soft SUSY breaking scalar masses, gaugino masses and trilinear couplings are unified at the GUT scale, so that the electroweak symmetry is broken radiatively. We show that the present constraints on the Higgs boson and superparticle masses from collider searches and precision measurements still allow for large regions of the mSUGRA parameter space where charginos, neutralinos, sleptons and top squarks as well as the heavier Higgs particles, are light enough to be produced at the next generation of $\ee$ linear colliders with center of mass energy around $\sqrt{s} \sim 800$ GeV, with sizeable cross sections. An important part of this parameter space remains even when we require that the density of the lightest neutralinos left over from the Big Bang, which we calculate using standard assumptions, falls in the range favored by current determinations of the Dark Matter density in the Universe. Already at a c.m. energy of 500 GeV, SUSY particles can be accessible in some parameter range, and if the energy is increased to $\sqrt{s} \simeq 1.2$ TeV, the $e^+e^-$ collider will have a reach for high precision studies of SUSY particles in a range that is comparable to the discovery range of the LHC.

hep-ph↗
arXiv2001

CP Violation in Tau Slepton Pair Production at Muon Colliders

We discuss in detail signals for CP violation in the Higgs boson and tau-slepton sectors through the production processes $\mu^+\mu^- \to \tilde{\tau}_i^- \tilde{\tau}_j^+$, where $i,j=1,2$ label the two $\tau$ slepton mass eigenstates in the minimal supersymmetric standard model. We assume that the soft breaking parameters of third generation sfermions contain CP violating phases, which induce CP violation in the Higgs sector through quantum corrections. We classify all the observables for probing CP violation in the Higgs boson and $\tau$ slepton sectors. These observables depend on the initial muon beam polarization, where we include transverse polarization states. If the heavy Higgs bosons can decay into tau slepton pairs, a complete determination of the CP properties of the neutral Higgs boson and $\tau$--slepton systems is possible. The interference between the Higgs boson and gauge boson contributions could also provide a powerful method for probing CP violation, if transversely polarized muon beams are available. We show in detail how to directly measure CP violation in the tau slepton system, under the assumption that the neutral Higgs mixing angles are determined through the on--shell production of the neutral Higgs bosons.

hep-ph↗