arXiv subjects

Manuel Drees

Publications and source records attributed to Manuel Drees.

At least 55 records · Page 3Linked to original sources

arXiv2012

Mitigation of the LHC Inverse Problem

The LHC inverse problem refers to the difficulties in determining the parameters of an underlying theory from data (to be) taken by the LHC experiments: if they find signals of new physics, and an underlying theory is assumed, could its parameters be determined uniquely, or do different parameter choices give indistinguishable experimental signatures? This inverse problem was studied before for a supersymmetric Standard Model with 15 free parameters. This earlier study found 283 indistinguishable pairs of parameter choices, called degenerate pairs, even if backgrounds are ignored. We can resolve all but 23 of those pairs by constructing a true χ^2 distribution using mostly counting observables. The elimination of systematic errors would even allow separating the residual degeneracies. Taking the Standard Model background into account we still can resolve 237 of the 283 "degenerate" pairs. This indicates that (some of) our observables should also be useful for the purpose of determining the values of SUSY parameters.

hep-ph↗

arXiv2012

Comment on the Updated CDF "Ghost" Events

In 2008 the CDF Collaboration announced the discovery of an excess of events with two or more muons, dubbed "ghost" events for their unusual properties. In a recent update, CDF finds that the azimuthal angle distribution between the primary (trigger) muons is significantly more back-to-back than that of all known sources of di-muon backgrounds. Here we show that this angular distribution cannot be reproduced in models where the muons are produced in the decays of relatively light X-particles: all models of this kind also predict a much broader distribution than that found by CDF. We conclude that the CDF measurement cannot be described via the annihilation of strongly interacting partons, and thus seems to be in conflict with basic tenets of QCD.

hep-ph↗

arXiv2012

Dark Matter Primordial Black Holes and Inflation Models

A broad range of single field models of inflation are analyzed in light of all relevant recent cosmological data, checking whether they can lead to the formation of long--lived Primordial Black Holes (PBHs) as candidate for dark matter. To that end we calculate the spectral index of the power spectrum of primordial perturbations as well as its first and second derivatives. PBH formation is possible only if the spectral index $n_S(k_0)$ increases significantly at small scales. Since current data indicate that the first derivative $α_S$ of the spectral index is negative at the pivot scale, PBH formation is only possible in the presence of a sizable and positive second derivative ("running of the running") $β_S$. Among the three small-field and five large-field models we analyze, only the "running-mass" model allows PBH formation, for a narrow range of parameters.

astro-ph.CO↗

arXiv2012

Mini--Review of Dark Matter: 2012

This is the mini-review on Dark Matter in the 2012 edition of the Particle Data Group's Review of Particle Properties. After briefly summarizing the arguments in favor of the existence of Dark Matter, we list possible candidates, ranging in mass from a fraction of an eV (e.g., axions) to many solar masses (e.g., primordial black holes), and discuss ways to detect them. The main emphasis is on Weakly Interacting Massive Particles (WIMPs). A large international effort is being made to detect them directly, or else to detect their annihilation products. We explain why we consider all claims to have established a positive signal for WIMPs in either direct or indirect detection to be premature. We also introduce the concept of a {\it WIMP safe} minimal mass; below this mass, the interpretation of a given direct search experiment depends strongly on the tail of the WIMP velocity distribution and/or on the experimental energy resolution.

hep-ph↗

arXiv2011

Running Spectral Index and Formation of Primordial Black Hole in Single Field Inflation Models

A broad range of single field models of inflation are analyzed in light of all relevant recent cosmological data, checking whether they can lead to the formation of long-lived Primordial Black Holes (PBHs). To that end we calculate the spectral index of the power spectrum of primordial perturbations as well as its first and second derivatives. PBH formation is possible only if the spectral index increases significantly at small scales, i.e. large wave number $k$. Since current data indicate that the first derivative $α_S$ of the spectral index $n_S(k_0)$ is negative at the pivot scale $k_0$, PBH formation is only possible in the presence of a sizable and positive second derivative ("running of the running") $β_S$. Among the three small-field and five large-field models we analyze, only one small-field model, the "running mass" model, allows PBH formation, for a narrow range of parameters. We also note that none of the models we analyze can accord for a large and negative value of $α_S$, which is weakly preferred by current data.

astro-ph.CO↗

arXiv2011

On the On-Shell Renormalization of the Chargino and Neutralino Masses in the MSSM

We discuss the choice of input parameters for the renormalization of the chargino and neutralino sector in the minimal supersymmetric standard model (MSSM) in the on-shell scheme. We show that one should chose the masses of a bino-like, a wino-like and a higgsino-like state as inputs in order to avoid large corrections to the masses of the other eigenstates in this sector. We also show that schemes where the higgsino-like input state is a neutralino are more stable than those where the mass of the higgsino-like chargino is used as input. The most stable scheme uses the masses of the wino-like chargino as well as the masses of the bino- and higgsino-like neutralinos as inputs.

hep-ph↗

arXiv2011

Relic Abundance of Asymmetric Dark Matter

We investigate the relic abundance of asymmetric Dark Matter particles that were in thermal equilibrium in the early universe. The standard analytic calculation of the symmetric Dark Matter is generalized to the asymmetric case. We calculate the asymmetry required to explain the observed Dark Matter relic abundance as a function of the annihilation cross section. We show that introducing an asymmetry always reduces the indirect detection signal from WIMP annihilation, although it has a larger annihilation cross section than symmetric Dark Matter. This opens new possibilities for the construction of realistic models of MeV Dark Matter.

hep-ph↗

arXiv2011

Running-Mass Inflation Model and Primordial Black Holes

We revisit the question whether the running-mass inflation model allows the formation of Primordial Black Holes (PBHs) that are sufficiently long-lived to serve as candidates for Dark Matter. We incorporate recent cosmological data, including the WMAP 7-year results. Moreover, we include "the running of the running" of the spectral index of the power spectrum, as well as the renormalization group "running of the running" of the inflaton mass term. Our analysis indicates that formation of sufficiently heavy, and hence long-lived, PBHs still remains possible in this scenario. As a by-product, we show that the additional term in the inflaton potential still does not allow significant negative running of the spectral index.

hep-ph↗

arXiv2010

CP-violating Supersymmetric Higgs at the Tevatron and LHC

We analyze the prospect for observing the intermediate neutral Higgs boson ($h_2$) in its decay to two lighter Higgs bosons ($h_1$) at the presently operating hadron colliders in the framework of the CP violating MSSM using the PYTHIA event generator. We consider the lepton+ 4-jets+ $\met$ channel from associate $W h_2$ production, with $W h_2 \ra W h_1 h_1 \ra \ell ν_\ell b \bar b b\bar b$. We require two, three or four tagged $b$-jets. We explicitly consider all relevant Standard Model backgrounds, treating $c$-jets separately from light flavor and gluon jets and allowing for mistagging. We find that it is very hard to observe this signature at the Tevatron, even with 20 fb$^{-1}$ of data, in the LEP--allowed region of parameter space due to the small signal efficiency, even though the background is manageable. At the LHC, a priori huge SM backgrounds can be suppressed by applying judiciously chosen kinematical selections. After all cuts, we are left with a signal cross section of around 0.5 fb, and a signal to background ratio between 1.2 and 2.9. According to our analysis this Higgs signal should be viable at the LHC in the vicinity of present LEP exclusion once 20 to 50 fb$^{-1}$ of data have been accumulated at $\sqrt{s}=14$ TeV.

hep-ph↗

arXiv2010

CP-violating MSSM Higgs at Tevatron and LHC

We analyze the prospect for observing the intermediate neutral Higgs boson ($h_2$) in its decay to two lighter Higgs bosons ($h_1$) at the presently operating hadron colliders in the framework of the CP violating MSSM using the PYTHIA event generator. We consider the lepton+ 4-jets+ $\met$ channel from associate $W h_2$ production, with $W h_2 \ra W h_1 h_1 \ra \ell ν_\ell b \bar b b\bar b$. We require two or three tagged $b$-jets. We explicitly consider all relevant Standard Model backgrounds, treating $c$-jets separately from light flavor and gluon jets and allowing for mistagging. We find that it is very hard to observe this signature at the Tevatron, even with 20 fb$^{-1}$ of data, in the LEP--allowed region of parameter space due to the small signal efficiency. At the LHC, a priori huge SM backgrounds can be suppressed by applying judiciously chosen kinematical selections. After all cuts, we are left with a signal cross section of around 0.5 fb, and a signal to background ratio between 1.2 and 2.9. According to our analysis this Higgs signal should be viable at the LHC in the vicinity of present LEP exclusion once 20 to 50 fb$^{-1}$ of data have been accumulated at $\sqrt{s}=14$ TeV.

hep-ph↗

arXiv2010

Hunting for CDF Multi-Muon "Ghost" Events at Collider and Fixed-Target Experiments

In 2008 the CDF collaboration discovered a large excess of events containing two or more muons, at least one of which seemed to have been produced outside the beam pipe. We investigate whether similar "ghost" events could (and should) have been seen in already completed experiments. The CDF di-muon data can be reproduced by a simple model where a relatively light X particle undergoes four-body decay. This model predicts a large number of ghost events in Fermilab fixed-target experiments E772, E789 and E866, applying the cuts optimized for analyses of Drell-Yan events. A correct description of events with more than two muons requires a more complicated model, where two X particles are produced from a very broad resonance Y. This model can be tested in fixed-target experiments only if the cut on the angles, or rapidities, of the muons can be relaxed. Either way, the UA1 experiment at the CERN ppbar collider should have observed O(100) ghost events.

hep-ph↗

arXiv2010

Direct and Indirect Detection of Neutralino Dark Matter and Collider Signatures in an $SO(10)$ Model with Two Intermediate Scales

We investigate the detectability of neutralino Dark Matter via direct and indirect searches as well as collider signatures of an $SO(10)$ model with two intermediate scales. We compare the direct Dark Matter detection cross section and the muon flux due to neutralino annihilation in the Sun that we obtain in this model with mSUGRA predictions and with the sensitivity of current and future experiments. In both cases, we find that the detectability improves as the model deviates more from mSUGRA. In order to study collider signatures, we choose two benchmark points that represent the main phenomenological features of the model: a lower value of $|μ|$ and reduced third generation sfermion masses due to extra Yukawa coupling contributions in the Renormalization Group Equations, and increased first and second generation slepton masses due to new gaugino loop contributions. We show that measurements at the LHC can distinguish this model from mSUGRA in both cases, by counting events containing leptonically decaying $Z^0$ bosons, heavy neutral Higgs bosons, or like--sign lepton pairs.

hep-ph↗

arXiv2009

Rapidity Gap Events for Squark Pair Production at the LHC

hep-ph↗

arXiv2009

Rapidity Gap Events in Squark Pair Production at the LHC

The exchange of electroweak gauginos in the $t-$ or $u-$channel allows squark pair production at hadron colliders without color exchange between the squarks. This can give rise to events where little or no energy is deposited in the detector between the squark decay products. We discuss the potential for detection of such rapidity gap events at the Large Hadron Collider (LHC). Our numerical analysis is divided into two parts. First, we evaluate in a simplified framework the rapidity gap signal at the parton level. The second part covers an analysis with full event simulation using PYTHIA as well as Herwig++, but without detector simulation. We analyze the transverse energy deposited between the jets from squark decay, as well as the probability of finding a third jet in between the two hardest jets. For the mSUGRA benchmark point SPS1a we find statistically significant evidence for a color singlet exchange contribution. The systematical differences between current versions of PYTHIA and HERWIG++ are larger than the physical effect from color singlet exchange; however, these systematic differences could be reduced by tuning both Monte Carlo generators on normal QCD di--jet data.

hep-ph↗

arXiv2009

The Thermal Abundance of Semi-Relativistic Relics

Approximate analytical solutions of the Boltzmann equation for particles that are either extremely relativistic or non-relativistic when they decouple from the thermal bath are well established. However, no analytical formula for the relic density of particles that are semi-relativistic at decoupling is yet known. We propose a new ansatz for the thermal average of the annihilation cross sections for such particles, and find a semi-analytical treatment for calculating their relic densities. As examples, we consider Majorana- and Dirac-type neutrinos. We show that such semi-relativistic relics cannot be good cold Dark Matter candidates. However, late decays of meta-stable semi-relativistic relics might have released a large amount of entropy, thereby diluting the density of other, unwanted relics.

hep-ph↗

arXiv2009

How Precisely Could We Identify WIMPs Model-Independently with Direct Dark Matter Detection Experiments

In this talk we present data analysis methods for reconstructing the mass and couplings of Weakly Interacting Massive Particles (WIMPs) by using directly future experimental data (i.e., measured recoil energies) from direct Dark Matter detection. These methods are independent of the model of Galactic halo as well as of WIMPs. The basic ideas of these methods and the feasibility and uncertainties of applying them to direct detection experiments with the next generation detectors will be discussed.

hep-ph↗

arXiv2009

Constraining the Spin-Independent WIMP-Nucleon Coupling from Direct Dark Matter Detection Data

Weakly Interacting Massive Particles (WIMPs) are one of the leading candidates for Dark Matter. For understanding the properties of WIMPs and identifying them among new particles produced at colliders (hopefully in the near future), determinations of their mass and their couplings on nucleons from direct Dark Matter detection experiments are essential. Based on our method for determining the WIMP mass model-independently from experimental data, we present a way to also estimate the spin-independent (SI) WIMP-nucleon coupling by using measured recoil energies directly. This method isindependent of the as yet unknown velocity distribution of halo WIMPs. In spite of the uncertainty of the local WIMP density (of a factor of ~ 2), at least an upper limit on the SI WIMP-nucleon coupling could be given, once two (or more) experiments with different target nuclei obtain positive signals. In a background-free environment, for a WIMP mass of 100 GeV its SI coupling on nucleons could in principle be estimated with a statistical error of only ~ 15% with just 50 events from each experiment.

hep-ph↗

arXiv2008

Neutralino Dark Matter in an SO(10) Model with Two-step Intermediate Scale Symmetry Breaking

We consider a supersymmetric Grand Unified Theory (GUT) based on the gauge group SO(10) suggested by Aulakh et al., which features two--step intermediate symmetry breaking, $SO(10) \to SU(4)_C \times SU(2)_L \times SU(2)_R \to SU(3)_C \times U(1)_{B-L} \times SU(2)_L \times SU(2)_R \to SU(3)_C \times SU(2)_L \times U(1)_Y$. {\bf $45, 54, 126+\overline{126}$} dimensional representations of Higgs superfields are employed to achieve this symmetry breaking chain. We also introduce a second, very heavy, pair of Higgs doublets, which modifies the Yukawa couplings of matter fields relative to minimal SO(10) predictions. We analyze the differences in the low energy phenomenology compared to that of mSUGRA, assuming universal soft breaking scalar masses, gaugino masses and trilinear couplings at the GUT scale. We find that thermal neutralino Dark Matter remains viable in this scenario, although for small and moderate values of $\tanβ$ the allowed region is even more highly constrained than in mSUGRA, and depends strongly on the the light neutrino masses.

hep-ph↗