arXiv subjects

Herbi Dreiner

Publications and source records attributed to Herbi Dreiner.

At least 19 recordsLinked to original sources

arXiv2025

A Proposal for the Lohengrin Experiment to Search for Dark Sector Particles at the ELSA Accelerator

We present a proposal for a future light dark matter search experiment at the Electron Stretcher Accelerator ELSA in Bonn: Lohengrin. It employs the fixed-target missing momentum based technique for searching for dark-sector particles. The Lohengrin experiment uses a high intensity electron beam that is shot onto a thin target to produce mainly SM bremsstrahlung and - in rare occasions - possibly new particles coupling feebly to the electron. A well motivated candidate for such a new particle is the dark photon, a new massive gauge boson arising from a new gauge interaction in a dark sector and mixing kinetically with the standard model photon. The Lohengrin experiment is estimated to reach sensitivity to couplings small enough to explain the relic abundance of dark matter in various models for dark photon masses between approximately 1 MeV and approximately 100 MeV.

hep-ex↗

arXiv2017

R-Parity Violation at the LHC

We investigate the phenomenology of the MSSM extended by a single R-parity violating coupling at the unification scale. For all R-parity violating couplings, we discuss the evolution of the particle spectra through the renormalization group equations and the nature of the lightest supersymmetric particle (LSP) within the CMSSM, as an example of a specific complete supersymmetric model. We use the nature of the LSP to classify the possible signatures. For each possible scenario we present in detail the current LHC bounds on the supersymmetric particle masses, typically obtained using simplified models. From this we determine the present coverage of R-parity violating models at the LHC. We find several gaps, in particular for a stau-LSP, which is easily obtained in R-parity violating models. Using the program CheckMATE we recast existing LHC searches to set limits on the parameters of all R-parity violating CMSSMs. We find that virtually all of them are either more strongly constrained or similarly constrained in comparison to the R-parity conserving CMSSM, including the $\bar U\bar D\bar D$ models. For each R-parity violating CMSSM we then give the explicit lower mass bounds on all relevant supersymmetric particles.

hep-ph↗

arXiv2015

A facility to Search for Hidden Particles at the CERN SPS: the SHiP physics case

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (Search for Hidden Particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, $τ\to 3μ$ and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the Standard Model and describe interactions between new particles and four different portals - scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the Standard Model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation

hep-ph↗

arXiv2014

CheckMATE: Confronting your Favourite New Physics Model with LHC Data

In the first three years of running, the LHC has delivered a wealth of new data that is now being analysed. With over 20 fb$^{-1}$ of integrated luminosity, both ATLAS and CMS have performed many searches for new physics that theorists are eager to test their model against. However, tuning the detector simulations, understanding the particular analysis details and interpreting the results can be a tedious task. CheckMATE (Check Models At Terascale Energies) is a program package which accepts simulated event files in many formats for any model. The program then determines whether the model is excluded or not at 95% C.L. by comparing to many recent experimental analyses. Furthermore the program can calculate confidence limits and provide detailed information about signal regions of interest. It is simple to use and the program structure allows for easy extensions to upcoming LHC results in the future. CheckMATE can be found at: http://checkmate.hepforge.org

hep-ph↗

arXiv2013

Contact Interactions Probe Effective Dark Matter Models at the LHC

Effective field theories provide a simple framework for probing possible dark matter (DM) models by reparametrising full interactions into a reduced number of operators with smaller dimensionality in parameter space. In many cases these models have four particle vertices, e.g. qqXX, leading to the pair production of dark matter particles, X, at a hadron collider from initial state quarks, q. In this analysis we show that for many fundamental DM models with s-channel DM couplings to qq-pairs, these effective vertices must also produce quark contact interactions (CI) of the form qqqq. The respective effective couplings are related by the common underlying theory which allows one to translate the upper limits from one coupling to the other. We show that at the LHC, the experimental limits on quark contact interactions give stronger translated limits on the DM coupling than the experimental searches for dark matter pair production.

hep-ph↗

arXiv2012

Constrained Supersymmetry after two years of LHC data: a global view with Fittino

We perform global fits to the parameters of the Constrained Minimal Supersymmetric Standard Model (CMSSM) and to a variant with non-universal Higgs masses (NUHM1). In addition to constraints from low-energy precision observables and the cosmological dark matter density, we take into account the LHC exclusions from searches in jets plus missing transverse energy signatures with about 5\,fb$^{-1}$ of integrated luminosity. We also include the most recent upper bound on the branching ratio $B_s\toμμ$ from LHCb. Furthermore, constraints from and implications for direct and indirect dark matter searches are discussed. The best fit of the CMSSM prefers a light Higgs boson just above the experimentally excluded mass. We find that the description of the low-energy observables, $(g-2)_μ$ in particular, and the non-observation of SUSY at the LHC become more and more incompatible within the CMSSM. A potential SM-like Higgs boson with mass around 126 GeV can barely be accommodated. Values for ${\cal B}(B_s\toμμ)$ just around the Standard Model prediction are naturally expected in the best fit region. The most-preferred region is not yet affected by limits on direct WIMP searches, but the next generation of experiments will probe this region. Finally, we discuss implications from fine-tuning for the best fit regions.

hep-ph↗

arXiv2011

Testing the CP-violating MSSM in stau decays at the LHC and ILC

We study CP violation in the two-body decay of a scalar tau into a neutralino and a tau, which should be probed at the LHC and ILC. From the normal tau polarization, a CP asymmetry is defined which is sensitive to the CP phases of the trilinear scalar coupling parameter $A_τ$, the gaugino mass parameter $M_1$, and the higgsino mass parameter $μ$ in the stau-neutralino sector of the Minimal Supersymmetric Standard Model. Asymmetries of more than 70% are obtained in scenarios with strong stau mixing. As a result, detectable CP asymmetries in stau decays at the LHC are found, motivating further detailed experimental studies for probing the SUSY CP phases.

hep-ph↗

arXiv2003

Bounds on Broken R-Parity from Leptonic Meson Decays

Investigating leptonic decays of pi^-, K^-, B^-, pi^0, K_L^0, B_s^0 we present new bounds on some products of two R-parity violating coupling constants. For mesons of a similar structure but so far poor experimental data we give the corresponding formulae, to be used in the future.

hep-ph↗

arXiv2002

Bounds on Broken R-Parity from NOMAD and CHORUS

We determine new constraints on the products of two R-parity violating coupling constants from the NOMAD and CHORUS experiments on nu_mu --> nu_tau oscillations. We obtain improved results from (a) lepton flavour violating (LFV) meson decays, (b) LFV deep-inelastic scattering and (c) a combination of the two.

hep-ph↗

arXiv2001

Attempts at Explaining the NuTeV Observation of Di-Muon Events

The NuTeV Collaboration has observed an excess in their di-muon channel, possibly corresponding to a long-lived neutral particle with only weak interactions and which decays to muon pairs. We show that this can {\it not} be explained by pair production of neutralinos in the target followed by their decay far downstream in the detector via a $LLE$ R-parity violating (RPV) operator, as suggested in the literature. In the parameter region allowed by LEP the event rate is far too small. We propose instead a new neutralino production method via $B$-mesons, which can fully explain the observation. This is analogous to neutrino production via $π$-mesons. This model can be completely tested and thus also possibly excluded with NOMAD data. If it is excluded, the NuTeV observation is most likely not due to physics beyond the Standard Model. Our model can also be tested at the current and future $B$-factories. This opens up a new way of testing a long-lived neutralino LSP at fixed-target experiments and thus a possibility to close the gap between collider and cosmological tests of R-parity violation. We also discuss a possible explanation in terms of a neutral heavy lepton mixing with the Standard Model neutrinos. The flavour structure of the observation can be accounted for but the production rate is far too low.

hep-ph↗

arXiv2000

Resonant Slepton Production in Hadron-Hadron Collisions

We consider the resonant production of sleptons via R-parity violation in hadron-hadron collisions followed by supersymmetric gauge decays of the sleptons. We look at decay modes which lead to the production of a like-sign dilepton pair. The dominant production mechanism giving this signature is the resonant production of a charged slepton followed by a decay to a charged lepton and a neutralino which then decays via R-parity violation. The discovery potential of this process at Run II of the Tevatron and the LHC is investigated using the HERWIG Monte Carlo event generator. We include the backgrounds from the MSSM. We conclude with a discussion of the possibility of extracting the lightest neutralino and slepton masses.

hep-ph↗

arXiv2000

Parton-Shower Simulations of R-parity Violating Supersymmetric Models

We study the colour connection structure of R-parity violating decays and production cross sections, and construct a Monte Carlo simulation of these processes including colour coherence effects. We then present some results from the implementation of these processes in the HERWIG Monte Carlo event generator. We include the matrix elements for the two-body sfermion and three-body gaugino and gluino decays as well as the two-to-two resonant hard production processes in hadron-hadron collisions.

hep-ph↗

arXiv2000

A supersymmetric solution to the KARMEN time anomaly

We interpret the KARMEN time anomaly as being due to the production of a (dominantly bino) neutralino with mass 33.9 MeV, which is the lightest supersymmetric particle but decays into 3 leptons through the violation of R-parity. For independent gaugino masses M_1 and M_2 we find regions in the (M_1, M_2, mu, tan beta) parameter space where such a light neutralino is consistent with all experiments. Future tests of this hypothesis are outlined.

hep-ph↗

arXiv2000

Resonant Slepton Production at the LHC

We consider the resonant production of charged sleptons at the LHC via R-parity violation followed by gauge decays to a charged lepton and a neutralino which then decays via R-parity violation. This gives a signature of two like-sign charged leptons. In the simulation we include the full hadronisation via Monte Carlo programs. We find a background, after cuts, of 5.1+/-2.5 events for an integrated luminosity of 10 fb^-1. A preliminary study of the signal suggests that couplings of 2.10^-3 for a smuon mass of 223 GeV and smuon masses of up to 540 GeV for couplings of 10^-2 can be probed.

hep-ph↗

arXiv1999

Resonant Slepton Production

We consider the production of resonant sleptons via R-parity Violation followed by gauge decays to a charged lepton and a neutralino which then decays via R-parity Violation. This gives a signature of two like-sign charged leptons. We find a background at run II of 0.14 +/- 0.13 events with an integrated luminosity of 2 fb^-1. This enables us to probe R-parity Violating couplings of 2.10^-3 for slepton mass of 100 GeV and up to slepton masses of 300 GeV for R-parity Violating couplings of 10^-2.

hep-ph↗

arXiv1999

Hide and Seek with Supersymmetry

This is the summary of a 90 minute introductory lecture on Supersymmetry presented at the Swiss Summer School on ``Hidden Symmetries and Higgs Phenomena'' at Zuoz, Engadin, in August 1998. I first review the hierarchy problem, and then discuss why we expect supersymmetry just around the corner, i.e. at or below $1\tev$. I focus on the specific example of the anomalous magnetic moment of the muon to show how supersymmetry can indeed hide just around the corner without already having been detected. An essential part of supersymmetry's disguise is the fact that it is broken. I end by briefly outlining how the disguise itself is also hidden.

hep-ph↗

arXiv1997

An Introduction to Explicit R-Parity Violation

I discuss the theoretical motivations for R-parity violation, review the experimental bounds and outline the main changes in collider phenomenology compared to conserved R-parity. I briefly comment on the effects of R-parity violation on cosmology.

hep-ph↗

arXiv1997

R-Parity Violating Supersymmetry at HERA

The phenomenology and prospects for a discovery of R-parity violating Supersymmetry at HERA are analysed. Emphasis is put on the direct resonant production of squarks by electron-quark fusion and all possible subsequent decay modes of the squarks are considered. In particular, the full consequences of the mixing in the supersymmetric gaugino-higgsino sector are taken into account. A rich phenomenology emerges for HERA which offers a unique sensitivity to new R-parity violating couplings and good discriminating power against free parameters of the theory.

hep-ph↗