arXiv subjects

M. M. Muhlleitner

Publications and source records attributed to M. M. Muhlleitner.

16 recordsLinked to original sources

arXiv2020

Higgs-Pair Production via Gluon Fusion at Hadron Colliders: NLO QCD Corrections

Higgs-pair production via gluon fusion is the dominant production mechanism of Higgs-boson pairs at hadron colliders. In this work, we present details of our numerical determination of the full next-to-leading-order (NLO) QCD corrections to the leading top-quark loops. Since gluon fusion is a loop-induced process at leading order, the NLO calculation requires the calculation of massive two-loop diagrams with up to four different mass/energy scales involved. With the current methods, this can only be done numerically, if no approximations are used. We discuss the setup and details of our numerical integration. This will be followed by a phenomenological analysis of the NLO corrections and their impact on the total cross section and the invariant Higgs-pair mass distribution. The last part of our work will be devoted to the determination of the residual theoretical uncertainties with special emphasis on the uncertainties originating from the scheme and scale dependence of the (virtual) top mass. The impact of the trilinear Higgs-coupling variation on the total cross section will be discussed.

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arXiv2015

Physics at the e+ e- Linear Collider

A comprehensive review of physics at an e+e- Linear Collider in the energy range of sqrt{s}=92 GeV--3 TeV is presented in view of recent and expected LHC results, experiments from low energy as well as astroparticle physics.The report focuses in particular on Higgs boson, Top quark and electroweak precision physics, but also discusses several models of beyond the Standard Model physics such as Supersymmetry, little Higgs models and extra gauge bosons. The connection to cosmology has been analyzed as well.

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arXiv2013

The measurement of the Higgs self-coupling at the LHC: theoretical status

Now that the Higgs boson has been observed by the ATLAS and CMS experiments at the LHC, the next important step would be to measure accurately its properties to establish the details of the electroweak symmetry breaking mechanism. Among the measurements which need to be performed, the determination of the Higgs self-coupling in processes where the Higgs boson is produced in pairs is of utmost importance. In this paper, we discuss the various processes which allow for the measurement of the trilinear Higgs coupling: double Higgs production in the gluon fusion, vector boson fusion, double Higgs-strahlung and associated production with a top quark pair. We first evaluate the production cross sections for these processes at the LHC with center-of-mass energies ranging from the present $\sqrt s=8$ TeV to $\sqrt s=100$ TeV, and discuss their sensitivity to the trilinear Higgs coupling. We include the various higher order QCD radiative corrections, at next-to-leading order for gluon and vector boson fusion and at next-to-next-to-leading order for associated double Higgs production with a gauge boson. The theoretical uncertainties on these cross sections are estimated. Finally, we discuss the various channels which could allow for the detection of the double Higgs production signal at the LHC and the accuracy on the self-coupling that could be ultimately achieved.

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arXiv2012

Theoretical Basis of Higgs-Spin Analysis in $H \to γγ$ and $Zγ$ Decays

We chart the theoretical basis of radiative decays of the Higgs boson, $H \to γγ$ and $Zγ$, for measuring the spin of the Higgs particle. These decay channels are complementary to other rare modes such as real/virtual $Z$-boson pair-decays. In systematic helicity analyses the angular distribution for zero-spin is confronted with hypothetical spin-$2^\pm$ and higher assignments to quantify the sensitivity.

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arXiv2010

The Hunt for New Physics at the Large Hadron Collider

The Large Hadron Collider presents an unprecedented opportunity to probe the realm of new physics in the TeV region and shed light on some of the core unresolved issues of particle physics. These include the nature of electroweak symmetry breaking, the origin of mass, the possible constituent of cold dark matter, new sources of CP violation needed to explain the baryon excess in the universe, the possible existence of extra gauge groups and extra matter, and importantly the path Nature chooses to resolve the hierarchy problem - is it supersymmetry or extra dimensions. Many models of new physics beyond the standard model contain a hidden sector which can be probed at the LHC. Additionally, the LHC will be a top factory and accurate measurements of the properties of the top and its rare decays will provide a window to new physics. Further, the LHC could shed light on the origin of neutralino masses if the new physics associated with their generation lies in the TeV region. Finally, the LHC is also a laboratory to test the hypothesis of TeV scale strings and D-brane models. An overview of these possibilities is presented in the spirit that it will serve as a companion to the Technical Design Reports (TDRs) by the particle detector groups ATLAS and CMS to facilitate the test of the new theoretical ideas at the LHC. Which of these ideas stands the test of the LHC data will govern the course of particle physics in the subsequent decades.

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arXiv2006

Decays of Supersymmetric Particles: the program SUSY-HIT (SUspect-SdecaY-Hdecay-InTerface)

We present the program package SUSY-HIT for the computation of supersymmetric particle decays within the framework of the Minimal Supersymmetric extension of the Standard Model. The code is based on two existing programs HDECAY and SDECAY for the calculation of the decay widths and branching ratios of, respectively, the MSSM Higgs bosons and the SUSY particles, and calls a program for the calculation of the SUSY particle spectrum such as SuSpect. Including all important higher order effects, the package allows the consistent calculation of the MSSM particle spectrum and decays with the presently highest level of precision.

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arXiv2006

Les Houches "Physics at TeV Colliders 2005'' Beyond the Standard Model working group: summary report

The work contained herein constitutes a report of the "Beyond the Standard Model'' working group for the Workshop "Physics at TeV Colliders", Les Houches, France, 2-20 May, 2005. We present reviews of current topics as well as original research carried out for the workshop. Supersymmetric and non-supersymmetric models are studied, as well as computational tools designed in order to facilitate their phenomenology.

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arXiv2005

Higgs Boson Search at e+e- and Photon Linear Colliders

The various search modes for the Higgs bosons of the Standard Model (SM) and its Minimal Supersymmetric Extension (MSSM) at the International Linear Collider (ILC) will be summarized briefly. In particular, as a unique discovery mode the production of heavy neutral MSSM Higgs bosons for medium values of $\tanβ$ in photon collisions will be presented. Furthermore, $τ^+τ^-$ fusion into MSSM Higgs bosons in the photon mode will be shown to give access to the mixing parameter $\tanβ$ with a precision of better than 10% for large values of this parameter.

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arXiv2005

Supersymmetry Parameter Analysis: SPA Convention and Project

High-precision analyses of supersymmetry parameters aim at reconstructing the fundamental supersymmetric theory and its breaking mechanism. A well defined theoretical framework is needed when higher-order corrections are included. We propose such a scheme, Supersymmetry Parameter Analysis SPA, based on a consistent set of conventions and input parameters. A repository for computer programs is provided which connect parameters in different schemes and relate the Lagrangian parameters to physical observables at LHC and high energy e+e- linear collider experiments, i.e., masses, mixings, decay widths and production cross sections for supersymmetric particles. In addition, programs for calculating high-precision low energy observables, the density of cold dark matter (CDM) in the universe as well as the cross sections for CDM search experiments are included. The SPA scheme still requires extended efforts on both the theoretical and experimental side before data can be evaluated in the future at the level of the desired precision. We take here an initial step of testing the SPA scheme by applying the techniques involved to a specific supersymmetry reference point.

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arXiv2005

Elements of Physics with a Photon Collider

After a brief description of the basic principle of a photon collider, we summarize the physics potential of such a facility at high energies. Unique opportunities are provided in supersymmetric theories for the discovery of heavy scalar and pseudoscalar Higgs bosons as well as selectrons and e-sneutrinos.

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arXiv2004

Determining tan(beta) in ττFusion to SUSY Higgs Bosons at a Photon Collider

We investigate ττfusion to light h and heavy H and A Higgs bosons in the Minimal Supersymmetric Standard Model (MSSM) at a photon collider as a promising channel for measuring large values of tan(beta). For standard design parameters of a photon collider an error Δtan(beta) \sim 1, uniformly for tan(beta) \gsim 10, may be expected, improving on complementary measurements at the LHC and e+e- linear colliders.

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arXiv2002

Identifying the Higgs Spin and Parity in Decays to Z Pairs

Higgs decays to Z boson pairs may be exploited to determine spin and parity of the Higgs boson, a method complementary to spin-parity measurements in Higgs-strahlung. For a Higgs mass above the on-shell ZZ decay threshold, a model-independent analysis can be performed, but only by making use of additional angular correlation effects in gluon-gluon fusion at the LHC and gamma-gamma fusion at linear colliders. In the intermediate mass range, in which the Higgs boson decays into pairs of real and virtual Z bosons, threshold effects and angular correlations, parallel to Higgs-strahlung, may be adopted to determine spin and parity, though high event rates will be required for the analysis in practice.

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arXiv2001

Measuring the Spin of the Higgs Boson

By studying the threshold dependence of the excitation curve and the angular distribution in Higgs-strahlung at e+e- colliders, e+e- -> HZ, the spin of the Higgs boson in the Standard Model and related extensions can be determined unambiguously in a model-independent way.

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arXiv2001

Testing Higgs Self-Couplings at High-Energy Linear Colliders

In order to verify the Higgs mechanism experimentally, the Higgs self-couplings have to be probed. These couplings allow the reconstruction of the characteristic Higgs potential responsible for the electroweak symmetry breaking. The couplings are accessible in a variety of multiple Higgs production processes. The theoretical analysis including the most relevant channels for the production of neutral Higgs boson pairs at high-energy and high-luminosity $e^+e^-$ linear colliders will be presented in this note.

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arXiv2001

Production of MSSM Higgs Bosons at Future $γγ$ Colliders

Future $γγ$ colliders allow the production of the heavy neutral MSSM Higgs bosons $H$ and $A$ as single resonances. The prospects of finding these particles in the $b\bar{b}$ and the neutralino-pair final states have been analysed. The $H,A$ bosons can be discovered for medium values of $\tanβ$ with masses up to 70--80% of the initial $e^\pm e^-$ c.m. energy. This production mode thus covers parts of the supersymmetric parameter space that are not accessible at other colliders.

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arXiv2000

Higgs Particles in the Standard Model and Supersymmetric Theories

This thesis presents a theoretical analysis of the properties of the Higgs bosons in the Standard Model (SM) and the minimal supersymmetric extension (MSSM), which can be investigated at the LHC and $e^+e^-$ linear colliders. The final goal is the reconstruction of the Higgs potential and thus the verification of the Higgs mechanism. MSSM Higgs boson production processes at future $γγ$ colliders are calculated in several decay channels. Heavy scalar and pseudoscalar Higgs bosons can be discovered in the $b\bar{b}$ final state in the investigated mass range 200 to 800 GeV for moderate and large values of $\tanβ$. The $τ^+τ^-$ channel provides a heavy Higgs boson discovery potential for large values of $\tanβ$. Several mechanisms that can be exploited at $e^+e^-$ linear colliders for the measurement of the lifetime of a SM Higgs boson in the intermediate mass range are analysed. In the $WW$ mode, the lifetime of Higgs scalars with masses below $\sim 160$ GeV can be determined with an error less than 10%. The reconstruction of the Higgs potential requires the measurement of the Higgs self-couplings. The SM and MSSM trilinear Higgs self-couplings are accessible in double and triple Higgs production. A theoretical analysis is presented in the relevant channels at the LHC and $e^+e^-$ linear colliders. For high luminosities, the SM trilinear Higgs self-coupling can be measured with an accuracy of 20% at a 500 GeV $e^+e^-$ linear collider. The MSSM coupling among three light Higgs bosons has to be extracted from continuum production. The other trilinear Higgs couplings are measurable in a restricted range of the MSSM parameter space. At the LHC, the $Hhh$ coupling can be probed in resonant decays.

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