arXiv subjects

Ulrich Ellwanger

Publications and source records attributed to Ulrich Ellwanger.

At least 19 recordsLinked to original sources

arXiv2024

NMSSM Explanation for Excesses in the Search for Neutralinos and Charginos and a 95 GeV Higgs Boson

The observed excesses in the search for neutralinos and charginos by ATLAS and CMS can be fitted simultaneously in the minimal supersymmetric standard model (MSSM) assuming a light higgsino mass, of magnitude less than about 250 GeV, and a compressed higgsino dominated neutralino and chargino spectrum, with $5-10\%$ mass splittings. However, light higgsinos as dark matter would have far too large direct detection cross sections. We consider the next-to-MSSM (NMSSM) with an additional singlino-like lightest supersymmetric particle (LSP) a few GeV below the next-to-lightest supersymmetric particle (NLSP). Sparticles prefer to decay first into the NLSP and remnants from the final decay into the LSP are too soft to contribute to the observed signals. Co-annihilation in the higgsino-sector can generate a relic density in the WMAP/Planck window. The singlino-like LSP has automatically a direct detection cross section below present and future sensitivities: a direct detection signal in the near future would exclude this scenario. The singlet-like Higgs scalar of the NMSSM can have a mass around 95 GeV and signal cross sections in the $b\bar{b}$ channel at LEP and in the $\gamma\gamma$ channel at the LHC compatible with the respective observations.

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arXiv2024

NMSSM with correct relic density and an additional 95 GeV Higgs boson

We investigate whether it is possible within the NMSSM to describe simultaneously a correct dark matter relic density complying with the latest null results from the LZ experiment, an extra Higgs boson with a mass of ~95 GeV visible in the bb channel at LEP and the diphoton channel at the LHC, and the deviation of the anomalous magnetic moment of the muon from its value within the Standard Model. We find that this is still possible for a variety of dark matter annihilation mechanisms, for singlino-like but also bino-like lightest supersymmetric particles. We show the signal rates of the extra Higgs boson and the dark matter detection rates as function of the dark matter mass and annihilation mechanism. The dark matter direct detection cross section may well fall below the neutrino floor. The masses of the lightest electroweakly interacting supersymmetric particles are typically not far above 100 GeV, but not excluded due to unconventional decays.

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arXiv2024

Benchmark Lines and Planes for Higgs-to-Higgs Decays in the NMSSM

A number of benchmark scenarios for NMSSM Higgs boson searches via Higgs-to-Higgs decays at the LHC have been proposed by the NMSSM Subgroup of the LHC HWG3. Some of them are already in use by the ATLAS and CMS collaborations for the interpretation of their results from Run 2. In this document we summarize the theory setup, the underlying procedures and reproduce the benchmark scenarios in table form.

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arXiv2023

Additional Higgs Bosons near 95 and 650 GeV in the NMSSM

Hints for an additional Higgs boson with a mass of about 95 GeV originate from LEP and searches in the diphoton channel by CMS and ATLAS. A search for resonant production of SM plus BSM Higgs bosons in the diphoton plus bb channel by CMS showed some excess for a 650 GeV resonance decaying into the SM Higgs plus a 95 GeV Higgs boson. We investigate whether these phenomena can be interpreted simultaneously within the NMSSM subject to the latest constraints on couplings of the SM Higgs boson, on extra Higgs bosons from the LHC, and on dark matter direct detection cross sections. We find that the hints for a 95 GeV Higgs boson in the diphoton channel by CMS and ATLAS and in the diphoton plus bb channel by CMS can be fitted simultaneously within the 2 sigma level.

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arXiv2022

$M_W$, Dark Matter and $a_μ$ in the NMSSM

We study regions in the parameter space of the NMSSM which are able to simultaneously explain the current measured values for the $W$ mass $M_W$ and the muon anomalous magnetic moment $a_μ$, and provide a dark matter relic density consistent with the observations as well as constraints from detection experiments. The corresponding regions feature light charginos, sleptons and staus in the 100-800~GeV range, at least some of them with masses below 150~GeV such that the electroweakly-interacting SUSY particles generate sufficiently large contributions to $M_W$. The LSP is always singlino-like with a mass below 140~GeV, and could possibly remain invisible even at future detection experiments. Decays of electroweak sparticles proceed through cascades via staus and/or sleptons which makes their detection challenging. We propose benchmark points for future searches of such sparticles. The lightest CP-even scalar may have a mass in the 95-98~GeV range with, however, modest signal rates in view of the mild excesses reported in this range at LEP and by CMS at the LHC.

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arXiv2022

Benchmark Planes for Higgs-to-Higgs Decays in the NMSSM

We present benchmark planes (or lines) with cross sections via gluon fusion for the processes $H\to h+H_{S}$, resonant Higgs pair and triple Higgs production, $A \to h+(A_S \to γγ)$, $A \to Z+ h$ and $A \to Z+ H_{S}$ within the Next-to-Minimal Supersymmetric Standard Model. Moreover we propose new searches for $H\to h+(H_{S} \to t\bar{t})$, $A \to Z+ (H_{S}\to h+h)$ and $A \to Z+ (H_{S}\to t\bar{t})$ for which possible cross sections are given. These allow the experimental collaborations to verify in the future which search channels cover yet unexplored regions of the parameter space. Expressions for the dominant contributions to trilinear Higgs couplings and Higgs--Z couplings are discussed which allow to identify the dominant processes contributing to a given final state.

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arXiv2021

Gradient Flow and Holography from a Local Wilsonian Cutoff

We consider the vacuum partition function of a 4d scalar QFT in a curved background as function of bare marginal and relevant couplings. A local UV cutoff $Λ(x)$ transforming under Weyl rescalings allows to construct Weyl invariant kinetic terms including Wilsonian cutoff functions. The local cutoff can be absorbed completely by a rescaling of the metric and the bare couplings. The vacuum partition function satisfies consistency conditions which follow from the Abelian nature of local redefinitions of the cutoff, and which differ from Weyl rescalings. These imply a gradient flow for beta functions describing the cutoff dependence of rescaled bare couplings. The consistency conditions allow to satisfy all but one Hamiltonian constraints required for a holographic description of the flow of bare couplings with the cutoff.

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arXiv2021

Weyl Consistency Conditions from a local Wilsonian Cutoff

A local UV cutoff $Λ(x)$ transforming under Weyl rescalings allows to construct Weyl invariant kinetic terms for scalar fields including Wilsonian cutoff functions. First we consider scalar fields in curved space-time with local bare couplings of any canonical dimension, and anomalous dimensions which describe their dependence on the UV cutoff. The local component of the UV cutoff plays the role of an additional coupling, albeit with a trivial constant $β$ function. This approach allows to derive Weyl consistency conditions for the corresponding anomalous dimensions which assume the form of an exact gradient flow. For renormalizable theories the Weyl consistency conditions are initially of the form of an approximate gradient flow for the $β$ functions, and we derive conditions under which it becomes the form of an exact gradient flow.

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arXiv2018

Gradient flows for $β$ functions via multi-scale renormalization group equations

Renormalization schemes and cutoff schemes allow for the introduction of various distinct renormalization scales for distinct couplings. We consider the coupled renormalization group flow of several marginal couplings which depend on just as many renormalization scales. The usual $β$ functions describing the flow with respect to a common global scale are assumed to be given. Within this framework one can always construct a metric and a potential in the space of couplings such that the $β$ functions can be expressed as gradients of the potential. Moreover the potential itself can be derived explicitly from a prepotential which, in turn, determines the metric. Some examples of renormalization group flows are considered, and the metric and the potential are compared to expressions obtained elsewhere.

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arXiv2018

The higgsino-singlino sector of the NMSSM: Combined constraints from dark matter and the LHC

A light singlino is a promising candidate for dark matter, and a light higgsino is natural in the parameter space of the NMSSM. We study the combined constraints on this scenario resulting from the dark matter relic density, the most recent results from direct detection experiments, LEP and the LHC. In particular limits from a recent search for electroweak production of charginos and neutralinos at $\sqrt{s}=13$ TeV after 35.9 fb$^{-1}$ by CMS and constraints on spin-independent dark matter-nucleon cross sections from XENON1T after one tonne$\times$year exposure are considered. We find that scenarios with higgsino masses below $\sim 250$ GeV as well as singlino masses below $\sim 100$ GeV are strongly constrained depending, however, on assumptions on the bino mass parameter $M_1$. Benchmark points and branching fractions for future searches at the LHC are proposed.

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arXiv2017

Simultaneous Search for Extra Light and Heavy Higgs Bosons via Cascade Decays

Models with extended Higgs sectors can contain several additional Higgs states, heavier or lighter than the SM Higgs boson. The couplings of lighter extra states to SM particles can be strongly reduced, leading to small cross sections for their direct production. Heavier extra states can have larger couplings to SM particles and, moreover, have large branching fractions into lighter extra states, notably into a SM-like Higgs boson accompagnied by another Higgs state which can be lighter or heavier than 125$\sim$GeV. Motivated by corresponding scenarios in the NMSSM we study the prospects for the discovery or exclusion of cascade decays $ggF \to H_3 \to H_2 + H_1$ in the $b\bar{b}b\bar{b}$, $b\bar{b}ττ$ and $b\bar{b}γγ$ final states where either $H_1$ or $H_2$ can be SM-like. Significant regions of the NMSSM parameter space can be tested by these searches. These are, however, not confined to models of the NMSSM type.

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arXiv2017

Present Status and Future Tests of the Higgsino-Singlino Sector in the NMSSM

The light higgsino-singlino scenario of the NMSSM allows to combine a naturally small $μ$ parameter with a good dark matter relic density. Given the new constraints on spin-dependent and spin-independent direct detection cross sections in 2016 we study first which regions in the plane of chargino- and LSP-masses below 300 GeV remain viable. Subsequently we investigate the impact of searches for charginos and neutralinos at the LHC, and find that the limits from run I do not rule out any additional region in this plane. Only the HL-LHC at 3000 fb$^{-1}$ will test parts of this plane corresponding to higgsino-like charginos heavier than 150 GeV and relatively light singlinos, but notably the most natural regions with lighter charginos seem to remain unexplored.

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arXiv2016

Possible Explanation of the Electron Positron Anomaly at 17 MeV in $^8Be$ Transitions Through a Light Pseudoscalar

We estimate the values of Yukawa couplings of a light pseudoscalar A with a mass of about 17 MeV, which would explain the $^8Be$ anomaly observed in the Atomki pair spectrometer experiment. The resulting couplings of A to up and down type quarks are about 0.3 times the coupling of the standard Higgs boson. Then constraints from K and B decays require that loop contributions to flavour changing vertices cancel at least at the 10% level. Constraints from beam dump experiments require the coupling of A to electrons to be larger than about 4 times the coupling of the standard Higgs boson, leading to a short enough A life time consistent with an explanation of the anomaly.

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arXiv2016

A 750 GeV Diphoton Signal from a Very Light Pseudoscalar in the NMSSM

The excess of events in the diphoton final state near 750 GeV observed by ATLAS and CMS can be explained within the NMSSM near the R-symmetry limit. Both scalars beyond the Standard Model Higgs boson have masses near 750 GeV, mix strongly, and share sizeable production cross sections in association with b-quarks as well as branching fractions into a pair of very light pseudoscalars. Pseudoscalars with a mass of ~ 210 MeV decay into collimated diphotons, whereas pseudoscalars with a mass of ~ 500-550 MeV can decay either into collimated diphotons or into three pi^0 resulting in collimated photon jets. Various such scenarios are discussed; the dominant constraints on the latter scenario originate from bounds on radiative Upsilon decays, but they allow for a signal cross section up to 6.7 fb times the acceptance for collimated multiphotons to pass as a single photon.

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arXiv2016

Higgs mass predictions of public NMSSM spectrum generators

The publicly available spectrum generators for the NMSSM often lead to different predictions for the mass of the standard model-like Higgs boson even if using the same renormalization scheme and two-loop accuracy. Depending on the parameter point, the differences can exceed 5 GeV, and even reach 8 GeV for moderate superparticle masses of up to 2 TeV. It is shown here that these differences can be traced back to the calculation of the running standard model parameters entering all calculations, to the approximations used in the two-loop corrections included in the different codes, and to different choices for the renormalization conditions and scales. In particular, the importance of the calculation of the top Yukawa coupling is pointed out.

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arXiv2016

Discovery Prospects of a Light Scalar in the NMSSM

We study a region in the NMSSM parameter space in which the mass of the SM-like Higgs boson is uplifted by ~4-17 GeV, allowing for stop masses and |A_t| <= 1 TeV alleviating the little fine tuning problem of the MSSM. An uplift of the mass of the SM-like Higgs boson is possible in two distinct regions in the NMSSM parameter space: Either for large lambda and small tan(beta) or, through singlet-doublet mixing, for small lambda and large tan(beta). For a mostly singlet-like Higgs state H_S with a mass below 125 GeV we investigate possible direct or indirect search channels at the run II of the LHC as function of the NMSSM-specific uplift of the mass of the SM-like Higgs boson: Direct production of H_S in gluon fusion with H_S decaying into diphotons, modified reduced couplings of the SM-like Higgs state, and the possible production of H_S in ggF -> A -> Z + H_S. We find that the region featuring singlet-doublet mixing can be tested if searches at the LHC at 13 TeV for BSM Higgs bosons in the mass range 88 - 102 GeV decaying into diphotons become sensitive to signal cross sections sigma(gg -> H_S -> diphotons) ~ 20 fb, or if measurements of the reduced coupling kappa_V(H_{SM}) of the SM Higgs boson to electroweak gauge boson exclude (or confirm) the region kappa_V(H_{SM}) <~ 0.93.

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arXiv2015

Possible explanation of excess events in the search for jets, missing transverse momentum and a Z boson in pp collisions

We study to which extent SUSY extensions of the Standard Model can describe the excess of events of 3.0 standard deviations observed by ATLAS in the on-Z signal region, respecting constraints by CMS on similar signal channels as well as constraints from searches for jets and E^{miss}_T. GMSB-like scenarios are typically in conflict with these constraints, and do not reproduce well the shape of the E^{miss}_T distribution of the data. An alternative scenario with two massive neutralinos can improve fits to the total number of events as well as to the H_T and E^{miss}_T distributions. Such a scenario can be realised within the NMSSM.

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arXiv2014

Excessive Higgs pair production with little MET from squarks and gluinos in the NMSSM

In the presence of a light singlino-like LSP in the NMSSM, the missing transverse energy - MET - signature of squark/gluino production can be considerably reduced. Instead, a pair of Higgs bosons is produced in each event. We propose benchmark points for such scenarios, which differ in the squark and gluino masses, and in their decay cascades. Events for these points are simulated for the run II of the LHC at 13 TeV centre of mass energy. After cuts on the transverse momenta of at least four jets, and requiring two tau-leptons from one Higgs decay, we find that the invariant mass of two b-jets from the other Higgs decay shows clear peaks above the background. Despite the reduced MET, this search strategy allows to see signals for sufficiently large integrated luminosities, depending on the squark/gluino masses.

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