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At least 1,081 records · Page 60
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DOE OSTI2020

Electroweak Phase Transition with Spontaneous $Z_2$-Breaking

This work investigates a simple, representative extension of the Standard Model with a real scalar singlet and spontaneous $Z_2$ breaking, which allows for a strongly first-order phase transition, as required by electroweak baryogenesis. We perform analytical and numerical calculations that systematically include one-loop thermal effects, Coleman-Weinberg corrections, and daisy resummation, as well as evaluation of bubble nucleation. We study the rich thermal history and identify the conditions for a strongly first-order electroweak phase transition with nearly degenerate extrema at zero temperature. This requires a light scalar with mass below 50 GeV. Exotic Higgs decays, as well as Higgs coupling precision measurements at the LHC and future collider facilities, will test this model. Additional information may be obtained from future collider constraints on the Higgs self-coupling. Gravitational-wave signals could potentially be probed by future gravitational wave experiments.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Observational signatures of Higgs inflation

We investigate the dependency of Higgs inflation on the non-renormalisable matching between the low energy Standard Model limit and the inflationary regime at high energies. We show that for the top mass range m {sub t} ∼> 171.8 GeV the scenario robustly predicts the spectral index n {sub s} ≅ 0.97 and the tensor-to-scalar ratio r ≅ 0.003. The matching is however non-trivial, even the best-fit values m {sub h} = 125.09 GeV and m {sub t} = 173.21 GeV require a jump δλ ∼ 0.01 in the Higgs coupling below the inflationary scale. For m {sub t} ∼< 171.8 GeV, the matching may generate a feature in the inflationary potential. In this case the predicted values of n {sub s} and r vary but the model is still falsifiable. For example, a detection of negative running of spectral index at level α {sub s} ∼< −0.01 would rule out Higgs inflation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Search for Dark Photons in Rare $Z$ Boson Decays with the ATLAS Detector

A search for events with a dark photon produced in association with a dark Higgs boson via rare decays of the standard model Z boson is presented, using 139 fb –1 of $\sqrt{s}$ = 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. The dark boson decays into a pair of dark photons, and at least two of the three dark photons must each decay into a pair of electrons or muons, resulting in at least two same-flavor opposite-charge lepton pairs in the final state. The data are found to be consistent with the background prediction, and upper limits are set on the dark photon’s coupling to the dark Higgs boson times the kinetic mixing between the standard model photon and the dark photon, α D ϵ 2 , in the dark photon mass range of [5, 40] GeV except for the $Υ$ mass window [8.8, 11.1] GeV. This search explores new parameter space not previously excluded by other experiments.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Taming the off-shell Higgs boson

We study the off-shell Higgs data in the process pp → h{sup (*)} → Z{sup (*)}Z{sup (*)} → 4l, to constrain deviations of the Higgs couplings. We point out that this channel can be used to resolve the long- and short-distance contributions to Higgs production by gluon fusion and can thus be complementary to pp→htt̄ in measuring the top Yukawa coupling. Our analysis, performed in the context of effective field theory, shows that current data do not allow drawing any model-independent conclusions. We study the prospects at future hadron colliders, including the high-luminosity LHC and accelerators with higher energy, up to 100 TeV. The available QCD calculations and the theoretical uncertainties affecting our analysis are also briefly discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Search for a light pseudoscalar Higgs boson in the boosted $\mu\mu\tau\tau$ final state in proton-proton collisions at $\sqrt{s}=$ 13 TeV

A search for a light pseudoscalar Higgs boson (a) decaying from the 125 GeV (or a heavier) scalar Higgs boson (H) is performed using the 2016 LHC proton-proton collision data at $ \sqrt{s} $ = 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{−1}$, collected by the CMS experiment. The analysis considers gluon fusion and vector boson fusion production of the H, followed by the decay H → aa → μμττ, and considers pseudoscalar masses in the range 3.6 < m$_{a}$< 21 GeV. Because of the large mass difference between the H and the a bosons and the small masses of the a boson decay products, both the μμ and the ττ pairs have high Lorentz boost and are collimated. The ττ reconstruction efficiency is increased by modifying the standard technique for hadronic τ lepton decay reconstruction to account for a nearby muon. No significant signal is observed. Model-independent limits are set at 95% confidence level, as a function of m$_{a}$, on the branching fraction (ℬ) for H → aa → μμττ, down to 1.5 (2.0) × 10$^{−4}$ for m$_{H}$ = 125 (300) GeV. Model-dependent limits on ℬ(H → aa) are set within the context of two Higgs doublets plus singlet models, with the most stringent results obtained for Type-III models. These results extend current LHC searches for heavier a bosons that decay to resolved lepton pairs and provide the first such bounds for an H boson with a mass above 125 GeV.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at $ \sqrt{s} $ = 13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at s√ = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb -1 , collected in 2017–2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb -1 , collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Searches for invisible decays of the Higgs boson in pp collisions at $\sqrt{s}$ = 7, 8, and 13 TeV

Searches for invisible decays of the Higgs boson are presented. The data collected with the CMS detector at the LHC correspond to integrated luminosities of 5.1, 19.7, and 2.3 fb$^{−1}$ at centre-of-mass energies of 7, 8, and 13 TeV, respectively. The search channels target Higgs boson production via gluon fusion, vector boson fusion, and in association with a vector boson. Upper limits are placed on the branching fraction of the Higgs boson decay to invisible particles, as a function of the assumed production cross sections. The combination of all channels, assuming standard model production, yields an observed (expected) upper limit on the invisible branching fraction of 0.24 (0.23) at the 95% confidence level. The results are also interpreted in the context of Higgs-portal dark matter models.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Search for Higgs boson pair production in the γγ$b\overline{b} $ final state with 13 TeV pp collision data collected by the ATLAS experiment

A search is performed for resonant and non-resonant Higgs boson pair production in the γγ$b\overline{b} $ final state. The data set used corresponds to an integrated luminosity of 36.1 fb -1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess relative to the Standard Model expectation is observed. The observed limit on the non-resonant Higgs boson pair cross-section is 0.73 pb at 95% confidence level. This observed limit is equivalent to 22 times the predicted Standard Model cross-section. The Higgs boson self-coupling (κ λ = λ HHH /λ HHH SM ) is constrained at 95% confidence level to -8.2 < κ λ < 13.2. For resonant Higgs boson pair production through X→HH→γγ$b\overline{b} $, the limit is presented, using the narrow-width approximation, as a function of mX in the range 260 GeV < m X < 1000 GeV. The observed limits range from 1.1 pb to 0.12 pb over this mass range. Open image in new window

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Enhanced lines and box-shaped features in the gamma-ray spectrum from annihilating dark matter in the NMSSM

We study spectral features in the gamma-ray emission from dark matter (DM) annihilation in the Next-to-Minimal Supersymmetric Standard Model (NMSSM), with either neutralino or right-handed (RH) sneutrino DM . We perform a series of scans over the NMSSM parameter space, compute the DM annihilation cross section into two photons and the contribution of box-shaped features, and compare them with the limits derived from the Fermi-LAT search for gamma-ray lines using the latest Pass 8 data. We implement the LHC bounds on the Higgs sector and on the masses of supersymmetric particles as well as the constraints on low-energy observables. We also consider the recent upper limits from the Fermi-LAT satellite on the continuum gamma-ray emission from dwarf spheroidal galaxies (dSphs). We show that in the case of the RH sneutrino the constraint on gamma-ray spectral features can be more stringent than the dSph bounds. This is due to the Breit-Wigner enhancement near the ubiquitous resonances with a CP even Higgs and the contribution of scalar and pseudoscalar Higgs final states to box-shaped features. By contrast, for neutralino DM, the di-photon final state is only enhanced in the resonance with a Z boson and box-shaped features are even more suppressed. Therefore, the observation of spectral features could constitute a discriminating factor between both models. In addition, we compare our results with direct DM searches, including the SuperCDMS and LUX limits on the elastic DM-nucleus scattering cross section and show that some of these scenarios would be accessible to next generation experiments. Thus, our findings strengthen the idea of complementarity among distinct DM search strategies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Search for doubly charged scalar bosons decaying into same-sign W boson pairs with the ATLAS detector

A search for doubly charged scalar bosons decaying into W boson pairs is presented. It uses a data sample from proton–proton collisions corresponding to an integrated luminosity of 36.1 fb -1 collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13 TeV in 2015 and 2016. This search is guided by a model that includes an extension of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons H &#x00B1;<!-- ± --> &#x00B1;<!-- ± --> . Those bosons are produced in pairs in proton–proton collisions and decay predominantly into electroweak gauge bosons H &#x00B1;<!-- ± --> &#x00B1;<!-- ± --> &#x2192;<!-- → --> W &#x00B1;<!-- ± --> W &#x00B1;<!-- ± --> . Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for H &#x00B1;<!-- ± --> &#x00B1;<!-- ± --> bosons with masses between 200 and 220 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Search for Dark Matter Produced in Association with a Higgs Boson Decaying to b b &#xAF; using 36 fb &#x2212; 1 of p p collisions at s = 13 TeV with the ATLAS Detector

CERN-LHC. Several extensions of the Standard Model predict associated production of dark-matter particles with a Higgs boson. Such processes are searched for in final states with missing transverse momentum and a Higgs boson decaying to a bb pair with the ATLAS detector using 36.1 fb-1 of pp collisions at a center-of-mass energy of 13 TeV at the LHC. The observed data are in agreement with the Standard Model and limits are placed on the associated production of dark-matter particles and a Higgs boson.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Vectorlike leptons: Muon g - 2 anomaly, lepton flavor violation, Higgs boson decays, and lepton nonuniversality

Here, we discuss the Standard Model (SM) with one family of vectorlike (VL) leptons, which couple to all three families of the SM leptons. We study the constraints on this model coming from the heavy-charged lepton mass bound, electroweak precision data, the muon anomalous magnetic moment, lepton flavor violation, Higgs boson decay constraints, and a recently measured lepton nonuniversality observable, R K * 0 , along with R K . We find that the strongest constraints are coming from the muon g - 2 , R μ μ = Γ ( h → μ μ ) / Γ ( h → μ μ ) SM , R γ γ and BR ( μ → e γ ) . While VL leptons couple to all three families of the SM leptons, the ratio of electron-VL to muon-VL coupling is constrained to be 〈 λ e / λ μ 〉 ≲ 10 - 4 . We also find that this model cannot fit the lepton nonuniversality discrepancies.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

125 GeV Higgs boson mass from 5D gauge-Higgs unification

In the context of a simple gauge-Higgs unification (GHU) scenario based on the gauge group SU( 3 )××U(1)" in a 5D flat space-time, we investigate the possibility of reproducing the observed Higgs boson mass of around 125 GeV. We introduce bulk fermion multiplets with a bulk mass and a (half-)periodic boundary condition. In our analysis, we adopt a low-energy effective theoretical approach of the GHU scenario, where the running Higgs quartic coupling is required to vanish at the compactification scale. Under this “gauge-Higgs condition,” we investigate the renormalization group evolution of the Higgs quartic coupling and find a relation between the bulk mass and the compactification scale so as to reproduce the 125 GeV Higgs boson mass. Through quantum corrections at the one-loop level, the bulk fermions contribute to the Higgs boson production and decay processes and deviate the Higgs boson signal strengths at the Large Hadron Collider experiments from the Standard Model (SM) predictions. Employing the current experimental data that show that the Higgs boson signal strengths for a variety of Higgs decay modes are consistent with the SM predictions, we obtain lower mass bounds on the lightest mode of the bulk fermions to be around 1 TeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Alignment without decoupling: The portal to light dark matter in the MSSM

Here we study light, thermal neutralino dark matter in the sub-GeV to ~65 GeV mass range in the minimal supersymmetric extension of the Standard Model. We consider realizations of the limit of alignment without decoupling in the Higgs sector where the heavier CP-even Higgs impersonates the observed Higgs state at 125 GeV, while the lighter CP-even Higgs is the mediator of dark matter annihilation. We single out three distinct and novel possibilities for light dark matter: (i) a neutralino with mass around half the light Higgs mass, in the sub-GeV to ~30 GeV mass range; (ii) a neutralino with a mass around half the pseudoscalar Higgs boson mass, in our examples around (60 – 65) GeV; (iii) a very light neutralino with mass around the light Higgs mass, pair annihilating to Higgs pairs. We discuss the implications of all these possibilities for indirect and direct dark matter detection experiments, and we demonstrate that all scenarios will be tested by next generation direct detection experiments. We also emphasize that the unique Higgs phenomenology of this setup warrants a dedicated search program at the LHC.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Minimal mirror twin Higgs

In a Mirror Twin World with a maximally symmetric Higgs sector the little hierarchy of the Standard Model can be significantly mitigated, perhaps displacing the cutoff scale above the LHC reach. We show that consistency with observations requires that the Z 2 parity exchanging the Standard Model with its mirror be broken in the Yukawa couplings. A minimal such effective field theory, with this sole Z 2 breaking, can generate the Z 2 breaking in the Higgs sector necessary for the Twin Higgs mechanism. The theory has constrained and correlated signals i n Higgs decays, direct Dark Matter Detection and Dark Radiation, all within reach of foreseen experiments, over a region of parameter space where the fine-tuning for the electroweak scale is 10-50%. For dark matter, both mirror neutrons and a variety of self-interacting mirror atoms are considered. Neutrino mass signals and the effects of a possible additional Z 2 breaking from the vacuum expectation values of B-L breaking fields are also discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Light flavor-singlet scalars and walking signals in N f = 8 QCD on the lattice

Based on the highly improved staggered quark action, we perform in this paper lattice simulations of N f = 8 QCD and confirm our previous observations, both of a flavor-singlet scalar meson (denoted as σ) as light as the pion and of various “walking signals” through the low-lying spectra, with higher statistics, smaller fermion masses m f , and larger volumes. We measure M π , F π , M ρ , M a 0 , M a 1 , M b 1 , M N , M σ , F σ , (φφ) (both directly and through the Gell-Mann-Oakes-Renner relation), and the string tension. The data are consistent with the spontaneously broken phase of the chiral symmetry, in agreement with the previous results: Ratios of the quantities to M π monotonically increase in the smaller m f region towards the chiral limit similarly to N f = 4 QCD, in sharp contrast to N f = 12 QCD where the ratios become flattened. We perform fits to chiral perturbation theory, with the value of F π found in the chiral limit extrapolation reduced dramatically to roughly 2/3 of the previous result, suggesting the theory is much closer to the conformal window. In fact, each quantity obeys the respective hyperscaling relation throughout a more extensive m f region compared with earlier works. The hyperscaling relation holds with roughly a universal value of the anomalous dimension, γ m ≃ 1, with the notable exception of M π with γ m ≃ 0.6 as in the previous results, which reflects the above growing up of the ratios towards the chiral limit. This is a salient feature (walking signal) of N f = 8, unlike either N f = 4, which has no hyperscaling relation at all, or N f = 12 QCD, which exhibits universal hyperscaling. The effective γ m Ξ γ m (m f ) of M π defined for each m f region has a tendency to grow towards unity near the chiral limit, in conformity with the Nambu-Goldstone boson nature, as opposed to the case of N f = 12 QCD where it is almost constant. We further confirm the previous observation of the light σ with mass comparable to the pion in the studied m f region. In a chiral limit extrapolation of the σ mass using the dilaton chiral perturbation theory and also using the simple linear fit, we find the value consistent with the 125 GeV Higgs boson within errors. Finally, our results suggest that the theory could be a good candidate for walking technicolor model, having anomalous dimension γ m ≃ 1 and a light flavor-singlet scalar meson as a technidilaton, which can be identified with the 125 GeV composite Higgs in the N f = 8 one-family model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

LHC SUSY and WIMP dark matter searches confront the string theory landscape

The string theory landscape of vacua solutions provides physicists with some understanding as to the magnitude of the cosmological constant. Similar reasoning can be applied to the magnitude of the soft SUSY breaking terms in supersymmetric models of particle physics: there appears to be a statistical draw towards large soft terms which is tempered by the anthropic requirement of the weak scale lying not too far from ~100 GeV. For a mild statistical draw of m softn with n = 1 (as expected from SUSY breaking due to a single F term) then the light Higgs mass is preferred at ~125 GeV while sparticles are all pulled beyond LHC bounds. We confront a variety of LHC and WIMP dark matter search limits with the statistical expectations from a fertile patch of string theory landscape. The end result is that LHC and WIMP dark matter detectors see exactly that which is expected from the landscape: a Standard Model-like Higgs boson of mass 125 GeV but as yet no sign of sparticles or WIMP dark matter. SUSY from the n = 1 landscape is most likely to emerge at LHC in the soft opposite-sign dilepton plus jet plus MET channel. Multi-ton noble liquid WIMP detectors should be able to completely explore the n = 1 landscape parameter space.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗