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At least 1,603 records · Page 89
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DOE OSTI2024

Measurement of the VH,H → ττ process with the ATLAS detector at 13 TeV

A measurement of the Standard Model Higgs boson produced in association with a W or Z boson and decaying into a pair of τ-leptons is presented. This search is based on proton-proton collision data collected at $\sqrt{s}$ = 13 TeV by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 140 fb -1 . For the Higgs boson candidate, only final states with at least one τ-lepton decaying hadronically (τ→hadrons+ν τ ) are considered. For the vector bosons, only leptonic decay channels are considered: Z→ℓℓ and W→ℓν ℓ , with ℓ=e,μ. An excess of events over the expected background is found with an observed (expected) significance of 4.2 (3.6) standard deviations, providing evidence of the Higgs boson produced in association with a vector boson and decaying into a pair of τ-leptons. The ratio of the measured cross-section to the Standard Model prediction is μ$_{VH}^{ττ}$=1.28$_{-0.29}^{+0.30}$(stat.)$_{-0.21}^{+0.25}$(syst.). This result represents the most accurate measurement of the VH(ττ) process achieved to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations

The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 1964, and CP violation in the weak interactions of quarks was soon established. Sakharov proposed that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. Yet, CP violation in quarks is too small to support this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter–antimatter disparity through a process called leptogenesis. Leptonic mixing, which appears in the standard model’s charged current interactions, provides a potential source of CP violation through a complex phase δ CP , which is required by some theoretical models of leptogenesis. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments. Until now, the value of δ CP has not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δ CP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δ CP , which is cyclic and repeats every 2π, is [-3.41, -0.03] for the so-called normal mass ordering and [-2.54, -0.32] for the inverted mass ordering. Our findings indicate CP violation in leptons and our method enables sensitive searches for matter–antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Deep and shallow inelastic scattering

In this session we focused on the higher energy deep and shallow inelastic particle interactions, DIS and SIS. DIS interactions occur when the energy of the incident particle beam is so large that the beam is able to penetrate the nucleons inside of the target nuclei. These interactions occur at the smallest level possible, that of the quark-gluon, or parton, level. SIS interactions occur in an intermediate energy range, just below the energy required for DIS interactions. The DIS cross section formula contains structure functions that describe our understanding of the underlying parton structure of nature. The full description of DIS interactions requires three structure functions: two may be measured in charged lepton or neutrino scattering, but one can only be extracted from neutrino DIS data. There are reasons to expect that the impact of nuclear effects could be different for neutrinos engaging in the DIS interaction, vs those felt by leptons. In fact, fits by the nCTEQ collaboration have found that the neutrino-Fe structure functions appear to differ from those extracted from lepton scattering data [1]. To better understand the global picture of DIS and SIS, we chose a three-pronged attack that examined recent experimental results, data fits, and latest theory predictions. Experimental results from neutrino and lepton scattering, as well as collider experiments, were presented.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

On the origin of X-ray spectra in luminous blazars

Gamma-ray luminosities of some quasar-associated blazars imply jet powers reaching values comparable to the accretion power even if assuming very strong Doppler boosting and very high efficiency of gamma-ray production. With much lower radiative efficiencies of protons than of electrons, and the recent reports of very strong coupling of electrons with shock-heated protons indicated by particle-in-cell simulations, the leptonic models seem to be strongly favored over the hadronic ones. However, the electron-proton coupling combined with the external-radiation-Compton (ERC) models of gamma-ray production in leptonic models predict extremely hard X-ray spectra, with energy indices α {sub x} ∼ 0. This is inconsistent with the observed 2-10 keV slopes of blazars, which cluster around α {sub x} ∼ 0.6. This problem can be resolved by assuming that electrons can be efficiently cooled down radiatively to non-relativistic energies, or that blazar spectra are entirely dominated by the synchrotron self-Compton (SSC) component up to at least 10 keV. Here, we show that the required cooling can be sufficiently efficient only at distances r < 0.03 pc. SSC spectra, on the other hand, can be produced roughly co-spatially with the observed synchrotron and ERC components, which are most likely located roughly at a parsec scale. We show that the dominant SSC component can also be produced much further than the dominant synchrotron and ERC components, at distances of ≳ 10 pc. Hence, depending on the spatial distribution of the energy dissipation along the jet, one may expect to see γ-ray/optical events with either correlated or uncorrelated X-rays. In all cases the number of e{sup +}e{sup –} pairs per proton is predicted to be very low. The direct verification of the proposed SSC scenario, and particularly the question of the co-spatiality of the SSC component with other spectral components, requires sensitive observations in the hard X-ray band. This is now possible with the deployment of the NuSTAR satellite, providing the required sensitivity to monitor the details of the hard X-ray spectra of blazars in the range where the ERC component is predicted to start dominating over the SSC component.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Multiwavelength variability properties of Fermi blazar S5 0716+714

S5 0716+714 is a typical BL Lacertae object. In this paper we present the analysis and results of long-term simultaneous observations in the radio, near-infrared, optical, X-ray, and γ-ray bands, together with our own photometric observations for this source. The light curves show that the variability amplitudes in γ-ray and optical bands are larger than those in the hard X-ray and radio bands and that the spectral energy distribution (SED) peaks move to shorter wavelengths when the source becomes brighter, which is similar to other blazars, i.e., more variable at wavelengths shorter than the SED peak frequencies. Analysis shows that the characteristic variability timescales in the 14.5 GHz, the optical, the X-ray, and the γ-ray bands are comparable to each other. The variations of the hard X-ray and 14.5 GHz emissions are correlated with zero lag, and so are the V band and γ-ray variations, which are consistent with the leptonic models. Coincidences of γ-ray and optical flares with a dramatic change of the optical polarization are detected. Hadronic models do not have the same natural explanation for these observations as the leptonic models. A strong optical flare correlating a γ-ray flare whose peak flux is lower than the average flux is detected. The leptonic model can explain this variability phenomenon through simultaneous SED modeling. Different leptonic models are distinguished by average SED modeling. The synchrotron plus synchrotron self-Compton (SSC) model is ruled out because of the extreme input parameters. Scattering of external seed photons, such as the hot-dust or broad-line region emission, and the SSC process are probably both needed to explain the γ-ray emission of S5 0716+714.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Search for decays of the Higgs boson into pair-produced pseudoscalar particles decaying into τ+τ−τ+τ− using pp collisions at s=13 TeV with the ATLAS detector

A search for a pair of low-mass pseudoscalars a that promptly decay into τ-leptons is presented using 140 fb −1 of proton–proton collision data at 13 TeV centre-of-mass energy recorded with the ATLAS detector at the Large Hadron Collider. The result is used to place constraints on exotic decays of the Higgs boson into four τ-leptons, H→aa→τ+τ−τ+τ−. This search focuses on events with either one or two τ-leptons decaying into hadrons and neutrinos, and the remaining three or two τ-leptons decaying into either electron or muon and neutrinos. No significant excess is observed above the expected Standard Model background and upper limits at the 95% confidence level on B(H→aa→τ+τ−τ+τ−) are set ranging from 0.06 to 0.23, depending on the mass ma ranging from 15 to 60 GeV.

Source record↗
DOE OSTI2016

Peccei-Quinn symmetry for Dirac seesaw and leptogenesis

We extend the DFSZ invisible axion model to simultaneously explain small Dirac neutrino masses and cosmic matter-antimatter asymmetry. After the Peccei-Quinn and electroweak symmetry breaking, the effective Yukawa couplings of the Dirac neutrinos to the standard model Higgs scalar can be highly suppressed by the ratio of the vacuum expectation value of an iso-triplet Higgs scalar over the masses of some heavy gauge-singlet fermions, iso-doublet Higgs scalars or iso-triplet fermions. The iso-triplet fields can carry a zero or nonzero hypercharge. Through the decays of the heavy gauge-singlet fermions, iso-doublet scalars or iso-triplet fermions, we can obtain a lepton asymmetry in the left-handed leptons and an opposite lepton asymmetry in the right-handed neutrinos. Since the right-handed neutrinos do not participate in the sphaleron processes, the left-handed lepton asymmetry can be partially converted to a baryon asymmetry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

A strong test of the dark matter origin of a TeV electron excess using icecube neutrinos

Due to the electroweak symmetry, high energy neutrinos and charged leptons are generically produced simultaneously in heavy dark matter decay or annihilation process. Correlating these two channels in dark matter indirect detections may provide important information on the intrinsic production mechanism. In this paper, we demonstrate this point by studying the tentative excess in the electron spectrum at 1.4 TeV reported by the DArk Matter Particle Explorer (DAMPE). A non-astrophysical scenario in which dark matter particles annihilate or decay in a local clump has been invoked to explain the excess. If e± annihilation channels in the final states are mediated by left-handed leptons as a component in the SU(2) L doublet, neutrinos with similar energies should have been simultaneously produced. Here we demonstrate that generic dark matter models can be decisively tested by the existing IceCube data. In case of a non-detection, such models would be excluded at the 5σ level by the five-year data for a point-like source and by the ten-year data for an extended source of dark matter particles with left-handed leptons. This serves as an example of the importance of correlating charged lepton and neutrino channels. It would be fruitful to conduct similar studies related to other approaches to the indirect detection of dark matter.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

The science case for an intermediate energy advanced and novel accelerator linear collider facility

It is widely accepted that the next lepton collider beyond a Higgs factory would require center-of-mass energy of the order of up to 15 TeV. Since, given reasonable space and cost restrictions, conventional accelerator technology reaches its limits near this energy, high-gradient advanced acceleration concepts are attractive. Advanced and novel accelerators (ANAs) are leading candidates due to their ability to produce acceleration gradients on the order of 1–100 GV/m, leading to compact acceleration facilities. However, intermediate energy facilities (IEF) are required to test the critical technology elements on the way towards multi-TeV-class colliders. Here a science case for a 20–100 GeV center-of-mass energy ANA-based lepton collider that can be a candidate for an intermediate energy facility is presented. The IEF can provide numerous opportunities for high energy physics studies including precision Quantum Chromodynamics and Beyond the Standard Model physics measurements, investigation of charged particle interactions with extreme electromagnetic fields, and exploring muon and proton beam acceleration. Possible applications of this collider include the studies of γγ and electron beam-fixed target/beamdump collider designs. Thus, the goal of the proposed IEF is to both carry out particle physics measurements in the 20-100 GeV ranges as well as to serve as an ANA demonstrator facility.

43 PARTICLE ACCELERATORS↗
DOE OSTI2025

Factorized QED and QCD contribution to deeply inelastic scattering

We present the first calculation of next-to-leading order (NLO) factorized QED and QCD contributions to the short-distance hard coefficients of inclusive lepton-hadron deep inelastic scattering (DIS) in a joint QED and QCD factorization approach. Unlike the traditional radiative correction approach to handle the collision-induced QED contributions to DIS, QED radiation from all charged leptons and quarks are treated equally, and their collinear sensitivities are systematically factorized into corresponding universal lepton and parton distribution functions. We demonstrate that the NLO factorized short-distance QED contribution is completely infrared safe and calculable without the need of any parameters other than the standard factorization scale in the same way as the factorized short-distance QCD contribution. We discuss the potential impact of this joint factorization approach on the extraction of partonic information from lepton-hadron DIS.

Deep inelastic scattering↗
DOE OSTI2021

Bulk viscosity from Urca processes: npeμ matter in the neutrino-trapped regime

In this work, we extend our previous study of the bulk viscosity of hot and dense npe matter induced by the Urca process in the neutrino trapped regime to npeμ matter by adding the muonic Urca processes as well as the purely leptonic electroweak processes involving electron-muon transition. Here, the nuclear matter is modeled in a relativistic density functional approach with two different parametrizations which predict neutrino dominated matter (DDME2 model) and anti-neutrino dominated matter (NL3 model) at temperatures for which neutrinos/anti-neutrinos are trapped. In the case of neutrino-dominated matter, the main equilibration mechanism is lepton capture, whereas in the case of antineutrino-dominated matter this is due to neutron decay. We find that the equilibration rates of Urca processes are higher than that of the pure leptonic processes, which implies that the Urca bulk viscosity can be computed with the leptonic reactions assumed to be frozen. We find that the bulk viscosity decreases with temperature as ζ~T –2 at moderate temperatures. At high temperatures this scaling breaks down by sharp drops of the bulk viscosity close to the temperature where the proton fraction is density-independent and the matter becomes scale-invariant. This occurs also when the matter undergoes a transition from the antineutrino-dominated regime to the neutrino-dominated regime where the bulk viscosity attains a local maximum. We also estimate the bulk viscous dissipation timescales and find that these are in the range ≳ 1 s for temperatures above the neutrino trapping temperature. These timescales would be relevant only for long-lived objects formed in binary neutron star mergers and hot proto-neutron stars formed in core-collapse supernovas.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Search for heavy right-handed Majorana neutrinos in the decay of top quarks produced in proton-proton collisions at s = 13 TeV with the ATLAS detector

A search for heavy right-handed Majorana neutrinos is performed with the ATLAS detector at the CERN Large Hadron Collider, using the 140 fb -1 of proton–proton collision data at $\sqrt{s}$ =13 TeV collected during Run 2. This search targets $t\overline{t}$ production, in which both top quarks decay into a bottom quark and a W boson, where one of the W bosons decays hadronically and the other decays into an electron or muon and a heavy neutral lepton. The heavy neutral lepton is identified through a decay into an electron or muon and another W boson, resulting in a pair of same-charge same-flavor leptons in the final state. This paper presents the first search for heavy neutral leptons in the mass range of 15–75 GeV using $t\overline{t}$ events. No significant excess is observed over the background expectation, and upper limits are placed on the signal cross sections. Assuming a benchmark scenario of the phenomenological type-I seesaw model, these cross section limits are then translated into upper limits on the mixing parameters of the heavy Majorana neutrino with Standard Model neutrinos.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Measurement of branching fractions and rate asymmetries in the rare decays B→K (*) l⁺l⁻

In a sample of 471×10⁶ BB¯¯¯ events collected with the BABAR detector at the PEP-II e⁺e⁻ collider we study the rare decays B→K (*) l⁺l⁻, where l⁺l⁻ is either e⁺e⁻ or μ⁺μ⁻. We report results on partial branching fractions and isospin asymmetries in seven bins of dilepton mass-squared. We further present CP and lepton-flavor asymmetries for dilepton masses below and above the J/ψ resonance. We find no evidence for CP or lepton-flavor violation. The partial branching fractions and isospin asymmetries are consistent with the Standard Model predictions and with results from other experiments.

Lees, J. P.↗
DOE OSTI2014

Measurements of normalized differential cross sections for t t ¯ production in p p collisions at ( s ) = 7 TeV using the ATLAS detector

We present measurements of normalized differential cross sections for top-quark pair production as a function of the top-quark transverse momentum, and of the mass, transverse momentum, and rapidity of the $t\bar{t}$ system, in proton–proton collisions at a center-of-mass energy of √ s=7 TeV. The data set corresponds to an integrated luminosity of 4.6 fb ₋1 , recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton + jets channel, requiring exactly one lepton and at least four jets with at least one of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector efficiency and resolution effects and are compared to several Monte Carlo simulations and theory calculations. The results are in fair agreement with the predictions in a wide kinematic range. Nevertheless, data distributions are softer than predicted for higher values of the mass of the $t\bar{t}$ system and of the top-quark transverse momentum. Lastly, the measurements can also discriminate among different sets of parton distribution functions.

43 PARTICLE ACCELERATORS↗
DOE OSTI2016

Unification of gauge, family, and flavor symmetries illustrated in gauged SU(12) models

In this study, to explain quark and lepton masses and mixing angles, one has to extend the standard model, and the usual practice is to put the quarks and leptons into irreducible representations of discrete groups. We argue that discrete flavor symmetries (and their concomitant problems) can be avoided if we extend the gauge group. In the framework of SU(12) we give explicit examples of models having varying degrees of predictability obtained by scanning over groups and representations and identifying cases with operators contributing to mass and mixing matrices that need little fine- tuning of prefactors. Fitting with quark and lepton masses run to the GUT scale and known mixing angles allows us to make predictions for the neutrino masses and hierarchy, the octant of the atmospheric mixing angle, leptonic CP violation, Majorana phases, and the effective mass observed in neutrinoless double beta decay.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

$L_\mu - L_\tau$ theory of Higgs flavor violation and $(g-2)_\mu$

Several experiments reported hints for the violation of lepton flavor or lepton flavor universality in processes involving muons. Most prominently, there is the hint for a nonzero rate of the flavor violating Higgs decay h→τμ at the LHC, as well as the hint for lepton flavor universality violation in rare B meson decays at the LHCb. In addition, also the long-standing discrepancy in the anomalous magnetic moment of the muon motivates new physics connected to muons. A symmetry which violates lepton flavor universality is Lμ-Lτ: the difference of muon number and tau number. We show that adding vectorlike fermions to a Lμ-Lτ theory generates naturally an effect in the anomalous magnetic moment of the muon and h→τμ, while effects in other τ→μ transitions are systematically suppressed by symmetry arguments. We find that if Lμ-Lτ is gauged it is possible to also accommodate the discrepant b→sμμ data while predicting a τ→3μ and a modified h→μμ rate within reach of upcoming experiments.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Dark matter and neutrino mass from the smallest non-Abelian chiral dark sector

All pieces of concrete evidence for phenomena outside the standard model (SM)—neutrino masses and dark matter—are consistent with the existence of new degrees of freedom that interact very weakly, if at all, with those in the SM. Here we propose that these new degrees of freedom organize themselves into a simple dark sector, a chiral SU(3) × SU(2) gauge theory with the smallest nontrivial fermion content. Similar to the SM, the dark SU(2) is spontaneously broken while the dark SU(3) confines at low energies. At the renormalizable level, the dark sector contains massless fermions—dark leptons—and stable massive particles—dark protons. We find that dark protons with masses between 10 and 100 TeV satisfy all current cosmological and astrophysical observations concerning dark matter even if dark protons are a symmetric thermal relic. The dark leptons play the role of right-handed neutrinos and allow simple realizations of the seesaw mechanism or the possibility that neutrinos are Dirac fermions. In the latter case, neutrino masses are also parametrically different from charged-fermion masses and the lightest neutrino is predicted to be massless. Since the new “neutrino” and “dark-matter” degrees of freedom interact with one another, these two new-physics phenomena are intertwined. Dark leptons play a nontrivial role in early Universe cosmology while indirect searches for dark matter involve, decisively, dark-matter annihilations into dark leptons. These, in turn, may lead to observable signatures at high-energy neutrino and gamma-ray observatories, especially once one accounts for the potential Sommerfeld enhancement of the annihilation cross section, derived from the low-energy dark-sector effective theory, a possibility we explore quantitatively in some detail.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Probing squeezed bino-slepton spectra with the Large Hadron Collider

In this study, we consider a minimal supersymmetric Standard Model scenario in which the only light superparticles are a binolike dark matter candidate and a nearly degenerate slepton. It is notoriously difficult to probe this scenario at the Large Hadron Collider (LHC), because the slepton pair-production process yields a final state with soft leptons and small missing transverse energy. We study this scenario in the region of parameter space where the mass difference between the lightest neutralino and the lightest slepton ( Δ m ) is ≲ 60 GeV , focusing on the process in which an additional radiated jet provides a transverse boost to the slepton pair. We then utilize the angular separation of the leptons from each other and from the missing transverse energy, as well as the angular separation between the jet and the missing transverse energy, to distinguish signal from background events. We also use the reconstructed ditau mass, the cos θ ℓ 1 ℓ 2 * variable, and for larger Δ m , a lower bound on the lepton p T . These cuts can dramatically improve both signal sensitivity and the signal-to-background ratio, permitting discovery at the LHC with reasonable integrated luminosity over the interesting region of parameter space. Using our search strategy the LHC will be able to exclude m μ ˜ ≈ 200 GeV for Δ m ≲ 60 GeV at 1.5 – 3 σ with 1000 fb - 1 of integrated luminosity. Although we focus on a particular model, the results generalize to a variety of scenarios in which the dark matter and a leptonic partner are nearly degenerate in mass, and especially to scenarios featuring a scalar mediator.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗