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DOE OSTI2023

Search for third-generation vector-like leptons in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

A search for vector-like leptons in multilepton (two, three, or four-or-more electrons plus muons) final states with zero or more hadronic $τ$-lepton decays is presented. The search is performed using a dataset corresponding to an integrated luminosity of 139 fb -1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. To maximize the separation of signal and background, a machine-learning classifier is used. No excess of events is observed beyond the Standard Model expectation. Using a doublet vector-like lepton model, vector-like leptons coupling to third-generation Standard Model leptons are excluded in the mass range from 130 GeV to 900 GeV at the 95% confidence level, while the highest excluded mass is expected to be 970 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Measurement of azimuthal anisotropy of muons from charm and bottom hadrons in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV with the ATLAS detector

Azimuthal anisotropies of muons from charm and bottom hadron decays are measured in Pb+Pb collisions at s NN = 5.02 TeV . The data were collected with the ATLAS detector at the Large Hadron Collider in 2015 and 2018 with integrated luminosities of 0.5 nb - 1 and 1.4 n b - 1 , respectively. The kinematic selection for heavy-flavor muons requires transverse momentum 4 <lt; p T < 30 GeV and pseudorapidity | η | < 2.0 . The dominant sources of muons in this p T range are semi-leptonic decays of charm and bottom hadrons. These heavy-flavor muons are separated from light-hadron decay muons and punch-through hadrons using the momentum imbalance between the measurements in the tracking detector and in the muon spectrometers. Azimuthal anisotropies, quantified by flow coefficients, are measured via the event-plane method for inclusive heavy-flavor muons as a function of the muon p T and in intervals of Pb+Pb collision centrality. Heavy-flavor muons are separated into contributions from charm and bottom hadron decays using the muon transverse impact parameter with respect to the event primary vertex. Non-zero elliptic ( v 2 ) and triangular ( v 3 ) flow coefficients are extracted for charm and bottom muons, with the charm muon coefficients larger than those for bottom muons for all Pb+Pb collision centralities. The results indicate substantial modification to the charm and bottom quark angular distributions through interactions in the quark-gluon plasma produced in these Pb+Pb collisions, with smaller modifications for the bottom quarks as expected theoretically due to their larger mass.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Search for the lepton-flavour violating decays B0 → K *0 μ ± e ∓ and ${B}_s^0$ → Φμ ± e ∓

A search for the lepton-flavour violating decays B 0 → K *0 μ ± e ∓ and \( {B}_s^0 \) → Φμ ± e ∓ is presented, using proton-proton collision data collected by the LHCb detector at the LHC, corresponding to an integrated luminosity of 9 fb - 1 . No significant signals are observed and upper limits of $$ {\displaystyle \begin{array}{c}\mathcal{B}\left({B}^0\to {K}^{\ast 0}{\mu}^{+}{e}^{-}\right)<5.7\times {10}^{-9}\left(6.9\times {10}^{-9}\right),\\ {}\mathcal{B}\left({B}^0\to {K}^{\ast 0}{\mu}^{-}{e}^{+}\right)<6.8\times {10}^{-9}\left(7.9\times {10}^{-9}\right),\\ {}\mathcal{B}\left({B}^0\to {K}^{\ast 0}{\mu}^{\pm }{e}^{\mp}\right)<10.1\times {10}^{-9}\left(11.7\times {10}^{-9}\right),\\ {}\mathcal{B}\left({B}_s^0\to \phi {\mu}^{\pm }{e}^{\mp}\right)<16.0\times {10}^{-9}\left(19.8\times {10}^{-9}\right)\end{array}} $$ are set at 90% (95%) confidence level. These results constitute the world’s most stringent limits to date, with the limit on the decay \( {B}_s^0 \) → Φμ ± e ∓ the first being set. In addition, limits are reported for scalar and left-handed lepton-flavour violating New Physics scenarios.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Test of CP-invariance of the Higgs boson in vector-boson fusion production and in its decay into four leptons

A search for CP violation in the decay kinematics and vector-boson fusion production of the Higgs boson is performed in the H → ZZ * → 4 ℓ ( ℓ = e, μ ) decay channel. The results are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb –1 . Matrix element-based optimal observables are used to constrain CP-odd couplings beyond the Standard Model in the framework of Standard Model effective field theory expressed in the Warsaw and Higgs bases. Differential fiducial cross-section measurements of the optimal observables are also performed, and a new fiducial cross-section measurement for vector-boson-fusion production is provided. All measurements are in agreement with the Standard Model prediction of a CP-even Higgs boson.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Search for the decay of the Higgs boson to a pair of light pseudoscalar bosons in the final state with four bottom quarks in proton-proton collisions at $ \sqrt{\textrm{s}} $ = 13 TeV

A search is presented for the decay of the 125 GeV Higgs boson (H) to a pair of new light pseudoscalar bosons (a), followed by the prompt decay of each a boson to a bottom quark-antiquark pair, $ \textrm{H}\to \textrm{aa}\to \textrm{b}\overline{\textrm{b}}\textrm{b}\overline{\textrm{b}} $. The analysis is performed using a data sample of proton-proton collisions collected with the CMS detector at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{−1}$. To reduce the background from standard model processes, the search requires the Higgs boson to be produced in association with a leptonically decaying W or Z boson. The analysis probes the production of new light bosons in a 15 < m$_{a}$ < 60 GeV mass range. Assuming the standard model predictions for the Higgs boson production cross sections for pp → WH and ZH, model independent upper limits at 95% confidence level are derived for the branching fraction $ \mathcal{B}\left(\textrm{H}\to \textrm{aa}\to \textrm{b}\overline{\textrm{b}}\textrm{b}\overline{\textrm{b}}\right) $. The combined WH and ZH observed upper limit on the branching fraction ranges from 1.10 for m$_{a}$ = 20 GeV to 0.36 for m$_{a}$ = 60 GeV, complementing other measurements in the μμττ, ττττ and bbℓℓ (ℓ = μ, τ) channels.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Measurement of the Z boson differential production cross section using its invisible decay mode (Z$\nu\bar{\nu}$) in proton-proton collisions at $\sqrt{s}=$ 13 TeV

Measurements of the total and differential fiducial cross sections for the Z boson decaying into two neutrinos are presented at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV. The data were collected by the CMS detector in 2016 and correspond to an integrated luminosity of 35.9 fb$^{-1}$. In these measurements, events are selected containing an imbalance in transverse momentum and one or more energetic jets. The fiducial differential cross section is measured as a function of the Z boson transverse momentum. The results are combined with a previous measurement of charged-lepton decays of the Z boson. The measured total fiducial cross section for events with Z boson transverse momentum greater than 200 GeV is $ {3000}_{-170}^{+180} $ fb.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Search for the baryon- and lepton-number violating decays B 0 → pμ⁻ and $B^0_s$ → pμ⁻

A search for the baryon- and lepton-number violating decays and $B$$^{0}_{s}$ →pμ – is performed at the LHCb experiment using data collected in proton-proton collisions at √s =7, 8 and 13 TeV, corresponding to integrated luminosities of 1, 2, and 6 fb –1 , respectively. No significant signal for B 0 → p⁢μ – and $B$$^{0}_{s}$ → p⁢μ – decays is found and the upper limits on the branching fractions are determined to be Β⁡(B 0 → pμ – ) < 2.6⁢(3.1) ×10 –9 and Β($B$$^{0}_{s}$ → pμ – ) < 12.1⁢(14.0) ×10 –9 , respectively, at 90% (95%) confidence level. These are the first limits on these decays to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Search for Scalar Leptoquarks Produced via τ -Lepton–Quark Scattering in p p Collisions at s = 13 TeV

The first search for scalar leptoquarks produced in τ -lepton–quark collisions is presented. It is based on a set of proton-proton collision data recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 138 fb − 1 . The reconstructed final state consists of a jet, significant missing transverse momentum, and a τ lepton reconstructed through its hadronic or leptonic decays. Limits are set on the product of the leptoquark production cross section and branching fraction and interpreted as exclusions in the plane of the leptoquark mass and the leptoquark- τ -quark coupling strength. © 2024 CERN, for the CMS Collaboration 2024 CERN

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Parton distribution function uncertainties in theoretical predictions for far-forward tau neutrinos at the Large Hadron Collider

New experiments dealing with neutrinos in the far-forward region at the Large Hadron Collider (LHC) are under design or already in preparation. Two of them, FASERν and SND@LHC, are expected to be active during Run 3 and have the potential to detect the interactions of ν and $\bar {v}$ that come from high-energy collisions in one of the LHC interaction points, extracted along the direction tangent to the beam line. Tau neutrinos and antineutrinos come predominantly from $D^{±}_{s}$ production in pp collisions, followed by the leptonic decay of these mesons. Neutrino pseudorapidities in the range of η > 6.9 and η > 8.9 are relevant to these future experiments. At such pseudorapidities at high energies, QCD theoretical predictions for the flux of ν τ plus $\bar {v}_τ$ rely on parton distribution functions (PDFs) in a combination of very small and large parton–x values. We evaluate PDF un certainties affecting the flux of ν τ + $\bar {v}_τ$ produced by $D^{±}_{s}$ decay in the far forward region at the LHC. Next-to-leading order (NLO) QCD uncertainties are included in the calculation of $D^{±}_{s}$ production and NLO PDF sets are used for consistency. The theoretical uncertainty associated with the 40 PDF sets of the PROSA19 group amounts to ±(20 – 30)% for the (ν τ + $\bar {v}_τ$) number of charged-current (CC) events. Scale uncertainties are much larger, resulting in a range of CC event predictions from ~70% lower to ~90% higher than the central prediction. A comparison of the predictions with those obtained using as input the central PDFs from the 3-flavour NLO PDF sets of the CT14, ABMP16 and NNPDF3.1 collaborations show that far-forward neutrino energy distributions vary by as much as a factor of ~2 – 4 relative to the PROSA19 predictions at TeV neutrino energies. The Forward Physics Facility in the high luminosity LHC era will provide data capable of constraining NLO QCD evaluations with these PDF sets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Measurements of Zγ +jets differential cross sections in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

Differential cross-section measurements of $Zγ$ production in association with hadronic jets are presented, using the full 139 fb -1 dataset of $\sqrt{s}$ = 13 TeV proton–proton collisions collected by the ATLAS detector during Run 2 of the LHC. Distributions are measured using events in which the $Z$ boson decays leptonically and the photon is usually radiated from an initial-state quark. Measurements are made in both one and two observables, including those sensitive to the hard scattering in the event and others which probe additional soft and collinear radiation. Different Standard Model predictions, from both parton-shower Monte Carlo simulation and fixed-order QCD calculations, are compared with the measurements. In general, good agreement is observed between data and predictions from MATRIX and MiNNLO PS , as well as next-to-leading-order predictions from MADGRAPH5_AMC@NLO and SHERPA.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Search for a new scalar resonance in flavour-changing neutral-current top-quark decays $t → qX (q = u, c)$, with $X → b\overline{b} $, in proton-proton collisions at $ \sqrt{s} $ = 13 TeV with the ATLAS detector

A search for flavour-changing neutral-current decays of a top quark into an up-type quark (either up or charm) and a light scalar particle X decaying into a bottom anti-bottom quark pair is presented. The search focuses on top-quark pair production where one top quark decays to qX, with X → $b\bar{b}$, and the other top quark decays according to the Standard Model, with the W boson decaying leptonically. The final state is thus characterised by an isolated electron or muon and at least four jets. Events are categorised according to the multiplicity of jets and jets tagged as originating from b-quarks, and a neural network is used to discriminate between signal and background processes. The data analysed correspond to 139 fb –1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC. The 95% confidence-level upper limits between 0.019% and 0.062% are derived for the branching fraction $\mathcal{B}$(t → uX) and between 0.018% and 0.078% for the branching fraction $\mathcal{B}$(t → cX), for masses of the scalar particle X between 20 and 160 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Measurement of the branching fraction of ${\textrm{D}}_{\textrm{s}}^{+}$ → τ + ν τ via τ + → μ+ν μ $\overline{\nu}$τ

Utilizing 7 . 33 fb - 1 of e + e - collision data taken at the center-of-mass energies of 4.128, 4.157, 4.178, 4.189, 4.199, 4.209, 4.219, and 4.226 GeV with the BESIII detector, the branching fraction of the leptonic decay ${D}_s^{+}$ → τ + ν τ via τ + → μ + ν μ $\overline{\nu}$ τ is measured to be ${\mathcal{B}}_{D_s^{+}\to {\tau}^{+}{\nu}_{\tau }}$ = (5 . 37 ± 0 . 17 stat ± 0 . 15 syst )%. Combining this branching fraction with the world averages of the measurements of the masses of τ + and ${D}_s^{+}$ as well as the lifetime of ${D}_s^{+}$, we extract the product of the decay constant of ${D}_s^{+}$ and the c → s Cabibbo-Kobayashi-Maskawa matrix element to be ${f}_{D_s^{+}}$ |V cs | = (246 . 7 ± 3 . 9 stat ± 3 . 6 syst ) MeV. Taking |V cs | from a global fit in the standard model we obtain ${f}_{D_s^{+}}$ = (253 . 4 ± 4 . 0 stat ± 3 . 7 syst ) MeV. Conversely, taking ${f}_{D_s^{+}}$ from lattice quantum chromodynamics calculations, we obtain |V cs | = 0 . 987 ± 0 . 016 stat ± 0 . 014 syst .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Search for single production of vector-like quarks decaying into W(ℓν)b in pp collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for single production of a vector-like quark Q, which could be either a singlet T, with charge $\frac{2}{3}$, or a Y from a (T, B, Y) triplet, with charge $-\frac{4}{3}$, is performed using data from proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to the full integrated luminosity of 140 fb −1 recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis targets Q → Wb decays where the W boson decays leptonically. The data are found to be consistent with the expected Standard Model background, so upper limits are set on the cross-section times branching ratio, and on the coupling of the Q to the Standard Model sector for these two benchmark models. Effects of interference with the Standard Model background are taken into account. For the singlet T, the 95% confidence level limit on the coupling strength κ ranges between 0.22 and 0.52 for masses from 1150 to 2300 GeV. For the (T, B, Y) triplet, the limits on κ vary from 0.14 to 0.46 for masses from 1150 to 2600 GeV.

beyond Standard Model↗
DOE OSTI2023

Observation of electroweak W + W – pair production in association with two jets in proton-proton collisions at √s = 13 TeV

An observation is reported of the electroweak production of a W + W – pair in association with two jets, with both W bosons decaying leptonically. The data sample corresponds to an integrated luminosity of 138 fb –1 of proton-proton collisions at √s = 13 TeV, collected by the CMS detector at the CERN LHC. Events are selected by requiring exactly two opposite-sign leptons (electrons or muons) and two jets with large pseudorapidity separation and high dijet invariant mass. Events are categorized based on the flavor of the final-state leptons. A signal is observed with a significance of 5.6 standard deviations (5.2 expected) with respect to the background-only hypothesis. The measured fiducial cross section is 10.2 ± 2.0 fb and this value is consistent with the standard model prediction of 9.1 ± 0.6 fb.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Search for nonresonant Higgs boson pair production in final state with two bottom quarks and two tau leptons in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for the nonresonant production of Higgs boson pairs (HH ) via gluon-gluon and vector boson fusion processes in final states with two bottom quarks and two tau leptons is presented. The search uses data from proton-proton collisions at a center-of-mass energy of $\sqrt{s}$ = 13 TeV recorded with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb -1 . Events in which at least one tau lepton decays hadronically are considered and multiple machine learning techniques are used to identify and extract the signal. The data are found to be consistent, within uncertainties, with the standard model (SM) predictions. Upper limits on the HH production cross section are set to constrain the parameter space for anomalous Higgs boson couplings. The observed (expected) upper limit at 95% confidence level corresponds to 3.3 (5.2) times the SM prediction for the inclusive HH cross section and to 124 (154) times the SM prediction for the vector boson fusion HH cross section. At 95% confidence level, the Higgs field self-coupling is constrained to be within -1.7 and 8.7 times the SM expectation, and the coupling of two Higgs bosons to two vector bosons is constrained to be within -0.4 and 2.6 times the SM expectation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Observation of gauge boson joint-polarisation states in W ± Z production from pp collisions at s = 13 TeV with the ATLAS detector

Measurements of joint-polarisation states of W and Z gauge bosons in W ± Z production are presented. The data set used corresponds to an integrated luminosity of 139 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. The W ± Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The simultaneous pair-production of longitudinally polarised vector bosons is measured for the first time with a significance of 7.1 standard deviations. The measured joint helicity fractions integrated over the fiducial region are $f$ 00 = 0.067 ± 0.010, $f$ 0T = 0.110 ± 0.029, $f$ T0 = 0.179 ± 0.023 and $f$ TT = 0.644 ± 0.032, in agreement with the next-to-leading-order Standard Model predictions. Individual helicity fractions of the W and Z bosons are also measured and found to be consistent with joint helicity fractions within the expected amounts of correlation. Both the joint and individual helicity fractions are also measured separately in W + Z and W - Z events. Inclusive and differential cross sections for several kinematic observables sensitive to polarisation are presented.

43 PARTICLE ACCELERATORS↗
DOE OSTI2023

Observation of $Wγγ$ triboson production in proton-proton collisions at s = 13 TeV with the ATLAS detector

This letter reports the observation of W(ℓν)γγ production in proton-proton collisions. This measurement uses the full Run 2 sample of events recorded at a center-of-mass energy of $\sqrt{s}$ = 13 TeV by the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 fb -1 . Events with a leptonically-decaying W boson and at least two photons are considered. The background-only hypothesis is rejected with an observed and expected significance of 5.6 standard deviations. The inclusive fiducial production cross section of W(eν)γγ and W(μν)γγ events is measured to be σ fid = 13.8 ± 1.1(stat) $^{+2.1}_{-2.0}$(syst) ± 0.1(lumi) fb, in agreement with the Standard Model prediction.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗