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DOE OSTI2021

Dynamics of Product Branching from Radical Intermediates in Elementary Combustion Reactions (Final Technical Report)

While the total rate constant for many elementary reactions is well-characterized, understanding the product branching in complex reactions presents a formidable challenge. To gain an incisive probe of such reactions, our experiments investigate the dynamics of the product channels that arise from transient radical intermediates along the bimolecular reaction coordinates. Our recent published work uses the methodology developed in my group in the last fifteen years, using both imaging and scattering apparatuses. The experiments generate a particular isomeric form of an unstable radical intermediate along a bimolecular reaction coordinate and study the branching between the ensuing product channels of the energized radical as a function of its internal rotational and vibrational energy under collision-less conditions. The experiments use a combination of: 1) measurement of product velocity and angular distributions in a crossed laser-molecular beam apparatus, with electron bombardment detection in my lab in Chicago or 2) with tunable vacuum ultraviolet photoionization detection at Taiwan's National Synchrotron Radiation Research Center (NSRRC), and 3) velocity map imaging using state-selective REMPI and single photon VUV ionization of radical intermediates and reaction products. We also employ tunable VUV photoionization detection in our imaging apparatus, using difference frequency four-wave mixing to produce photoionization light tunable from 8 to 10.8 eV. This year of support funded our work to photolytically generate the radical intermediate important in the OH + propene reaction when OH adds to an end carbon. Our other major result this past year was on the photoionization detection of vinoxy radicals. Our experiments at the NSRRC definitively show that vinoxy dissociatively ionizes to both CH 3 + and HCO + , and establishes the absolute partial photoionization cross section of vinoxy to CH 3 + at 10.5 and 11.44 eV. This should allow experimentalists detecting vinoxy radicals produced in bimolecular reactions such as OH + propene to get good branching fractions to that product channel. Finally we initiated studies of the photodissociation of 1-bromo-2-chloroethane and 1,1-bromochloroethane at 193 nm at the NSRRC.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Strong Interaction Studies with Medium Energy Probes

The PI, Kam Seth, and his group have engaged in nuclear and medium-energy particle physics for several decades. This report is the final one in a long successful scientific effort. During the last funding cycle, the Seth group contributed in important ways to the commissioning of the GlueX experiment at JLAB. Prior to joining GlueX, they were long-standing members of the CLEO Collaboration. Their scientific production includes measurements of hadronic cross sections pertinent to the (g-2) muon anomaly, the measurement of certain rare branching fractions of the Omega-minus baryon, and the observation of the first exclusive photoproduction of J/psi in Gluex. Two postdocs, Sean Dobbs and Ting Xiao, secured good faculty positions and are successfully pursuing their own research agendas.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Theoretical analysis of QOOH combustion reaction pathways

QOOH radicals are key intermediates in the chain of reactions leading to the autoignition of hydrocarbons and oxygenated organic compounds. They are thought to undergo two main reactions: OH elimination to form a cyclic ether and HO2 elimination to form an alkene. However, theoretical analysis of various substituted hydroperoxyalkyl radicals has found two new pathways: OH transfer and internal H abstraction assisted OH elimination. To determine the importance of these new pathways, their barrier heights for several substituted alkanes were calculated using various quantum chemical theories and compared to those of the well-known pathways. Several cases revealed possible competition with the well-known pathways. Rate coefficients were calculated for propyl systems but further studies will need to complete rate coefficients and branching fractions for all systems analyzed to understand these new reactions’ role in autoignition.

33 ADVANCED PROPULSION SYSTEMS↗

DOE OSTI2026

Exploring the Energy Frontier through Precision Tests and Fast Tracking with the CMS Detector (Final Technical Report)

This Early Career Award supported a research program using the CMS experiment at the CERN LHC to probe physics beyond the Standard Model in the top quark and Higgs boson sectors, alongside detector and trigger developments for the High-Luminosity LHC (HL-LHC) upgrade. The program (i) searched for charged lepton flavor violation (LFV) in the top quark sector with the full CMS Run-2 data set, placing the world’s strongest limits to date on the $t → eµq\ (q = u/c)$ branching fraction; (ii) developed preliminary analysis methods toward a boosted $t\bar{t}H(b\bar{b})$ measurement of the top quark Yukawa coupling and its CP properties; (iii) made leading contributions to the hardware-based Level-1 (L1) track finding system for the upgraded CMS detector for HL-LHC; and (iv) developed novel L1 trigger algorithms, notably a displaced vertex trigger enabling new searches for exotic long-lived particles.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2026

Study of Unboosted Higgs Pair Production with HH to bbWW Decay with the CMS experiment at the LHC

Measurement of the Higgs boson self-interaction through the production of Higgs boson (H) pairs is critical for understanding the shape of the Higgs potential and the stability of the universe. Higgs pair production (HH) where one H decays to b quarks (H to bb) and the other to hadronically decaying W bosons (H to WW*→qqqq) has a large branching fraction with the cost of high background from jets produced through quantum chromodynamics (QCD). While previous all-hadronic HH→bbWW analyses focused on the few percent of highly energetic HH events with collimated decay products, this first study of its kind targets the more challenging 95$\%$ of HH to bbWW* in which decay products can be reconstructed as separate jets providing sensitivity to a complementary region of phase space. This poster focuses on generator level measurements of the fractions of HH to bbWW* signal events are fully or partially reconstructed and what fraction are impacted by contamination from high momentum jets from initial or final state radiation. Partially reconstructed events are further classified according to whether the H to bb, H to WW*, or W to qq are fully reconstructed

Gillespie, Emma [U. Louisville]↗

DOE OSTI2020

Reply to Robinson and Michaud, arXiv:2002.08893

We respond to Robinson and Michaud's (RM) comment (arXiv:2002.08893) on our recent preprint arXiv:2002.06937, in which we discuss recent excesses in low-threshold dark matter searches, and offer a potential unifying dark matter interpretation. We thank RM for their feedback, which highlights the critical need for future measurements to directly calibrate plasmon charge yields for low $\sim$ 10 eV energy depositions. RM objected to our assertion that plasmons generated at energy scales below 100~eV may have a large branching fraction into phonons. As we argue below, the points raised by RM do not invalidate our primary conclusions, as they pertain to a much different energy scale than we discuss in our paper.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2020

Search for $\mu^- \rightarrow e^+$ conversion: what can be learned from the SINDRUM-II positron data on a gold target

In their 2006 paper setting the current limit on \mumemconv\ conversion search on a gold target \cite{sindrum_ii:Bertl2006}, the SINDRUM-II collaboration published, along with the electron momentum distribution, the momentum distribution of reconstructed positrons. Near the positron spectrum endpoint, there is a statistically significant excess of observed events over the expected background. We estimate that in the region 88 MeV/c < p < 95 MeV/c there are 13 events with an expected background of about 1-1.5 event, which has not been discussed by the authors. Those 13 events form a bump with a width consistent with the experimental resolution, making one think of a $\mu^- \rightarrow e^+$ conversion signal. However, the reconstructed position of the bump is about 1 MeV/c, or $\sim\ 4\sigma_p$, lower than the expected $\mu^- Au \rightarrow e^+ Ir $ signal, which strongly discourages the exotic interpretation. The excess, however, could be due to an exclusive dipole radiative muon capture (RMC) transition $^{197}Au(GS) ~\rightarrow~ ^{197}Pt(GS)$ with the branching fraction of about $2\cdot 10^{-4}$. Such a transition would not be resolved by the existing RMC measurements. We conclude that the exclusive RMC transitions could significantly modify the positron spectrum near the kinematic endpoint, to fully exploit the physics potential of the upcoming experiments such as Mu2e and COMET, a better theoretical understanding of the RMC spectrum on nuclei is needed. A high-resolution measurement of the RMC photon spectra has to be carried out. Without that, the sensitivity of the searches for $\mu^- \rightarrow e^+$ and $\mu^- \rightarrow e^-$ might be severely limited by unknown probabilities of RMC transitions to the exclusive low-lying states of the daughter nuclei.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2024

Model-Independent Description of $B\rightarrow D \pi \ell \nu$ Decays

In this contribution we present a novel, model-independent description of semileptonic $B\rightarrow D \pi \ell \nu$ decays. In addition, we discuss recent developments in the understanding of coupled-channel $D \pi$-$D \eta$-$D_s K$ S-wave scattering and, for the first time, apply them to semileptonic decays. We not only obtain model-independent predictions for kinematic distributions in $B\rightarrow D \pi \ell \nu$ decays, but also rule out the hypothesis that the gap between the inclusive $B\rightarrow X\ell\nu$ branching fraction and the sum over exclusive channels is made up predominantly by $B\rightarrow D^{(\ast)} \eta \ell \nu$ decays.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2026

Axion-like particle at the 10 GeV scale: Higgs decays to wide jets and photons

If axion-like particles (ALPs) exist, they may have renormalizable couplings only to the Higgs boson ($h^0$) or to fields beyond the Standard Model. An ALP $A_h$ lighter than about 60 GeV would allow the $h^0 \to A_h A_h$ decay, with a branching fraction determined by the explicit global symmetry breaking responsible for the main contributions to the $A_h$ mass ($M_A$). New heavy fields that carry color and electric charge, such as squarks, can mediate $A_h$ decays at one loop mostly into gluons, but also into photons. Exploring LHC sensitivity to $A_h$, we show that the $h^0 \to A_h A_h \to (gg)(γγ)$ channel leads to a diphoton resonance at $M_A$ whose production rate is consistent with a $3.5σ$ excess reported by a CMS search at $M_A \approx 13.6$ GeV. For $M_A$ of order 10 GeV, the $h^0 \to A_h A_h \to 4g$ cascade decay leads to two wide jets (each with 2-prong substructure) that form a resonance at 125 GeV.

Dobrescu, Bogdan A. [Fermilab] (ORCID:000000034291↗

DOE OSTI2023

Evidence for the singly Cabibbo-suppressed decay Ω 0 c → Ξ¯π + and search for Ω 0 c → Ξ¯ K + and Ω¯K + decays at Belle

Using a data sample of 980 fb -1 collected with the Belle detector at the KEKB asymmetric-energy e + e - collider, we study for the first time the singly Cabibbo-suppressed decays Ω 0 c → Ξ¯π + and Ω¯K + and the doubly Cabibbo-suppressed decay Ω 0 c → Ξ¯ K + . Evidence for an Ω 0 c signal in the Ω 0 c → Ξ¯π + mode is reported with a significance of 4.5σ including systematic uncertainties.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

Observation of the ${K}^{+}\to {\pi}^{+}\nu \overline{\nu}$ decay and measurement of its branching ratio

A measurement of the ${K}^{+}\to {\pi}^{+}\nu \overline{\nu}$ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This dataset was recorded, after modifications to the beamline and detectors, at a higher instantaneous beam intensity with respect to the 2016–2018 data taking. Combining NA62 data collected in 2016–2022, a measurement $\mathcal{B} ({K}^{+}\to {\pi}^{+}\nu \overline{\nu}) = \large{(}13.0^{+3.3}_{-3.0}\large{)}$ x $10^{-11}$ of is reported. With 51 signal candidates observed and an expected background of $18^{+3}_{-2}$ events, $\mathcal{B} ({K}^{+}\to {\pi}^{+}\nu \overline{\nu})$ becomes the smallest branching ratio measured with a signal significance above 5σ.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

First observation and study of the $K^± → π^0π^0μ^±ν$ decay

The NA48/2 experiment at CERN reports the first observation of the $K^± → π^0π^0μ^±$ν decay based on a sample of 2437 candidates with 15% background contamination collected in 2003–2004. The decay branching ratio in the kinematic region of the squared dilepton mass above 0.03 GeV 2 /c 4 is measured to be (0.65 ± 0.03) × 10 –6 . The extrapolation to the full kinematic space, using a specific model, is found to be (3.45 ± 0.16) × 10 –6 , in agreement with chiral perturbation theory predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2022

Study of charmonium and charmonium-like contributions in $\mathrm{B}^+$ → $\mathrm{J/ψηK^+}$ decays

A study of B+ → J/ψηK + decays, followed by J/ψ → μ + μ – and η → γγ, is performed using a dataset collected with the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb –1 The J/ψη mass spectrum is investigated for contributions from charmonia and charmonium-like states. Evidence is found for the B + →(ψ2(3823) → J/ψη)K + and B + → (ψ(4040) → J/ψη)K + decays with significance of 3.4 and 4.7 standard deviations, respectively. This constitutes the first evidence for the ψ 2 (3823) → J/ψη decay.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2023

Improved calorimetric particle identification in NA62 using machine learning techniques

Measurement of the ultra-rare $K^+$ → $π$ +$ν$$\overline{ν}$ decay at the NA62 experiment at CERN requires high-performance particle identification to distinguish muons from pions. Calorimetric identification currently in use, based on a boosted decision tree algorithm, achieves a muon misidentification probability of 1.2×10 -5 for a pion identification efficiency of 75% in the momentum range of 15–40 GeV/c. In this work, calorimetric identification performance is improved by developing an algorithm based on a convolutional neural network classifier augmented by a filter. Muon misidentification probability is reduced by a factor of six with respect to the current value for a fixed pion-identification efficiency of 75%. Alternatively, pion identification efficiency is improved from 72% to 91% for a fixed muon misidentification probability of 10 -5 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2020