Inclusive nonresonant multilepton probes of new phenomena at s = 13 TeV
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
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Interpreting LHCb’s Λ b → Λ c τ ν ¯ measurement and puzzles in semileptonic Λ b decays
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Study of the muon decay-in-flight in the τ - → μ - ν ¯ μ ν τ decay to measure the Michel parameter ξ '
We present the first measurement of the Michel parameter ξ' in the $\tau$ - → μ¯ $\overline{v}$ μ v $\tau$ decay using the full data sample of 988 fb -1 collected by the Belle detector operating at the KEKB asymmetric energy e + e - collider. The method is based on the reconstruction of the μ¯ → e¯$\overline{v}$ e v μ decay-in-flight in the Belle central drift chamber and relies on the correlation between muon spin and its daughter electron momentum. We study the main sources of the background that can imitate the signal decay, such as kaon and pion decays-in-flight and charged particle scattering on the detector material. Highly efficient methods of their suppression are developed and applied to select 165 signal-candidate events. We obtain ξ' = 0.22±0.94±0.42 where the first uncertainty is statistical, and the second one is systematic. The result is in agreement with the Standard Model prediction of ξ' = 1.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for Long-Lived Particles Decaying in the CMS End Cap Muon Detectors in Proton-Proton Collisions at s = 13 TeV
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Weakly supervised anomaly detection with event-level variables
We introduce a new topology for weakly supervised anomaly detection searches, diobject plus X. In this topology, one looks for a resonance decaying to two standard model particles produced in association with other anomalous event activity (X). This additional activity is used for classification. We demonstrate how anomaly detection techniques which have been developed for dijet searches focusing on jet substructure anomalies can be applied to event-level anomaly detection in this topology. To robustly capture event-level features of multiparticle kinematics, we employ new physically motivated variables derived from the geometric structure of a collision’s phase space manifold. As a proof of concept, we explore the application of this approach to several benchmark signals in the di-𝜏 and di-𝜇 plus X final states. We demonstrate that our anomaly detection approach can reach discovery-level significances for signals that would be missed in a conventional bump-hunt approach.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for the nonresonant production of Higgs boson pairs via gluon fusion and vector-boson fusion in the $b\bar{b}$τ + τ – final state in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector
A search for the nonresonant production of Higgs boson pairs in the HH → $b\overline{b}$$\tau^{+}\tau^{-}$ channel is performed using 140 fb -1 of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The analysis strategy is optimized to probe anomalous values of the Higgs boson self-coupling modifier $\kappa_{\tau}$ and of the quartic HHVV (v = W,Z) coupling modifier $\kappa$ 2V . No significant excess above the expected background from Standard Model processes is observed. An observed (expected) upper limit μ HH < 5.9(3.3) is set at 95% confidence-level on the Higgs boson pair production cross section normalized to its Standard Model prediction. The coupling modifiers are constrained to an observed (expected) 95% confidence interval of -3.1 <$\kappa_{\tau}$ < 9.0 (-2.5 < $\kappa_{\tau}$ < 9.3) and -0.5 <$\kappa$ 2V < 2.7 (-0.2 <$\kappa$ 2V < 2.4), assuming all other Higgs boson couplings are fixed to the Standard Model prediction. The results are also interpreted in the context of effective field theories via constraints on anomalous Higgs boson couplings and Higgs boson pair production cross sections assuming different kinematic benchmark scenarios.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for lepton-number- and baryon-number-violating tau decays at Belle
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
First Measurement of R ( X τ / ℓ ) as an Inclusive Test of the b → c τ ν Anomaly
We measure the tau-to-light-lepton ratio of inclusive B -meson branching fractions R ( X τ / ℓ ) ≡ B ( B → X τ ν ) / B ( B → X ℓ ν ) , where ℓ indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed B meson and a charged lepton candidate from 189 fb − 1 of electron-positron collision data collected with the Belle II detector. We find R ( X τ / ℓ ) = 0.228 ± 0.016 ( stat ) ± 0.036 ( syst ) , in agreement with standard-model expectations. This is the first direct measurement of R ( X τ / ℓ ) . Published by the American Physical Society 2024
Physics↗
Search for lepton flavour violating decays of a neutral heavy Higgs boson to $\mu\tau$ and e$\tau$ in proton-proton collisions at $\sqrt{s}=$ 13 TeV
A search for lepton flavour violating decays of a neutral non-standard-model Higgs boson in the μτ and eτ decay modes is presented. The search is based on proton-proton collisions at a center of mass energy $ \sqrt{s} $ = 13 TeV collected with the CMS detector in 2016, corresponding to an integrated luminosity of 35.9 fb$^{−1}$. The τ leptons are reconstructed in the leptonic and hadronic decay modes. No signal is observed in the mass range 200–900 GeV. At 95% confidence level, the observed (expected) upper limits on the production cross section multiplied by the branching fraction vary from 51.9 (57.4) fb to 1.6 (2.1) fb for the μτ and from 94.1 (91.6) fb to 2.3 (2.3) fb for the eτ decay modes.[graphic not available: see fulltext]
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
New oscillation and scattering constraints on the tau row matrix elements without assuming unitarity
The tau neutrino is the least well measured particle in the Standard Model. Most notably, the tau neutrino row of the lepton mixing matrix is quite poorly constrained when unitarity is not assumed. In this paper, we identify data sets involving tau neutrinos that improve our understanding of the tau neutrino part of the mixing matrix, in particular ν ${\tau}$ appearance in atmospheric neutrinos. We present new results on the elements of the tau row leveraging existing constraints on the electron and muon rows for the cases of unitarity violation, with and without kinematically accessible steriles. We also show the expected sensitivity due to upcoming experiments and demonstrate that the tau neutrino row precision may be comparable to the muon neutrino row in a careful combined fit.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for vector-like leptons with long-lived particle decays in the CMS muon system in proton-proton collisions at $\sqrt{s}$ = 13 TeV
A first search is presented for vector-like leptons (VLLs) exclusively decaying into a light long-lived pseudoscalar boson and a standard model τ lepton. The pseudoscalar boson is assumed to have a mass below the τ + τ − threshold, so that it decays exclusively into two photons. It is identified using the CMS muon system. The analysis is carried out using a data set of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment in 2016–2018, corresponding to an integrated luminosity of 138 fb −1 . Selected events contain at least one pseudoscalar boson decaying electromagnetically in the muon system and at least one hadronically decaying τ lepton. No significant excess of data events is observed compared to the background expectation. Upper limits are set at 95% confidence level on the vector-like lepton production cross section as a function of the VLL mass and the pseudoscalar boson mean proper decay length. The observed and expected exclusion ranges of the VLL mass extend up to 700 and 670 GeV, respectively, depending on the pseudoscalar boson lifetime.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Rocks, water, and noble liquids: Unfolding the flavor contents of supernova neutrinos
Measuring core-collapse supernova neutrinos, both from individual supernovae within the Milky Way and from past core collapses throughout the Universe (the diffuse supernova neutrino background, or DSNB), is one of the main goals of current and next generation neutrino experiments. Detecting the heavy-lepton flavor (muon and tau types, collectively v x ) component of the flux is particularly challenging due to small statistics and large backgrounds. Further, while the next galactic neutrino burst will be observed in a plethora of neutrino channels, allowing us to measure a small number of v x events, only upper limits are anticipated for the diffuse v x flux even after decades of data taking with conventional detectors. However, paleo detectors could measure the time-integrated flux of neutrinos from galactic core-collapse supernovae via flavor-blind neutral current interactions. In this work, we show how combining a measurement of the average galactic core-collapse supernova flux with paleo detectors and measurements of the DSNB electron-type neutrino fluxes with the next-generation water Cherenkov detector Hyper-Kamiokande and the liquid noble gas detector DUNE will allow to determine the mean supernova vx flux parameters with precision of order ten percent. Realizing this potential requires both the cosmic supernova rate out to z~1 and the integrated Galactic supernova rate over the last ~1 Gyr to be established at the ~10% level.
79 ASTRONOMY AND ASTROPHYSICS↗
Test of lepton universality in beauty-quark decays
The standard model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown that a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton–proton collision data collected with the LHCb detector at CERN’s Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the standard model, such as a new fundamental interaction between quarks and leptons.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Hint of a new scalar interaction in LHCb data?
Abstract We explain recent LHCb measurements of the lepton universality ratios, $$R_{D^{(*)}}^{\tau /\ell }\equiv \frac{\mathcal {B}(\bar{B} \rightarrow D^{(*)+} \tau ^- \bar{\nu }_\tau )}{\mathcal {B}(\bar{B} \rightarrow D^{(*)+}\ell ^- \bar{\nu }_\ell )}$$ R D ( ∗ ) τ / ℓ ≡ B ( B ¯ → D ( ∗ ) + τ - ν ¯ τ ) B ( B ¯ → D ( ∗ ) + ℓ - ν ¯ ℓ ) and $${R(\Lambda _c^+)}^{\tau /\ell } \equiv \frac{\mathcal {B}(\Lambda _b \rightarrow \Lambda _c^+ \tau ^- \bar{\nu }_{\tau })}{\mathcal {B}(\Lambda _b \rightarrow \Lambda _c^+ \ell ^- \bar{\nu }_{\ell })}$$ R ( Λ c + ) τ / ℓ ≡ B ( Λ b → Λ c + τ - ν ¯ τ ) B ( Λ b → Λ c + ℓ - ν ¯ ℓ ) with $$\ell =\mu $$ ℓ = μ , via new physics that affects $$R_D^{\tau /\ell }$$ R D τ / ℓ and $$R(\Lambda _c^+)^{\tau /\ell }$$ R ( Λ c + ) τ / ℓ but not $$R_{D^*}^{\tau /\ell }$$ R D ∗ τ / ℓ . The scalar operator in the effective theory for new physics is indicated. We find that the forward-backward asymmetry and $$\tau $$ τ polarization in $$\bar{B} \rightarrow D^+ \tau ^{-} \bar{\nu }_{\tau }$$ B ¯ → D + τ - ν ¯ τ and $$\Lambda _b \rightarrow \Lambda _c^+ \tau ^- \bar{\nu }_{\tau }$$ Λ b → Λ c + τ - ν ¯ τ decays are significantly affected by the scalar interaction. We construct a simple two Higgs doublet model as a realization of our scenario and consider lepton universality in semileptonic charm and top decays, radiative B decay, B -mixing, and $$Z \rightarrow b \bar{b}$$ Z → b b ¯ .
Physics↗
Lepton flavor violation by three units
The conservation of lepton flavor is a prediction of the Standard Model and is still an excellent approximate symmetry despite our observation of neutrino oscillations. Lepton flavor violation by one or two units has been discussed for decades, with several dedicated experiments exploring the vast model landscape but no discoveries so far. Here, we explore operators and processes that violate at least one lepton flavor by three units and identify testable signatures. In the Standard Model effective field theory, such operators already arise at mass dimension 7 and can be tested through their contributions to Michel parameters in leptonic decays. True neutrinoless charged-lepton flavor violation arises at mass dimension 10 and can realistically only be seen in the tau decay channels 𝜏 → $𝑒𝑒𝑒\bar{𝜇}\bar{𝜇}$ or 𝜏 → $𝜇𝜇𝜇\bar{𝑒}\bar{𝑒}$, for example in Belle II. Testable rates for these tau decays require light new particles and subsequently predict an avalanche of remarkably clean but so-far unconstrained collider signatures.
Feynman diagrams↗
Neutron Identification Capabilities in MicroBooNE Through the Application of Machine Learning with Blips
Neutrinos (ν) are subatomic particles first observed in 1956 by LANL physicists Clyde Cowan and Frederick Reines but first theorized by Wolfgang Pauli in 1930. Neutrinos are the least massive known particle and are classified as leptons with 3 flavors corresponding to their leptonic counterparts (electron, muon, and tau). We know that they are abundant, 65 billion neutrinos travel through your fingertip every second, and elusive, a single neutrino could fly through a lightyear of lead without interacting at all. Though, there is much still unknown and a better characterization of these “ghostly” particles can give us clues as to the matter/anti-matter asymmetry in the early universe and possible glimpses into new physics. To measure a particle that is extremely light and rarely interacting, physicists have developed an extremely sensitive detector known as a Liquid Argon Time Projection Chamber (LArTPC). The fiducial volume (or TPC) is bombarded with neutrinos, some of which interact with argon (Ar) atoms to produce particles that in turn excite and ionize the Ar. The products of these are free electrons which then drift through the TPC’s applied magnetic field towards a multi-plane wire readout system. The electrons’ charge is collected at this anode and the light from the initial interactions is collected by photomultiplier tubes (PMTs). In conjunction, these mechanisms allow LArTPCs to achieve millimeter spatial resolution and sub-MeV energy thresholds. The detector of interest in this study is the MicroBooNE Experiment at Fermilab. MicroBooNE is an above ground LArTPC with dimensions of approximately 10m × 2.5m × 2.3m, about the size of a school bus. Its purpose is to study neutrinos, so to improve rates of measured ν interactions, the detector is squarely in the path of the Booster Neutrino Beam (BNB) at Fermilab. A major challenge in neutrino studies is energy reconstruction, much of the neutrino’s original energy is lost in interactions that the detector is not sensitive to, often due to low-energy products. The initial goal of this analysis was to better identify neutrons, the main source of poor energy reconstruction in neutrino events. Because neutrons are neutral particles, like neutrinos, we can only directly measure the products of their interactions in LArTPCs. Most of these products are low-energy signals and while each one contributes a negligible amount of energy, collectively these signals make up most of the lost energy in each neutrino event. We define these signals as blips; point-like, isolated depositions of charge in the detector. Blips have MeV-scale energies and are the size of a single hit (charge deposition) or a cluster of a few hits on at least two wire planes. Blips are the principal detector features used to study low-energy physics; thus, they are the key to unlocking information not only about neutrons but gamma photons, supernova and solar neutrinos as well as helping us better identify certain particles. Therefore, this analysis strives to use blips for improved neutron identification (ID) and characterization.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of the τ lepton polarization in Z boson decays in proton-proton collisions at $ \sqrt{s} $ = 13 TeV
The polarization of τ leptons is measured using leptonic and hadronic τ lepton decays in Z → τ$^{+}$τ$^{−}$ events in proton-proton collisions at $ \sqrt{s} $ = 13 TeV recorded by CMS at the CERN LHC with an integrated luminosity of 36.3 fb$^{−1}$. The measured τ$^{−}$ lepton polarization at the Z boson mass pole is $ {\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $ = −0.144 ± 0.006 (stat) ± 0.014 (syst) = −0.144 ± 0.015, in good agreement with the measurement of the τ lepton asymmetry parameter of A$_{τ}$ = 0.1439 ± 0.0043 = $ -{\mathcal{P}}_{\tau}\left(\textrm{Z}\right) $ at LEP. The τ lepton polarization depends on the ratio of the vector to axial-vector couplings of the τ leptons in the neutral current expression, and thus on the effective weak mixing angle sin$^{2} {\theta}_{\textrm{W}}^{\textrm{eff}} $, independently of the Z boson production mechanism. The obtained value sin$^{2} {\theta}_{\textrm{W}}^{\textrm{eff}} $ = 0.2319 ± 0.0008(stat) ± 0.0018(syst) = 0.2319 ± 0.0019 is in good agreement with measurements at e$^{+}$e$^{−}$ colliders.[graphic not available: see fulltext]
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Looking forward to lepton-flavor-violating ALPs
We assess the status of past and future experiments on lepton flavor violating (LFV) muon and tau decays into a light, invisible, axion-like particle (ALP), a . We propose a new experimental setup for MEG II, the MEGII-fwd , with a forward calorimeter placed downstream from the muon stopping target. Searching for μ → ea decays MEGII-fwd is maximally sensitive to LFV ALPs, if these have nonzero couplings to right-handed leptons. The experimental set-up suppresses the (left-handed) Standard Model background in the forward direction by controlling the polarization purity of the muon beam. The reach of MEGII-fwd is compared with the present constraints, the reach of Mu3e and the Belle-II reach from τ → ℓa decays. We show that a dedicated experimental campaign for LFV muon decays into ALPs at MEG II and Mu3e will be able to probe the ALP parameter space in an unexplored region well beyond the existing astrophysical constraints. We study the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle. To this extent we discuss the majoron in low-scale seesaw setups and introduce the LFV QCD axion, the LFV axiflavon and the leptonic familon, paying particular attention to the cases where the LFV ALPs constitute cold dark matter.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗