Standard Model Value Research Articles

Transverse spin asymmetries and the electron Yukawa coupling at an FCC-ee

We show that measurements of single transverse-spin asymmetries at an e+e− collider can enhance the sensitivity to the electron Yukawa coupling, possibly enabling observation of the Standard Model value for this quantity. We demonstrate that the significance in both the bb¯ and semileptonic WW final states can be enhanced by factors of up to 3 compared to inclusive cross-section determinations of this coupling for transversely polarized electrons. If the positrons can be simultaneously longitudinally polarized, even at the level of 30%, the significance can be enhanced by a factor of 5 or more. The method utilizes quantum interference between the Higgs signal and the continuum background and is also applicable in other WW and ZZ final states. Published by the American Physical Society 2024

Angular analysis of B→K*e+e− in the low- q2 region with new electron identification at Belle

We perform an angular analysis of the B→K*e+e− decay for the dielectron mass squared, q2, range of 0.0008–1.1200 GeV2/c4 using the full Belle dataset in the K*0→K+π− and K*+→KS0π+ channels, incorporating new methods of electron identification to improve the statistical power of the dataset. This analysis is sensitive to contributions from right-handed currents from physics beyond the Standard Model by constraining the Wilson coefficients C7(′). We perform a fit to the B→K*e+e− differential decay rate and measure the imaginary component of the transversality amplitude to be ATIm=−1.27±0.52±0.12, and the K* transverse asymmetry to be AT(2)=0.52±0.53±0.11, with FL and ATRe fixed to the Standard Model values. The resulting constraints on the value of C7′ are consistent with the Standard Model within a 2σ confidence interval. Published by the American Physical Society 2024

Determination of the Relative Sign of the Higgs Boson Couplings to W and Z Bosons Using WH Production via Vector-Boson Fusion with the ATLAS Detector.

The associated production of Higgs and W bosons via vector-boson fusion is highly sensitive to the relative sign of the Higgs boson couplings to W and Z bosons. In this Letter, two searches for this process are presented, using 140 fb^{-1} of proton-proton collision data at sqrt[s]=13 TeV recorded by the ATLAS detector at the LHC. The first search targets scenarios with opposite-sign couplings of the W and Z bosons to the Higgs boson, while the second targets standard model-like scenarios with same-sign couplings. Both analyses consider Higgs boson decays into a pair of b quarks and W boson decays with an electron or muon. The data exclude the opposite-sign coupling hypothesis with a significance beyond 5σ, and the observed (expected) upper limit set on the cross section for vector-boson fusion WH production is 9.0 (8.7) times the standard model value at 95% confidence level.

New renormalization scheme in the two Higgs doublet models

We propose a new renormalization scheme in the two Higgs doublet models with a softly-broken Z2 symmetry and CP-conservation in the Higgs sector. In this scheme, counterterms for mixing angles of the Higgs bosons are determined by using the decay rates of the discovered Higgs boson h, i.e., h→τ+τ− and h→ZZ⁎→Zℓ+ℓ− at next leading order (NLO) instead of using the renormalized two-point functions which are adopted in the previous scheme. We require that the decay rates at NLO are determined to be the corresponding predictions at NLO in the Standard Model (SM) times square of the scaling factor which describes the deviation of h couplings at tree level from the SM value. The mixing angles then maintain the meaning of the “alignmentness”, i.e., how the properties of h are close to the SM predictions, while they lose such meaning in the previous scheme. We compare the predictions of the decay rates at NLO given in the new scheme and those in the previous scheme.

Search for pair production of boosted Higgs bosons via vector-boson fusion in the [formula omitted] final state using pp collisions at [formula omitted] with the ATLAS detector

A search for Higgs boson pair production via vector-boson fusion is performed in the Lorentz-boosted regime, where a Higgs boson candidate is reconstructed as a single large-radius jet, using 140 fb−1 of proton–proton collision data at s=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Only Higgs boson decays into bottom quark pairs are considered. The search is particularly sensitive to the quartic coupling between two vector bosons and two Higgs bosons relative to its Standard Model prediction, κ2V. This study constrains κ2V to 0.55<κ2V<1.49 at the 95% confidence level. The value κ2V=0 is excluded with a significance of 3.8 standard deviations with other Higgs boson couplings fixed to their Standard Model values. A search for new heavy spin-0 resonances that would mediate Higgs boson pair production via vector-boson fusion is carried out in the mass range of 1–5 TeV for the first time under several model and decay-width assumptions. No significant deviation from the Standard Model hypothesis is observed and exclusion limits at the 95% confidence level are derived.

B→D* and Bs→Ds* vector, axial-vector and tensor form factors for the full q2 range from lattice QCD

We compute the complete set of Standard Model (SM) and tensor B→D*ℓν¯ and Bs→Ds*ℓν¯ semileptonic form factors across the full kinematic range of the decays using second generation MILC nf=2+1+1 highly improved staggered quark (HISQ) gluon field configurations and HISQ valence quarks, with the method. Lattice spacings range from 0.09 to 0.044 fm with pion masses from ≈300 MeV down to the physical value and heavy quark masses ranging between ≈1.5mc and 4.1mc≈0.9mb; currents are normalized nonperturbatively. Using the recent untagged B→D*ℓν¯ℓ data from Belle and Bs→Ds*μν¯μ from LHCb together with our form factors, we determine a model independent value of Vcb=39.03(56)exp(67)latt×10−3, in agreement with previous exclusive determinations and in tension with the most recent inclusive result at the level of 3.6σ. We also observe a ≈1σ tension between the shape of the differential decay rates computed using our form factors and those measured by Belle. We compute a purely theoretical Standard Model value for the ratio of semitauonic and semimuonic decay rates, R(D*)=0.273(15), which we find to be closer to the recent Belle measurement and heavy flavor averaging group average than theory predictions using fits to experimental differential rate data for B→D*ℓν¯ℓ. Determining Vcb from our form factors and the experimental total rate for B→D*ℓν also gives a value in agreement with inclusive results. We also compute the longitudinal polarization fraction for the semitauonic mode, FLD*=0.395(24), which is in tension at the level of 2.2σ with the recent Belle measurement. Our calculation combines B→D* and Bs→Ds* lattice results in a simultaneous chiral continuum extrapolation, maintaining correlations between both modes. We then give results for both B→D* and Bs→Ds*, with the Bs→Ds* results superseding our previous lattice computation. We also give the chiral perturbation theory needed to analyze the tensor form factors. Published by the American Physical Society 2024

Accessing CKM suppressed top decays at the LHC

We propose an extension of the existing experimental strategy for measuring branching fractions of top quark decays, targeting specifically t\to j_q Wt→jqW, where j_qjq is a light quark jet. The improved strategy uses orthogonal bb- and qq-taggers, and adds a new observable, the number of light-quark-tagged jets, to the already commonly used observable, the fraction of bb-tagged jets in an event. Careful inclusion of the additional complementary observable significantly increases the expected statistical power of the analysis, with the possibility of excluding |V_{tb}|=1|Vtb|=1 at 95\%95% C.L. at the HL-LHC, and accessing directly the standard model value of |V_{td}|^2+|V_{ts}|^2|Vtd|2+|Vts|2.

New physics interpretations for nonstandard values of h→Zγ

Current measurements of the h→Zγ signal strength invite us to speculate about possible new physics interactions that exclusively affect μZγ without altering the other signal strengths. Additional consideration of tree unitarity enables us to correlate the nonstandard values of μZγ with an upper limit on the scale of new physics. We find that even when μZγ deviates from the Standard Model value by only 20%, the scale of new physics should be well within the reach of the LHC. Published by the American Physical Society 2024

Improved Tensor Current Limit from ^{8}B β Decay Including New Recoil-Order Calculations.

A precision measurement of the β^{+} decay of ^{8}B was performed using the Beta-decay Paul Trap to determine the β-ν angular correlation coefficient a_{βν}. The experimental results were combined with new abinitio symmetry-adapted no-core shell-model calculations to yield the second-most precise measurement from Gamow-Teller decays, a_{βν}=-0.3345±0.0019_{stat}±0.0021_{syst}. This value agrees with the standard model value of -1/3 and improves uncertainties in ^{8}B by nearly a factor of 2. By combining results from ^{8}B and ^{8}Li, a tight limit on tensor current coupling to right-handed neutrinos was obtained. A recent global evaluation of all other precision β decay studies suggested a nonzero value for right-handed neutrino coupling in contradiction with the standard model at just above 3σ. The present results are of comparable sensitivity and do not support this finding.

Spin-flavor oscillations of relic neutrinos in primordial magnetic field

The neutrino magnetic moment operator clasps a tiny but nonzero value within the standard model (SM) of particle physics and rather enhanced values in various new physics models. This generation of the magnetic moment (μν) is through quantum loop corrections which can exhibit spin-flavor oscillations in the presence of an external magnetic field. Also, several studies predict the existence of a primordial magnetic field (PMF) in the early Universe, extending back to the era of Big Bang nucleosynthesis (BBN) and before. The recent North American Nanohertz Observatory for Gravitational Waves measurement can be considered as a strong indication of the presence of these PMFs. In this work, we consider the effect of the PMF on the flux of relic neutrinos. For Dirac neutrinos, we show that half of the active relic neutrinos can become sterile due to spin-flavor oscillations well before becoming nonrelativistic owing to the expansion of the Universe and also before the timeline of the formation of galaxies and hence intergalactic fields, subject to the constraints on the combined value of μν and the cosmic magnetic field at the time of neutrino decoupling. For the upper limit of PMF allowed by the BBN, this can be true even if the experimental bounds on μν approach a few times its SM value. Published by the American Physical Society 2024

Lepton universality in a model with three generations of sterile Majorana neutrinos

The extension of the Standard Model lepton sector by three right-handed Majorana neutrinos [heavy neutral leptons (HNLs)] with masses up to GeV scale is considered. While the lightest heavy neutral lepton is the dark matter particle with mass of the order of 5 keV, the remaining two HNLs ensure standard (active) neutrino mass generation by means of the seesaw type I mechanism. Two heavy sterile neutrinos with quasidegenerate masses up to 5 GeV can induce the deviation of lepton universality violation parameter in the decays of π+ and K+ mesons from the Standard Model value. Contours are obtained for the permissible values of this parameter within the framework of two mixing scenarios, taking into account the lifetime boundary for heavy neutral lepton from big bang nucleosynthesis in the Universe. When calculating the HNL decay width in the framework of the model with six Majorana neutrinos, three active and three heavy, both two-particle and three-particle lepton decays, essential for masses below the mass of the pion, were taken into account. When calculating the decay widths, the limiting case known as the “Dirac limit” is not used. The results based on the explicit form of mixing matrices for three HNL generations and the diagram technique for Majorana neutrinos, which explicitly take into account the interference terms for diagrams with identical mass states, can lead to some differences in lifetime from the results using the Dirac limit and the displacement of the corresponding experimental exclusion contours of the “mass-mixing” type. For the second mixing scenario, a mass region of 460 MeV&lt;M&lt;485 MeV has been found that allows violation of lepton universality in charged kaon decays at the level observed in the experiment. Published by the American Physical Society 2024

Explaining the muon g – 2 discrepancy by the two-Higgs-doublet model

The anomalous magnetic dipole moment of muon (from now on called muon g – 2) is one of the precision measurements sensitive to new physics. Recent measurement, performed by the Muon g – 2 Collaboration of Fermilab, differs from the Standard Model (SM) value calculated by the Muon g – 2 Theory Initiative Group at the combined statistical significance of 5.1σ. Taking at face value, such discrepancy is caused by new physics. In this paper, we explain the discrepancy in the context of the two-Higgs-doublet model. We shall systematically classify every scenario that may induce muon g – 2. Their compatibility with relevant constraints, e.g. the oblique parameters and/or lepton-favor universality will also be discussed. We shall also show that the two-Higgs-doublet model can be extended to the Zee model, giving rise to radiative neutrino masses.

Cosmological Distance Measurement of Twelve Nearby Supernovae IIP with ROTSE-IIIb

We present cosmological analysis of 12 nearby (z < 0.06) Type IIP supernovae (SNe IIP) observed with the ROTSE-IIIb telescope. To achieve precise photometry, we present a new image-differencing technique that is implemented for the first time on the ROTSE SN photometry pipeline. With this method, we find up to a 20% increase in the detection efficiency and significant reduction in residual rms scatter of the SN lightcurves when compared to the previous pipeline performance. We use the published optical spectra and broadband photometry of well-studied SNe IIP to establish temporal models for ejecta velocity and photospheric temperature evolution for our SNe IIP population. This study yields measurements that are competitive with other methods even when the data are limited to a single epoch during the photospheric phase of SNe IIP. Using the fully reduced ROTSE photometry and optical spectra, we apply these models to the respective photometric epochs for each SN in the ROTSE IIP sample. This facilitates the use of the Expanding Photosphere Method (EPM) to obtain distance estimates to their respective host galaxies. We then perform cosmological parameter fitting using these EPM distances, from which we measure the Hubble constant to be 72.9−4.3+5.7kms−1Mpc−1 , which is consistent with the standard ΛCDM model values derived using other independent techniques.

Constraining the Higgs trilinear coupling from an SU(2) quadruplet with bounded-from-below conditions

Integrating out a heavy scalar can cause the Higgs trilinear coupling to deviate from its Standard Model value: a good example is provided by an SU(2) quadruplet. Constraints on the full theory, however, can limit the size of the deviation. We show that the bounded-from-below conditions for the Standard Model extended by an SU(2) quadruplet strongly constrain the ℤ2-breaking Higgs portal and can bound the Higgs trilinear coupling close to its Standard Model value. For TeV-scale quadruplet masses in models with custodial symmetry violation, these constraints can be a few times stronger than constraints from electroweak precision measurements. For the custodial quadruplet, these are the strongest theoretical constraints available.

Implications of an enhanced B→Kνν¯ branching ratio

Rare decays mediated by b→sνν¯ transitions have been reported by the Belle II experiment. The branching ratio of the decay B+→K+νν¯ is found to be enhanced with respect to the standard model value. If taken at face value, the implications are profound; either lepton flavor universality is violated at the (multi)-TeV-scale, or light new physics is involved. This holds in general if B(B+→K+νν¯) exceeds 1.2×10−5(1.3×10−5) at 1σ (2σ), which tightens with a decreasing upper limit on B(B→K*νν¯), that is in reach of the Belle II experiment. In view of the strong constraints on electron-muon universality violation in |Δb|=|Δs|=1 processes, viable explanations are heavy, (5–10)-TeV tree-level new physics mediators that couple only to tau-flavors, or lepton flavor violating ones. In addition, couplings of similar size to both left- and right-handed quarks are generically required, implying nonminimal beyond the standard model sectors which are carefully balanced against flavor constraints. The decay Bs0→invisibles can shed light on whether new physics is light or heavy. In the former case, branching ratios can be as large as 10−5. Published by the American Physical Society 2024

Higgs boson pair production at NLO in the Powheg approach and the top quark mass uncertainties

We present a new Monte Carlo code for Higgs boson pair production at next-to-leading order in the Powheg-Box Monte Carlo framework. The code is based on analytic results for the two loop virtual corrections which include the full top quark mass dependence. This feature allows to freely assign the value of all input parameters, including the trilinear Higgs boson self coupling, as well as to vary the renormalization scheme employed for the top quark mass. We study the uncertainties due to the top-mass renormalization scheme allowing the trilinear Higgs boson self coupling to vary around its Standard Model value including parton shower effects. Results are presented for both inclusive and differential observables.

Effects of Kaluza-Klein neutrinos on RD and [formula omitted

Recent measurements of RD and RD⁎ by the LHCb collaboration show deviations from their respective Standard Model values. These semileptonic B meson decays, associated with b→cτν¯ transition, are pointing toward new physics beyond the Standard Model via leptonic flavor universality violation. In this paper, we show that such anomaly can be resolved by the cummulative Kaluza-Klein (KK) modes of singlet right-handed neutrino which propagates in the large extra dimensional space. We found that the number of extra dimension should be 2 to explain RD and RD⁎. We show that both RD and RD⁎ constraint the energy scale MF of this extra dimension which are compatible with the limits from lepton flavor violating tau decays. In contrast, our findings are in tension with the limits coming from the neutrino experiments which set the most stringent lower bound on MF. The future measurements of RD(⁎)exp with reduced uncertainties will exclude this extra dimensional model with right-handed neutrino propagating in the bulk, if the central values stay.

Observation of the γγ→ττ Process in Pb+Pb Collisions and Constraints on the τ-Lepton Anomalous Magnetic Moment with the ATLAS Detector.

This Letter reports the observation of τ-lepton-pair production in ultraperipheral lead-lead collisions Pb+Pb→Pb(γγ→ττ)Pb and constraints on the τ-lepton anomalous magnetic moment a_{τ}. The dataset corresponds to an integrated luminosity of 1.44 nb^{-1} of LHC Pb+Pb collisions at sqrt[s_{NN}]=5.02 TeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a τ-lepton decay, an electron or charged-particle track(s) from the other τ-lepton decay, little additional central-detector activity, and no forward neutrons. The γγ→ττ process is observed in Pb+Pb collisions with a significance exceeding 5 standard deviations and a signal strength of μ_{ττ}=1.03_{-0.05}^{+0.06} assuming the standard model value for a_{τ}. To measure a_{τ}, a template fit to the muon transverse-momentum distribution from τ-lepton candidates is performed, using a dimuon (γγ→μμ) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for a_{τ} is -0.057<a_{τ}<0.024.

Light-quark Yukawa couplings from off-shell Higgs production

Yukawa couplings of the first quark generation are notoriously difficult to constrain due to their small values within the Standard Model. Here we propose Higgs off-shell production, with the Higgs boson decaying to four leptons, as a probe of the up- and down-quark Yukawa couplings. Using kinematic discriminants similar to the ones employed in the Higgs width measurements we find that the down (up) Yukawa coupling can be constrained to a factor of 156 (260) times its Standard Model value at the high-luminosity LHC assuming only experimental systematic uncertainties. Off-shell Higgs production hence provides better sensitivity to the first-generation quark Yukawa couplings with respect to other probes such as Higgs+jet or Higgs pair production.

Extended analysis of neutrino-dark matter interactions with small-scale CMB experiments

We explore an extension of the standard ΛCDM model by including an interaction between neutrinos and dark matter, and making use of the ground based telescope data of the Cosmic Microwave Background (CMB) from the Atacama Cosmology Telescope (ACT). An indication for a non-zero coupling between dark matter and neutrinos (both assuming a temperature independent and T2 dependent cross-section) is obtained at the 1σ level coming from the ACT CMB data alone and when combined with the Planck CMB and Baryon Acoustic Oscillations (BAO) measurements. This result is confirmed by both fixing the effective number of relativistic degrees of freedom in the early Universe to the Standard Model value of Neff=3.044, and allowing Neff to be a free cosmological parameter. Furthermore, when performing a Bayesian model comparison, the interacting νDM (+Neff) scenario is mostly preferred over a baseline ΛCDM (+Neff) cosmology. The preferred value is then used as a benchmark and the potential implications of dark matter’s interaction with a sterile neutrino are discussed.