A new fourteen page preprint summarizing results from several of the main subjects discussed at the Moriond 2024 conference has a highly concentrated wealth of results from the last year, some of which were first announced at the conference. The paper is Barbara Clerbaux, "Experimental Summary of the Moriond 2024 conference - Electroweak Interaction & Unified Theories" arXiv:2409.07120 (September 11, 2024).
This year's results generally strongly vindicate the Standard Model of Particle Physics, although there are a few minor experimental tensions with it. I summarize the results further below.
The Higgs Boson
The latest (full run 2) mass measurement from CMS and from ATLAS are mH = 125.04 ± 0.11 (stat) ± 0.05 (syst) GeV for the H→ZZ→4ℓ decay channel and mH = 125.17 ± 0.11 (stat) ± 0.09 (syst) GeV for the H→γγ decay channel, respectively, the main uncertainties coming from the lepton and photon energy scales. Figure 1 presents the various H mass measurements of ATLAS and the final run 1 and run 2 combination, leading to a relative precision of 0.09%. The H mass uncertainty target for the HL-LHC is about 20 MeV.
The tiny width predicted in the SM of 4.1 MeV is much smaller than the experimental mass resolution of about 1 to 2 GeV. However BSM contributions could bring a significant enhancement of the H width. ATLAS and CMS deduced an indirect limit on the H width using the ratio of the off-shell and on-shell cross section measurements.
Various other measurements of Higgs boson decays and couplings are basically consistent with the Standard Model predictions for those properties.
A wide scope of new BSM H boson searches has been released by ATLAS and CMS. No excess are observed above the SM prediction, however still a large amount of phase space is available for extended H sectors. In the search for low mass H→γγ, CMS observes an excess of local (global) significance of 2.9σ (1.3σ) at a mass of 95.4 GeV, ATLAS observes a local significance of 1.7σ at 95.4 GeV.
The 95.4 GeV excess is not statistically significant globally, or in the combined CMS and ATLAS measurements. Notably, the bump that is observed is very close to the mass of the Z boson, which is 91.188(2) GeV plus the mass of the b quark which is 4.183(7) GeV (according the latest Particle Data Group estimate), the sum of which is 95.371(7) GeV, which to three significant digits is 95.4 GeV.
Since a bb decay is the most common form of Higgs boson decay, and Z boson-photon decays are also possible, one possibility, for example, is that this bump represents the decays of a real and a virtual Higgs boson pair in which one of the b quark decay products and a photon are missed by the LHC detectors and misinterpreted in the subsequent analysis.
The Electroweak Precision Results
The W mass is extracted from the W boson transverse mass and pT distributions. The obtained value, mW = 80366.5 ± 15.9 MeV, has an impressive precision of less than 0.02%. The W mass result, shown in Figure 4 (left) is in good agreement with the SM and does not confirm the higher value of the W mass obtained in 2022 by the CDF data re-analysis measurement. The ATLAS analysis is also sensitive to the W width, measured to be ΓW =2202±47 MeV. ATLAS performed a comprehensive study of events with jets and large missing transverse energy (MET) in the final state, providing a measurement of the differential Z→ ν¯ ν cross section as a function of the Z boson pT. The W mass is also measured by LHCb with uncertainties that are anti-correlated to that of ATLAS and CMS. Using about a third of the available run 2 dataset, the value of mW = 80354±32 MeV is obtained by LHCb, with the target to have an expected statistical precision with the full run 2 dataset of about 14 MeV. . . .
The sin2θℓ eff measurement is a CMS highlighted new result presented at the conference. The mixing angle sin2θℓ eff is a key parameter of the SM and is calculated using other precise experimental inputs to be sin2θℓeff(SM)=0.23155±0.00004. Up to now the most precise measurements come from LEP and SLD, and differ between each other by about 3σ. The new CMS analysis uses the Drell-Yan events with electron or muon pairs in the final state. In case of the electron channel, the very forward calorimeters up to a pseudorapidity value of |η| = 4.36 are added in the event selection, increasing significantly the measurement precision of the forward-backward asymmetry in the lepton decay angle. From this, a value of sin2 θℓ eff=0.23157±0.00031 is extracted, reaching a comparable precision as the LEP and SLD measurements, as shown in Figure 4 (right). . . .
The test of lepton flavour universality (LFU) in W decays is the highlighted new result by ATLAS. The analysis uses the top-quark pair events and compares the occurrence of W decays in the muon and the electron final states. To reduce as much as possible the systematics uncertainties, the ratio R = BR(W→µν)/ BR(W→eν) is measured and normalised to the corresponding ratio for the Z boson BR(Z→µµ)/ BR(Z→ee). The ratio R obtained is presented in Figure 5(left), the value is in agreement with 1 with a relative uncertainty of 0.45%. This is the most precise single measurement for this ratio to date and is also more precise than the previous PDG (particle data group) average.
The photon-induced production of a pair of tau leptons is observed for the first time in proton-proton collisions by CMS at 5.3σ. . . . Modifying the tau lepton magnetic moment modifies the γγ → ττ cross section and modifies the pT and mass distributions of the signal. A very precise measurement of the tau lepton anomalous magnetic moment is extracted and presented in Figure 5(right), in good agreement with the expected SM value given as the dashed vertical line. The measurement does not show evidence for the presence of new physics that would modify its value.
Top Quark Physics
Measurements of top quark properties have been reported by ATLAS and CMS. . . . A new combination of the ATLAS and CMS top quark mass measurements leads to mt = 172.52 ± 0.14 (stat) ± 0.30 (syst) GeV, the dominant systematics uncertainty coming from the b-quark jet energy scale. . . . The ttZ+tWZ cross section measurement has a small tension with the SM prediction (being slightly above at a 2σ level). The new ATLAS result on the t¯ tγ production is in agreement with the SM.
Quantum entanglement in top events are new results that generated excitement and discussion during the conference. ATLAS and CMS presented their latest analysis results from top-antitop events in the dilepton decay channels. Top-quark pairs at the LHC are mainly unpolarised, with their spins being strongly correlated. The spin information can be measured via the final state particle angular variables. The spin correlation depends on the mass of the top-antitop system mt¯t and on the angular variables. A system is considered as being in a quantum entanglement state if D < −1/3, where D is defined as the trace of the spin correlation matrix divided by 3. . . . entanglement is observed with > 5σ at low mt¯t. The CMS analysis shows in addition that when a t¯t bound state (toponium, a colour singlet pseudo-scalar state) is included in the simulation, the agreement between the measurement and the SM simulations improves in the threshold mass region.
Beyond The Standard Model Physics
[N]o deviation from the SM expectation has been observed[.]
Flavor Physics
The LHCb and CMS experiments made measurements of CP violation in b quark and charm quark decays that increase the precision with which the CP violating parameter in the CKM matrix has been measured.
The LHCb and Belle/Belle II experiments looked at lepton flavor universality violations in semi-leptonic decays of b quarks to charm quarks, in results that put the global average measurement in mild tension with the Standard Model prediction of lepton flavor universality (at a 3.2 sigma level). The great spread of the experimental results, however, casts doubt on the meaningfulness of a global average measurement.
Belle/Belle II improved the accuracy with which the branching fractions of ten kinds of B meson decays and measured the branching fractions of four more kinds of B meson decays for the first time.
BESIII mostly measured charmed hadron decays, improving the precision with which the CKM matrix element for charm to strange quark transition probability is known and examining the possibility of lepton flavor universality violations:
Using this measurement together with input from lattice QCD calculation, the CKM matrix element |Vcs| is determined with a precision of 1.4%. When combined with the tau decay channel analysis, the precision on |Vcs| value improves to 1.0%. Lepton flavour universality tests have also been performed in leponic and semi-leptonic decays of charm mesons. No violation was observed at the 1.5% precision level.
Neutrino Physics
Multiple experiments including NOvA, T2K, and Super-Kamiokande studied neutrino oscillation parameters. The precision of the measurement of the mass difference between the second and third neutrino mass eigenstates was improved. A normal mass ordering is favored, but only inconclusively. CP violation in neutrino oscillations has also been largely confirmed, but its magnitude has large uncertainties.
Efforts to detect neutrinoless double-beta (0νββ) at the CUORE and Legend experiments continued to come up empty, increasing the minimum half-life for neutrinoless double-beta decay. At CUORE:
The limit obtained for the half-live time of 130Te, based on data taken from 2017-2023 . . . is T1/2 0ν > 3.8 x 10^25 yr at 90% CL, which is the most stringent limit for the 130Te to date. The corresponding limit on the effective Majorana mass assuming a light Majorana neutrino-exchange is mββ < 70-240 meV.
The Legend experiment using enriched germanium detector 76Ge has a . . . ultimate goal is to reach sensitivity for a half-life time of this nucleus beyond 10^28 years, corresponding to a neutrino effective mass measurement of about 18 meV.
Neutrinoless double beta decay, if discovered, would be strong evidence that the neutrino is a Majorana particle with Majorana mass, and would represent the first evidence of non-conservation of lepton number ever observed. But, given the increasing evidence that the neutrino masses are masses are very small, with the lightest neutrino mass probably well under 18 meV, we shouldn't expect to be able to detect neutrinoless double beta decay in the near to medium term, even if neutrinos do have Majorana mass.
The Faser experiment at the LHC has as its goal:
to measure SM neutrino interaction cross sections at unexplored TeV energies, as well as to search for long-lived BSM particles (e.g. axion-like particles ALPs). . . . New results were presented on neutrino (νe and νµ) interaction cross sections. . . . This represents the first detection of νe at the LHC. Results on limits on ALPs were also shown for a luminosity of 57.7 fb−1, excluding uncovered parameter space (in the coupling and mass plane) significantly. . . .
The present neutrino mass limit of 0.8 eV from the Katrin experiment was reminded and the future release of the neutrino mass limit with 0.5 eV sensitivity expected for mid-2024 was presented, together with the R&D for the Katrin++ project to reach the inverted ordering mass scale.
The Katrin result is now outdated as previously reported at this blog. The new limit is actually 0.45 eV.
An interesting (small) deficit of events was observed by the IceCube experiment in the muon antineutrino survival probability for atmospheric neutrinos, that can be fitted with the addition of a fourth neutrino family (the p-value for the null hypothesis is 3.1%).
The potentially anomalous IceCube results have been credibly explained as the result of flawed modeling. See also this July 2024 paper reaching a contrary conclusion.
Dark Matter Searches
Searches for dark matter (DM) by the Lux-Zeplin experiment at the Sanford Underground Research Facility and by the PandaX experiment at the China Jinping Underground Laboratory were reported.
None of the experiments detected any dark matter and the parameter space excluded by these direct dark matter detection experiments was expanded.
Muon g-2
Updates on anomalous magnetic moment of the muon defined as aµ = (gµ −2)/2 were also discussed. It is a very sensitive variable to new physics, as the quantum effects arise from virtual particle contributions from all known and potentially unknown particles. The long-standing discrepancy between the experimental measurements and the theory predictions has been scrutinised during the conference. The Fermilab Muon g-2 experiment is providing improved measurements, currently at a precision of 0.2 ppm. A lot of efforts are dedicated to the SM calculation, and more specifically on the hadronic vacuum polarisation contribution. New results on lattice QCD have been presented and when taken into account, the SM prediction for aµ falls better in line with the experimental results. However these computations are complicated, and lattice QCD results from other groups are expected to be public soon. A discussion will then take place on the inclusion or not of these results in the official SM calculation.
As mentioned in previous posts on the determination of the SM prediction for muon g-2, it is actually pretty clear that the experimental results confirm the SM prediction, and that the previously anomaly was a result of inaccurate experimental data that was used to substitute for some particular difficult Lattice QCD calculations.
No comments:
Post a Comment