Moriond electroweak takes stock of open questions

6 April 2023
Many exciting results were presented at this year's Moriond electroweak session. Credit: L Fayard

The 57th Recontres de Moriond conference on electroweak interactions and unified theories, which took place from 18 to 25 March on the Alpine slopes of La Thuile in Italy, saw over 150 physicists meet in person for week packed with physics. More than 100 talks on the latest experimental results and theoretical ideas  were actively debated, not only during the sessions but also during the breaks and meal times, in a stimulating and congenial atmosphere. The talks covered all the important areas of electroweak physics, with experiment and theory providing complementary approaches to some of the most pressing problems in particle physics and cosmology.

Neutrinos first
Neutrino masses and mixing provide a unique window on the only new physics so far seen beyond the Standard Model. The measured mass differences and mixing parameters provide a consistent picture suggesting the presence of a new scale potentially at approximately 1015 GeV. However, to complete this picture two fundamental elements are missing: the absolute mass scale of neutrinos and the determination, via neutrinoless double beta decay, of whether  neutrinos have a Majorana nature. Also of fundamental importance are the mass-squared ordering of  neutrinos, the maximality (or not) of atmospheric mixing, and the measurement of leptonic CP violation. All these questions were addressed by a range of new experimental results, many of which were presented for the first time.

NOvA and T2K presented a very consistent picture of the PMNS framework with a slight preference of the normal over the inverted ordering

The KATRIN collaboration reported an absolute upper limit on the electron-neutrino mass of 800 meV and is expected to reach a limit of 200 meV eventually. With a detailed analysis of their tritium decay spectrum, the team was also able to exclude rapid oscillations of electron neutrinos with potential sterile neutrinos and to set a limit on cosmic-neutrino local over-densities. The KamLandZEN, CUPID-Mo and Majorana Demonstrator experiments showed first results on neutrinoless double-beta decay searches in different systems.  KamLandZEN  had the largest number of radionuclei, providing upper limits on the effective electron neutrino mass between 36 and 156 meV (depending on model assumptions) and is expected to reach 20 meV with more data. CUPID-Mo and Majorana Demonstrator experiments are expected to eventually reach stronger limits down to approximately 10 meV. The latter experiment, based on germanium detectors, also reported interesting bounds on models for wave-function collapse.


The long-baseline νμ oscillation experiments NOvA and T2K presented analyses of their latest intermediate dataset, showing a very consistent picture of the PMNS framework with a slight preference (at the one or two standard-deviation level) of the normal over the inverted ordering and the upper over the lower octant for θ23. Both experiments are sensitive to electron-neutrino appearance. NOvA, however, provided the first evidence for electron anti-neutrino appearance and a first long-baseline measurement of sin2θ23, in very good agreement with the reactor neutrino data. Both experiments exclude CP conserving values of δCP of 0 or π at 90% confidence. IceCUBE with its DeepCORE extension also presented stunning atmospheric neutrino-oscillation results comparable with SuperKamiokande and long-baseline experiment sensitivities. All these experiments provide strong supporting evidence of the validity of the three neutrino-flavour paradigm.

Longstanding neutrino anomalies were discussed in detail. The reactor-neutrino deficit interpretation in terms of the existence of a sterile neutrino species is incompatible with several short baseline data. The significance of the LSND and MiniBooNE short-baseline low-energy excess was revisited in the light of new backgrounds. The long-standing gallium anomaly was further verified and confirmed by the independent experiment BEST. The BEST observations are, however, also not compatible with a simple sterile-neutrino oscillation pattern. The PROSPECT reactor-neutrino experiment also showed first results excluding the gallium anomaly in terms of an oscillation with a sterile neutrino. Finally, a peaking anomaly, in the range 5-7 MeV, was observed by several experiments (including RENO, DayaBay, NEOS, Chooz and PROSPECT). This anomaly cannot be easily interpreted in terms of fundamental neutrino physics. Instead, nuclear models have been discussed in detail and should be looked at carefully.

Finally, the results of CONUS, a Coherent neutrino scattering experiment based on high precision germanium detectors,  set limits on light vector mediators and the neutrino magnetic moment.

The three-neutrino paradigm is standing tall with some anomalies that  need to be further clarified, in particular the BEST gallium anomaly.

On the theoretical side, it was shown that leptogenesis is possible for any right-handed neutrino masses above about 0.1 GeV, which, if light enough, can be probed by the proposed SHiP experiment at CERN, as well as FCC-ee and HL-LHC. Neutrino experiments such as COHERENT were analysed in the framework of Standard Model Effective Field Theory.

The IceCUBE experiment also showed splendid multi-messenger results from high- and ultrahigh-energy neutrino observations and pointed out their ability to probe the Standard Model with ultrahigh-energy neutrinos that have travelled cosmic distances. These neutrinos are expected to be even mixtures of the three neutrino species; any deviation would be a clear sign of new physics. The cosmic-neutrino data also highlighted the missing data in neutrino-nucleon interactions in the range of a few 100 GeV to 10 TeV. At this year’s Moriond conference, the birth of collider neutrino physics was also presented, with the first results from the FASERν and SND experiments. FASERν showed the first unambiguous observation of neutrinos from proton-proton collisions at LHC point 1.

Overall the three neutrino paradigm is standing tall with some anomalies that still need to be further clarified, in particular the BEST gallium anomaly.

From neutrinos to quarks

From a theoretical point of view, neutrino and heavy-quark physics are two sides of the same coin: they provide information related to the flavour problem, namely the unexplained origin of quark and lepton families, masses and mixings. The fact that in the Standard Model fermion mass hierarchies arise from Yukawa couplings does not make it more satisfactory. The recently observed anomalies in semi-leptonic B decays exhibiting unexpected lepton-flavour patterns have raised numerous speculations and have in particular suggested that the flavour scale might be right around the corner at the TeV scale, motivating models discussed at the conference involving a new Z’ gauge boson or a scalar or a vector leptoquark from a twin Pati-Salam theory of flavour.

However, the recent results from LHCb on the main anomalies have shed new light on the question. LHCb discussed their recent reanalysis of the R(K) and R(K*) ratio of decay rates of B→K(*)μμ /ee with the inclusion of an additional background from misidentified electrons are now in excellent agreement with the Standard Model. LHCb also presented a new result on the measurement of the R(D*) ratio of decay rates including fully hadronic τ decays and a new combined measurement of the R(D) and R(D*) ratios. With these new measurements from LHCb the R(D*) ratio agrees with the Standard Model predictions. A tension at the 3 standard deviations level is still observed, mostly due to the R(D) ratio.

Alternatively, D-meson decays were extensively discussed as a promising new playground for discovering new physics due to the richness of new data available, and the efficiency of the GIM mechanism for the charm quark and SU(3) flavour symmetry leading to easily verifiable null tests of the Standard Model.

Results of various rare decay and new resonance searches were presented by LHC experiments, with for example the ambitious searches of the extremely rare decay mode of the D meson in two muons, the observation by the CMS experiment of the decay of the η meson to four muons and the search for  states decaying to di-charmonium states as J/ψ/J/ψ or J/ψ/ψ2S to four muons, which could correspond to four charm tetra-quark states.

Leaving no stone unturned, the LHC experiments have presented a whole host of new results of searches for new phenomena beyond the Standard Model

A highlight of the conference was the strong contribution from the Belle II experiment in all areas of heavy flavour physics, including: several measurements of b→s transitions, including a fully inclusive measurement; several time dependent CP-violation observables, which yield precisions on the CKM parameter sin(2β) on a par with the current world’s best measurements in those channels; as well as new input to the |Vub| and |Vcb| puzzle (the tension between exclusive and inclusive measurements which suffer from different theoretical uncertainties), with an exclusive measurement in the golden B→πlν mode and an inclusive measurement of the B→D*lν decay.


LHCb presented nice new results in the bsss transition in the φφ channel showing that no CP- violating effect is seen, with results separated in different polarisation modes. LHCb also presented a new measurement of the CKM angle γ in the B±→D[Kπ±ππ±]h± (h = π, K) channel and an overall combination yielding a precision of approximately 3.7º.

Finally, a status report was given by the KOTO experiment which is searching for the extremely rare KLπνν process. The two first runs (starting in 2015 until 2018) have allowed the collaboration to identify two new backgrounds and provide methods to mitigate them since 2019.  With these improvements the KOTO experiment should reach sensitivities at the 10-10 level, close to the expected branching fraction in the Standard Model of 3×10-11. All measurements shown so far are compatible with the CKM paradigm.

Also in the quark sector, the latest measurements and the prospects in measurements of the neutron electric dipole moment were presented, providing strong constraints on new physics scenarios at high energy scales.

Lattice-QCD studies have made remarkable progress in recent years, with hadronic contributions to  muon g-2 being more or less under control, more so in the case of light-by-light contributions, which agree well with other results, and less so regarding the hadronic vacuum polarisation with  errors being driven down by the BMW collaboration, which by itself seems to lead to more consistency with the FNAL and BNL results. However, the BMW results are not yet fully confirmed either by other lattice groups or the R-ratio from experiment, with the recent VEPP data being out of line with previous experiments.

Higher precision from lattice calculations has also led to the so-called Cabibbo anomaly reported at Moriond, whereby the unitarity of the first row of the CKM matrix seems to be violated by 2.7σ. If confirmed by future experiments and lattice calculations, this could be a signal for new physics.

In addition, in the lepton flavour sector Belle II presented their first and already the world’s most precise tau-mass measurement, which agrees with previous measurements. With only approximately half the luminosity accumulated by the Belle experiment, Belle II presented measurements surpassing the Belle precision, thus displaying the excellent performance of the experiment.

Dark searches

A variety of dark-matter candidates were discussed including: primordial black hole with improved limits using 21 cm hydrogen astronomy; weakly interacting massive particles (WIMPs) from new electroweak fermion multiplets with heavier masses; heavy singlet dilaton-like scalars; keV neutrinos from an inverse seesaw model; axions or axion-like particles with an extended window of masses arising from non-standard cosmology; and ultralight dark matter such as dark photons whose interactions with the detector could be simulated by the software package DarkELF. An interesting proposal for axion detectors that can double up as high-frequency gravitational wave detectors was also discussed.

A flurry of results of searches for dark-sector particles at the LHC, Belle II, Babar, NA62, BES and PADME were shown.

The XENONnT collaboration presented new results, unblinded for the occasion, with an exposure of 95.1 days corresponding to 1.1 tonne-year. LZ also presented their latest results with a similar exposure. The two experiments, along with the PandaX xenon-based experiment, are now exploring new territory at low WIMP-nucleon cross sections.

These very low cross sections motivate further searches for the existence of a dark sector with dark photons or axion-like particles. A flurry of results of searches for dark-sector particles at the LHC, Belle II, Babar, NA62, BES and the PADME experiment were shown. PADME, a fixed-target e+e experiment, also presented their ability to directly probe an anomaly which was also seen in 12C and 4He.

Theories of new heavy particles were also discussed, ranging from an analysis of the minimal supersymmetric Standard Model which showed that gluinos of 1 TeV and stop squarks of 500 GeV could still have escaped detection, to theories of two Higgs-doublet models plus a Higgs singlet, which might be responsible for the 95 GeV diphoton events, to the observation that vector-like fermions (which come in opposite chirality pairs) have the right properties to avoid a metastable universe.

Electroweak searches at the LHC

The LHC experiments presented results from a host of searches for new phenomena beyond the Standard Model, leaving no stone unturned. These looked for signatures of models motivated by theories addressing the shortcomings of the Standard Model, astrophysical and cosmological observations such as dark matter that could be interpreted as the existence of a fundamental field, and experimental anomalies observed such as in the lepton-flavour or muon g-2 anomalies. These searches place very important limits on the presence of new phenomena up to the few-TeV scale. With 20 times more data, the High-Luminosity LHC (HL-LHC) will provide invaluable opportunities to significantly increase the search domain and bring potential for discoveries.

The LHC experiments also presented a series of new results based on W and Z production, coinciding very well with the 40th anniversary of the W and Z boson discoveries at the CERN SppS. The CMS collaboration showed a measurement of the τ polarisation. This measurement can be directly translated in terms of a measurement of the weak mixing angle with a precision of approximately 10%, which is close to the precision reached by e+e experiments. The CMS collaboration also presented a measurement of the invisible width of the Z boson that is more precise than the direct invisible-width measurements performed at LEP. ATLAS showed the precise measurement of the Z boson transverse momentum differential cross section integrated over the full phase space of leptons produced in the Z decay, and with it was able to provide the current most precise measurement of αS with a precision comparable to the current world average or estimates using lattice QCD. ATLAS also presented a new measurement of the W boson mass using a re-analysis of 7 TeV data collected in 2011, yielding a value slightly lower (by 10 MeV) and with a precision improved to 16 MeV, thus increasing the experimental tension with the recently published CDF measurement.

The LHC results have already obtained precision and sensitivity to processes that were thought to be unreachable prior to the start of operations.

ATLAS and CMS also showed results for more complex and rare processes equally highlighting the remarkable progresses made at the precision frontier. Both experiments showed an observation of the four top quarks production process and ATLAS presented the observation of two new tri-boson production processes, WZγ and Wγγ. ATLAS also presented a new measurement of the associated production of a W boson in association with a pair of top quarks which is a key background to numerous very important processes, as for instant the associated production of a Higgs boson with a pair of top quarks.

The results presented at this year’s Moriond elctroweak session show how LHC results have already obtained precision and sensitivity to processes that were thought to be unreachable prior to the start of operations. An outstanding example discussed in detail was the progress made in the search for di-Higgs production by ATLAS and CMS, a cornerstone of the HL-LHC physics programme to constrain the Higgs boson trilinear self-coupling. These results showed that combined, experiments should reach the sensitivity for the observation of this process at the LHC. Another example which was also discussed is the race to reach sensitivity to the Higgs-boson decays to charm quarks, where new methods based on deep learning techniques are making significant progress.

To further improve on the expected precision reach at the HL-LHC, intermediate goals at Run 3 are extremely important. Both ATLAS and CMS presented new results on measurements of Z boson, top, and Higgs boson production with LHC Run 3 data taken in 2022.

This year’s Moriond conference showed an extraordinary harvest of new results, giving an opportunity to take stock on the open questions and see the remarkable progress made since last year.


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