Speculations on the W-mass measurement at CDF

The W mass determination at the Tevatron CDF experiment reported a deviation from the SM expectation at the 7σlevel.We discuss a few possible interpretations and their collider implications.We perform electroweak global fits under various frameworks and assumptions.We consider three types of electroweak global fits in the effective-field-theory framework:the S-T,S-T-δG_(F),and eight-parameter flavor-universal one.We discuss the amounts of tensions between different m_(W)measurements reflected in these fits and the corresponding shifts in central values of these parameters.With these electroweak fit pictures in hand,we present a few different classes of models and discuss their compatibility with these results.We find that while explaining the m_(W)discrepancy,the single gauge boson extensions face strong LHC direct search constraints unless the Z′is fermiophobic(leptophobic),which can be realized if extra vector fermions(leptons)mix with the SM fermions(leptons).Vector-like top partners can partially generate the needed shift to the electroweak observables.The compatibility with the top squark is also studied in detail.We find that the non-degenerate top squark soft masses enhance the needed operator coefficients,enabling an allowed explanation compatible with current LHC measurements.Overall,more theoretical and experimental developments are highly in demand to reveal the physics behind this discrepancy.

Optimizing Higgs factories by modifying the recoil mass

It is difficult to measure the WW-fusion Higgs production process （e＋e-→vvh） at a lepton collider with a center of mass energy of 240-250 GeV due to its small rate and tile large background from the Higgsstrahlung proeess with an invisible Z （e＋e- → hZ,Z-4→vv）. We construct a modified recoil mass variable, mprecoil, defined using only the 3-momentum of the reconstructed Higgs particle, and show that it can separate the WW-fusion and Higgsstrahhlng events bettcr than the original recoil mass variable m recoil. Consequently, the variable can be used to improve the overall precisions of the extracted Higgs couplings, in both the conventional framework and the effective-field-theory framework. We also explorc the application of the mprecoil variable in the inclusive cross section mcasurements of the Higgsstrahlung process, while a quantitive analysis is left for future studies.

Probing top-quark couplings indirectly at Higgs factories

We perform a global effective-field-theory analysis to assess the combined precision of Higgs couplings, triple gauge-boson couplings, and top-quark couplings, at future circular e＋e- colliders, with a focus on runs below the tt-production threshold. Deviations in the top-quark sector entering as one-loop corrections are consistently taken into account in the Higgs and diboson processes. We find that future lepton colliders running at center-of-mass energies below the tt production threshold can still provide useful information on top-quark couplings, by measuring virtual top-quark effects. With rate and differential measurements, the indirect individual sensitivity achievable is better than at the high-luminosity LHC. However, strong correlations between the extracted top-quark and Higgs couplings are also present and lead to much weaker global constraints on top-quark couplings. This implies that a direct probe of top-quark couplings above the t~ production threshold is also helpful for the determination of Higgs and triple-gauge-boson couplings. In addition, we find that below the e＋e- -＋tth production threshold, the top-quark Yukawa coupling can be determined by its loop corrections to all Higgs production and decay channels. Degeneracy with the ggh coupling can be resolved, and even a global limit is competitive with the prospects of a linear collider above the threshold. This provides an additional means of determining the top-quark Yukawa coupling indirectly at lepton colliders.

Precision Higgs physics at the CEPC

The discovery of the Higgs boson with its mass around 125 GeV by the ATLAS and CMS Collaborations marked the beginning of a new era in high energy physics.The Higgs boson will be the subject of extensive studies of the ongoing LHC program.At the same time,lepton collider based Higgs factories have been proposed as a possible next step beyond the LHC,with its main goal to precisely measure the properties of the Higgs boson and probe potential new physics associated with the Higgs boson.The Circular Electron Positron Collider(CEPC)is one of such proposed Higgs factories.The CEPC is an e^+e^- circular collider proposed by and to be hosted in China.Located in a tunnel of approximately 100 km in circumference,it will operate at a center-of-mass energy of 240 GeV as the Higgs factory.In this paper,we present the first estimates on the precision of the Higgs boson property measurements achievable at the CEPC and discuss implications of these measurements.

Probing top-quark operators with precision electroweak measurements

In the standard model effective field theory,operators involving the top quark are generally difficult to probe and can generate sizable loop contributions to electroweak precision observables measured by past and future lepton colliders.Could the high precision of electroweak measurements compensate for loop suppression and provide competitive reaches on these operators?Would the inclusion of these contributions introduce too many additional parameters for a meaningful global electroweak analysis to be performed?In this paper,we perform a detailed phenomenological study to address these two important questions.Focusing on eight dimension-6 operators that generate anomalous couplings between electroweak gauge bosons and third-generation quarks,we calculate their one loop contributions to e^(+)e^(-)→ff processes,both on and off the Z-pole,and the e^(+)e^(-)→WW process.A global analysis is performed with these eight operators and those that contribute to the above processes at tree level using measurements at the LEP,SLC,and several low energy experiments.We find that although current electroweak precision measurements are sensitive to the one-loop effects of top-quark operators,it is difficult to separate them from the operators that contribute at tree level,making a global analysis rather challenging.Under further assumptions(for instance,new physics contributes to only third generation quark operators and the S and T parameters),competitive reaches may be obtained in a global fit.Another important finding of our study is that the two operators that generate the dipole interactions of the bottom quark have a significant impact on the Z-pole measurements and should not be omitted.We also discuss the implications of the recently reported W-boson mass measurement at the CDF for our results.Finally,we estimate the reaches of future lepton colliders in probing top-quark operators with precision electroweak measurements.