Higgs couplings and precision electroweak data

Abstract

In light of the discovery of a Higgs-like particle at the LHC, we revisit the status of the precision electroweak data, focusing on two discrepant observables: 1) the long-standing 2.4σ deviation in the forward-backward asymmetry of the bottom quark A b F B , and 2) the 2.3σ deviation in R b , the ratio of the Z → bb partial width to the inclusive hadronic width, which is now in tension after a recent calculation including new two-loop electroweak corrections. We consider possible resolutions of these discrepancies. Taking the data at face value, the most compelling scenario is that new physics directly affects A b F B and R b , bringing the prediction into accord with the measured values. We propose a modified 'Beautiful Mirrors' scenario which contains new vector-like quarks that mix with the b quark, modifying the Zbb vertex and thus correcting A b F B and R b . We show that this scenario can lead to modifications to the production rates of the Higgs boson in certain channels, and in particular a sizable enhancement in the diphoton channel. We also describe additional collider tests of this scenario.The ATLAS and CMS experiments have recently discovered a new boson with properties closely resembling those of the Standard Model (SM) Higgs boson [1,2]. The focus of the experimental collaborations now turns to determining the properties of this state, such as its quantum numbers and its couplings to other SM particles. This is an extremely important task since any deviation from the SM Higgs boson predictions will unambiguously point to new physics (NP) at the TeV scale. In this respect it is intriguing that both experiments observe a slight enhancement in the h → γγ channel, though with the current dataset this enhancement is not statistically significant [3, 4].If, as the current data suggest, this new state is indeed the Higgs boson, its mass m h ∼ 125 GeV will be in accord with the expectation indirectly suggested by the precision electroweak data, which favors a light SMlike Higgs boson. Of the many precision measurements used to test the electroweak sector, most of them are in good agreement with the SM predictions, see for example [5],[6], [7]. However, there are a couple of notable discrepancies. The forward-backward asymmetry of the bottom quark A b F B measured at the Z-pole at LEP1 constitutes a long-standing exception. The measured value and SM prediction are [5], [7],and thus exhibit a 2.4σ discrepancy. Furthermore, a recent calculation of R b , the ratio of the Z → bb partial width to the inclusive hadronic width, which includes new two-loop electroweak corrections, now puts the prediction in tension with the measured value [8]. The measurement and prediction read [5], [7],displaying a 2.3σ discrepancy.It is a matter of debate whether these deviations call for NP. Among a large ensemble of measurements, such as that used in the electroweak fit, one may expect an occasional discrepancy of this size, which could be the result of a statistical fluctuation or unknown systematic error. However, a possib...

Higgs at last

“…lines have shown that the coefficients of the leading operators in this expansion can already be meaningfully constrained [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

“…3 See also [15][16][17][18][19]43] for previous studies of the impact of electroweak precision tests in the context of the 126 GeV Higgs. …”

Light top partners and precision physics

“…Constraints on the corrections to the Z boson couplings to the bottom quark. The ellipses show the exclusion contours at 68% and 95% confidence level [79,80]. The vertical band shows the expected size of the corrections to the g b R coupling.…”

Exotic leptons: Higgs, flavor and collider phenomenology

“…On the other hand, extensions of the SM with vector-like leptons which couple to the Higgs, can both enhance and suppress the h → γγ decay rate, because in contrast to chiral leptons, vector-like leptons can interfere also constructively with the W contribution. Colored new matter is additionally constrained by data, because it also affects the gluon fusion production cross section of the Higgs, which in turn leads to modifications of all Higgs signal strengths [7,9,14,25,26,28,[43][44][45][46][47].…”