Search

After observing the Higgs boson by the ATLAS and CMS experiments at the LHC, accurate measurements of its properties, which allow us to study the electroweak symmetry breaking mechanism, become a high priority for particle physics. The most promising of extracting the Higgs self-coupling at hadron colliders is by examining the double Higgs production, especially in the $b \bar{b} \gamma \gamma$ channel. In this work, we presented full loop calculation for both SM and New Physics effects of the Higgs pair production to next-to-leading-order (NLO), including loop-induced processes $gg\to HH$, $gg\to HHg$, and $qg \to qHH$. We also included the calculation of the corrections from diagrams with only one QCD coupling in $qg \to qHH$, which was neglected in the previous studies. With the latest observed limit on the HH production cross-section, we studied the constraints on the effective Higgs couplings for the LHC at center-of-mass energies of 14 TeV and a provisional 100 TeV proton collider within the Future-Circular-Collider (FCC) project. To obtain results better than using total cross-section alone, we focused on the $b \bar{b} \gamma \gamma$ channel and divided the differential cross-section into low and high bins based on the total invariant mass and $p_{T}$ spectra. The new physics effects are further constrained by including extra kinematic information. However, some degeneracy persists, as shown in previous studies, especially in determining the Higgs trilinear coupling. Our analysis shows that the degeneracy is reduced by including the full NLO corrections., Comment: 28 pages, 24 figures

A sequential fourth generation is still viable, but the t' and b' quarks are constrained to be not too far apart in mass. The t'{\to}bW and b'{\to}tW decay channels are still being pursued at the Tevatron, which would soon be surpassed by the LHC. We use a convolution method with up to five-body final state to study t' and b' decays. We show how the two decay branches for m_{b'} below the tW threshold, b'{\to}tW^* and t^*W, merge with b'{\to}tW above the threshold. We then consider the heavy-to-heavy transitions b'{\to}t^{\prime(*)}W^{(*)} (or t'{\to}b^{\prime(*)}W^{(*)}), as they are not suppressed by quark mixing. We find that, because of the threshold sensitivity of the branching fraction of t'{\to}b'W^* (or b'{\to}t'W^*), it is possible to measure the strength of the CKM mixing element V_{t'b} (or V_{tb'}), especially when it is rather small. We urge the experiments to pursue and separate the t'{\to}b'W^* (or b'{\to}t'W^*) decay in their search program.

Heavy $t' \to bW$ is currently being searched for at the Tevatron, but a broader spectrum should be explored at the LHC. For $m_{b'} < m_{t'}$, we discuss the two decay branches, $b' \to tW^*$ and $t^*W$, below the $tW$ threshold, and how they merge with $b' \to tW$ above threshold. We use a convolution method with five body final state, and compare the production cross section with PYTHIA. This can be extended for a $b'$ with mass heavier than $t'$. A similar discussion is given for $t'$ decay, considering $t' \to b^{(*)}W^{(*)}$ processes, besides the $t' \to bW$ considered by CDF., Comment: Proceedings for poster at ICHEP08, Philadelphia, USA, July 2008. 4 pages, LaTeX, 4 figures