Time： 2:30PM, Jan. 7th 2016 Thursday
Place: Theoretical Physics Division，319
Speaker：Dr. Yan Wang 王彦 (IHEP, CAS)
Title: 750 GeV Diphoton Excess from Cascade Decay
Abstract: Motivated by the recent 750 GeV diphoton excess observed by the ATLAS and CMS collaborations, we explain this excess with cascade decay. Model-independent constraints and predictions on the allowed couplings for generating the observed diphoton excess are studied in detail, and the compatibility between Run 1 and Run 2 data is considered simultaneously. We demonstrate that the possible four photon signal can be used to test this scenario, and also explain the interesting deviation for a diphoton mass of about 1.6 TeV by ATLAS, where the local significance is 2.8 $\sigma$.
Speaker：Dr. Jiayin Gu 顾嘉荫 (IHEP, CAS)
Title: Running after Diphoton
Abstract: A very plausible explanation for the recently observed diphoton excess at the 13 TeV LHC is a (pseudo)scalar with mass around 750 GeV, which couples to a gluon pair and to a photon pair through loops involving vector-like quarks (VLQs). To accommodate the observed rate, the required Yukawa couplings tend to be large. A large Yukawa coupling would rapidly run up with the scale and quickly reach the perturbativity bound, indicating that new physics, possibly with a strong dynamics origin, is near by. The case becomes stronger especially if the ATLAS observation of a large width persists. In this paper we study the implication on the scale of new physics from the 750 GeV diphoton excess using the method of Renormalization Group running with careful treatment of different contributions and perturbativity criterion. Our results suggest that the scale of new physics is generically not much larger than the TeV scale, in particular if the width of the scalar is large. Introducing multiple copies of VLQs, lowing the VLQ masses (~ 400 GeV) and enlarging VLQ electric charges (to 5/3) help reduce the required Yukawa couplings and can push the cutoff scale to higher values. Nevertheless, if the width of the 750 GeV resonance turns out to be larger than about 1 GeV, it is very hard to increase the cutoff scale beyond a few TeVs. This is a strong hint that new particles in addition to the 750 GeV resonance and the vector like quarks should be around the TeV scale.
Speaker：Dr. Jue Zhang 张珏 (IHEP, CAS)
Title：Electroweak Vacuum Stability and Diphoton Excess at 750 GeV
Abstract：In this talk I will first introduce a simple and straightforward model that explains the recent diphoton excess by extending the Standard Model with a singlet scalar and a pair of vector-like fermions. Then, I will focus on the electroweak vacuum stability problem, and show that a successful explanation for the diphoton excess and the absolute stability of electroweak vacuum cannot be achieved simultaneously in this economical setup.
