# Overview on the recent di-photon excess at the LHC Run 2

Friday, 8 January 2016 from to (Asia/Seoul)
at IBS CTPU ( Seminar Room )
KAIST Munji Campus, Faculty Wing 3rd Floor, 193, Munji-ro, Yuseong-gu, Daejeon 34051
 Description The candidate for a new signature toward the physics beyond the Standard Model at the LHC With 3.6 fb-1 (ATLAS) and 2.6 fb-1​ (CMS) data of the LHC Run 2, experimental collaborations reported that there is an excess in the di-photon channel, at the level of local significance ~ 3 sigma (global significance ~ 2.0 sigma).  We would like to discuss on the details of this excess, possible BSM scenarios and its phenomenological impact. Our main topics to be discussed are following,  Overview on various possibilities Phenomenological implications Invited speakers :  Won Sang Cho (IBS CTPU) Kwang Sik Jeong (Pusan National Univesity) Kunio Kaneta (IBS CTPU) Hyung Do Kim (Seoul National University) Pyungwon Ko (KIAS) Hyun Min Lee (Chung-Ang University) Kenji Nishiwaki (KIAS) Jong-Chul Park (Chungnam National University) Andrew Spray (IBS CTPU) Yeo Woong Yoon (Konkuk University)   Organizers :   Link to IBS CTPU : http://ctpu.ibs.re.kr/html/ctpu_en/ Sponsored by
Go to day
• Friday, 8 January 2016
• 09:00 - 10:00 Registration and Coffee
• 10:00 - 12:00 Talks: Morning session (session chair: Myeonghun Park)
• 10:00 Interpretation of the diphoton resonance in effective field theory 30'
We discuss the potential interpretation of the diphoton excess at the LHC with a new resonance in effective field theory and present a concrete example with Kaluza-Klein graviton as the diphoton resonance, playing a role of the mediator of dark matter. We also show how the spin information of the resonance can be extracted from the kinetic distribution of the decay products.
 Speaker: Prof. Hyun Min Lee (Chung-Ang University) Material:
• 10:30 Glue to light signal of a new particle 30'
Any new particle charged under SU(3)C and carrying electric charge will leave an imprint in the di-photon invariant mass spectrum as it can mediate gg to gamma gamma process through loops. The combination of properties of loop functions and gluon pdfs results in a peak-like feature in the di-photon invariant mass around twice the mass of a given particle. Using recent ATLAS analysis, we set upper limits on the combined SU(3)C and electric charge of new particles and indicate future prospects. We briefly discuss the possibility that the excess of events in the di-photon invariant mass spectrum around 750 GeV originates from loops of a particle with mass around 375 GeV.

(My talk is based on the paper, arXiv:1512.08221.)
 Speaker: Prof. Hyung Do Kim (Seoul National University) Material:
• 11:00 QCD axion from aligned axions and diphoton excess 30'
We argue that the QCD axion can arise from many aligned axions with decay constants much smaller than the conventional axion window. If the typical decay constant is of O(100) GeV to 1 TeV, one or more of the axions or saxions may account for the recently found diphoton excess at ∼750  GeV. Our scenario predicts many axions and saxions coupled to gluons with decay constants of order the weak scale, and therefore many collider signatures by heavy axions and saxions will show up at different energy scales. In particular, if the inferred broad decay width is due to multiple axions or saxions, a non-trivial peak structure may become evident when more data is collected. We also discuss cosmological implications of the aligned QCD axion scenario.
 Speaker: Prof. Kwang Sik Jeong (Pusan National University) Material:
• 11:30 Radion (Diaton) Explanations 30'  Speaker: Dr. Andrew Spray (IBS CTPU) Material:
• 12:00 - 13:30 Lunch
• 13:30 - 15:30 Talks: Afternoon session 1/2 (session chair: Andrew Spray)
• 13:30 LHC 750 GeV Diphoton excess in a radiative seesaw model 30'
We investigate a possibility for explaining the recently announced 750 GeV diphoton excess by the ATLAS and the CMS experiments at the CERN LHC in a model with multiple doubly-charged particles, which was originally suggested for explaining tiny neutrino masses through a three-loop effect in a natural way. The enhanced radiatively generated effective coupling of a new singlet scalar S with diphoton, with multiple charged particles in the loop, enlarges the production rate of S in pp→S+X via photon fusion process and also the decay width of S→γγ even without assuming a tree level production mechanism. Very interestingly, we find that the branching ratio of S→γγ is  about 60% fixed by quantum numbers and the model is consistent with all the constraints from 8TeV LHC data.
 Speaker: Dr. Kenji Nishiwaki (KIAS) Material:
• 14:00 Dark matter indirect signature and 750 GeV diphoton resonance 30'
Motivated by the recently reported diphoton resonance at 750 GeV, we study a new axion-like bosonic portal model of dark matter physics. When the resonance particle is identified as the pseudo-scalar mediator, via which the standard model sector would interact with the dark matter sector, the data from collider physics would provide profound implications to dark matter phenomenology. In this work, we first identify the preferred parameter space of the suggested portal model from the results of the 13 TeV LHC run, and then we examine the dark matter signature taking into account the data from cosmic-ray experiments including Fermi-LAT dwarf galaxy gamma-ray search, HESS gamma-line search, and future CTA diffuse gamma-ray and gamma-line searches.
 Speaker: Prof. Jong-Chul Park (Chungnam National University) Material:
• 14:30 The 750 GeV Diphoton Excess May Not Imply a 750 GeV Resonance 30'
We discuss non-standard interpretations of the 750 GeV diphoton excess recently reported by the
ATLAS and CMS Collaborations which do not involve a new, relatively broad, resonance with a
mass near 750 GeV. Instead, we consider the sequential cascade decay of a much heavier, possibly
quite narrow, resonance into two photons along with one or more invisible particles. The resulting
diphoton invariant mass signal is generically rather broad, as suggested by the data. We examine
three specific event topologies | the \antler", the \sandwich", and the 2-step cascade decay, and
show that they all can provide a good t to the observed published data. In each case, we delineate
the preferred mass parameter space selected by the best t. In spite of the presence of invisible
particles in the nal state, the measured missing transverse energy is moderate, due to its anti-
correlation with the diphoton invariant mass. We comment on the future prospects of discriminating
with higher statistics between our scenarios, as well as from more conventional interpretations
 Speaker: Dr. Won Sang Cho (IBS CTPU) Material:
• 15:00 The 750 GeV diphoton excess and its implications for Z’ 30'
We discuss a possible connection between the 750 GeV diphoton excess and a heavy neutral gauge boson (Z') in view of the idea of the grand unified theory. The unification of the gauge coupling constants is a guiding principle in our scenario. There, the gauged B-L symmetry is naturally introduced. We point out a simple fact that a new scalar boson of 750 GeV would imply a TeV-scale Z' which might be within the reach of the LHC Run 2 experiments.
 Speaker: Dr. Kunio Kaneta (IBS CTPU) Material:
• 15:30 - 16:00 Coffee
• 16:00 - 17:00 Talks: Afternoon session 2/2 (session chair: Won Sang Cho)
• 16:00 Possible implications of 750 GeV diphoton excess on BSM 30'
I describe possible BSM scenarios which can accommodate the 750 GeV diphoton excess in a generic way.
 Speaker: Prof. Pyungwon Ko (KIAS) Material:
• 16:30 How Resonance-Continuum Interference Changes 750 GeV Diphoton Excess: Signal Enhancement and Peak Shift 30'
We discuss that the interference between 750 diphoton resonance and continuum background has two considerable effects: (1) enhancing or suppressing diphoton signal rate due to the imaginary-part interference and (2) distorting resonance shape due to the real-part interference. We find that the resonance contribution to the 750 GeV excess can be enhanced by a factor of 2(1.6) for 3(6) fb signal rate and the 68%(95%) CL best-fit mass range can shift by 1-4 (any ${\cal O}$(1)) GeV.
 Speaker: Dr. Yeo Woong Yoon (Konkuk University) Material:
• 17:00 - 18:00 Discussion