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Register a new userColor-octet Companions of a 750 GeV Heavy Pion
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Color octet bosons are a universal prediction of models in which the 750 GeV diphoton resonance corresponds to a pion of a QCD-like composite sector. We show that the existing searches for dijet and photon plus jet resonances at the LHC constrain single productions of color octet states and can be translated into stringent limits on the 750 GeV diphoton rate. For a minimal 5 + 5bar model, the 750 GeV diphoton signal cross section at the 13 TeV LHC is constrained to be below around 5 fb. Future LHC searches for the photon plus jet resonances can establish evidence of a new color-octet state with 20/fb and validate a pion-like explanation for the 750 GeV resonance.
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We point out a potential common origin of the recently observed 750 GeV diphoton resonance and a Weakly Interacting Massive Particle (WIMP) candidate. In a dark QCD sector with an unbroken dark G-parity, the diphoton resonance could be a dark G-even pion, while the WIMP could be the lightest dark G-odd pion. Both particles are Standard Model gauge singlets and have the same decay constant. For the dark pion decay constant of around 500 GeV, both the diphoton excess at the LHC and the dark matter thermal abundance can be accommodated in our model. Our model predicts additional dark G-even and dark G-odd color-octet pions within reach of the 13 TeV LHC runs. For the 5 + 5bar model, compatible with the Grand Unified Theories, the WIMP mass is predicted to be within (613, 750) GeV.
We propose a hypothetical heavy leptonium, the scalar bound state of an exotic lepton-antilepton pair, as a candidate for the recent 750 GeV resonance in the early LHC Run 2 data. Such a para-leptonium is dominantly produced via photon-photon fusion at the LHC and decays into a photon pair with a significant branching fraction. In addition, our model predicts a companion spin-1 ortho-leptonium bound state, which can decay to $W^+W^-$, $fbar{f}$ and three photons. Under the LHC and the electroweak precision observables bounds, we find that the observed excess of 750 GeV diphoton events can be explained within $2sigma$ accuracy for $Y_{L} approx 4.8 - 7.2$ for the minimal case in our scenario. The observation of the ortho-leptonium in the dilepton channel will be the smoking gun for our scenario.
We revisit models where a heavy chiral 4th generation doublet of fermions is embedded in a class of two Higgs doublets models (2HDM) with a discrete $Z_2$ symmetry, which couples the heavy scalar doublet only to the 4th generation fermions and the light one to the Standard Model (SM) fermions - the so-called 4G2HDM introduced by us several years ago. We study the constraints imposed on the 4G2HDM from direct searches of heavy fermions, from precision electroweak data (PEWD) and from the measured production and decay signals of the 125 GeV scalar, which in the 4G2HDM corresponds to the lightest CP-even scalar h. We then show that the recently reported excess in the $gammagamma$ spectrum around 750 GeV can be accommodated by the heavy CP-even scalar of the 4G2HDM, H, resulting in a unique choice of parameter space: negligible mixing (sinalpha ~ O(0.001)) between the two CP-even scalars h,H and heavy 4th generation quark and lepton masses m_t,m_b < 400 GeV and $m_{ u},m_{tau}$ > 900 GeV, respectively. Whether or not the 750 GeV gamma gamma resonance is confirmed, interesting phenomenology emerges in q - Higgs systems (q=t,b), that can be searched for at the LHC. For example, the heavy scalar states of the model, S=H,A,H^+, may have BR(S -> qq) ~ O(1), giving rise to observable qq signals on resonance, followed by the flavor changing q decays t->uh (u=u,c) and/or b->dh (d=d,s,b). This leads to distinct high jet-multiplicity signatures, with or without charged leptons, of the form qq -> (nj + mb + lW)_S (j and b being light and b-quark jets, respectively), with n+m+l =6-8 and unique kinematic features. It is also shown that the 4G2HDM can easily accommodate the interesting recent indications of a percent-level branching ratio in the lepton-flavor-violating (LFV) decay $h to tau mu$ of the 125 GeV Higgs, if confirmed.
We study kinematic distributions that may help characterise the recently observed excess in diphoton events at 750 GeV at the LHC Run 2. Several scenarios are considered, including spin-0 and spin-2 750 GeV resonances that decay directly into photon pairs as well as heavier parent resonances that undergo three-body or cascade decays. We find that combinations of the distributions of the diphoton system and the leading photon can distinguish the topology and mass spectra of the different scenarios, while patterns of QCD radiation can help differentiate the production mechanisms. Moreover, missing energy is a powerful discriminator for the heavy parent scenarios if they involve (effectively) invisible particles. While our study concentrates on the current excess at 750 GeV, the analysis is general and can also be useful for characterising other potential diphoton signals in the future.
We examine the scenario of a warped extra dimension containing bulk SM fields in light of the observed diphoton excess at 750 GeV. We demonstrate that a spin-2 graviton whose action contains localized kinetic brane terms for both gravity and gauge fields is compatible with the excess, while being consistent with all other constraints. The graviton sector of this model contains a single free parameter, once the mass of the graviton is fixed. The scale of physics on the IR-brane is found to lie in the range of a $sim$ few TeV, relevant to the gauge hierarchy. There remains significant flexibility in the coupled gauge/fermion KK sectors to address the strong constraints arising from precision measurements.
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