Summary of Suggestions
We have attempted to organize our findings across many decay channels by considering them in the context of several well-motivated classes of models. Each class exhibits certain relations among the branching ratios for the various possible exotic decays. For each class, we have determined which searches are both feasible and likely to be sensitive to the presence of new physics in the near- or medium-term. From these results we have come to the following list of suggestions. For more details, please refer to Section 20 of our Survey of Exotic Higgs Decays, arXiv:1312.4992We find that the following searches are highly motivated within the 7 and 8 TeV data set as well as within future data sets. In some cases, especially in regimes where the objects are collimated, searches have already been done by ATLAS and/or CMS, though not always with the full data set.
- Search for h→ ZD ZD → (l+l−)(l+l−) across the full range of kinematically allowed ZD masses, including regimes where the leptons are collimated (forming simple "lepton-jets"). This could also be interpreted as a search for h→ ZD Z′D if the dilepton pairs have different masses, or as h→ ZD ZD + MET, for small MET, if the condition m4l=mh is relaxed.
- Search for h→ Z ZD → (l+l−)(l+l−) across the full range of kinematically allowed ZD masses, including regimes where the leptons are collimated (forming a simple "lepton-jet"). This search should also be interpreted as a search for h→ Z a→ (l+l−)(μ+μ−).
- Search for h→l+l− + MET, including regimes where the leptons are collimated, and including the cases where there is a resonance in mll. Benchmark models include h→ XY→ ZD YY or a Y Y, h→ XX→ a a(′) YY for ma < 2mτ, h→ XX→ Z* Z* YY, where Y is invisible and Z* is an off-shell Z boson.
- Search for h→l+l− l+l− + MET, including regimes where the leptons are collimated, and including the cases where there is a resonance in mll. Benchmark models include h→ XX→ ZD ZD YY, h→ XX→ aa(′) YY for ma < 2 mτ, h→ XX→ Z* Z* YY, where Y is invisible and Z* is an off-shell Z.
- Search for h→ aa→ (bb)(μ+μ−) across the full range of kinematically allowed a masses, including regimes where the bb pair tend to merge. If possible, searches for h→ a a′, where ma > 2mb > ma′, could be considered, in which case the leptons may be collimated.
- Search for h→ aa→ (τ+τ−)(μ+μ−) across the full range of kinematically allowed a masses, including regimes where the leptons are collimated. A search for h→ aa → (τ+τ−)(τ+τ−) may not be as powerful, but deserves to be investigated further.
- Search for h→ aa→ (γγ)(γγ), including regimes where the photons are collimated. This could also be interpreted as a search for h→ a a′ if the diphoton pairs have different masses, or as h→ aa +MET, for small MET, if the condition m4γ=mh is relaxed.
- Search for h→γγ+MET, including the cases where there is a resonance in mγγ. Benchmark models include h→ XY→ a YY, h → XX→ a a(′) YY, h→ XX→ (γY)(γY), where Y is invisible.
- h→ XY→ γYY where Y is invisible, giving γ+ MET.
- h→ aa→ (bb)(bb).
- h→ aa→ (bb)(τ+τ−), perhaps in VBF production.
- For several searches, boosted h recoiling against a leptonically-decaying W or Z is expected to be necessary. Presumably even the higher lepton pT thresholds required at Run 2 will not much affect these searches.
- However, many searches that we have not studied directly (high multiplicity of soft particles, long-lived particles, etc.) will require as many events as possible be retained under triggers on the lepton in Wh (and tt h) and on the jets in VBF. Keeping the one-lepton trigger thresholds low, or combining one lepton or VBF dijet triggers with signatures of unusual Higgs decay final states, is critically important for achieving high sensitivity.
- Many of our searches involve triggering on two or more leptons, possibly soft and possibly collimated; these issues have been well-explored already in Run I and should remain a priority.
- For h→ l+l− + MET, if the leptons are soft and the MET is substantial, then a VBF-based search may be essential, in which triggering off a combination of the VBF jets, the MET, and the soft leptons may be needed.
- The same issues apply to photons; triggering on multiple photons, possibly collimated, and on softer photons in combination with VBF jets and MET may be important.
- We have not studied them here, but final states with leptons and at least one photon are possible; this may have trigger implications for any combined lepton and photon trigger pathway.
- Triggering in the VBF context is also potentially important for other difficult modes, such as bb ττ, bb +MET, etc., but more theory studies are needed.
- Although the search at CMS for γ+ MET is expected to benefit from a data parking trigger in the 2012 data, the trigger challenge for this final state in Run 2 is very severe, and a thorough study is needed to determine if it is both feasible and worth the bandwidth. The VBF channel may be helpful here.
For further studyFirst, we did not study two-body decays such as h→ τμ or h→ Zγ, but these have been studied extensively in the literature. More exotic decays that have received varying degrees of attention include
- h→ 2→ 6 e.g. decays of the Higgs to neutralinos that decay via R-parity violation to three jets, etc.
- h to > 4 leptons, τs, bs; decays such as h→ 6τ or 8b have been suggested in the literature , but both theoretical and experimental study has been limited, though CDF has looked for decays of the Higgs to many soft leptons .
- h to complex lepton jets (i.e. with > 2 tracks), including both purely electronic, purely muonic, purely leptonic with a mix of muons and electrons, and mixed leptonic/hadronic jets (see for example ).
- Decays to one or more photonic jets (consisting of ≥ 2 collimated photons) need more experimental study; theory studies include [4,5,6].
- h decaying to long-lived particles with decays in flight [7,8,9]. There have been a number of searches for specific final states at particular decay lifetimes, but not a coherent program that covers all cases.
- Further studies in more difficult channels, such as bb ττ, bb + MET, ττ+ MET, jjγγ, are needed particularly in the context of VBF production. If such studies reveal VBF can yield significant improvements in sensitivity, then developing triggers for 2015 aimed at these final states may offer a significant advantage.
- Also well-motivated are studies of exotic decays in the tth associated production channel, which can be competitive with Wh,Zh for non-SM Higgs decays. The combinatoric backgrounds that make this channel difficult for a SM Higgs may be significantly reduced for certain non-SM decay modes , and the hard leptons and b jets from the t decays offer another inclusive trigger pathway.
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