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**Exotic Higgs Decays**

Interested people: Adam, Roberto, Filip, Jack Gunion, Sabine, Grégory, Beranger, Kirtimaan, Yun Jiang, Aoife, Lorenzo, Alex A., Nazila, Andreas-1, Andreas-2, Giacomo, Aldo...  (please add your name!)

Finding exotic (not predicted by the SM) Higgs decays could be the shortest route to new physics. 
Indirect constraints (via visible decays) allow for ~25% branching fraction into exotic states (if the Higgs production rate is as in the SM ), or even up to ~50% with some conspiracy (if the Higgs production rate is enhanced).  The SM Higgs width is just 4 MeV, so even very weakly coupled new physics can a significant branching fraction for exotic decays; e.g. for a new scalar X coupled as c|H|^2 |X|^2 one has BR(h->X*X)=10% BR for c~0.01. Given a fairly large Higgs production cross section at the LHC even decays with a tiny branching fraction can be effectively probed; note that the Higgs was discovered in the diphoton (BR~10^-3) and 4-lepton (BR~10^-4) channels. 


The goal of this project is to  1) classify possible exotic  Higgs decays, 2) Check existing constraints on the couplings mediating exotic decays, so as to see whether they leave room a non-negligible branching fraction, 3) Prompt experimental searches for the motivated decays that are not being searched for by experiments     

To facilitate a systematic exploration one can classify the decays according to the number of visible SM particles the Higgs decays to. Below are some examples, you're welcome to add more.  
   

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0 particle: 

h -> invisible

searched for at the LHC  in monojet, vbf, and Z/W associated production.
 
Seems there's nothing left to do on this front. 

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1 particle:

- h -> gamma+MET or h -> Z+MET  

E.g these decays could be induced by the operator -c|H|^2/v^2  F_{\mu \nu} X_{\mu \nu} where 
X_\mu is a dark vector boson that subsequently decays to the DM particles.  gamma+MET decays could also arise from the cascade decays in h-> neutralino+gravitino -> 2xgravitino+gamma in SUSY, see  1203.4563.
Z+MET decays can occur e.g. in inverse see-saw models (see e.g., 1209.4803) as a cascade decay h-> nu N -> nu Z nu.    

To do: check the constraints on the effective operator, find the maximum allowed branching fraction, check whether some LHC searches (monophotons? mono-Z?) could pick up this decay, devise experimental strategy. Check contraints from existing H-> ZZ  

__Interested people:__ Adam, Lorenzo, ...


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2 particle:

- flavor violating decays to quarks, probably hopeless given the constraints from flavor violation, see e.g. 1209.1397 

- lepton flavor violating decays (h->tau mu, h-> tau e, h-> mu e), the first two are promising, see e.g. 1209.1397. Also, as before, lepton flavour violating decays can occur in inverse see-saw models (see e.g., 1209.4803) as a cascade decay h-> nu N -> nu l W -> nu l l' nu', where the flavour of l and l' are not related. These decays are searched for by experiment and has been quite thoroughly studied theoretically, so probably nothing to do. 

- cascade decays h->X a -> X f fbar, where X is invisible.  
Note that this decay cannot be realized in the 2HDM+singlet models in which we require X to be a single  additional singlet that is a stable DM candidate by means of imposing Z_2 symmetry so that single S terms are absent in the Lagrangian.   In this case, X would have to be two S states, i.e. H->SS h or something of the sort.  This would yield invisible + 2 SM particles. A related scenario is H->h h where one h decays to f fbar and the other h decays to SS, i.e. h has a mixture of regular and invisible decays.

To do: concrete models, existing constraints and experimental strategies. In particular, interesting to recast the SUSY dilepton+MET searches in this context. 

__Interested people:__  Jack, Aoife, Andreas, Lorenzo

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3 particle:  

-  h -> f1 F(*) -> Z f1 f2,  where f1 f2 are SM fermions, and F is a heavy BSM fermion with couplings y h F f1, g F \gamma_\mu f2 Z_\mu. F could be charged or not charged, colored or not charged (several possibilities with different f1 f2). For this case must check LEP limits on Z^(*)-> f F.  If F must be very heavy then BR for such decay would be very small.


To do: existing constraints, e.g. from h->ZZ*4l; limits on F from direct searches (LEP and LHC), precision contraints on F, collider strategies   

__Interested people:__ Aldo, Giacomo, Aldo 

- h -> Z/A X - > Z/A f fbar  where X is an intermediate boson , and f could be b, tau, mu, e, etc. 
To do: existing constraints, e.g. from h->ZZ*4l; limits on F from direct searches, precision contraints on F, collider strategies 

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4 particle: 

- classic h->aa or hlhl->4f decays (where hl is a lighter CP-even higgs). 

- should we consider the variant in which the a's or hl's are unhiggs type continuum states that would have possibly escaped detection in Z^*->aa since no definite mass peaks in e.g. 2b+2b final state.  Rather just a continuum of bb masses.
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To do: provide theory references for the decays already studied in the literature :

- h -> mu tau and h -> tc  [[http://fr.arxiv.org/abs/0906.1990]]

- h -> X + Z_d where X = Z, Z_d or gamma  [[http://fr.arxiv.org/abs/1304.4935]]

- h -> aa -> 4 taus using subjets [[http://arxiv.org/abs/arXiv:1106.4545]]

- h -> partially (in)visible, signature driven subjet analysis [[http://arxiv.org/abs/arXiv:1209.0494]]
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