Les Houches

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__Interested people:__ //Jose Miguel No, Jose Zurita, Stefania Gori, Ken Mimasu, Linda Finco, Sijing Zhang, Susan Gascon-Shotkin, Grégory Moreau, Haiying Cai, Sylvain Fichet, Thomas Flacke, Stefan Liebler, Daniele Barducci, Andrei Angelescu, Jon Butterworth//


**Down to which masses the LHC can test (pseudo-)scalars?**

Below 80-90GeV, for a generic (pseudo-)scalar $\phi$ (light Higgs, radion, dilaton, axion,...), the decay $\phi$ → Z*Z is kinematically closed and $\phi$ → $\gamma\gamma$ becomes challenging to trigger on, so that the gluon-gluon Fusion mechanism to produce $\phi$ is not promising anymore (see for instance [[https://arxiv.org/abs/1607.08653|1607.08653]]).



**1. Associated production ($pp \to V \phi$)** //[contact: Grégory Moreau]//

Motivation: the produced Vector boson V helps to trigger on the signal (for example: Radion+Z production within the RS model studied in [[https://arxiv.org/abs/1702.03984|1702.03984]]).

Context here: EFT with on-shell scalar ($\phi$) and scalar couplings to vector bosons only so look at the decay $\phi \to \gamma \gamma$ [here $\phi$ is not a stable DM candidate leading to missing $E_T$]. 
Looking at scalar couplings to fermions, other production and decay modes ($\phi \to \tau \tau$ or $\phi \to b \bar b$ [reconstructing the invariant scalar mass]) could be studied as well.

Gauge invariant (pseudo-)scalar couplings to ZZ, WW, $\gamma\gamma$, $\gamma$Z, gluon-gluon: $$\frac{\phi}{f_H}\vert D_\mu H\vert^2   
\ , \ \ \frac{\phi}{f_Z} Tr[V_{\mu\nu} V^{\mu\nu}] 
\ \ and \ \ \frac{\phi}{\tilde f_Z} Tr[V_{\mu\nu} \tilde V^{\mu\nu}] \ .$$ 

Two other effective parameters: $\phi$ mass ($m_\phi$) and Branching for $\phi \to \gamma \gamma$ ($B_\gamma$).

Exhaustive list of hard processes (no focus on W production for EXP reasons):

  * $q \bar q$ → Z*/$\gamma$* → Z $\phi$ followed by Z→$\ell^+\ell^- \ $ [to be first looked at this Les Houches session] 
  * $q \bar q$ → $Z^{(*)}$ → $\gamma$ $\phi \ $ [existing analysis from ATLAS?]
  * $q \bar q$ → $\gamma$* → $\gamma$ $\phi \ $ [[https://arxiv.org/pdf/1409.4792.pdf|1409.4792]]
  * $q \bar q$ → gluon* → gluon $\phi \ $ [[https://arxiv.org/pdf/1409.4792.pdf|1409.4792]]

A first work Plan: 
(1) implement the effective model into FeynRules
(2) produce kinematical distributions for the signal with MadGRAPH (directly from the LHE files with madAnalysis for example)
(3) interface the DELPHES detector response simulator (at the MadGRAPH level)
(4) compare distributions for SM-like couplings ($1/f_H$) and kinetic-like couplings ($1/f_Z$, $1/\tilde f_Z$) [also to develop a discrimination test between e.g. a light Higgs and a radion] 
(5) simulate SM background events as well (main physical one: Drell-Yan + double ISR/FSR ?) 
(6) optimise a set of selection cuts
(7) obtain a sensitivity plot (prospective for Run 2) in the parameter space: $m_\phi$, $1/f_H$, $1/f_Z$, $1/\tilde f_Z$ [=couplings for the considered gauge bosons] for $B_\gamma$=100% (plot to be simply rescaled accordingly to the wanted $B_\gamma$ value then). (+motivate EXP analysis of exclusion limits from Run 1 data?).

Goal: derive a generic plot that can be recast to any specific theoretical scenario with a light (pseudo-)scalar, and, determine its complementarity with LEP(,EWPT) bounds.

Tools: Some of us are looking at this using [[2017:groups:tools:contur_for_light_scalar_particles|Contur]].

 
**2. Complementarity with "single" scalar production** [same context / strategy as in point **1.**]

  * VBF(+ISR/FSR): q q → q q $\phi$ [for low $m_\phi$, possible tag on the 2 jets?] 
  * ggF+ISR: g g → $\phi$ with a gluon radiated off one of the 2 initial state gluons //[credit: Thomas Flacke]//. A scalar recoiling against a hard jet could allow to trigger on it. 

For example, motivated in extended Higgs sectors (e.g. light Higgses in 2HDM) when the 125GeV is SM like (2HDM “alignment limit”).



**3. Exotic Higgs decays: What are the holes?**  //[contact: Stefania Gori, Jose Miguel No]//

h → a a, Z a, a a' with a, a' scalar/pseudoscalar (for Z a, a is pseudoscalar)

  * Current searches for h → a a: 4 muon, 2 mu 2 b, 4 tau, 2 tau 2 mu, 4 b…
  * Interesting to look at signatures with resolved/not resolved photons: 2b 2gamma, 4 gamma (for unresolved 2 gamma pairs, measurement of h→gamma gamma could yield weak constraint)
  * h → Z a (a → gamma gamma) has been initially studied by Bauer, Neubert, Thamm in [[https://arxiv.org/pdf/1704.08207|1704.08207]], for a being an ALP (axion-like particle)
  * h → a a': if ma is different from ma', h → a a searches would miss it (also depends on final state)
   
    
h→chi1 chi2

      * Decays involving MET? (e.g. chi1 invisible); (Also considerable for h → a a' and h → Z a)

Exotic Higgs decays into LLP (displaced vertices). Role of LHCb (also BaBar, Belle II)
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