Les Houches

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Interested people: Jose Miguel No, Jose Zurita, Stefania Gori, Ken Mimasu, Grégory Moreau, Sylvain Fichet

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 is not promising anymore (see for instance 1607.08653).

1. Associated production (pp → 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 1702.03984).

Context here: EFT with on-shell scalar (phi) and scalar couplings to vector bosons considered so look at the decay phi → diphoton [here phi is not a stable DM candidate leading to missing E_T].

Other decay modes to be studied could be: phi → tau tau and phi → b bbar [reconstructing the invariant scalar mass].

Gauge invariant (pseudo-)scalar couplings to ZZ, WW, gamma-gamma, gamma-Z, gluon-gluon: (phi/Lambda_1)[D_mu H]^2 [for scalars only] and (phi/Lambda_2) (Trace) V_{mu nu} (\tilde) V^{mu nu}.

Two other effective parameters: phi mass (m_phi) and Branching for phi → gamma gamma (B_ga).

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

• q qbar → Z*,gamma* → Z phi, Z→l+l- [to be first looked at this Les Houches session]
• q qbar → Z(*) → gamma phi [existing analysis from ATLAS ?]
• q qbar → gamma* → gamma phi 1409.4792
• q qbar → gluon* → gluon phi 1409.4792

A first work Plan: (1) implement the effective model into FeynRules (2) interface the DELPHES detector response simulator (at the MadGRAPH level) (3) produce kinematical distributions for the signal with MadGRAPH (4) compare distributions for SM-like couplings (1/Lambda_1) and kinetic-like couplings (1/Lambda_2) [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 3-dimension parameter space: m_phi, 1/Lambda_1, 1/Lambda_2 [=couplings for the considered gauge bosons] for B_ga=100% (plot to be simply rescaled accordingly to the wanted B_ga 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.

2. Complementarity with phi “single” 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: gluon gluon → phi with a gluon radiated off one of the 2 initial state gluons [credit: Thomas Flacke]. A phi 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 125 GeV is SM like (2HDM “alignment limit”).

3. Exotic Higgs decays: What are the holes? [contact: Stefania Gori]

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 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)