Illuminating dark photons with high-energy colliders

Abstract

High-energy colliders offer a unique sensitivity to dark photons, the mediators of a broken dark U(1) gauge theory that kinetically mixes with the Standard Model (SM) hypercharge. Dark photons can be detected in the exotic decay of the 125 GeV Higgs boson, h → ZZ D → 4 , and in Drell-Yan events, pp → Z D → . If the dark U(1) is broken by a hidden-sector Higgs mechanism, then mixing between the dark and SM Higgs bosons also allows the exotic decay h → Z D Z D → 4 . We show that the 14 TeV LHC and a 100 TeV proton-proton collider provide powerful probes of both exotic Higgs decay channels. In the case of kinetic mixing alone, direct Drell-Yan production offers the best sensitivity to Z D , and can probe 9 × 10 −4 (4 × 10 −4 ) at the HL-LHC (100 TeV pp collider). The exotic Higgs decay h → ZZ D offers slightly weaker sensitivity, but both measurements are necessary to distinguish the kinetically mixed dark photon from other scenarios. If Higgs mixing is also present, then the decay h → Z D Z D can allow sensitivity to the Z D for 10 −9 − 10 −6 (10 −10 − 10 −7 ) for the mass range 2m µ < m Z D < m h /2 by searching for displaced dark photon decays. We also compare the Z D sensitivity at pp colliders to the indirect, but model-independent, sensitivity of global fits to electroweak precision observables. We perform a global electroweak fit of the dark photon model, substantially updating previous work in the literature. Electroweak precision measurements at LEP, Tevatron, and the LHC exclude as low as 3 × 10 −2 . Sensitivity can be improved by up to a factor of ∼ 2 with HL-LHC data, and an additional factor of ∼ 4 with ILC/GigaZ data.

A tale of two portals: testing light, hidden new physics at future e + e − colliders

“…The four lepton final state has also been used to constrain the exotic decay H 0 →ZK, which gives sensitivity to from (2.24). The current bound using 8 TeV data is weak, with the strongest sensitivity for mK ≈ 30 GeV giving 0.05, while the improvement at the HL-LHC is expected to reach 0.01 [8]. These gains are mainly…”

“…which give 8) where the kinetic terms are now canonically normalized and only one further unitary rotation is needed to diagonalize the mass matrix. We remark that | | < c W is required to ensure the kinetic mixing matrix in eq.…”

“…These exotic decays can also give fully visible and semi-visible signatures [8,[71][72][73][74][75] when theK or S particle decays to SM final states, which provide additional handles for Higgs collider phenomenology. Those references concentrate on the scenario where the decay to SM final states are dominant, e.g.…”

“…Therefore, such constraints should be modified by the relevant branching ratios in the DDP model because the DDP model includes a DM decay. If visible decays ofK dominate, though, the search for h → 2a → 4µ [76] constrains the process H 0 →KK, and bounds λ HP 0.01 for mK 10 GeV [8], while the bound strengthens to λ HP 0.001 [8] for mK 10 GeV from recasting the differential distributions in 8 TeV h → ZZ * → 4 data [77]. The coupling λ HP = λ HP m 2 H 0 /|m 2 H 0 − m 2 S |, is roughly the same as λ HP if m S is not close to or much larger than m H 0 .…”

“…Both of these marginal operators have been studied autonomously at electron colliders in the hidden photon context [5][6][7][8][9][10][11][12][13][14][15][16][17] and the hidden scalar context [18][19][20][21][22][23]. Some works study both operators in tandem [24][25][26] or adopt an effective operator approach [27].…”