Higgs and Dark Photon Searches

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1 Helsinki Institute of Physics [ [hep-ph]], Sanjoy Biswas, Emidio Gabrielli, M. H., Barbara Mele LCWS15, November, 215

2 Contents 1 Introduction 2 e + e H γ

3 Dark Photons Dark photons appear in several beyond the Standard Model physics scenarios, where a new U(1) gauge group is added to the SM. Massive dark photons can be dark matter candidates, while massless dark photons can appear in models of self-interacting dark matter. (Cusp-vs-core, missing satellites.) Unbroken U(1) results in a massless dark photon. Motivated e.g. in a model for radiative origin of the SM Yukawa couplings. [arxiv: [hep-ph]]

4 Coupling to the SM Dark photons can couple to the SM particles via the kinetic mixing operator F µνf µν, or via loop-induced dimension 5 operators. The kinetic mixing of massless Dark Photons can be transformed away by field redefinitions. Generally this results in millicharges for the particles initially charged under the hidden U(1). If the tree-level kinetic mixing is set to zero, the possible loop-induced mixing vanishes on-shell. If there are particles charged under both the hidden and the SM U(1), there will be loop-induced couplings between the dark photon and the SM.

5 Coupling to the SM Couplings to the Higgs can be generated via messenger particles charged under U(1) U(1). H γ γ H Similar diagrams will also contribute to the H γγ, H ZZ decay widths. Effective Lagrangian: L DPH = α π γ γ ( Cγ γ v γµν γ µν H + C Z γ v Z µν γ ) µν H + C γ γ v γµν γ µν H H γ Z

6 Higgs to New Physics Branching Ratios 35 3 R i R 2 = Br(H Z γ) Br(H Zγ) SM R 1 = Br(H γ γ) Br(H γγ) SM C γ γ, C Z γ

7 Contents 1 Introduction 2 e + e H γ

8 Higgs + Dark Photon production The process e + e H γ is generated by the s-channel diagram: e + e γ/z γ C V γ H We look at the final state H b b, so that the signal is two b-jets plus missing energy.

9 Inclusive Production Cross Section C Γ Γ 1, C Z Γ.79 Σ fb C Γ Γ 1, C Z Γ C Γ Γ, C Z Γ

10 Event Selection Initial event selection: Two b-jets with p T > 2 GeV, η < 2.5, and R(bb) >.4 Missing energy /E > 4 GeV. The main SM background is the ν νb b production, including the on shell ZH ν νb b. There is also a subdominant contribution from ν νq q, where both light jets are misstagged as b-jets. We assume 8% b-tagging efficiency and a miss-tag rate of 1 2 for light jets.

11 Jet Pair Invariant Mass Distribution Events/5 GeV H γ ν νb b ν νq q Rj = 1.5 σ(e) E = 3% E 5 1 M jj (GeV) 15 2 We require M jj within 1% of the peak value of the simulated signal events. The distributions shown are normalized to one.

12 Missing Mass Distribution Events/5 GeV H γ ν νb b ν νq q particle level Events/5 GeV H γ ν νb b ν νq q Rj = 1.5 σ(e) E = 3% E Mmiss (GeV) Mmiss (GeV) Missing mass M miss = /E 2 /p 2 distributions for parton level events and after PYTHIA and jet energy smearing. We require M miss < 4 GeV

13 Missing Energy Distribution Events/5 GeV H γ ν νb b Rj = 1.5 σ(e) E = 3% E /E (GeV) Missing energy distributions after applying the cuts on M jj and M miss. We require 4 GeV < /E < 1 GeV.

14 Signal and Backgrounds After Cuts Process Cross section (fb) Acceptance (%) H γ (C Z γ = ) Cγ γ H γ (C γ γ = ) CZ 2 γ 17.3 H γ (C Z γ =.79 C γ γ ) Cγ γ SM ν νb b The cross section and acceptance after the cuts for the signal and SM background. The ν νq q background is negligible.

15 Discovery Reach C Z Γ.79C Γ Γ 8 C Z Γ Significance Σ 6 4 5Σ C Γ Γ 2 2Σ The projected sensitivity for the effective couplings for 1 4 fb 1 at a 24 GeV e + e collider.

16 Discovery Reach C Z Γ.79C Γ Γ C Z Γ C Γ Γ C Γ Γ, C Z Γ The projected 5σ-sensitivity for the effective couplings for a 24 GeV e + e collider.

17 Conclusions The Higgs boson can act as a portal to a hidden sector responsible for e.g. dark matter, flavor hierarchy, EWSB etc. Production of a SM Higgs in association with a dark photon is a signature of such scenario. The effective coupling C γ γ can be probed down to values corresponding to BR(H γ γ) O(1%) in future e + e colliders.