Search for doubly-charged Higgs boson production at HERA

Doubly-charged Higgs bosons (H^±±) appear when the Higgs sector of the Standard Model is extended by one or more triplet(s) with non-zero hypercharge. Examples are provided by some Left-Right Symmetric models, or their supersymmetric extensions, which are of particular interest since they provide a mechanism to generate small non-zero neutrino masses. Such models can lead to a doubly-charged Higgs boson light enough to be produced at the existing colliders. The Higgs triplet(s) may be coupled to matter fields via Yukawa couplings which are generally not related to the fermion masses. A non-vanishing coupling of a doubly-charged Higgs to an electron would allow its single production in ep collisions at HERA. This possibility has been investigated with a search for doubly-charged Higgs bosons decaying into a high mass pair of same charge leptons, one of them being an electron. The analysis is based on ep data collected by the H1 experiment between 1994 and 2000, which amount to a luminosity of up to 118 pb^-1.

In a previous model independent multi-electron analysis, H1 observed six events with a di-electron mass above 100 GeV, a region where the Standard Model expectation is small. Out of the six events, only one is compatible with the signature of a doubly-charged Higgs boson. No electron-muon or electron-tau event is found in this mass domain. Therefore this analysis places new limits on the H^±± production cross section, its mass and its Yukawa couplings h_el to an electron-lepton pair.

Upper limits on the H^±± production cross section times branching ratio are derived as a function of the H^±± mass and are shown in Figure a. The best sensitivity is obtained for a H^±± produced and decaying via h_eμ, with upper limits around 0.05 pb.

If the doubly-charged Higgs boson couples only to an electron pair (Figure b) this analysis rules out H^±± masses below 138 GeV for a coupling h_ee of the electromagnetic strength, h_ee ~ 0.3. The result is compared to the bounds obtained from searches for H^±± pair production at LEP and by the CDF experiment, and to both the indirect and direct limits obtained by the OPAL experiment, the latter being the most stringent. The OPAL experiment has also set similar stringent constraints on h_ee independently of the Higgs decay mode. These constraints also exclude a sizeable H^±± production at HERA via h_ee followed by the H^±± decay via h_μμ or h_ττ, which is consistent with the non-observation of a resonance signal in the μμ and ττ final states in the present H1 data.

Assuming that the doubly-charged Higgs boson couples only to an electron-muon (electron-tau) pair, this analysis allows masses below 141 GeV (112 GeV) to be ruled out for h_eμ ~ 0.3 (h_eτ ~ 0.3), as shown in Figure c (Figure d). The H1 limits extend the excluded region in the electron-muon and electron-tau channels to masses that are beyond those reached in previous searches for pair production at LEP and the Tevatron.

Last Update 19.01.2007 Bob Olivier