Two-step electroweak symmetry-breaking: theory meets experiment

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Springer

Received: February 3, 2020 Accepted: April 15, 2020 Published: May 11, 2020

Nicole F. Bell,a Matthew J. Dolan,a Leon S. Friedrich,a Michael J. Ramsey-Musolfb,c,d and Raymond R. Volkasa a

ARC Centre of Excellence for Particle Physics at the Terascale, School of Physics, The University of Melbourne, Melbourne, VIC 3010, Australia b Amherst Center for Fundamental Interactions, Department of Physics, University of Massachusetts Amherst, Amherst, MA 01003, U.S.A. c Tsung-Dao Lee Institute and School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China d Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A.

E-mail: [email protected], [email protected], [email protected], [email protected], [email protected] Abstract: We study the phenomenology of a hypercharge-zero SU(2) triplet scalar whose existence is motivated by two-step electroweak symmetry-breaking. We consider both the possibility that the triplets are stable and contribute to the dark matter density, or that they decay via mixing with the standard model Higgs boson. The former is constrained by disappearing charged track searches at the LHC and by dark matter direct detection experiments, while the latter is constrained by existing multilepton collider searches. We find that a two-step electroweak phase transition involving a stable triplet with a negative quadratic term is ruled out by direct detection searches, while an unstable triplet with a mass less than 230 GeV is excluded at 95% confidence level. Keywords: Beyond Standard Model, Higgs Physics ArXiv ePrint: 2001.05335

c The Authors. Open Access, Article funded by SCOAP3 .

https://doi.org/10.1007/JHEP05(2020)050

JHEP05(2020)050

Two-step electroweak symmetry-breaking: theory meets experiment

Contents 1

2 Model 2.1 Z2 symmetric model 2.2 Z2 broken model 2.3 Perturbative unitarity and perturbativity 2.4 RGEs and running constraints 2.5 Phase transition requirements 2.6 Higgs diphoton rate

3 4 5 6 7 8 9

3 Stable triplet phenomenology 3.1 Disappearing tracks 3.2 Dark matter direct detection

11 11 13

4 Unstable triplet phenomenology 4.1 Production processes 4.2 Decay channels 4.3 Collider searches 4.4 Collider constraints

16 16 18 20 20

5 Conclusion

23

1

Introduction

The origin of the baryon asymmetry of the universe is a major open problem in particle physics and cosmology. Successful baryogenesis mechanisms require extensions to the standard model (SM), as it has neither enough charge-parity (CP) violation nor does it provide the necessary out-of-equilibrium conditions. Electroweak baryogenesis provides one possible solution, and is particularly attractive as its association with new electroweak scale physics means it is testable experimentally via collider searches [1] and electric dipole moment (EDM) measurements (for a review, see, e.g. [2]). There has been recent interest in the possibility of multi-step electroweak phase transitions [3–8]. In such scenar