Searching for heavy leptophilic Z′: from lepton colliders to gravitational waves
Arnab Dasgupta, P. S. Bhupal Dev, Tao Han, Rojalin Padhan, Si Wang, Keping Xie
Abstract
A bstract We study the phenomenology of leptophilic Z ′ gauge bosons at the future high-energy e + e − and μ + μ − colliders, as well as at the gravitational wave observatories. The leptophilic Z ′ model, although well-motivated, remains largely unconstrained from current low-energy and collider searches for Z ′ masses above $$ \mathcal{O} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>O</mml:mi> </mml:math> (100 GeV), thus providing a unique opportunity for future lepton colliders. Taking $$ \textrm{U}{(1)}_{L_{\alpha }-{L}_{\beta }} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>U</mml:mi> <mml:msub> <mml:mfenced> <mml:mn>1</mml:mn> </mml:mfenced> <mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>α</mml:mi> </mml:msub> <mml:mo>−</mml:mo> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>β</mml:mi> </mml:msub> </mml:mrow> </mml:msub> </mml:math> ( α , β = e , μ , τ ) models as concrete examples, we show that future e + e − and μ + μ − colliders with multi-TeV center-of-mass energies provide unprecedented sensitivity to heavy leptophilic Z ′ bosons. Moreover, if these U(1) models are classically scale-invariant, the phase transition at the U(1) symmetry-breaking scale tends to be strongly first-order with ultra-supercooling, and leads to observable stochastic gravitational wave signatures. We find that the future sensitivity of gravitational wave observatories, such as advanced LIGO-VIRGO and Cosmic Explorer, can be complementary to the collider experiments, probing higher Z ′ masses up to $$ \mathcal{O} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>O</mml:mi> </mml:math> (10 4 TeV), while being consistent with naturalness and perturbativity considerations.