SU(2) Triplet Scalars

• SU(2) triplet scalars are scalar fields transforming as a triplet under SU(2) that extend the Higgs sector with additional neutral and charged components.
• The scalar potential and doublet–triplet mixing lead to unique collider signals, including long-lived charged tracks and enhanced Higgs-to-diphoton decay rates.
• Electroweak precision tests, especially constraints from the ρ parameter, tightly limit the triplet vev, enabling dark matter candidates in the x₀ = 0 regime.

An SU(2) triplet scalar is a scalar field that transforms as a triplet under the SU(2) gauge symmetry, and appears as an extension to the Standard Model Higgs sector or in other gauge scenarios. The inclusion of such scalars—either real (hypercharge Y = 0) or complex (Y = ±1 or Y = 2)—fundamentally modifies the vacuum structure, spectrum, and phenomenology of the theory. These modifications affect electroweak precision constraints, collider observables, and can provide new candidates for dark matter, all while enabling distinctive multi-lepton or photon final states at high-energy colliders.

1. Model Construction and Scalar Potentials

The minimal extension of the Standard Model incorporating an SU(2) triplet introduces two main scalar multiplets:

• An SU(2) doublet Higgs $H = (\phi^+, \phi^0)^T$.
• A real SU(2) triplet $$\Sigma = (1/2) \begin{pmatrix} \Sigma^0 & \sqrt{2}\Sigma^+ \ \sqrt{2}\Sigma^- & -\Sigma^0 \end{pmatrix}$$.

The most general renormalizable scalar potential with these fields is: $$V(H,\Sigma) = -\mu^2\,H^\dagger H + \lambda_0\,(H^\dagger H)^2 - M_\Sigma^2\,\mathrm{Tr}\,\Sigma^2 + \lambda_1\,\mathrm{Tr}\,\Sigma^4 + \lambda_2\,(\mathrm{Tr}\,\Sigma^2)^2 + \alpha\,(H^\dagger H)\mathrm{Tr}\,\Sigma^2 + \beta\, H^\dagger \Sigma^2 H + a_1\,H^\dagger \Sigma H$$ where $a_1$ and $a_2 \equiv \alpha + \beta/2$ parameterize the doublet–triplet mixing. After electroweak symmetry breaking (EWSB), the vacuum expectation values (vevs) are, in general, $H = (0,\, (v_0 + h^0 + i\xi^0)/\sqrt{2})^T$ and $$\Sigma = (1/2)\begin{pmatrix} x_0 + \sigma^0 & \sqrt{2} \Sigma^+ \ \sqrt{2} \Sigma^- & -x_0 - \sigma^0 \end{pmatrix}$$.

The model's scalar spectrum includes:

• A neutral scalar $H_1$ (mostly doublet-like),
• A neutral scalar $H_2$ (mostly triplet-like),
• Charged scalars $H^\pm$ (mixtures of doublet and triplet components).

The charged and neutral scalar mass parameters and their mixings are governed by the minimization of the scalar potential and by the parameters controlling doublet–triplet couplings.

2. Vacuum Structure and Constraints

The vacuum structure is determined by simultaneous minimization conditions: $$(-\mu^2 + \lambda_0 v_0^2 - \tfrac{a_1x_0}{2} + \tfrac{a_2 x_0^2}{2}) v_0 = 0$$

$$(-M_\Sigma^2 x_0 + b_4 x_0^3 - \tfrac{a_1 v_0^2}{4} + \tfrac{a_2 v_0^2 x_0}{2}) = 0$$

with $b_4 = \lambda_2 + 1/2\lambda_1$. Electroweak precision constraints—most stringently from the $\rho$ parameter, $\delta\rho = (2x_0/v_0)^2$—force the triplet vev $x_0$ to be much smaller than the doublet vev. Empirically, $\delta \rho \lesssim 0.001$ so $x_0 \lesssim 4$ GeV.

Two viable regimes emerge:

• $x_0 \ll v_0$: Small mixing, nonvanishing triplet vev, scalar mass eigenstates are admixtures.
• $x_0 = 0$: Exact vanishing triplet vev. The doublet sector is SM-like, and the scalar spectrum retains a conserved $\Sigma \to -\Sigma$ $Z_2$ symmetry, making the triplet sector stable.

Direct searches at colliders set additional mass bounds (e.g., $M_{H^\pm} \gtrsim 100$ GeV, $M_{H_1} \gtrsim 114$ GeV), while perturbativity and unitarity restrict the quartic couplings.

3. Collider Phenomenology: Signatures of Triplet Scalars

The presence of light triplet charged scalars—as well as the nearly degenerate neutral states—generates distinctive collider signatures:

• Drell–Yan Pair Production: $H^+ H^-$ and $H^+ H_2$ can be produced at observable rates due to their electroweak gauge couplings.
• Long-lived Charged Tracks: For $x_0 = 0$, the charged scalar $H^\pm$ decays to $H_2$ plus a soft pion with a long lifetime due to small (radiative) mass splitting (~166 MeV). This leads to one or two disappearing or kinked tracks in the detector, accompanied by missing transverse energy—an unambiguous signature for non-minimal Higgs sectors.
• Diphoton Higgs Signal Excess: The process $H_1 \rightarrow \gamma\gamma$ receives new loop contributions from the light charged scalar $H^\pm$:

$$\Gamma(H_1 \to \gamma\gamma) \propto \left| A_W + A_t + A_{H^\pm} \right|^2$$

where $A_{H^\pm}$ depends on the $H_1 H^+ H^-$ coupling (mainly $a_2 v_0$) and charged scalar mass. For appropriate $M_{H^\pm} < 200$ GeV and sizable, negative $a_2$, the diphoton rate can be significantly enhanced relative to the SM, potentially up to a factor of 2.

All such effects are intertwined with the specific vertex structures (including $H^\pm Z W^\mp$, $H_2 W^+W^-$) induced by the SU(2) multiplet quantum numbers.

4. Dark Matter in the $x_0 = 0$ Limit

When $x_0 = 0$, the triplet sector possesses an exact $Z_2$ symmetry, $\Sigma \to -\Sigma$, leading to the stability of the triplet's neutral component ($H_2$). As such, $H_2$ becomes a viable cold dark matter candidate, provided all dimension-3/4 operators respect this symmetry.

Key points for the dark matter scenario:

• The neutral state $H_2$ is absolutely stable if $x_0 = 0$.
• Radiative corrections split $H^\pm$ and $H_2$ by $\approx 166$ MeV; $H^\pm$ decay to $H_2$ plus soft pions with a long lifetime.
• Colliders could detect long-lived charged tracks from $H^\pm$ decays, potentially unique to such a scenario.
• For thermal relic abundance consistent with observations, $M_{H_2} \sim 2.5$ TeV is required, but lighter $H_2$ can be viable in a multicomponent DM scenario.

5. Precision Electroweak and Theoretical Constraints

Electroweak precision observables, most importantly the $\rho$ parameter, place severe constraints on the triplet vev and hence the possible scalar mixings and mass spectrum. Quantitatively,

$$\rho = \frac{M_W^2}{M_Z^2 \cos^2 \theta_W} = 1 + \delta \rho$$

with $\delta \rho = (2x_0/v_0)^2$. The fits require $x_0 \ll v_0$, supporting the $x_0 = 0$ regime as especially attractive both phenomenologically and for dark matter.

Perturbativity and unitarity bound quartic couplings (e.g., $\lambda_0$, $a_2$) to not become large, and direct collider bounds forbid light charged scalars below $\sim$100 GeV.

6. Theoretical Implications and Model Discrimination

SU(2) triplet scalar extensions introduce phenomena not present in doublet-only models. The accidental $Z_2$ symmetry for $x_0=0$ is directly responsible for dark matter stability. The characteristic collider signature of long-lived (hundreds of mm) charged tracks is an unambiguous marker of such a spectrum. Enhanced Higgs-to-diphoton rates provide additional constraints and possible observables for early LHC data.

Summary of selected key formulae: | Formula Type | Expression | |----------------------------------------------|---------------------------------------------------------------------------------------------| | Tree-level scalar minimization | $$(-\mu^2 + \lambda_0 v_0^2 - \frac{a_1 x_0}{2} + \frac{a_2 x_0^2}{2}) v_0 = 0$$ | | $\rho$ parameter deviation | $\delta \rho = \left( \frac{2 x_0}{v_0} \right)^2$ | | Charged scalar mass at $x_0=0$ | $M_{H^\pm}^2 = -M_\Sigma^2 + \frac{a_2 v_0^2}{2}$ | | Higgs diphoton decay amplitude | $$\Gamma(H_1 \to \gamma\gamma) \propto |A_W + A_t + A_{H^\pm}|^2$$ |

7. Outlook and Relevance to New Physics Searches

The SU(2) triplet extension of the Standard Model Higgs sector, as formulated above, is characterized by:

• A robust mechanism for dark matter stability tied to a symmetry of the triplet sector.
• Distinctive LHC signatures, particularly from long-lived charged scalars and enhanced Higgs diphoton rates.
• Direct correlation between the vacuum structure, scalar spectrum, and experimental observables, offering multiple independent avenues for discovery or exclusion.
• Severe constraints from electroweak precision measurements, anchoring the viable parameter space.

The interplay between the scalar potential, vacuum symmetry, dark matter phenomenology, and collider signatures underscores the importance of SU(2) triplet scalars in the search for physics beyond the Standard Model, especially in the dark matter and Higgs sectors (0811.3957).