diff --git a/Physlib.lean b/Physlib.lean
index d1bd5fb76..a0450d782 100644
--- a/Physlib.lean
+++ b/Physlib.lean
@@ -189,6 +189,7 @@ public import Physlib.Particles.StandardModel.AnomalyCancellation.NoGrav.One.Lin
 public import Physlib.Particles.StandardModel.AnomalyCancellation.Permutations
 public import Physlib.Particles.StandardModel.Basic
 public import Physlib.Particles.StandardModel.Fermions.QuarkDoublet
+public import Physlib.Particles.StandardModel.Fermions.UpSinglet
 public import Physlib.Particles.StandardModel.HiggsBoson.Basic
 public import Physlib.Particles.StandardModel.HiggsBoson.EffectivePotential
 public import Physlib.Particles.StandardModel.HiggsBoson.Potential
diff --git a/Physlib/Particles/StandardModel/Fermions/UpSinglet.lean b/Physlib/Particles/StandardModel/Fermions/UpSinglet.lean
new file mode 100644
index 000000000..8fe9c0071
--- /dev/null
+++ b/Physlib/Particles/StandardModel/Fermions/UpSinglet.lean
@@ -0,0 +1,211 @@
+/-
+Copyright (c) 2026 Joseph Tooby-Smith. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Joseph Tooby-Smith
+-/
+module
+
+public import Physlib.Particles.StandardModel.Basic
+public import Physlib.Relativity.Tensors.ComplexTensor.Basic
+/-!
+# Up-type singlets
+
+In this module we define the type corresponding to
+the target vector space of an up-type singlet quark field in the Standard Model.
+
+On this type we define a representation of the Lorentz group, and a
+representation of the Standard Model gauge group.
+
+-/
+
+@[expose] public section
+
+namespace StandardModel
+
+open TensorProduct
+
+/-- The vector space of an up-type singlet quark field in the Standard Model.
+  These live in the (3, 1)_{4} representation of the gauge group. -/
+@[ext]
+structure UpSinglet where
+  /-- The underlying value of the up-type quark field in the tensor product space. -/
+  val : Fermion.RightHandedWeyl ⊗[ℂ] EuclideanSpace ℂ (Fin 3)
+
+namespace UpSinglet
+
+/-!
+
+## Equivalence with the underlying tensor product space
+
+-/
+
+/-- The linear equivalence between `UpSinglet` and its underlying tensor product space. -/
+def valEquiv : UpSinglet ≃ Fermion.RightHandedWeyl ⊗[ℂ] EuclideanSpace ℂ (Fin 3) where
+  toFun := val
+  invFun := fun m => ⟨m⟩
+
+/-!
+
+## The structure of a module
+
+The AddCommGroup and module instances are inherited from the underlying tensor product space.
+-/
+
+instance : AddCommGroup UpSinglet := Equiv.addCommGroup valEquiv
+
+instance : Module ℂ UpSinglet := Equiv.module ℂ valEquiv
+
+/-- The linear equivalence between `UpSinglet` and its underlying tensor product space. -/
+def valLinEquiv : UpSinglet ≃ₗ[ℂ]
+    Fermion.RightHandedWeyl ⊗[ℂ] EuclideanSpace ℂ (Fin 3) where
+  toFun := val
+  invFun := fun m => ⟨m⟩
+  map_add' := by intros; rfl
+  map_smul' := by intros; rfl
+
+@[simp]
+lemma valLinEquiv_apply (q : UpSinglet) : valLinEquiv q = q.val := rfl
+
+lemma valLinEquiv_symm_apply (m : Fermion.RightHandedWeyl ⊗[ℂ] EuclideanSpace ℂ (Fin 3)) :
+    valLinEquiv.symm m = ⟨m⟩ := rfl
+
+@[simp]
+lemma val_add (q1 q2 : UpSinglet) : (q1 + q2).val = q1.val + q2.val := rfl
+
+@[simp]
+lemma val_smul (r : ℂ) (q : UpSinglet) : (r • q).val = r • q.val := rfl
+
+/-!
+
+## Lorentz group representation
+
+-/
+open Matrix MatrixGroups
+
+open Representation in
+/-- The representation of the Lorentz group on the space of up-type quark fields. -/
+noncomputable def repLorentzGroup : Representation ℂ (SL(2,ℂ)) UpSinglet where
+  toFun Λ :=  valLinEquiv.symm ∘ₗ
+      (TensorProduct.map (Fermion.RightHandedWeyl.rep Λ)
+        (trivial ℂ (SL(2,ℂ)) (EuclideanSpace ℂ (Fin 3)) Λ))
+      ∘ₗ valLinEquiv
+  map_one' := by
+    ext q
+    simp [Module.End.one_eq_id]
+  map_mul' Λ1 Λ2 := by
+    ext1 q
+    simp [TensorProduct.map_map, Module.End.mul_eq_comp]
+
+/-!
+
+## The representation of the Standard Model gauge group
+
+-/
+
+/-- The action of the full Standard Model gauge group on up-type quark fields. -/
+noncomputable def repGaugeGroupI : Representation ℂ GaugeGroupI UpSinglet where
+  toFun g := valLinEquiv.symm ∘ₗ
+        (TensorProduct.map
+        (LinearMap.id (M := Fermion.RightHandedWeyl)) -- action on the Lorentz indices
+        g.toSU3.1.toEuclideanLin) -- SU(3) action
+      ∘ₗ LinearMap.lsmul ℂ _ (g.toU1.1 ^ 4 : ℂ) -- U(1) action
+      ∘ₗ valLinEquiv
+  map_one' := by
+    ext q
+    simp [valLinEquiv_symm_apply]
+  map_mul' g1 g2 := by
+    ext q
+    simp [smul_smul, mul_comm, TensorProduct.map_map, valLinEquiv_symm_apply]
+    ring_nf
+
+lemma repGaugeGroupI_tmul (g : GaugeGroupI) (ψ : Fermion.RightHandedWeyl)
+    (v : EuclideanSpace ℂ (Fin 3)) :
+    repGaugeGroupI g ⟨ψ ⊗ₜ v⟩ = ⟨g.toU1 ^ 4 • ψ ⊗ₜ (g.toSU3.1.toEuclideanLin v)⟩ := rfl
+
+open Fermion in
+/-- The action of the full gauge group on a tensor product of basis elements, expanded as a
+  sum over the columns of the `SU(3)` matrix. -/
+lemma repGaugeGroupI_tmul_basis_eq_sum (g : GaugeGroupI) (k : Fin 2) (i : Fin 3) :
+    repGaugeGroupI g ⟨RightHandedWeyl.basis k ⊗ₜ[ℂ] EuclideanSpace.basisFun (Fin 3) ℂ i⟩ =
+      ∑ i' : Fin 3, (g.toU1.1 ^ 4  * g.toSU3.1 i' i)
+      • (⟨RightHandedWeyl.basis k ⊗ₜ[ℂ] EuclideanSpace.basisFun (Fin 3) ℂ i'⟩ : UpSinglet) := by
+  apply valLinEquiv.injective
+  apply (((RightHandedWeyl.basis).tensorProduct
+    (EuclideanSpace.basisFun (Fin 3) ℂ).toBasis)).repr.injective
+  ext ⟨⟨k, l⟩, m⟩
+  simp only [EuclideanSpace.basisFun_apply, repGaugeGroupI_tmul, Submonoid.smul_def,
+    SubmonoidClass.coe_pow, valLinEquiv_apply, map_smul, Finsupp.coe_smul, Pi.smul_apply,
+    Module.Basis.tensorProduct_repr_tmul_apply, OrthonormalBasis.coe_toBasis_repr_apply,
+    EuclideanSpace.basisFun_repr, ofLp_toLpLin, PiLp.ofLp_single, toLin'_apply, mulVec_single,
+    MulOpposite.op_one, col_apply, one_smul, Module.Basis.repr_self, smul_eq_mul, map_sum,
+    Finsupp.coe_finsetSum, Finset.sum_apply, PiLp.single_apply, ite_mul, one_mul, zero_mul, mul_ite,
+    mul_zero, Finset.sum_ite_eq, Finset.mem_univ, ↓reduceIte]
+  ring
+
+open Fermion in
+lemma repGaugeGroupI_eq_iff_mul_eq {g1 g2 : GaugeGroupI} :
+    repGaugeGroupI g1 = repGaugeGroupI g2 ↔ ∀ i i',
+    g1.toU1.1 ^ 4 * g1.toSU3.1 i' i = g2.toU1.1 ^ 4 * g2.toSU3.1 i' i := by
+  let b := (RightHandedWeyl.basis).tensorProduct (EuclideanSpace.basisFun (Fin 3) ℂ).toBasis
+  constructor
+  · intro h i i'
+    have h' := congrFun (congrArg (fun f => f.1) h)
+      ⟨RightHandedWeyl.basis 0 ⊗ₜ[ℂ] EuclideanSpace.basisFun (Fin 3) ℂ i⟩
+    simp only [Fin.isValue, LinearMap.coe_toAddHom, repGaugeGroupI_tmul_basis_eq_sum] at h'
+    replace h' := congrArg b.repr (congrArg valLinEquiv h')
+    simpa [Module.Basis.tensorProduct_repr_tmul_apply, -Fin.sum_univ_two, b] using
+      congrArg (fun f => f (0, i')) h'
+  · intro h
+    apply (valLinEquiv.symm.eq_comp_toLinearMap_iff (repGaugeGroupI g1) (repGaugeGroupI g2)).mp
+    apply b.ext
+    rintro ⟨i, k⟩
+    have h1 := repGaugeGroupI_tmul_basis_eq_sum g1 i k
+    have h2 := repGaugeGroupI_tmul_basis_eq_sum g2 i k
+    simp only [EuclideanSpace.basisFun_apply] at h1 h2
+    simp [valLinEquiv_symm_apply, h1, h2, b, h]
+
+lemma mem_repGaugeGroupI_ker_iff_eq {g : GaugeGroupI} :
+    g ∈ repGaugeGroupI.ker ↔ ∃ a : ℂ, g.toSU3.1 = a • 1 ∧  a * g.toU1.1 ^ 4 = 1 := by
+  rw [MonoidHom.mem_ker, ← MonoidHom.map_one repGaugeGroupI, repGaugeGroupI_eq_iff_mul_eq]
+  constructor; swap
+  · rintro ⟨a, h1, h2⟩ i i'
+    simp only [Matrix.smul_apply, smul_eq_mul, h1, map_one, OneMemClass.coe_one]
+    linear_combination h2 * (1 : Matrix _ _ ℂ) i' i
+  · intro h
+    use g.toSU3.1 0 0
+    simp only [map_one, OneMemClass.coe_one, Fin.forall_fin_succ, Fin.isValue,
+      Fin.succ_zero_eq_one, IsEmpty.forall_iff, and_true, one_apply_eq, mul_one, ne_eq, one_ne_zero,
+      not_false_eq_true, one_apply_ne, mul_zero, mul_eq_zero, zero_ne_one, Fin.succ_one_eq_two,
+      Fin.reduceEq] at h
+    refine ⟨?_, ?_⟩
+    · ext i j
+      fin_cases i <;> fin_cases j <;> simp <;> grind
+    · grind
+
+lemma gaugeGroup_subgroup_ℤ₆_le_ker_repGaugeGroupI :
+    GaugeGroupQuot.subgroup .ℤ₆ ≤ repGaugeGroupI.ker := by
+  simp only [GaugeGroupQuot.subgroup, gaugeGroupℤ₆SubGroup, SetLike.le_def, MonoidHom.mem_range,
+    gaugeGroupℤ₆Hom_apply, Subtype.exists, mem_repGaugeGroupI_ker_iff_eq, forall_exists_index]
+  rintro g x hx ⟨rfl⟩
+  use  (x ^ 2)
+  simp only [gaugeGroupℤ₆OfRoot_toSU3, gaugeGroupℤ₆SU3OfRoot_eq_mul_id, gaugeGroupℤ₆OfRoot_toU1,
+    gaugeGroupℤ₆UnitaryOfRoot_coe, true_and]
+  field_simp
+  exact (mem_rootsOfUnity' 6 x).mp hx
+
+lemma gaugeGroup_subgroup_le_ker_repGaugeGroupI (Q : GaugeGroupQuot) :
+    Q.subgroup ≤ repGaugeGroupI.ker := Q.subgroup_le_subgroup_ℤ₆.trans
+  gaugeGroup_subgroup_ℤ₆_le_ker_repGaugeGroupI
+
+/-- The action of the Standard Model gauge group, potentially quotiented by
+  a discrete factor on quark fields. -/
+noncomputable def repGaugeGroup : (Q : GaugeGroupQuot) →
+    Representation ℂ (GaugeGroup Q) UpSinglet
+  | .I => repGaugeGroupI
+  | .ℤ₆ => QuotientGroup.lift _ repGaugeGroupI (gaugeGroup_subgroup_le_ker_repGaugeGroupI .ℤ₆)
+  | .ℤ₂ => QuotientGroup.lift _ repGaugeGroupI (gaugeGroup_subgroup_le_ker_repGaugeGroupI .ℤ₂)
+  | .ℤ₃ => QuotientGroup.lift _ repGaugeGroupI (gaugeGroup_subgroup_le_ker_repGaugeGroupI .ℤ₃)
+
+end UpSinglet
+
+end StandardModel