How would I keep a beam facing Horizontal whilst Face Camera is on?

I have a beam connecting from the RootRigAttachment of a character to a part using the following script:

local plr = game.Players.LocalPlayer
local beam = script:WaitForChild("Beam"):Clone()

local function pathToPart(beam, a0, part)
	local a1 = part:FindFirstChildOfClass("Attachment")
	if a1 then
		beam.Attachment0 = a0
		beam.Attachment1 = a1
	else
		warn("No attachment was inserted into "..part:GetFullName())
	end
	a0.Orientation = Vector3.new(0,0,-90)
end

plr.CharacterAdded:Connect(function(char)
	
	local root = char:WaitForChild("HumanoidRootPart")
	local a0 = root:WaitForChild("RootRigAttachment")
	pathToPart(beam, a0, workspace:WaitForChild("Part"))
	beam.Parent = char
end)

Currently, the beam faces wherever the player’s camera is pointing because the Face Camera option is enabled.

I understand that I could disable Face Camera and adjust the attachment’s orientation. However, if I do this, the beam rotates along with the character. How can I ensure the beam remains horizontal, regardless of the camera’s direction? Below is an image showing what I mean by “horizontal”: