Only Get the horizontal plane of lookVector?

I'm making a script where when you double tap the "w" key, you get propelled at the direction your camera is facing. I only want it to propel you in a horizontal direction, (x and z) and not a vertical direction (y). (All the code is in a LocalScript inside of StarterCharacter Scripts) Here is my Code so far:

--Getting the Camera
    game.Players.PlayerAdded:connect(function(player)
        script.LocalScript:clone().Parent = player.CharacterAdded:wait()
    end)    


--Local Variables   
local cam = workspace.CurrentCamera
local player = game.Players.LocalPlayer
local Char = player.Character
local Humanoid = Char:WaitForChild("Humanoid")
local Mouse = player:GetMouse()
local lastTapTime = 0
local clicked = false
local Trail = game.ServerStorage.Trail:Clone()
local Mouse = player:GetMouse()


_G.Energy = 100



--Moving the Humanoid
    --Double Tap
enabled = true
Mouse.KeyDown:connect(function(key)
    if not enabled and clicked then
        return
    end
    if key == "w" and _G.Energy >= 25 then
    enabled = false

    local function onTap()
        if tick() - lastTapTime < 0.2 then
            print("double Click!")
            clicked = true
                --What happends when the user double taps
            player.Character.UpperTorso.Velocity = cam.CFrame.lookVector * 300
            Trail.Parent = Char.HumanoidRootPart
            local attachment0 = Instance.new("Attachment", Char.Head)
            attachment0.Name = "TrailAttachment0"
            local attachment1 = Instance.new("Attachment", Char.HumanoidRootPart)
            attachment1.Name = "TrailAttachment1"
            Trail.Attachment0 = attachment0
            Trail.Attachment1 = attachment1
            wait(0.3)
            Trail:Remove()
            _G.Energy = _G.Energy - 25
            wait(0.5)   
            end
        end

        onTap()
        lastTapTime = tick()

        wait(0.5)
        enabled = true
        clicked = false
    end
end)

I'd also like to point out that this script doesnt work when I publish the game, and play it from the site. Any solutions are greatly appreciated!!