code: def mgamma(mu, lower=False):
      r"""Returns a Dirac gamma matrix `\gamma^\mu` in the standard
      (Dirac) representation.
  
      Explanation
      ===========
  
      If you want `\gamma_\mu`, use ``gamma(mu, True)``.
  
      We use a convention:
  
      `\gamma^5 = i \cdot \gamma^0 \cdot \gamma^1 \cdot \gamma^2 \cdot \gamma^3`
  
      `\gamma_5 = i \cdot \gamma_0 \cdot \gamma_1 \cdot \gamma_2 \cdot \gamma_3 = - \gamma^5`
  
      References
      ==========
  
      .. [1] https://en.wikipedia.org/wiki/Gamma_matrices
  
      Examples
      ========
  
      >>> from sympy.physics.matrices import mgamma
      >>> mgamma(1)
      Matrix([
      [ 0,  0, 0, 1],
      [ 0,  0, 1, 0],
      [ 0, -1, 0, 0],
      [-1,  0, 0, 0]])
      """
      if mu not in (0, 1, 2, 3, 5):
          raise IndexError("Invalid Dirac index")
      if mu == 0:
          mat = (
              (1, 0, 0, 0),
              (0, 1, 0, 0),
              (0, 0, -1, 0),
              (0, 0, 0, -1),
          )
      elif mu == 1:
          mat = (
              (0, 0, 0, 1),
              (0, 0, 1, 0),
              (0, -1, 0, 0),
              (-1, 0, 0, 0),
          )
      elif mu == 2:
          mat = (
              (0, 0, 0, -I),
              (0, 0, I, 0),
              (0, I, 0, 0),
              (-I, 0, 0, 0),
          )
      elif mu == 3:
          mat = (
              (0, 0, 1, 0),
              (0, 0, 0, -1),
              (-1, 0, 0, 0),
              (0, 1, 0, 0),
          )
      elif mu == 5:
          mat = ((0, 0, 1, 0), (0, 0, 0, 1), (1, 0, 0, 0), (0, 1, 0, 0))
      m = Matrix(mat)
      if lower:
          if mu in (1, 2, 3, 5):
              m = -m
      return m
  
imports: from sympy.core.numbers import I
  from sympy.matrices.dense import MutableDenseMatrix
  from sympy.utilities.decorator import deprecated
label_desc: Use image node_id 3 for calling a global function with example usage: mgamma(mu, lower) and returns: m