Merge pull request #346 from NNPDF/issue_#201

Test scale variation schemes difference.

tests/eko/scale_variations/test_diff.py

@@ -0,0 +1,184 @@
+"""Test ``ModSV=exponentiated`` kernel vs ``ModSV=expanded``.
+
+We test that the quantity :math:`ker_A / ker_B` for truncated solution,
+is always higher order difference.
+
+For simplicity we do FFNS nf=4.
+"""
+
+import numpy as np
+
+from eko import basis_rotation as br
+from eko.beta import beta_qcd_as2, beta_qcd_as3
+from eko.couplings import CouplingEvolutionMethod, Couplings, CouplingsInfo
+from eko.kernels import non_singlet, singlet
+from eko.quantities.heavy_quarks import QuarkMassScheme
+from eko.scale_variations import expanded, exponentiated
+from ekore.anomalous_dimensions.unpolarized.space_like import gamma_ns, gamma_singlet
+
+NF = 4
+Q02 = 1.65**2
+Q12 = 100**2
+EV_METHOD = "truncated"
+
+
+def compute_a_s(q2, order):
+    sc = Couplings(
+        couplings=CouplingsInfo(
+            alphas=0.1181,
+            alphaem=0.007496,
+            scale=91.00,
+            max_num_flavs=4,
+            num_flavs_ref=4,
+        ),
+        order=order,
+        method=CouplingEvolutionMethod.EXPANDED,
+        masses=np.array([0.0, np.inf, np.inf]),
+        hqm_scheme=QuarkMassScheme.POLE,
+        thresholds_ratios=np.array([1.0, 1.0, 1.0]),
+    )
+    # the multiplication for xif2 here it's done explicitly outside
+    return sc.a_s(scale_to=q2)
+
+
+def scheme_diff_ns(g, a0, a1, L, order):
+    """:math:`ker_A / ker_B` for truncated non-singlet expansion."""
+
+    b0 = beta_qcd_as2(NF)
+    b1 = beta_qcd_as3(NF)
+    if order == (1, 0):
+        # series of (1.0 + b0 * L * a0) ** (g[0] / b0), L->0
+        diff = 1 + a0 * g[0] * L + 1 / 2 * a0**2 * g[0] * (-b0 + g[0]) * L**2
+    elif order == (2, 0):
+        # this term is formally 1 + as^2
+        diff = (
+            1
+            - (a1**2 * g[0] * L * (-b1 * g[0] + b0 * (g[1] + b0 * g[0] * L))) / b0**2
+            + (
+                (a0 * a1 * g[0] * L)
+                * (-2 * b1 * g[0] + b0 * (3 * b0 * g[0] * L + g[1] * 2))
+            )
+            / b0**2
+            + (a0**2 * L / (2 * b0**5))
+            * (
+                +3 * b0**6 * g[0] * L
+                + b0**5 * (-3 * g[0] * g[0] * L + g[1] * 2)
+                + (2 * b0**3) * (+b1 * g[0] * (g[0]))
+                + b0**4 * (-2 * g[0] * g[1])
+            )
+        )
+    return diff
+
+
+def scheme_diff_s(g, a0, a1, L, order):
+    """:math:`ker_A / ker_B` for truncated singlet expansion."""
+
+    b0 = beta_qcd_as2(NF)
+    b1 = beta_qcd_as3(NF)
+    if order == (1, 0):
+        # series of exp(log(1.0 + b0 * L * a0) * g[0] / b0)[0], L->0
+        diff = np.eye(2) + a0 * g[0] * L + 1 / 2 * a0**2 * g[0] @ (-b0 + g[0]) * L**2
+    elif order == (2, 0):
+        # this term is formally 1 + as^2
+        diff = (
+            np.eye(2)
+            - (a1**2 * g[0] * L @ (-b1 * g[0] + b0 * (g[1] + b0 * g[0] * L))) / b0**2
+            + (
+                (a0 * a1 * g[0] * L)
+                @ (-2 * b1 * g[0] + b0 * (3 * b0 * g[0] * L + g[1] * 2))
+            )
+            / b0**2
+            + (a0**2 * L / (2 * b0**5))
+            * (
+                +3 * b0**6 * g[0] * L
+                + b0**5 * (-3 * g[0] @ g[0] * L + g[1] * 2)
+                + (2 * b0**3) * (+b1 * g[0] @ (g[0]))
+                + b0**4 * (-2 * g[0] @ g[1])
+            )
+        )
+    return diff
+
+
+def test_scale_variation_a_vs_b():
+    r"""Test ``ModSV=exponentiated`` kernel vs ``ModSV=expanded``."""
+
+    # let's use smaller scale variation to
+    # keep the expansions under control
+    for xif2 in [0.9, 1.1]:
+        L = np.log(xif2)
+        for order in [(1, 0), (2, 0)]:
+            # compute values of alphas
+            a0 = compute_a_s(Q02, order)
+            a1 = compute_a_s(Q12, order)
+            a0_b = a0
+            a1_b = compute_a_s(Q12 * xif2, order)
+            a0_a = compute_a_s(Q02 * xif2, order)
+            a1_a = a1_b  # for FFNS these 2 will coincide
+            for n in [2.0, 3.0, 10.0]:
+                # Non singlet kernels
+                gns = gamma_ns(
+                    order,
+                    br.non_singlet_pids_map["ns+"],
+                    n,
+                    NF,
+                    n3lo_ad_variation=(0, 0, 0, 0, 0, 0, 0),
+                )
+
+                # build scheme B solution
+                ker_b = non_singlet.dispatcher(
+                    order, EV_METHOD, gns, a1_b, a0_b, NF, ev_op_iterations=1
+                )
+                ker_b = ker_b * expanded.non_singlet_variation(gns, a1_b, order, NF, L)
+
+                # build scheme A solution
+                gns_a = exponentiated.gamma_variation(gns.copy(), order, NF, L)
+                ker_a = non_singlet.dispatcher(
+                    order, EV_METHOD, gns_a, a1_a, a0_a, NF, ev_op_iterations=1
+                )
+
+                ns_diff = scheme_diff_ns(gns, a0, a1, L, order)
+                np.testing.assert_allclose(
+                    ker_a / ker_b,
+                    ns_diff,
+                    err_msg=f"{L=},{order=},{n=},non-singlet",
+                    rtol=2e-5 if order == (1, 0) else 3e-3,
+                )
+
+                # Singlet kernels
+                gs = gamma_singlet(
+                    order, n, NF, n3lo_ad_variation=(0, 0, 0, 0, 0, 0, 0)
+                )
+
+                # build scheme B solution
+                ker_b = singlet.dispatcher(
+                    order,
+                    EV_METHOD,
+                    gs,
+                    a1_b,
+                    a0_b,
+                    NF,
+                    ev_op_iterations=1,
+                    ev_op_max_order=1,
+                )
+                ker_b = expanded.singlet_variation(gs, a1_b, order, NF, L, 2) @ ker_b
+
+                # build scheme A solution
+                gs_a = exponentiated.gamma_variation(gs.copy(), order, NF, L)
+                ker_a = singlet.dispatcher(
+                    order,
+                    EV_METHOD,
+                    gs_a,
+                    a1_a,
+                    a0_a,
+                    NF,
+                    ev_op_iterations=1,
+                    ev_op_max_order=1,
+                )
+
+                s_diff = scheme_diff_s(gs, a0, a1, L, order)
+                np.testing.assert_allclose(
+                    np.diag(ker_a @ np.linalg.inv(ker_b)),
+                    np.diag(s_diff),
+                    err_msg=f"{L=},{order=},{n=},singlet",
+                    rtol=2e-4 if order == (1, 0) else 9e-3,
+                )