This is a supplement to Thomas's answer from March. I don't yet have enough reputation to add a comment, so let me write something not quite so brief but hopefully still clear.

Although certain lattice QCD calculations with physical masses are currently feasible (and underway), many projects still need to use heavier masses. This is particularly true of work involving multiple baryons like the dineutron studies mentioned in the question. For example, the recent arXiv:1706.06550 still uses ~800-MeV pions.

A significant factor complicating these calculations at lighter masses is a signal-to-noise problem. To over-simplify a 'classic' argument attributed to Peter Lepage, the signal in these calculations decays like $\exp[-m_N t]$ where $m_N$ is the mass of the hadron of interest (let's say a neutron) and $t$ is the time since its creation. The noise $\sigma^2(t)$, however, decays like $\exp[-m_L t]$ where $t$ is the same but $m_L$ is the mass of the lightest state that can be created by the **square** of the creation operator. The square of a neutron creation operator couples not only to a neutron--antineutron pair, but also to three pions. Therefore one expects to see (and does see) the signal-to-noise ratio degrading $\sim \exp\left[-(m_N - 3m_{\pi}/2)t\right]$.

This problem essentially vanishes in the limit of infinite quark mass, $m_q \to \infty$, where we expect $m_{\pi} \simeq \frac{2}{3}m_N$ just from counting valence quarks. One the other hand, it's most severe in the opposite limit $m_q \to 0$ where $m_{\pi} \to 0$ while $m_N$ remains non-zero. Since no clever trick has so far been found to solve this problem (although some efforts are ongoing), the exponential degradation of the signal-to-noise ratio at lighter masses would naively require a compensating exponential increase in statistics and hence in computational cost. Heavier-than-physical masses move us farther from the worst case and make calculations feasible with existing technology.

As an aside, the alert reader may be wondering why we don't just make $t$ smaller (i.e., measure the nucleon(s) at very short times after their creation). This indeed has been an important source of progress. It's easier said than done, however, because the creation operators couple to all states with the appropriate quantum numbers. So one ends up with "excited state" artifacts that decay like $\exp[-m_H t]$ with $m_H > m_N$. Therefore $t$ needs to be large enough for the excited-state contaminations to decay away but small enough for the desired signals to remain visible above the noise. (This special range of $t$ is sometimes called the golden window.) In order to be able to use smaller $t$ folks have had to construct improved creation operators that have more overlap with the desired ground state and less overlap with the unwanted excited states. This has been done with significant success (mentioned briefly in Section III.A of the arXiv:1706.06550 linked above).

As another addendum to Thomas's answer, let me also add that although $m_{\pi}^{-1} \simeq 1.4~\mbox{fm}$ "is not that large", to avoid finite-size artifacts lattice QCD calculations require $L^4$ lattices with $L$ several times larger than $m_{\pi}^{-1}$. For the simplest calculations (where physical masses are feasible) it can suffice to have $L\cdot m_{\pi} \gtrsim 4$. With a representative lattice spacing $a \approx 0.1~\mbox{fm}$, this $L \simeq 5.6~\mbox{fm}$ implies lattices of size at least $60^4$, corresponding to Dirac operator matrices of dimension $\gtrsim 4\cdot 12\cdot 60^4 \sim 6\times 10^8$. Inverting tens of thousands of roughly $\mbox{billion}\times\mbox{billion}$ (sparse) matrices is feasible, but not easy.

Of course, things get harder when the lattice volume needs to be big enough to contain two (possibly unbound) baryons. The calculation in arXiv:1706.06550 therefore uses $L\cdot m_{\pi} \simeq 20$, mostly achieved thanks to the heavier-than-physical masses but also helped by a rather large lattice spacing $a \approx 0.15~\mbox{fm}$ (which increases the discretization artifacts contaminating the results).

This post imported from StackExchange Physics at 2017-11-22 17:13 (UTC), posted by SE-user David Schaich