Time : 2022/03/17 (Thu.) 12:30
Title: Examining diquarks and doubly heavy tetraquarks with lattice QCD

Abstract:
Recently LHCb announced the observation of a structure about 300 keV
below the $DD^*$ threshold, thereby indicating a tetraquark candidate
with doubly charmed quark content. Such doubly heavy tetraquarks have
been at the center of a discussion in both phenomenology and on the
lattice in recent years. Their discovery at LHCb gives experimental
credence and confirms the first member of this new family of exotic
hadrons.

Their existence can be motivated by invoking diquarks as effective
degrees of freedom within QCD and indeed the observed heavy baryon
spectrum served as catalyst for the new studies, in particular in
lattice QCD, that sparked their discussion. In this seminar I will
discuss and review the lattice studies in this direction.

Turning the time order of studies on its head, I start by presenting a
recent study of diquarks on the lattice in the background of a heavy
static quark, i.e. in a gauge-invariant formalism, with quark masses
down to almost physical pion masses in full QCD. Determining mass
differences between diquark channels as well as diquark-quark ones, we
establish the special status of the "good" scalar, $\bar{3}_F$,
$\bar{3}_c$, $J^P=0^+$, diquark. Additionally, we find attractive
quark-quark spatial correlations only in the "good" diquark channel,
further confirming the predicted attractive effect. We extract a
corresponding diquark size of $\sim 0.6~\rm{fm}$ and perform a first
exploration of its shape, which is shown to be spherical.

With these results in hand I review the previously published lattice
studies of doubly heavy tetraquarks and their status in the flavour
channels $qq'\bar Q\bar Q'$ with $q,q'=u,d,s$ and $Q,Q'=b,c$ quarks.
Gathering and comparing results from the community a consistent picture
for these doubly heavy tetraquarks is emerging in which $ud \bar b \bar
b$ tetraquarks are strong-interaction stable and the $ud \bar b \bar c$
as well as the $ud \bar c \bar c$ could be shallow bound states.

In the heavy regime, the gathered results provide support for the
diquark picture and predict the existence of doubly heavy tetraquarks.
Whether this is true for the not-so-heavy $ud \bar c \bar c$ being the
candidate now observed by LHCb remains an open question and I will
discuss possible avenues to pin down the binding mechanism as well as
structure of doubly heavy tetraquarks on the lattice in the future.