Doubly heavy tetraquarks in nature and in lattice QCD - examining their properties and the role of diquarks

  • Event Date: 2022-09-26
  • High energy phenomenology
  • Speaker: Anthony Francis (NYCU)  /  Host: Dr. Girish Kumar
    Place: R517, New Physics Building, NTU

Title:Doubly heavy tetraquarks in nature and in lattice QCD - examining their properties and the role of diquarks
Time:2022/09/26 (Mon.) 12:30
Place:R517, New Physics Building, NTU

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.

Given these developments I review the status of doubly heavy tetraquarks on
the lattice 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.

Their existence can be motivated by invoking diquarks as effective degrees
of freedom within QCD. Exploring this idea further I present a recent
lattice study in a gauge-invariant formalism. We find attractive
quark-quark spatial correlations only in the scalar "good" diquark channel,
confirming the predicted attractive effect. Furthermore we find they are
almost spherical and of hadronic size.

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.