A Charmed Tetraquark at LHC
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A Charmed Tetraquark at LHC∗ Amit Roy The quark model of hadrons (i.e., particles whose interactions are mediated by strong interactions) was postulated by Gell-Mann and Zweig in 1964 in which the mesons (like pion, Kaon, etc.) were formed out of qq¯ pairs and the baryons (proton, neutron, etc.) out of qqq triplets [1]. The quarks interact via gluons and are subject to confinement inside the nuclear volume according to current theories. Apart from their mass, spin and electric charge, the quarks have additional quantum numbers, flavour and colour, assigned to them as they obey certain symmetries. There are six flavours, viz., up(u), down(d), strange(s), charm(c), bottom or beauty(b) and top or truth(t). Out of these, the first three are light quarks, and the last three are much heavier with the top being the heaviest. Each flavour can have three colours, red, blue, and green so that the hadrons are colour neutral. The colours have nothing to do with the visible colours in optics but are akin to the charge in electromagnetism. This model successfully describes the properties of hadrons and their interactions and is known as quantum chromodynamics (QCD)—the theory of strong interactions. Even the early quark model did not preclude the hadrons to be composed of four or more quarks. States composed of quarks and gluons beyond conventional mesons (qq) ¯ and baryons (qqq) are termed exotic hadrons.
Study of exotic hadrons can provide new insights into the internal structure and the confinement mechanism and also act as a unique probe to non-perturbative behaviour of QCD. The first experimental evidence for an exotic hadron candidate was the X(3872) state observed in 2003 by the Belle collaboration [2]. Since then, a series of novel states consistent with four-quark composition have been discovered. The LHCc collaboration has also observed resonances interpreted to be pentaquark states. But the first unambiguous experimental evidence of the existence of these exotic hadrons came, when the Z(4430)− particle, first observed by the Belle collaboration, was shown to be composed of four quarks (c¯cdu¯ ) by the LHCc collaboration in 2014 [3]. All hadrons observed so far, including those of exotic nature, contain at most two heavy charm (c) or two bottom (b) quarks, whereas many QCD-motivated phenomenological models also predict the existence of states consisting of four heavy quarks, i.e. Xq1q2q3q4 , where qi is a c or a b quark. Search for such heavy tetraquarks has been going on at LHCb since the Large Hadron Collider (LHC) started operating in 2009. The motivation for this search is that hadrons containing the heavy flavours (charm and bottom) are easier to understand in terms of QCD compared to light quark hadrons. Due to their large masses, the heavy quarks can be treated non-relativistically in the hadron rest frame. This allows for the construction of reasonable QCD potential models and also makes lattice QCD calculations easier. The heavy quark hadrons have a high probability for decay to fi-
∗ Vol.25, No.8, DOI: https://doi.org
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