LHCb gets closer to discovering the second doubly charmed baryon

Abstract

Recently the LHCb Collaboration published the results of a search for the doubly charmed baryon Ξ + cc [1]. No significant signal is seen in the mass range from 3.4 to 3.8 GeV. To put this result in context, the Ξ ++ cc baryon was seen by LHCb in decay modes Λ c K − π + π + [2] (2017) and Ξ + c π + [3] (2018). The weighted average of the Ξ ++ cc mass is 3621.24 ± 0.65 (stat.) ± 0.31 (syst.) MeV [3].The Ξ ++ cc and Ξ + cc have the quark content ccu and ccd, respectively. Under the isospin symmetry of the strong interactions they form an isodoublet, like the proton and the neutron. Isospin breaking in hadron masses is a very small effect [4]. Consequently we have firm reasons to expect that the Ξ ++ cc − Ξ + cc mass difference is quite small, O(1.5) MeV [5]. The production rates of Ξ ++ cc and Ξ + cc should be similar, as the bottleneck -the production of the cc diquark -is the same in both cases. Consequently we know Ξ + cc exists in the vicinity of 3620 MeV. A claimed Ξ + cc at 3518.7±1.7 MeV [7,8] is unlikely to be the isospin partner of the established Ξ ++ cc , and has not been confirmed by any other experiment.The search was a "blind analysis", i.e., it was performed with the whole procedure defined before inspecting the data in the 3400 to 3800 MeV mass range. A search for a Ξ + cc signal was performed and the significance of the signal as a function of the Ξ + cc mass was evaluated. If the global significance, after considering the look-elsewhere effect, was found to be above 3σ, the Ξ + cc mass was measured; otherwise, upper limits were set on the production rates for different CM energies.As can be seen from Fig. 2 in Ref.[1], the data exhibit several peaks, but the most significant one occurs just where it is expected. The largest local significance, corresponding to 3.1σ (2.7σ after considering systematic uncertainties), occurs around 3620 MeV. However, the lookelsewhere effect [6], intrinsic to the LHCb search procedure, reduces this to 1.7σ. We believe that in this case the look-elsewhere effect may be overstated, because the peak shows up nearly (but not precisely) where expected. The result of a fit, as given in the Supplementary Material of Ref.[1], is M(Ξ + cc ) = 3623.4 ± 1.7 MeV, a bit larger than M(Ξ ++ cc ), in contrast to the prediction of Ref. [5] and nearly all the others quoted there which find M(Ξ + cc ) less than but within a few MeV of M(Ξ ++ cc ). The upper limit on Ξ + cc production (or the significance of a signal) increases with shorter assumed lifetime, as seen in Table 6 and Fig. 6 of Ref. [1]. As a result of the internal cd → su process in the decay of Ξ + cc , its lifetime is several times shorter than that of Ξ ++ cc : For example, Ref.[9] finds τ (Ξ + cc ) = 53 fs, and τ (Ξ ++ cc ) = 185 fs. (The latter was measured by LHCb to be 256 +24 −22 ± 14 fs [10].) The validity of the prediction of M(Ξ ++ cc ) [9] and the signal of its isospin partner not far from its predicted mass [5] lend credence to an estimate of the mass of the ccūd tetraquark using †

Mass inequalities for baryons with heavy quarks

“…[3]. (Hints of a possible isospin partner Ξ + cc decaying to Λ c K − π + have also been found [4,5].) In this approach, one adds up constituent-quark masses and spin-spin interactions [6][7][8] and corrects for the additional binding in any quark pair both of whose members are at least as heavy as a strange quark [3].…”