Last week I met Marek Karliner at the ICNFP 2013 conference in Crete, where we both enjoyed a nice friendly atmosphere, great food, and a wonderful peaceful location. Professor Marek Karliner is the chair of the Institute of Theoretical Physics of Tel Aviv in Israel. Since he agrees that outreach in physics is an important service that researchers should provide to the community, I was able to convince him to write for this blog the short article which you find below, on the interesting topic of baryons containing two heavy quarks - TD.


Baryons with two heavy quarks: bbq, ccq, bcq - Why they are interesting, and how to look for them experimentally[1,2]


From the point of view of QCD there is nothing exotic about baryons containing two heavy quarks (b or c, generically denoted by Q) and one light quark (u or d, generically denoted by q). Heavy quarks decay only by weak interaction, with a characteristic lifetime orders of magnitude larger than the typical QCD timescale, so from the point of view of strong interactions the QQq baryons are stable, just like protons, neutrons and hyperons. Thus these doubly-heavy baryons must exist.

Still, producing and discovering them is significant experimental challenge. One has to
produce two  pairs which need to rearrange themselves, so as to form QQ and   diquarks, rather than the more favorable configuration of two color singlets. Then the QQ diquark needs to pick up a an light quark q, to make a QQq baryon.

At first it seems that such processes likely to be very rare. But there is an experimental indication that they occur quite frequently. This indication is based on the copious production of the doubly-heavy B_c = ( ) mesons by the LHC and Tevatron experiments, suggesting that simultaneous production of  and  pairs which are close to each other in space and in rapidity and can coalesce to form doubly-heavy hadrons is not too rare. This is an encouraging sign for the prospects of producing and observing the bbq, ccq and bcs baryons. ATLAS and CMS and especially LHCb probably have the best chance of discovering these states.

Among the doubly-heavy baryons the double-bottom baryons bbq have a unique and a spectacular decay mode, with two J/ψ mesons in the final state and essentially no background. This mode is mediated by both b quarks decaying via

 

and yields



with all final state hadrons coming from the same vertex.

This unique signature is however hampered by a very low rate. It is both a challenge and a opportunity for LHCb and for other experiments in which large numbers of heavy quarks are produced[2].

Experimental discovery of doubly-heavy baryons will be a superb testing ground for various theoretical approaches that have been proposed for dealing with nonperturbative aspects of QCD spectrum, such as lattice, quark-models, large-N_c, etc.

In addition to being interesting in their own right, experimental observation of doubly-heavy QQq baryons can provide crucial constraints on the possible existence of hypothetical
exotic hadrons, such as doubly-heavy tetraquarks . This is because there are strong parallels between these two types of hadrons. In both types of systems there is a light color triplet -- a quark or an anti-diquark -- bound to a heavy diquark.

In the last few years it became possible to accurately predict at the level of 2-3 MeV the masses of heavy baryons containing the b-quark: Σ_b (bqq), Ξ_b(bsq) and Ω_b(bss) [3,4,5]. These predictions used as input the masses of the B, B_s, D and D_s mesons, together with the masses of the corresponding c-baryons Σ_c(cqq), Ξ_c(csq) and Ω_c(css).

An analogous approach to the masses of the , suggested the relation

m( ) = m(Ξ_ccu) + m(Λ_c) - m(D⁰) - 1/4 [m(D^*)-m(D)]


Thus once the mass of the ccq baryon is known, one can immediately compute the mass of a ccud tetraquark and see whether it is above or below the threshold for two D mesons. A completely analogous reasoning can be applied to the bbq baryon and the tetraquark, to see if it is above or below the BB threshold. Since the b quark is much heavier than the c quark, the bbud tetraquark has an a priori better chance of being below the BB threshold. If it is below threshold, it can only decay weakly, and will be stable as far as strong interactions are concerned. A discovery of such a stable exotic hadron would be a spectacular finding indeed.

References:

[1] M. Karliner and S. Nussinov, "The doubly heavies:  and  tetraquarks and QQq baryons,''JHEP 1307, 153 (2013) [arXiv:1304.0345 [hep-ph]].

[2] M. Karliner, H. J. Lipkin and N. A. Tornqvist, "New hadrons with heavy quarks,'' Acta Phys. Polon. Supp. 6, 181 (2013).

[3] M. Karliner and H. J. Lipkin, arXiv:hep-ph/0307243; condensed version in
Phys. Lett. B575 (2003) 249.

[4] M. Karliner, B. Keren-Zur, H. J. Lipkin and J. L. Rosner, "Predictions for masses of Ξ_b baryons,'' arXiv:0706.2163 [hep-ph].

[5] M. Karliner, B. Keren-Zur, H. J. Lipkin and J. L. Rosner, "The Quark Model and b Baryons,'' Annals Phys. 324, 2 (2009) [arXiv:0804.1575 [hep-ph]].