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The present meaning of pentaquark contains several different kinds
of particles. For reading a quite short paper that explains this
matter,
click here.
It turns out the the recent CERN's LHCb pentaquark discovery is
no big deal. Furthermore, the nonexistence of the original pentaquarks is
still a QCD problem.
Below you can see a discussion that refers to the original
pentaquark notion.
A pentaquark is a system of four quarks and one antiquark. It can be
regarded as a system which consists of a baryon and a meson.
The problem
addressed here is the existence of strongly bound states of
pentaquarks. In spite of many experimental attempts to detect strongly bound
pentaquarks, there is still no confirmation of the existence of these
objects (see the PDG report
here).
Note that the Θ+ candidate is an
antibound state of a neutron and a K+.
The Regular Charge-Monopole Theory (RCMT) predicts that strongly bound
pentaquarks do not exist. This conclusion relies on the similarity
between electricity of charges and magnetism of
magnetic monopoles. Now, the third
postulate
of the RCMT states that
baryons are like neutral nonionized atoms. Thus, for example, bound
states of nucleons are like atoms in a liquid drop. This conclusion
explains the underlying structure of nuclei, which are bound states
of nucleons. Here, the typical binding energy per nucleon is 8 MeV.
This quantity is very far below hadronic energies which are measured
by hundreds of MeV. Hence, no strongly bound state of baryons is
expected to exist.
The case of a pentaquark differs from that of two nucleons. Here, the
free meson lowest energy state has a total spin J=0. (All other mesonic
states are excited by hundreds of MeV.) Hence, it is expected that a
meson-baryon bound state and a molecular
bound state of a noble gas and another atom,
take analogous structure. For this reason, the
pentaquark binding energy should be much less then 2.2 MeV, which is
the proton-neutron (deuteron) binding energy.
This value is very far below that of strongly bound pentaquark.
In principle, QCD allows the existence of strongly bound pentaquarks.
Moreover, QCD proponents have suggested their existence [1,2]
and not even one QCD expert has argued to the
contrary. Now, the
Particle Data Group (PDG) is the authorized institute for a
confirmation of the
existence of particles and of their physical properties.
As pointed out at the beginning of this page, the 2008
PDG report states the complete failure
of attempts to detect (unbound states of) pentaquarks.
A fortiori, no
strongly bound pentaquark exists. This failure provides
yet another reason for questioning the validity of QCD.
The following text explains why the pentaquark
detection which was recently (2015) announced by CERN
is irrelevant to the QCD pentaquark.
(See
here
).
Further physical aspects of pentaquarks are discussed
here.
The following points summarize the pentaquark issue:
-
Many dedicated experimental searches, which have been carried out
during about 30 years, have failed
to detect the strongly bound pentaquark state whose existence has been
predicted by QCD proponents.
-
Unlike ordinary persons, QCD proponents do not regard a failure as
a failure and declare that the Standard Model (which contains QCD)
has "no confirmed conflicts with any existing experiments!" (see e.g. [3]).
References:
[1] C. Gignoux, B. Silvestre-Brac and J. M. Richard, Phys. Lett.
193, 323 (1987).
[2] H. J. Lipkin, Phys. Lett. 195, 484 (1987).
[3] http://higgs.ph.ed.ac.uk/sites/default/files/Strassler_Looking%20Beyond%20SM.pdf
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