Pentaquarks search in NOMAD

Existence of pentaquarks - hadrons build of 4 quarks and one anti-quark was predicted in a brilliant paper by Dmitry Diakonov, Viktor Petrov and Maxim Polyakov [1].

Penta-classics. 2006. Bochum. From left to right: Viktor Petrov, Dmitry Diakonov, Maxim Polyakov.

Many experiments looked for these new states. Some of them found, some did not.

The NOMAD experiment also took a part in this race. The road was not easy...

In 2004 our report‎ claimed for an evidence for the $\Theta^+$ state based on a sample with cleaner protons loosing however 6 times in the final statistics. However later we realized that in a less pure sample we must see pentaquarks even more clearly and we performed this time a blind analysis maximazing the sensitivity to the pentaquark state. The abstract of our article accepted for Eur.Phys.C reads:

A search for exotic $\Theta^+$ baryon via $\Theta^+\to p+K^0_S$ decay mode in the NOMAD $\nu_\mu N$ data is reported. The special background generation procedure was developed. The proton identification criteria are tuned to maximize the sensitivity to the $\Theta^+$ signal as a function of $x_F$ which allows to study the $\Theta^+$ production mechanism. We do not observe any evidence for the $\Theta^+$ state in the NOMAD data. We provide an upper limit on $\Theta^+$ production rate at 90% CL as $2.13\cdot 10^{-3}$ per neutrino interaction.