Exotic Multiquark Systems
Experimental Verification of the NN-decoupled Dibaryon Resonance d*1(1956) Production in Proton-Proton Collisions Below the Pion Production Threshold
Tech Area / Field
- PHY-PFA/Particles, Fields and Accelerator Physics/Physics
- INS-DET/Detection Devices/Instrumentation
3 Approved without Funding
Joint Institute of Nuclear Research, Russia, Moscow reg., Dubna
- Institute of Physical-Technical Problems, Russia, Moscow reg., Dubna
- University of Groningen / Kernfysisch Versneller Insituut, The Netherlands, Groningen\nMassachusetts Institute of Technology (MIT) / Physics Department, USA, MA, Cambridge\nUniversity of British Columbia / Department of Physics and Astronomy, Canada, BC, Vancouver\nOsaka University / Research Center for Nuclear Physics, Japan, Osaka\nCNRS / Institut de Physique Nucléaire - Orsay, France, Orsay
Project summaryThe project is aimed at solving the problem associated with the existence of novel multiquark states of matter qualitatively different from usual hadrons and nuclei. Its primary aim is to make crucial experimental verification of the existence of the NN-decoupled dibaryon resonance d*1 and to determine its parameters. Strong evidence for the existence of this resonance that is a highly likely candidate for an exotic six-quark state has recently been found by the authors of the given project [1,2].
In the modern theory of strong interactions, quantum chromodynamics (QCD), it is postulated that only colorless, composite systems of quarks (q), antiquarks (q- ), and gluons (g), called hadrons, can exist in nature as free particles. Ordinary hadrons are either bound systems of three quarks (baryons) or a quark and an antiquark (mesons). Meanwhile, QCD admits that along with ordinary hadrons (3q or qq- ), exotic hadrons, in particular those with the baryon number B=2 made of six quarks, can also exist in nature. An important feature of these novel six-quark systems is that they can be composed of colored quark clusters separated in space. Unfortunately, their static parameters (masses, widths and quantum numbers) are still not calculable within framework of the exact QCD theory since they are determined by a domain of large distances between quarks where QCD becomes nonperturbative. For this reason, most, if not all, predictions for the static parameters of the exotic 6q systems have been obtained with the help of several effective models of quark interaction [4-6]. Of course, the predictions obtained are highly model dependent. Thus, the experimental discovery of any exotic six-quark state would provide valuable information for stringent test of nonperturbative QCD models.
Most promising candidates for the exotic six-quark states are dibaryon resonances not coupled to the nucleon-nucleon (NN) channel, in particular those with zero strangeness. These are nonstrange, two-baryon hadrons (2B) with quantum numbers which are forbidden for the NN states by the Pauli principle. A few experiments have searched for these exotic dibaryon resonances, but none has provided any convincing evidence for their existence [7-11]. The main drawback of the methods of the previous searches for the dibaryon resonances in question is a very large but not well enough determined background due to non-resonant processes of reactions used. The authors of the project have proposed a new and effective method of searching for low-lying, nonstrange, NN-decoupled dibaryon resonances [12,13] which is free of such a drawback. In this method the direct two-photon production process in proton-proton collisions (pp®ggX) at energy below the pion production threshold (pNN) is used as a source of information about sought resonances. In accordance with our present-day knowledge such a process presents the double pp-bremsstrahlung reaction. However, if the resonances in question exist in nature, then the process pp®ggX may proceed through the mechanism that directly involves the radiative production pp®g2B and decay 2B®gpp modes of these resonances. In pp collisions with energies less than the pNN threshold this mechanism of production and decay of such resonances with masses MRЈ2mp + mp (mp- and mp- the proton and pion mass, respectively) would be unique for the resonances with quantum numbers I(JP)=1(1+,3+, etc.) or dominant for the resonances with I≥2. The main advantage of the pp®ggX reaction with respect to those used in the past experiments is that it, at least in principle, is well enough understood theoretically and has an extremely small cross section [14,15]. Therefore, in an experiment registering two photons of this process in coincidence one can hope for a good signal-to-background ratio.
As a result of a study of the process pp®ggX at an energy of 216 MeV, carried out for the first time by the authors of the given project , strong evidence for the existence of the NN-decoupled dibaryon resonance with a mass MR»1956 MeV was found. This resonance, called d*1, is believed to be formed in the radiative process pp®gd*1 and to decay via the radiative channel d*1pp®gpp. Its presence and, consequently, the new mechanism of photon production in NN collisions is also supported  by energy spectra of hard photons emitted in NN and nucleon-nuclear (NA) collisions below the pNN threshold measured in a few laboratories of the world [18,21].
So, there is a set of experimental data testifying to the presence of the NN-decoupled dibaryon resonance d*1. Nevertheless, to draw a final conclusion about its existence and to determine its parameters (mass, width, and quantum numbers) additional careful studies of the process pp®ggX below the pNN threshold are needed. For crucial verification of the existence of the dibaryon resonance d*1 we plan to perform high-precision measurements of two energy spectra of photons from this process for two proton energies, namely, 215 and 235 MeV. In the case of the d*1 resonance production in pp collisions, positions of a characteristic g line associated with its formation should be shifted in these spectra relative one another by a definite value. These spectra will also enable us to determine a more precise value of its mass and to estimate its total width. To establish the most probable quantum numbers of the resonance d*1 we plan to measure the angular distributions of both photons from the process pp®ggX, and the photon yield dependence on the angle between these photons at 215 MeV. In addition, calculations of the corresponding energy and angular distributions of photons from the process pp®ggX will be made by the Monte-Carlo method for the resonance d*1 with different sets of quantum numbers. The analysis of the experimental data and their comparison with the results of the calculations will allow us to determine its most probable quantum numbers. Thus, as a result of the planned investigations the crucial experimental verification of the existence of the dibaryon resonance d*1 will be made and more accurate values of its mass and width, and its most probable quantum numbers will be determined. In addition, the existence of the new mechanism of photon production in NN collisions will be confirmed.
The experimental discovery of the exotic six-quark state d*1 with subsequent reliable determination of its parameters will provide valuable information for stringent test of QCD theory, critical verification of the existing models and developing new, more realistic models of nonperturbative QCD. Moreover, the observation of such a qualitatively new type of nuclear matter would give important information on the confining interaction in hadrons composed of colored quark clusters. Experimental confirmation of the presence of the new mechanism of two hard photon production in NN collisions related to the existence of the resonance d*1 will require revision of our theoretical understanding of the fundamental photon emission process in nucleon-nucleon interaction. Experimental information we plan to obtain in this project will also be important for interpreting results of other experiments dealing with both single and two hard photon production processes in nucleon-nucleon, nucleon-nucleus, and nucleus-nucleus collisions. The analysis of the photon energy spectra of the npg, pdg, and pAg reactions we have carried out earlier  directly confirms this statement. In addition, this information will also be very useful for interpretation of the results of the experimental studies of the space-time evolution of processes of heavy-ions collisions at intermediate energies by using the so-called Hanbury Brown-Twiss interferometry technique. Since it is based on the measurement of a correlation function of two hard photons produced in such collisions, ignoring the contribution to this function from the mechanism in question can lead to incorrect results.
In order to carry out the planned measurements one needs an effective experimental setup making it possible to detect pairs of photons in the energy range from 5 to 150 MeV with high energy and angular resolutions and providing effective background suppression. In this connection the experimental setup we have used in the previous measurements  should significantly be improved. We plan to develop and manufacture a new spectrometer of photon pairs which will permit us to detect photons of interest with the energy resolution about 5%, new data acquisition system, and a new liquid hydrogen target with a total thickness of windows at least half that of our old target. This target will ensure at least two times lower background of photons produced in collisions of protons with the windows of the target.
The photon pair spectrometer which will be developed and manufactured in this project can be used for various measurements in nuclear and particle physics of intermediate energies.
The leading institute of the present project is JINR, which has great experience in experimental and theoretical studies in nuclear physics and physics of elementary particles. In the project JINR will participate in all its tasks and coordinate its fulfillment. The qualification of the project executants is fairly high. Well-known Russian specialists, including 7 candidate and doctor of science, having large experience in work on the project’s subject will participate in it.
The foreign collaborators of the project will participate in the composition of work plans, exchange information, expert evaluation of work results, arrangement of scientific meetings and publications on the project’s subject.
The given project meets the ISTC objectives. It directs the Russian weapon specialist’s efforts to peaceful activities, promotes integration of Russian scientist in the international scientific community, and supports fundamental and applied researches and development of modern technologies in the field of elementary particle and nuclear physics. Labor expenses of weapon specialists make up over 60% of those of all project participants.
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