Physics Program

with the parameters b= (3.04±0.06)% and c = (0.62±0.05)% according to measure-

ments with one specific configuration [PANDA17b].

2.3. Physics Program

The physics program of PANDA covers a large spectrum of particles for understanding the effects of QCD in a better way. Studying bound states of quarks inside the QCD is important to evaluate theoretical predictions of the particle spectra with effective field theories or LQCD. The major goals of the PANDA physics program are described for instance in [PANDA09]. The most important studies regarding the open charm sector and exotic matter will be briefly summarized in the following.

2.3.1. Open Charm Studies

One part of the physics program in PANDA is the study of the open charm sector. An open charm state results from a decay of a charmonium meson with ¯cc into two mesons

with one charm and one light quark each. One example is the D+meson that consists of

one c and one ¯d quark. These states are interesting regarding strong decay modes and the

investigation of the CP violation in the weak interaction. They might be also a possible hint for physics beyond the SM. The results of previous and ongoing measurements in experiments like BaBar or Belle agree in general with the theoretical predictions. How- ever, some discrepancies have been observed in the past due to experimental limitations or large background levels.

While e± colliders produce less hadronic background they have the disadvantage of

not being able to create non vector states. This follows from the fact that the exchanged

photon is a vector boson with spin S=1 and the angular momentum has to be conserved.

Hence, the production of excited charmonium states with all possible quantum numbers

can only be possible with ¯pp reactions. In contrast to e± colliders the mass and width

resolution does not depend on the detector resolution of PANDA but on the phase-space cooled antiproton momenta which is excellent in the HESR. Additionally, the PANDA detector has the ability to scan the resonance mass and can achieve an excellent PID in combination with a good vertex resolution. These conditions are important to study some of the already known open charm states in a more detailed way.

2.3.2. Exotic Matter & Nuclear Form Factors

Another physics goal is to find exotic matter with quantum numbers that cannot be taken by ordinary mesons. Two examples of exotic states are states with the forbidden quan-

tum numbers JPC = 0−−or JPC = 2+−. They could be taken by hypothetical glueballs,

hybrids, or multiquark systems.

The high momentum resolution of the HESR allows a mass resolution of better than

∆m/m= 10−4_{. This makes a search for exotic states with a center-of-mass energy in the}

2. PANDA

mass region of observable states has been restricted to values below 2.2GeV/c2because

of experimental limitations. These small mass regions contain the disadvantage of a high density of ordinary two-quark states.

Additionally, the high luminosity of the HESR is needed for a high production yield as most of these states are formed with cross sections in the order of nanobarn. With the foreseen luminosity of the HESR several thousand exotic particles can be created per day which is an outstanding quality of PANDA compared to other experiments. Together with a very good antiproton beam energy definition the decay widths of exotic particles are going to be measured precisely down to values in the order of 10 keV.

A analysis of simulation data including the sample exotic meson candidate X(3872)

with a rest mass of 3871.68±0.17 MeV/c2_{for performance studies of the PANDA detec-}

tor has been done already. This particle has been discovered by Belle in 2003 and does not

fit into the standard quark model because of its quantum numbers JPC =1++which are

not allowed in the SM. The obtained results look promising to achieve the desired physics goals with the given detector configuration according to the hadronic background.

In addition to exotic particle states, nuclear form factors are planned be measured with PANDA. In general, two possible reactions can be used: The space-like

e−+p→e−+p (2.9)

and the time-like

¯p+p→e++e− (2.10)

region. The names are related to the orientation of the related Feynman diagrams. The electromagnetic form factors in the time-like region by using ¯pp collisions have not been studied until now due to several reasons such as the low intensity of existing antiproton beams or missing precision in the measurement of the momentum transfer

q2. The HESR is designed to provide all necessary specifications for the determination of

nuclei form factors in the time-like region and makes determination of the form factors

possible. Previous measurement of the reaction e+e− _→ ¯pp up to an energy of 4.5 GeV

have been performed with BABAR [AtBC07].

2.3.3. Hypernuclear Physics

Hyperons are baryons like protons or neutrons which can be created by exchanging one u or d with one of the heavier quarks. One well-known hyperon is the so-called Λ baryon with the quark content uds which has been discovered already several decades ago. It

has a rest mass of 1115.683±0.006 MeV/c2 and a positive intrinsic parity. QCD rules

forbid a decay of a ground state Λ hyperon via the strong interaction. The preferred decay channels are the following decays that involve the weak interaction:

Λ_→ p+π− (2.11)

Λ_→n+π0 (2.12)

It has to be taken into account that the probability for a decay into π− is by a factor of