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K* Meson Production at CLAS Exploring Strangeness Production
and
the Search for "Missing Resonances"
Huge efforts have been going on to understand the strong nuclear force. At high energy regions, this force has been successfully described by the quantum chromodynamic (QCD), the universally accepted theory of the strong interaction, which explains nuclear particles in terms of quarks and gluons, i.e. these fundamental particiles form the basic degrees of freedom of the QCD. In this energy region quarks, within a particle, seem close together and therefore the interaction between them becomes arbitrarily weak, i.e. in high-energy scattering the quarks move within nucleons as free, non-interacting, particles. This "asymptotic freedom" simplifies the theory and allows using perturbative techniques, i.e. perturbative quantum chromodynamic (pQCD).

However, the strong force, at medium energies, is still one of the unsolved problems in particle physics. In this non-perturbative region, hadrons (mesons and baryons) degrees of freedom dominate the physics and the QCD calculation techniques can not be simplified. That is, full complexity of QCD emerges, including nonlinearity and confinement. This represnts an obstacle for understanding hadronic phenomena at a fundamental level. And therefore, since the QCD Lagrangian cannot be solved in the low-energy regime and for bound states, quark models have been developed in order to predict the properties of hadronic states. Thus, the primary goals of hadron physics are to determine the relevant degrees of freedom at different scales and to relate them to parameters and fundamental fields of the QCD. My research activities are in this energy region, where we try to understand the non-perturbative QCD.

Understanding the structure of the nucleon and its excited states is very important to understand the underlaying theory of the strong interactions,

• the ground state nucleon can be studied using elastic scattering of electrons off the nucleon (protons or neutrons) in order to obtain the electric/magnetic charge distributions. The inclusive electron scattering spectrum shows 4 resonance regions above the elastic peak, but we cannot separate different resonances that make up the 2nd and higher resonance peaks. Even at the first resonant region there is a significant non-resonant background under the dominant Δ(1232) peak. And therefore exclusive measurements with a large angular coverage (in the hadronic c.m.) are necessary to separate background from different overlapping resonances.
• study of excited states: Because the lifetimes of the excited states are too short (to make a target of excited nucleons), they can only be studied via the transition from the ground state into the nucleon resonances.

One of the research areas, at Ohio University , is in medium-energy nucleon-structure physics, using experiments performed in Hall B, at JLAB. We have been trying to explore the production of strange quarks in order to better understand the structure of the nucleon and fundamental aspects of the strong nuclear force. In particular, I'm interested in the electromagnetic production of strange particles via electron and photon-induced reactions. I have been looking for nucleon resonances, N*, as well as trying to better understand the K^*0 production mechanism using K^*0 electro- and photoproduction experiments. In these experiments, a photon of "known energy" strikes a proton at rest, and we have two major energy regions,

1. at low energies: we obtain "resonances" that decay into final state particles.
2. at very high energies (Perturbative Region): the photon interacts with a quark of the proton (QCD).
   The above two energy regimes are traditionally separated using the invariant mass, W, of the hadronic final state. A region of prominent nucleon resonances is observed at W < 2 GeV, while there are no noticeable resonances for higher W, in particular when the 4-momentum transfer squared, Q², is large.
3. resonance energies (non-perturbative Region): between the above two energy regions it is difficult to see "resonances" because of the large number of resonances, where the QCD is not valid, i.e non-perturbative region. This suggests a "resonances region", resonance physics. Physicists at Jefferson Lab. probe energies both "in" and "above" the resonance region to observe the transition from ``resonance physics" to ``perturbative physics". Read more..