Publication List (via the inSPIRE database)

Curriculum Vitae (2024)

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Discussion of Selected Publications

Charged Pion Form Factor:

H.P. Blok, T. Horn, G.M. Huber, et al., ``Charged Pion Form Factor between Q2=0.60 and 2.45 GeV2, I: Measurement of the Cross Section for the 1H(e,e'π+)n Reaction'', Physical Review C 78 (2008) 045202 1-25 [Download PDF].

G.M. Huber, H.P. Blok, T. Horn, et al., ``Charged Pion Form Factor between Q2=0.60 and 2.45 GeV2, II: Determination of, and Results for, the Pion Form Factor'', Physical Review C 78 (2008) 045203 1-16 [Download PDF].

I am the co-spokesperson (with D. Gaskell - JLab) of an experimental program that aims to significantly improve our knowledge of the pion charge form factor in the space-like region. Because the pion has a relatively simple qbar-q valence structure, this observable is of particular interest – it is one of the most direct ways of testing QCD-based models in the non-perturbative regime. Thus, it is an observable that all QCD-based calculations use as a first test case (the `positronium atom' of QCD). Using electron beams up to 5 GeV energy at JLab, we acquired data and published the first high-quality pion form factor data (with well-understood and quantified systematic uncertainties) since the seminal work of Brauel et al., at DESY, in 1979. Our data, spanning the range Q2=0.6-2.45 GeV2, are where theoretical calculations for Fπ begin to diverge and constrain the treatment of soft versus hard contributions to the pion wave function. The publications from this work have had continued impact, to date gathering over 600 citations (source: inspirehep.net). As a result of the impact of this work, we have received the highest possible endorsement of the JLab PAC to continue these studies with the upgraded 12 GeV electron beam at JLab, including the awarding of A scientific priority in 2010, and being identified as a ``high impact'' experiment in a 2014 review of the JLab 12 GeV scientific program.

Exclusive, Forward π+/- Electroproduction from Deuterium:

G.M. Huber, H.P. Blok, C. Butuceanu, D. Gaskell, T. Horn, et al., ``Separated Response Function Ratios in Exclusive, Forward π+/- Electroproduction'', Physical Review Letters 112 (2014) 182501 1-6 [Download PDF].

G.M. Huber, H.P. Blok, C. Butuceanu, D. Gaskell, T. Horn, et al., ``Separated Response Function in Exclusive, Forward π+/- Electroproduction on Deuterium'', Physical Review C 91 (2015) 015202 1-23 [Download PDF].

The study of exclusive π+/- electroproduction on the nucleon, including separation of the structure functions, is a very useful tool in the study of hadronic structure. In contrast to inclusive (e,e') or photoproduction measurements, the transverse momentum (size) of the scattering constituent and the resolution at which it is probed can be varied independently. Furthermore, ratios of separated response functions can be formed for which nonperturbative corrections may partially cancel, yielding insight into soft-hard factorization at modest Q2. Our results are intriguing, as the ratio of transverse cross sections RT=σπ-T/σπ+T suggests a transition between pion knockout and quark knockout mechanisms as the Mandelstam variable -t is increased. The ratio of longitudinal cross sections RL=σπ-L/σπ+L is also of interest as it confirms the dominance of the pion-pole diagram at low -t, which is necessary for the extraction of the charged pion form factor from electroproduction data. The analysis of these data were very difficult due to the careful rate-corrections required to produce accurate charged pion ratios.

ρ0 In-Medium Spectral Function Studies:

G.M. Huber, G.J. Lolos, Z. Papandreou, ``Probing the ρ0 Mass Modification in the Subthreshold Region on 3He'', Physical Review Letters 80 (1998) 5285-5288 [Download PDF].

G.M. Huber, et al., ``In-medium ρ0 Spectral Function Study via the 2H, 3He, 12C(γ, π+π-) reaction'', Physical Review C 68 (2003) 065202 1-27 [Download PDF].

During the years 1991-1996, my research was largely performed at the Institute for Nuclear Study of the University of Tokyo. At that time, they had a tagged photon facility and large solid angle detector, TAGX, which was unique in the world for its combination of beam energy (endpoint energy up to 1.2 GeV), large solid angle, wide tagging acceptance, and respectable duty factor (up to 20%), making it optimally suited to answer a number of important questions in intermediate-energy physics. There, I was co-spokesman of two experiments to investigate the prediction of many QCD-based models that the mass and/or width properties of the ρ0 meson are modified in the nuclear medium. Our experiments were performed in the energy region for tagged photons between 600 and 1120 MeV, which is mostly below the ρ0 threshold on the free proton of 1083 MeV. This has a number of advantages.
  • In this subthreshold region, the reaction cannot proceed unless the Fermi momentum of the struck bound nucleon is utilized. Thus, the ρ0 mesons observed in these experiments will be due to the interaction of the photon with a single nucleon in the nucleus, and the coherent γ-nucleus production mechanism is naturally suppressed. This coherent production mechanism takes place essentially within the Coulomb field outside the nucleus and is not sensitive to nuclear medium effects.
  • The ρ0 mesons will be produced with very little boost, and so will mostly decay inside the nucleus. If the produced ρ0 exits the nucleus before it decays, medium modifications cannot be observed. This explains why such effects were not observed in the early CERN experiments on nuclei. This has the further advantage that, for the energies accessible at INS, it is not necessary to perform experiments on heavy nuclei in order to ensure that the ρ0 meson is suitably affected by the nuclear environment. Use of a moderately light target nucleus, like 3He, or 12C$, provides a respectable nuclear matter density despite the small number of nucleons, minimizing the artificial widening of the ρ0 resonance due to pion-nucleus final state interactions.
Thus, if there is a modification of the ρ0 spectral function in nuclear matter, it should be observable with 1 GeV photons on moderately light nuclei. Our data provide adequate systematics across a variety of light nuclei with different densities to discern the medium-modification effect.

ΔNN Nuclear Wavefunction Analysis:

G.M. Huber, G.J. Lolos, et al., ``Probing the ΔNN Component of 3He'', Physical Review C 62 (2000) 044001 1-8 [Download PDF].

This manuscript presents our investigation to identify any ΔNN component in the 3He ground state wavefunction, by comparing pπ+/pπ- photoproduction ratios in the 1 GeV region. This follows an idea originally raised by Lipkin and Lee in 1987 [Phys.Lett.B 182 (1987) 22], but with a number of refinements. This work resulted after I realized that data we had from a separate experiment could contain evidence of this small physical effect. This required a completely different analysis of the data, and a very extensive simulation effort. A lower limit to the configuration probability of 1.5%+/-0.6%+/-0.5% was extracted. Because of the sophistication of the analysis, this result was believed by the accepting referee to be more definitive than any prior work on this topic.