Promotiebericht, Nikhef, 25-02-2008

Swimming in the virtual sea of quarks and gluons to reveal the structure of the proton

Nikhef PhD student Gabriel Grigorescu studied the inner structure of the proton at the particle collider HERA in Germany. He measured charm production in electron-proton collisions in regions of phase-space unexplored so far with the ZEUS experiment. His results are useful for the physics at the LHC as they are not exactly as the theory predicts. Gabriel will defend his thesis on March 28 at the University of Amsterdam.

The most abundant element in the Universe is hydrogen, which constitutes about 75% of the luminous matter. The proton is the nucleus of the hydrogen atom. Protons were created in large numbers at about 10^-6 seconds after the Big Bang. They are believed to be stable. The proton is built from three valence quarks uud) which carry the proton quantum numbers. These quarks are kept together by gluons, the messengers of the strong nuclear force. The quarks constantly radiate and absorb gluons which, in turn, can split into pairs of quarks and anti-quarks or other gluons. This virtual sea of quarks and gluons determines many of the proton's properties.

The inner structure of the proton can be studied in great detail with electron-proton scattering. HERA, an e^–p collider, accelerates both protons and electrons to high energies and then collides them against each other. From the resulting collision fragments, information can be inferred about the structure of the proton, while at the same testing predictions of QCD.

The present analysis measures the production of charm quarks in deep inelastic scattering, with HERA II data. The measurement relies on accurate calibration of the microvertex detector as well as on analysis-dependent vertexing techniques. D⁰ charm mesons are reconstructed via the decay channel D⁰ → K+π. For the first time at ZEUS, charm signal is shown for values of the meson transverse momentum lower than 3 GeV. Charm production cross-sections are measured and compared to NLO theoretical predictions. A good agreement in shape is seen but the overall normalization is about 20-25% lower than the theoretical predictions performed with gluon distributions extracted from inclusive DIS. If this deficit is due to a gluon density mismatch, then any cross-section dependent on gluon-gluon fusion at the Large Hadron Collider will be less than expected by about 40%.

Left plot: D⁰ invariant mass reconstruction for mesons with a transverse momentum in the range of 1.5 < P_T(D⁰) < 3 GeV. 5483 candidates are found in a data sample of 127.35 pb^–1, recorded by ZEUS in 2005. Right plot: the measured differential cross-section dσ/dx w.r.t. Bjorken x for the process e^–p → e^–+D⁰+X, for the kinematic range 5 < Q² < 1000 GeV² and P_T(D⁰) < 20 GeV, is compared to the second independent analysis and to the NLO theoretical prediction.

Measurement of Charm Production in Deep Inelastic Scattering at HERA II by Gabriel Grigorescu

The defense will take place on Friday, March 28 2008, at 10:00 am in Aula der Universiteit, Singel 411, hoek Spui, Amsterdam.
Promotores: prof. dr. P. Kooijman, prof. dr. E. Koffeman.
Contact: gabrielg@nikhef.nl