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Register a new userObservation of the beam-size effect at HERA
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Authors
Krzysztof Piotrzkowski
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A precise measurement of the spectrum of the photons from $ep$ bremsstrahlung with the ZEUS luminosity monitor at HERA is reported. The measurement shows a reduced rate compared to the Bethe-Heitler spectrum for photon energies below 5~GeV. This suppression, called the beam-size effect, is explained by the finite transverse size of the beam overlap relative to the typical impact parameter in the process of $ep$ bremsstrahlung at HERA energies.
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Results on open charm and beauty production and on the search for top production in high-energy electron-proton collisions at HERA are reviewed. This includes a discussion of relevant theoretical aspects, a summary of the available measurements and measurement techniques, and their impact on improved understanding of QCD and its parameters, such as parton density functions and charm- and beauty-quark masses. The impact of these results on measurements at the LHC and elsewhere is also addressed.
The parameters of the electroweak theory are determined in a combined electroweak and QCD analysis using all deep-inelastic $e^+p$ and $e^-p$ neutral current and charged current scattering cross sections published by the H1 Collaboration, including data with longitudinally polarised lepton beams. Various fits to Standard Model parameters in the on-shell scheme are performed. The mass of the $W$ boson is determined as $m(W)=80.520pm 0.115$ GeV. The axial-vector and vector couplings of the light quarks to the $Z$ boson are also determined. Both results improve the precision of previous H1 determinations based on HERA-I data by about a factor of two. Possible scale dependence of the weak coupling parameters in both neutral and charged current interactions beyond the Standard Model is also studied. All results are found to be consistent with the Standard Model expectations.
The high-precision HERA data allows searches up to TeV scales for Beyond the Standard Model contributions to electron-quark scattering. Combined measurements of the inclusive deep inelastic cross sections in neutral and charged current $ep$ scattering corresponding to a luminosity of around 1 fb$^{-1}$ have been used in this analysis. A new approach to the beyond the Standard Model analysis of the inclusive $ep$ data is presented; simultaneous fits of parton distribution functions together with contributions of new physics processes were performed. Results are presented considering a finite radius of quarks within the quark form-factor model. The resulting 95% C.L. upper limit on the effective quark radius is $0.43cdot 10^{-16}$ cm.
Exclusive photoproduction of $rho^0(770)$ mesons is studied using the H1 detector at the $ep$ collider HERA. A sample of about 900000 events is used to measure single- and double-differential cross sections for the reaction $gamma p to pi^{+}pi^{-}Y$. Reactions where the proton stays intact (${m_Y{=}m_p}$) are statistically separated from those where the proton dissociates to a low-mass hadronic system ($m_p{<}m_Y{<}10$ GeV). The double-differential cross sections are measured as a function of the invariant mass $m_{pipi}$ of the decay pions and the squared $4$-momentum transfer $t$ at the proton vertex. The measurements are presented in various bins of the photon-proton collision energy $W_{gamma p}$. The phase space restrictions are $0.5 < m_{pipi} < 2.2$ GeV, ${vert tvert < 1.5}$ GeV${}^2$, and ${20 < W_{gamma p} < 80}$ GeV. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the $m_{pipi}$ dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the $rho^{0}(770)$ meson mass and width at $m_rho = 770.8 {}^{+2.6}_{-2.7}$ (tot) MeV and $Gamma_rho = 151.3 {}^{+2.7}_{-3.6}$ (tot) MeV, respectively. The model is used to extract the $rho^0(770)$ contribution to the $pi^{+}pi^{-}$ cross sections and measure it as a function of $t$ and $W_{gamma p}$. In a Regge asymptotic limit in which one Regge trajectory $alpha(t)$ dominates, the intercept $alpha(t{=}0) = 1.0654 {}^{+0.0098}_{-0.0067}$ (tot) and the slope $alpha^prime(t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }$ (tot) GeV${}^{-2}$ of the $t$ dependence are extracted for the case $m_Y{=}m_p$.
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J. Chwastowski
, J. Figiel (H. Niewodniczanski Inst. of Nuclearn Physics Polish Academy of Sciences
, ul. Radzikowskiego
2003
Selected aspects of photoproduction in ep scattering at the HERA collider, studied with the ZEUS detector, are presented. The results are interpreted in the formalism of Vector Dominance Model, Regge theory and perturbative Quantum Chromodynamics.
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