Measurement of dijet production at low Q 2 at HERA

Abstract

The measurement of triple differential dijet cross sections in e ± p interactions in the H1 experiment in the region of photon virtualities 2 < Q 2 < 80 GeV 2 , inelasticities 0.1 < y < 0.85, jet transverse energies E * T 1 > 7 GeV, E * T 2 > 5 GeV, and pseudorapidities −2.5 < η * 1 , η * 2 < 0 is presented. The analysis was done in the γ * p centre-of-mass frame using an integrated luminosity of 57.6 pb −1. The data are compared with Monte Carlo event generators based on LO QCD cross sections and NLO QCD parton level calculations. The latter fail to describe the region of low Q 2 and low jet transverse energies, in contrast to the LO Monte Carlo generators that include direct and resolved photon interactions. The effects of resolved photons with longitudinal polarisation and initial and final state parton showers are investigated in detail. The data show clear evidence for effects that go beyond the fixed-order NLO QCD calculations. First of all, I would like to thank to the supervisor of my Thesis, Jaroslav Cvach, for his guidance, continuous support and encouragement and for great working conditions. During the time of our collaboration, he became more than a teacher and colleague for me. I am also grateful to Jiří Chýla, Marek Taševský, Alice Valkárová and Hannes Jung for many valuable discussions, suggestions and for they constant support, which largely exceeded their duty and even the usual level of collaboration. I especially appreciate the help from Jiří Chýla, who spent a lot of time with me even though he was occupied with other duties. I would also like to thank to

Determination of the strong coupling constant $α_s(M_Z)$ in next-to-next-to-leading order QCD using H1 jet cross section measurements

“…The value of α s (m Z ) from 'H1 inclusive jets' has a significantly reduced experimental uncertainty compared to the results for the individual data sets. The cut μ > 28 GeV results for inclusive jets in α s (m Z ) = 0.1152 (20) exp (27) th , which is consistent with the world average [2,108].…”

“…In the present analysis, the value with the smallest total uncertainty is obtained in a fit to H1 jets restricted to μ > 42 GeV with the result α s (m Z ) = 0.1168 (22) exp (20) theo and a value of χ 2 /n dof = 37.6/40. This result, however, is obtained from a very limited number of measurements, the precision of which is limited by statistical uncertainties.…”

“…The cross section predictions account for the α s dependence in the two components of the calculations, the partonic cross sections and the parton distribution functions (PDFs). The strong coupling constant is determined to be α s (m Z ) = 0.1157 (20) exp (29) th , where the jet data are restricted to high scales μ > 28 GeV. Uncertainties due to the input PDFs or the hadronisation corrections are found to be small, and the largest source of uncertainty is from scale variations of the NNLO calculations.…”

“…Measurements of inclusive jet and dijet cross sections in NC DIS have been performed at HERA by the H1 [15][16][17][18][19][20][21][22][23][24] and ZEUS [25][26][27][28][29][30][31][32] collaborations during different data taking periods and for different centre-of-mass energies. In general, the predictions in pQCD provide a good description of these data.…”

Diffractive Dijet Production with a Leading Proton in ep Collisions at HERA

“…A somewhat different shape is observed for data and theory as a function of Q 2 , however the deviations are covered by the uncertainties. Resolved photon [65] and higher twist contributions [66] are expected to change the DIS cross sections at small Q 2 . The predicted shape in y also differs from the observation, such that at high y smaller cross sections are predicted than observed.…”