arXiv subjects

Andreas van Hameren

Publications and source records attributed to Andreas van Hameren.

25 records · Page 2Linked to original sources

arXiv2016

Four-jet production in single- and double-parton scattering within high-energy factorization

We perform a first study of 4-jet production in a complete high-energy factorization (HEF) framework. We include and discuss contributions from both single-parton scattering (SPS) and double-parton scattering (DPS). The calculations are performed for kinematical situations relevant for two experimental measurements (ATLAS and CMS) at the LHC. We compare our results to those reported by the ATLAS and CMS collaborations for different sets of kinematical cuts. The results of the HEF approach are compared with their counterparts for collinear factorization. For symmetric cuts the DPS HEF result is considerably smaller than the one obtained with collinear factorization. The mechanism leading to this difference is of kinematical nature. We conclude that an analysis of inclusive 4-jet production with asymmetric $p_T$-cuts below 50 GeV would be useful to enhance the DPS contribution relative to the SPS contribution. In contrast to the collinear approach, the HEF approach nicely describes the distribution of the $ΔS$ variable, which involves all four jets and their angular correlations.

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arXiv2015

Production of two charm quark-antiquark pairs in single-parton scattering within the $k_t$-factorization approach

We present first results for the $2 \to 4$ single-parton scattering $g g \to c \bar c c \bar c$ subprocess for the first time fully within the $k_t$-factorization approach. In this calculation we have used the Kimber-Martin-Ryskin unintegrated gluon distribution which effectively includes some class of higher-order gluon emissions, and an off-shell matrix element squared calculated using recently developed techniques. The results are compared with our earlier result obtained within the collinear-factorization approach. Only slightly larger cross sections are obtained than in the case of the collinear approach. Inclusion of transverse momenta of gluons entering the hard process leads to a much stronger azimuthal decorrelation between $c c$ and $\bar c \bar c$ than in the collinear-factorization approach. A comparison to predictions of double parton scattering (DPS) results and the LHCb data strongly suggests that the assumption of two fully independent DPS ($g g \to c \bar c \otimes g g \to c \bar c$) may be too approximate.

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arXiv2014

Three jet production and gluon saturation effects in p-p and p-Pb collisions within high-energy factorization

We analyze three jet production in the central-forward and forward rapidity regions in proton-proton and proton-lead collisions at LHC energies. Our calculation relies on high-energy factorization with a single off-shell gluon obeying small x evolution equation which includes saturation. The calculations are made using two independent Monte Carlo codes implementing tree-level gauge invariant off-shell matrix elements. We calculate differential cross sections for azimuthal decorrelations and unbalanced jet transverse momenta and discuss them in the context of differences in the evolution of the unintegrated gluon densities.

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arXiv2014

Charm meson production and double parton interactions at the LHC

We discuss production of open charm mesons in proton-proton collisions at the LHC. The cross section for inclusive production of $c \bar c$ pairs is calculated within the $k_{\perp}$-factorization approach in which a major part of higher-order corrections is belived to be effectively included. We use and test the applicability of several unintegrated gluon distributions. Numerical results of the $k_{\perp}$-factorization approach are compared to NLO pQCD collinear predictions. Inclusive differential distributions in transverse momentum and (pseudo)rapidity of several charmed mesons are presented and compared to recent results of the ALICE, ATLAS and LHCb collaborations. We also examine production of neutral charmed meson-antimeson pairs ($D^0 {\overline D^0}$) in unique kinematics of forward rapidities of the LHCb experiment. Kinematical correlations in azimuthal angle $φ_{D^0 {\overline D^0}}$, invariant mass $M_{D^0 {\overline D^0}}$ and rapidity difference $Y_{D^0 {\overline D^0}}$ distributions are calculated. Moreover, we also discuss production of two pairs of $c \bar c$ within a simple formalism of double-parton scattering (DPS). We compare results of calculations of single-parton scattering (SPS) and double-parton scattering (DPS) for production of $c \bar c c \bar c$ and for $D^0-D^0$ meson-meson correlations. We compare our predictions for double charm production with recent results of the LHCb collaboration for azimuthal angle $φ_{D^0 D^0}$ distribution, dimeson invariant mass $M_{D^0 D^0}$ and rapidity distance between mesons $Y_{D^0 D^0}$. The obtained results clearly certify the dominance of DPS in the production of events with double charm.

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arXiv2014

Single-parton scattering versus double-parton scattering in the production of two $c \bar c$ pairs and charmed meson correlations at the LHC

We compare results of exact calculations of single parton scattering (SPS) and double parton scattering (DPS) for production of $c \bar c c \bar c$ and for $D$ meson correlations. The SPS calculations are performed in collinear approximation with exact matrix element for $g g \to c \bar c c \bar c$ and $q \bar q \to c \bar c c \bar c$ subprocesses. It is shown that the contribution of gluon-gluon subprocess is about factor 50 larger than that for quark-antiquark annihilation. The new results are compared with results of previous calculation with the approximate matrix element for $g g \to c \bar c c \bar c$ in the high-energy approximation. The cross section for the present exact calculation is bigger only at small invariant masses and small rapidity difference between two $c$ quarks (or two $\bar c$ antiquarks). We compare correlations in rapidities of two $c$ (or two $\bar c$) for DPS and SPS contributions. Finally we compare our predictions for $D$ mesons with recent results of the LHCb collaboration for invariant mass, rapidity distance between mesons and dimeson invariant mass. The predicted shapes are similar to the measured ones, however, some strength seems to be lacking. Our new calulations clearly confirm the dominance of DPS in the production of events with double charm.

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arXiv2012

Multi-gluon helicity amplitudes with one off-shell leg within high energy factorization

Basing on the Slavnov-Taylor identities, we derive a new prescription to obtain gauge invariant tree-level scattering amplitudes for the process g*g->Ng within high energy factorization. Using the helicity method, we check the formalism up to several final state gluons, and we present analytical formulas for the the helicity amplitudes for N=2. We also compare the method with Lipatov's effective action approach.

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arXiv2011

Complete off-shell effects in top quark pair hadroproduction with leptonic decay at next-to-leading order

Results for next-to-leading order QCD corrections to the pp(p\bar{p}) -> t \bar{t} -> W^+W^- b\bar{b} -> e^{+} ν_{e} μ^{-} \barν_μ b \bar{b} +X processes with complete off-shell effects are presented for the first time. Double-, single- and non-resonant top contributions of the order {\cal{O}}(α_{s}^3 α^4) are consistently taken into account, which requires the introduction of a complex-mass scheme for unstable top quarks. Moreover, the intermediate W bosons are treated off-shell. Comparison to the narrow width approximation for top quarks, where non-factorizable corrections are not accounted for is performed. Besides the total cross section and its scale dependence, several differential distributions at the TeVatron run II and the LHC are given. In case of the TeVatron the forward-backward asymmetry of the top is recalculated afresh. With inclusive selection cuts, the forward-backward asymmetry amounts to A^{t}_{FB} = 0.051 +/- 0.0013. Furthermore, the corrections with respect to leading order are positive and of the order 2.3% for the TeVatron and 47% for the LHC. A study of the scale dependence of our NLO predictions indicates that the residual theoretical uncertainty due to higher order corrections is 8% for the TeVatron and 9% for the LHC.

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