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| 2. |
- Alwall, Johan, et al.
(författare)
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Improved description of charged Higgs boson production at hadron colliders
- 2004
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Ingår i: Journal of High Energy Physics. - : Springer Science and Business Media LLC. - 1029-8479. ; 0412:050
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Tidskriftsartikel (refereegranskat)abstract
- We present a new method for matching the two twin-processes gb→H±t and gg→H±tb in Monte Carlo event generators. The matching is done by defining a double-counting term, which is used to generate events that are subtracted from the sum of these two twin-processes. In this way we get a smooth transition between the collinear region of phase space, which is best described by gb→H±t, and the hard region, which requires the use of the gg→H±tb process. The resulting differential distributions show large differences compared to both the gb→H±t and gg→H±tb processes illustrating the necessity to use matching when tagging the accompanying b-jet.
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| 3. |
- Alwall, Johan, 1975-
(författare)
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Quark Distributions and Charged Higgs Boson Production : Studies of Proton Structure and New Physics
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Standard Model describes all elementary particles known today, but at larger energies it will have to be complemented with new particles and interactions. To be able to distinguish new physics at proton colliders such as LHC at CERN, it is essential to have an appropriate description of the colliding protons and their interactions. The study of the proton is important also in itself, to get a better understanding of the non-perturbative aspects of the strong interaction.In paper I-IV of this thesis, a model for the non-perturbative dynamics of quarks and gluons is developed, based on quantum fluctuations in hadrons. The parton distributions of the proton are given by momentum fluctuations, with sea quark distributions generated by fluctuations into baryon-meson pairs. This model can reproduce proton structure function data, as well as measured asymmetries between up and down valence quark distributions and between the anti-up and anti-down sea. It provides an intrinsic charm quark component as indicated by data. It also predicts an asymmetry in the strange sea of the proton, which can explain the NuTeV anomaly first attributed to new physics beyond the Standard Model.Charged Higgs bosons are predicted by several theories for new physics, including Supersymmetry. At proton colliders, the predicted dominant production mechanism is in association with top and bottom quarks. In paper V-VII, different contributions to this production are studied, and an algorithm is developed for combining the two dominant processes gb -> tH+/- and gg -> tbH+/-. The algorithm gives a smooth transition from small to large transverse momenta of the b-quark, which is important when the b-quark is observed. It also gives arguments for the choice of factorisation scale in the process.
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| 5. |
- Alwall, Johan, et al.
(författare)
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The pp -> tbH+/- process at the Tevatron in Herwig and Pythia simulations
- 2004
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Ingår i: The European Physical Journal C: Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 39, s1, s. s37-s39
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Tidskriftsartikel (refereegranskat)abstract
- Charged Higgs boson production in association with a top quark could be the first indication of the existence of Higgs particles. The Tevatron Run-II started data-taking in April 2001 at $\sqrt s=1960$ GeV and could probe the existence of a charged Higgs boson beyond the current mass limit. We study the $\rm p\bar p \rightarrow tbH^\pm$ production process with Monte Carlo simulations in HERWIG and PYTHIA, comparing expected cross sections and basic selection variables.
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| 7. |
- Alwall, Johan, et al.
(författare)
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Interpretation of electron-proton scattering at low Q2
- 2004
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Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693. ; 596, s. 77-83
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Tidskriftsartikel (refereegranskat)abstract
- Low-Q2 photons do not resolve partons in the proton, which gives problems when applying the deep inelastic scattering formalism, such as an unphysical, negative gluon density extracted from data. Considering instead hadronic fluctuations of the photon, we show that the generalised vector meson dominance model (GVDM) gives a good description of the measured cross section at low Q2, i.e., reproduces F2(x,Q2), using only few parameters with essentially known values. Combining GVDM and parton density functions makes it possible to obtain a good description of F2 data over the whole range of x and Q2.
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| 10. |
- Alwall, Johan
(författare)
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Quark Asymmetries and Intrinsic Charm in Nucleons
- 2005
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We have developed a physical model for the non-perturbative x-shape of parton density functions in the proton, based on Gaussian fluctuations in momenta, and quantum fluctuations of the proton into meson-baryon pairs. The model describes the proton structure function and gives a natural explanation of observed quark asymmetries, such as the difference between the anti-up and anti-down sea quark distributions and between the up and down valence distributions. We find an asymmetry in the momentum distribution of strange and anti-strange quarks in the nucleon, large enough to reduce the NuTeV anomaly to a level which does not give a significant indication of physics beyond the standard model. We also consider charmed fluctuations, and show that they can explain the excess at large x in the EMC F2c data.
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