arXiv subjects

Johan Rathsman

Publications and source records attributed to Johan Rathsman.

At least 37 records · Page 2Linked to original sources

arXiv2011

Discerning Secluded Sector gauge structures

New fundamental particles, charged under new gauge groups and only weakly coupled to the standard sector, could exist at fairly low energy scales. In this article we study a selection of such models, where the secluded group either contains a softly broken U(1) or an unbroken SU(N). In the Abelian case new γv gauge bosons can be radiated off and decay back into visible particles. In the non-Abelian case there will not only be a cascade in the hidden sector, but also hadronization into new πv and ρv mesons that can decay back. This framework is developed to be applicable both for e+e- and pp collisions, but for these first studies we concentrate on the former process type. For each Abelian and non-Abelian group we study three different scenarios for the communication between the standard sector and the secluded one. We illustrate how to distinguish the various characteristics of the models and especially study to what extent the underlying gauge structure can be determined experimentally.

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arXiv2009

2HDMC - Two-Higgs-Doublet Model Calculator

This manual describes the public code 2HDMC which can be used to perform calculations in a general, CP-conserving, two-Higgs-doublet model (2HDM). The program features simple conversion between different parametrizations of the 2HDM potential, a flexible Yukawa sector specification with choices of different Z_2-symmetries or more general couplings, a tree-level decay library including all two-body - and some three-body - decay modes for the Higgs bosons, and the possibility to calculate observables of interest for constraining the 2HDM parameter space, as well as theoretical constraints from positivity and unitarity. The latest version of the 2HDMC code and full documentation is available from: http://www.isv.uu.se/thep/MC/2HDMC

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arXiv2009

A Monte Carlo Model for 'Jet Quenching'

We have developed the Monte Carlo simulation program JEWEL 1.0 (Jet Evolution With Energy Loss), which interfaces a perturbative final state parton shower with medium effects occurring in ultra-relativistic heavy ion collisions. This is done by comparing for each jet fragment the probability of further perturbative splitting with the density-dependent probability of scattering with the medium. A simple hadronisation mechanism is included. In the absence of medium effects, we validate JEWEL against a set of benchmark jet measurements. For elastic interactions with the medium, we characterise not only the medium-induced modification of the jet, but also the jet-induced modification of the medium. Our main physics result is the observation that collisional and radiative medium modifications lead to characteristic differences in the jet fragmentation pattern, which persist above a soft background cut. We argue that this should allow to disentangle collisional and radiative parton energy loss mechanisms by measuring the n-jet fraction or a class of jet shape observables.

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arXiv2008

Colour rearrangements in B-meson decays

We present a new model, based on colour rearrangements, which at the same time can describe both hidden and open charm production in B-meson decays. The model is successfully compared to both inclusive decays, such as B to J/psi X and B to D_s X, as well as exclusive ones, such as B to J/psi K^(*) and B to D^(*) D^(*)K. It also gives a good description of the momentum distribution of direct J/psi's, especially in the low-momentum region, which earlier has been claimed as a possible signal for new exotic states.

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arXiv2008

Parton Energy Loss Without Transverse Momentum Broadening

The JEWEL 1.0 Monte Carlo simulates jet evolution in a medium with a microscopic description of splitting and scattering processes. In the framework of this model we investigate the transverse momentum broadening due to medium effects in different scenarios. Depending on assumptions about hadronisation, we observe either a small increase or even a slight decrease of the mean transverse momentum, but no sizeable broadening. This appears to be a natural consequence of a model formulation which conserves energy and momentum microscopically at each splitting and at each scattering.

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arXiv2008

Associated charged Higgs and W boson production in the MSSM at the CERN Large Hadron Collider

We investigate the viability of observing charged Higgs bosons (H^+/-) produced in association with W bosons at the CERN Large Hadron Collider, using the leptonic decay H^+ -> tau^+ nu_tau and hadronic W-decay, within different scenarios of the Minimal Supersymmetric Standard Model (MSSM) with both real and complex parameters. Performing a parton level study we show how the irreducible Standard Model background from W+2 jets can be controlled by applying appropriate cuts and find that the size of a possible signal depends on the cuts needed to suppress QCD backgrounds and misidentifications. In the standard maximal mixing scenario of the MSSM we find a viable signal for large tan(beta) and intermediate H^+/- masses (~m_t) when using optimistic cuts whereas for more pessimistic ones we only find a viable signal for very large tan(beta) (>~50). We have also investigated a special class of MSSM scenarios with large mass-splittings among the heavy Higgs bosons where the cross-section can be resonantly enhanced by factors up to one hundred, with a strong dependence on the CP-violating phases. Even so we find that the signal after cuts remains small except for small masses (~< m_t) with optimistic cuts. Finally, in all the scenarios we have investigated we have only found small CP-asymmetries.

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arXiv2007

Higgs boson plus photon production at the LHC: a clean probe of the b-quark parton densities

Higgs boson production in association with a high pT photon at the CERN Large Hadron Collider is analyzed, in the framework of the MSSM model, for the heavier neutral Higgs bosons. The request of an additional photon in the exclusive Higgs boson final state selects b-quark pairs among the possible initial partonic states, since gluon-gluon initial states are not allowed by C-parity conservation. Hence, the measurement of cross sections for neutral Higgs boson plus photon production can provide a clean probe of the b-quark density in the proton as well as of the b-quark Yukawa coupling. The suppression of the production rates by the b-quark electromagnetic coupling can be compensated by the enhanced Higgs boson Yukawa coupling to b's in the large tan(beta) regime. The Higgs boson decay into a tau-lepton pair is considered, and irreducible backgrounds with corresponding signal significances are evaluated.

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arXiv2007

New angles on top quark decay to a charged Higgs

To properly discover a charged Higgs Boson ($H^\pm$) requires its spin and couplings to be determined. We investigate how to utilize $\ttbar$ spin correlations to analyze the $H^\pm$ couplings in the decay $t\to bH^+\to bτ^+ν_τ$. Within the framework of a general Two-Higgs-Doublet Model, we obtain results on the spin analyzing coefficients for this decay and study in detail its spin phenomenology, focusing on the limits of large and small values for $\tanβ$. Using a Monte Carlo approach to simulate full hadron-level events, we evaluate systematically how the $H^\pm\toτ^\pmν_τ$ decay mode can be used for spin analysis. The most promising observables are obtained from azimuthal angle correlations in the transverse rest frames of $t(\bar{t})$. This method is particularly useful for determining the coupling structure of $H^\pm$ in the large $\tanβ$ limit, where differences from the SM are most significant.

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arXiv2007

Associated charged Higgs and W boson production in the MSSM at the LHC

We investigate the associated production of charged Higgs bosons (H^\pm) and W bosons at the CERN Large Hadron Collider, using the leptonic decay H^+ -> tau^+ nu_tau and hadronic W decay, within different scenarios of the Minimal Supersymmetric Standard Model (MSSM) with both real and complex parameters. Performing a parton level study we show how the irreducible Standard Model background from W + 2 jets can be controlled by applying appropriate cuts. In the standard m_h^max scenario we find a viable signal for large tan beta and intermediate H^\pm masses (~ m_t). In MSSM scenarios with large mass-splittings among the heavy Higgs bosons the cross-section can be resonantly enhanced by factors up to one hundred, with a strong dependence on the CP-violating phases.

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arXiv2007

H^\pm W^\mp production in the MSSM at the LHC

We investigate the viability of observing charged Higgs bosons (H^\pm) produced in association with W bosons at the CERN Large Hadron Collider, using the leptonic decay H^+ -> tau^+ nu_tau and hadronic W decay, within the Minimal Supersymmetric Standard Model. Performing a parton level study we show how the irreducible Standard Model background from W + 2 jets can be controlled by applying appropriate cuts. In the standard m_h^max scenario we find a viable signal for large tan beta and intermediate H^\pm masses (~ m_t).

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arXiv2007

Heavy Quark Energy Loss Through Soft QCD Scattering in the QGP

A strong suppression of non-photonic electrons in Au+Au collisions is observed at RHIC. This is in disagreement with the expected dominance of the energy loss via gluon radiation, which predicts a much weaker suppression of heavy flavours due to the dead cone effect. However, collisional energy loss is also important, as demonstrated recently by the Soft Colour Interaction Jet Quenching model. Based on this model we show that collisional energy loss of charm quarks gives a suppression of electrons of the observed magnitude, but the contribution from beauty decays, which dominates at intermediate and large pT, results in a somewhat weaker overall suppression of electrons than observed.

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arXiv2006

Jet Quenching from soft QCD Scattering in the Quark-Gluon Plasma

We show that partons traversing a quark-gluon plasma can lose substantial amounts of energy also by scatterings, and not only through medium-induced radiation as mainly considered previously. Results from Monte Carlo simulations of soft interactions of partons, emerging from a hard scattering, through multiple elastic scatterings on gluons in an expanding relativistic plasma show a sizeable jet quenching which can account for a substantial part of the effect observed in RHIC data.

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arXiv2003

The ppbar -> tbH^+/- Process at the Tevatron in HERWIG and PYTHIA Simulations

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 a centre-of-mass energy of 1960 GeV and could probe the existence of a charged Higgs boson beyond the current mass limit. We study the ppbar -> tbH^+/- production process with Monte Carlo simulations in HERWIG and PYTHIA, comparing expected cross sections and basic selection variables.

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arXiv2003

On the Behavior of the Effective QCD Coupling alpha_tau(s) at Low Scales

The hadronic decays of the tau lepton can be used to determine the effective charge alpha_tau(m^2_tau') for a hypothetical tau-lepton with mass in the range 0 < m_tau' < m_tau. This definition provides a fundamental definition of the QCD coupling at low mass scales. We study the behavior of alpha_tau at low mass scales directly from first principles and without any renormalization-scheme dependence by looking at the experimental data from the OPAL Collaboration. The results are consistent with the freezing of the physical coupling at mass scales s = m^2_tau' of order 1 GeV^2 with a magnitude alpha_tau ~ 0.9 +/- 0.1.

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arXiv2002

The thrust and heavy-jet mass distributions in the two-jet region

Dressed Gluon Exponentiation (DGE) is used to calculate the thrust and the heavy-jet mass distributions in e+e- annihilation in the two-jet region. We perform a detailed analysis of power corrections, taking care of the effect of hadron masses on the measured observables. In DGE the Sudakov exponent is calculated in a renormalization-scale invariant way using renormalon resummation. Neglecting the correlation between the hemispheres in the two-jet region, we express the thrust and the heavy-jet mass distributions in terms of the single-jet mass distribution. This leads to a simple description of the hadronization corrections to both distributions in terms of a single shape function, whose general properties are deduced from renormalon ambiguities. Matching the resummed result with the available next-to-leading order calculation, we get a good description of the thrust distribution in a wide range, whereas the description of the heavy-jet mass distribution, which is more sensitive to the approximation of the phase space, is restricted to the range rho_H<1/6. This significantly limits the possibility to determine alpha_s from this observable. However, fixing alpha_s by the thrust analysis, we show that the power corrections for the two observables are in good agreement.

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arXiv2001

Resummation and power corrections by Dressed Gluon Exponentiation

Event-shape distributions in $e^+e^-$ annihilation offer a unique lab oratory for understanding perturbative and non-perturbative aspects of QCD. Dressed Gluon Exponentiation is a resummation method designed to evaluate differ ential cross sections close to a kinematic threshold and provide the basis for parametrization of power corrections. The method and its application in the case of the thrust and the heavy jet mass distributions in the two-jet region are briefly presented.

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arXiv2001

Renormalon resummation and exponentiation of soft and collinear gluon radiation in the thrust distribution

The thrust distribution in e+e- annihilation is calculated exploiting its exponentiation property in the two-jet region t = 1-T << 1. We present a general method (DGE) to calculate a large class of logarithmically enhanced terms, using the dispersive approach in renormalon calculus. Dressed Gluon Exponentiation is based on the fact that the exponentiation kernel is associated primarily with a single gluon emission, and therefore the exponent is naturally represented as an integral over the running coupling. Fixing the definition of Lambda is enough to guarantee consistency with the exact exponent to next-to-leading logarithmic accuracy. Renormalization scale dependence is avoided by keeping all the logs. Sub-leading logs, that are usually neglected, are factorially enhanced and are therefore important. Renormalization-group invariance as well as infrared renormalon divergence are recovered in the sum of all the logs. The logarithmically enhanced cross-section is evaluated by Borel summation. Renormalon ambiguity is then used to study power corrections in the peak region Qt \gsim Lambda, where the hierarchy between the renormalon closest to the origin (~1/Qt) and others (~1/(Qt)^n) starts to break down. The exponentiated power-corrections can be described by a shape-function, as advocated by Korchemsky and Sterman. Our calculation suggests that the even central moments of the shape-function are suppressed. Good fits are obtained yielding alpha_s^{MSbar} (M_Z) = 0.110 \pm 0.001, with a theoretical uncertainty of ~5%.

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arXiv2001

The static PQCD potential with modified boundary conditions

We calculate the potential between two static quarks in QCD using modified boundary conditions for the perturbative expansion. Through a change of the Feynman iepsilon prescription we effectively add a "sea" of gluons to the asymptotic states with energies below a given scale Lambda. We find that the standard result for the static potential gets corrections of order Lambda^2/Q^2 both at small and large momentum transfers Q^2. After resummation of the infrared sensitive corrections we find that the running coupling alpha_s(Q^2) freezes in the infrared and that the exchanged gluon gets an effective tachyonic mass. We verify that identical results are obtained in the Coulomb and Feynman gauges.

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