Johan Alwall - Academia.edu (original) (raw)

Papers by Johan Alwall

Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to ... more Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to be able to find those particles, an accurate description of their production is needed. In Monte Carlo simulations of charged Higgs boson production at hadron colliders, the two tree-level processes gb->H+/-t and gg->H+/-tb are used. Since those processes overlap in the collinear region of the phase-space of the outgoing b-quark, care must be taken not to introduce double-counting if both processes are to be used together. In this talk I present a method for matching these processes, developed by Johan Rathsman and myself. The method also allows for investigations of the factorization scale dependence of the processes and a better understanding of which factorization scale to choose to get a reliable description of charged Higgs production.

Journal of High Energy Physics, 2009

We present a study of effects of QCD radiation in the production of heavy colored states, employi... more We present a study of effects of QCD radiation in the production of heavy colored states, employing inclusive multi-jet samples obtained by matching matrix elements and parton showers. We discuss several examples showing that matched samples are in general not only more accurate than a parton shower alone, but also often indispensable to make reliable predictions of beyond the Standard Model signals.

The generalised vector meson dominance model (GVDM) gives a good description of F_2 data at very ... more The generalised vector meson dominance model (GVDM) gives a good description of F_2 data at very low Q^2. At intermediate Q^2 a GVDM component avoids problems when applying the large-Q^2 DIS formalism, such as a negative gluon distribution in the proton. The fluctuations of the exchanged photon into vector mesons is also a natural part of a model with hadronic fluctuations of the target proton, which gives a good description of the non-perturbative x-shape of the proton's parton density functions at the starting scale Q^2_0 for DGLAP evolution.

This manual describes how to use the MatCHig code for matching the charged Higgs boson production... more This manual describes how to use the MatCHig code for matching the charged Higgs boson production processes gg->tbH+/- and gb->tH+/-. A negative term, correcting for the double-counting between these processes, is implemented as an external process to PYTHIA, allowing the Monte Carlo generation of matched events. Results from the matching were published in JHEP 0412, 050. The code can be downloaded from http://www.isv.uu.se/thep/MC/matchig/ .

How and Where to Go Beyond the Standard Model, 2007

Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to ... more Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to be able to find those particles, an accurate description of their production is needed. In Monte Carlo simulations of charged Higgs boson production at hadron colliders, the two tree-level processes gb → H ± t and gg → H ± tb are used. Since those processes overlap in the collinear region of the phase-space of the outgoing b-quark, care must be taken not to introduce double-counting if both processes are to be used together. In this talk I present a method for matching these processes, developed by Johan Rathsman and myself. The method also allows for investigations of the factorization scale dependence of the processes and a better understanding of which factorization scale to choose to get a reliable description of charged Higgs production.

Advances in High Energy Physics

Dark matter presents perhaps one of the most compelling direct indications of new physics beyond ... more Dark matter presents perhaps one of the most compelling direct indications of new physics beyond the standard model with three generations of fermions. In this paper, we survey several scenarios for dark matter in association with a fourth generation of chiral matter. The surveyed scenarios include stable heavy neutrino dark matter, composite dark matter consisting of stable heavy quarks, heavy quarks as mediators between the dark and visible sectors, and the four-generation standard model with the minimal addition of a stable real scalar field. We discuss the basic properties of the models, direct search constraints on their dark matter, and their collider phenomenology, as well as the possible effects of dark matter on the searches for a Higgs boson in the presence of four generations. We also comment on the potential implication of the recent observation of a Higgs-like new particle at the LHC.

We have developed a physical model for the non-perturbative x-shape of parton density functions i... more 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.


We have developed a physical model for the non-perturbative x-shape of parton density functions i... more 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 also 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.

Arxiv preprint arXiv:0704.2307, 2007

The Atlas discovery potential for the hadronic τ decay of a heavy charged Higgs boson, H ± → τ ± ... more The Atlas discovery potential for the hadronic τ decay of a heavy charged Higgs boson, H ± → τ ± ντ , is presented. A new matched production algorithm for the processes gg → tbH ± and gb → tH ± is used allowing to span the investigated mass range from m ± H < mt up to m ± H mt with a consistent treatment of the transition region. For the considered charged Higgs boson masses from 165 to 600 GeV, H ± → τ ± ντ is the most relevant decay channel together with H ± → tb. Whereas the latter suffers from large irreducible backgrounds, the τ decay channel offers a decisively cleaner signature. This is the first Atlas Full Simulation study of the H ± → τ ± ντ channel. For the background, Atlas Fast Simulation is used for the investigation of a greater variety of channels than in previous studies, emphasizing the need for new selection cuts with additional discriminating power between signal and background. Such cuts are introduced and it is shown that current limits from direct charged Higgs boson searches can be substantially extended already with a few years of LHC data. * Electronic address: bjarte.mohn@ift.uib.no † Electronic address:

Advances in High Energy Physics, 2013

Ever since the emergence of the third generation of fermions in the 1970s, which was only complet... more Ever since the emergence of the third generation of fermions in the 1970s, which was only completed in 1995 with the discovery of the top quark, the heaviest fermion known to date, the question has lingered: can there be more fermion generations in Nature, that is, a 4th generation? By this one means a repetition of the left-handed (weak) doublet and right-handed singlet pattern that is observed for the first three generations.

... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather ... more ... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather well with ... pure parton-shower approach are then missing and therefore the transition between jet multiplicities is not as well ... C 33, 23 1986 ; OPAL Collaboration, MZ Akrawy et al., ibid ...

This thesis is meant as an introduction to the mathematics of Supersymmetry, giving a thorough de... more This thesis is meant as an introduction to the mathematics of Supersymmetry, giving a thorough derivation of concepts such as superspace, superfields and the Lagrangians of superfields, followed by a survey of the supersymmetric theories for extensions of the Standard Model of weak interactions, as well as the present status of experimental searches for supersymmetric particles.

... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather ... more ... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather well with ... pure parton-shower approach are then missing and therefore the transition between jet multiplicities is not as well ... C 33, 23 1986 ; OPAL Collaboration, MZ Akrawy et al., ibid ...

Top is a very important probe to test theories for the electroweak symmetry breaking such as two-... more Top is a very important probe to test theories for the electroweak symmetry breaking such as two-Higgs doublet models. One process of great interest in theories with extended Higgs sectors is the associated production of top, bottom and charged Higgs, described by gb → H -t (where the extra b is described by parton showering) and gg → H -tb. To get a reliable description of this production when the extra b-quark is observed, these processes must be matched to remove double-counting, a problem similar to that in the single top processes pp → tbW ± and ub → dt. In this talk, we present our method for doing this matching at a fully differential level, and its implementation in the event generator Pythia. The implementation files can be downloaded from http://www.isv.uu.se/thep/MC/matchig/ .

Physical Review D, 2012

The production and decay of a new heavy vector boson, a chromophilic Z vector boson, is described... more The production and decay of a new heavy vector boson, a chromophilic Z vector boson, is described. The chromophilic Z couples only to two gluons, but its two-body decays are absent, leading to a dominant decay mode of Z → qqg. The unusual nature of the interaction predicts a cross-section which grows with m Z for a fixed coupling and an accompanying gluon with a coupling that rises with its energy. We study the ttg decay mode, proposing distinct reconstruction techniques for the observation of an excess and for the measurement of m Z . We estimate the sensitivity of current experimental datasets.

Phys Rev D, 2007

We demonstrate the possibility of studying weakly interacting new particles in weak-boson fusion,... more We demonstrate the possibility of studying weakly interacting new particles in weak-boson fusion, using the example of supersymmetric same-sign charginos. This signal could establish the existence of Majorana neutralinos and give access to their electroweak couplings. It can be observed over (supersymmetric) QCD backgrounds provided the charginos are light and not too close to the squark mass. We finally show how same-sign fermion production can be distinguished from same-sign scalars or vectors arising in other models of new physics.

We determine the prospects for finding dark matter at the Tevatron and LHC through the production... more We determine the prospects for finding dark matter at the Tevatron and LHC through the production of exotic 4th generation quarks T' that decay through T' \to t X, where X is dark matter. The resulting signal of t \bar{t} + \met has not previously been considered in searches for 4th generation quarks, but there are both general and specific dark matter motivations for this signal, and with slight modifications, this analysis applies to any scenario where invisible particles are produced in association with top quarks. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and 600 GeV, and the dark matter's mass may be anywhere below m_T'. We simulate the signal and main backgrounds with MadGraph/MadEvent-Pythia-PGS4. For the Tevatron, we find that an integrated luminosity of 20 fb^-1 will allow 3\sigma discovery up to m_T' = 400 GeV and 95% exclusion up to m_T' = 455 GeV. For the 10 TeV LHC with 300 pb^-1, the discovery and exclusion sensitivities rise to 490 GeV and 600 GeV. These scenarios are therefore among the most promising for dark matter at colliders. Perhaps most interestingly, we find that dark matter models that can explain results from the DAMA, CDMS and CoGeNT Collaborations can be tested with high statistical significance using data already collected at the Tevatron and have extraordinarily promising implications for early runs of the LHC.

Physical Review D, 2007

We demonstrate the possibility of studying weakly interacting new particles in weak boson fusion,... more We demonstrate the possibility of studying weakly interacting new particles in weak boson fusion, using the example of supersymmetric same-sign charginos. This signal could establish the existence of Majorana neutralinos and give access to their electroweak couplings. It can be observed over (supersymmetric) QCD backgrounds provided the charginos are light and not too close to the squark mass. We finally show how same-sign fermion production can be distinguished from samesign scalars or vectors arising in other models of new physics. * Electronic address: alwall@slac.stanford.edu † Electronic address: rain@pas.rochester.edu ‡ Electronic address: tilman.plehn@cern.ch

A physical model for parton densities in hadrons, based on Gaussian momentum fluctuations of part... more A physical model for parton densities in hadrons, based on Gaussian momentum fluctuations of partons and hadronic baryon-meson fluctuations, is presented. The model has previously been shown to describe proton structure function data, and is now applied to sea quark asymmetries and shown to describe the dbar-ubar asymmetry of the proton. By considering fluctuations involving strange quarks, the model gives an asymmetry between the momentum distributions of s and sbar, which would reduce the significance of the NuTeV anomaly.

Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to ... more Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to be able to find those particles, an accurate description of their production is needed. In Monte Carlo simulations of charged Higgs boson production at hadron colliders, the two tree-level processes gb->H+/-t and gg->H+/-tb are used. Since those processes overlap in the collinear region of the phase-space of the outgoing b-quark, care must be taken not to introduce double-counting if both processes are to be used together. In this talk I present a method for matching these processes, developed by Johan Rathsman and myself. The method also allows for investigations of the factorization scale dependence of the processes and a better understanding of which factorization scale to choose to get a reliable description of charged Higgs production.

Journal of High Energy Physics, 2009

We present a study of effects of QCD radiation in the production of heavy colored states, employi... more We present a study of effects of QCD radiation in the production of heavy colored states, employing inclusive multi-jet samples obtained by matching matrix elements and parton showers. We discuss several examples showing that matched samples are in general not only more accurate than a parton shower alone, but also often indispensable to make reliable predictions of beyond the Standard Model signals.

The generalised vector meson dominance model (GVDM) gives a good description of F_2 data at very ... more The generalised vector meson dominance model (GVDM) gives a good description of F_2 data at very low Q^2. At intermediate Q^2 a GVDM component avoids problems when applying the large-Q^2 DIS formalism, such as a negative gluon distribution in the proton. The fluctuations of the exchanged photon into vector mesons is also a natural part of a model with hadronic fluctuations of the target proton, which gives a good description of the non-perturbative x-shape of the proton's parton density functions at the starting scale Q^2_0 for DGLAP evolution.

This manual describes how to use the MatCHig code for matching the charged Higgs boson production... more This manual describes how to use the MatCHig code for matching the charged Higgs boson production processes gg->tbH+/- and gb->tH+/-. A negative term, correcting for the double-counting between these processes, is implemented as an external process to PYTHIA, allowing the Monte Carlo generation of matched events. Results from the matching were published in JHEP 0412, 050. The code can be downloaded from http://www.isv.uu.se/thep/MC/matchig/ .

How and Where to Go Beyond the Standard Model, 2007

Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to ... more Many extensions of the Standard Model predict the existence of charged Higgs bosons. In order to be able to find those particles, an accurate description of their production is needed. In Monte Carlo simulations of charged Higgs boson production at hadron colliders, the two tree-level processes gb → H ± t and gg → H ± tb are used. Since those processes overlap in the collinear region of the phase-space of the outgoing b-quark, care must be taken not to introduce double-counting if both processes are to be used together. In this talk I present a method for matching these processes, developed by Johan Rathsman and myself. The method also allows for investigations of the factorization scale dependence of the processes and a better understanding of which factorization scale to choose to get a reliable description of charged Higgs production.

Advances in High Energy Physics

Dark matter presents perhaps one of the most compelling direct indications of new physics beyond ... more Dark matter presents perhaps one of the most compelling direct indications of new physics beyond the standard model with three generations of fermions. In this paper, we survey several scenarios for dark matter in association with a fourth generation of chiral matter. The surveyed scenarios include stable heavy neutrino dark matter, composite dark matter consisting of stable heavy quarks, heavy quarks as mediators between the dark and visible sectors, and the four-generation standard model with the minimal addition of a stable real scalar field. We discuss the basic properties of the models, direct search constraints on their dark matter, and their collider phenomenology, as well as the possible effects of dark matter on the searches for a Higgs boson in the presence of four generations. We also comment on the potential implication of the recent observation of a Higgs-like new particle at the LHC.

We have developed a physical model for the non-perturbative x-shape of parton density functions i... more 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.


We have developed a physical model for the non-perturbative x-shape of parton density functions i... more 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 also 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.

Arxiv preprint arXiv:0704.2307, 2007

The Atlas discovery potential for the hadronic τ decay of a heavy charged Higgs boson, H ± → τ ± ... more The Atlas discovery potential for the hadronic τ decay of a heavy charged Higgs boson, H ± → τ ± ντ , is presented. A new matched production algorithm for the processes gg → tbH ± and gb → tH ± is used allowing to span the investigated mass range from m ± H < mt up to m ± H mt with a consistent treatment of the transition region. For the considered charged Higgs boson masses from 165 to 600 GeV, H ± → τ ± ντ is the most relevant decay channel together with H ± → tb. Whereas the latter suffers from large irreducible backgrounds, the τ decay channel offers a decisively cleaner signature. This is the first Atlas Full Simulation study of the H ± → τ ± ντ channel. For the background, Atlas Fast Simulation is used for the investigation of a greater variety of channels than in previous studies, emphasizing the need for new selection cuts with additional discriminating power between signal and background. Such cuts are introduced and it is shown that current limits from direct charged Higgs boson searches can be substantially extended already with a few years of LHC data. * Electronic address: bjarte.mohn@ift.uib.no † Electronic address:

Advances in High Energy Physics, 2013

Ever since the emergence of the third generation of fermions in the 1970s, which was only complet... more Ever since the emergence of the third generation of fermions in the 1970s, which was only completed in 1995 with the discovery of the top quark, the heaviest fermion known to date, the question has lingered: can there be more fermion generations in Nature, that is, a 4th generation? By this one means a repetition of the left-handed (weak) doublet and right-handed singlet pattern that is observed for the first three generations.

... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather ... more ... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather well with ... pure parton-shower approach are then missing and therefore the transition between jet multiplicities is not as well ... C 33, 23 1986 ; OPAL Collaboration, MZ Akrawy et al., ibid ...

This thesis is meant as an introduction to the mathematics of Supersymmetry, giving a thorough de... more This thesis is meant as an introduction to the mathematics of Supersymmetry, giving a thorough derivation of concepts such as superspace, superfields and the Lagrangians of superfields, followed by a survey of the supersymmetric theories for extensions of the Standard Model of weak interactions, as well as the present status of experimental searches for supersymmetric particles.

... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather ... more ... has the strongest tendency towards 4 jets, as could be expected, whereas ME PS agrees rather well with ... pure parton-shower approach are then missing and therefore the transition between jet multiplicities is not as well ... C 33, 23 1986 ; OPAL Collaboration, MZ Akrawy et al., ibid ...

Top is a very important probe to test theories for the electroweak symmetry breaking such as two-... more Top is a very important probe to test theories for the electroweak symmetry breaking such as two-Higgs doublet models. One process of great interest in theories with extended Higgs sectors is the associated production of top, bottom and charged Higgs, described by gb → H -t (where the extra b is described by parton showering) and gg → H -tb. To get a reliable description of this production when the extra b-quark is observed, these processes must be matched to remove double-counting, a problem similar to that in the single top processes pp → tbW ± and ub → dt. In this talk, we present our method for doing this matching at a fully differential level, and its implementation in the event generator Pythia. The implementation files can be downloaded from http://www.isv.uu.se/thep/MC/matchig/ .

Physical Review D, 2012

The production and decay of a new heavy vector boson, a chromophilic Z vector boson, is described... more The production and decay of a new heavy vector boson, a chromophilic Z vector boson, is described. The chromophilic Z couples only to two gluons, but its two-body decays are absent, leading to a dominant decay mode of Z → qqg. The unusual nature of the interaction predicts a cross-section which grows with m Z for a fixed coupling and an accompanying gluon with a coupling that rises with its energy. We study the ttg decay mode, proposing distinct reconstruction techniques for the observation of an excess and for the measurement of m Z . We estimate the sensitivity of current experimental datasets.

Phys Rev D, 2007

We demonstrate the possibility of studying weakly interacting new particles in weak-boson fusion,... more We demonstrate the possibility of studying weakly interacting new particles in weak-boson fusion, using the example of supersymmetric same-sign charginos. This signal could establish the existence of Majorana neutralinos and give access to their electroweak couplings. It can be observed over (supersymmetric) QCD backgrounds provided the charginos are light and not too close to the squark mass. We finally show how same-sign fermion production can be distinguished from same-sign scalars or vectors arising in other models of new physics.

We determine the prospects for finding dark matter at the Tevatron and LHC through the production... more We determine the prospects for finding dark matter at the Tevatron and LHC through the production of exotic 4th generation quarks T' that decay through T' \to t X, where X is dark matter. The resulting signal of t \bar{t} + \met has not previously been considered in searches for 4th generation quarks, but there are both general and specific dark matter motivations for this signal, and with slight modifications, this analysis applies to any scenario where invisible particles are produced in association with top quarks. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and 600 GeV, and the dark matter's mass may be anywhere below m_T'. We simulate the signal and main backgrounds with MadGraph/MadEvent-Pythia-PGS4. For the Tevatron, we find that an integrated luminosity of 20 fb^-1 will allow 3\sigma discovery up to m_T' = 400 GeV and 95% exclusion up to m_T' = 455 GeV. For the 10 TeV LHC with 300 pb^-1, the discovery and exclusion sensitivities rise to 490 GeV and 600 GeV. These scenarios are therefore among the most promising for dark matter at colliders. Perhaps most interestingly, we find that dark matter models that can explain results from the DAMA, CDMS and CoGeNT Collaborations can be tested with high statistical significance using data already collected at the Tevatron and have extraordinarily promising implications for early runs of the LHC.

Physical Review D, 2007

We demonstrate the possibility of studying weakly interacting new particles in weak boson fusion,... more We demonstrate the possibility of studying weakly interacting new particles in weak boson fusion, using the example of supersymmetric same-sign charginos. This signal could establish the existence of Majorana neutralinos and give access to their electroweak couplings. It can be observed over (supersymmetric) QCD backgrounds provided the charginos are light and not too close to the squark mass. We finally show how same-sign fermion production can be distinguished from samesign scalars or vectors arising in other models of new physics. * Electronic address: alwall@slac.stanford.edu † Electronic address: rain@pas.rochester.edu ‡ Electronic address: tilman.plehn@cern.ch

A physical model for parton densities in hadrons, based on Gaussian momentum fluctuations of part... more A physical model for parton densities in hadrons, based on Gaussian momentum fluctuations of partons and hadronic baryon-meson fluctuations, is presented. The model has previously been shown to describe proton structure function data, and is now applied to sea quark asymmetries and shown to describe the dbar-ubar asymmetry of the proton. By considering fluctuations involving strange quarks, the model gives an asymmetry between the momentum distributions of s and sbar, which would reduce the significance of the NuTeV anomaly.