| 1. |
- Danielsson, Ulf, et al.
(författare)
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Heavy photophilic scalar at the LHC from a varying electromagnetic coupling
- 2017
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Ingår i: Nuclear Physics B. - : Elsevier BV. - 0550-3213 .- 1873-1562. ; 919, s. 569-582
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the phenomenology of a heavy scalar ϕ of the type involved in Bekenstein's framework for varying electromagnetic coupling theories, with the difference that the scalar in our model has a large mass. The model has only two free parameters, the mass Mϕ of the scalar and the scale Λ of new physics. The scalar is dominantly produced through photon-photon fusion at the LHC and leads to a diphoton final state. It can also be produced by quark-antiquark fusion in association with a photon or a fermion pair. Its dominating decay is to diphotons, but it also has a large three-body branching to a fermion pair and a photon, which can provide an interesting search channel with a dilepton-photon resonance. We derive exclusion limits on the Mϕ−Λplane from the latest 13 TeV LHC diphoton resonance search data. For a benchmark mass of Mϕ∼1 TeV, we find a lower limit on Λ of 18 TeV. We discuss the more complex possibility of varying couplings in the full electroweak theory and comment on the possibility that the new physics is related to extra dimensions or string theory.
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| 2. |
- Danielsson, Ulf, et al.
(författare)
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Varying gauge couplings and collider phenomenology
- 2019
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Ingår i: Physical Review D. Particles and fields. - : American Physical Society. - 0556-2821 .- 1089-4918. ; 100:5
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we investigate a natural extension of the Standard Model that involves varying coupling constants. This is a general expectation in any fundamental theory such as string theory, and there are good reasons for why new physics could appear at reachable energy scales. We investigate the collider phenomenology of models with varying gauge couplings where the variations are associated with real singlet scalar fields. We introduce three different heavy scalar fields that are responsible for the variations of the three gauge couplings of the Standard Model. This gives rise to many interesting collider signatures that we explore, resulting in exclusion limits based on the most recent LHC data, and predictions of the future discovery potential at the high-luminosity LHC.
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| 3. |
- Ekstedt, Andreas, et al.
(författare)
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Constraining minimal anomaly free U(1) extensions of the Standard Model
- 2016
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Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; 1611
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Tidskriftsartikel (refereegranskat)abstract
- We consider a class of minimal anomaly free U(1) extensions of the Standard Model with three generations of right-handed neutrinos and a complex scalar. Using electroweak precision constraints, new 13 TeV LHC data, and considering theoretical limitations such as perturbativity, we show that it is possible to constrain a wide class of models. By classifying these models with a single parameter, κ, we can put a model independent upper bound on the new U(1) gauge coupling gz. We find that the new dilepton data puts strong bounds on the parameters, especially in the mass region MZ′≲3 TeV.
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| 4. |
- Ekstedt, Andreas, et al.
(författare)
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Hard color singlet BFKL exchange and gaps between jets at the LHC
- 2017
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- We explore the perturbative QCD dynamics of hard parton-parton scattering through the exchange of a color singlet two-gluon ladder as described by the BFKL equation, resulting in a rapidity gap between two high transverse momentum jets. Implementing this in a complete Monte Carlo event simulation that also accounts for additional QCD processes at softer scales provides dynamical modeling of gap survival probabilities, which makes possible a detailed comparison with data on such jet-gap-jet events. New data from CMS at the LHC extend the dynamic range of the previous Tevatron data, and can be reproduced reasonably well provided that the Soft Color Interaction model is modified based on the idea of reduced resolution power of softer gluon exchanges. This indicates the need for further theoretical developments in connection with other color exchange processes related to rapidity gaps in the hadronic final state.
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| 5. |
- Ekstedt, Andreas, et al.
(författare)
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Minimal anomalous U(1) theories and collider phenomenology
- 2018
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Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; 1802, s. 152-
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Tidskriftsartikel (refereegranskat)abstract
- We study the collider phenomenology of a neutral gauge boson Z′ arising in minimal but anomalous U(1) extensions of the Standard Model (SM). To retain gauge invariance of physical observables, we consider cancellation of gauge anomalies through the Green-Schwarz mechanism. We categorize a wide class of U(1) extensions in terms of the new U(1) charges of the left-handed quarks and leptons and the Higgs doublet. We derive constraints on some benchmark models using electroweak precision constraints and the latest 13 TeV LHC dilepton and dijet resonance search data. We calculate the decay rates of the exotic and rare one-loop Z′ decays to ZZ and Z-photon modes, which are the unique signatures of our framework. If observed, these decays could hint at anomaly cancellation through the Green-Schwarz mechanism. We also discuss the possible observation of such signatures at the LHC and at future ILC colliders.
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| 6. |
- Ekstedt, Andreas, et al.
(författare)
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Nucleon parton distributions from hadronic quantum fluctuations
- 2019
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 100:3
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Tidskriftsartikel (refereegranskat)abstract
- A physical model is presented for the nonperturbative parton distributions in the nucleon. This is based on quantum fluctuations of the nucleon into baryon-meson pairs convoluted with Gaussian momentum distributions of partons in hadrons. The hadronic fluctuations, here developed in terms of hadronic chiral perturbation theory, occur with high probability and generate sea quarks as well as dynamical effects also for valence quarks and gluons. The resulting parton momentum distributions f(x,Q20) at low momentum transfers are evolved with conventional Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equations from perturbative QCD to larger scales. This provides parton density functions f(x,Q2) for the gluon and all quark flavors with only five physics-motivated parameters. By tuning these parameters, experimental data on deep-inelastic structure functions can be reproduced and interpreted. The contribution to sea quarks from hadronic fluctuations explains the observed asymmetry between ¯u and ¯d in the proton. The strange-quark sea is strongly suppressed at low Q2, as observed.
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| 7. |
- Ekstedt, Andreas
(författare)
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Phenomenology of new Neutral Vector Bosons and Parton Distributions from Hadronic Fluctuations
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Higgs particle was first predicted in 1964, and was discovered in the summer of 2012 at the Large Hadron Collider (LHC). This discovery was the latest in a long list of successful Standard Model predictions spanning the last fifty years. However, some of the Standard Models predictions, such as massless neutrinos, are not in agreement with experiment. Thus, extensions of the Standard Model should be considered. Furthermore, some issues, such as how quarks are bound within the proton, are difficult to study from first principles.In paper I and II of this thesis, a class of models that contains a new TeV scale neutral vector boson is studied. The parameter space of this class of models is constrained using electroweak precision constraints and 13 TeV LHC data. Gauge anomalies are cancelled both by choosing appropriate fermion charges, and by adding Green-Schwarz terms.The Higgs mechanism is often studied at leading order, but there are also important radiative corrections. These radiative corrections, which change the ground state energy, can both be IR divergent and gauge dependent. In paper III it is shown how to solve both of these problems. In particular, IR divergences are shown to be spurious.In paper IV of this thesis, rapidity gaps at the LHC are explained by using a colour singlet two-gluon ladder exchange (BFKL). These exchanges, together with a soft-gluon model, are implemented in a complete Monte Carlo simulation, and reproduce observed rapidity gaps at the LHC.The momentum distributions of bound partons, quarks and gluons, are described by parton distribution functions (PDFs). In paper V and VI of this thesis, a physically motivated model for PDFs is presented. This model can reproduce proton structure function data, and gives a possible solution to the proton spin puzzle.
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| 8. |
- Ghaderi, Hazhar
(författare)
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The interplay between quark and hadronic degrees of freedom and the structure of the proton
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We study the low-energy sector of the strong interaction which is the least understood part of the Standard Model, the theory that describes the interactions of all known particles. The ideal particles for this study are the proton and the neutron, collectively called the nucleon. They make up the nucleus of all the atoms of our world and understanding them has been of high priority ever since their discovery. We show that one cannot neglect the effects of other hadrons, such as neutrons and pions when studying the proton. A large part of the proton's hadronic wavefunction is shown to consist of the wavefunctions of other hadrons. In other words, when probing the proton there is a sizeable probability that one is probing some other hadron surrounding the proton as a quantum fluctuation.The nucleon itself consists of elementary particles known as quarks and gluons, collectively called partons. Exactly how the properties of these partons make up the properties of the nucleon has been the subject of active research ever since their discovery. Two main issues are the flavor asymmetry of the proton sea and the spin structure of the nucleon. To address these questions we study the interplay between the partonic and hadronic degrees of freedom. We introduce a model based on a convolution between hadronic quantum fluctuations as described by chiral perturbation theory, and partonic degrees of freedom motivated by a physical model of the nucleon having only few physically constrained parameters.We present the hadronic distribution functions and the parton distribution functions. The results are in agreement with a large set of experimental data. These include the structure functions of the proton and the neutron. Agreement with the sum rules of the spin structure functions offers new insight into the spin structure of the nucleon.
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| 9. |
- Ingelman, Gunnar, et al.
(författare)
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Dynamic colour screening in diffractive deep inelastic scattering
- 2016
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Ingår i: PHYSICAL REVIEW D. - 2470-0010. ; 93:9
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We present a novel Monte Carlo implementation of dynamic color screening via multiple exchanges of semisoft gluons as a basic QCD mechanism to understand diffractive electron-proton scattering at the HERA collider. Based on the kinematics of individual events in the standard QCD description of deep inelastic scattering at the parton level, which at low x is dominantly gluon initiated, the probability is evaluated for additional exchanges of softer gluons resulting in an overall color singlet exchange leading to a forward proton and a rapidity gap as the characteristic observables for diffractive scattering. The probability depends on the impact parameter of the soft exchanges and varies with the transverse size of the hard scattering subsystem and is therefore influenced by different QCD effects. We account for matrix elements and parton shower evolution either via conventional DGLAP log Q(2) evolution with collinear factorization or CCFM small x evolution with k(perpendicular to) factorization and discuss the sensitivity to the gluon density distribution in the proton and the importance of large log x contributions. The overall result is, with only two model parameters which have theoretically motivated values, a satisfactory description of the observed diffractive cross section at HERA obtained in a wide kinematical range.
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| 10. |
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