arXiv subjects

Andreas Ekstedt

Publications and source records attributed to Andreas Ekstedt.

23 records · Page 2Linked to original sources

arXiv2019

Nucleon parton distributions from hadronic quantum fluctuations

A physical model is presented for the non-perturbative 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 $f(x,Q_0^2)$ at low momentum transfers are evolved with conventional DGLAP equations from perturbative QCD to larger scales. This provides parton density functions $f(x,Q^2)$ 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 $\bar{u}$ and $\bar{d}$ in the proton. The strange-quark sea is strongly suppressed at low $Q^2$, as observed.

hep-ph↗

arXiv2019

On the relationship between gauge dependence and IR divergences in the $\hbar$-expansion of the effective potential

Perturbative calculations of the effective potential evaluated at a broken minimum, $ V_{\text{min}} $, are plagued by difficulties. It is hard to get a finite and gauge invariant result for $ V_{\text{min}} $. In fact, the methods proposed to deal with gauge dependence and IR divergences are orthogonal in their approaches. Gauge dependence is dealt with through the $ \hbar $-expansion, which establishes and maintains a strict loop-order separation of terms. On the other hand, IR divergences seem to require a resummation that mixes the different loop orders. In this paper we test these methods on Fermi gauge Abelian Higgs at two loops. We find that the resummation procedure is not capable of removing all divergences. Surprisingly, the $ \hbar $-expansion seems to be able to deal with both the divergences and the gauge dependence. In order to isolate the physical part of $ V_{\text{min}} $, we are guided by the separation of scales that motivated the resummation procedure; the key result is that only hard momentum modes contribute to $ V_{\text{min}} $.

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arXiv2017

Minimal anomalous $\mathrm{U}(1)$ theories and collider phenomenology

We study the collider phenomenology of a neutral gauge boson $Z'$ arising in minimal but anomalous $\mathrm{U}(1)$ 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 $\mathrm{U}(1)$ extensions in terms of the new $\mathrm{U}(1)$ 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.

hep-ph↗

arXiv2017

Hard color singlet BFKL exchange and gaps between jets at the LHC

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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arXiv2016

Constraining minimal anomaly free $\mathrm{U}(1)$ extensions of the Standard Model

We consider a class of minimal anomaly free $\mathrm{U}(1)$ 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 $\mathrm{U}(1)$ gauge coupling $g_z$. We find that the new dilepton data puts strong bounds on the parameters, especially in the mass region $M_{Z'}\lesssim 3~ \mathrm{TeV}$.

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