Supersymmetry status and phenomenology at the Large Hadron Collider

Large Hadron Collider (LHC) has a great chance to finally reveal supersymmetry which remains a compelling theory for over 30 years in spite of lack of its discovery. It might be around the corner the present LHC era with sensitive dark matter search experiments and international linear collider hopefully coming up in the near future.      

Extraction of the neutron charge form factor G<Superscript>n</Superscript><Subscript>E</Subscript>(Q<Superscript>2</Superscript>) from the charge form factor of deuteron G<Superscript>d</Superscript><Subscript>C</Subscript>(Q<Superscript>2</Superscript>)

We extract the neutron charge form factor G ⁿ _E(Q ²) from the charge form factor of deuteron G ^d _C(Q ²) obtained from T ₂₀(Q ²) data at 0≤Q ²≤ 1.717 (GeV/c)². The extraction is based on the relativistic impulse approximation in the instant form of the relativistic Hamiltonian dynamics. Our results (12 new points) are compatible with existing values of the neutron charge form factor of other authors. We propose a fit for the whole set (36 points) taking into account the data for the slope of the form factor at Q ² = 0. Received: 26 July 2002 / Accepted: 18 September 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: krutov@ssu.samara.ru RID="b" ID="b"e-mail: troitsky@theory.sinp.msu.ru Communicated by V.V. Anisovich     

Top quark spin correlations at the Tevatron and the LHC

Spin correlations of top quarks produced in hadron collisions have not been observed experimentally with large significance. In this Letter, we propose a new variable that may enable demonstration of the existence of spin correlations with 3–4 σ$3\text{–}4\sigma$ significance using just a few hundred dilepton events both at the Tevatron and the LHC. Such number of dilepton events has been observed at the Tevatron. At the LHC, it will become available once integrated luminosity of a few hundred inverse picobarns is collected.     

An effective approach to same sign top pair production at the LHC and the forward-backward asymmetry at the Tevatron

We study the phenomenology of same sign top pair production at the LHC in a model-independent way. The complete set of dimension six operators involving two top (or anti-top) quarks is introduced and the connection with all possible t- or s-channel heavy particle exchanges is established. Only in the former case, same and opposite sign top pair production can be related. We find that while current Tevatron data disfavor t-channel models, other production mechanisms are viable and can be tested at the LHC.     

Cold dark matter hypotheses in the MSSM

We perform a Bayesian model selection analysis in the R-parity conserving MSSM to compare two different assumptions: whether the lightest neutralinos make all or only part of the cold dark matter. This corresponds to either imposing full WMAP relic density limits or just its upper bound for constraining the MSSM parameters. We consider several realisations of the MSSM, namely, three GUT-scale SUSY breaking scenarios with a handful of parameters corresponding to the CMSSM, anomaly mediation and the large volume string scenarios as well as the weak-scale 25-parameter phenomenological MSSM (pMSSM). The results give a data-based quantitative evidence for a multicomponent cold dark matter. The pMSSM posterior samples indicate that the choice of imposing full WMAP limits or just its upper bound affects mostly the gaugino–higgsino content of the neutralino and, against naive expectations, essentially not any other sector.     

F-theory and experiment

There is currently a wide gulf between string theory and experiment. This is compounded by the existence of a ‘landscape’ of solutions to the string theory equations of motion, many of which are in gross violation of experimental constraints. To make contact with experiment, it is then necessary to impose some additional criteria to find potentially realistic solutions. In this article we show that requiring that the forces of the Standard Model unify, while remaining decoupled from gravity, imposes sharp restrictions. A promising class of string based models which meet both these criteria are known as F-theory GUTs. We review the main ingredients of these constructions, and how they make contact with experiment.     

New Electroweak Formulation Fundamentally Accounting for the Effect Known as “Maximal Parity-Violation”

The electroweak scheme is wholly recast, in the framework of a relativistic quantum field formalism being a covariant fermion–antifermion extension of the usual one for massive spin- point fermions. The new formalism is able to reread the “maximal P-violation” effect in a way restoring P and C symmetries themselves: it provides a natural “chiral field” approach, which gives evidence of the existence of a pseudoscalar (extra) charge variety anticommuting with the scalar (ordinary) one and just underlying the “maximally P-violating” phenomenology. Its zero-mass limit leads to a strict “chiral” particle theory, which remodels any massless spin- fermion and corresponding antifermion as two mere pseudoscalar-charge eigenstates being the simple mirror images of each other. On such a basis, the (zero-mass) electroweak primary fermions are all redefined to be (only left-handed) “chiral” particles (with right-handed complements just standing for their antiparticles) and to carry at most scalar charges subjected as yet to a maximal uncertainty in sign: it is only by acquiring mass, and by gaining an extra helicity freedom degree, that they now may also manifest themselves as “Dirac” particles, with sharp scalar-charge eigenvalues. The fermion-mass appearance is thus made herein a dynamical condition strictly necessary to obtain actual superselected scalar-charge (and first, electric-charge) eigenstates. A pure “internal” mass-generating mechanism, relying only on would-be-Goldstone bosons (even to yield fermion masses) and no longer including an “external” Higgs contribution, is adopted accordingly. This is shown to be a self-consistent mechanism, which still maintains both renormalizability and unitarity. It involves a P-breaking in the neutral-weak-current sector (due to the Weinberg mixing) while it leaves the charged-current couplings truly P-invariant even in the presence of a (standardly parametrized) CP-violation.