Showing papers on "Elementary particle published in 2008"

Low-scale B?L extension of the standard model

Abstract: The fact that neutrinos are massive indicates that the standard model (SM) requires extension. We propose a low-energy (TeV) B–L extension of the SM, which is based on the gauge group . We show that this model provides a natural explanation for the presence of three right-handed neutrinos in addition to an extra-gauge boson and a new scalar Higgs. Therefore, it can lead to very interesting phenomenological implications different from the SM results which can be tested at the LHC. Also we analyze the muon anomalous magnetic moment in this class of models. We show that one loop with exchange Z' may give dominant new contribution ~few × 10−11.

The muon g-2 and the bounds on the Higgs boson mass

Abstract: After a brief review of the muon g-2 status, we analyze the possibility that the present discrepancy between experiment and the standard model (SM) prediction may be due to hypothetical errors in the determination of the hadronic leading-order contribution to the latter. In particular, we show how an increase of the hadroproduction cross section in low-energy e{sup +}e{sup -} collisions could bridge the muon g-2 discrepancy, leading however to a decrease on the electroweak upper bound on M{sub H}, the SM Higgs boson mass. That bound is currently M{sub H} 114.4 GeV (95% C.L.). By means of a detailed analysis we conclude that this solution of the muon g-2 discrepancy is unlikely in view of current experimental error estimates. However, if this turns out to be the solution, the 95% C.L. upper bound on M{sub H} is reduced to about 130 GeV which, in conjunction with the experimental lower bound, leaves a narrow window for the mass of this fundamental particle.

Nonstandard Higgs Boson Decays

Abstract: This review summarizes the motivations for and the phenomenological consequences of nonstandard Higgs boson decays, with an emphasis on final states containing a pair of non–standard model particles that subsequently decay to standard model particles. Typically these non–standard model particles form part of a “hidden” sector, for example a pair of neutral Higgs bosons or a pair of unstable neutralinos. We emphasize that such decays allow for a Higgs substantially below the standard model Higgs Large Electron-Positron Collider limit of 114 GeV. A Higgs with standard model WW, ZZ, and top couplings and a mass near 100 GeV eliminates the fine-tuning problems of many beyond the standard model theories, in particular supersymmetric models, and leads to excellent consistency with precision electroweak data.

Wilson line correlator in the MV model: relating the glasma to deep inelastic scattering

Abstract: In the color glass condensate framework the saturation scale measured in deep inelastic scattering of high energy hadrons and nuclei can be determined from the correlator of Wilson lines in the hadron wavefunction. These same Wilson lines give the initial condition of the classical field computation of the initial gluon multiplicity and energy density in a heavy ion collision. In this paper the Wilson line correlator in both adjoint and fundamental representations is computed using exactly the same numerical procedure as has been used to calculate gluon production in a heavy ion collision. In particular the discretization of the longitudinal coordinate has a large numerical effect on the relation between the color charge density parameter g2μ and the saturation scale Qs. Our result for this relation is Qs≈0.6g2μ, which results in the classical Yang–Mills value for the “gluon liberation coefficient” c≈1.1.

The thermal production of strange and non-strange hadrons in e + e − collisions

Abstract: The thermal multihadron production observed in different high energy collisions poses two basic problems. (1) Why do even elementary collisions with comparatively few secondaries (e + e − annihilation) show thermal behavior? (2) Why is there in such interactions a suppression of strange particle production? We show that the recently proposed mechanism of thermal hadron production through Hawking–Unruh radiation can naturally account for both. The event horizon of color confinement leads to thermal behavior, but the resulting temperature depends on the strange quark content of the produced hadrons, causing a deviation from full equilibrium and hence a suppression of strange particle production. We apply the resulting formalism to multihadron production in e + e − annihilation over a wide energy range and make a comprehensive analysis of the data in the conventional statistical hadronization model and the modified Hawking–Unruh formulation. We show that this formulation provides a very good description of the measured hadronic abundances, fully determined in terms of the string tension and the bare strange quark mass; it contains no adjustable parameters.

Cosmic Strings

Abstract: Cosmic strings are linear concentrations of energy that form whenever phase transitions in the early universe break axial symmetries as originally shown by Kibble They are the result of frustrated order in the quantum fields responsible for elementary particles and their interactions For about two decades, motivation for their study was provided by the possibility that they could be behind the density inhomogeneities that led to the observed large-scale structures in the universe Precision observations, particularly of the cosmic microwave background radiation, have limited strings to a sub-dominant role in structure formation More recently, interest has been revived with the realization that there may be strong links between field theory cosmic strings and fundamental strings The latter are the supposed ultimate building blocks of matter, and in their original context of superstring theory were thought to be microscopic However, in its modern version---sometimes referred to as M-theory---it is possible and perhaps even mandatory to have macroscopic (cosmological-sized) fundamental strings

Electroweak corrections using effective field theory : Applications to the CERN LHC

Abstract: Electroweak Sudakov logarithms at high energy, of the form ({alpha}/sin{sup 2}{theta}{sub W}){sup n}log{sup m}s/M{sub Z,W}{sup 2}, are summed using effective theory (EFT) methods. The exponentiation of Sudakov logarithms and factorization is discussed in the EFT formalism. Radiative corrections are computed to scattering processes in the standard model involving an arbitrary number of external particles. The computations include nonzero particle masses such as the t-quark mass, electroweak mixing effects which lead to unequal W and Z masses and a massless photon, and Higgs corrections proportional to the top-quark Yukawa coupling. The structure of the radiative corrections, and which terms are summed by the EFT renormalization group is discussed in detail. The omitted terms are smaller than 1%. We give numerical results for the corrections to dijet production, dilepton production, tt production, and squark pair production. The purely electroweak corrections are significant--about 15% at 1 TeV, increasing to 30% at 5 TeV, and they change both the scattering rate and angular distribution. The QCD corrections (which are well-known) are also computed with the EFT. They are much larger--about a factor of 4 at 1 TeV, increasing to a factor of 30 at 5 TeV. Mass effects are also significant; the qq{yields}tt ratemore » is enhanced relative to the light-quark production rate by 40%.« less

Dark matter candidate in a heavy higgs model: direct detection rates

Abstract: We investigate direct detection rates for Dark Matter candidates arise in a SU(2)L×U(1)Y with an additional doublet Higgs proposed by Barbieri, Hall and Rychkov. We refer to this model as "Heavy Higgs Model". The Standard Model Higgs mass comes out from this model is very heavy, so there is very slim chance that there is no Higgs boson mass below 200 GeV. The additional Higgs boson develops neither any VEV due to the choice of coefficient of the scalar potential of the model nor it has any coupling with fermions due to the incorporation of a discrete parity symmetry. Thus, the neutral components of the extra doublet are stable and can be considered as probable candidate of Cold Dark Matter. We have made calculations for three different types of Dark Matter experiments, namely, 76Ge (like GENIUS), DAMA (NaI) and XENON (131Xe). Also demonstrated the annual variation of Dark Matter detection in case of all three

Studying the MSSM Higgs sector by forward proton tagging at the LHC

Abstract: We show that the use of forward proton detectors at the LHC installed at 220 m and 420 m distance around ATLAS and/or CMS can provide important information on the Higgs sector of the MSSM. We analyse central exclusive production of the neutral \(\mathcal{CP}\)-even Higgs bosons h and H and their decays into bottom quarks, τ leptons and W bosons in various MSSM benchmark scenarios. Using plausible estimates for the achievable experimental efficiencies and the relevant background processes, we find that the prospective sensitivity of the diffractive Higgs production will allow one to probe interesting regions of the MA–tanβ parameter plane of the MSSM. Central exclusive production of the \(\mathcal{CP}\)-even Higgs bosons of the MSSM may provide a unique opportunity to access the bottom Yukawa couplings of the Higgs bosons up to masses of MH≲ 250 GeV. We also discuss the prospects for identifying the \(\mathcal{CP}\)-odd Higgs boson, A, in diffractive processes at the LHC.

Vacuum configurations for renormalizable non-commutative scalar models

Abstract: In this paper we find non-trivial vacuum states for the renormalizable non-commutative φ4 model. An associated linear sigma model is then considered. We further investigate the corresponding spontaneous symmetry breaking.

The Thermodynamic Universe

Abstract: Using Planck scale oscillators in the background dark energy in a model that parallels the theory of phonons, we deduce the Planck mass, the elementary particle mass scale, the mass of the Universe and a recently discovered residual energy in the cosmic background. We also deduce the Beckenstein temperature formula for black holes. Finally we show that the model explains the four minute time lag in thearrival of gamma photons from a recently observed gamma flare by the MAGIC telescope.

HELAS and MadGraph/MadEvent with spin-2 particles

Abstract: Fortran subroutines to calculate helicity amplitudes with massive spin-2 particles (massive gravitons), which couple to the standard model particles via the energy momentum tensor, are added to the HELAS (HELicity Amplitude Subroutines) library. They are coded in such a way that arbitrary scattering amplitudes with one graviton production and its decays can be generated automatically by MadGraph and MadEvent, after slight modifications. All the codes have been tested carefully by making use of the invariance of the helicity amplitudes under the gauge and general coordinate transformations.

Electroweak Precision Constraints on Vector-like Fermions

Abstract: We calculate the oblique electroweak corrections and confront them with the experiments in an extension of the standard model. The new fields added are a vector-like weak doublet and a singlet fermion. After electroweak symmetry breaking there is a mixing between the components of the new fields, but there is no mixing allowed with the standard fermions. Four electroweak parameters, $\hat{S}$ , $\hat{T}$ , W and Y, are presented in the formalism of Barbieri et al.; these are the generalization of the Peskin–Takeuchi S, T and U. The vector-like extension is slightly constrained. $\hat{T}$ requires the new neutral fermion masses not to be very far from each other, allowing for higher mass differences for higher masses and smaller mixing. $\hat{S}$ , W and Y give practically no constraints on the masses. This extension can give a positive contribution to $\hat{T}$ , allowing for a heavy Higgs boson in electroweak precision tests of the standard model.

Strong Limits on the DFSZ Axion Mass with G117-B15A

Abstract: We compute rates of period change () for the 215 s mode in G117-B15A and the 213 s mode in R548, first for models without axions, and then for models with axions of increasing mass We use the asteroseismological models for G117-B15A and R548 we derived in an earlier publication For G117-B15A, we consider two families of solutions, one with relatively thick hydrogen layers and one with thin hydrogen layers Given the region of parameter space occupied by our models, we estimate error bars on the calculated values using Monte Carlo simulations Together with the observed for G117-B15A, our analysis yields strong limits on the DFSZ axion mass Our thin hydrogen solutions place an upper limit of 135 meV on the axion, while our thick hydrogen solutions relaxes that limit to 265 meV

Supersymmetry in Elementary Particle Physics

Abstract: These lectures give a general introduction to supersymmetry, emphasizing its application to models of elementary particle physics at the 100 GeV energy scale. I discuss the following topics: the construction of supersymmetric Lagrangians with scalars, fermions, and gauge bosons, the structure and mass spectrum of the Minimal Supersymmetric Standard Model (MSSM), the measurement of the parameters of the MSSM at high-energy colliders, and the solutions that the MSSM gives to the problems of electroweak symmetry breaking and dark matter.

The K ℓ3 scalar form factors in the standard model

Abstract: We discuss the predictions of the standard model for the scalar form factors of K l3 decays. Our analysis is based on the results of chiral perturbation theory, large N c estimates of low-energy couplings and dispersive methods. It includes a discussion of isospin-violating effects of strong and electromagnetic origin.

A QCD motivated model for soft interactions at high energies

Abstract: In this paper we develop an approach to soft scattering processes at high energies which is based on two elements: the Good–Walker mechanism for low mass diffraction and multi-pomeron interactions for high mass diffraction. The principal idea, which allows us to specify the theory for pomeron interactions, is that the so called soft processes occur at rather short distances (r 2 ∝1/〈p t 〉2 ∝ α′ℙ≈0.01 GeV−2), where perturbative QCD is valid. The value of the pomeron slope α′ℙ is obtained from a fit to the experimental data. Using this theoretical approach, we suggest a model that fits all soft data in the ISR-Tevatron energy range: total, elastic, single and double diffractive cross sections, as well as the t dependence of the differential elastic cross section, and the mass dependence of single diffraction. In this model we calculate the survival probability of diffractive Higgs production, and we obtain a value for this observable that is smaller than 1% at the LHC energy range.

The "Top Priority" at the LHC

Abstract: The LHC will be a top-quark factory. With 80 million pairs of top quarks and an additional 34 million single tops produced annually at the designed high luminosity, the properties of this particle will be studied to a great accuracy. The fact that the top quark is the heaviest elementary particle in the Standard Model with a mass right at the electroweak scale makes it tempting to contemplate its role in electroweak symmetry breaking, as well as its potential as a window to unknown new physics at the TeV scale. We summarize the expectations for top-quark physics at the LHC, and outline new physics scenarios in which the top quark is crucially involved.

Discrete and continuous symmetries in multi-Higgs-doublet models

Abstract: We consider the Higgs sector of multi-Higgs-doublet models in the presence of simple symmetries relating the various fields. We construct basis-invariant observables which may in principle be used to detect these symmetries for any number of doublets. A categorization of the symmetries into classes is required, which we perform in detail for the case of two and three Higgs doublets.

A new type of CP symmetry, family replication and fermion mass hierarchies

Abstract: We study a two-Higgs doublet model with four generalised CP symmetries in the scalar sector. Electroweak symmetry breaking leads automatically to spontaneous breaking of two of them. We require that these four CP symmetries can be extended from the scalar sector to the full Lagrangian and call this requirement the principle of maximal CP invariance. The Yukawa interactions of the fermions are severely restricted by this requirement. In particular, a single fermion family cannot be coupled to the Higgs fields. For two fermion families, however, this is possible. Enforcing the absence of flavour-changing neutral currents, we find degenerate masses in both families or one family massless and one massive. In the latter case the Lagrangian is highly symmetric, with the mass hierarchy being generated by electroweak symmetry breaking. Adding a third family uncoupled to the Higgs fields and thus keeping it massless we get a model which gives a rough approximation of some features of the fermions observed in Nature. We discuss a number of predictions of the model which may be checked in future experiments at the LHC.

Six-Dimensional Gauge-Higgs Unification with an Extra Space S^2 and the Hierarchy Problem(Physics of elementary particles and fields)

Abstract: We calculate one-loop radiative correction to the mass of Higgs identified with the extra space components of the gauge field in a six-dimensional massive scalar QED compactified on a two-sphere. The radiatively induced Higgs mass is explicitly shown to be finite for arbitrary bulk scalar mass M . Furthermore, the remaining finite part also turns out to vanish, at least for the case of small M , thus suggesting that the radiatively induced Higgs mass exactly vanishes, in general. The non-zero “Kaluza–Klein” modes in the gauge sector are argued to have a Higgs-like mechanism and quantum mechanical N =2 supersymmetry, while the Higgs zero modes, as supersymmetric states, have a close relation with monopole configuration.

Weak productions of new charmonium in semileptonic decays of B c

Abstract: We study the weak productions of novel heavy mesons, such as eta'(c), h(c), h'(c), X'(c0), X (3940), Y (3940), X (3872), and Y (4260), in the semileptonic B(c) decays. Since there is still no definite answer for the components of X (3940), Y (3940), X (3872), Y (4260) so far, we will assign them as excited charmonium states with the possible quantum numbers constrained by the current experiments. As for the weak transition form factors, we calculate them in the framework of the light- cone QCD sum rules approach, which has proven to be a powerful tool to deal with the nonperturbative hadronic matrix element. Our results indicate that different interpretations of X ( 3940 ) can result in a remarkable discrepancy of the production rate in the B(c) decays, which would help to clarify the inner structure of the X ( 3940 ) with the forthcoming LHC- b experiments. Besides, the predicted large weak production rates of X ( 3872 ) and Y ( 3940 ) in B(c) decays and the small semileptonic decay rate for B(c) -> Y ( 4260 ) all depend on their quantum number J(PC) assignments. Moreover, the S - D mixing of various vector charmonium states in the weak decay of B(c) is also discussed in this work. The future experimental measurements of these decays will test the inner structures of these particles, according to our predictions here.

In medium T-matrix for nuclear matter with three-body forces - binding energy and single particle properties

Abstract: We present spectral calculations of nuclear matter properties including three-body forces. Within the in-medium T-matrix approach, implemented with the CD-Bonn and Nijmegen potentials plus the three-nucleon Urbana interaction, we compute the energy per particle in symmetric and neutron matter. The three-body forces are included via an effective density dependent two-body force in the in-medium T-matrix equations. After fine tuning the parameters of the three-body force to reproduce the phenomenological saturation point in symmetric nuclear matter, we calculate the incompressibility and the energy per particle in neutron matter. We find a soft equation of state in symmetric nuclear matter but a relatively large value of the symmetry energy. We study the the influence of the three-body forces on the single-particle properties. For symmetric matter the spectral function is broadened at all momenta and all densities, while an opposite effect is found for the case of neutrons only. Noticeable modification of the spectral functions are realized only for densities above the saturation density. The modifications of the self-energy and the effective mass are not very large and appear to be strongly suppressed above the Fermi momentum.

Power law in a microcanonical ensemble with scaling volume fluctuations

Abstract: Volume fluctuations are introduced in a statistical modeling of relativistic particle collisions. The microcanonical ensemble is used, and the volume fluctuations are assumed to have specific scaling properties. This leads to the KNO scaling of the particle multiplicity distributions as measured in p + p interactions. A striking prediction of the model is a power law form of the single particle momentum spectrum at high momenta. Moreover, the mean multiplicity of heavy particles also decreases as a function of the particle mass according to a power law. Finally, it is shown that the dependence of the momentum spectrum on the particle mass and momentum reduces to the dependence on the particle energy. These results resemble the properties of particle production in collisions of high energy particles.

The η ′ meson from lattice QCD

Abstract: We study the flavour-singlet pseudoscalar mesons from first principles using lattice QCD. With Nf=2 flavours of light quarks, this is the so-called η2 meson and we discuss the phenomenological status of this meson. Using maximally twisted-mass lattice QCD, we extract the mass of the η2 meson at two values of the lattice spacing for lighter quarks than previously discussed in the literature. We are able to estimate the mass value in the limit of light quarks with their physical masses.

The "top Priority" at the Lhc

Abstract: The LHC (Large Hadron Collider) will be a top-quark factory. With 80 million pairs of top quarks and an additional 34 million single tops produced annually at the designed high luminosity, the properties of this particle will be studied to a great accuracy. The fact that the top quark is the heaviest elementary particle in the Standard Model with a mass right at the electroweak scale makes it tempting to contemplate its role in electroweak symmetry breaking, as well as its potential as a window to unknown new physics at the TeV scale. We summarize the expectations for top-quark physics at the LHC, and outline new physics scenarios in which the top quark is crucially involved.