Showing papers on "Elementary particle published in 2010"

Review of Particle Physics

Abstract: This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2158 new measurements from 551 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on neutrino mass, mixing, and oscillations, QCD, top quark, CKM quark-mixing matrix, V-ud & V-us, V-cb & V-ub, fragmentation functions, particle detectors for accelerator and non-accelerator physics, magnetic monopoles, cosmological parameters, and big bang cosmology.

Two-loop soft anomalous dimensions for single top quark associated production with a W- or H-

Abstract: I present results for the two-loop soft anomalous dimensions for associated production of a single top quark with a W boson or a charged Higgs boson. The calculation uses expressions for the massive cusp anomalous dimension, which are presented in different forms, and it allows soft-gluon resummation at next-to-next-to-leading-logarithm (NNLL) accuracy. From the NNLL resummed cross section I derive approximate NNLO cross sections for bg{yields}tW{sup -} and bg{yields}tH{sup -} at LHC energies of 7, 10, and 14 TeV.

New model of inflation with nonminimal derivative coupling of standard model Higgs boson to gravity.

Abstract: In this Letter we show that there is a unique nonminimal derivative coupling of the standard model Higgs boson to gravity such that it propagates no more degrees of freedom than general relativity sourced by a scalar field, reproduces a successful inflating background within the standard model Higgs parameters, and finally does not suffer from dangerous quantum corrections.

Spin determination of single-produced resonances at hadron colliders

Abstract: We study the production of a single resonance at the LHC and its decay into a pair of Z bosons. We demonstrate how full reconstruction of the final states allows us to determine the spin and parity of the resonance and restricts its coupling to vector gauge bosons. Full angular analysis is illustrated with the simulation of the production and decay chain including all spin correlations and the most general couplings of spin-zero, -one, and -two resonances to Standard Model matter and gauge fields. We note implications for analysis of a resonance decaying to other final states.

Fat jets for a light higgs boson.

Abstract: At the LHC associated top quark and Higgs boson production with a Higgs boson decay to bottom quarks has long been a heavily disputed search channel. Recently, it has been found not to be viable. We show how it can be observed by tagging massive Higgs bosons and top jets. For this purpose we construct boosted top and Higgs taggers for standard-model processes in a complex QCD environment.

Chiral three-nucleon forces and neutron matter

Abstract: We calculate the properties of neutron matter and highlight the physics of chiral three-nucleon forces. For neutrons, only the long-range $2\ensuremath{\pi}$-exchange interactions of the leading chiral three-nucleon forces contribute, and we derive density-dependent two-body interactions by summing the third particle over occupied states in the Fermi sea. Our results for the energy suggest that neutron matter is perturbative at nuclear densities. We study in detail the theoretical uncertainties of the neutron matter energy, provide constraints for the symmetry energy and its density dependence, and explore the impact of chiral three-nucleon forces on the $S$-wave superfluid pairing gap.

Higgs production via vector-boson fusion at NNLO in QCD

Abstract: We present the total cross sections at next-to-next-to-leading order in the strong coupling for Higgs boson production via weak-boson fusion. Our results are obtained via the structure function approach, which builds upon the approximate, though very accurate, factorization of the QCD corrections between the two quark lines. The theoretical uncertainty on the total cross sections at the LHC from higher order corrections and the parton distribution uncertainties are estimated at the 2% level each for a wide range of Higgs boson masses.

Three-loop static potential.

Abstract: We compute the three-loop corrections to the potential of two heavy quarks. In particular, we consider in this Letter the purely gluonic contribution which provides, in combination with our previous fermion corrections, the complete answer at three loops.

Recoil Polarization Measurements of the Proton Electromagnetic Form Factor Ratio to Q^2 = 8.5 GeV^2

Abstract: Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon’s quark constituents; indeed, recent proton data have attracted intense theoretical interest In this Letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers Q2=52, 67, and 85 GeV2 By extending the range of Q2 for which GEp is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the nonperturbative regime

Dominant QCD backgrounds in Higgs boson analyses at the LHC: a study of pp --> tt + 2 jets at next-to-leading order.

Abstract: We report the results of a next-to-leading order simulation of top quark pair production in association with two jets. With our inclusive cuts, we show that the corrections with respect to leading order are negative and small, reaching 11%. The error obtained by scale variation is of the same order. Additionally, we reproduce the result of a previous study of top quark pair production in association with a single jet.

Higgs production in gluon fusion at next-to-next-to-leading order QCD for finite top mass

Abstract: The inclusive Higgs production cross section from gluon fusion is calculated through NNLO QCD, including its top quark mass dependence. This is achieved through a matching of the 1/M t expansion of the partonic cross sections to the exact large- $\hat{s}$ limits which are derived from k T -factorization. The accuracy of this procedure is estimated to be better than 1% for the hadronic cross section. The final result is shown to be within 1% of the commonly used effective theory approach, thus confirming earlier findings.

Heavy-light meson spectroscopy and Regge trajectories in the relativistic quark model

Abstract: The masses of the ground, orbitally and radially excited states of heavy-light mesons are calculated within the framework of the QCD-motivated relativistic quark model based on the quasipotential approach. Both light (q=u,d,s) and heavy (Q=c,b) quarks are treated fully relativistically without application of the heavy-quark 1/m Q expansion. The Regge trajectories in the (M 2,J) and (M 2,n r ) planes are investigated and their parameters are obtained. The results are in good agreement with available experimental data except for the masses of the anomalous $D^{*}_{s0}(2317)$ , D s1(2460) and $D_{sJ}^{*}(2860)$ states.

Higgs boson look-alikes at the LHC

Abstract: The discovery of a Higgs particle is possible in a variety of search channels at the LHC. However, the true identity of any putative Higgs boson will, at first, remain ambiguous until one has experimentally excluded other possible assignments of quantum numbers and couplings. We quantify the degree to which one can discriminate a standard model Higgs boson from “look-alikes” at, or close to, the moment of discovery at the LHC. We focus on the fully-reconstructible golden decay mode to a pair of Z bosons and a four-lepton final state. Considering both on-shell and off-shell Z’s, we show how to utilize the full decay information from the events, including the distributions and correlations of the five relevant angular variables. We demonstrate how the finite phase space acceptance of any LHC detector sculpts the decay distributions, a feature neglected in previous studies. We use likelihood ratios to discriminate a standard model Higgs from look-alikes with other spins or nonstandard parity, CP, or form factors. For a resonance mass of 200 GeV/c^2, we achieve a median discrimination significance of 3σ with as few as 19 events, and even better discrimination for the off-shell decays of a 145 GeV/c^2 resonance.

Combination of Tevatron searches for the standard model Higgs boson in the W+W- decay mode

Abstract: We combine searches by the CDF and D0 Collaborations for a Higgs boson decaying to W+W-. The data correspond to an integrated total luminosity of 4.8 (CDF) and 5.4 (D0) fb(-1) of p (p) over bar collisions at root s = 1.96 TeV at the Fermilab Tevatron collider. No excess is observed above background expectation, and resulting limits on Higgs boson production exclude a standard model Higgs boson in the mass range 162-166 GeV at the 95% C.L.

Quirky composite dark matter

Abstract: We propose a new dark matter candidate, 'quirky dark matter', that is a scalar baryonic bound state of a new non-Abelian force that becomes strong below the electroweak scale. The bound state is made of chiral quirks: new fermions that transform under both the new strong force as well as in a chiral representation of the electroweak group, acquiring mass from the Higgs mechanism. Electric charge neutrality of the lightest baryon requires approximately degenerate quirk masses which also causes the charge radius of the bound state to be negligible. The abundance is determined by an asymmetry that is linked to the baryon and lepton numbers of the universe through electroweak sphalerons. Dark matter elastic scattering with nuclei proceeds through Higgs exchange as well as an electromagnetic polarizability operator which is just now being tested in direct detection experiments. A novel method to search for quirky dark matter is to look for a gamma-ray 'dark line' spectroscopic feature in galaxy clusters that result from the quirky Lyman-alpha or quirky hyperfine transitions. Colliders are expected to dominantly produce quirky mesons, not quirky baryons, consequently large missing energy is not the primary collider signal of the physics associated with quirky dark matter.

Spontaneous Symmetry Breaking and Nambu-Goldstone Bosons in Quantum Many-Body Systems

Abstract: Spontaneous symmetry breaking is a general principle that constitutes the underlying concept of a vast number of physical phenomena ranging from ferromagnetism and superconductivity in condensed matter physics to the Higgs mechanism in the standard model of elementary particles. I focus on manifestations of spontaneously broken symmetries in systems that are not Lorentz invariant, which include both nonrelativistic systems as well as relativistic systems at nonzero density, providing a self-contained review of the properties of spontaneously broken symmetries specific to such theories. Topics covered include: (i) Introduction to the mathematics of spontaneous symmetry breaking and the Goldstone theorem. (ii) Minimization of Higgs-type potentials for higher-dimensional representations. (iii) Counting rules for Nambu–Goldstone bosons and their dispersion relations. (iv) Construction of effective Lagrangians. Specific examples in both relativistic and nonrelativistic physics are worked out in detail.

Wilson Fermions and Axion Electrodynamics in Optical Lattices

Abstract: We show that ultracold Fermi gases in optical superlattices can be used as quantum simulators of relativistic lattice fermions in 3+1 dimensions. By exploiting laser-assisted tunneling, we find an analogue of the so-called naive Dirac fermions, and thus provide a realization of the fermion doubling problem. Moreover, we show how to implement Wilson fermions, and discuss how their mass can be inverted by tuning the laser intensities. In this regime, our atomic gas corresponds to a phase of matter where Maxwell electrodynamics is replaced by axion electrodynamics: a 3D topological insulator.

Helicity fractions of W bosons from top quark decays at next-to-next-to-leading order in QCD

Abstract: Decay rates of unpolarized top quarks into longitudinally and transversally polarized W bosons are calculated to second order in the strong coupling constant {alpha}{sub s}. Including the finite bottom quark mass and electroweak effects, the standard model predictions for the W-boson helicity fractions are F{sub L}=0.687(5), F{sub +}=0.0017(1), and F{sub -}=0.311(5).

Anomalous quartic WWγγ, ZZγγ, and trilinear WWγ couplings in two-photon processes at high luminosity at the LHC

Abstract: We study the W/Z pair production via two-photon exchange at the LHC and give the sensitivities on trilinear and quartic gauge anomalous couplings between photons and W/Z bosons for an integrated luminosity of 30 and 200 fb{sup -1}. For simplicity and to obtain lower backgrounds, only the leptonic decays of the electroweak bosons are considered.

Explorations of the top quark forward-backward asymmetry at the Tevatron

Abstract: We consider the recent measurement of the top quark forward-backward asymmetry at the Fermilab Tevatron, which shows a discrepancy of slightly more than $2\ensuremath{\sigma}$ compared to the standard model prediction. We find that $t$-channel exchange of a color sextet or triplet scalar particle can explain the measurement, while leaving the cross section for $t\overline{t}$ production within measured uncertainties. Such particles have good discovery prospects by study of the kinematic structure of $t\overline{t}+\mathrm{\text{jets}}$ at the LHC.

Penning traps as a versatile tool for precise experiments in fundamental physics

Abstract: This review article describes the trapping of charged particles. The main principles of electromagnetic confinement of various species from elementary particles to heavy atoms are briefly described. The preparation and manipulation with trapped single particles, as well as methods of frequency measurements, providing unprecedented precision, are discussed. Unique applications of Penning traps in fundamental physics are presented. Ultra-precise trap-measurements of masses and magnetic moments of elementary particles (electrons, positrons, protons and antiprotons) confirm CPT-conservation, and allow accurate determination of the fine-structure constant α and other fundamental constants. This together with the information on the unitarity of the quark-mixing matrix, derived from the trap-measurements of atomic masses, serves for assessment of the Standard Model of the physics world. Direct mass measurements of nuclides targeted to some advanced problems of astrophysics and nuclear physics are also presented.

Diffractive electroproduction of \rho and \phi mesons at HERA

Abstract: Diffractive electroproduction of rho and phi mesons is measured at HERA with the H1 detector in the elastic and proton dissociative channels. The data correspond to an integrated luminosity of 51 pb^-1. About 10500 rho and 2000 phi events are analysed in the kinematic range of squared photon virtuality 2.5 < Q^2 < 60 GeV^2, photon-proton centre of mass energy 35 < W < 180 GeV and squared four-momentum transfer to the proton |t| < 3 GeV^2. The total, longitudinal and transverse cross sections are measured as a function of Q^2, W and |t|. The measurements show a transition to a dominantly "hard" behaviour, typical of high gluon densities and small q\bar{q} dipoles, for Q^2 larger than 10 to 20 GeV^2. They support flavour independence of the diffractive exchange, expressed in terms of the scaling variable (Q^2 + M_V^2)/4, and proton vertex factorisation. The spin density matrix elements are measured as a function of kinematic variables. The ratio of the longitudinal to transverse cross sections, the ratio of the helicity amplitudes and their relative phases are extracted. Several of these measurements have not been performed before and bring new information on the dynamics of diffraction in a QCD framework. The measurements are discussed in the context of models using generalised parton distributions or universal dipole cross sections.

A Unitarity-Conserving Higgs Inflation Model

Abstract: Scalar field models of inflation based on a large nonminimal coupling to gravity $\ensuremath{\xi}$, in particular, Higgs inflation, may violate unitarity at an energy scale $\ensuremath{\Lambda}\ensuremath{\sim}{M}_{p}/\ensuremath{\xi}\ensuremath{\ll}{M}_{p}$ In this case the model is incomplete at energy scales relevant to inflation Here we propose a new unitarity-conserving model of Higgs inflation The completion of the theory is achieved via additional interactions which are proportional to products of the derivatives of the Higgs doublet The resulting model differs from the original version of Higgs inflation in its prediction for the spectral index, with a classical value $n=0974$ In the case of a nonsupersymmetric model, quantum corrections are likely to strongly modify the tree-level potential, suggesting that supersymmetry or a gauge singlet scalar inflaton is necessary for a completely successful model

Confinement of fractional quantum number particles in a condensed-matter system

Abstract: The concept of confinement states that in certain systems the constituent particles are bound together by an interaction for which the strength increases with increasing particle separation. One of the consequences of this is that these individual particles cannot be observed directly. The most famous example of confinement is found in particle physics where baryons and mesons are produced by the confinement of quarks. However, similar phenomena can occur in condensed-matter physics systems such as spin ladders that consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S=1/2, are confined by even an infinitesimal interchain coupling. Most ladders studied so far cannot illustrate this process because the large strength of their interchain coupling suppresses the spinon excitations at all energy scales. Here we present neutron scattering experiments for a weakly coupled ladder material. At high energies the behaviour of this system approaches that of individual chains, but at low energies it is dominated by the integral spin excitations of strongly coupled chains. The composition of integral quantum number particles such as protons and neutrons from the strong confinement of fractional quantum number particles such as quarks is well known in high-energy physics. Now, similar behaviour has been found in condensed-matter physics, in the excitation spectra of a weakly coupled spin-ladder compound.

Vacuum Properties of Mesons in a Linear Sigma Model with Vector Mesons and Global Chiral Invariance

Abstract: We present a two-flavor linear sigma model with global chiral symmetry and vector and axial-vector mesons. We calculate {pi}{pi} scattering lengths and the decay widths of scalar, vector, and axial-vector mesons. It is demonstrated that vector and axial-vector meson degrees of freedom play an important role in these low-energy processes and that a reasonable theoretical description requires globally chirally invariant terms other than the vector-meson mass term. An important question for meson vacuum phenomenology is the quark content of the physical scalar f{sub 0}(600) and a{sub 0}(980) mesons. We investigate this question by assigning the quark-antiquark {sigma} and a{sub 0} states of our model with these physical mesons. We show via a detailed comparison with experimental data that this scenario can describe all vacuum properties studied here except for the decay width of the {sigma}, which turns out to be too small. We also study the alternative assignment f{sub 0}(1370) and a{sub 0}(1450) for the scalar mesons. In this case the decay width agrees with the experimental value, but the {pi}{pi} scattering length a{sub 0}{sup 0} is too small. This indicates the necessity to extend our model by additional scalar degrees of freedom.

Nuclear physics from lattice QCD at strong coupling.

Abstract: We study numerically the strong coupling limit of lattice QCD with one flavor of massless staggered quarks. We determine the complete phase diagram as a function of temperature and chemical potential, including a tricritical point. We clarify the nature of the low temperature dense phase, which is strongly bound ``nuclear'' matter. This strong binding is explained by the nuclear potential, which we measure. Finally, we determine, from this first-principles limiting case of QCD, the masses of ``atomic nuclei'' up to $A=12$ ``carbon''.

The impact of PDF and alphas uncertainties on Higgs Production in gluon fusion at hadron colliders

Abstract: We present a systematic study of uncertainties due to parton distributions (PDFs) and the strong coupling on the gluon-fusion production cross section of the standard model Higgs at the Tevatron and LHC colliders. We compare procedures and results when three recent sets of PDFs are used, CTEQ6.6, MSTW08, and NNPDF1.2, and we discuss specifically the way PDF and strong coupling uncertainties are combined. We find that results obtained from different PDF sets are in reasonable agreement if a common value of the strong coupling is adopted. We show that the addition in quadrature of PDF and ${\ensuremath{\alpha}}_{s}$ uncertainties provides an adequate approximation to the full result with exact error propagation. We discuss a simple recipe to determine a conservative $\mathrm{PDF}+{\ensuremath{\alpha}}_{s}$ uncertainty from available global parton sets, and we use it to estimate this uncertainty on the given process to be about 10% at the Tevatron and 5% at the LHC for a light Higgs.

Phenomenology of a light scalar: the dilaton

Abstract: We make use of the language of nonlinear realizations to analyze electroweak symmetry breaking scenarios in which a light dilaton emerges from the breaking of a nearly conformal strong dynamics and compare the phenomenology of the dilaton to that of the well-motivated light composite Higgs scenario. We argue that - in addition to departures in the decay/production rates into massless gauge bosons mediated by the conformal anomaly - characterizing features of the light dilaton scenario are enhancements in off-shell events at high invariant mass involving two longitudinally polarized vector bosons and a dilaton, and tree-level flavor violating processes. Accommodating both electroweak precision measurements and flavor constraints appears especially challenging in the ambiguous scenario in which the Higgs and the dilaton fields strongly mix. We show that warped higgsless models of electroweak symmetry breaking are explicit and tractable realizations of this limiting case. The relation between the naive radion profile often adopted in the study of holographic realizations of the light dilaton scenario and the actual dynamical dilaton field is clarified in the Appendix.

Discovering the Higgs Boson in new physics events using jet substructure.

Abstract: We demonstrate that the Higgs boson can be discovered at the large hadron collider in new physics events using boosted kinematics, b-tagging and jet substructure. This method superbly identifies the lightest Higgs boson in the minimal supersymmetric standard model. Two case studies are considered where Higgs is produced in association with superpartners that decay to a light gravitino, however generalizations to other spectra and models is anticipated. In some circumstances, discovery of the lightest Higgs is possible before conventional search strategies uncover convincing evidence.

Higgs boson pair production in new physics models at hadron, lepton, and photon colliders

Abstract: We study Higgs boson pair production processes at future hadron and lepton colliders including the photon collision option in several new physics models; i.e., the two-Higgs-doublet model, the scalar leptoquark model, the sequential fourth generation fermion model and the vectorlike quark model. Cross sections for these processes can deviate significantly from the standard model predictions due to the one-loop correction to the triple Higgs boson coupling constant. For the one-loop induced processes such as $gg\ensuremath{\rightarrow}hh$ and $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}hh$, where $h$ is the (lightest) Higgs boson and $g$ and $\ensuremath{\gamma}$ respectively represent a gluon and a photon, the cross sections can also be affected by new physics particles via additional one-loop diagrams. In the two-Higgs-doublet model and scalar leptoquark models, cross sections of ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}hhZ$ and $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}hh$ can be enhanced due to the nondecoupling effect in the one-loop corrections to the triple Higgs boson coupling constant. In the sequential fourth generation fermion model, the cross section for $gg\ensuremath{\rightarrow}hh$ becomes very large because of the loop effect of the fermions. In the vectorlike quark model, effects are small because the theory has decoupling property. Measurements of the Higgs boson pair production processes can be useful to explore new physics through the determination of the Higgs potential.