13061892
Rapidity-separation dependence and the large next-to-leading corrections to the BFKL equation
Recent concerns about the very large next-to-leading logarithmic (NLL) corrections to the BFKL equation are addressed by the introduction of a physical rapidity-separation parameter $\ensuremath{\Delta}.$ At the leading logarithm (LL) this parameter enforces the constraint that successive emitted gluons have a minimum separation in rapidity, ${y}_{i+1}\ensuremath{-}{y}_{i}g\ensuremath{\Delta}.$ The most significant effect is to reduce the BFKL Pomeron intercept from the standard result as $\ensuremath{\Delta}$ is increased from 0 (standard BFKL). At NLL this $\ensuremath{\Delta}$-dependence is compensated by a modification of the BFKL kernel, such that the total dependence on $\ensuremath{\Delta}$ is formally next-to-next-to-leading logarithmic. In this formulation, as long as $\ensuremath{\Delta}\ensuremath{\gtrsim}2.2$ (for ${\ensuremath{\alpha}}_{s}=0.15),$ (i) the NLL BFKL Pomeron intercept is stable with respect to variations of $\ensuremath{\Delta},$ and (ii) the NLL correction is small compared to the LL result. Implications for the applicability of the BFKL resummation to phenomenology are considered.
17634589
Radiative electroweak symmetry breaking in a Little Higgs model
We present a new Little Higgs model, motivated by the deconstruction of a five-dimensional gauge-Higgs model. The approximate global symmetry is SO(5)0 × SO(5)1, breaking to SO(5), with a gauged subgroup of [SU(2)0L × U(1)0R] × O(4)1, breaking to SU(2)L ×U(1)Y . Radiative corrections produce an additional small vacuum misalignment, breaking the electroweak symmetry down to U(1)EM. Novel features of this model are: the only un-eaten pseudo-Goldstone boson in the effective theory is the Higgs boson; the model contains a custodial symmetry, which ensures that $ \widehat{T} = 0 $ at tree-level; and the potential for the Higgs boson is generated entirely through one-loop radiative corrections. A small negative mass-squared in the Higgs potential is obtained by a cancellation between the contribution of two heavy partners of the top quark, which is readily achieved over much of the parameter space. We can then obtain both a vacuum expectation value of v = 246 GeV and a light Higgs boson mass, which is strongly correlated with the masses of the two heavy top quark partners. For a scale of the global symmetry breaking of f = 1 TeV and using a single cutoff for the fermion loops, the Higgs boson mass satisfies 120 GeV ≲ MH ≲ 150 GeV over much of the range of parameter space. For f raised to 10 TeV, these values increase by about 40 GeV. Effects at the ultraviolet cutoff scale may also raise the predicted values of the Higgs boson mass, but the model still favors MH ≲ 200GeV.
118429632
Higgs Boson Cross Section from CTEQ-TEA Global Analysis
We study the uncertainties of the Higgs boson production cross section through the gluon fusion subprocess at the LHC (with $\sqrt s=7, 8$ and $14$ TeV) arising from the uncertainties of the parton distribution functions (PDFs) and of the value of the strong coupling constant $\alpha_s(M_Z)$. These uncertainties are computed by two complementary approaches, based on the Hessian and the Lagrange Multiplier methods within the CTEQ-TEA global analysis framework. We find that their predictions for the Higgs boson cross section are in good agreement. Furthermore, the result of the Lagrange Multiplier method supports the prescriptions we have previously provided for using the Hessian method to calculate the combined PDF and $\alpha_s$ uncertainties, and to estimate the uncertainties at the $68\%$ confidence level by scaling them from the $90\%$ confidence level.
17272111
Effects of fermion localization in Higgsless theories and electroweak constraints
Abstract Extra-dimensional Higgsless models with electroweak symmetry breaking through boundary conditions generically have difficulties with electroweak precision constraints, when the fermions are localized to the “branes” in the fifth dimension. In this Letter we show that these constraints can be relaxed by allowing the light fermions to have a finite extent into the bulk of the fifth dimension. The T and U electroweak parameters can be naturally suppressed by a custodial symmetry, while the S parameter can be made to vanish through a cancellation, if the leakage into the bulk of the light gauge fields and the light left-handed fermion fields are of the same size. This cancellation is possible while allowing realistic values for the first two generations of fermion masses, although special treatment is probably required for the top quark. We present this idea here in the context of a specific continuum theory-space model; however, it can be applied to any five-dimensional Higgsless model, either with a flat or a warped background.
17090149
Higgsless electroweak symmetry breaking from theory space
We investigate unitarity of W+W− scattering in the context of theory space models of the form U(1) × [SU(2)]N × SU(2)N+1, which are broken down to U(1)EM by non-linear Σ fields, without the presence of a physical Higgs Boson. By allowing the couplings of the U(1) and the final SU(2)N+1 to vary, we can fit the W and Z masses, and we find that the coefficient of the term in the amplitude that grows as E2/mW2 at high energies is suppressed by a factor of (N+1)−2. In the N+1→∞ limit the model becomes a 5-dimensional SU(2) gauge theory defined on an interval, where boundary terms at the two ends of the interval break the SU(2) down to U(1)EM. These boundary terms also modify the Kaluza-Klein (KK) mass spectrum, so that the lightest KK states can be identified as the W and Z bosons. The T parameter, which measures custodial symmetry breaking, is naturally small in these models. Depending on how matter fields are included, the strongest experimental constraints come from precision electroweak limits on the S parameter.
6678978
Ultrahigh-energy neutrino–nucleon cross sections and perturbative unitarity
Abstract Unitarity relates the total cross section for neutrino–nucleon scattering to the neutrino–nucleon forward scattering amplitude. Assuming the validity of the perturbative expansion of the forward amplitude in the weak coupling constant, we derive a unitarity bound on the inelastic cross section. The inelastic cross section saturates this bound at a typical neutrino energy E ν ≃10 8  GeV. This implies that calculations of the inelastic cross section that use current parton distribution functions and lowest order weak perturbation theory are unreliable above this energy.
118327970
Universal O(N) scaling and the chiral critical line in (2+1)-flavor QCD with small chemical potentials
We show that for small values of the chemical potential the curvature of the phase transition line can be deduced from an analysis of scaling properties of the chiral condensate and its susceptibilities. We make use of a recent analysis of the magnetic equation of state in (2+1)-flavor QCD where a connection between the QCD parameters and the universal scaling fields could be established. The remaining dependence of the reduced temperature on the chemical potential can be fixed by an analysis of a mixed susceptibility, obtained from a derivative with respect to quark mass and chemical potential. We extract this dependence which describes the curvature of the phase transition line, at two values of the cut-off, $aT=1/4$ and $1/8$. We find that cut-off effects are small for the curvature parameter and determine the transition line in the chiral limit to leading order in the light quark chemical potential. We obtain $T_c(\mu_B)/T_c(0) = 1 - 0.00656(66) (\mu_B/T)^2 +{\cal O}(\mu_B^4)$.
110755788
Fermilab linac injector, revisited
The Fermilab Linac has been operating at 200 MeV since 1971 and was upgraded to 400 MeV in 1993 with a peak current of /spl sim/35 mA. The injector consists of a Haefely Cockcroft-Walton, H/sup -/ source, an high gradient accelerating column (750 kV), 4 and 10 meter long transfer lines and a single gap 201.24 MHz cavity. The performance of the injector has been very gratifying although some tuning must be done approximately daily. The purpose of this report is to present representative experimental data and numerical simulations that are used as a guide for improvements to the Fermilab Linac. Today, the Fermilab Linac delivers a 400 MeV H/sup -/ beam with a peak current over 45 mA.
118820620
Magnetic field measurement and compensation in the Recycler Electron Cooler
Cooling of 8.9‐GeV/c antiprotons in the Recycler Electron Cooler requires a round 4.34‐MeV electron beam with a small angular spread propagating through a 20‐m long cooling section. To confine the electron beam tightly and to keep its total transverse angles below 0.2 mrad the cooling section is immersed in a solenoidal field of 50–200 G. The field was measured with a compass‐based sensor (transversal) and a hall‐probe (longitudinal) after installation of the solenoids into the Recycler tunnel. For the field strength of 105 G, the transverse field components were compensated to the level that provided corresponding dipole beam oscillations below 0.1 mrad, which in turn allowed the first cooling of antiprotons in the GeV energy range. This paper discusses the field measurements and compensation scheme including the results of dipole oscillation measurements.
32943617
Measuring the Aperture and Locating Beam Obstructions in the Fermilab Accelerator
A method which uses the coasting beam at the injection energy to scan the aperture in a large accelerator such as the Main Accelerator at Fermilab will be described. This scanning procedure serves two purposes. It provides a record of the aperture size around the entire accelerator which can be used for future reference and comparison. And it is highly useful in locating beam obstructions which may be caused by a collapsed vacuum chamber or some small object in the chamber to within a few meters. A high degree of accuracy has been achieved in finding and removing even very small obstructions. This method uses several computer programs, the main accelerator correction dipoles and beam position detectors, and some simple calculations.
119007148
Conserved charge fluctuations with HISQ fermions
We calculate cumulants of fluctuations of net-baryon number, net-electric charge and net-strangeness, in the framework of lattice regularized QCD. We use a highly improved staggered quark (HISQ) action on lattices with temporal extent of N_tau=6,8 and 12 and almost physical quark masses. By means of a Taylor expansion in various chemical potentials and under demanding both strangeness neutrality as well as the correct isospin asymmetry, we evaluate these fluctuations at conditions met in heavy ion collisions. Cumulants of net-electric charge fluctuations can, in principle, also be measured in heavy ion experiments. We therefore propose a method to extract freeze-out parameters, such as the freeze-out temperature and baryon chemical potential, based on a comparison of lattice results and experimental measurements of two different ratios of net-electric charge cumulants. As this method involves only (lattice) QCD results and experimental measurements it is model independent and, for the purpose of extracting freeze-out parameters, does not require any input from hadron resonance gas model calculations.