122794487
Status of the Fermilab electron cooling project
Abstract A prototype of a 4.3-MeV electron cooling system has been assembled at Fermilab as part of the on-going R&D program in high-energy electron cooling. This electron cooler prototype will not demonstrate the actual cooling but it would allow to determine if the electron beam properties are suitable for antiproton beam cooling. An electron beam is accelerated by a 5-MV Pelletron (Van de Graaff type) accelerator and transported to a prototype cooling section. The cooling would take place in a 20-m long solenoid flanked on both sides by supply and return beam-lines—a total of 60 m of transport channel. This paper describes the status of the electron cooling R&D program.
119968235
Direct current surface plasma source with high emission current density
A surface plasma source for long-time dc operation with efficient production of negative ions and high emission current density has been designed and tested. Several modifications of the discharge system, beam extraction, cesium supply, and source design have been tested for long-time operation. The emission current density of H− ions has been increased to j=1 A/cm2 in the dc mode of operation with a discharge voltage, Ud=80 V; a discharge current, Id=0.9 A and a discharge power, P=70 W. Corresponding efficiency of the current density production S=j/P=15 A/cm2 kW is much higher than in modern positive ion sources. Source features determining an operational lifetime will be discussed.
118674965
Operation and improvements of the Fermilab 400 MeV Linac
The 400 MeV Fermilab Linac Upgrade commissioning began August 28, 1993. High energy physics collider operation (run 1b) began in November 1993 and ended March 1, 1996. The Linac, operating at 98% reliability, provided 400 MeV H{sup -} beam to the Booster and 66 MeV H{sup -} beam to the Neutron Therapy Facility. During this time, the beam intensity, which initially was administratively set to 35 MA, rose to a peak of 50 mA while losses decreased significantly. This paper discusses the Linac operation and reliability since the Upgrade.
723137
Status of the Fermilab electron cooling project
The first stage of the Fermilab Electron Cooling R&D program is now complete: technology necessary to generate hundreds of milliamps of electron beam current at MeV energies has been demonstrated. Conceptual design studies show that with an electron beam current of 200 mA and with a cooling section of 20 m electron cooling in the Fermilab Recycler ring can provide antiproton stacking rates suitable for the Tevatron upgrades beyond Run II luminosity goals. A prototype of such an electron cooling system is now being built at Fermilab as part of the continuing R&D program. This paper describes the electron cooling system design as well as the status of the Fermilab electron cooling R&D program.
121242219
Attainment of an MeV-range, DC electron beam for the Fermilab cooler
To prepare a beam generation device for Fermilab's future Recycler Electron Cooling system, an experimental set-up with a simplified beam line has been commissioned at Fermilab. Stable operation was achieved at an electron energy of 3.5 MeV and a DC beam current of up to 0.5 A. The main reason for interruptions of the operation was found to be microsecond long bursts of the cathode current. While the frequency of the interruptions is determined primarily by a flow of secondary ions, the resulting reduction in the duty factor depends on the beam optics, the protection systems, and the tube electric field strength.
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.
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.
859441
A Monte Carlo Solution to the BFKL Equation
A simple solution to the BFKL equation is obtained as a series in the number of real gluons emitted with transverse momentum greater than some small cutoff μ. This solution reveals physics inside the BFKL ladder which is hidden in the standard inclusive solution, and lends itself to a straightforward Monte Carlo implementation. With this approach one can explore new useful physical observables, which are shown to be independent of the cutoff μ. In addition, this approach allows the imposition of kinematic constraints (such as energy conservation) which are important at finite energies. The distribution of ∑ E⊥ of particles in a central rapidity bin between two widely-spaced jets is presented as an example. The BFKL (Balitsky-Fadin-Kuraev-Lipatov) equation [1], which systematically resums powers of αs times large rapidity intervals or logarithms of Feynman x in perturbative QCD, has recently moved from the purely theoretical realm to the phenomenological arena. The classic prediction of the BFKL resummation is the rise of F2(x) at small x in deep inelastic scattering. Unfortunately, due to the resiliency of the parton density functions and DGLAP [2] evolution equations, this observation [3, 4] is still open to interpretation [5]. Alternatively, by tagging jets on both ends of a large rapidity interval, as suggested by Mueller and Navalet [6], it is possible to unambiguously isolate the effects of the BFKL ladder from the parton density functions. Predictions of this kind include the rise in the cross section as a function of the rapidity interval, both in hadron-hadron [6] and leptonhadron colliders [7]. In particular, recent preliminary results from H1 [3] on deep inelastic scattering with a tagged jet at fixed xj show an intriguing rise in the cross section with decreasing xBJ that is not well-explained either by a fixed-order matrix element calculation [8] or by parton shower Monte Carlo simulations. The BFKL prediction [9], however, seems to be in good agreement. Other observables related to the kinematics of the tagging jets, such as the decorrelation in azimuthal angle [10], have also been considered. With the advent of BFKL phenomenology, it has become a necessity to understand the range of validity and the errors inherent in the BFKL approximations. The calculation of the next-to-leading logarithmic (NLL) corrections to the BFKL matrix elements is currently under way [11]. Short of these full NLL corrections, it has been shown in [12] that some of the largest corrections to the asymptotic the-
13413480
The chiral critical point in 3-flavour QCD
Abstract We determine the second order endpoint of the line of first order phase transitions, which occur in the light quark mass regime of 3-flavour QCD at finite temperature, and analyze universal properties of this chiral critical point. A detailed analysis of Binder cumulants and the joint probability distributions of energy like and ordering-field like observables confirms that the chiral critical point belongs to the universality class of the three-dimensional Ising model. From a calculation with improved gauge and staggered fermion actions we estimate that the transition is first order for pseudo-scalar meson masses less than about 200 MeV.
15521004
The Di-Photon Background to a Light Higgs Boson at the LHC
Recent years have seen a significant advance in our ability to calculate two-loop matrix elements. In this talk we describe an application of this breakthrough to improve our understanding of the background to the search for a light Higgs boson at the LHC. In particular, we focus on the QCD corrections to the gluon fusion subprocess gg to gamma gamma, which forms an important component of the background in the di-photon channel. We find that the K factor for this subprocess is significantly smaller than estimated previously.
5495552
The infrared behavior of one-loop gluon amplitudes at next-to-next-to-leading order
For the case of n-jet production at next-to-next-to-leading order in the QCD coupling, in the infrared divergent corners of phase space where particles are collinear or soft, one must evaluate (n + 1)-parton final-state one-loop amplitudes through O(ǫ2), where ǫ is the dimensional regularization parameter. For the case of gluons, we present to all orders in ǫ the required universal functions which describe the behavior of one-loop amplitudes in the soft and collinear regions of phase space. An explicit example is discussed for three-parton production in multi-Regge kinematics that has applications to the next-to-leading logarithmic corrections to the BFKL equation. ∗On leave of absence from I.N.F.N., Sezione di Torino, Italy. The quest to obtain ever increasing precision in perturbative QCD requires calculations to higher orders in the QCD coupling, αs. Over the years, significant effort has been expended on computing the next-to-leading order (NLO) contributions to multi-jet rates within perturbative QCD. An important next step in the endeavor to obtain higher precision would be the computation of next-to-next-to-leading-order (NNLO) contributions to multi-jet rates. As an example, although the NLO contributions to ee → 3 jets have been computed for some time now [1, 2], the NNLO contributions have yet to be obtained. A calculation of these NNLO contributions would be needed to further reduce the theoretical uncertainty in the determination of αs from event shape variables [3]. Considerable effort has also been expended in the computation of leading and next-to-leading logarithmic contributions to the BFKL equations for parton evolution at small x [4, 5]. These two problems are, of course, connected since the logarithms that are resummed in the BFKL equations also appear at fixed orders of perturbation theory. Indeed, the one-loop Lipatov vertex may be extracted [6, 7] from one-loop five-gluon helicity amplitudes [8]. In order to compute a cross section at NNLO, three series of amplitudes are required in the squared matrix elements: a) tree-level, one-loop, and two-loop amplitudes for the production of n particles; b) tree-level and one-loop amplitudes for the production of n + 1 particles; c) tree-level amplitudes for the production of n + 2 particles. For the case of NNLO ee → 3 jets the five-parton final-state tree [9] amplitudes, as well as four-parton final-state one-loop amplitudes exist in both helicity [10] and squared matrixelement forms [11], but as we discuss below, in order to be used in NNLO computations additional terms enter because of infrared issues. For the required two-loop three-parton final-state amplitudes no computations exist, as yet. Indeed, no two-loop amplitude computations exist for cases containing more than a single kinematic variable, except in the special cases of maximal supersymmetry [12]. Besides these amplitudes it is also important to have a detailed understanding of the infrared singularities that arise from virtual loops and from unresolved real emission when the momenta of particles become either soft or collinear. (For hadronic initial states infrared divergences are also associated with initial-state parton distribution functions.) These infrared divergences show themselves as poles in the dimensional regularization parameter ǫ = (4 − D)/2. By the Kinoshita-Lee-Nauenberg theorem [13] the infrared singularities must cancel for sufficiently inclusive physical quantities. However, it is only when the various contributions are combined that the infrared singularities cancel. At NLO the structure of the infrared singularities has been extensively studied. The singularities occur in a universal way, i.e. independent of the particular particle production amplitude considered. Accordingly, soft singularities have been accounted for by universal soft functions [14, 15], and collinear singularities by universal splitting functions [16]. A detailed discussion of the infrared singularities at NLO for ee → jets may be found, for example, in ref. [17].