C. Royon

The H1 lead/scintillating-fibre calorimeter

Abstract The backward region of the H1 detector has been upgraded in order to provide improved measurement of the scattered electron in deep inelastic scattering events. The centerpiece of the upgrade is a high-resolution lead/scintillating-fibre calorimeter. The main design goals of the calorimeter are: good coverage of the region close to the beam pipe, high angular resolution and energy resolution of better than 2% for 30 GeV electrons. The calorimeter should be capable of providing coarse hadronic energy measurement and precise time information to suppress out-of-time background events at the first trigger level. It must be compact due to space restrictions. These requirements were fulfilled by constructing two separate calorimeter sections. The inner electromagnetic section is made of 0.5 mm scintillating plastic fibres embedded in a lead matrix. Its lead-to-fibre ratio is 2.3:1 by volume. The outer hadronic section consists of 1.0 mm diameter fibres with a lead-to-fibre ratio of 3.4:1. The mechanical construction of the new calorimeter and its assembly in the H1 detector are described.

Performance of an electromagnetic lead/scintillating-fibre calorimeter for the H1 detector

Abstract The properties of final modules of a high resolution lead/scintillating-fibre calorimeter to upgrade the backward region of the H1 detector were studied with electrons in the energy range from 2–60 GeV. The electromagnetic calorimeter consists of scintillating fibres with a diameter of 0.5 mm embedded in a lead matrix. This small fibre radius, in combination with a lead-to-fibre ratio of 2.27:1, ensures excellent energy resolution which has been measured to be δ/E=7.1%/ E/GeV ⊕ 1.0% . The spatial resolution as a function of energy for impact points at the center of a cell is given by 4.4 mm/ E/GeV + 1.0 mm . The time resolution was found to be better than 0.4 ns.

Popping out the Higgs boson off vacuum at Tevatron and LHC

Abstract In the prospect of diffractive Higgs production at the LHC collider, we give an extensive study of Higgs boson, dijet, diphoton and dilepton production at hadronic colliders via diffraction at both hadron vertices. Our model, based on non-factorizable pomeron exchange, describes well the observed dijet rate observed at Tevatron runxa0I. Taking the absolute normalization from data, our predictions are given for diffractive processes at Tevatron and LHC. Stringent tests of our model and of its parameters using data being taken now at Tevatron runxa0II are suggested. These measurements will also allow to discriminate between various models and finally to give precise predictions on diffractive Higgs boson production cross-section at the LHC.

Sensitivity to the standard model Higgs boson in exclusive double diffraction

We use a Monte Carlo implementation of recently developed models of double diffraction to assess the sensitivity of the LHC experiments to standard model Higgs bosons produced in exclusive double diffraction. The signal is difficult to extract, due to experimental limitations related to the first level trigger, and to contamination by inclusive double diffractive background. Assuming these difficulties can be overcome, the expected signal-to-background ratio is presented as a function of the experimental resolution on the missing mass. With a missing mass resolution of 2 GeV, a signal-to-background ratio of about 0.5 is obtained; a resolution of 1 GeV brings a signal to background ratio of 1. This result is lower than previous estimates, and the discrepancy is explained.

The ATLAS Forward Physics project

We describe the main components of the ATLAS Forward Physics project, namely the movable beam pipe, the tracking and timing detectors which allow to detect intact protons in the final state at the LHC. The position detector is composed on 6 layers of 3D silicon detectors readout by FE-I4 chips developped for ATLAS. The fast timing detector is built from a quartz-based Cerenkov detector coupled to a microchannel plate photomultiplier tube, followed by the electronic elements that amplify, measure, and record the time of the event along with a stabilized reference clock signal, ensuring a time resolution of 10-15 picoseconds.

Hadronic response and e / pi separation with the H1 lead / fiber calorimeter

Hadronic response and electron identification performance of the new H1 lead-scintillating fibre calorimeter are investigated in the 1 to 7 GeV energy range using data taken at the CERN Proton Synchrotron. The energy response to minimum ionizing particles and interacting pions are studied and compared to Monte Carlo simulations. The measured energy of pions interacting either in the electromagnetic or in the hadronic section is found to scale linearly with the incident energy, providing an energy resolution σE ∼ 38% within a depth of one interaction length and σE ∼ 29% for a total depth of two interaction lengths. Several electron identification estimators are studied and combined as a function of energy and impact point. The probability for pions to be misidentified as electrons of any measured energy above 1 GeV ranges from 5% (for 2 GeV incident pions) to 0.4% (at 7 GeV) for an electron detection efficiency of 90%. The probability for pions of a given energy to be misidentified as electrons of the same energy falls to 0.25% at 7 GeV.

Confronting next-leading BFKL kernels with proton structure function data

We propose a phenomenological study of the Balitsky-Fadin-Kuraev-Lipatov (BFKL) approach applied to the data on the proton structure function F_2 measured at HERA in the small-x_{Bjorken} region. In a first part we use a simplified ``effective kernel approximation leading to few-parameter fits of F_2. It allows for a comparison between leading-logs (LO) and next-to-leading logs (NLO) BFKL approaches in the saddle-point approximation, using known resummed NLO-BFKL kernels. The NLO fits give a qualitatively satisfactory account of the running coupling constant effect but quantitatively the chi squared remains sizeably higher than the LO fit at fixed coupling. In a second part, a comparison of theory and data through a detailed analysis in Mellin space (x_{Bjorken} ->omega) leads to a more model independent approach to the resummed NLO-BFKL kernels we consider and points out some necessary improvements of the extrapolation at higher orders.

Decisive test for the pomeron at the Tevatron

We propose a new measurement to be performed at the Tevatron which can be decisive to distinguish between pomeron-based and soft color interaction models of hard diffractive scattering.

Hard diffraction and the nature of the Pomeron

Abstract We ask the question whether the quark and gluon distributions in the Pomeron obtained from QCD fits to hard diffraction processes at HERA can be dynamically generated from a state made of valence-like gluons and sea quarks as input. By a method combining backward Q2-evolution for data exploration and forward Q2-evolution for a best fit determination, we find that the diffractive structure functions published by the H1 Collaboration at HERA can be described by a simple valence-like input at an initial scale of order μ2∼2.3– 2.7 GeV 2 . The parton number sum rules at the initial scale μ2 for the H1 fit gives 2.1±0.1±0.1 and 0.13±0.01±0.02 for gluon and sea quarks, respectively, corresponding to an initial Pomeron state made of (almost) only two gluons. It has flat gluon density leading to a plausible interpretation in terms of a gluonium state.

Series tests of fine mesh photomultiplier tubes in magnetic fields of up to 1.2 Tesla

Abstract The new lead/scintillating-fibre calorimeter (“SpaCal”) for the backward region of the H1 experiment at HERA (DESY) is equipped with fine mesh phototubes which operate in a magnetic field close to 1 T. A large sample of these tubes of the types Hamamatsu R5505 and R5506, and Hamamatsu R2490-05, have been tested in fields of up to 1.2T. We have investigated the cathode homogeneity with and without magnetic field, the gain loss under the influence of the magnetic field, and stability with time. For a subsample of tubes, we have performed additional studies on stability with respect to temperature changes, variation of gain as a function of the magnetic field, high voltage discharges, single photo-electron response, and linearity. We finally summarize the experience with these tubes after one year of operation in the experiment.