HERA-B Experiment Research Articles

Preservation of the HERA-B Collaboration heritage

We present the concept of the data preservation developed for the HERA-B experiment which studied proton-nucleus interactions of the 920 GeV/c beam at HERA/DESY. The full analysis chain (starting from raw data) is to be preserved based on the rolling DPHEP model with open access to the data. We have frozen the full working analysis software environment using chroot-jail virtualization and report first experience of porting the software to a newer OS/compiler.

Diffraction on nuclei: Effects of nucleon correlations

The cross sections for a variety of diffractive processes in proton-nucleus scattering, associated with large gaps in rapidity, are calculated within an improved Glauber-Gribov theory, where the inelastic shadowing corrections are summed to all orders by employing the dipole representation. The effects of nucleon correlations, leading to a modification of the nuclear thickness function, are also taken into account. Numerical calculations are performed for the energies of the HERA-B experiment, and the RHIC and LHC colliders, and for several nuclei. It is found that whereas the Gribov corrections generally make nuclear matter more transparent, nucleon correlations act in the opposite direction and have important effects in various diffractive processes.

Kinematic distributions and nuclear effects of J/ψ production in 920 GeV fixed-target proton-nucleus collisions

Measurements of the kinematic distributions of J/ψ mesons produced in p–C, p–Ti and p–W collisions at $\sqrt{s}=41.6~\mathrm{GeV}$ in the Feynman-x region −0.34<x F <0.14 and for transverse momentum up to p T =5.4 GeV/c are presented. The x F and p T dependencies of the nuclear suppression parameter, α, are also given. The results are based on 2.4×105 J/ψ mesons reconstructed in both the e + e − and μ + μ − decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of x F for the first time. The average value of α in the measured x F region is 0.981±0.015. The data suggest that the strong nuclear suppression of J/ψ production previously observed at high x F turns into an enhancement at negative x F .

Angular distributions of leptons from J/ψ’s produced in 920 GeV fixed-target proton-nucleus collisions

A study of the angular distributions of leptons from decays of J/ψ’s produced in p-C and p-W collisions at $\sqrt{s}=41.6\mbox{~GeV}$ has been performed in the J/ψ Feynman-x region −0.34<x F <0.14 and for J/ψ transverse momenta up to 5.4 GeV/c. The data were collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The results, based on a clean selection of 2.3×105 J/ψ’s reconstructed in both the e + e − and μ + μ − decay channels, indicate that J/ψ’s are produced polarized. The magnitude of the effect is maximal at low p T . For p T >1 GeV/c a significant dependence on the reference frame is found: the polar anisotropy is more pronounced in the Collins-Soper frame and almost vanishes in the helicity frame, where, instead, a significant azimuthal anisotropy arises.

Luminosity determination at HERA-B

A detailed description of an original method used to measure the luminosity accumulated by the HERA-B experiment for a data sample taken during the 2002–2003 HERA running period is reported. We show that, with this method, a total luminosity measurement can be achieved with a typical precision, including overall systematic uncertainties, at a level of 5% or better. We also report evidence for the detection of δ -rays generated in the target and comment on the possible use of such delta rays to measure luminosity.

The electromagnetic calorimeter of the HERA-B experiment

The electromagnetic calorimeter of the HERA-B experiment built at the HERA proton accelerator at DESY (Hamburg) is described. The construction characteristics of the detector, of the related front-end, readout, trigger and service electronics are discussed together with the constraints and the motivations which inspired the design philosophy. The detector performance are presented as obtained from the analysis of the data acquired during the HERA-B running period, including calibration procedures and achievements and the electron identification capability exploiting a method, proposed here for the first time, based on the observation of the associated bremsstrahlung γ . Finally, some observed physical signals and a short overview of the main obtained physics results are presented.

The outer tracker detector of the HERA-B experiment. Part III: Operation and performance

In this paper we describe the operation and performance of the HERA-B Outer Tracker, a 112 674 channel system of planar drift tube layers. The performance of the HERA-B Outer Tracker system fullfilled all requirements for stable and efficient operation in a hadronic environment, thus confirming the adequacy of the honeycomb drift tube technology and of the front-end readout system. The detector was stably operated with a gas gain of 3 × 10 4 in an Ar / CF 4 / CO 2 (65:35:5) gas mixture, yielding a good efficiency for triggering and track reconstruction, larger than 95% for tracks with momenta above 5 GeV / c . The hit resolution of the drift cells was 300– 320 μ m and the relative momentum resolution can be described as: σ ( p ) / p [ % ] = ( 1.61 ± 0.02 ) + ( 0.0051 ± 0.0006 ) · p [ GeV / c ] . At the end of the HERA-B running no aging effects in the Outer Tracker cells were observed.

Charmonium, Open Charm and Beauty Production at HERA-B

The HERA-B experiment has collected data on proton-induced collisions with three fixed nuclear targets at 41.6 GeV centre-of-mass energy. Preliminary results of an ongoing, detailed study of charmonium production include the determination of the nuclear dependence of the J / ψ production kinematics (extending for the first time into the negative- x F region) and precise measurements of the feeddown components due to χ c , ψ ′ and b-hadron decays. A new analysis of the J / ψ decay angular distribution finds significant polarization effects in the low-momentum region, marginally touched by previous experiments. Measurements of the D-meson cross sections are consistent with the absence of nuclear suppression in open charm production.

A statistical method for luminosity monitoring in high energy collider experiments

A statistical method which uses a combination of two subdetectors to monitorthe luminosity in high energy interactions is presented. To illustrateits performance, this method was applied to random triggered minimum bias data collected in the commissioning period of the HERA-B experiment in spring 2000. It is found that luminosity estimates with an intrinsic systematic error of 3% can be obtained.

Large rapidity gap processes in proton-nucleus collisions

The cross sections for a variety of channels of proton-nucleus interaction associated with large gaps in rapidity are calculated within the Glauber-Gribov theory. We found inelastic shadowing corrections to be dramatically enhanced for such events. We employ the light-cone dipole formalism which allows to calculate the inelastic corrections to all orders of the multiple interaction. Although Gribov corrections are known to make nuclear matter more transparent, we demonstrate that in some instances they lead to an opaqueness. Numerical calculations are performed for the energies of the HERA-B experiment and the RHIC-LHC colliders.

Measurement of the J/ψ production cross section in 920 GeV/c fixed-target proton–nucleus interactions

The mid-rapidity (dsigma_(pN)/dy at y=0) and total sigma_(pN) production cross sections of J/Psi mesons are measured in proton-nucleus interactions. Data collected by the HERA-B experiment in interactions of 920 GeV/c protons with carbon, titanium and tungsten targets are used for this analysis. The J/Psi mesons are reconstructed by their decay into lepton pairs. The total production cross section obtained is sigma_(pN)(J/Psi) = 663 +- 74 +- 46 nb/nucleon. In addition, our result is compared with previous measurements.

HERA-B results on heavy flavour production in 920 GeV proton–nucleus interactions

In this report, recent results from the HERA-B experiment on heavy flavour production in proton–nucleus interactions using a 920 GeV proton beam are presented. The topics include the production of the charmonium states J/ψ, ψ′ and χc, of the D-meson states D0, D±, D*, the open beauty production cross section ( production) and the ϒ production cross section. The nuclear dependence for the differential cross sections of charmonium production is discussed. The results are mainly preliminary. The report closes with a summary of the HERA-B search for pentaquark states with strangeness, which yielded quite stringent upper limits for the production of such states in hadronic environments.

The Outer Tracker detector of the HERA-B experiment. Part II: Front-end electronics

The HERA-B Outer Tracker is a large detector with 112,674 drift chamber channels. It is exposed to a particle flux of up to 2 × 10 5 cm - 2 s - 1 thus coping with conditions similar to those expected for the LHC experiments. The front-end readout system, based on the ASD-8 chip and a customized TDC chip, is designed to fulfil the requirements on low noise, high sensitivity, rate tolerance, and high integration density. The TDC system is based on an ASIC which digitizes the time in bins of about 0.5 ns within a total of 256 bins. The chip also comprises a pipeline to store data from 128 events which is required for a deadtime-free trigger and data acquisition system. We report on the development, installation, and commissioning of the front-end electronics, including the grounding and noise suppression schemes, and discuss its performance in the HERA-B experiment.

Search for the flavor-changing neutral current decay [formula omitted] with the HERA-B detector

We report on a search for the flavor-changing neutral current decay D0→μ+μ− using 50×106 events recorded with a dimuon trigger in interactions of 920 GeV protons with nuclei by the HERA-B experiment. We find no evidence for such decays and set a 90% confidence level upper limit on the branching fraction B(D0→μ+μ−)<2.0×10−6.

Aging tests with GEM-MSGCs

The Inner Tracker of the HERA-B experiment at the Deutsches Elektronen-Synchrotron is a position-sensitive detector which has to cope with a particle flux up to 2×10 4 MIPs mm −2 s −1 and a radiation dose up to 1 Mrad/year ( 10 kGy/year ). The system consists of Microstrip-Gas-Chambers using a Gas Electron Multiplier as preamplifier structure in the gas volume. During the R&D phase, long-term aging measurements with X-rays were performed to study which electrode material and counting gas do not lead to aging of the detector over long running periods.

A large ultra-clean gas system with closed loop for the high-rate Outer Tracker at HERA-B

The gas system for the Outer Tracker of the HERA-B experiment at DESY produces the desired counting gas mixture Ar/CF 4/CO 2 65:30:5 and circulates it through the detector at a flow rate of 20 m 3/ h , i.e. ∼1 vol/ h . It controls flows and regulates pressures in all 26 OTR half-superlayers, purifies the gas upon return from the detector, and automatically performs a quantitative analysis of main and trace (O 2, N 2, H 2O) gas components for the common input and the outputs of all half-superlayers. The first running experience and the strategies employed during system construction to avoid any detector aging possibly induced by the gas system are discussed. The large system with major gas purification stations was constructed using only non-outgassing, “clean” materials and devices, such as stainless steel, PEEK, baked Viton, and metal bellows pumps. An epoxy glue was used extensively as a non-outgassing sealing material in applications with up to 100 bar pressure.

Humidity dependence of anode corrosion in HERA-B Outer Tracker Chambers operated with Ar/CF 4/CO 2

The Outer Tracker of the HERA-B experiment consists of drift tubes folded from gold-coated polycarbonate foils (honeycomb) and is operated with the gas mixture Ar/CF 4/CO 2. The choice of the materials and the drift gas was the result of an extended R&D program launched to avoid both anode aging (gain loss) and cathode aging (Malter effect). However, other aging effects were observed at high irradiation doses: rising conductivity of the wire-supporting strips and wire corrosion. In this paper, we show that these two aging effects could also be prevented by keeping water at a proper concentration (about 300 ppm ). In an attempt to interpret the test results, the production and reaction probability of molecular species produced via CF 4 dissociation in the avalanche are semi-quantitatively estimated.

Aging studies for the muon detector of HERA-B

The severe radiation environment of the HERA-B experiment leads to a maximum accumulated charge on a wire, within the muon detector, of 200 mC/cm wire . For operation in this high-intensity environment, the main criteria for the gas choice turned out to be stability against aging. We report recent results of aging studies performed by irradiating aluminum proportional wire chambers filled with Ar/CF 4/CH 4 (74:20:6), Ar/CF 4/CH 4 (67:30:3), Ar/CF 4/CO 2 (65:30:5) Ar/CF 4 (70:30), CF 4/CH 4 (90:10) and CF 4/CH 4 (80:20) mixtures in three different experimental setups: laboratory conditions ( 55 Fe and 106 Ru sources), a 100 MeV α -beam and the high-rate HERA-B environment (secondaries from interactions of 920 GeV protons with target nucleus). Our experience shows that the aging rate depends not only on the total collected charge, but, in addition, on the mode of operation (high voltage and/or gas gain) and area of irradiation. Effects of CF 4 addition on aging performance of gaseous detectors are summarized. Possible application of these results for operation of large scale gaseous detectors in the high rate environment is discussed.

A RICH multiplicity veto for the HERA-B experiment

We present the design and commissioning of a new multiplicity veto for the HERA-B detector, a fixed-target spectrometer originally designed to study the physics of B mesons in proton-nucleus interactions. The HERA-B trigger is a message-driven multilevel track trigger. The first level trigger (FLT) consists of custom-made electronics, and the higher trigger levels are implemented as PC farms. The multiplicity veto has been designed to reject high-multiplicity events before they enter the trigger chain. A veto signal is generated based on the comparison of the number of photons in part of the HERA-B ring-imaging Cerenkov counter (RICH) with a programmable threshold. The RICH multiplicity veto is a modular system. First the hits in 256 detector channels are summed by base sum cards (BSC), then FED sum cards (FSC) sum the subtotals of up to eight BSCs. Finally, the veto board (VB) takes the veto decision based on the sum of up to 14 FSCs. The RICH multiplicity veto has been successfully installed and commissioned in HERA-B. The measured veto efficiency is (99.9991/spl plusmn/0.0001)%, and the system is used in the routine data-taking of the HERA-B experiment.

Architecture of the HERA-B data acquisition system

The HERA-B experiment was dedicated for the measurement of charge conjugation space inversion (parity) (CP)-violation in decays of neutral B-mesons and for investigating the physics of charmed particles. One of the experimental requirements is a highly selective on-line filtering of data due to high interaction rates and a low signal-to-background ratio. This demands a hierarchical trigger and a high bandwidth data acquisition system. The challenge for the data acquisition (DAQ) system is a readout free of deadtime, which requires an unprecedented speed of storing and processing the data. In this paper, we will outline the general architecture and hardware implementation of the HERA-B DAQ.