HERA Storage Ring Research Articles

Straightening of superconducting HERA dipoles for the any-light-particle-search experiment ALPS II

At DESY the ALPS II experiment is being installed in the HERA tunnel to search for axion like particles. A laser beam will be injected into the magnetic field of a string of superconducting (s.c.) dipole magnets, available from the HERA proton storage ring, to produce axion like particles. After passing a light tight wall, the ALPs can reconvert into photons in a second string of HERA s.c. dipoles. The sensitivity of the experiment will be increased by two mode-matched optical cavities before and behind the wall. The dipoles for the HERA storage ring are curved, suited for stored proton beam. However, the curvature of the magnets limits the aperture and hence the performance of the optical resonators beyond a certain length. As the sensitivity of the search scales with the length of the magnetic field, the aperture for the optical resonators inside the HERA dipoles was increased by straightening the curved magnet yoke. The procedure of straightening the s.c. HERA dipoles is described in this report.

Status and commissioning results of the helium refrigerator plant for the European XFEL

The European XFEL project is under construction at DESY in Germany. The superconducting XFEL linac is supplied by the XFEL helium refrigerator plant. This plant consists of two existing refrigerators, which were in service for the HERA storage ring until 2007. Since the XFEL linear accelerator requires cryogenic cooling at 2K, the existing cryogenic infrastructure had to be modified. Two of the three existing HERA helium refrigerators were required to cover the design heat load of the XFEL-linac. The refrigerator infrastructure was extended by a 2 K cooling loop, whose main component consists of a string of four cold compressors generating approximately 1.7 kW isothermal cooling capacity at 2K. The step by step commissioning and extension of the accelerator as well as the future upgrade option of the heat load demanded an extremely high turn down ability, a particular challenge for 2K cold compressor strings. The commissioning of the helium refrigerator infrastructure is underway and should be completed soon. The current status of this project, commissioning results and particular challenges are presented.

Multiplicities of charged pions and kaons from semi-inclusive deep-inelastic scattering by the proton and the deuteron

Multiplicities in semi-inclusive deep-inelastic scattering are presented for each charge state of \pi^\pm and K^\pm mesons. The data were collected by the HERMES experiment at the HERA storage ring using 27.6 GeV electron and positron beams incident on a hydrogen or deuterium gas target. The results are presented as a function of the kinematic quantities x_B, Q^2, z, and P_h\perp. They represent a unique data set for identified hadrons that will significantly enhance our understanding of the fragmentation of quarks into final-state hadrons in deep-inelastic scattering.

Measurement of the virtual-photon asymmetry A 2 and the spin-structure function g 2 of the proton

A measurement of the virtual-photon asymmetry A_2(x,Q^2) and of the spin-structure function g_2(x,Q^2) of the proton are presented for the kinematic range 0.004 < x < 0.9 and 0.18 GeV^2 < Q^2 < 20 GeV^2. The data were collected by the HERMES experiment at the HERA storage ring at DESY while studying inclusive deep-inelastic scattering of 27.6 GeV longitudinally polarized leptons off a transversely polarized hydrogen gas target. The results are consistent with previous experimental data from CERN and SLAC. For the x-range covered, the measured integral of g_2(x) converges to the null result of the Burkhardt-Cottingham sum rule. The x^2 moment of the twist-3 contribution to g_2(x) is found to be compatible with zero.

Investigation of heavy-quark production in proton-nucleus collisions with the HERA-B detector

Results obtained with the aid of the HERA-B detector, which operated at an internal target in a 920-GeV proton beam of the HERA storage ring at DESY (Hamburg, Germany), are discussed. About 164 million dilepton-triggered and 210 million minimum-bias events were collected over the data-taking time. These vast statistics made it possible to study in detail charmonium production in proton—nucleus collisions. Inclusive cross sections for open-charm, open-beauty, and ϒ-meson production weremeasured. The results are compared with the respective predictions of theoretical models.

Accelerator vacuum systems at DESY

The research center DESY in Germany is one of the leading accelerator centers worldwide. At the facilities located in Hamburg more than 3000 scientists from all over the world do research with photons and in the field of particle physics. Since 1992 the 6-km-long HERA storage ring was used for high energy collisions of electrons/positrons and protons. It consists of two rings. The one for the 27.5 GeV electrons/positrons uses normal conducting magnets. The other one bends the up to 920 GeV protons with superconducting magnets. Data taking at HERA has been terminated only recently. Synchrotron light for 36 experimental stations is generated by the 4.5 GeV storage ring DORIS. The 2.3 km long storage ring PETRA is presently rebuilt into PETRA III, one of the most brilliant X-ray sources worldwide, to start user operation in 2009. Therefore the vacuum system is completely replaced. The construction of the European X-ray Free Electron Laser XFEL, a new international research facility, just has started next to DESY. Its extremely intense X-ray laser flashes with tunable wavelengths down to 0.1 nm will open up completely new experimental possibilities for nearly all fields of natural sciences. The basic process adopted to generate the X-ray pulses is SASE (Self-Amplified Spontaneous Emission). Therefore electron bunches are brought to high energy of about 20 GeV through a superconducting linear accelerator, and conveyed to up to 250 m long undulators where the X-rays are generated. The beam vacuum system of this 3.4 km long straight facility contains sections operated at room temperature as well as at 2 K in the areas of the superconducting accelerating structures. In addition to standard UHV requirements the vacuum system needs to preserve the particle cleanliness of the superconducting cavity surfaces. Further challenges are the undulator vacuum chambers filling more than 750 m with extreme requirements to the surface quality. Unique research opportunities worldwide are offered already now by the 250 m long free-electron laser FLASH, the prototype for the XFEL, which is under operation since several years as a user facility. The vacuum systems of the various DESY accelerator facilities will be discussed in this paper in more detail.

Extraction of g1p, g1d and δg/g from HERMES data

HERMES collected deep-inelastic scattering (DIS) events of 27.6 GeV longitudinally polarised positrons off longitudinally polarised hydrogen and deuterium gas targets internal the HERA storage ring. Double-spin asymmetries A∥p,d of cross sections in inclusive DIS allow the precise extraction of spin structure functions of the proton g1p(x,Q2) and the deuteron g1d(x,Q2) in the kinematic range 0.0041 ≤ x ≤ 0.9 and 0.18 GeV ≤ Q2 ≤ 20GeV2. Furthermore, double-spin asymmetries of inclusive production of charged hadrons as a function of transverse momentum pt have been measured. For pt > 1 GeV the asymmetries are sensitive to the spin dependent gluon distribution function δg. Using a Leading Order Monte Carlo model to obtain information on the background asymmetry and background process kinematics, δg/g has been extracted.

A vertex trigger based on cylindrical multiwire proportional chambers

This article describes the technical implementation and the performance of the z-vertex trigger (CIP2k), which is part of the H1-experiment at HERA. The HERA storage ring and collider was designed to investigate electron (and positron) proton scattering at a center-of-mass energy of 320 GeV. To improve the sensitivity for detecting non-standard model physics and other high momentum transfer phenomena, the HERA ring has been ungraded between 2000 and 2003 to increase the specific luminosity for the experiments. In order to cope with the increased event and background rate the experiments were upgraded, too. The CIP2k trigger system is based on a set of five cylindrical multiwire proportional chambers with cathode pad readout, and allows to distinguish between events induced by beam background and ep-interactions at the first trigger stage. The trigger decision is calculated dead-time free with a latency of 1.5 μ s in parallel to the beam clock at 10.4 MHz. The trigger-logic is realized in large field programmable gate arrays (FPGA) using the hardware description language Verilog. The system is operational since October 2003. It suppresses background events with high efficiency and provides event timing information, as designed.

Measurement of D0, D+, Ds + and D*+ production in fixed target 920 GeV proton–nucleus collisions

The inclusive production cross sections of the charmed mesons D0,D+,Ds + and D*+ have been measured in interactions of 920 GeV protons on C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of transverse momentum and Feynman’s x variable are given for the central rapidity region and for transverse momenta up to pT=3.5 GeV/c. The atomic mass number dependence and the leading to non-leading particle production asymmetries are presented as well.

K*0 and φ meson production in proton–nucleus interactions at $\sqrt{s}=41.6\text{GeV}$

The inclusive production cross sections of the strange vector mesons K*0, K*0bar, and phi have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to pT=3.5 GeV/c. The atomic number dependence is parametrised as sigma(pA) = sigma(pN)*A**alpha, where sigma(pN) is the proton-nucleon cross section. Within the phase space accessible, alpha(K*0) = 0.86+/-0.03, alpha(K*0bar) = 0.87+/-0.03, and alpha(phi) = 0.96+/-0.02. The total proton-nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are sigma(pN->K*0) = 5.06+/-0.54 mb, sigma(pN->K*0bar) = 4.02+/-0.45 mb, and sigma(pN->phi) = 1.17+/-0.11 mb. The Cronin effect is observed for the first time for vector mesons containing strange quarks; compared to the measurements of Cronin et al. for K+- mesons, the measured values of alpha for phi mesons coincide with those of K- mesons for all transverse momenta, while the enhancement for K*0 / K*0bar mesons is smaller.

Precise determination of the spin structure functiong1of the proton, deuteron, and neutron

Precise measurements of the spin structure functions of the proton $g_1^p(x,Q^2)$ and deuteron $g_1^d(x,Q^2)$ are presented over the kinematic range $0.0041 \leq x \leq 0.9$ and $0.18 $ GeV$^2$ $\leq Q^2 \leq 20$ GeV$^2$. The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function $g_1^n$ is extracted by combining proton and deuteron data. The integrals of $g_1^{p,d}$ at $Q^2=5$ GeV$^2$ are evaluated over the measured $x$ range. Neglecting any possible contribution to the $g_1^d$ integral from the region $x \leq 0.021$, a value of $0.330 \pm 0.011\mathrm{(theo.)}\pm0.025\mathrm{(exp.)}\pm 0.028$(evol.) is obtained for the flavor-singlet axial charge $a_0$ in a leading-twist NNLO analysis.

A Measurement of the ψ′ to J/ψ production ratio in 920 GeV proton-nucleus interactions

Ratios of the ψ′ over the J/ψ production cross sections in the dilepton channel for C, Ti and W targets have been measured in 920 GeV proton-nucleus interactions with the HERA-B detector at the HERA storage ring. The ψ′ and J/ψ states were reconstructed in both the μ+μ- and the e+e- decay modes. The measurements covered the kinematic range -0.35≤xF≤0.1 with transverse momentum pT≤4.5 GeV/c. The angular dependence of the ratio has been used to measure the difference of the ψ′ and J/ψ polarization. All results for the muon and electron decay channels are in good agreement: their ratio, averaged over all events, is Rψ′(μ)/Rψ′(e)=1.00±0.08±0.04. This result constitutes a new, direct experimental constraint on the double ratio of branching fractions, (B′(μ)B(e))/(B(μ)B′(e)), of ψ′ and J/ψ in the two channels. The ψ′ to J/ψ production ratio is almost constant in the covered xF range and shows a slow increase with pT.

Exclusive vector meson production and DVCS at HERMES

The HERMES results from the hard exclusive processes such as ρ o and φ meson production from hydrogen and Deeply Virtual Compton Scattering (DVCS) from different targets are presented. The data were collected in the scattering of the 27.6 GeV longitudinally polarized lepton beam of the HERA storage ring off an internal gas target. The measurement of the longitudinal cross-section σ L for exclusive production of ρ o and φ mesons and DVCS related beam-spin(A LU ), target-spin(A UL ), and beam-charge(A C ) azimuthal asymmetries in the single photon electroproduction process were found to be in agreement with theoretical predictions based on the GPD formalism. PACS: 13.60.-f – 13.60.Le – 13.88.+e – 14.20.Dh – 24.85.+p

A study of aging effects in the gas-monitoring proportional counters of the BAC calorimeter in the ZEUS experiment

The multi-cell proportional chambers in the backing calorimeter of the ZEUS experiment at the HERA storage ring are supplied with an Ar/CO 2 gas mixture by an open gas system. Flow proportional counters with built-in 55 Fe sources are used as gas system monitoring detectors. The results of the measurements of the aging effects of the gas-monitoring counters are presented.

The Longitudinal Polarimeter at HERA

The design, construction and operation of a Compton back-scattering laser polarimeter at the HERA storage ring at DESY are described. The device measures the longitudinal polarization of the electron beam between the spin rotators at the HERMES experiment with a fractional systematic uncertainty of 1.6%. A measurement of the beam polarization to an absolute statistical precision of 0.01 requires typically 1 min when the device is operated in the multi-photon mode. The polarimeter also measures the polarization of each individual electron bunch to an absolute statistical precision of 0.06 in approximately 5 min . It was found that colliding and non-colliding bunches can have substantially different polarizations. This information is important to the collider experiments H1 and ZEUS for their future longitudinally polarized electron program because those experiments use the colliding bunches only.

Measurement of longitudinal spin transfer toΛhyperons in deep-inelastic lepton scattering

Spin transfer in deep-inelastic Lambda electroproduction has been studied with the HERMES detector using the 27.6 GeV polarized positron beam in the HERA storage ring. For an average fractional energy transfer <z> = 0.45, the longitudinal spin transfer from the virtual photon to the Lambda has been extracted. The spin transfer along the Lambda momentum direction is found to be 0.11 +/- 0.17 (stat) +/- 0.03 (sys); similar values are found for other possible choices for the longitudinal spin direction of the Lambda. This result is the most precise value obtained to date from deep-inelastic scattering with charged lepton beams, and is sensitive to polarized up quark fragmentation to hyperon states. The experimental result is found to be in general agreement with various models of the Lambda spin content, and is consistent with the assumption of helicity conservation in the fragmentation process.

Exclusive leptoproduction of $\rho^0$ mesons from hydrogen at intermediate virtual photon energies

Measurements of the cross section for exclusive virtual-photoproduction of $\rho^0$ mesons from hydrogen are reported. The data were collected by the HERMES experiment using 27.5 GeV positrons incident on a hydrogen gas target in the HERA storage ring. The invariant mass W of the photon-nucleon system ranges from 4.0 to 6.0 GeV, while the negative squared four-momentum $Q^2$ of the virtual photon varies from 0.7 to 5.0 GeV $^2$ . The present data together with most of the previous data in the intermediate W-domain are well described by a model that infers the W-dependence of the cross section from the dependence on the Bjorken scaling variable x of the unpolarized structure function for deep-inelastic scattering. In addition, a model calculation based on Off-Forward Parton Distributions gives a fairly good account of the longitudinal component of the $\rho^0$ production cross section for $Q^2>2$ GeV $^2$ .

HERA-B: physics potential and prospects

HERA-B is a hadroproduction experiment located at DESY in Hamburg, Germany. The experiment produces B mesons and baryons by inserting thin wire targets into the halo of the proton beam circulating in the HERA storage ring. The B decays are studied to search for evidence of CP violation and constrain the angle β and possibly γ and α of the CKM unitarity triangle. The experiment also produces B s mesons; these decays are studied to measure or constrain the mass difference Δ m s and width difference Δ Γ s between the two B s 0/ B ̄ s 0 mass eigenstates. Finally, the large number of B's produced allows HERA-B to search for rare and forbidden decays such as B→ K (∗) l 1 + l 2 − . The experiment is scheduled to begin running in early 2000.

Measurement of the spin asymmetry in the photoproduction of pairs of high- p(T) hadrons at HERMES

We present a measurement of the longitudinal spin asymmetry A(||) in photoproduction of pairs of hadrons with high transverse momentum p(T). Data were accumulated by the HERMES experiment using a 27.5 GeV polarized positron beam and a polarized hydrogen target internal to the HERA storage ring. For h(+)h(-) pairs with p(h(1))(T)>1.5 GeV/c and p(h(2))(T)>1.0 GeV/c, the measured asymmetry is A(||) = -0. 28+/-0.12(stat)+/-0.02(syst). This negative value is in contrast to the positive asymmetries typically measured in deep inelastic scattering from protons, and is interpreted to arise from a positive gluon polarization.

Future semi-inclusive experiments with internal targets

The physics prospects for semi-inclusive hadron production experiments in the deep-inelastic lepton scattering domain are discussed. It is argued that the luminosity that can be obtained with interal gaseous targets at the HERA storage ring is sufficient to obtain quantitatively significant data on various physics subjects. Assuming that the HERMES spectrometer is used for such measurements, count-rate estimates have been evaluated for experiments on quark propagation in nuclei and neutron structure functions with recoil detection. The physics potential of such measurements is further illustrated by the preliminary ρ0-production data that were obtained during the first year of data taking at HERMES in 1995.