HERMES Experiment Research Articles

The luminosity monitor of the HERMES experiment at DESY

A detector is described which measures the luminosity of the HERMES experiment at DESY. It is based on the coincident detection of electron–positron and photon pairs, or electron pairs, originating from the interaction of the beam positrons, or electrons, with the electrons of the atomic gas target. It consists of two calorimeters with radiation hard NaBi(WO 4) 2 crystals. Properties of the monitor, investigated in an electron test beam, and its performance in the experiment are presented.

Double-spin asymmetry in the cross section for exclusive ρ0 production in lepton–proton scattering

Evidence for a positive longitudinal double-spin asymmetry <A_1^rho> = 0.24 +-0.11 (stat) +-0.02 (syst) in the cross section for exclusive diffractive rho^0(770) vector meson production in polarised lepton-proton scattering was observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA positron beam was scattered off a longitudinally polarised pure hydrogen gas target. The average invariant mass of the photon-proton system has a value of <W> = 4.9 GeV, while the average negative squared four-momentum of the virtual photon is <Q^2> = 1.7 GeV^2. The ratio of the present result to the corresponding spin asymmetry in inclusive deep-inelastic scattering is in agreement with an early theoretical prediction based on the generalised vector meson dominance model.

Multiwire proportional chambers in the HERMES experiment

The multiwire proportional chambers of the HERMES spectrometer are described. Each chamber consists of a series anode–cathode planes which measure position along axes at −30°, 0° , and +30° relative to horizontal. The anode wire spacing is 2 mm , while the anode–cathode plane spacing is 4 mm . The chambers are located in the spectrometer magnet gap which imposed strong constraints on their design. A nonflammable gas mixture of Ar(65%), CO 2(30%) and CF 4(5%) is used and an efficiency plateau 500 V wide is obtained. The readout is performed with the LeCroy PCOS4 data acquisition system modified according to the specific requirements of the HERMES experiment. A dedicated water cooling system has been implemented to control and stabilize the surface temperature of the electronic chips mounted on the on-chamber preamplifier boards. A spatial resolution of ≃600 μm (rms) per plane has been measured. These chambers are essential for tracking low momentum particles which do not enter the drift chambers after the spectrometer magnet; these particles principally result from the decay of various unstable hadrons. The momentum and angular resolutions in tracking these low momentum particles has been studied and found to be similar to those obtained for high momentum tracks which traverse the entire tracking system of the HERMES spectrometer.

Investigation of single spin asymmetries in [formula omitted] electroproduction

The azimuthal single target-spin asymmetries for π + production in semi-inclusive deep inelastic scattering of leptons off longitudinally polarized protons are evaluated using two main approaches available in the literature. It is shown that the approximation where the twist-2 transverse quark spin distribution in the longitudinally polarized nucleon is small enough to be neglected leads to a consistent description of all existing asymmetries observed by the HERMES experiment.

Hadron formation in deep-inelastic positron scattering in a nuclear environment

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy $\nu$ and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio $R_M^{h}$ at low $\nu$ and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the $\nu$ -dependence of $R_M^{h}$ is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the $\nu$ - and z-dependence of $R_M^{h}$ .

First experience with the HERMES silicon detector

A silicon detector system (the Lambda Wheels) is being added to the HERMES experiment at DESY. The acceptance of the front region will be enlarged in order to increase the fraction of Λ 0, Λ c, and J/Ψ-particles which can be reconstructed. The detector is located inside the ring vacuum. Two disks cover the area between 4.5 and 16 cm from the beam. The detector consists of 12 identical modules. This report describes some details of the design, the cooling and the read-out of the detector. A prototype of a single module has been operated since the beginning of 2000. A signal-to-noise ratio of about 10 has been obtained for minimum-ionizing particles.

Soft gluon resummation for polarized deep-inelastic production of heavy quarks

We present the threshold resummation for the cross section of heavy quark production in polarized deep-inelastic scattering to next-to-leading logarithmic accuracy and in single-particle inclusive kinematics. We expand our resummed result in α s to next-to-leading and next-to-next-to-leading order and study the impact of these higher order corrections on the charm structure function g 1 c in the kinematical range accessible to the HERMES and COMPASS experiments.

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 $^2$ .

Dual radiator RICH of HERMES for π, K, p identification

The HERMES experiment uses the 27.5 GeV beam of longitudinally polarized electrons of the HERA accelerator together with an internal gas target to study the spin structure of the nucleon. The ability to identify hadrons in coincidence with the scattered electron is crucial for the determination of the contributions of the different quark flavors to the nucleon spin. For this purpose, a Ring Imaging CHerenkov (RICH) detector was developed. It is able to identify pions, kaons and protons over the relevant kinematic range of 2–16 GeV by using two radiators, silica aerogel and C 4F 10 gas. This detector was installed in May 1998 in the HERMES experiment. It is the first RICH detector in an actual experiment that uses aerogel as a radiator.

The lambda wheels, a silicon vertex detector for HERMES

A silicon detector system is being added to the HERMES experiment at DESY. The detector is located inside the ring vacuum, and increases the fraction of Λ 0, Λ c +, and J/Ψ-particles which can be reconstructed. This report describes some details of the mechanical design and the read-out of the detector.

Nuclear shadowing and coherence length for longitudinal and transverse photons

Motivated by the recent results for DIS off nuclei from the HERMES experiment we have performed a systematic study of shadowing for transverse and longitudinal photons. We found that the coherence length which controls the onset of nuclear shadowing at small ${x}_{\mathrm{Bj}}$ is much longer for longitudinal than transverse photons, and is much shorter for shadowing of gluons. The light-cone Green function approach we apply properly treats shadowing in the transition region ${x}_{\mathrm{Bj}}&gt;0.01.$ It also incorporates the nonperturbative effects and is legitimate at small ${Q}^{2}.$ We calculate nuclear shadowing and compare with data from the HERMES and NMC experiments. Although we expect different nuclear shadowing for longitudinal and transverse photons, numerically it cannot explain the strong effect observed by the HERMES Collaboration.

Radiative tail from the quasielastic peak in deep inelasticscattering of polarized leptons off polarized 3He

The contribution of the radiative tail from the quasielastic peakto low-order radiative correction to deep inelasticscattering ofpolarized leptons by polarized 3He is calculated within the sum-rulesformalism and y-scaling hypothesis. A numerical analysisis carried out under the conditions of the HERMES experiment.

Latest results from HERA

Highlights are presented of the latest measurements from the H1, ZEUS and HERMES experiments at HERA.

Recent spin physics results from the HERMES experiment

The HERMES experiment at HERA/DESY studies the spin structure of the nucleon through deep inelastic scattering of longitudinally polarised positrons off polarised H̄, D̄ and 3 He targets. In addition to inclusive measurements which aim at a precise determination of the spin structure functions g 1 and g 2 of the proton and the neutron, HERMES focuses on semi-inclusive measurements, taking advantage of the unique capability of the experiment to reconstruct and identify final state hadrons. A selection of recent results from measurements with longitudinally polarised targets are presented, including polarised quark distributions and a first extraction of the polarisation of gluons. Also presented are results from single-spin asymmetries which allow to complete the investigation of the nucleon spin structure at and beyond leading twist level. Finally, first evidence is presented for (double-)spin asymmetries in the diffractive production of vector mesons.

The HERMES silicon project

A silicon detector system will be added to the HERMES experiment at DESY. The acceptance of the front region will be enlarged with a view to increase the fraction of Λ 0, Λ c +, and J/Ψ-particles which can be reconstructed. The detector is located inside the ring vacuum. This report describes some details of the design, the cooling and the read-out of the detector. The first parts of the detector have recently been installed in the experiment. It is expected that the full detector will be installed in the fall of 1999.

Single-spin azimuthal asymmetries in the “Reduced twist-3 approximation”

We consider the single-spin azimuthal asymmetries recently measured at the HERMES experiment for charged pions produced in semi-inclusive deep inelastic scattering of leptons off longitudinally polarized protons. Guided by the experimental results and assuming a vanishing twist-2 transverse quark spin distribution in the longitudinally polarized nucleon, denoted as “reduced twist-3 approximation”, a self-consistent description of the observed single-spin asymmetries is obtained. In addition, predictions are given for the z dependence of the single target-spin asymmetry.

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.

Vector meson production at HERMES

Exclusive incoherent electroproduction of vector mesons from 1H, 2H, 3He and 14N has been studied by the HERMES experiment. Results for electroproduction cross sections, decay angle distributions, double spin asymmetries and nuclear transparency are presented.

Nuclear effects on R= σL/ σT in deep-inelastic scattering

Cross section ratios for deep-inelastic scattering from 14N and 3He with respect to 2H have been measured by the HERMES experiment at DESY using a 27.5 GeV positron beam. The data cover a range in the Bjorken scaling variable x between 0.013 and 0.65, while the negative squared four-momentum transfer Q 2 varies from 0.5 to 15 GeV 2. The data are compared to measurements performed by NMC, E665, and SLAC on 4He and 12C, and are found to be different for x<0.06 and Q 2<1.5 GeV 2. The observed difference is attributed to an A-dependence of the ratio R= σ L / σ T of longitudinal to transverse deep-inelastic scattering cross sections at low x and low Q 2.

Highlights from HERMES

A selection of recent results from the HERMES experiment is presented. The flavor decomposition of the nucleon spin is one of the highlights. Polarized semi-inclusive deep inelastic scattering (DIS) data are used to extract the polarization of the up, down and sea quarks separately and as a function of the scaling variable x. The polarization of the up (down) quarks is positive (negative) and its absolute value increases with x. The polarization of the sea quarks is compatible with zero. The first and second moments of the flavor spin distributions have been extracted and compared to predictions and to results from other experiments. A second semi-inclusive result from HERMES concerns the (unpolarized) flavor asymmetry of the light quark sea. The significant non-zero result shows an enhancement of down antiquarks compared to up antiquarks. This enhancement has not been naively expected in QCD but is consistent with the recent observation that the Gottfried sum rule is broken. HERMES discovered a new nuclear effect in DIS by comparing inclusive DIS cross sections of nitrogen, helium and deuterium. It was found that the ratio of the longitudinal to transverse photoabsorption cross section is very different for the different nuclei in certain kinematic areas. A natural explanation of this effect points to quark-gluon correlations in nuclear matter.