HERMES Experiment Research Articles

Event generator for jet tomography in electron-ion collisions

We develop the first event generator, the electron-heavy-ion-jet-interaction-generator (eHIJING), for the jet tomography study of electron-ion collisions. In this generator, energetic jet partons produced from the initial hard scattering undergo multiple collisions with the nuclear target. The collision rate is proportional to the transverse-momentum-dependent (TMD) gluon density in the nucleus, which is given by a simple model inspired by the physics of gluon saturation. Medium-modified QCD splitting functions within the higher-twist (HT) and generalized higher-twist (GHT) frameworks are utilized to simulate parton showering in the nuclear medium that takes into account the non-Abelian Landau-Pomeranchuck-Midgal interference effect. Employing eHIJING, we revisit hadron production in semi-inclusive deep inelastic scattering (SIDIS) as measured by EMC, HERMES, and recent CLAS experiments. eHIJING with both GT and GHT frameworks gives reasonably good descriptions of these experimental data. Predictions for experiments at the future electron-ion colliders are also provided. It is demonstrated that future measurements of the transverse momentum broadening of single hadron spectra can be used to map out the two-dimensional kinematic (Q2, xB) dependence of the jet transport coefficient q^ in cold nuclear matter. Published by the American Physical Society 2024

T -odd proton-helicity asymmetry in semi-inclusive DIS in perturbative QCD

We compute the single-spin asymmetry ${A}_{\mathrm{UL}}$ in semi-inclusive deep-inelastic scattering of unpolarized leptons and longitudinally polarized protons at a large transverse momentum of the produced hadron. Our calculation is performed in collinear factorization at the lowest order of QCD perturbation theory. For photon exchange, the asymmetry is $T$-odd and receives contributions from the interference of the tree level and one-loop absorptive amplitudes. We consider the behavior of the spin asymmetry at low transverse momentum where contact to the formalism based on transverse momentum dependent distribution functions can be made. We also present some phenomenological results relevant for the COMPASS and HERMES experiments and the future Electron-Ion Collider.

Hadronization and Color Transparency

In this paper, the earlier studies by us on the production of hadrons in a nuclear environment are reviewed. A string-breaking model for the initial production of hadrons and a quantum-kinetic Giessen-Boltzmann-Uehling-Uhlenbeck (GiBUU) transport model are used to describe the final state interactions of the newly formed (pre)hadrons. The latter are determined both by the formation times and by the time-development of the hadron–hadron cross section. First, it is shown that only a linear time dependence is able to describe the available hadronizatin data. Then, the results are compared with detailed data from HERMES and Jefferson Laboratory (JLAB) experiments; a rather good agreement is reached for all reactions, studied without any tuning of parameters. Predictions of spectra for pions and kaons for JLAB experiments at 12 GeV are also repeated. Finally, the absence of color transparency (CT) effects in the recent experiment on proton transparencies in quasi-elastic (QE) scattering events on nuclei is discussed. We propose to look instead for CT effects on protons in semi-inclusive deep inelastic scattering (SIDIS) events.

Kotzinian-Mulders effect in semi-inclusive DIS within TMD factorization * *Supported by the National Natural Science Foundation of China (12075043)

In this study, we investigate the Kotzinian-Mulders effect under semi-inclusive deep inelastic scattering (SIDIS) within the framework of transverse momentum dependent (TMD) factorization. The asymmetry is contributed by the convolution of the Kotzinian-Mulders function and the unpolarized fragmentation function . As a TMD distribution, the Kotzinian-Mulders function in the coordinate space in the perturbative region can be represented as the convolution of the C-coefficients and the corresponding collinear correlation function. The Wandzura-Wilczek approximation is used to obtain this correlation function. We perform a detailed phenomenological numerical analysis of the Kotzinian-Mulders effect in the SIDIS process within TMD factorization at the kinematics of the HERMES and COMPASS experiments. We observe that the obtained -, -, and -dependent Kotzinian-Mulders effects are basically consistent with the HERMES and COMPASS measurements. We also make predictions at EIC and EicC kinematics.

Determination of unpolarized pion fragmentation functions using semi-inclusive deep-inelastic-scattering data

We present MAPFF1.0, a determination of unpolarised charged-pion fragmentation functions (FFs) from a set of single-inclusive $e^+e^-$ annihilation and lepton-nucleon semi-inclusive deep-inelastic-scattering (SIDIS) data. FFs are parametrised in terms of a neural network (NN) and fitted to data exploiting the knowledge of the analytic derivative of the NN itself w.r.t. its free parameters. Uncertainties on the FFs are determined by means of the Monte Carlo sampling method properly accounting for all sources of experimental uncertainties, including that of parton distribution functions. Theoretical predictions for the relevant observables, as well as evolution effects, are computed to next-to-leading order (NLO) accuracy in perturbative QCD. We exploit the flavour sensitivity of the SIDIS measurements delivered by the HERMES and COMPASS experiments to determine a minimally-biased set of seven independent FF combinations. Moreover, we discuss the quality of the fit to the SIDIS data with low virtuality $Q^2$ showing that, as expected, low-$Q^2$ SIDIS measurements are generally harder to describe within a NLO-accurate perturbative framework.

Contribution of exclusive diffractive processes to the measured azimuthal asymmetries in SIDIS

Hadron leptoproduction in Semi-Inclusive measurements of Deep-Inelastic Scattering (SIDIS) on unpolarised nucleons allows one to get information on the intrinsic transverse momentum of quarks in a nucleon and on the Boer-Mulders function through the measurement of azimuthal modulations in the cross section. These modulations were recently measured by the HERMES experiment at DESY on proton and deuteron targets, and by the COMPASS experiment using the CERN SPS muon beam and a 6LiD target. In both cases, the amplitudes of the cos⁡ϕh and cos⁡2ϕh modulations show strong kinematic dependences for both positive and negative hadrons. It has been known since some time that the measured final-state hadrons in those SIDIS experiments receive a contribution from exclusive diffractive production of vector mesons, particularly important at large values of z, the fraction of the virtual photon energy carried by the hadron. In previous measurements of azimuthal asymmetries this contribution was not taken into account, because it was not known that it could distort the azimuthal modulations. Presently, a method to evaluate the contribution of the exclusive reactions to the azimuthal asymmetries measured by COMPASS has been developed. The subtraction of this contribution results in a better understanding of the kinematic effects, and the remaining non-zero cos⁡2ϕh modulation gives indication for a non-zero Boer-Mulders effect.

Overview of recent HERMES results

The HERMES experiment has collected a wealth of data using the 27.6 GeV polarized HERA lepton beam and various polarized and unpolarized gaseous targets. This allows for a series of unique measurements of observables sensitive to the multidimensional (spin) structure of the nucleon, in particular semi-inclusive deep-inelastic scattering (SIDIS) measurements.In this contribution, transverse single-spin asymmetry, longitudinal single- and double-spin asymmetries in SIDIS will be presented. The azimuthally uniform double-spin asymmetries using longitudinally polarized nucleons constrain the flavour dependence of the quark-spin contribution to the nucleon spin. For a first time, such asymmetries are explored differential in three dimensions in Bjorken-x and the in the hadron kinematics z and Ph± (which respectively represent the energy fraction and transverse momentum of the final-state hadron) simultaneously. This approach increases the quark-flavour sensitivity and allows to probe the transverse-momentum dependence of the helicity distribution. The measurement of hadron charge-difference asymmetries allows, under certain simplifying assumptions, the direct extraction of valence quark polarisations.The azimuthal modulation of this double-spin as well as of the single-(beam)spin asymmetry probe novel quark-gluon-quark correlations through twist-3 distribution and fragmentation functions. Also here, asymmetries are explored in several dimensions. Furthermore, in case of the beam-spin asymmetry, results for electro-produced protons and antiprotons have become available. The beam-spin asymmetries for pions are compared to similar measurements for pions at CLAS and unidentified hadrons at COMPASS.

Beam-helicity asymmetries for single-hadron production in semi-inclusive deep-inelastic scattering from unpolarized hydrogen and deuterium targets

A measurement of beam-helicity asymmetries for single-hadron production in deep-inelastic scattering is presented. Data from the scattering of 27.6 GeV electrons and positrons off gaseous hydrogen and deuterium targets were collected by the HERMES experiment. The asymmetries are presented separately as a function of the Bjorken scaling variable, the hadron transverse momentum, and the fractional energy for charged pions and kaons as well as for protons and anti-protons. These asymmetries are also presented as a function of the three aforementioned kinematic variables simultaneously.

Medium-indused modification of kaons spectra measured in SIDIS at HERMES

The predicted high sensitivity of the nuclear modification factor for K– in SIDIS due to the QCD based effect of medium-induced flavour conversion in the fragmentation function is studied at HERMES experiment. Unlike π+, π– and K+ nuclear modification factor for K– is assumed to increase at high value of Bjorken variable xB and a hadron fraction energy z. The experimental signature of this phenomenon is an enhanced K– nuclear modification factor compared with those of K+ for high xB and z range. The z spectra of nuclear attenuation ratios for charged kaons in different slices of xB extracted on Ne, Kr and Xe targets will be presented and discussed.

Updating spin-dependent Regge intercepts

We use new high statistics data from CLAS and COMPASS on the nucleon's spin structure function at low Bjorken x and low virtuality, Q^2 < 0.5 GeV^2, together with earlier measurements from the SLAC E-143, HERMES and GDH experiments to estimate the effective intercept(s) for spin dependent Regge theory. We find alpha_{a_1} = 0.31 \pm 0.04 for the intercept describing the high-energy behaviour of spin dependent photoabsorption together with a new estimate for the high-energy part of the Gerasimov-Drell-Hearn sum-rule, -15 \pm 2 mu b from photon-proton centre-of-mass energy greater than 2.5 GeV. Our value of alpha_[a_1} suggests QCD physics beyond a simple straight-line a_1 trajectory.

Investigation of difference of fundamental diagrams in pedestrian flow

In this paper the difference of fundamental diagrams in pedestrian flow is investigated. A new measurement method in which the moving back is considered as negative contribution to pedestrian flow is proposed based on previous study. Firstly the comparison of different measurement methods is made and small discrepancy in the fundamental diagrams of the same experiment is observed. In order to remove the effect of different measurement methods on final results, we compare the fundamental diagrams of Chinese and German experiments by using the same measurement method and surprisingly large difference is found. From the analysis of experimental video, it is observed the motivation and competitiveness of participants in two experiments are quite different, which plays a dominant role on the large difference between fundamental diagrams. To make it more tenable, we further analyze two German experiments (Hermes experiment and BaSiGo experiment) in which the participants have the same average age. The free velocity is adopted to measure pedestrian’s motivation in the experiment and the analysis results confirm that the discrepancy of motivation and competitiveness of participants in different experiments leads to the large difference in the fundamental diagrams. The study may be helpful to understand the effect of pedestrian behavior on global flow and microscopic dynamics.

Nonperturbative functions for SIDIS and Drell–Yan processes

We update the well-known BLNY fit to the low transverse momentum Drell–Yan lepton pair productions in hadronic collisions, by considering the constraints from the semi-inclusive hadron production in deep inelastic scattering (SIDIS) from HERMES and COMPASS experiments. We follow the Collins–Soper–Sterman (CSS) formalism with the [Formula: see text]-prescription. A nonperturbative form factor associated with the transverse momentum dependent quark distributions is found in the analysis with a new functional form different from that of BLNY. This releases the tension between the BLNY fit to the Drell–Yan data with the SIDIS data from HERMES/COMPASS in the CSS resummation formalism.

Measurement of multiplicities of charged hadrons, pions and kaons in DIS at COMPASS

Precise measurements of multiplicities of charged hadrons, pions and kaons in deep inelastic scattering were performed. The data were obtained by the COMPASS Collaboration by scattering 160 GeV muons off an isoscalar 6LiD target. The results were obtained in three-dimensional bins of the Bjorken scaling variable x, the relative virtual-photon energy y, and the fraction z of the virtual-photon energy carried by the produced hadron. A leading-order pQCD analysis was performed using the pion multiplicity results to extract quark fragmentation functions into pions. The results for the sum of the z-integrated multiplicities for pions and for kaons, differ from earlier results from the HERMES experiment. The results from the sum of the z-integrated K+ and K- multiplicities at high x point to a value of the non-strange quark fragmentation function larger than obtained by the earlier DSS fit.

Extraction of helicity amplitude ratios from exclusive ρ0-meson electroproduction on transversely polarized protons

Exclusive ρ0-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV2 < Q2 < 7.0 GeV2, 3.0 GeV < W < 6.3 GeV, and −t′ < 0.4 GeV2. Using an unbinned maximum-likelihood method, 25 parameters are extracted. They determine the real and imaginary parts of the ratios of certain helicity amplitudes (describing ρ0-meson production by a virtual photon) and the dominant amplitude without the nucleon-helicity flip. The latter amplitude describes the production of a longitudinal ρ0 meson by a longitudinal virtual photon. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to . The ratios of nucleon-helicity-non-flip amplitudes to are found to be in good agreement with those from the previous HERMES analysis. A comparison of the extracted amplitude ratios with the Goloskokov-Kroll model shows the necessity to add pion exchange amplitudes with positive πρ form factor to the amplitudes based on generalized parton distributions to improve the HERMES data description.

Overview of recent HERMES results

An overview of more recent and important results from the HERMES experiment is presented in this paper. HERMES collected a wealth of data using the 27.6 GeV polarized HERA lepton beam and various polarized and unpolarized gaseous targets. This allows for a series of unique measurements of observables sensitive to the multidimensional structure of the nucleon. Amongst them are the semi-inclusive deep-inelastic scattering measurements of azimuthal modulations sensitive to the leading-twist transverse momentum distributions and the distributions sensitive to convolutions of the twist-2 and twist-3 distribution and fragmentation functions. They all provide information on the three-momentum-dependent quark distributions. Knowledge about the quark distribution as a function of longitudinal momentum and transverse position in impact-parameter space can be accessed, e.g., through exclusive vector–meson leptoproduction. Particularly, the measurement of spin-density-matrix elements on an unpolarized hydrogen target and the measurement of azimuthal modulations on transversely polarized hydrogen target for the ω–meson production are described in this paper. The results of the measurement of ratios of helicity amplitudes for the ρ0 –meson leptoproduction on transversely polarized hydrogen target are also presented.

Ratios of helicity amplitudes for exclusive rho ^0 electroproduction on transversely polarized protons

Exclusive rho ^0-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV^2<Q^2< 7.0 GeV^2, 3.0 GeV <W< 6.3 GeV, and -t^prime < 0.4 GeV^2. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing rho ^{0}-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude F_{0frac{1}{2}0frac{1}{2}}, which describes the production of a longitudinal rho ^{0}-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to F_{0frac{1}{2}0frac{1}{2}}. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the pi -rho transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 rho ^0 spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.

Small-x Behavior of Non-singlet Spin Structure Function and Bjorken Sum Rule with pQCD Correction up to NNLO and Higher Twist Correction

A calculation of the non-singlet part of spin dependent structure function, $xg_1^{NS}(x,Q^2)$ and associated sum rule, the Bjorken Sum rule up to next-next-to-leading order(NNLO) is presented. We use a unified approach incorporating Regge theory and the theoretical framework of perturbative Quantum Chromodynamics. Using a Regge behaved model with $Q^2$ dependent intercept as the initial input, we have solved the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) evolution equation up to NNLO at small-$x$ for $xg_1^{NS}(x,Q^2)$ and the solutions are utilised to calculate the polarised Bjorken sum rule(BSR). We have also extracted the higher twist contribution to BSR based on a simple parametrisation. These results for both of $xg_1^{NS}(x,Q^2)$ and BSR, along with higher twist corrections are observed to be consistent with the available data taken from SMC, E143, HERMES, COMPASS and JLab experiments. In addition, our results are also compared with that of other theoretical and phenomenological analysis based on different models and a very good agreement is also observed in this regard. Further a very good consistency between our calculated results and theoretical QCD predictions of BSR is also achieved.

Ratios of helicity amplitudes for exclusiveρ0electroproduction on transversely polarized proton

Exclusive ρ 0 -meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV 2 Q 2 2 , 3.0 GeV t ′ 2 . Using an unbinned maximum-likelihood method, 25 parameters are extracted. They determine the real and imaginary parts of the ratios of certain helicity amplitudes describing ρ 0 -meson production by a virtual photon, where the denominator is the dominant amplitude . The latter is the nucleon-helicity-non-flip amplitude, which describes the production of a longitudinal ρ 0 meson by a longitudinal virtual photon. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to . The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis.

Overview of recent HERMES results

An overview of more recent and important results from the HERMES experiment are presented in this paper. HERMES collected a wealth of data using the 27.6 GeV polarized HERA lepton beam and various polarized and unpolarized gaseous targets. This unique data set opens the door to the measurements of observables sensitive to the multidimensional structure of the nucleon. Among them are semi-inclusive deep-inelastic scattering measurements of azimuthal modulations sensitive to the transverse momentum distributions, such as the leading- twist Sivers and Collins distributions and distributions sensitive to the convolutions of the twist-2 and twist-3 functions. They all provide an information on the three-momentum-dependent quark distributions. Knowledge on the quark distribution as a function of longitudinal momentum and transverse position in impact-parameter space can be accessed, e.g., through exclusive ω—meson leptoproduction, particularly through the measurement of spin density matrix elements and the measurement of azimuthal modulations on transversely polarized proton target. The measurement of Bose-Einstein correlations of hadron pairs in quasi-real leptoproduction are also presented. The transverse polarization of Λ hyperons measured again in quasi-real leptoproduction regime are presented as well. Finally, the new analysis for the search on pentaquark at HERMES are mentioned.

DVCS at HERMES

Deeply virtual Compton scattering process was studied widely at the HERMES experiment through measurements of various cross-section asymmetries. These measurements exploited the rich data collected at HERMES on scattering a longitudinally polarized lepton (electron/positron) beam off unpolarized, longitudinally and transversely polarized hydrogen targets, unpolarized and longitudinally polarized deuterium targets, as well as unpolarized nuclear targets. The operation of a recoil detector during the last two years of HERMES running enabled for the full kinematic reconstruction of the events of exclusively produced real photons and a clean (with a background well below the 1[Formula: see text]) measurement of the beam-helicity asymmetry. Also, first measurement of the beam-helicity asymmetry related to the associated deeply virtual Compton scattering process, where the proton is excited to a [Formula: see text]-resonance state, was possible using the recoil detector information.