Measurement of unpolarized semi-inclusive pi+ electroproduction off the proton

TL;DR

This work measures unpolarized pi+ electroproduction on the proton with the CLAS detector, delivering complete five-fold differential cross sections over a broad kinematic range and enabling separation of phi-dependent terms. The phi-independent component aligns with perturbative QCD expectations for current fragmentation, while the phi-dependent terms are small, dominated by Cahn/ Berger-type contributions, with Boer-Mulders effects being small within uncertainties. The analysis also explores target fragmentation via fracture functions and assesses whether spectator diquark fragmentation mirrors anti-quark fragmentation seen in e+e− data, finding qualitative symmetry. The results provide crucial constraints on transverse momentum distributions, pQCD accuracy at moderate Q^2, and the role of target fragmentation in SIDIS, offering valuable inputs for refining fragmentation models and TMD phenomenology.

Abstract

Semi-inclusive pi+ electroproduction on protons has been measured with the CLAS detector at Jefferson Lab. The measurement was performed on a liquid-hydrogen target using a 5.75 GeV electron beam. The complete five-fold differential cross sections were measured over a wide kinematic range including the complete range of azimuthal angles between hadronic and leptonic planes, phi, enabling us to separate the phi-dependent terms. Our measurements of phi-independent term of the cross section at low Bjorken x were found to be in fairly good agreement with pQCD calculations. Indeed, the conventional current fragmentation calculation can account for almost all of the observed cross section, even at small pi+ momentum. The measured center-of-momentum spectra are in qualitative agreement with high energy data, which suggests a surprising numerical similarity between the spectator diquark fragmentation in the present reaction and the anti-quark fragmentation measured in e+e- collisions. We have observed that the two phi-dependent terms of the cross section are small. Within our precision the cos(2phi) term is compatible with zero, except for low-z region, and the measured cos(phi) term is much smaller in magnitude than the sum of the Cahn and Berger effects.

Figures (30)

• Figure 1: Schematic representation of current (a) and target (b) fragmentation processes in the virtual photon-proton center of momentum frame neglecting transverse momenta of particles. Blobs represent non-perturbative functions: the parton density function $f_i(x)$, parton fragmentation function $D_i^h(z)$ and fracture function $M_i^h(x,z)$ as given in Eq. \ref{['eq:lo_pqcd']}. $X_1$ and $X_2$ indicate two components of the undetected final state hadronic system $X$.
• Figure 2: Definition of the azimuthal angle between the leptonic and hadronic planes $\phi$ and hadron momentum components $p_T$ and $p_\parallel$.
• Figure 3: Kinematical regions covered by the present experiment in different independent variables. Numbers on the plots give base-10 logarithm of the number of events.
• Figure 4: Contour plot of particle momentum $\textbf{p}$ from tracking versus particle energy deposited in the calorimeter $EC_{\rm tot}$ normalized by $|\textbf{p}|$. Events on the left correspond to pions and those on the right to electrons. Only fiducial cuts were applied. The Cherenkov detector, providing the basic electron identification, allows to identify clearly the electrons up to $|\textbf{p}| \approx 2.7$ GeV/c. The dashed line shows the cut applied to the data to remove remaining pion contamination at $|\textbf{p}| > 2.7$ GeV/c.
• Figure 5: Measured squared mass of positive hadrons (left) and squared missing mass for the $e^-h^+$-system in the region of the $\pi^+n$ exclusive peak (right). The shaded area indicates hadrons identified as pions. The exclusive $\pi^+n$ was removed from the analysis by a cut $M_X^2 > 1.08$ (GeV/c$^2$)$^2$.