Hadron Formation in Deep-Inelastic Positron Scattering in a Nuclear Environment

Abstract

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 νand the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio R_M^h at low νand at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the ν-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 ν- and z-dependence of R_M^h.

Figures (5)

• Figure 1: Scatter plot of the hadron (or pion) energy $E_h$ ($E_\pi$) and the energy transfer $\nu$. Lines representing constant values of $z$ are shown as well.
• Figure 2: Charged hadron multiplicity ratio $R_M^{h}$ as a function of $\nu$ for values of $z$ larger than 0.2. In the upper panel the CERN EMC and SLAC osborne data for Cu are compared to various phenomenological calculations taken from the original publications EMCpavel. In the lower panel the HERMES data on $^{14}$N are represented by solid squares, while the open star represents the CERN data point on $^{12}$C and the open square the SLAC data point on $^{12}$C. The error bars represent the statistical uncertainty only. The systematic uncertainty of the HERMES data is $\le$ 3%. The curves are described in the text.
• Figure 3: The multiplicity ratio as a function of $z$ for all charged pions (open circles) and all charged hadrons including pions (closed squares). The full curve represents a gluon-bremsstrahlung model calculation for pions. The dotted, dashed and dot-dashed curves represent phenomenological formation-time calculations.
• Figure 4: The multiplicity ratio as a function of $\nu$ for charged pions with $z>0.5$. The solid curve represents a gluon-bremsstrahlung model calculation. The dotted curve is the result of a one time-scale model calculation assuming a $(1-z) \nu$-dependence of the formation time.
• Figure 5: Multiplicity ratios for hadrons including pions (top panel) and identified pions (bottom panel) as a function of $\nu$. The open (closed) squares represent the positive (negative) hadrons. Identified pions are represented by open (positive) and closed (negative) circles. The curves are parameterizations of the data using the one time-scale model assuming $t_f^h = c_h (1-z) \nu$.