Elsevier

Physics Letters B

Volume 522, Issues 3–4, 13 December 2001, Pages 233-239
Physics Letters B

Measurement of the Σ− charge radius by Σ−–electron elastic scattering

https://doi.org/10.1016/S0370-2693(01)01285-0Get rights and content

Abstract

The Σ− mean squared charge radius has been measured in the space-like Q2 range 0.035–0.105 GeV2/c2 by elastic scattering of a Σ− beam off atomic electrons. The measurement was performed with the SELEX (E781) spectrometer using the Fermilab hyperon beam at a mean energy of 610 GeV/c.

We obtain 〈r2ch〉Σ−=(0.61±0.12(stat.)±0.09(syst.)) fm2. The proton and π− charge radii were measured as well and are consistent with results of other experiments. Our result agrees with the recently measured strong interaction radius of the Σ−.

Introduction

The systematic measurement of the static properties of hadrons has led to a better understanding of their fundamental structure. However, their finite size— a consequence of the confinement of quarks inside a spatial volume—has not been thoroughly explored. Sizes of hadrons may be probed by their strong and electromagnetic interactions. Most commonly the electromagnetic charge radius is measured in elastic electron–hadron scattering. For unstable hadrons the inverse kinematics can be applied using a suitable hadronic beam. So far, electromagnetic radii have been established only for the proton [1], neutron [2], π− [3], and K− [4]. The difference between the pion and the kaon radius indicates a dependence on the strangeness content. A systematic study of the radii of hyperons with different strangeness will therefore enhance our understanding of the relative sizes of hadrons as bound quark systems. Theoretical predictions for the Σ− charge radius have been divergent [5]. The most recent efforts suggest 〈r2ch〉Σ−=(0.67±0.03)fm2 [6] on the one hand and 〈r2ch〉Σ−=(0.54±0.09)fm2 [7] on the other. The feasibility of probing the Σ− radius by inverse electron scattering has been demonstrated at CERN [8]. We present here the first high-statistics measurement of the Σ− charge radius.

Section snippets

The E781/SELEX experiment

The primary objective of SELEX is the study of charm hadroproduction and spectroscopy of charm baryons in the forward hemisphere. The experiment was built as a 3-stage magnetic spectrometer as shown in Fig. 1. Here we only describe those features of the apparatus which are relevant to hadron–electron scattering.

The 800 GeV/c proton beam from the Fermilab Tevatron was steered onto a Be target to produce a beam consisting of equal numbers of Σ− and π− at 610 GeV/c mean momentum. The hyperon beam

Kinematics

The differential cross section of the elastic scattering of a spin 1/2 baryon (mass M) off an electron (mass m) can be approximated by the relation (ℏ=c=1) [10] dσdQ2=4πα2Q41−Q2Q2maxF2Q2 up to corrections of the order m2/(s−M2). Here, s denotes the center of mass energy and Q2 the four momentum transfer from the hadron to the electron. Q2 has a kinematically allowed maximum value which depends on the beam momentum. For instance, Q2max is 0.2 GeV2/c2 for 610 GeV/c Σ−. For m⪡M the squared form

Analysis and results

Out of 77 million triggers with a Σ− a sample of 5010 Σ−–electron scattering events was extracted as follows: the events containing one electron were selected and the particle trajectories combined to check if they formed vertices inside the target material. The event was accepted if it contained exactly three tracks forming one vertex consisting of the beam track, the electron candidate, and the scattered hadron candidate.

At a given beam energy, momenta and scattering angles of the outgoing

Discussion and conclusion

To put our results in perspective, we compare the electromagnetic charge radii to radii derived from total hadron–proton cross sections (strong interaction radii [5], Fig. 4). The latter have been normalized to the mean squared charge radius of the proton as measured by SELEX at s=34 GeV and the total pp cross section [16] interpolated to this energy. Both the Σ− and π− squared strong interaction radii are calculated from total cross sections measured by SELEX [17] as well. The mean squared

Acknowledgements

The authors are indebted to the staff of Fermi National Accelerator Laboratory and for invaluable technical support from the staffs of collaborating institutions, especially F. Pearsall, D. Northacker, and J. Zimmer. This project was supported in part by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, Consejo Nacional de Ciencia y Tecnologı́a (CONACyT), the Conselho Nacional de Desenvolvimento Cientı́fico e Tecnológico, the Fondo de Apoyo a la Investigación (UASLP),

References (18)

  • R. Rosenfelder

    Phys. Lett. B

    (2000)
  • S. Amendolia

    Phys. Lett. B

    (1986)
  • B. Povh et al.

    Phys. Lett. B

    (1990)
  • E.J. Hackett-Jones et al.

    Phys. Lett. B

    (2000)
  • S. Kopecky

    Phys. Rev. Lett.

    (1995)
  • S. Amendolia

    Nucl. Phys. B

    (1986)
  • B. Kubis et al.

    Eur. Phys. J. C

    (2001)
  • M. Adamovich

    Eur. Phys. J. C

    (1999)
  • I. Eschrich
There are more references available in the full text version of this article.

Cited by (0)

1

Now at Imperial College, London SW7 2BZ, U.K.

2

Present address: Boston Consulting Group, München, Germany.

3

Present address: Siemens Medizintechnik, Erlangen, Germany.

4

Present address: Deutsche Bank AG, Eschborn, Germany.

5

Deceased.

6

Present address: Infineon Technologies AG, München, Germany.

7

Now at University of Karlsruhe, Karlsruhe, Germany.

8

Now at Instituto de Fı́sica da Universidade Estadual de Campinas, UNICAMP, SP, Brazil.

9

Now at Universität Freiburg, Freiburg, Germany.

10

Now at Physik-Department, Technische Universität München, Garching, Germany.

11

Present address: Lucent Technologies, Naperville, IL.

12

Now at Instituto de Fı́sica Teórica da Universidade Estadual Paulista, São Paulo, Brazil.

13

Now at Max-Planck-Institut für Physik, München, Germany.

14

Present address: SPSS Inc., Chicago, IL.

15

Now at University of Alabama at Birmingham, Birmingham, AL 35294.

16

Present address: DOE, Germantown, MD.

View full text
Copyright © 2001 Elsevier Science B.V. All rights reserved.