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Physics Letters B 750 (2015) 184–193

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Study of the quasi-free np →npπ

−

reaction with a deuterium

beam at 1.25 GeV/nucleon

G. Agakishiev

, A. Balanda

, D. Belver

, A.V. Belyaev

, A. Blanco

, M. Böhmer

J.L. Boyard

, P. Braun-Munzinger

d,3

, P. Cabanelas

, E. Castro

, S. Chernenko

, T. Christ

M. Destefanis

, J. Díaz

, F. Dohrmann

, A. Dybczak

, L. Fabbietti

, O.V. Fateev

P. Finocchiaro

, P. Fonte

b,2

, J. Friese

, I. Fröhlich

, T. Galatyuk

e,3

, J.A. Garzón

R. Gernhäuser

, A. Gil

, C. Gilardi

, K. Göbel

, M. Golubeva

, D. González-Díaz

F. Guber

, M. Gumberidze

, T. Hennino

, R. Holzmann

, A. Ierusalimov

, I. Iori

l,5

A. Ivashkin

, M. Jurkovic

, B. Kämpfer

f,4

, T. Karavicheva

, D. Kirschner

, I. Koenig

W. Koenig

, B.W. Kolb

, R. Kotte

, F. Krizek

, R. Krücken

, W. Kühn

, A. Kugler

A. Kurepin

, A. Kurilkin

g,∗

, P. Kurilkin

, V. Ladygin

g,∗

, S. Lang

, J.S. Lange

, K. Lapidus

T. Liu

, L. Lopes

, M. Lorenz

, L. Maier

, A. Mangiarotti

, J. Markert

, V. Metag

B. Michalska

, J. Michel

, E. Morinière

, J. Mousa

, C. Müntz

, L. Naumann

J. Otwinowski

, Y.C. Pachmayer

, M. Palka

, Y. Parpottas

n,6

, V. Pechenov

O. Pechenova

, J. Pietraszko

, W. Przygoda

, B. Ramstein

o,∗

, A. Reshetin

, A. Rustamov

A. Sadovsky

, P. Salabura

, A. Schmah

j,1

, E. Schwab

, Yu.G. Sobolev

, S. Spataro

k,7

B. Spruck

, H. Ströbele

, J. Stroth

h,d

, C. Sturm

, A. Tarantola

, K. Teilab

, P. Tlusty

M. Traxler

, R. Trebacz

, H. Tsertos

, V. Wagner

, T. Vasiliev

, M. Weber

M. Wisniowski

, T. Wojcik

, J. Wüstenfeld

, S. Yurevich

, Y. Zanevsky

, P. Zhou

Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud, 95125 Catania, Italy

LIP-Laboratório de Instrumentação e Física Experimental de Partículas, 3004-516 Coimbra, Portugal

Smoluchowski Institute of Physics, Jagiellonian University of Cracow, 30-059 Kraków, Poland

GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany

Technische Universität Darmstadt, 64289 Darmstadt, Germany

Institut für Strahlenphysik, Helmholtz-Zentrum Dresden-Rossendorf, 01314 Dresden, Germany

Joint Institute of Nuclear Research, 141980 Dubna, Russia

Institut für Kernphysik, Goethe-Universität, 60438 Frankfurt, Germany

Excellence Cluster ‘Origin and Structure of the Universe’ , 85748 Garching, Germany

Physik Department E12, Technische Universität München, 85748 Garching, Germany

II. Physikalisches Institut, Justus Liebig Universität Giessen, 35392 Giessen, Germany

Istituto Nazionale di Fisica Nucleare, Sezione di Milano, 20133 Milano, Italy

Institute for Nuclear Research, Russian Academy of Science, 117312 Moscow, Russia

Department of Physics, University of Cyprus, 1678 Nicosia, Cyprus

Institut de Physique Nucléaire (UMR 8608), CNRS/IN2P3 – Université Paris Sud, F-91406 Orsay Cedex, France

Nuclear Physics Institute, Academy of Sciences of Czech Republic, 25068 Rez, Czech Republic

LabCAF. Dpto. Física de Partículas, Univ. de Santiago de Compostela, 15706 Santiago de Compostela, Spain

Instituto de Física Corpuscular, Universidad de Valencia-CSIC, 46971 Valencia, Spain

Corresponding authors.

E-mail

addresses: akurilkin@jinr.ru (A. Kurilkin), vladygin@jinr.ru (V. Ladygin), ramstein@ipno.in2p3.fr (B. Ramstein).

Also at Lawrence Berkeley National Laboratory, Berkeley, USA.

Also at ISEC Coimbra, Coimbra, Portugal.

Also at ExtreMe Matter Institute EMMI, 64291 Darmstadt, Germany.

Also at Technische Universität Dresden, 01062 Dresden, Germany.

Also at Dipartimento di Fisica, Università di Milano, 20133 Milano, Italy.

Also at Frederick University, 1036 Nicosia, Cyprus.

Also at Dipartimento di Fisica Generale and INFN, Università di Torino, 10125 Torino, Italy.

http://dx.doi.org/10.1016/j.physletb.2015.09.016

0370-2693/

SCOAP

G. Agakishiev et al. / Physics Letters B 750 (2015) 184–193 185

a r t i c l e i n f o a b s t r a c t

Article history:

Received

13 March 2015

Received

in revised form 4 September 2015

Accepted

7 September 2015

Available

online 11 September 2015

Editor:

D.F. Geesaman

Keywords:

Two-pion

production

np collisions

Resonance

excitations

The tagged quasi-free np → npπ

−

reaction has been studied experimentally with the High Acceptance

Di-Electron Spectrometer (HADES) at GSI at a deuteron incident beam energy of 1.25 GeV/nucleon

(

√

s ∼ 2.42 GeV/c for the quasi-free collision). For the ﬁrst time, differential distributions of solid

statistics for π

−

production in np collisions have been collected in the region corresponding to the

large transverse momenta of the secondary particles. The invariant mass and angular distributions for the

np →npπ

−

reaction are compared with different models. This comparison conﬁrms the dominance of

the t-channel with  contribution. It also validates the changes previously introduced in the Valencia

model to describe two-pion production data in other isospin channels, although some deviations are

observed, especially for the π

−

invariant mass spectrum. The extracted total cross section is also in

much better agreement with this model. Our new measurement puts useful constraints for the existence

of the conjectured dibaryon resonance at mass M ∼2.38 GeV and with width  ∼ 70 MeV.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

The two-pion production in nucleon–nucleon (NN) collisions is

a rich source of information about the baryon excitation spectrum

and the baryon–baryon interactions. In addition to the excitation

of a resonance decaying into two pions, which can also be studied

in the π N → ππN [1] and γ N →ππN [2] reactions, the simul-

taneous

excitation of two baryons can be investigated in the NN

reactions.

By giving access to single and double baryon excitation

processes, which both play an important role in the NN dynamics

in the few GeV energy range and contribute signiﬁcantly to meson

and dilepton production, the two-pion production appears as a key

process towards a better understanding of hadronic processes. In

comparison to the one-pion decay mode, it presents a different se-

lectivity

with respect to the various resonances. In particular, the

excitation of baryonic resonances coupled to the ρ meson can be

studied with the two pions in the isospin 1 channel. This is of

utmost interest for a better understanding of the dilepton produc-

tion

in nucleon–nucleon reactions, where these couplings manifest

clearly [3–5], and also in nucleon matter due to the expected mod-

iﬁcations

of the ρ meson spectral functions [6]. Finally, the com-

parison

of two-pion production in pp and np channels could shed

some light on the origin of the surprisingly large isospin depen-

dence

of the dilepton emission observed by the HADES experiment

[7]. In particular, the ρ production mechanism via  ﬁnal state

interaction, which does not contribute in the pp channel, was re-

cently

proposed as an explanation for the different dilepton yield

measured in pp and pn channels [8]. It is therefore important to

check the description of the double  process in the two-pion pro-

duction

channels.

Additionally,

following the intriguing results obtained by the

WASA Collaboration in the double pionic fusion reactions, a re-

newed

interest on the study of the two-pion production in NN

collisions

was sparked, in order to check the possible contribution

of a dibaryon resonance [9,10].

The

answer to all these open questions requires systematic

two-pion production measurements both in proton–proton and

neutron–proton collisions. Concerning proton–proton collisions,

a signiﬁcant amount of data has been accumulated for various

two-pion ﬁnal channels in bubble chamber experiments [11–23]

for

proton incident energies from the threshold up to 2.85 GeV.

Precise differential cross-sections have also been obtained recently

at CELSIUS and COSY up to 1.4 GeV [24–34], with an emphasis

on the π

production. The data base for the pn reaction from

the bubble chamber experiments is even more scarce [14,18,19,21,

35].

Very recently, however, precise measurements of total and dif-

ferential

cross sections for the np → ppπ

−

and np → npπ

became available from WASA at COSY at neutron energies from

1.075 to 1.36 GeV [36,37]. In the np → npπ

−

channel, differ-

ential

cross-sections are also known from Dubna measurements

[38,39], covering the beam incident energy range from 0.624 to

4.346 GeV.

Since

the chiral perturbation theory calculations for two-pion

production in NN collisions are available only near threshold

[40], several phenomenological models have been suggested for

the analysis of the double pion production in NN collisions in

the GeV energy range. The ﬁrst theoretical developments related

to the two-pion production were based on the one-pion exchange

(OPE) model [41]. The reggeized π exchange model (OPER) [42,

43],

which uses the partial wave analysis results for π N elas-

tic

scattering [44], constitutes its most recent and most elaborate

modiﬁcation. Lagrangian models were also introduced. The Valen-

cia

model by Alvarez-Ruso et al. [45] was ﬁrst developed. It aimed

at a description of NN collisions at energies lower than 1.4 GeV and

included N(1440) and (1232) excitations. The Cao et al. model

[46], developed after the publication of new data at COSY and CEL-

SIUS,

has a larger range of applicability due to the inclusion of

resonances with mass up to 1.72 GeV. Both models qualitatively

reproduce the very fast increase of the cross section above thresh-

old

in the different two-pion production channels and predict the

dominance of two processes above 1GeV: the excitation of the

N(1440) resonance and subsequent decay into π or Nσ and the

double  excitation.

However,

both models [45,46] have failed to reproduce the

spectra for the pp → pp π

reaction at beam energies

above 1.0 GeV [32,34], which motivated the development of the

so-called “modiﬁed Valencia model” [32], providing a much im-

proved

description of these data. This new model has been used

by the WASA Collaboration for the interpretation of the double pi-

onic

fusion reactions, after some additional changes to take into

account the deuteron formation. However, the observed resonant

behavior of the cross section of the pn →dπ

[9,10], associated

with a structure at low π

invariant mass (the so-called ABC

effect) could not be explained by such an approach and were in-

terpreted

as being due to a dibaryon resonance in the I = 0 NN

channel,

with a mass of 2.37 GeV/c

and a width of 70 MeV. This

hypothesis was further supported by the isospin decomposition

of the pn → dππ reaction [47]. The latter provided a consistent

description of both I = 0 and I = 1 channels by taking into ac-

count

the resonant contribution in addition to the conventional

t-channel processes described by the “modiﬁed Valencia model”.

Even more recently, the pn → ppπ

−

[36] reaction was also

consistently described with the same model. The accuracy of d

∗

resonance hypothesis is also supported by the SAID partial wave

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