Measurement of exclusive $${\varvec{{{{\pi ^+\pi ^-}}}}}$$ and $${\varvec{{{{\rho ^0}}}}}$$ meson photoproduction at HERA
Authors: Andreev V. , Baghdasaryan A. , Baty A. , Begzsuren K. , Belousov A. , Bolz A. , Boudry V. , Brandt G. , Britzger D. , Buniatyan A. , Bystritskaya L. , Campbell A. J. , Cantun Avila K. B. , Cerny K. , Chekelian V. , Chen Z. , Contreras J. G. , Cvach J. , Dainton J. B. , Daum K. , Deshpande A. , Diaconu C. , Eckerlin G. , Egli S. , Elsen E. , Favart L. , Fedotov A. , Feltesse J. , Fleischer M. , Fomenko A. , Gal C. , Gayler J. , Goerlich L. , Gogitidze N. , Gouzevitch M. , Grab C. , Grebenyuk A. , Greenshaw T. , Grindhammer G. , Haidt D. , Henderson R. C. W. , Hladkỳ J. , Hoffmann D. , Horisberger R. , Hreus T. , Huber F. , Jacquet M. , Janssen X. , Jung A. W. , Jung H. , Kapichine M. , Katzy J. , Kiesling C. , Klein M. , Kleinwort C. , Kogler R. , Kostka P. , Kretzschmar J. , Krücker D. , Krüger K. , Landon M. P. J. , Lange W. , Laycock P. , Lebedev A. , Levonian S. , Lipka K. , List B. , List J. , Li W. , Lobodzinski B. , Malinovski E. , Martyn H.-U. , Maxfield S. J. , Mehta A. , Meyer A. B. , Meyer H. , Meyer J. , Mikocki S. , Mondal M. M. , Morozov A. , Müller K. , Naumann Th. , Newman P. R. , Niebuhr C. , Nowak G. , Olsson J. E. , Ozerov D. , Park S. , Pascaud C. , Patel G. D. , Perez E. , Petrukhin A. , Picuric I. , Pitzl D. , Polifka R. , Radescu V. , Raicevic N. , Ravdandorj T. , Reimer P. , Rizvi E. , Robmann P. , Roosen R. , Rostovtsev A. , Rotaru M. , Sankey D. P. C. , Sauter M. , Sauvan E. , Schmitt S. , Schmookler B. A. , Schoeffel L. , Schöning A. , Sefkow F. , Shushkevich S. , Soloviev Y. , Sopicki P. , South D. , Spaskov V. , Specka A. , Steder M. , Stella B. , Straumann U. , Sykora T. , Thompson P. D. , Traynor D. , Truöl P. , Tseepeldorj B. , Tu Z. , Valkárová A. , Vallée C. , Van Mechelen P. , Wegener D. , Wünsch E. , Žáček J. , Zhang J. , Zhang Z. , Žlebčík R. , Zohrabyan H. , Zomer F. 
Published: December 2020

Exclusive photoproduction of $${{\rho ^0}} (770)$$

ρ
0

(
770
)

mesons is studied using the H1 detector at the ep collider HERA. A sample of about 900,000 events is used to measure single- and double-differential cross sections for the reaction $$\gamma p \rightarrow \pi ^{+}\pi ^{-}Y$$

γ
p
→

π
+

π
-

Y

. Reactions where the proton stays intact ($${{{m_Y}} {=}m_p}$$

m
Y

=

m
p

) are statistically separated from those where the proton dissociates to a low-mass hadronic system ($$m_p{<}{{m_Y}} {<}10~{{\text {GeV}}} $$

m
p

<

m
Y

<
10

GeV

). The double-differential cross sections are measured as a function of the invariant mass $$m_{\pi \pi }$$

m

π
π

of the decay pions and the squared 4-momentum transfer t at the proton vertex. The measurements are presented in various bins of the photon–proton collision energy $${{W_{\gamma p}}} $$

W

γ
p

. The phase space restrictions are $$0.5\le m_{\pi \pi } \le 2.2~{{\text {GeV}}} $$

0.5
≤

m

π
π

≤
2.2

GeV

, $$\vert t\vert \le 1.5~{{\text {GeV}^2}} $$

|
t
|

≤
1.5

GeV
2

, and $$20 \le W_{\gamma p} \le 80~{{\text {GeV}}} $$

20
≤

W

γ
p

≤
80

GeV

. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the $${m_{\pi \pi }}$$

m

π
π

dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the $${{\rho ^0}} (770)$$

ρ
0

(
770
)

meson mass and width at $$m_\rho = 770.8{}^{+2.6}_{-2.7}~({\text {tot.}})~{{\text {MeV}}} $$

m
ρ

=
770.8

-
2.7

+
2.6

(
tot.
)

MeV

and $$\Gamma _\rho = 151.3 {}^{+2.7}_{-3.6}~({\text {tot.}})~{{\text {MeV}}} $$

Γ
ρ

=
151.3

-
3.6

+
2.7

(
tot.
)

MeV

, respectively. The model is used to extract the $${{\rho ^0}} (770)$$

ρ
0

(
770
)

contribution to the $$\pi ^{+}\pi ^{-}$$

π
+

π
-

cross sections and measure it as a function of t and $${W_{\gamma p}}$$

W

γ
p

. In a Regge asymptotic limit in which one Regge trajectory $$\alpha (t)$$

α
(
t
)

dominates, the intercept $$\alpha (t{=}0) = 1.0654\ {}^{+0.0098}_{-0.0067}~({\text {tot.}})$$

α

(
t
=
0
)

=
1.0654

-
0.0067

+
0.0098

(
tot.
)

and the slope $$\alpha ^\prime (t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }~({\text {tot.}}) ~{{\text {GeV}^{-2}}} $$

α
′

(
t
=
0
)

=
0.233

-
0.074

+
0.067

(
tot.
)

GeV

-
2

of the t dependence are extracted for the case $$m_Y{=}m_p$$

m
Y

=

m
p

.