Measurement of exclusive $${\varvec{{{{\pi ^+\pi ^-}}}}}$$ π + π - and $${\varvec{{{{\rho ^0}}}}}$$ ρ 0 meson photoproduction at HERA
The European Physical Journal C(2020)
摘要
Exclusive photoproduction of
$${{\rho ^0}} (770)$$
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
$$\gamma p \rightarrow \pi ^{+}\pi ^{-}Y$$
. Reactions where the proton stays intact (
$${{{m_Y}} {=}m_p}$$
) are statistically separated from those where the proton dissociates to a low-mass hadronic system (
$$m_p{<}{{m_Y}} {<}10~{{\text {GeV}}} $$
). The double-differential cross sections are measured as a function of the invariant mass
$$m_{\pi \pi }$$
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
$${{W_{\gamma p}}} $$
. The phase space restrictions are
$$0.5\le m_{\pi \pi } \le 2.2~{{\text {GeV}}} $$
,
$$\vert t\vert \le 1.5~{{\text {GeV}^2}} $$
, and
$$20 \le W_{\gamma p} \le 80~{{\text {GeV}}} $$
. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the
$${m_{\pi \pi }}$$
dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the
$${{\rho ^0}} (770)$$
meson mass and width at
$$m_\rho = 770.8{}^{+2.6}_{-2.7}~({\text {tot.}})~{{\text {MeV}}} $$
and
$$\Gamma _\rho = 151.3 {}^{+2.7}_{-3.6}~({\text {tot.}})~{{\text {MeV}}} $$
, respectively. The model is used to extract the
$${{\rho ^0}} (770)$$
contribution to the
$$\pi ^{+}\pi ^{-}$$
cross sections and measure it as a function of t and
$${W_{\gamma p}}$$
. In a Regge asymptotic limit in which one Regge trajectory
$$\alpha (t)$$
dominates, the intercept
$$\alpha (t{=}0) = 1.0654\ {}^{+0.0098}_{-0.0067}~({\text {tot.}})$$
and the slope
$$\alpha ^\prime (t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }~({\text {tot.}}) ~{{\text {GeV}^{-2}}} $$
of the t dependence are extracted for the case
$$m_Y{=}m_p$$
.
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