A third workshop on small-x physics, within the Small-x Collaboration, was held in Hamburg in May 2004 with the aim of overviewing recent theoretical progress in this area and summarizing the ...experimental status.
The purpose of this volume is to gather the latest experimental results from the H1, ZEUS and HERMES collaborations and to capture new trends in HERA phenomenology. The presentations are by experts ...for experts, but are suitable for a mixed readership of both theoreticians and experimentalists. H1 members also cover ZEUS results and vice versa. This is the place where discrepancies between experimental data and theoretical predictions are pointed out and ventilated and where projects to be launched in the future are identified.
Charged particle multiplicity distributions in positron-proton deep inelastic scattering at a centre-of-mass energy
s
=
319
GeV are measured. The data are collected with the H1 detector at HERA ...corresponding to an integrated luminosity of 136 pb
-
1
. Charged particle multiplicities are measured as a function of photon virtuality
Q
2
, inelasticity
y
and pseudorapidity
η
in the laboratory and the hadronic centre-of-mass frames. Predictions from different Monte Carlo models are compared to the data. The first and second moments of the multiplicity distributions are determined and the KNO scaling behaviour is investigated. The multiplicity distributions as a function of
Q
2
and the Bjorken variable
x
bj
are converted to the hadron entropy
S
hadron
, and predictions from a quantum entanglement model are tested.
A precision measurement of jet cross sections in neutral current deep-inelastic scattering for photon virtualities
5.5
<
Q
2
<
80
GeV
2
and inelasticities
0.2
<
y
<
0.6
is presented, using data taken ...with the H1 detector at HERA, corresponding to an integrated luminosity of
290
pb
-
1
. Double-differential inclusive jet, dijet and trijet cross sections are measured simultaneously and are presented as a function of jet transverse momentum observables and as a function of
Q
2
. Jet cross sections normalised to the inclusive neutral current DIS cross section in the respective
Q
2
-interval are also determined. Previous results of inclusive jet cross sections in the range
150
<
Q
2
<
15
,
000
GeV
2
are extended to low transverse jet momenta
5
<
P
T
jet
<
7
GeV
. The data are compared to predictions from perturbative QCD in next-to-leading order in the strong coupling, in approximate next-to-next-to-leading order and in full next-to-next-to-leading order. Using also the recently published H1 jet data at high values of
Q
2
, the strong coupling constant
α
s
(
M
Z
)
is determined in next-to-leading order.
The parameters of the electroweak theory are determined in a combined electroweak and QCD analysis using all deep-inelastic
e
+
p
and
e
-
p
neutral current and charged current scattering cross ...sections published by the H1 Collaboration, including data with longitudinally polarised lepton beams. Various fits to Standard Model parameters in the on-shell scheme are performed. The mass of the
W
boson is determined as
m
W
=
80.520
±
0.115
GeV
. The axial-vector and vector couplings of the light quarks to the
Z
boson are also determined. Both results improve the precision of previous H1 determinations based on HERA-I data by about a factor of two. Possible scale dependence of the weak coupling parameters in both neutral and charged current interactions beyond the Standard Model is also studied. All results are found to be consistent with the Standard Model expectations.
The measurement of the jet cross sections by the H1 collaboration had been compared to various predictions including the next-to-next-to-leading order (NNLO) QCD calculations which are corrected in ...this erratum for an implementation error in one of the components of the NNLO calculations. The jet data and the other predictions remain unchanged. Eight figures, one table and conclusions are adapted accordingly, exhibiting even better agreement between the corrected NNLO predictions and the jet data.
Abstract
The determination of the strong coupling constant
$$\alpha _{\mathrm{s}} (m_{\mathrm{Z}})$$
α
s
(
m
Z
)
from H1 inclusive and dijet cross section data 1 exploits perturbative QCD predictions ...in next-to-next-to-leading order (NNLO) 2–4. An implementation error in the NNLO predictions was found 4 which changes the numerical values of the predictions and the resulting values of the fits. Using the corrected NNLO predictions together with inclusive jet and dijet data, the strong coupling constant is determined to be
$$\alpha _{\mathrm{s}} (m_{\mathrm{Z}}) =0.1166\,(19)_{\mathrm{exp}}\,(24)_{\mathrm{th}}$$
α
s
(
m
Z
)
=
0.1166
(
19
)
exp
(
24
)
th
. Complementarily,
$$\alpha _{\mathrm{s}} (m_{\mathrm{Z}})$$
α
s
(
m
Z
)
is determined together with parton distribution functions of the proton (PDFs) from jet and inclusive DIS data measured by the H1 experiment. The value
$$\alpha _{\mathrm{s}} (m_{\mathrm{Z}}) =0.1147\,(25)_{\mathrm{tot}}$$
α
s
(
m
Z
)
=
0.1147
(
25
)
tot
obtained is consistent with the determination from jet data alone. Corrected figures and numerical results are provided and the discussion is adapted accordingly.
Abstract
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
.
Exclusive photoproduction of
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
γ
p
→
π
+
π
-
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
GeV
). The double-differential cross sections are measured as a function of the invariant mass
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
γ
p
. The phase space restrictions are
0.5
≤
m
π
π
≤
2.2
GeV
,
|
t
|
≤
1.5
GeV
2
, and
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
π
π
dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the
ρ
0
(
770
)
meson mass and width at
m
ρ
=
770.8
-
2.7
+
2.6
(
tot.
)
MeV
and
Γ
ρ
=
151.3
-
3.6
+
2.7
(
tot.
)
MeV
, respectively. The model is used to extract the
ρ
0
(
770
)
contribution to the
π
+
π
-
cross sections and measure it as a function of
t
and
W
γ
p
. In a Regge asymptotic limit in which one Regge trajectory
α
(
t
)
dominates, the intercept
α
(
t
=
0
)
=
1.0654
-
0.0067
+
0.0098
(
tot.
)
and the slope
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
.
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