354 research outputs found
Isocurvature forecast in the anthropic axion window
We explore the cosmological sensitivity to the amplitude of isocurvature
fluctuations that would be caused by axions in the "anthropic window" where the
axion decay constant f_a >> 10^12 GeV and the initial misalignment angle
Theta_i << 1. In a minimal Lambda-CDM cosmology extended with subdominant
scale-invariant isocurvature fluctuations, existing data constrain the
isocurvature fraction to alpha < 0.09 at 95% C.L. If no signal shows up, Planck
can improve this constraint to 0.042 while an ultimate CMB probe limited only
by cosmic variance in both temperature and E-polarisation can reach 0.017,
about a factor of five better than the current limit. In the parameter space of
f_a and H_I (Hubble parameter during inflation) we identify a small region
where axion detection remains within the reach of realistic cosmological
probes.Comment: 14 pages, 4 figures; v2: matches published versio
Energy dependence of the saturation scale and the charged multiplicity in pp and AA collisions
A natural framework to understand the energy dependence of bulk observables
from lower energy experiments to the LHC is provided by the Color Glass
Condensate, which leads to a "geometrical scaling" in terms of an energy
dependent saturation scale Q_s. The measured charged multiplicity, however,
seems to grow faster (~\sqrt{s}^0.3) in nucleus-nucleus collisions than it does
for protons (~\sqrt{s}^0.2), violating the expectation from geometric scaling.
We argue that this difference between pp and AA collisions can be understood
from the effect of DGLAP evolution on the value of the saturation scale, and is
consistent with gluon saturation observations at HERA.Comment: RevTeX, 8 pages, 4 figures. V2: modified discussion of fragmentation,
published in EPJ
Nuclear shadowing in Glauber-Gribov theory with Q2-evolution
We consider deep inelastic scattering off nuclei in the Regge limit within
the Glauber-Gribov model. Using unitarized parton distribution functions for
the proton, we find sizeable shadowing effects on the nuclear total and
longitudinal structure functions, and , in the low-x limit.
Extending a fan-diagram analysis for the large-mass region of coherent
diffraction off nuclei to high Q2, we also find significant shadowing effects
in this kinematical regime. Finally, we discuss shortcomings of our approach
and possible extensions of the model to other kinematical regimes.Comment: 16 pages, 9 figure
Generalized parton distributions and Deeply Virtual Compton Scattering in Color Glass Condensate model
Within the framework of the Color Glass Condensate model, we evaluate quark
and gluon Generalized Parton Distributions (GPDs) and the cross section of
Deeply Virtual Compton Scattering (DVCS) in the small- region. We
demonstrate that the DVCS cross section becomes independent of energy in the
limit of very small , which clearly indicates saturation of the DVCS
cross section. Our predictions for the GPDs and the DVCS cross section at
high-energies can be tested at the future Electron-Ion Collider and in
ultra-peripheral nucleus-nucleus collisions at the LHC.Comment: 20 pages, 8 Figure
Strangeness Enhancement in and Interactions at SPS Energies
The systematics of strangeness enhancement is calculated using the HIJING and
VENUS models and compared to recent data on , and
collisions at CERN/SPS energies (). The HIJING model is used to
perform a {\em linear} extrapolation from to . VENUS is used to
estimate the effects of final state cascading and possible non-conventional
production mechanisms. This comparison shows that the large enhancement of
strangeness observed in collisions, interpreted previously as possible
evidence for quark-gluon plasma formation, has its origins in non-equilibrium
dynamics of few nucleon systems. % Strangeness enhancement %is therefore traced
back to the change in the production dynamics %from to minimum bias
and central collisions. A factor of two enhancement of at
mid-rapidity is indicated by recent data, where on the average {\em one}
projectile nucleon interacts with only {\em two} target nucleons. There appears
to be another factor of two enhancement in the light ion reaction relative
to , when on the average only two projectile nucleons interact with two
target ones.Comment: 29 pages, 8 figures in uuencoded postscript fil
Limiting fragmentation in hadron-hadron collisions at high energies
Limiting fragmentation in proton-proton, deuteron-nucleus and nucleus-nucleus
collisions is analyzed in the framework of the Balitsky-Kovchegov equation in
high energy QCD. Good agreement with experimental data is obtained for a wide
range of energies. Further detailed tests of limiting fragmentation at RHIC and
the LHC will provide insight into the evolution equations for high energy QCD.Comment: 28 pages, 10 figures (2 new figures, text slightly expanded, and some
additional references
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Search for lepton-flavor violation at HERA
A search for lepton-flavor-violating interactions and has been performed with the ZEUS detector using the entire HERA I
data sample, corresponding to an integrated luminosity of 130 pb^{-1}. The data
were taken at center-of-mass energies, , of 300 and 318 GeV. No
evidence of lepton-flavor violation was found, and constraints were derived on
leptoquarks (LQs) that could mediate such interactions. For LQ masses below
, limits were set on , where
is the coupling of the LQ to an electron and a
first-generation quark , and is the branching ratio of
the LQ to the final-state lepton ( or ) and a quark . For
LQ masses much larger than , limits were set on the four-fermion
interaction term for LQs that couple to an electron and a quark
and to a lepton and a quark , where and are
quark generation indices. Some of the limits are also applicable to
lepton-flavor-violating processes mediated by squarks in -Parity-violating
supersymmetric models. In some cases, especially when a higher-generation quark
is involved and for the process , the ZEUS limits are the most
stringent to date.Comment: 37 pages, 10 figures, Accepted by EPJC. References and 1 figure (Fig.
6) adde
Multijet production in neutral current deep inelastic scattering at HERA and determination of alpha_s
Multijet production rates in neutral current deep inelastic scattering have
been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2.
The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s)
= 318 GeV using the ZEUS detector and correspond to an integrated luminosity of
82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster
algorithm in the longitudinally invariant inclusive mode. Measurements of
differential dijet and trijet cross sections are presented as functions of jet
transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with
E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD
calculations describe the data well. The value of the strong coupling constant
alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections,
is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.)
{+0.0064}_{-0.0046}(th.)Comment: 22 pages, 5 figure
Measurement of (anti)deuteron and (anti)proton production in DIS at HERA
The first observation of (anti)deuterons in deep inelastic scattering at HERA
has been made with the ZEUS detector at a centre-of-mass energy of 300--318 GeV
using an integrated luminosity of 120 pb-1. The measurement was performed in
the central rapidity region for transverse momentum per unit of mass in the
range 0.3<p_T/M<0.7. The particle rates have been extracted and interpreted in
terms of the coalescence model. The (anti)deuteron production yield is smaller
than the (anti)proton yield by approximately three orders of magnitude,
consistent with the world measurements.Comment: 26 pages, 9 figures, 5 tables, submitted to Nucl. Phys.
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