3,026 research outputs found
Spin asymmetry at large x_F and k_T
We suggest that the large single spin asymmetries observed at high momentum
fractions x_F and transverse momenta k_T of the pion in p^\uparrow p ->
\pi(x_F,k_T)+X arise from the coherence of the soft interactions with the hard
parton scattering process. Such coherence can be maintained if x_F -> 1 as k_T
-> \infty, while k_T^2(1-x_F) ~ \Lambda_{QCD}^2 stays fixed. Analogous
coherence effects have been seen experimentally in the Drell-Yan process at
high x_F. We find that the p^\uparrow p -> \pi X production amplitudes have
large dynamic phases and that helicity flip contributions are unsuppressed in
this limit, giving rise to potentially large single spin asymmetries.Comment: 3 pages, 1 figure. Talk given at EPS HEP 2007, Mancheste
Electroweak phase transition in technicolor
Several phenomenologically viable walking technicolor models have been
proposed recently. I demonstrate that these models can have first order
electroweak phase transitions, which are sufficiently strong for electroweak
baryogenesis. Strong dynamics can also lead to several separate transitions at
the electroweak scale, with the possibility of a temporary restoration and an
extra breaking of the electroweak symmetry. First order phase transitions will
produce gravitational waves, which may be detectable at future experiments.Comment: 6 pages, 4 figures. Talk at PASCOS 2010 conference, Valencia, 19-23
July 201
N-point functions in rolling tachyon background
We study n-point boundary correlation functions in Timelike Boundary
Liouville theory, relevant for open string multiproduction by a decaying
unstable D-brane. We give an exact result for the one-point function of the
tachyon vertex operator and show that it is consistent with a previously
proposed relation to a conserved charge in string theory. We also discuss when
the one-point amplitude vanishes. Using a straightforward perturbative
expansion, we find an explicit expression for a tachyon n-point amplitude for
all n, however the result is still a toy model. The calculation uses a new
asymptotic approximation for Toeplitz determinants, derived by relating the
system to a Dyson gas at finite temperature.Comment: 30 pages, 2 figures. v2: minor corrections, comments and reference
added. v3: revised version, added spatial momentum dependence and references,
to appear in PR
Hydrogen Atom in Relativistic Motion
The Lorentz contraction of bound states in field theory is often appealed to
in qualitative descriptions of high energy particle collisions. Surprisingly,
the contraction has not been demonstrated explicitly even in simple cases such
as the hydrogen atom. It requires a calculation of wave functions evaluated at
equal (ordinary) time for bound states in motion. Such wave functions are not
obtained by kinematic boosts from the rest frame. Starting from the exact
Bethe-Salpeter equation we derive the equal-time wave function of a
fermion-antifermion bound state in QED, i.e., positronium or the hydrogen atom,
in any frame to leading order in alpha. We show explicitly that the bound state
energy transforms as the fourth component of a vector and that the wave
function of the fermion-antifermion Fock state contracts as expected.
Transverse photon exchange contributes at leading order to the binding energy
of the bound state in motion. We study the general features of the
corresponding fermion-antifermion-photon Fock states, and show that they do not
transform by simply contracting. We verify that the wave function reduces to
the light-front one in the infinite momentum frame.Comment: 20 pages, 10 figures; v2: some changes in discussion, accepted for
publication in Phys.Rev.
Mapping EK Draconis with PEPSI - Possible evidence for starspot penumbrae
We present the first temperature surface map of EK Dra from
very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and
Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Changes in
spectral line profiles are inverted to a stellar surface temperature map using
our Map code. The long-term photometric record is employed to compare our
map with previously published maps. Four cool spots were reconstructed, but no
polar spot was seen. The temperature difference to the photosphere of the spots
is between 990 and 280K. Two spots are reconstructed with a typical solar
morphology with an umbra and a penumbra. For the one isolated and relatively
round spot (A), we determine an umbral temperature of 990K and a penumbral
temperature of 180K below photospheric temperature. The umbra to photosphere
intensity ratio of EK Dra is approximately only half of that of a comparison
sunspot. A test inversion from degraded line profiles showed that the higher
spectral resolution of PEPSI reconstructs the surface with a temperature
difference that is on average 10% higher than before and with smaller surface
areas by 10-20%. PEPSI is therefore better suited to detecting and
characterising temperature inhomogeneities. With ten more years of photometry,
we also refine the spot cycle period of EK Dra to 8.90.2 years with a
continuing long-term fading trend. The temperature morphology of spot A so far
appears to show the best evidence for the existence of a solar-like penumbra
for a starspot. We emphasise that it is more the non-capture of the true umbral
contrast rather than the detection of the weak penumbra that is the limiting
factor. The relatively small line broadening of EK Dra, together with the only
moderately high spectral resolutions previously available, appear to be the
main contributors to the lower-than-expected spot contrasts when comparing to
the Sun.Comment: Accepted for A&
Gravitational Techniwaves
We investigate the production and possible detection of gravitational waves
stemming from the electroweak phase transition in the early universe in models
of minimal walking technicolor. In particular we discuss the two possible
scenarios in which one has only one electroweak phase transition and the case
in which the technicolor dynamics allows for multiple phase transitions.Comment: 30 pages, 5 figures. v2: minor changes, references added, title
changed in journa
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