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Date: Mon, 22 Jan 2007 14:33:13 -0500
From: physnews@aip.org
To: sondheim@PANIX.COM
Subject: Physics News Update 809

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 809  22 January 2007  by Phillip F. Schewe, Ben Stein, Turner
Brinton,and Davide Castelvecchi        www.aip.org/pnu

GRAVITATIONAL WAVE BACKGROUND.  In the standard model of cosmology,
the early universe underwent a period of fantastic growth.  This
inflationary phase, after only a trillionth of a second, concluded
with a violent conversion of energy into hot matter and radiation.
This �reheating� process also resulted in a flood of gravitational
waves.  (Interestingly, some cosmologists would identify the �big
bang�with this moment and not the earlier time=0 moment.)  Let�s
compare this gravitational wave background (GWB) with the more
familiar cosmic microwave background (CMB).  The GWB dates from the
trillionth-of-a-second mark, while the CMB sets in around 380,000
years later when the first atoms formed.  The CMB represents a
single splash of photons which were (at that early time) in
equilibrium with the surrounding atoms-in-the-making; the microwaves
we now see in the sky were (before being redshifted to lower
frequencies owing to the universe�s expansion) ultraviolet waves and
were suddenly freed to travel unimpeded through space.  They are now
observed to be mostly at a uniform temperature of about 3 K, but the
overall map of the microwave sky does bear the faint imprint of
matter inhomogeneities (lumps) existing even then.
What, by contrast, does the GWB represent?  It stems from three
different production processes at work in the inflationary era:
waves stemming from the inflationary expansion of space itself;
waves from the collision of bubble-like clumps of new matter at
reheating after inflation; and waves from the turbulent fluid mixing
of the early pools of matter and radiation, before equilibrium among
them (known as thermalization) had been achieved.  The gravity waves
would never have been in equilibrium with the matter (since gravity
is such a weak force there wouldn�t be time to mingle adequately);
consequently the GWB will not appear to a viewer now to be at a
single overall temperature.
A new paper by Juan Garcia-Bellido and Daniel Figueroa (Universidad
Autonoma de Madrid) explain how these separate processes could be
detected and differentiated in modern detectors set up to see
gravity waves, such as LIGO, LISA, or BBO (Big Bang Observer).
First, the GWB would be redshifted, like the CMB.  But because of
the GWB�s earlier provenance, the reshifting would be even more
dramatic: the energy (and frequency) of the waves would be
downshifted by 24 orders of magnitude.  Second, the GWB waves would
be distinct from gravity waves from point sources (such as the
collision of two black holes) since such an encounter would release
waves with a sharper spectral signal.  By contrast the GWB from
reheating after inflation would have a much broader spectrum,
centered around 1 Hz to 1 GHz depending on the scale of inflation.
Garcia-Bellido (34-91-497-4896, juan.garciabellido@uam.es) suggests
that if a detector like the proposed BBO could disentangle the
separate signals of the end-of-inflation GWB, then such a signal
could be used as a probe of inflation and could help explore some
fundamental issues as matter-antimatter asymmetry, the production of
topological defects like cosmic strings, primoridal magnetic fields,
and possibly superheavy dark matter.  (Physical Review Letters,
upcoming article; see also http://lattice.ft.uam.es/)

TOMOGRAPHY OF PROTONS.  In medical imaging, such as MRI, a planar
slice of tissue can be imaged in longitudinal space.  A
three-dimensional image of structure in the body is built up from a
composite of planar views.  By analogy, physicists at the Jefferson
Lab in Virginia are attempting to image the quarks inside protons,
one planar slice at a time in momentum space, with the goal being
the formation of a three dimensional quark map of the proton.  In
the case of proton tomography, the �microscope� consists of an
intense beam of electrons which strikes a hydrogen target.  An
electron can scatter from a proton in many ways, but here a single
collision is sought, a rather rare event called deeply virtual
Compton scattering (DVCS); the incoming electron scatters by sending
a virtual photon (a high energy gamma ray) out ahead of it.  This
scatters not from the proton as a whole, but from one of the
elementary quarks that together with the gluons are the building
blocks of the proton. The quark re-emits a gamma ray but does not
otherwise change its identity.  In this way the original target
proton retains intact.  Thus the overall reaction is as follows: an
electron and proton collide and out comes an electron, proton, and
gamma ray; the outgoing electron and gamma are detected, and from
this a lot about the status of quarks inside the proton can be
gleaned.  For example, the spatial position of the quark inside the
proton (transverse to the direction of the virtual photon) can be
related to the angles and energies of the outgoing gamma ray.  It�s
as if a quark had been removed from one place inside the proton and
then returned to another place.
In one important sense the Jefferson Lab experiment is not like
medical imaging.  In conventional microscopy, decreasing the
wavelength of the illumination source allows one to see finer
details, and this is great when looking at the interior of tumors or
cells.  But the structures inside a proton, quarks, are pointlike,
beyond the resolving power of any probe. Therefore, the structure of
protons can be probed but not that of quarks.  In proton tomography,
the  momentum transferred (actually the square of the transfer
momentum, or Q^2) from electron to quark in the form of a virtual
gamma ray should, up to a point, provide better spatial resolution.
Beyond a certain level, however, a larger Q^2 does not get you
greater resolving power.  What this means is that the gamma is no
longer probing the proton as a whole but rather individual quarks.
The best one can do is to map out the probabilities for the presence
of quarks with a certain momentum to reside at various places inside
the proton; this is analogous to the �orbital� clouds used to depict
the likely position of electrons in various energy levels inside
atoms.
Indeed, perhaps the most important thing achieved in the present
experiment is to affirm that the scattering becomes independent of
Q^2 above a level of about 2 GeV^2.  This says that true tomography
of the proton is proceeding. DVCS events, which have been seen in
other experiments before but never with the exactitude employed
here, are rare.  Nevertheless, the Jefferson physicists were able to
muster a million of them.
With a requested upgrade in electron beam energy, the researchers
hope to carry their map of the proton to quarks which carry a higher
share of the proton�s momentum.  This in turn will allow the JLab
physicists to explore the origin of proton mass and spin.  (Munoz
Camacho et al., Physical Review Letters, 31 December 2006; contact
Carlos Munoz Camacho, cmunoz@clipper.ens.fr, now also at Los Alamos
National Lab, 505-606-6-7 )

***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
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Physics News Update appears approximately once a week.

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