[tt] Physics News Update 861

Thu Apr 17 06:25:56 UTC 2008

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From: physnews at aip.org
Date: Wed, 16 Apr 2008 11:44:33 -0400
To: eugen at LEITL.ORG
Subject: Physics News Update 861
Reply-to: physnews at aip.org

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 861   April 15, 2008      www.aip.org/pnu
by Phillip F. Schewe and Jason S. Bardi

FINDING THE HIGGS BOSON is the imperative of the two most powerful
particle accelerators ever built---the Tevatron at Fermilab, now
reaching the peak of its decades-long performance, and the Large
Hadron Collider (LHC) at CERN, where beams will circulate for the
first time around a 27-km track within the next few months.  The
Higgs has not yet been discovered, but at this week*s meeting of the
American Physical Society (APS) in St. Louis dozens of talks
referred to the status of the Higgs search.  Why is the Higgs so
important?  Because it is thought to pervade the universal vacuum;
not, as with the old aether, to provide a material substrate for the
propagation of electromagnetic waves, but rather to interact with
particles and confer mass upon them.  The Higgs* ministrations are
usually hidden away in the vacuum, but if enough energy is brought
to bear in a tiny volume of space---at the point where two energetic
particles collide---then the Higgs can be turned into an actual
particle whose existence can be detected.  Theoretical calculations
made using the standard model of particle physics combined with
previous experiments serve to limit the possible range of masses for
the Higgs particle.  Right now that mass is thought to be larger
than 114 GeV but smaller than about 190 GeV.
The Tevatron delivers more than enough energy to create a particle
in that energy range.  The main issue, then, is luminosity, or the
density of beam particles crashed together per second.  The Tevatron
recently established a record high luminosity: 3.1 x 10^32 per cm^2
per second.
That would a Higgs event look like? One speaker at the meeting,
Brian Winer (Ohio State), said that the *most Higgs-like Higgs
event* seen so far was on in which (it is surmised) the
proton-antiproton collision at the Tevatron had created a fireball
which then decayed into a W boson (one of the carriers of the weak
nuclear force) and a Higgs particle.  The Higgs in turn quickly
decayed into a bottom-antibottom quark pair whose combined mass
amounted to 120 GeV.  By itself such an event does not constitute a
discovery. Successfully observing the Higgs involves finding an
inventory of candidate events substantially larger than the number
of expected background events from collisions which to not produce a
Higgs particle but which mimic some features of the Higgs.
Time (and luminosity) will tell whether the Tevatron accumulates
enough Higgs candidate events to establish a
statistically-satisfactory *discovery.*  One Tevatron physicist,
Dmitri Denisov (denisovd at fnal.gov) summarized the likely status of
things when the experiments (the CDF and D0 detector groups) start
to wrap up in the year 2010.  The luminosity, he said, would
probably be twice what it is now and that 4 to 8 times more data
would be analyzed than is available today.
The Higgs, if it exists, is expected to show up in abundance at the
LHC, where the collision energy is much higher than at the
Tevatron.  Abraham Seiden (abs at scipp.ucsc.edu) of UC Santa Cruz
summarized the current status of the LHC.  In the CERN lab
scientists and engineers are now chilling down the magnets which
steer protons around their proper trajectory to the
near-absolute-zero temperatures needed for operating in a
superconducting mode.  Although designed to produce proton beams at
7 TeV, the accelerator will at first hold to a more conservative  5
TeV.  As for the present schedule, Seiden quoted a recent CERN
report specifying mid June as th
e time when the machine would be
cooled and ready to circulate beams around the ring and August as
the time when actual particle collisions will commence.  However,
several scientists at the meeting, when asked, were somewhat
skeptical that this timeline would be met.
As for the prospective scenario for discoveries at LHC in coming
years, Seiden said that finding evidence for a supersymmetric
particle (one of a large family of hypothetical particles) might be
possible as early as the year 2009, while finding the Higgs might be
possible by 2010.

***********
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