[News-releases] TRIUMF & Canadian Scientists Help Measure Proton's Weak Charge

[Tim Meyer]

News Release | For Immediate Release | 17 Sep 2013, 5:00 p.m. PDT

TRIUMF & CANADIAN SCIENTISTS HELP MEASURE PROTON’S WEAK CHARGE

(Newport News, VA, USA) --- An international team including Canadian
researchers at TRIUMF has reported first results for the proton’s weak
charge in Physical Review Letters (to appear in the October 18, 2013 issue)
based on precise new data from Jefferson Laboratory, the premier U.S.
electron-beam facility for nuclear and particle physics in Newport News,
Virginia.

The Q-weak experiment used a high-energy electron beam to measure the weak
charge of the proton—a fundamental property that sets the scale of its
interactions via the weak nuclear force. This is distinct from but analogous
to its more familiar electric charge (Q), hence, the experiment’s name:
‘Q-weak.’  Following a decade of design and construction, Q-weak had a
successful experimental run in 2010–12 in Hall C at Jefferson Laboratory.
Data analysis has been underway ever since.

“Nobody has ever attempted a measurement of the proton’s weak charge
before,” says Roger Carlini, Q-weak’s spokesperson at Jefferson Laboratory,
“due to the extreme technical challenges to reach the required sensitivity.
The first 4% of the data have now been fully analyzed and already have an
important scientific impact, although the ultimate sensitivity awaits
analysis of the complete experiment.”

The first result, based on Q-weak’s commissioning data set, is Q_W^p= 0.064
± 0.012.  This is in good agreement with the theoretical prediction of
0.0710 ± 0.0007 based on our current understanding of fundamental
interactions, which physicists refer to as the Standard Model.

In order to measure the proton’s weak charge, experimenters had to exploit
the weak interaction’s unique property of parity violation, closely related
to mirror symmetry.  The Q-weak collaboration built an apparatus to detect
the scattered electrons with unprecedented sensitivity, allowing them to
measure the tiny asymmetry in the electron scattering rate that depends on
the longitudinal polarization of the electron beam. The success of the
experiment relied on Jefferson Laboratory’s world renowned ‘parity quality’
beam properties.  When the spin of the beam particles is reversed with
respect to their direction of motion, the changes to its other properties
can be kept amazingly small -- for example, the beam moves less than the
width of an atom, on average.  

To achieve the required statistical precision for Q-weak, the CEBAF
accelerator at Jefferson Laboratory was pushed to new limits of high
intensity polarized beam delivery, and the liquid hydrogen (proton) target
built for Q-weak was able to absorb 1.7 kW of beam power while maintaining
uniform density at a temperature of only 20 degrees above absolute zero,
making it the world’s highest power liquid hydrogen target to date.  The
apparatus contained many interleaved diagnostic systems to monitor and
diagnose beam conditions during data taking.  All of these systems worked
extremely well, as demonstrated by the first results reported here.

The Canadian group has played a leading role in the Q-weak experiment since
its inception – a proposal submitted to Jefferson Laboratory’s Program
Advisory Committee 12 years ago.  Dr. Shelley Page (U. Manitoba) is a
co-spokesperson for Q-weak; Dr. Willem T.H van Oers led the construction of
the large magnetic spectrometer for Q-Weak in collaboration with scientists
and engineers from TRIUMF, the University of Manitoba, MIT-Bates Laboratory,
and Jefferson Laboratory.  In all, the Canadian group contributed about 15%
of the Q-weak effort in terms of manpower and resources, focused on design
and construction of the experimental apparatus, planning and execution of
the measurements, and analysis of the data. Extending a successful design
that was developed for the TRIUMF proton-proton parity violation experiment
(TRIUMF E497), essentially all of the low noise integrating electronics used
for Q-weak were built at TRIUMF, including readout of the novel diamond
strip detector that was used in a new Compton polarimeter, provided by the
University of Winnipeg group. Over the years, more than $3M of NSERC support
has been provided through the subatomic physics Project Grant program to the
Canadian group (which also includes scientists from the University of
Winnipeg, University of Northern BC, and TRIUMF) – funds were used to build
equipment and support student and postdoctoral researchers’ salaries and
travel to carry out the measurements at Jefferson Laboratory.

The really great news is that Q-weak has about 25 times more data in hand,
which is currently undergoing analysis.  This will allow experimenters to
further shrink the error bar, which will then be small enough to severely
constrain possible physics beyond the Standard Model. The final result could
reveal hints of new interaction-mediating particles, and would be
complementary to searches at the highest energy scales such as at the Large
Hadron Collider project in Geneva, Switzerland.

This first determination of the proton’s weak charge was carried out by a
team of 97 researchers from 23 institutions in the U.S., Canada, and Europe.
It was made possible by funding from the US Department of Energy and the
U.S. National Science Foundation, the Natural Sciences and Engineering
Research Council of Canada, university matching contributions from The
College of William and Mary, Virginia Tech, George Washington University,
and Louisiana Tech University, and technical and engineering support from
Jefferson Laboratory, TRIUMF, and MIT-Bates laboratories.  

For more information, please visit http://www.jlab.org/qweak.  

--30--


****************************************************************************
***************************
Timothy I. Meyer, Ph.D.
Head, Strategic Planning & Communications
TRIUMF -- Accelerating Science for Canada | 
    Un accélérateur de la démarche scientifique canadienne
4004 Wesbrook Mall | Vancouver, BC  V6T 2A3 | CANADA
Tel: 604.222.7674
Fax: 604.222.3791
Mobile (w/call fwd): 604.235.1925
E-mail: tmeyer at triumf.ca
WWW: http://www.triumf.ca
****************************************************************************
***************************

-------------- next part --------------
A non-text attachment was scrubbed...
Name: NR-ProtonWkCharge-17-Sep-2013.pdf
Type: application/pdf
Size: 31609 bytes
Desc: not available
Url : http://lists.triumf.ca/pipermail/news-releases/attachments/20130917/43e6f928/attachment-0001.pdf