The CoolerCSB Experiment at the Indiana University

1
The Cooler-CSB Experiment at the Indiana
University Cooler Synchrotron A Test of Charge
Symmetry Breaking and Isospin Conservation via
the Reaction dd Y ap0
C. Allgower, V. Anferov, A.D. Bacher, G.P.A.
Berg, J. Doskow, C.C. Foster, W. Fox, D.
Friesel, A. Gardestig, C. Horowitz, C. Lavelle,
H. Nann, J. Olmsted, T, Rinckel, E.J. Stephenson,
and K. Solberg Indiana University Cyclotron
Facility, Bloomington, IN 47408 M.A.
Pickar Minnesota State University at Mankato,
Mankato, MN 56002 B. Chujko, A. Kuznetsov, V.
Medvedev, D. Patalakha, A. Prudkoglyad, and P.A.
Semenov Institute for High Energy Physics,
Protvino, Moscow Region, Russia 142284 P.V.
Pancella Western Michigan University, Kalamazoo,
MI 49001 A. Smith Hillsdale College, Hillsdale,
MI 49242 H.M. Spinka Argonne National
Laboratory, Argonne, IL 60439 S. Shastry State
University of New York, Plattsburgh, NY 12901 J.
Rapaport Ohio University, Athens, OH 45701 G.A.
Miller University of Washington, Seattle, WA
98195 U. van Kolck California Institute of
Technology, Pasadena, CA 91109
spokesperson for CE-82 spokesperson for Letter of
Intent (CE-78) technical manager theory
2
Outline
I. Background Concepts
II. Some Isospinology
III. A Little Theory Relevant to
IV. Description of the Cooler-CSB Experiment
V. Some Previews of Preliminary Data
3
Threshold Energy
m3 - mn
m1
m2
To make a reaction go in a fixed target
experiment, m1 has to hit m2 with enough energy
to make the particles in the final state.
The minimum kinetic energy required is called the
threshold energy
Tthr - Q
2m2
Examples
Tthr 225.4 MeV
p 0
3He
d
p


Tthr 198.7 MeV
Tthr 1120 MeV
4
in the lab close to threshold
g
cone angle
a
p 0
.
d
d
g
For a fixed target experiment just above
threshold,

• a particles emerge within a narrow cone about
  the 0-degree line.
• (Spectrometer with small forward acceptance
  will catch every a.)

• low-energy p0 quickly decays into two photons
  which emerge nearly back to back in the lab.

Therefore, the apparatus must identify a forward
a in coincidence with two photons that have a
large opening angle between them.
5
Isospin
A property of the strong interaction which
identifies particle states and can be used to
generalize ways in which particles of different
types interact. The algebra of isospin is
isomorphic to that of spin
Examples
Light Quarks
Light Baryons
Light Mesons
h0
h0


p
I3 1
(
p 0
I3 0
p -
I3 -1
)
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In terms of isospin operators, the N-N
interaction can be written as follows
Vg
VNN

VI
VII
VIII
VIV




VEM

Vg

Coulomb Interaction
Vg
Q(1)
Q(2),



I3(1)
I3(2)
(
)



I3(1)
I3(2)


for pp, 0 for np, nn
VEM

Other electromagnetic terms, dipole-dipole, etc.
E. M. Henley, G. A. Miller, Mesons in Nuclei,
p. 405, Amsterdam, North-Holland (1979)
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Isospin-Conserving Pieces
Example
VI Isospin Symmetric Force
Scattering Length
lim s 4 p a2
k 0
(I0) np is different from (I1) pp, np, nn, all
of which are the same.(e.g. You can have a bound
I0 deuteron and three I1 unbound states)
NN Scattering Lengths for I1 States
(Coulomb-corrected)
VII Charge Symmetric Force
pp
-17.3
0.4 fm
-
VII c (
)
I3(1)
I3(2)
0.3 fm
-18.8
nn
For (I1), nn and pp are same, but np(I1) is
different.
-23.75
0.09 fm
np
These numbers suggest that VI is large, VII
is small, and VIII is smaller still.
VIII Charge Antisymmetric Force
VIII d (
)
I3(1)

I3(2)
No effect on np, but now nn and pp are different.
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Isospin-Nonconserving Piece
VIV Isospin Antisymmetric Force
-
VIV
e (


)
I3(1)
I3(2)
f (
)
x
No effect on nn, pp, but now in np, I0 and I1
are mixed.
VIV allows you to violate conservation of isospin
in nuclear reactions.
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Charge Symmetry
Physics is unchanged under interchange of p,n (or
u,d).
eipI
PCS

2
Charge Conjugates
(PCS leaves the state unchanged)
-

PCS
PCS

d

p 0

a


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In a reaction that involves only charge conjugate
particles, one would expect charge symmetry to
hold.

• The reaction

(I0)
(I0)
(I0)
(I1)
p 0
cannot happen because the is an I 1
particle, and so the reaction violates
conservation of isospin.

• There is no other law of physics that forbids
  this reaction from
• occurring above the reaction threshold.

• If a signal for this reaction were observed,
  then it would imply
• that VIV is nonzero.

Note The reaction
is allowed, because the eta is an I 0 particle.
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There are two basic mechanisms for how the
reaction might go
a
a
a
p 0
p 0
p 0
I1?
(I1)
p
h


I1?
d
d
d
d
d
d
Threshold
Mixing

• Near threshold (Td 225.4 MeV) Threshold
  should dominate.
• (Note Threshold amplitude is proportional
  to the u,d quark
• mass difference, so it is an interesting QCD
  probe.)

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dmN
Strong Int.
Connections to Other Measurements
Coulomb
Mixing
Neutron-Proton Mass Difference


U. von Kolck, et. al., Phys. Lett. B 493, 65
(2000)
IUCF Cooler-CSB Measurement
a
b
c
dmN
Mixing
Threshold
(a, b, c are being determined by a collaboration
of Chiral Perturbation theorists)
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Challenges of Measuring
Cross Section

• Need d beam, d target.

• Need to measure luminosity (calibrated dd
  elastic system)
• (Scintillators located at lab angle of 45
  degrees measure dd elastic
• scattering to provide normalization for cross
  section determination.)

• Need to detect the forward a (magnetic
  spectrometer, particle ID)

• Need to detect photons from p0 decays (arrays of
  Pb glass counters)

• and oh, by the way ...

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IUCF Cooler Synchrotron
Location of Cooler-CSB
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Gas Jet Target
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Scintillators DE-2 E Veto-1 Veto-2

• COOLER-CSB MAGNETIC CHANNEL
• separate all 4He for total cross section
  measurement
• determine 4He 4-momentum (using TOF and position)
• detect one or both decay gs from p0 in Pb-glass
  array

Pb-glass array 250 detectors from IUCF and ANL
(Spinka) scintillators for cosmic trigger
Scintillator DE-1
Focussing Quads
MWPC
MWPCs
Target D2 jet
Beam tensor polarized deuterons
20E Septum Magnet
Separation Magnet removes 4He at 12.5E from beam
at 6E
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Basic Trigger Logic for Magnetic Channel
From the known timing for 3He po and detailed
energy loss calculations, one can deduce the
correct timing for the 4He po case. This is
crucial for detecting a rare signal in a hail of
particles coming from various background
processes, each of which has a large cross
section.
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• Need to reject 8-9 orders of magnitude of
  background from ...

Method of Rejection
d d
d d
elastics
d n p
breakup
n n p p
big backgrounds, but can be eliminated by good
spectrometer design, good particle ID, software
cuts.
3He n
allowed nuclear reactions
3H p
4He g
.
.
.
non-target-related junk, ...
4He h
allowed meson production
Running just above threshold for a p0 puts you
below threshold for all of these no problem.
4He 2p0
d n n p
breakup meson production
d p p p-
double radiative alpha production
True background must be subtracted.
4He g g
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Double Radiative Alpha Production is ...

• a true background to our reaction of
• interest,

a
g
g

• a reaction that has never been
• unambiguously seen experimentally,

• and of great theoretical interest by
• itself. (We have a fair-sized group
• of theorists working on cross section
• estimates for both a g g and a po.)

A past experiment at Saclay done near the eta
production threshold (1100 MeV) reported a
positive result for a po, but later it was
argued that they probably saw only a g g
instead.
d
d
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SEPARATION OF ap0 AND agg EVENTS
Calculate missing mass from the four-momentum
measured in the magnetic channel alone, using TOF
for z-axis momentum and MWPC X and Y for
transverse momentum.
High-statistics Monte Carlo simulation for
illustration. Experimental errors included in
peak width.
MWPC spacing 2 mm
ap0 peak sTOT 20 pb
Y-position (cm)
agg prediction from G_at_rdestig
MWPC1 X-position (cm)
agg background (16 pb)
TOF resolution sGAUSS 100 ps
Difference is due to acceptance of
channel. Acceptance widths are angle 70
mr (H and V) momentum 10
missing mass (MeV)
Cutoff controlled by available energy
above threshold.
Time of Flight ()E1 - )E2) (ns)
NOTE Pb-glass no further help with background
separation.
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SENSITIVITY LIMITS
COUNT RATE ESTIMATE target thickness 2.5 H
1015 cm-2 average beam current 2 mA
luminosity 3 H 1031 cm-2s-1 Cooler duty factor
0.6 2-g efficiency (Pb-glass) 0.33 total
cross section 10 pb dd Y ap0 rate 5 /day
CROSS SECTION ESTIMATE measured sTOT for pd Y
3Hep0 of s/h 12.6 mb pion momentum fraction h
pp/mp 0.20 overlap calculation to add neutron
spectator 0.31 based on K.R. Greider,
Phys. Rev. 122, 1919 (1961). isospin violation
scaled from np Y dp0 0.35 based on U.
van Kolck, J.A. Niskanen, and G.A. Miller,
Phys. Lett. B 493, 65 (2000) and private
communications total cross section 5 20 pb
Simulated data for sTOT 2 pb and 2 months
running.
p0 missing mass peak
POLARIZATION vertical quantization axis 85 of
maximal tensor polarization Azz 1
(maximal) (radiative capture has vanishing
analyzing power)
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Milestones

• First test run (Nov. 1999) deuteron beam,
  scintillators, no magnetic channel

• IUCF technical review (Feb. 2000) (final
  approval for construction go-ahead)

• Deuteron polarization run, PINTEX detector (Feb.
  2001) (1st pol. d beam)

• Pb glass cosmic ray test stand (5/2000-1/2001)
  (Commissioned 256 counters)

• Channel construction (completed April 2001)
  (septum magnet built in house)

• Channel commissioning run (May 2001) (proton
  beam and no Pb glass,
• measuring the reaction p d 3He po
  just above threshold)

• Pb glass commissioning run (Jan. 2002) (same as
  May run, but now with Pb
• glass arrays installed.)

• First dd run (Mar. 2002) (Calibration of dd
  elastic luminosity systems,
• start of production running for d d
  4He po )

• Production running (May-July 2002) (Take the
  real data, analyze it!!!!)

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Raw Hit Patterns in Channel Front Wire Chamber
0.7 MeV above threshold (0.951o cone)
0.8 MeV below threshold (thr 198.7 MeV)
(Charge 1 Junk)
1.4 MeV above threshold (1.550o cone)
3.1 MeV above threshold (1.2o cone) (thr225.4
MeV)
(Charge 1 Junk)
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Numbers of Pb Glass Hit, Beam Left vs. Beam Right
Cosmic Rays
0.8 MeV below threshold (thr 198.7 MeV)
0.7 MeV above threshold (0.951o cone)
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Cone Propagation Down the Channel
Second Wire Chamber (Downstream of Septum Magnet)
First Wire Chamber (Upstream of Septum Magnet)
Third Wire Chamber (Back End of Channel)
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Missing Mass Resolution
Mass of po 134.98 MeV/c2
Resolution is 100 KeV.
Mass (Mev/c2 100)
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d d 4He po
Picking Out Alphas from Scintillator Raw Data
Raw Data
Requirements

• gt2 Hits in Pb
• Glass, and
• 130 MeV/c2
• Missing Mass

DE1 Energy vs. DE2 Energy
Alphas from dd 4He po will go through DE1,
DE2, and stop in E.
The Above Window Picks Out a Pretty Clean
Locus of Alphas.
DE2 Energy vs. E Energy
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d d 4He po
Picking Out Alphas from Scintillator Raw Data
Raw Data (3 Days, 3/2002)
Requirements
Events Inside En. vs. En. Window in DE1 vs. DE2
Pb Glass Hits, Beam Right vs. Beam Left
Alphas from dd 4He po will go through DE1,
DE2, and stop in E.
Lots of Alphas with Photon Pairs in
the Coincidence, but So Far No Clear Peak for an
Excess at the po Mass.
Missing Mass (Mev/c2 100)
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Summary

• The reaction

is a severe and clean test of
fundamental properties of isospin in the strong
interaction.

• Doing the experiment at threshold isolates the
  Threshold mechanism,
• thus simplifying the theoretical treatment.
  It also eliminates many
• potential sources of background.

• We have built an apparatus that has the
  required sensitivity to see
• the signal, if there is one. Preliminary
  data online show evidence for
• a clean sample of events from d d a
  g g, and in May-July
• 2002 we will see any po signal larger than
  a few picobarns.

• Results from this experiment, coupled with
  results from TRIUMF
• experiment E704 and calculations from chiral
  perturbation theory
• will help to unravel the relative sizes of
  the Coulomb and Strong
• pieces that contribute to the neutron proton
  mass difference.
• This in turn can also help determine the u,d
  quark mass difference.