GSI - 20.08.04 P. Lenisa - Univ. Ferrara and INFN
1
PAXPAX PPolarizedolarized AAntiproton Entiproton Exxperimentperiment
PAX CollaborationPAX Collaborationwww.fz-juelich.de/ikp/pax
Spokespersons:
Paolo Lenisa [email protected] Rathmann [email protected]
Status reportStatus report
2
OutlineOutline
• Extracted beam vs internal target (vs collider)
• Transversity measurement by Drell-Yan – Rates– Angular distribution– Background
• Detector concept• Conclusions
3
Extracted beam vs internal Extracted beam vs internal targettarget
Lext=7.51024 x 1.3106 = 1.0 1031 cm-2 s-1
Extracted beam:
Lext=t x Npbart = areal density (15 g/cm2 NH3)
Internal target
Lint= t x f x Npbar
t = areal densityf = revolution frequencyNpbar = number of pbar stored in HESR
Lint= 7.21014 x 6105 x 4.91010 = 2.1 1031 cm-2 s-1
Production rate of polarized antiprotons (P = 2 B) cannot exceed:
Npbar = 1.0107/e2 = 1.3106 pbar/s
Drell-Yan events rate: NDY=L x
DY
extDYDY NN 1.2int
Polarized beam luminosity:Polarized beam luminosity:
4
extDY
extDY
ext
NNPQdTTA
171
Extracted beam:
d=3/17 Q=0.85 P=0.3
intint
int 31
DYDY NNPQdTTA
Internal target:
d=1 Q=0.85 P=0.3
Extracted beam vs internal Extracted beam vs internal targettarget
Statistical uncertainty in Statistical uncertainty in AATTTT
DYNPQdTTA
1d = diluition factorQ = proton target polarizationP = antiproton beam polarization
2.8
1.2
3
17
3
17
int
int
extDY
extDY
DY
extDY
A
extA
N
N
N
N
TT
TT
factor 67 in measuring time!
5
Transversity measurement with Transversity measurement with Drell-Yan lepton pairsDrell-Yan lepton pairs
p pqL
q
l+
l-q2=M2
qT
Polarized antiproton beam → polarized proton target (both transverse)
q
qF
MxqMxqMxqMxqexxsMdxdM
d 22
21
22
21
2
212
2
2
2
,,,,)(9
4
1) Events rate.
),(),(
),(),(ˆ
22
21
221
211
MxuMxu
MxhMxha
d
dA
uu
TTTT
2) Angular distribution.
6
Drell-Yan cross section and event Drell-Yan cross section and event raterate
q
qF
MxqMxqMxqMxqexxsMdxdM
d 22
21
22
21
2
212
2
2
2
,,,,)(9
4 •M2 = s x1x2 •xF=2qL/√s = x1-x2
• Mandatory use of the invariant mass region below the J/ (2 to 3 GeV).•22 GeV preferable to 15 GeV
•x1x2 =
M2/s
15 GeV22 GeV
M>2 GeV
M>4 GeV
22 GeV
15 GeV
M (GeV/c2)
2 k events/day
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AATTTT asymmetry: angular distribution asymmetry: angular distribution
),(),(
),(),(ˆ
22
21
221
211
MxuMxu
MxhMxha
d
dA
uu
TTTT
The asymmetry is large in the large acceptance detector (LAD)
2cos)cos1(
sin),(
2
2
TTa
•The asymmetry is maximal for angles =90°
•The asymmetry has a cos(2) azimuthal asymmetry.
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Theoretical predictionTheoretical prediction
0.15
0.2
0.25 TT
TT
a
A
Anselmino, Barone, Drago, Nikolaev (hep-ph/0403114
v1)
T=22 GeV
T=15 GeV
0.3
0 0.6xF=x1-x2
0.40.2
Asymmetry amplitude Angular modulation
FWD: lab < 8°
LAD: 8° < lab < 50°
P=Q=1
LAD
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Estimated signalEstimated signal•120 k events sample
• 60 days at L=2.1 1031 cm2 s-2, P = 0.3, Q = 0.85
Events under J/y can double the statistics. Good momentum resolution
requested
LAD
LAD
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BackgroundBackground
mbpp
50
nbDY 1 108-109 rejection factor against background
• DY pairs can have non-zero transverse momentum (<pT> = 0.5 GeV)
coplanarity cut between DY and beam not applicable
• Background higher in the forward direction (where the asymmetry is lower).
• Background higher for than for e (meson decay)
hadronic absorber needed for inhibits additonal physics chan.
•Sensitivity to charge helps to subtract background from wrong-charge pairs
Magnetic field envisaged
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Background for Background for Xeepp
Average multiplicity: 4 charged + 2 neutral particle per event.
Combinatorial background from meson decay.
Prelim. estimation of most of the processes shows background under control.
pp21hh X
eeK /0
/
ee0
eeK //0
ee
ee
21hh
…
13
• Background higher for than for e
Background for Background for Xeepp
Preliminary PYTHIA result (2109 events)
• Background from charge coniugated mesons negligible for e.
e
x1000
x100
Total background
x1000
x100
e
Background origin
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Detector concept Detector concept
• Drell-Yan process requires a large acceptance detector
•Good hadron rejection needed
•102 at trigger level, 104 after data analysis for single track.
•Magnetic field envisaged
•Increased invariant mass resolution with respect to simple calorimeter
•Improved PID through E/p ratio
•Separation of wrong charge combinatorial background
•Toroid?
•Zero field on axis compatible with polarized target.
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Possible solution: 6 Possible solution: 6 superconducting coils superconducting coils
Sperconducting coils for the target do not affect azimuthal acceptance.
(8 coils solution also under study)
• 800 x 600 mm coils
• 3 x 50 mm section (1450 A/mm2)
• average integrated field: 0.6 Tm
• free acceptance > 80 %
GSI - 20.08.04 P. Lenisa - Univ. Ferrara and INFN
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Conclusions Conclusions • Internal target ideal to fully exploit the limited production of polarized antprotons
• 22 GeV preferred to 15 GeV
• Angular distribution of events mainly interests large acceptance detector
• Electrons favoured over muons for additional physics
• Background seems not a problem, but more detailed studies necessary
• A toroid magnet might be the proper choice for the polarized target.
•The collider represents an attractive perspetive (background to be studied).
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eeK 0
eeK 0
KKpp
ee000pp
eL eK 000 KKpp
eL eK 0
XKKpp
XeeDDpp
ee000pp
5.002.055.0000000 2
DYDalitz
pp
pp
DYDY
BRS
B
9000
pp
Example
ee000pp
0.01
M > 2GeV
Dalitz veto through unpaired e
wrong charge
10 nb @ GeV2
Background for Background for Xeepp
Combinatorial background from meson decay:
Direct estimation of candidate processes shows negligible contribution.
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Performance of Polarized Internal Performance of Polarized Internal TargetsTargets
PT = 0.795 0.033
HERMES
H Transverse Field (B=297 mT)
HERMES
H
D
PT = 0.845 ± 0.028
Longitudinal Field (B=335 mT)
HERMES: Stored Positrons PINTEX: Stored Protons
H
Fast reorientation in a weak field (x,y,z)
Targets work very reliably (many months without service)
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Detector ConceptDetector ConceptTwo complementary parts:
1. Forward Detector
• ±80 acceptance• unambiguous
identification of leading particles
• precise measurement of their momenta
• measurement of angles (θ,φ) and energies of Drell-Yan pairs
2. Large Acceptance Detector
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Count rate estimateCount rate estimateUncertainty of ATT depends on target and beam polarization
(|P|=0.05, |Q|~0.9)
N
22
NQP
1TTA
resonant J/Ψ contribution (2 higher rate) ½ times number of days
T = 15 GeV
T = 22 GeV
number
of sources
states during buildup
Feed tube/cell tube
Average Luminosity[1031 cm-2s-
1]
Number of days to Number of days to achieve above errorsachieve above errors
EM only P(2·τb)=0.05
EM + hadronic
P(2·τb)=0.10
1 e() p() Standard/Round 0.56 214 54
2 e() p() Standard/Round 0.72 166 42
2 e() p() Low Conductance/Round 1.90 63 16
2 e() p() Low Conductance/Elliptical
0.95 - 32For single spin asymmetries L ~ 10 times larger
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Cost EstimateCost Estimate
• Forward Spectrometer: – HERMES Spectrometer magnet plus detectors– Magnet possibly available after 2007
• Large Acceptance detector:– Structure of E835 detector assumed, using HERMES figures +
HERMES recoil detector • Target:
– Parts of the HERMES + ANKE Targets can be recuperated ( 20% Reduction)
• Infrastructure: – based on HERMES figures for platform, support structures,
cablingm cooling, water lines, gas supply lines and a gas house, cold gas supply lines, electronic trailer with air conditioning
Forward Spectrometer a la HERMES 12.0 M€
Large Acceptance Detector 2.6 M€
Target 1.8 M€
Infrastructure (cabling, cooling, platform, shielding)
3.0 M€
Total 19.4 M€
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Requirements for PAX at HESRRequirements for PAX at HESR
PAX needs a separate experimental areaa. Storage cell target requires low-β section (β=0.2
m)b. Polarization buildup requires a large acceptance
angle at the target (Ψacc = 10 mrad)c. HESR must be capable to store polarized
antiprotons• Slow ramping of beam energy needed
1. Optimization of polarization buildup2. Acceleration of polarized beam to highest energies
d. The experiment would benefit from higher energy (22 GeV)
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Final RemarkFinal Remark
Polarization data has often been the graveyard of fashionable theories. If theorists had their way, they might
just ban such measurements altogether out of self-protection. J.D. Bjorken
St. Croix, 1987
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Physics PerformancePhysics Performance
• Luminosity– Spin-filtering for two beam lifetimes: P > 5%
– N(pbar) = 5·1011 at fr~6·105 s-1
– dt = 5·1014 cm-2
1231105.110
1)0( scmdfNtL trp
Time-averaged luminosity is about factor 3 lower • beam loss and duty cycle
Experiments with unpolarized beam• L factor 10 larger
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Beam lifetimes in HESRBeam lifetimes in HESR
The lifetime of a stored beam is given byfd)(
1
t0Cb
2
1
2
1
vm2
ed
d
d2acc
42p0
4
.RuthC
max
min
)pp(tot0
(Target thickness =dt=5·1014 atoms/cm2)
1 400.8 800.6 1200.4 1600.2 20000
1
2
3
4
5
6
7
8
9
10
11
kinetic energy [MeV]
beam
lif
etim
e [h
]
10.89
0.214
T 20 103
T 10 103
T 5 103
T 1 103
2 1031 T
400 800 1200 T (MeV)
2
4
6
8be
am li
lfetim
e τ b
(h)
Ψacc = 1 mrad
5 mrad10 mrad
10 20 mrad
In order to achieve highest polarization in the antiproton beam, acceptance angles of Ψacc = 10 mrad are needed.
28
Low Conductance Feed TubeLow Conductance Feed Tube
Method tested successfully but not optimized during development of FILTEX/HERMES
Atomic Beam Source (Heidelberg 1991).
H1H2
~3
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Puzzle from FILTEX TestPuzzle from FILTEX Test
Observed polarization build-up: dP/dt = ± (1.24 ± 0.06) x 10-2 h-1
Expected build-up: P(t)=tanh(t/τ1),
1/τ1=σ1Qdtf=2.4x10-2 h-1
about factor 2 larger!
σ1 = 122 mb (pp phase shifts)Q = 0.83 ± 0.03dt = (5.6 ± 0.3) x 1013cm-2
f = 1.177 MHz
Three distinct effects:
1. Selective removal through scattering beyond θacc=4.4 mrad σR=83 mb
2. Small angle scattering of target protons into ring acceptance σS=52 mb
3. Spin transfer from polarized electrons of the target atoms to the stored protons
σE=-70 mb Horowitz & Meyer, PRL 72, 3981 (1994)
H.O. Meyer, PRE 50, 1485 (1994)
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Spin transfer from electrons to Spin transfer from electrons to protonsprotons
epep
020
p2
ep2
e
pa2ln2sin
2C
mp
m14
2
1
Horowitz & Meyer, PRL 72, 3981 (1994)
H.O. Meyer, PRE 50, 1485 (1994)
α fine structure constantλp=(g-2)/2=1.793 anomalous magnetic momentme, mp rest massesp cm momentuma0 Bohr radiusC0
2=2πη/[exp(2πη)-1] Coulomb wave functionη=-zα/ν Coulomb parameter (neg. for anti-protons)v relative lab. velocity between p and ez beam charge number
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Antiproton PolarizerAntiproton Polarizer
Exploit spin-transfer from polarizedelectrons of the target to antiprotons
orbiting in HESR
Expected Buildup
dt=5·1014 atoms/cm2, Pelectron=0.9
1 10 100 1 103
1 104
1 105
0.01
0.1
1
10
100
1 103
181.621
0.022
etr T
1.5 1045 T
10 100 1000 T (MeV)
e
(mba
rn)
100
10
1
T=500 MeV
T=800 MeV
Goal
antip
roto
n P
olar
izat
ion
(%)
t (h)10 20 30
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PolarimetryPolarimetry
Different schemes to determine target and beam polarization
1. Suitable target polarimeter (Breit-Rabi or Lamb-Shift) to measure target polarization
2. At lower energies (500-800 MeV) analyzing power data from PS172 are available.
Therefrom a suitable detector asymmetry can be calibrated→ effective analyzing power
•Beam and target analyzing powers are identical• measure beam polarization using an unpolarized
target•Export of beam polarization to other energies
• target polarization is independent of beam energy
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Beam Polarimeter Configuration for HESRBeam Polarimeter Configuration for HESR
Detection system for p-pbar elastic scattering+ simple, i.e. non-magnetic + Polarized Internal Storage Cell Target
- magnetic guide field (Qx,Qy,Qz)+ azimuthal symmetry (polarization
observables)+ large acceptance
Storage cell
Ex: EDDA at COSYEx: EDDA at COSY
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Polarization Conservation in a Polarization Conservation in a Storage RingStorage Ring
HESR design must allow for storage of polarized particles!
Indiana CoolerH.O. Meyer et al., PRE 56, 3578 (1997)
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Spin Manipulation in a Storage Spin Manipulation in a Storage RingRing
SPIN@COSY (A. Krisch et. al)– Frequent spin-flips reduce systematic errors– Spin-Flipping of protons and deuterons by artifical
resonance •RF-Dipole
– Applicable at High Energy Storage Rings (RHIC, HESR)
Stored protons:P(n)=Pi()n
=(99.3±0.1)%
36
Single Spin AsymmetriesSingle Spin Asymmetries
Several experiments have observed unexpectedly large single spin asymmetries in pbar-p at large values of xF ≥ 0.4 and
moderate values of pT (0.7 < pT < 2.0 GeV/c)
E704 Tevatron FNAL 200GeV/c
xF
NN
NN
P
1A
beamN
π+
π-
Large asymmetries originate from valence quarks: sign of AN related to u and d-quark polarizations
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Proton Electromagnetic Formfactors Proton Electromagnetic Formfactors
• Measurement of relative phases of magnetic and electric FF in the time-like region– Possible only via SSA in
the annihilation pp → e+e-
• Double-spin asymmetry– independent GE-Gm
separation– test of Rosenbluth
separation in the time-like region
2
p2
2E
22M
2M
*E
y
m4/q
/|G|)(sin|G|)(cos1
)GGIm()2sin(A
38
Extension of the “safe” regionExtension of the “safe” region
eeqq
/Jqq unknown vector coupling, but same Lorentzand spinor structureas other two processes
Unknown quantities cancel in the ratios for ATT, but helicity structure remains!
Cross section increases by two orders from M=4 to M=3 GeV
→ Drell-Yan continuum enhances sensitivity of PAX to ATTAnselmino, Barone, Drago, Nikolaev(hep-ph/0403114 v1)