Transcript Folie 1 - fe.infn.it

PAX
Polarized Antiproton Experiment
Status report
PAX Collaboration
www.fz-juelich.de/ikp/pax
Spokespersons:
Paolo Lenisa
Frank Rathmann
GSI - 20.08.04
[email protected]
[email protected]
P. Lenisa - Univ. Ferrara and
INFN
1
Outline
• Extracted beam vs internal target (vs collider)
• Transversity measurement by Drell-Yan
– Rates
– Angular distribution
– Background
• Detector concept
• Conclusions
2
Extracted beam vs internal target
Polarized beam luminosity:
Extracted beam:
Production rate of polarized antiprotons (tP = 2 tB) cannot exceed:
Npbar = 1.0107/e2 = 1.3106 pbar/s
Lext=t
x
Npbar
t = areal density (15 g/cm2 NH3)
Lext=7.51024 x 1.3106 = 1.0  1031 cm-2 s-1
Internal target
Lint= t
x f x
Npbar
t = areal density
f = revolution frequency
Npbar = number of pbar stored in HESR
Lint= 7.21014 x 6105 x 4.91010 = 2.1  1031 cm-2 s-1
Drell-Yan events rate: NDY=L
x sDY
int
ext
 N DY
 2.1N DY
3
Extracted beam vs internal target
Statistical uncertainty in ATT

ATT
d = diluition factor
Q = proton target polarization
P = antiproton beam polarization
1

d  Q  P  N DY
Extracted beam:

d=3/17 Q=0.85 P=0.3
Internal target:
TT
ATT

d=1 Q=0.85 P=0.3
 Aext

int
A
ext
17
17
ext
N DY

3
ext
N DY
3
 8.2
int
ATT


1
d Q P  N
ext
DY
1
d Q P  N
int
DY

17

3
ext
N DY
int
N DY
factor 67 in measuring time!
TT
int
N DY
ext
2.1N DY
4
Transversity measurement with Drell-Yan
lepton pairs
Polarized antiproton beam → polarized proton target (both transverse)
l+
q
p
l-
q2=M2
qT
p
qL
1) Events rate.

d 2s
4 2
2
2
2
2
2


e
q
x
,
M
q
x
,
M

q
x
,
M
q
x
,
M

q
1
2
1
2
dM 2 dxF 9M 2 s( x1  x2 ) q

 


2) Angular distribution.
ds
h1u ( x1 , M 2 )h1u ( x2 , M 2 )
ATT 
 aˆTT
ds
u( x1 , M 2 )u( x2 , M 2 )
5
Drell-Yan cross section and event rate
2
d 2s
4 2
2
2
2
2
2 •M = s x1x2

  eq q x1 , M q x2 , M  q x1 , M q x2 , M
•xF=2qL/√s = x1-x2
dM 2 dxF 9M 2 s( x1  x2 ) q


 


2 k events/day
22 GeV
15 GeV
22 GeV
15 GeV
M>4 GeV
M>2 GeV
M (GeV/c2)
•x1x2 = M2/s
• Mandatory use of the invariant mass region below the J/y (2 to 3 GeV).
•22 GeV preferable to 15 GeV
6
Collider ring (15 GeV)
L > 1030cm-2s-1 to get the same rates
7
ATT asymmetry: angular distribution
ATT 
ds
h ( x1 , M )h ( x2 , M )
 aˆTT
ds
u( x1 , M 2 )u( x2 , M 2 )
u
1
2
u
1
2

aTT ( ,  ) 

sin 

2

(1  cos2  )
 cos2 

•The asymmetry is maximal for angles  =90°
•The asymmetry has a cos(2) azimuthal asymmetry.
The asymmetry is large in the large acceptance detector (LAD)
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Theoretical prediction
Asymmetry amplitude
0.3
0.25
0.2
0.15
0
Angular modulation
A TT
aˆ TT
T=15 GeV
T=22 GeV
Anselmino, Barone, Drago,
Nikolaev (hep-ph/0403114 v1)
0.2
0.4
xF=x1-x2
LAD
0.6
FWD: lab < 8°
LAD: 8° < lab < 50°
P=Q=1
9
Estimated signal
•120 k events sample
• 60 days at L=2.1 1031 cm2 s-2, P = 0.3, Q = 0.85
LAD
LAD
Events under J/y can double the statistics.
 Good momentum resolution requested
10
Background
s p p  50mb
s DY  1nb
 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 m than for e (meson decay)
 hadronic absorber needed for m  inhibits additonal physics chan.
•Sensitivity to charge helps to subtract background from wrong-charge pairs
 Magnetic field envisaged
11
 
p
p

e
e X
Background for
Average multiplicity: 4 charged + 2 neutral particle per event.
Combinatorial background from meson decay.
 0  e e
p p  h1h2 X
h1h2 
K  /    0e /  e
K 0    / e /  e
    ee
  e e
…
Prelim. estimation of most of the processes shows background under control.
12
 
p
p

e
e X
Background for
Preliminary PYTHIA result (2109 events)
Total background
x1000
Background origin
x1000
x100
x100
e
e
m
m
• Background higher for m than for e
• Background from charge coniugated mesons negligible for e.
13
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.
14
Possible solution: 6 superconducting coils
• 800 x 600 mm coils
• 3 x 50 mm section (1450 A/mm2)
• average integrated field: 0.6 Tm
• free acceptance > 80 %
Sperconducting coils for the
target do not affect azimuthal
acceptance.
(8 coils solution also under study)
15
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).
GSI - 20.08.04
P. Lenisa - Univ. Ferrara and
INFN
16
17
 
p
p

e
e X
Background for
Combinatorial background from meson decay:
Direct estimation of candidate processes shows negligible contribution.
p p   0 0
 0  e e


0 0
K


e
0 e
pp  

p p  KExample
K
K    0e e
0.01
0
K L   e e
0
pp  K 0 K
B 0 0
 0  e e
M > 2GeV
sK L0 sp p e e
 
2


 BRDalitz  
 0.55  0.02  0.5
S
s DY s p p
 DY
 DY 
pp  K K X

0
0
10 nb @ GeV2


pp  D D  e e X
s  0 0

 90
s pp
0
wrong
charge
0
Dalitz veto
through unpaired e
18
Performance of Polarized Internal Targets
HERMES: Stored Positrons
H
Longitudinal Field (B=335 mT)
PT = 0.845 ± 0.028
D
HERMES
H
PINTEX: Stored Protons
H
Fast reorientation
in a weak field (x,y,z)
Transverse Field (B=297 mT)
PT = 0.795  0.033
HERMES
Targets work very reliably (many months without service)
19
Detector Concept
Two complementary parts:
1. Forward Detector
• ±80 acceptance
• unambiguous identification
of leading particles
• precise measurement of
their momenta
2. Large Acceptance Detector
•
measurement of angles
(θ,φ) and energies of
Drell-Yan pairs
20
Count rate estimate
Uncertainty of ATT depends on
target and beam polarization
(|P|=0.05, |Q|~0.9)
 ATT 
1
22

PQ N
N
T = 15 GeV
T = 22 GeV
number
of
sources
states
during
buildup
Feed tube/cell tube
Average
Luminosity
[1031 cm-2s-1]
1
e() p()
Standard/Round
2
e() p()
2
2
Number of days to achieve
above errors
EM only
P(2·τb)=0.05
EM + hadronic
P(2·τb)=0.10
0.56
214
54
Standard/Round
0.72
166
42
e() p()
Low Conductance/Round
1.90
63
16
e() p()
Low Conductance/Elliptical
0.95
-
32
For single spin asymmetries
L ~ 10 times larger
resonant J/Ψ contribution (2  higher
rate)
21
 ½ times number of days
Cost Estimate
•
•
•
•
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€
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
22
Requirements for PAX at HESR
PAX needs a separate experimental area
a. 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 buildup
2. Acceleration of polarized beam to highest energies
d. The experiment would benefit from higher energy
(22 GeV)
23
24
Final 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
25
Physics 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
1
31
 2 1
L(t  0)   N p  f r  d t  1.5 10 cm s
10
Time-averaged luminosity is about factor 3 lower
• beam loss and duty cycle
Experiments with unpolarized beam
• L factor 10 larger
26
Beam lifetimes in HESR
1
tb 
(sC  s0 )  d t  f
The lifetime of a stored beam is given by
 max
 1
e4
1
 ds 

s C   
 
 d 
2 4 
2
d  Ruth.
20 m p v  2y acc 2 
 min 
In order to achieve highest
polarization in the antiproton
beam, acceptance angles of
Ψacc = 10 mrad are needed.
11
20 mrad
10
9
8
5 mrad
10 mrad
8
7
 T  2 01 0
t
beam lifetime [h]
(Target thickness =
dt=5·1014 atoms/cm2)
10
beam lilfetime τb (h)
s0  stot (pp)
10.89
3

6
 T  1 01 0
t
3

 T  5 1 0
t

 T  1 1 0
t

3
6
5
3
4
4
Ψacc = 1 mrad
3
2
2
1
0.214
0
1
400.8
1
400
800.6
1200.4
T
kinetic energy [MeV]
800
1200
1600.2
2000
3
T (MeV)
27
2 1 0
Low Conductance Feed Tube
Method tested successfully but not optimized during
development of FILTEX/HERMES
Atomic Beam Source (Heidelberg 1991).
H2
H1
~3
28
Puzzle 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 ± 0.03
dt = (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
Horowitz & Meyer, PRL 72, 3981 (1994)
σE=-70 mb
H.O. Meyer, PRE 50, 1485 (1994)
29
Spin transfer from electrons to protons


pe  pe
se
2


4

1   p m e  2   
1
 2 ln2pa0 
 
 C0    sin 
2

2 
p mp
2







Horowitz & Meyer, PRL 72, 3981 (1994)
H.O. Meyer, PRE 50, 1485 (1994)
α
λp=(g-2)/2=1.793
me, mp
p
a0
C02=2πη/[exp(2πη)-1]
η=-zα/ν
v
z
fine structure constant
anomalous magnetic moment
rest masses
cm momentum
Bohr radius
Coulomb wave function
Coulomb parameter (neg. for anti-protons)
relative lab. velocity between p and e
beam charge number
30
Antiproton Polarizer
Exploit spin-transfer from polarized
electrons of the target to antiprotons
orbiting in HESR
Expected Buildup
dt=5·1014 atoms/cm2, Pelectron=0.9
1 10
181.621
antiproton Polarization (%)
3
100
se (mbarn)
100
10
10
s etr T 
1
1
0.1
0.022 0.01
1
5
10
1 10
3
100
T
10
100 1000
1 10
4
1 10
5
T=500 MeV
Goal
T=800 MeV
10
20
30
t (h)
4
1.510
T (MeV)
31
Polarimetry
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
32
Beam 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
Ex: EDDA at COSY
Storage cell
33
Polarization Conservation in a Storage Ring
Indiana Cooler
H.O. Meyer et al.,
PRE 56, 3578 (1997)
HESR design must allow for storage of polarized particles!
34
Spin Manipulation in a Storage Ring
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)%
35
Single 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
π+
π-
xF
AN 
1
Pbeam
N  N
N  N
Large asymmetries originate
from valence quarks: sign of
AN related to u and d-quark
polarizations
36
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-
sin(2)  Im(G *E  G M )
Ay 
1  cos2 () | G M |2  sin 2 () | G E |2 / t t

t  q 2 / 4m p


2
• Double-spin asymmetry
– independent GE-Gm
separation
– test of Rosenbluth
separation in the time-like
region
37
Extension of the “safe” region
qq  J / 
qq   *
qq  e  e 
unknown vector coupling,
but same Lorentz
and spinor structure
as 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 ATT
Anselmino, Barone, Drago, Nikolaev
(hep-ph/0403114 v1)
38