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 lenisa@mail.desy.deFrank Rathmann f.rathmann@fz-juelich.de

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

7

Collider ring (15 GeV)Collider ring (15 GeV)

L > 1030cm-2s-1 to get the same rates

8

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.

9

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

10

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

11

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

12

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

14

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.

15

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

16

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).

17

18

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.

19

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)

20

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

22

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€

23

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)

24

25

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

26

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

27

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

29

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)

30

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

31

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

32

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

33

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

34

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)

35

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

37

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

*qq

/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)