GSI P. Lenisa - Univ. Ferrara and INFN 1 PAX Polarized Antiproton Experiment PAX Collaboration Spokespersons: Paolo Frank Rathmann Status report

2 Outline Extracted beam vs internal target (vs collider) Transversity measurement by Drell-Yan –Rates –Angular distribution –Background Detector concept Conclusions

3 Extracted beam vs internal target L ext =7.5  x 1.3  10 6 = 1.0  cm -2 s -1 Extracted beam: L ext =t x N pbar t = areal density (15 g/cm 2 NH 3 ) Internal target L int = t x f x N pbar t = areal density f = revolution frequency N pbar = number of pbar stored in HESR L int = 7.2  x 6  10 5 x 4.9  = 2.1  cm -2 s -1 Production rate of polarized antiprotons (  P = 2  B ) cannot exceed: N pbar = 1.0  10 7 /e 2 = 1.3  10 6 pbar/s Drell-Yan events rate: N DY =L x  DY Polarized beam luminosity:

4 Extracted beam: d=3/17 Q=0.85 P=0.3 Internal target: d=1 Q=0.85 P=0.3 Extracted beam vs internal target Statistical uncertainty in A TT d = diluition factor Q = proton target polarization P = antiproton beam polarization factor 67 in measuring time!

5 Transversity measurement with Drell-Yan lepton pairs p p qLqL q l+l+ l-l- q 2 =M 2 qTqT Polarized antiproton beam → polarized proton target (both transverse) 1) Events rate. 2) Angular distribution.

6 Drell-Yan cross section and event rate M 2 = s x 1 x 2 x F =2q L /√s = x 1 -x 2 Mandatory use of the invariant mass region below the J/  (2 to 3 GeV). 22 GeV preferable to 15 GeV x 1 x 2 = M 2 /s 15 GeV 22 GeV M>2 GeV M>4 GeV 22 GeV 15 GeV M (GeV/c 2 ) 2 k events/day

7 Collider ring (15 GeV) L > cm -2 s -1 to get the same rates

8 A TT asymmetry: angular distribution The asymmetry is large in the large acceptance detector (LAD) The asymmetry is maximal for angles  =90 ° The asymmetry has a cos(2  ) azimuthal asymmetry.

9 Theoretical prediction Anselmino, Barone, Drago, Nikolaev (hep-ph/ v1) T=22 GeV T=15 GeV x F =x 1 -x Asymmetry amplitudeAngular modulation FWD:  lab < 8° LAD: 8° <  lab < 50° P=Q=1 LAD

10 Estimated signal 120 k events sample 60 days at L=2.1  cm 2 s -2, P = 0.3, Q = 0.85 Events under J/y can double the statistics.  Good momentum resolution requested LAD

11 Background  rejection factor against background DY pairs can have non-zero transverse momentum ( = 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 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. …

13 Background higher for  than for e Background for Preliminary PYTHIA result (2  10 9 events) Background from charge coniugated mesons negligible for e. e  x1000 x100 Total background x1000 x100 e  Background origin

14 Detector concept Drell-Yan process requires a large acceptance detector Good hadron rejection needed 10 2 at trigger level, 10 4 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 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/mm 2 ) average integrated field: 0.6 Tm free acceptance > 80 %

GSI P. Lenisa - Univ. Ferrara and INFN 16 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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18 Example 0.01 M > 2GeV Dalitz veto through unpaired e wrong charge 10 GeV 2 Background for Combinatorial background from meson decay: Direct estimation of candidate processes shows negligible contribution.

19 Performance of Polarized Internal Targets P T =  HERMES H Transverse Field (B=297 mT) HERMES H D P T = ± Longitudinal Field (B=335 mT) HERMES: Stored PositronsPINTEX: Stored Protons H Fast reorientation in a weak field (x,y,z) Targets work very reliably (many months without service)

20 Detector Concept Two complementary parts: 1. Forward Detector ±8 0 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

21 Count rate estimate Uncertainty of A TT depends on target and beam polarization (|P|=0.05, |Q|~0.9) 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 [10 31 cm -2 s -1 ] Number of days to achieve above errors EM only P(2·τ b )=0.05 EM + hadronic P(2·τ b )=0.10 1e(  ) p(  )Standard/Round e(  ) p(  )Standard/Round e(  ) p(  )Low Conductance/Round e(  ) p(  )Low Conductance/Elliptical For single spin asymmetries L ~ 10 times larger

22 Cost 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 HERMES12.0 M€ Large Acceptance Detector2.6 M€ Target1.8 M€ Infrastructure (cabling, cooling, platform, shielding) 3.0 M€ Total19.4 M€

23 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 (Ψ acc = 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)

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25 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

26 Physics Performance Luminosity –Spin-filtering for two beam lifetimes: P > 5% –N(pbar) = 5·10 11 at f r ~6·10 5 s -1 –d t = 5·10 14 cm -2 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 HESR The lifetime of a stored beam is given by (Target thickness = d t =5·10 14 atoms/cm 2 ) T (MeV) beam lilfetime τ b (h) Ψ acc = 1 mrad 5 mrad 10 mrad mrad In order to achieve highest polarization in the antiproton beam, acceptance angles of Ψ acc = 10 mrad are needed.

28 Low Conductance Feed Tube Method tested successfully but not optimized during development of FILTEX/HERMES Atomic Beam Source (Heidelberg 1991). H1H1 H2H2 ~3 

29 Puzzle from FILTEX Test Observed polarization build-up: dP/dt = ± (1.24 ± 0.06) x h -1 Expected build-up: P(t)=tanh(t/τ 1 ), 1/τ 1 =σ 1 Qd t f=2.4x10 -2 h -1  about factor 2 larger! σ 1 = 122 mb (pp phase shifts) Q = 0.83 ± 0.03 d t = (5.6 ± 0.3) x cm -2 f = MHz Three distinct effects: 1.Selective removal through scattering beyond θ acc =4.4 mrad σ R  =83 mb 2.Small angle scattering of target protons into ring acceptance σ S  =52 mb 3.Spin transfer from polarized electrons of the target atoms to the stored protons σ E  =-70 mb Horowitz & Meyer, PRL 72, 3981 (1994) H.O. Meyer, PRE 50, 1485 (1994)

30 Spin transfer from electrons to protons Horowitz & Meyer, PRL 72, 3981 (1994) H.O. Meyer, PRE 50, 1485 (1994) α fine structure constant λ p =(g-2)/2=1.793anomalous magnetic moment m e, m p rest masses pcm momentum a 0 Bohr radius C 0 2 =2πη/[exp(2πη)-1]Coulomb wave function η=-zα/νCoulomb parameter (neg. for anti-protons) vrelative lab. velocity between p and e zbeam charge number

31 Antiproton Polarizer Exploit spin-transfer from polarized electrons of the target to antiprotons orbiting in HESR Expected Buildup d t =5·10 14 atoms/cm 2, P electron = T (MeV)  e  (mbarn) T=500 MeV T=800 MeV Goal antiproton Polarization (%) t (h)102030

32 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 ( 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 HESR Detection system for p-pbar elastic scattering + simple, i.e. non-magnetic + Polarized Internal Storage Cell Target -magnetic guide field (Q x,Q y,Q z ) + azimuthal symmetry (polarization observables) + large acceptance Storage cell Ex: EDDA at COSY

34 Polarization Conservation in a Storage Ring HESR design must allow for storage of polarized particles! Indiana Cooler H.O. Meyer et al., PRE 56, 3578 (1997)

35 Spin Manipulation in a Storage Ring (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)=P i (  ) n   =(99.3±0.1)%

36 Single Spin Asymmetries Several experiments have observed unexpectedly large single spin asymmetries in pbar-p at large values of x F ≥ 0.4 and moderate values of p T (0.7 < p T < 2.0 GeV/c) E704 Tevatron FNAL 200GeV/c xFxF π+π+ π-π- Large asymmetries originate from valence quarks: sign of A N related to u and d-quark polarizations

37 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 G E -G m separation –test of Rosenbluth separation in the time-like region

38 Extension of the “safe” region unknown vector coupling, but same Lorentz and spinor structure as other two processes Unknown quantities cancel in the ratios for A TT, 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 A TT Anselmino, Barone, Drago, Nikolaev (hep-ph/ v1)