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Polarized Antiprotons Experiments
The PAX Collaboration 178 Collaborators 36
institutions (15 EU, 21 NON-EU)
dr. Paolo Lenisa Università di Ferrara and INFN -
ITALY
Project X Workshop FNAL, 01/26/08
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Study of the proton spin
Physics with Polarized Antiprotons
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Study of the proton spin
Quark structure of the nucleon
unpolarised quarks and nucleons
transversely polarised quarks and nucleons
longitudinally polarised quarks and nucleons
Only glimpse
Well known
Known
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Transversity

• Probes relativistic nature of quarks
• No gluon analog for spin-1/2 nucleon
• Different evolution than
• Sensitive to valence quark polarization

h1 is chirally odd -gt it needs a chirally odd
partner
Inclusive DIS Semi-inclusive DIS
Drell-Yan
HERMES,COMPASS,JLab
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h1 from p-p Drell-Yan
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The PAX proposal at FAIR (phase II)
EXPERIMENT Asymmetric collider polarized
protons in HESR (p15 GeV/c) polarized
antiprotons in CSR (p3.5 GeV/c)
(Same s as Main Injector beam on fixed target)
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h1 from p-p Drell-Yan

• u-dominance
• h1ugth1d

PAX M2/sx1x20.02-0.3
valence quarks (ATT large 0.2-0.3 )
RHIC tx1x2M2/s10-3 ? Exploration of the sea
quark content (polarizations small!) ATT very
small ( 1 )
First direct measurement of h1 for valenc
quarks No competitive processes
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Physics with Polarized Antiprotons
Proton electromagnetic form-factors
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Electromagnetic Form Factors
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Space-like and Time-like regions

• FFs are analytical functions. of t q2 -Q2.

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Space-Like FFs proton data
Proton Electromagnetic Form-Factors data
Time-Like FFs proton data

• JLab results dramatically changed picture of the
  Nucleon
• GEp/GMp decreases with Q2

Is the proton round?
- Q2 dependence suggests different charge and
magnetization spatial distributions inside the
nucleon
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Space-Like FFs proton data
Proton Electromagnetic Form-Factors data
Time-Like FFs proton data
Is the proton round?
Expected Q2 behaviour reached quite early,
however ... ... there is still a factor of
2 between timelike and spacelike.
Additional direct measurement needed
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The PAX proposal - Phase I
EXPERIMENT Fixed target experiment polarized
antiprotons protons in CSR (Eklt2.5 GVe) fixed
polarized protons target
s range covered by both Main Injector and
Antiproton Accumulator beams on fixed target
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Double polarized pbar-p annihilation
E. Tomasi, F. Lacroix, C. Duterte, G.I. Gakh,
EPJA 24, 419(2005)

• Most contain mduli GE, GM
• Independent GE-GM separation
• Test of Rosenbluth separation in the time-like
  region
• Access to GE-GM phase
• Very sensitive to different models

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Theoretical models
Spacelike
Timelike
E. Tomasi, F. Lacroix, C. Duterte, G.I. Gakh,
EPJA 24, 419(2005)
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(Single Spin Asymmetry)
A. Z. Dubnickova et al. Nuovo Cimento A109, 241
(1996)
S.J. Brodsky et al. PRD 69, 054022 (2004)

• Single-spin asymmetry in pp ? ee-
• Measurement of relative phases
• of magnetic and electric FF in
• the time-like region
• - Also sensitive to different models

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Physics with Polarized Antiprotons
Polarized pbar-p hard-scattering
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Hard p-p polarized scattering
T10.85 GeV
The greatest asymmetries in hadron physics ever
seen by a human being (Brodsky)
D.G. Crabb et al., PRL 41, 1257 (1978)
One of the unsolved mysteries of hadron
physics(Brodsky, 2005)
It would be very interesting to perform these
measurements with polarized antiprotons.
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Physics with Polarized Antiprotons

• Spectroscopy of hadrons
• Use of polarization degrees of freedom to
  decrease number of contributing amplitudes

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J/y,y production
NRQCD
Able to reproduce the unpolarized xsec
Fixed target exp
Fails in predicting polarization vs pT at CDF
F. Maltoni et al., hep-ph/0601203
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Physics with Polarized Antiprotons

• Spectroscopy of hadrons
• Use of polarization degrees of freedom to
  decrease number of contributing amplitudes

• Low-t proton-antiproton scattering
• Investigation of spin and isospin dependence of
  nucleon-antinucleon interaction at low energy

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More single-spin asymmetries
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Single-spin asymmetries
Correlation functions
Sivers effect number of partons in polarized
proton depends on P (p x k-)
PDFs
Boer-Mulders effect polarization of partons in
unpolarized proton depends on Pq (p x k-)
Collins effect fragmentation of polarized quark
depends on Pq (pq x k-)
FFs
Polarizing FF polarization of hadrons from
unpolarized partons depends on PL (pq x k-)
These effects may generate SSA
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Sivers from SIDIS
ep?hX
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The Sivers function
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Summary

• The physics case for polarization expeiments with
  antiprotons is outstanding
• Two options to perform polarization experiments
  with pbar at FNAL
• Single spin-asymmetries
• Fixed target in AA or MI
• Double spin-asymmetries
• Polarizer ring (F. Rathmann -gt next talk)
  experimental ring

• The FNAL pbar-source is a pretious treasure for
  the world hadron-physics community
• Factor 10 higher real intensity than the FAIR
  source on the paper in ten years
• Future of CERN pbar-source unclear
• AD ring low energy, no stacking capability