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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 Physics with Polarized Antiprotons Study of the proton spin Study of the proton spin

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slide1

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

Polarized Antiprotons Experiments

study of the proton spin

Physics with

Polarized Antiprotons

Study of the proton spin

Polarized Antiprotons Experiments

slide3

Study of the proton spin

h1=

Quark structure of the nucleon

unpolarised quarks

and nucleons

longitudinally polarised

quarks and nucleons

transversely polarised

quarks and nucleons

Only glimpse

Well known

Known

Polarized Antiprotons Experiments

slide4

transversely polarised

quarks and nucleons

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 -> it needs a chirally odd partner

Inclusive DIS Semi-inclusive DIS Drell-Yan

HERMES,COMPASS,JLab

Polarized Antiprotons Experiments

slide5

M invariant Mass

of lepton pair

Inclusive DIS Semi-inclusive DIS Drell-Yan

h1 from p-p Drell-Yan

Polarized Antiprotons Experiments

slide6

The PAX proposal at FAIR (phase II)

s=200 GeV2

EXPERIMENT:

Asymmetric collider:

polarized protons in HESR (p=15 GeV/c)

polarized antiprotons in CSR (p=3.5 GeV/c)

(Same “s” as Main Injector beam on fixed target)

Polarized Antiprotons Experiments

slide7

h1 from p-p Drell-Yan

  • u-dominance
  • |h1u|>|h1d|

PAX : M2/s=x1x2~0.02-0.3

valence quarks

(ATTlarge ~ 0.2-0.3)

RHIC: τ=x1x2=M2/s~10-3

→ Exploration of the sea quark content(polarizations small!)

ATT very small (~ 1 %)

First direct measurement of h1 for valenc quarks

No competitive processes

Polarized Antiprotons Experiments

slide8

Physics with

Polarized Antiprotons

Proton electromagnetic form-factors

Polarized Antiprotons Experiments

slide9

Electromagnetic Form Factors

- Describe int. structure of the nucleon

  • Information about proton ground state
  • Test for models of nucleon structure

- Wavelength tunable with Q2:

< 0.1 GeV2 integral quantities

0.1-10 GeV2 internal structure

> 20 GeV2 pQCD scaling

p

p’

q=k’-k

k’

k

  • One-photon-exchange approximation:
  • Pauli-Dirac (F1 and F2) or Sachs (GE and GM)
  • GM(q2) = F1(q2) + F2(q2)
  • GE(q2) = F1(q2) + t F2(q2)t=q2/4M2
  • In the Breit reference system, Sachs FFs are the Fourier transform of the charge and magnetization distributions

k

(

)

(

)

m

m

mn

=

g

+

s

2

2

J

F

q

F

q

i

q

n

1

2

2

M

Polarized Antiprotons Experiments

space like and time like regions

t=q2>0 (timelike)

t=q2<0 (spacelike)

real function

complex function

Annihilation

Scattering

_

e+ + e- => h+ h

e- + h => e- + h

_

Space-like and Time-like regions
  • FFs are analytical functions. of t = q2 = -Q2.

Polarized Antiprotons Experiments

space like ffs proton data

Rosenbluth

Polarization

Proton Electromagnetic Form-Factors: data

Space-Like FFs: proton 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

Polarized Antiprotons Experiments

space like ffs proton data12

Rosenbluth

Polarization

Proton Electromagnetic Form-Factors: data

Space-Like FFs: proton 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

Polarized Antiprotons Experiments

slide13

The PAX proposal - Phase I

EXPERIMENT:

Fixed target experiment:

polarized antiprotons protons in CSR (Ek<2.5 GVe)

fixed polarized protons target

“s” range covered by both Main Injector and Antiproton Accumulator beams on fixed target

Polarized Antiprotons Experiments

double polarized pbar p annihilation
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

Polarized Antiprotons Experiments

theoretical models

Magnetic

Electric

Magnetic

Electric

proton

neutron

VDM : IJL

F. Iachello..PLB 43, 191 (1973)

QCD inspired

Bosted PRC 51, 409 (1995)

Extended VDM

E.L.Lomon PRC 66, 045501 2002)

Hohler

NPB 114, 505 (1976)

Theoretical models

Spacelike

Timelike

E. Tomasi, F. Lacroix, C. Duterte, G.I. Gakh, EPJA 24, 419(2005)

Polarized Antiprotons Experiments

single spin asymmetry
(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 → e+e-
    • Measurement of relative phases

of magnetic and electric FF in

the time-like region

- Also sensitive to different models

Polarized Antiprotons Experiments

slide17

Physics with

Polarized Antiprotons

Polarized

pbar-p

hard-scattering

Polarized Antiprotons Experiments

slide18

P

Beam

Target

Hard p-p polarized scattering

T=10.85 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.

Polarized Antiprotons Experiments

slide19

Physics with

Polarized Antiprotons

Further perspectives …

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

Polarized Antiprotons Experiments

j y y production
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

Polarized Antiprotons Experiments

slide21

Physics with

Polarized Antiprotons

Further perspectives …

  • 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

Polarized Antiprotons Experiments

slide22

More single-spin asymmetries

Polarized Antiprotons Experiments

slide23

π

Pq

k┴

pq

q

P

k┴

pp

q

Pq

k┴

pp

PL

L

k┴

pq

Single-spin asymmetries

Correlation functions

Sivers effect = number of partons in polarized proton depends onP· (p x k┴)

PDFs

Boer-Mulders effect = polarization of partons in unpolarized proton depends onPq· (p x k┴)

Collins effect = fragmentation of polarized quark depends onPq· (pqx k┴)

FFs

Polarizing FF = polarization of hadrons from unpolarized partons depends onPL· (pqx k┴)

These effects may generate SSA

Polarized Antiprotons Experiments

sivers from sidis

Sivers from hadron scattering

pp→pX

BNL-AGS √s = 6.6 GeV 0.6 < pT < 1.2p↑p

E704 √s = 20 GeV 0.7 < pT < 2.0p↑p

ep→hX

Sivers from SIDIS

Polarized Antiprotons Experiments

slide25

Test of Universality

A.V. Efremov et al.,

Phys. Lett. B 612, 233 (2005)

M. Anselmino et al.,

Phys. Rev. D72, 094007 (2005)

The Sivers function

Polarized Antiprotons Experiments

slide26

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 -> 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

Polarized Antiprotons Experiments