Transverse Hyperon Polarization Revisited

Stephan Paul

Physik Department

TU-München

Overview

• Polarization Phenomena revisited• Experimental Techniques• Present status

• ‘New’ measurements• Hyperon beams • Exclusive production• Photo-production• Polarized beams• other observations

• Future measurements• charmed baryons

• Summary

Hyperon Polarization

Measurements of hyperon polarization have been performed in a large variety of beams:• protons

unpolarized on fixed target (largest existing data set)polarized on fixed targetcollider

• antiprotons high energy on fixed targetat threshold (LEAR)

• pions• kaons• hyperons• neutrinos• Z0-decays• polarized photons (quasireal and highly virtual)

rich set of data - difficult to describe coherently

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

• Two types of experiments contribute– ´Beam stop´ experiments - thick target followed by magnetic

extraction channel with small acceptance

Experiments

– Spectrometer experiments – thin target with large acceptance spectrometer

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

How to determine the Polarization

• Polarization phenomena studied since 20 years– hyperons ideal tool: self analyzing via weak decays– most of the work done at FNAL

• How to determine polarization ?

dN / d cos = 1/2 (1 + cos )

• Fit to decay distribution returns polarization

n = kbeam k^

B B’ + M e.g. p

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Need large analyzing power

Data Set

• High energy reactions (mostly inclusive)– Production characterized by kinematic variables

•

• pT

– Problem: • in many production experiments xF and pT experimentally correlated

• Often low statistics no separation of variables

– Exception• Hyperon beam experiment WA89 (and polarization) new

features !

s

px L

F

2

• Low Energy Reactions (mostly exclusive)–Strong correlation with other reaction products

–Strong energy dependence

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Polarization of Hyperons in Proton Beams

! Relative sign of polarisation !

xF

pT

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

• |polarization| increases with p shows small p dependence

• polarization saturates at p > 1 GeV/c (typically)...

• polarization increases linearly with xF

• polarization increases for exclusive production

• small energy dependence (s 4-62 GeV/c)– negative of– independence for – positive for

• small (significant ?) target dependence (A)• relative sign of polarization according to SU(6) w.f.

• polarization of antihyperons (except )• polarization of all hyperons (except )

• polarization also in K, and e- beams

Features of Hyperon Polarization

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

New Proton Data

• 800 GeV proton beam– Observe + polarization (in beamline)– Strong correlation of xF and pT

– Strong nuclear mass effect (not seen by WA89!)• Thick production target used x ~I (thin one x ~0.03 I by WA89)

– Drop of polarization at large xF (thus large pT )

pT ~ 1.4 pT ~ 1.83 pT ~ 2.1

Submitted to PRD

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Hyperon beam

330 GeV/c - beam at CERN SPS (-spectrometer)• forward spectrometer• large data sample (optimized for charm)• Contamination of direct production measured

15-20% come from 0 35% come from *

0

2000

4000

6000

x 102

-25- 12.50 12.52 5

0.1<xF<0.2

0

5000

10000

15000

-25- 12.50 12.52 5

0.8<xF<0.9

m (MeV/c2)

reconstructed

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

reconstructed

0

20000

40000

-25 -12.5 0 12.5 25

0.0<xF<0.2

0

2000

4000

-25 -12.5 0 12.5 25

0.7<xF<0.9

m- (MeV/c2)

Hyperon beam

330 GeV/c - beam at CERN SPS (-spectrometer)• forward spectrometer

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Hyperon beam

• Polarization rises for pT>0.3• Polarization only xF>0.3• :

– positive polarization– breakdown for pT>1.3– 50% stem from decays

• :– mostly negative polarization– shift of sign for xF>0.6– beam at large xF

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Comparison to other data

• Breakdown of polarization for large pT already observed elsewhere ?– Mostly hidden by integration over second variable (xF or pT

respectively)

– Only prominent for (partially from -decay)

– See FNAL data on polarization effect already observed

xF ~0.45

xF ~0.55

800 GeV

400 GeV

Claim for energy dependence...

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Exclusive Reactions

a) experiment at AGS (27.5 GeV) pp pK

b) Experiments R608 at ISR/CERN (31+31 GeV/c) and E690/FNAL (800 GeV) pp pf Kexclusive

ISR

800 GeV FNAL

Diffractive production

PRL 88,6 (2002)

800 GeV FNAL

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Hyperon Polarization in DIS

for xF<0

transversep-beam transverse

Nucl.Phys.B588:3-36,2000

-production of -hyperons• longit. polarized beam• nuclear target• separate n and p-induced interaction

– 8500 – transverse polarization only for xF<0– polarization increases with pT (until 0.5 GeV/c)– polarization mainly from production off neutrons

influence of feed-down from * in p production ? !!

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Photoproduction

• COMPASS (160 GeV -beam)– Select small Q2, quasi-real -production– Use longitudinally polarized target

(sum over runs with alternate polarizations)

– use bias-cancelling method– 160 K (mostly small xF)

Small polarizaion at xF ~0

– 80 K No significant polarizaion observed

– ~ 8 x present statistics on tape...

..... See J. Friedrichs talk in parallel session

• HERMES (27 GeV e-beam)– No full kinematic reconstruction ...– P ~ 5.5 0.6 1.6% (2002 talk)..... new results see talk in parallel session

COMPASS 2002

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Spin Transfer

• So far only spontaneous polarization in production (fragmentation)

• Can we control the spin ? Check for spin transfer– Use transversely polarized proton beams

• Accelerate polarized protons – low energy only (3.7 GeV)Saclay – DISTO experiment

• Use protons from unpolarized decays – high energy (200 GeV) FNAL - E704

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Spin Transfer

PRL 83:1534-1537,1999 DISTO

Low energy: Long distance exchange processes

FNAL Experiment E704

Correlation xF and pT !

xF ~ xF ~

PRL 78:4003-4006,1997

High energy: hard scattering

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

exclusive

Polarization in pp annihilation

PRL 89 (2002) 212302

• pp annihilation into • Use polarized p target

Singlet Fraction

Dnn = Knn Triplet production

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Hyperon Polarization – Models and Theory

No coherent description of all phenomena• Dynamic Models

– Lund String Fragmentation– De Grand-Mietinnen

• Distribution Functions (Hard processes, DIS)– Fragmentation Functions– Structure Functions

• Exchange Models (Soft processes, low energy)

Common feature: – Sign of polarization for different hadrons SU(6) w.f.

(constituent quarks)– Some calculations use modifications (spin of valence quarks)

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Model descriptions

understanding of polarization requires understanding of particle production

Any description needs to correlate pT and polarization

creation of pT creates quantization axis

classification:

• fragmentation or recombination modelscorrelation spin-orbital angular momentum– kinematical (Lund, De Grand Mietinnen, Swed)– intrinsic 3P0 - model

• dissociation models (diffraction)interference spin flip/non-flip amplitudes

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Examples:

a) Lund-model:

pT = kT + qT

string fragmentation

ss in string breaking recombines with struck quark

L kT x q

b) deGrand/Miettinnen:

• recombination with s-quarks from sea in projectile• sea quarks have to catch up with valence quarks• no kT in hadron needed

sign of acceleration sign of polarizationin -production s-quark acceleratedin -production (uu)1 diquark decelerated – s-quark spin anti spin

Dynamical Explanations

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Low Energy Schemes

e) -exchange : particle production in peripheral interactions dominated by OPE

relate reaction pp K + X to p Kand to polarization phenomena observed therein

f).... Other models e.g. comprise the role of angular momentum of valence quarks

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Polarization and Distribution/Fragmentation

• Assume hard scattering mechanism for hyperon productionConsider pT as ´trigger bias´ and parametrize process

– Polarized Quark Distribution Functions• Consider initial k of quarks in hadron

• Take g1T(xBj, k ) – correlate quark spin with k

• Polarization ~ k / pT

• Polarization should decrease at large pT.... Scale ?

– Polarized Fragmentation Functions• Assume unique PFF for each quark species and fold it with baryon w.f.

• Consider final k of quarks in hadron

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Anselmino et al.,

Other Reactions .....• Polarization Transfer in pp

• Polarized target: Transversity

transversity fragmentation function

transversity function

• production of polarized

hh

X

lh

X

D

ISIntroduction

General

Wisdom

New

Data

Theory

Extension to

Charm

Charmed-Baryons

• BIS-2: using 70 GeV/c neutrons

decay asymmetry not known

c -++ andc pK0 -+

| P | = 0.5 ± 0.2

p = 0.43 GeV/c

• R608: using 32 GeV/c protons at ISR

observation of decay asymmetry for

c -++

| P | : 3 effect (xF > 0.5)

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Only two measurements exists:

• photo-production on polarized targets:

c-quark production via -g-fusion c-quark transmits spin direction of gluonc transmits spin direction of c-quark

probe g-polarization in polarized nucleon

• charm in hadro-production

Tests for intrinsic charmed Fock-states

• c polarization proceeds via – fragmentation (low polarization ?)– recombination (high polarization a la DeGrand)

expect strong xF-dependence (large at small xF)• Brodsky: look for large polarizations at very high xF

analogue to large polarization observed for J/

Why Measure c - Polarization

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

G via c polarization

• original idea: – unpolarizedon polarized target

problem: c difficult to detect

• modified idea: – unpolarized protons on polarized target

c production via g g fusion

Mcc = x1x2 s

x2 desired: ~ 0.1Mcc > 4 GeV

xF = (x1-x2)

– measure longitudinal polarization of c

– depend on polarized charm-fragmentation function assume large due to large mc

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Conclusions

Hyperon Polarization: Large data set existing• General trend of polarization P(pT, xF) confirmed, but :• Firm observation of breakdown of polarization at ‚large‘ pT

• Clear establishment of effect of exclusive production• Polarization also in Leptoproduction

separate target and projectile fragmentation effects – data are coming in

• Needed: High statistics data – separating pT and xF

– exclusive and semi-inclusive– Coverage of full xF range– Energies...select region of ‚hard‘ processes

• Polarized c – probe for G using polarized target in p or -beams– probe production and polarization process– tests for intrinsic charm

new

new

• For cascade decays

dNp / dcos = 1/2 (1 + | cos )

analyze polarization of daughter baryon () (polarization transfer)

22with

many experiments measure

Cascading Decays

B´ B´´ + M

B B´ + M

= P + [+ (1-) P] ^^

1 + P^

and not the angular distribution of

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Hyperon Decay Properties

hyperon B.R. [%] asymmetry p

64

0.642

p

51.5

-0.980

n

48.5

0.068

n

100

-0.068

100

-

100

-0.293

100

-0.264

K

68

0.026

Overview of decay asymmetries for hyperons

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Polarization with Meson-Beams

• Medium energy beams

• - and K- beams

• use backward production (xF < 0)– Polarization in target fragmentation independent on projectile

Longitudinal Spin Transfer in DIS

Acta Phys.Polon.B33:3791-3796,2002

longitudinal

Nomad

PR D 64 2001

HERMES

• probe longitudinal quark d.o.f• diquark polarisation and s-quark association• require DIS kinematics

IntroductionG

eneral W

isdomN

ew D

ataT

heoryE

xtension to C

harm

Energy Dependence of Polarization

• Fixed target (270 GeV beam):• s ~ 25 GeV x1 ~ 0.65• xF = 0.6 p(c) ~ 180 GeV/c

suitable for larger x2 , decay point reconstructable

• Collider (RHIC with singly polarized beam)• s ~ 100-500 GeV x1 ~ 0.35 - 0.16• xF = 0.25-0.06 p(c) ~ 12 GeV/c

sign of xF chooses kinematics (size of x2)problem: typical p is small - bad acceptance must go to large Mcc

• suitable for smaller x2 , charm tagging necessary• fast flip of beam polarization bias cancelling ‘easy’

c Production in Experiments

Introduction

General

Wisdom

New

Data

Theory

Extension to

Charm

Low energy Kaon beam....

K- beam – 8.35 GeV

Nucl.Phys.B148:18,1979

xF < 0 xF > 0

Low energy Kaon – Semi-exclusive

Nuovo Cim.A44:345,1978

How to measure c -polarization ?

• Asymmetry parameter of multibody decays unclear

• Use decays with known e.g. c + = - 0.98 ± 0.19

problem: c + / tot = 0.79 ± 0.18 %

Use: • 300 GeV/c p-beam • intensity: 108/spill• 10,000 c + in 50 days

BUT: no good trigger possible, difficult to reconstruct

use multibody decays and determine

in COMPASS :

c) dynamical polarization:multiple scattering of massive quark in gluon field

polarization effective scattering plane

Pq ~ - n mq / E²cm

i) massive s-quark from sea scatters polarization recombination with diquark from projectile

ii) massive constituent quark scatters polarization correlation with s-quark spin via 3P0 - model (constituenten quark = current quark + 3P0 sea) s-quark polarization s-quark fragmentationd) quasi diffractive production : produced via triple Regge exchange - direct production (no polarization) - dissociation of virtual hyperons into + interference from andleads to polarization

follow example of ‘previous’ experiments:

use : c -++ BR = 2.9 ± 0.6 % and c +-+ BR = 3.0 ± 0.6 %

about 30,000 in each channel reconstructed (50 days)

• first determine -parameter for different reactions

– determine polarization of hyperon ()– difficult to predict (see paper by Bjorken)– may vary over region in Dalitz plot - (q2)

dN / dcos = 1/2 (1 + cos )

using unpolarized c

in COMPASS :

Introduction

General

Wisdom

New

Data

Theory

Extension to

Charm

ConclusionsI) existing data on hyperon polarization :

• ‘universal’ features of polarization observed• polarization pattern (B/B) hints for diquark effects• transversal asymmetry ( and ) light quark effects• no coherent understanding of observed phenomena dynamics of production not understood

II) future possibilities probing -polarization

• low s exclusive reactions (COSY)• probe of ‘strange’ sea in nucleon using:

polarized target in polarized photon beamslongitudinally polarized protons

III) polarized c

• probe for G usingpolarized target in p or -beams

• probe production and polarization process• tests for intrinsic charm