transverse hyperon polarization revisited stephan paul physik department tu-münchen
TRANSCRIPT
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
• 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
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 :
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