IX International Conference on Hypernuclear and Strange Particle Physics

IX International Conference onHypernuclear and Strange Particle Physics

Johannes Gutenberg-UniversitätMainz, October 10 - 14, 2006

Marco MaggioraDipartimento di Fisica ``A. Avogadro'' and INFN - Torino, Italy

SPIN OBSERVABLES IN BARYON-BARYON INTERACTIONS

@ DISTO

DISTO @ Saturne: polarised proton beam up to T = 2.9 Gev

Acceptance:

• 2-cm thick unpolarised LH2 target• S170 magnet (<14.7 KGauss, = 120 , = 20)• semi-cylindrical 1mm-square scintillating fibers triplets inside magnet• MWPC planar triplets outside magnet• scintillator hodoscopes vertically and horizontally segmented• scintillator hodoscopes as polarimeter slabs• doped water Cerenkov counters

Á § :±

µ § ±

2.40

2.34

1.79

1.58

Detected Prongs Reaction p

sthr

~pp ! pK ~ pK p¼¡

pK p¼¡ ~pp ! pK ~

~pp ! pK ¤

~pp ! pK ¤

pK p¼¡ ¼ ¼

pK ¼ ¼¡ ¡ ¼

pK ¼¡ p ¼ ¼¡

pK ¼ ¼¡ ¡ ¼

pK p¼¡ ¼

pK ¼¡ por ¼ ¼¡

~ ° ! p¼¡ °

GOAL: first exclusive (kinematically complete) measurements with a polarised beam for

Hyperon production @ DISTO

Hyperon events’ topology – Data @ 2.94 , 3.31 and 3.67 GeV

complete reconstruction

missing

0 and/or missing

pK

pK

pK

pK

• DECAY VERTEX:

• REACTION VERTEX: pK

1 positive track1 negative track

2 positives tracks

~ ! p¼¡

Spin observables

n ~pB£ ~pj~pB£ ~p j

Depolarisation

Analysing power

d¾if d¾P BS¤

nB y

DY Y ¡d¾"" d¾##¢

¡¡d¾"# d¾#"¢

d¾"" d¾## d¾"# d¾#"

AY ¡d¾"" d¾"#¢

¡¡d¾#" d¾##¢

d¾"" d¾"# d¾#" d¾##

Y dependence on the spin of the proton from the beam~Sp

AY d¾SB "¡ d¾SB #

d¾SB " d¾SB #

transfer of the transverse component of to ~Sp ~SY

DY Y d¾N SF ¡ d¾SF

d¾N SF d¾SF

Spin observables: experimental asymmetries

NPB "¡ NPB #

NPB " N PB # ¡N "" N "#¢

¡¡N#" N ##¢

N "" N "# N#" N ## AY PB

Self-analysing weak decay:

® : § :N µ¤ N P ® µ¤

NN SF ¡ NSF

NN SF N SF ¡N "" N ##¢

¡¡N "# N #"¢

N "" N ## N "# N #" DY Y PBcosÁ

= 15.5

most of events are acquired at high cos()

n ² nB Á

pp elastic scattering: ping pong trigger

Elastic proton-proton scattering p" p ! pp

pp elastic scattering acquired for some spills every run beam polarisation continuos evaluation

data validation

" AY PB

rcross q rL

rR

" r¡ r

rL N

PB "L

NPB #L

rR N

PB "R

NPB #R

L and R on one side only free from L/R acceptance

PB↑and PB

↓ simultaneously acquired PB PB

" PB#

¾" »

Poor data from literatureIn particular @ 3.67 GeV/c

Errors for spin obsdo not include

contribution from

¾PB » : ¥

¾PB

Hyperon production : event reconstruction

Unrefitted M- p Reffitted MpK

.

DYY evaluation: accounting for hyperon contamination

1. select exclusive and ‘s from 0 decay through a MpK cut

2. evaluate separately and

3. estimate relative contaminations , the fraction of and 0 in each gate, evaluated throught fit functions integration in each gate.

Of course is

4. extract the corrected DYY values from the linear combinations:

1. no cos dependence ) direct evaluation of and 0 yealds

f ; ;

f i f i

DYY EVALUATION: 0.7 · |cos|· 1.

AY EVALUATION: 0.7 · |cos|· 1.

DY Y ~ ! ~ °DY Y

D Y Y f

DY Y f DY Y ~ ! ~ °

D Y Y f

DY Y f DY Y ~ ! ~ °

DYY @ 2.94 , 3.31 and 3.67 GeV/c – Exclusive Λ production

Exclusive Λ production:

1. DYY sizeable and negative2. smooth energy dependence

- 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7

Λ from decay:

1. DYY strongly different from the one of exclusive Λ

2. positive and negative values3. smooth energy dependence

DYY @ 2.94 , 3.31 and 3.67 GeV/c – Λ from Σ0 decay

~ ! ~ °

- 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7

AY @ 2.94 , 3.31 and 3.67 GeV/c – Exclusive Λ production


1. effects less important vs DYY

2. smooth energy dependence

- 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7

Λ from decay:

1. differences from exclusive Λ AY


AY @ 2.94 , 3.31 and 3.67 GeV/c – Λ from Σ0 decay

~ ! ~ °

- 0.7 ≤ xF ≤ 0.9 ; 0. ≤ pT ≤ 750 Mev/c ; |cos φΛ| ≤ 0.7

DYY @ 2.94 , 3.31 and 3.67 GeV/c – Exclusive Λ production

Inclusive Λ production:

• high energy ) DYY > 0


• lower energy ) DYY < 0

Exclusive Λ from Σ0 decay:

• lower energy ) ≠ DYY

Different reaction mechanism?

FIRST EXCLUSIVE DATA AVAILABLE,FIRST DATA IN THE TFR (xF < 0 )

DYY theoretical predictions – Exclusive Λ production

Polarisation transfer mechanism:

DIRECT FUSION MODEL[1]

DYY SIZEABLE AND POSITIVE

[1] C. Boros and Liang Zuo-tang, Phys. Rev. D53 (1996) 2279

.~Sp k ~S DY Y »


LAGET’S OBE MODEL[1]

NEGATIVE DYY CAN BE INTERPRETED

[1] J. M. Laget, Phys. Lett. B259 (1991) 24.

xF << 0 (TFR):DNN = 0

xF >> 0 (BFR):kaon ) DNN = -1pion ) DNN = +1

Conclusions

• first exclusive data available• first data in the TFR (xF < 0 ) region• first data for the exclusive selection of the Λ from Σ0 decay


1. DYY sizeable and negative2. smooth energy dependence3. DYY not easily interpreted in a

quark-model framework4. OBE approach can account

with FSI for DYY < 0

Λ from decay:

1. DYY strongly different from the one of exclusive Λ

2. positive and negative values3. smooth energy dependence

~ ! ~ °

Both Λ production:

1. effects less important vs DYY


Question time

QUESTION TIME

4body event reconstruction @ DISTO

pattern reconstruction and track fitting iteration: pattern recognition provides candidate for the fitting stage; input is the 12D coordinates vector

track fitting: 5D parameter vector (x,y): coordinates of the intersection point with z = 0a: inclination of track at z = 0starting angle in (x-z) plane : inverse momentum perpendicolar to B

detector coordinated depend smootly on all parameters

pxz ppx pz

~p

~x


track fitting by lookup table: goal is inverting in

look-up table:5D lattice that provide for tracks track coordinates

consist in linear interpolation of the lattice toobtain

2 minimisation:F inversion is performed minimizing:

iteration stops if do not change to nearest grid point

~p G~x

~p

~x F ~p

F ~p~x

Âmin

2

4 P

j 2fvalid coordsg

µxm

j ¡ xj ~p¾j

¶3

5

min ~p ~x


kinematically constrained refit:

1. global 4 tracks – 2 vertices refit

2. same approach, inversion of

3. more complex parameter 18D • 3 (x12, y12, z12) for the reaction vertex• six (1, a1, pxz,1, 2, a2, pxz,2 to describe momenta of the

two track emerging from the reaction vertex• 3 (x12, y12, z12) for decay vertex• six (3, a3, pxz,3, 4, a4, pxz,4 to describe momenta of the

two track emerging from the decay vertex

4. 4 degrees of freedom are constrained kinematically

~x F ~p~p

Hyperon production: reconstruction @ DISTO

kinematic constrains:

1. reconstructed momentun parallel to the joiner of the reaction and decay verteces ) 2 parameters

2. reconstructed M- p at decay vertex is M 1.115 GeV/c2 ) 1 parameter

3. M4B= 0 ( or ) or M4B= M ( ) ) 1 parameter

14D

~pp ! pK ~ ~pp ! pK ~

~pp ! pK ~ ¤

~p

Âmin

2

4d ~vreac;~bzr eac¾

d

P

j 2valid coordsg

µxm

j ¡ xj ~pcon¾j

¶3

5

min“soft” constraint on reaction vertex along the beam

Hyperon production: event selection

One of the most effective cuts in event selection is the kinematically constrained refit itself!

1. M- p = M

2.

3. M4B = 0 or M4B = M

Additional cuts:

1. veto

2. |

| < 0.15 rad, decay proton momentun in LF

3. p ID for positive track from decay vertex

4. p

< 1 GeV/c ) missing a at most

5. |vz| < 3.5 cm ) Klegecell veto

~p k ~VRVD

Kinematic region restricted to:

-0.7 ≤ xF ≤ 0.9pT ≤ 750 MeV/c

|cos | < 0.7

Particle identification @ DISTO

particle identication: iterative process for tagging candidates

+ veto: pp+- is

major background

Particle identification @ DISTO

particle identication: combined Cerenkon and hodoscopes tagging

very small + contaminationin hyperon sample

Spin observables: inferring polarisation

Self-analysing weak decay:

NU R¡ N µ¤d µ¤

P ®

NU¡ NDNU ND

P ";#

sign of PB

® : § :N µ¤ N P ® µ¤

Geometrical interpretation of AN:

L ";L # R#;R"Á ! Á ¼N "

L N #R ;N #

L N "R

ALN

PB

N "L ¡ N#

LN "

L N#L

PB

N "L ¡ N "

RN "

L N "R


LAGET’S OBE MODEL[1]

NEGATIVE DYY CAN BE INTERPRETED

[1] J. M. Laget, Phys. Lett. B259 (1991) 24.

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IX International Conference on Hypernuclear and Strange Particle Physics