Spin physics in COMPASS - uni-mainz.de · Spin physics in COMPASS RICH characteristics mirror wall...

Spin physics in COMPASS

COMPASS

• Introduction.

• Spectrometer.

• Event reconstruction.

• Physics analysis.

Spin physics

in COMPASS.

A.Korzeneva

Mainz University

January 19, 2004

aon leave from JINR

Kernphysikseminar, 19.01.04 A.Korzenev


INTRODUCTION



More than 220 physicists from 30 institutes

TelAviv

Saclay

(SINS & TU)

CERN

Warsaw

BielefeldBochumBonn(ISKP&PI)ErlangenFreiburgHeidelbergMainzMunchen(LMU,TU)

Torino(University & INFN)Trieste(University & INFN)

Prague(CU,CUT,TUL)

Lisbon

Dubna

State University)Moscow(INR,LPI,

Protvino

Helsinki

CalcuttaBurdwan

Nagoya



Physics goals

• Spin physics with muon beam

– ∆G/G from high pT hadrons and open charm production

– Quark flavor separation via semi-inclusive analysis

– Transverse quark distribution ∆T q via single spin asymmetry

– Λ and Λ polarization

– Exclusive vector meson production

• Physics with hadron beam

– Primakoff effect

– Exotic QCD states (glueballs, hybrids)

– Doubly charmed baryons



The CERN µ-beam

*

*

π, Κ µ

+µ

TargetT6

p beam

from SPS

Decay channel

600 m100 m 330 m

µ momentum selection

100 m

hadronabsorber

+++

selectionπ

Scraper

Scraper

COMPASSTarget

νµ

µ+

π, Κ++

π, Κ++p

θJ=0

• Energy 160 GeV

• 5s spill every 14.4s

• Intensity 2 · 108 µ/spill

• Natural polarization due to parity

violation in the week decay of par-

ent hadrons Pµ =-76%



COMPASSSPECTROMETER



GEM& MWPC

Trigger Hodoscopes

SM2

beam

ECal & HCal

SM1

Straws

Target

RICH

Scifi

MWPC

MicroMegas

Drift chamber

.

.Spectrometer with 2 stages

SAS: small scattering angles, p>5 GeV/c(SM1: 1Tm, SM2: 4.4 Tm)

(SciFi, Silicons, GEM & Micromegas)

SAS: large angles & small momenta(MWPC, Straw & Drift chambers).

.Electromagnetic & hadron calorimeters..Particle identification: RICH & FTarget angular acceptance 70 mrad

µ

50m



The Polarized Target

(70mrad)AcceptanceSMC

He-Precooler3

COMPASSAcceptance(180mrad)

Dilution refrigerator Targets

SolenoidSuperconducting

1m

• Two 60 cm long target cells with

opposite polarization

• Target material 6LiD

– Maximum polarization: 57%

– Dilution factor: ∼50%

• 2.5 T solenoid field (homogeneity:

±1.5 · 10−5)

• 3He/4He dilution refrigerator

(Tmin ≈50 mK)

• SMC magnet is currently used

– Hadron acceptance: 70 mrad



RICH characteristics

mirrorwall

photondetectorts

beam

3.3 m

5.3

m

6.6 m

Performance:

• Angular resolution σ1γ=1.4 mrad.

• Photons per ring 〈nγ〉 = 14

• Two segmented spherical mirrors.

• Photon detectors are MWPC’s with

CsI photocathodes. Total active sur-

face 5.3 m2. Pad size 8×8 mm2.

• Analog 2D read-out.

• Radiator gas C4F10 for momenta

3-65 GeV.



RICH PID performance

Event − Display m [GeV]0 0.2 0.4 0.6 0.8 1 1.2

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

m [GeV]0 0.2 0.4 0.6 0.8 1 1.2

0

500

1000

1500

2000

2500

Zoom

πK

p

Mass Spectrum

20

10

30

40

50

0 10 15 25 35 4020 305p [GeV/c]

[mra

d]θ

pπ

e,µ

K



Detectors positioning

Particle rate is highest on the beam axis and decreases outward ⇒

• Tracking system is subdivided into

a set of nested detectors of increas-

ing rate capabilities.

• Large aperture detectors are pro-

tected against high rate either by

physical hole or by deactivation of

the central region. .

Silicon microstripdetectors

Scintillating Fiber &

GEM detectorsMicroMeGaS &

MWPC

Drift cell chambers,Steel drift tubes &

Silicon Scintillating Micro- GEM Drift Straw

microstrips Fibers MeGas chambers

resolution 15 µm 150 µm 70 µm 70 µm 170 µm 270 µm

Size



Trigger concept

y=0.3

Magnet

z2

z1

zm

zo

z

x

Halo µ

Signal

Trigger

H4

H5

Target

HCAL

Beam

ThresholdMatrixCoincidenceHodoscopes

scattered µ

geometric property of scattered.. minimal energy deposition in hadron

µ

Semi−inclusive triggers ( Q < 5 (GeV/c) )2 2

calorimeter to reject

3. events with low energy halo tracks

1. radiative eventsµ e2. scattering

geometric property of scattered. µ

Inclusive triggers ( Q > 0.5 (GeV/c) )22

.

.32x32 coincidence matrix.

coincidence width < 3ns.

hodoscope time resolution 130 ps..



EVENTRECONSTRUCTION



Track finding program

1. reconstruction in

2. reconstruction in space.projections.

Selection of group of clusterswhich seems consistent with belonging to a straight line

Pattern Recognition

3. straight line fit of tracksegments without multiple

Track Bridging

material (hadron absorber).

1. bridging through magnets

2. bridging through thick

Magnet

Joining of track segmentsfrom different zones

into one track

scattering.

account multiple scattering.

Final fit of tracks in mag−netic field taking into

Track Fitting



Search for primary interaction

and V0 decay points

Target

beam

• Fit with constraint on intersection

of all tracks in one point is used.

• V0 is a neutral particle which decay

into to charged.

K0S → π+ π−

Λ → p π−

• Estimation of the vertex position.

• Estimation of momentum of all

tracks at the vertex.

• Rejection of tracks belonging to an-

other vertices or to background.

Targetbeam

+

−



PHYSICS ANALYSIS

• ∆G/G from high pT hadrons

• Open charm production

• Quark flavor separation via semi-inclusive analysis

• Transversity

• Λ and Λ polarization

• Exclusive vector meson production



Nucleon Spin Puzzle

d

uu

γ ∗

h

XN

µµ (E,p) ’(E’,p’)

q

Static Quark Model ∆Σ = 1

Baryons week ∆Σ = 0.58 ± 0.03

decays assuming ∆s = 0

DIS ∆Σ = 0.24 ± 0.03

∆s = −0.11 ± 0.01

Due to axial anomaly ∆Σ interpreta-

tion is difficult: ∆Σ → ∆Σ − 3αs

2π∆G

• Nucleon Spin

1

2= 1

2∆Σ + ∆G+ 〈Lz〉

∆G - gluon contribution.

〈Lz〉 - orbital angular momentum

of q and G.

∆Σ - quarks spin.

∆Σ = ∆u + ∆u + ∆d + ∆d + ∆s + ∆s

G∆-1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

q∆

-0.2

0

0.2

0.4

0.6

0.8

Σ∆

s∆



Double-Spin asymmetry

• Polarized DIS cross section in one-

photon exchange approximation

σ = σ ±1

2∆σ

• Structure functions

σ = aF1(x, Q2) + bF2(x, Q2)

∆σ = αg1(x, Q2) + βg2(x, Q2)

• Double spin asymmetry in case of lon-

gitudinally polarized beam and target

A|| =∆σ||

2σ, A1 ' DA||

• Virtual photon - proton asymmetry A1:

γ ∗

σ = Σ1/2 e qq q2

uu

d

u

du

γ ∗

σ = Σ3/2 e qq q2

A1 =σ1/2 − σ3/2

σ1/2 + σ3/2

=

∑qe2

q∆q∑

qe2

qq

• Measured in many experiments-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

10–2 10–1 1

A1d

x

SMC (1995)SMC (1994)SMC (1992)

Q2 > 1 GeV2

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6SMC (Q2 > 1.0 GeV2 )SMC (Q2 > 0.2 GeV2 )E143 (Q2 > 1.0 GeV2 )

10–2 10–1 1

x10–3

A1d

(a)



adau

can be neglectedassuming

Araw =Nu − Nd

Nu + Nd

=aunu − adnd + fPb(aunuPu + adndPd)A||

aunu + adnd + fPb(aunuPu − adndPd)A||

A′raw =

N ′u − N ′

d

N ′u + N ′

d

=a′unu − a′dnd − fPb(a

′unuP

′u + a′dndP

′d)A||

a′unu + a′dnd − fPb(a′unuP ′u − a′dndP

′d)A||

Nu = auΦnuσ(1 + fPbPuA||)

Nd = adΦndσ(1 − fPbPdA||)

N ′u = a′uΦ

′nuσ(1 − fPbP′uA||)

N ′d = a′dΦ

′ndσ(1 + fPbP′dA||)

A|| =1

fPbPt

1

2(Araw − A′

raw) − Afalse||

Afalse|| =

1

fPbPt

1

2

r − 1

r + 1−

r′ − 1

r′ + 1

r =

aunu

adnd

P N P Nb u u d dP

beam

every 8h

Upstream Downstreamcell cell

reversal

P’ N’ P’ N’P’b u u dd

Asymmetry extraction



∆G/G from high pt pairs

h1

γ

q

*

h2 h2

h1γ

q

*

q

Gh2

h1

q

γ*

q

G

Leading process Gluon radiation (Compton) Photon Gluon Fusion

Measured asymmetry:

A =N↑↓ − N↑↑

N↑↓ + N↑↑= PµPT fAµN→hhN

AµN→hhN = A1〈aLL〉LP RLP + A1〈aLL〉

GRRGR

+∆G

G〈aLL〉

PGF RPGF

〈aLL〉process is analyzing power of

process.

A1 = g1/F1

Rprocess = σprocess/σtotal



∆G/G from high pt pairs

]2

m [GeV/c0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

1

10

102

103

104

105

preliminary

[GeV/c]tp0 1 2 3 4 5 6

1

10

102

103

104

preliminary

]2

m [GeV/c0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

0

10000

20000

30000

40000

50000

preliminary

• µ, µ′+2 hadrons

• in plots only 5% of 2002

• Statistics of 2002 data:

Q2 > 1 GeV2 18K ev

all Q2 160K ev

• Accuracy:

Q2 > 1 GeV2 δ(∆GG

) ≈ 0.3

all Q2 δ(∆GG

) ≈ 0.1



Open charm production

h2

h1γ*

c

G

c

*D D+ 0 π+

s

K π− +

*D D− 0 π−

s

K π−+

∆MKππ = MKππs− (MKπ + Mπs

)

• 3.1 < ∆MKππ < 9.1 MeV

• statistics: 317 D0 tagged by

D∗. (MeV)0D-MπKM-400 -300 -200 -100 0 100 200 300 400

(M

eV

)π

-Mπ

k-M

sπ

πk

M

-10

0

10

20

30

40

50

20

40

60

80

100

120

140

160

(MeV)oD-MπKM

-400 -300 -200 -100 0 100 200 300 4000

50

100

150

200

250

300

350

Preliminary

(MeV)π-MπK-MsππKM

-10 0 10 20 30 40 500

20

40

60

80

100

120

140

160

180

200

220

Preliminary

Data 2002. Selection cuts:

• zD0 > 0.2.

• | cos(θ∗)| < 0.85

• 10 < pk < 35 GeV



Flavor separation via

semi-inclusive asymmetries

• Inclusive Asymmetry:

A1 =σ↑↓ − σ↑↑

σ↑↓ + σ↑↑

LO=

∑q

e2q(∆q(x) + ∆q(x))

∑q

e2q(q(x) + q(x))

• Semi-Inclusive Asymmetry:

Ah1 =

σh↑↓ − σh

↑↑

σh↑↓

+ σh↑↑

LO=

∑q

e2q(∆q(x)

∫Dh

q dz + ∆q(x)∫

Dhq dz)

∑q

e2q(q(x)

∫Dh

q dz + q(x)∫

Dhq dz)

du

γ ∗

’µ

q(x,Q )2∆

D (z,Q )hq

2

µ

q(x,Q )2

XN

q h

• Due to isospin symmetry of deuteron

only {∆u+ ∆d,∆u+ ∆d,∆s} can be

extracted assuming ∆s = ∆s.

• If combined with data from

proton target full flavor sep-

aration is possible.



Quark flavors separation

0

0.2

10-1

x⋅∆u

-0.2

0

10-1

x⋅∆d

-0.1

0

10-1

x⋅∆u–

-0.1

0

10-1

x⋅∆d–

-0.1

0

10-1

x⋅∆s

0.03 0.1 0.6x

-0.2

-0.1

0

0.1

0.2

10 -1

0.03 0.1 0.6x

x(∆u–-∆d

–)

• HERMES: hep-ex/0307064

• Low x region is important

• COMPASS limit with Q2 > 1 GeV2 is

xmin = 0.003

Bjx

10-2

10-1

1

s∆x

-0.2

-0.15

-0.1

-0.05

-0

0.05

0.1

0.15

0.2

COMPASSHERMEShep-ex/0210049

Error estimates for COMPASSs,K-,K+,K-,h+,hµ2002 run using

asymmetries

-x s(

x)



Transversity and Collins asymmetry

• PDF q(x), ∆q(x), ∆T q(x) (f1(x), g1(x),

h1(x)) fully describe the nucleon structure

at twist-2 level.

• f1(x) and g1(x) are measured in inclusive

DIS.

• h1(x) is chiral odd (helicity flip), so re-

quires associative hadron production.

+

− +

+

+−

− −

• In polarized semi-inclusive DIS chiral odd

H⊥1 is coupled to h1. It affects azimuthal

dependence of cross section.

s’Φ

Φc

x y

z

µ

µ’

γ *p

Th

S

S’

h

s’Φ

Φc

ααS

S’

y

x ( µ−µ plane )

γ *

p hT

’

(a)

(b)



Transversity and Collins asymmetry

• Experimental asymmetry

N↑ − N↓

N↑ + N↓= ε sin ΦC

ε = AUT · PT · f · DNN

PT - target polarization

DNN - spin transfer coefficient

f - dilution factor

AUT - Collins asymmetry

• Selection cuts

Q2 > 1GeV 2/c2 0.1 < y < 0.9

zh > 0.25 phT > 0.1GeV/c

• 106 DIS events with transverse

polarization (2002)

x0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

(%

) >

CΦ

< si

n

UT

A

-20

-15

-10

-5

0

5

10

15

20

25

30

for negative leading hadron >CΦ< sin

UTThe Collins asymmetry A

COMPASS 2002 expected statistical error

Efremov prediction

PRELIMINARY

x0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

(%

) >

CΦ

< si

n

UT

A

-20

-15

-10

-5

0

5

10

15

20

25

30

for negative leading hadron >CΦ< sin

UTThe Collins asymmetry A

COMPASS 2002 expected statistical error

Efremov prediction

PRELIMINARY



Λ and Λ production

α-1 -0.5 0 0.5 1

[G

eV/c

]t

p

0

0.05

0.1

0.15

0.2

0.25PRELIMINARY

s0K

0Λ 0Λ

2/c2>1.0 GeV2

Q

• Measure:

– Longitudinal spin transfer in current frag-

mentation region.

– Access to intrinsic strangeness of the nu-

cleon in target fragmentation region.

• Selection criteria:

– Decay vertex is outside of the target.

– pt > 23 MeV/c.

– Q2 > 1 GeV2, 0.2 < y < 0.9.



Λ and Λ production

0.45 0.5 0.55 0.6 0.65

A.U

.

0

1

2

3

4

5

6 0K

260±22608

-1 -0.5 0 0.5 1

DA

TA

/MC

0

0.5

1

1.5

-1 -0.5 0 0.5 1

DA

TA

/MC

0

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

1.1 1.15 1.2

A.U

.

0

0.5

1

1.5

2

2.5 Λ

123±3603

-1 -0.5 0 0.5 1

DA

TA

/MC

0

0.5

1

1.5

-1 -0.5 0 0.5 1

DA

TA

/MC

0

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

-1 -0.5 0 0.5 10

0.5

1

1.5

]2

m [GeV/c

1.1 1.15 1.2

A.U

.

0

0.5

1

1.5

2 Λ

99±2013

)x*Θcos(

-1 -0.5 0 0.5 1

DA

TA

/MC

0

0.5

1

1.5

2

)x*Θcos(

-1 -0.5 0 0.5 1

DA

TA

/MC

0

0.5

1

1.5

2

)y*Θcos(

-1 -0.5 0 0.5 10

0.5

1

1.5

2

)y*Θcos(

-1 -0.5 0 0.5 10

0.5

1

1.5

2

)z*Θcos(

-1 -0.5 0 0.5 10

0.5

1

1.5

2

)z*Θcos(

-1 -0.5 0 0.5 10

0.5

1

1.5

2

P R E L I M I N A R Y

• 1/6 of 2002 data.

• In total: 22K Λ and 12K Λ.

• dNd cos θi

= Ntot

2(1 + αPi cos θi)

• Polarization seems consistent

with 0.



Exclusive ρ0 and φ production

P

V

µµ

γ

2

W2

Q

t

’

pp’

*

Cuts:

• 3 outgoing tracks in

primary vertex: µ′, h+, h−.

• −2 < ∆E < 2.5 GeV

• |t′| < 0.5 GeV

Kinematic range:

10−3 < Q2 < 10GeV2

7.5 < W < 16GeV

E [GeV]∆-5 0 5 10 15

0

10000

20000

30000

40000

50000

60000

70000

80000

Missing Energy

∆ M −M2M

E=2 2

p

p

x

mass cut statistics (1/6 of 2002)

ρ0 0.5< mππ <1 GeV 1.3·106

φ |mKK − mφ| < 9 MeV 42·103

m [GeV]0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

(770)ρ

* Preliminary

* No MC corrections

* 1/6 of 2002 statistics

invariant mass-π+π

m [GeV]0.98 1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 1.16 1.18

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

(1020)φ * Preliminary* No MC corrections* 1/6 of 2002 statistics

invariant mass.-K+K



Exclusive ρ0 production

p’

π−

π+

γ * γ *

p’

� � ��

� � ��

� ��

p

ρ0

p

π

π

• No acceptance correction.

• Non-exlusive contribution is

subtracted.

• Fit with Soding parameteri-

zation.

• Non-resonant contribution

decreases with encrease of

|t′| or Q2.

Interference between resonant and non-resonant

π+π− production.

) [GeV]-π+πm(0.6 0.8 1 1.2

-5000

0

5000

10000

15000

20000

25000

[0 ,0.02]∈-t’

) [GeV]-π+πm(0.6 0.8 1 1.2

-5000

0

5000

10000

15000

20000

25000

[0.02,0.07]∈-t’

) [GeV]-π+πm(0.6 0.8 1 1.2

0

5000

10000

15000

[0.07,0.19]∈-t’

) [GeV]-π+πm(0.6 0.8 1 1.2

-2000

0

2000

4000

6000

8000

10000

[0.19,0.5]∈-t’

) [GeV]-π+πm(0.6 0.8 1 1.2

-5000

0

5000

10000

15000

20000

>=0.022<Q

) [GeV]-π+πm(0.6 0.8 1 1.2

-2000

0

2000

4000

6000

8000

10000

12000

14000

16000

>=0.132<Q

) [GeV]-π+πm(0.6 0.8 1 1.2

-500

0

500

1000

1500

2000

2500

>=0.412<Q

) [GeV]-π+πm(0.6 0.8 1 1.2

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

>=1.272<Q

PRELIMINARY

dN/d

mdN

/dm

2Q, |t’|



Angular distributions in ρ0 production

• Test of SCHC at low Q2.

• Access to GPD at high Q2.

π−

γ *

p’t

γ * center−of−mass framep

π+ρ0

ρ0 rest frame

p’π+

π−

µ

µ’p

φ

Φ

θ

• No acceptance correction.

• pt > 0.15 GeV/c, Q2 > 0.05 (GeV/c)2

• Measurement of density matrix elements gives

access to helicity transfer mechanism.

W (cos θ) = 3/4[(1 − r0400) + (3r04

00 − 1) cos2 θ]

)θcos(-1 -0.5 0 0.5 10

0.2

0.4

0.6

0.8

>=0.1252<Q

)θcos(-1 -0.5 0 0.5 10

0.2

0.4

0.6

0.8

>=0.4052<Q

)θcos(-1 -0.5 0 0.5 10

0.2

0.4

0.6

0.8

>=1.062<Q

)θcos(-1 -0.5 0 0.5 10

0.2

0.4

0.6

0.8

>=3.312<Q

[rad]ψ0 1 2 3 4 5 6

0

0.1

0.2

0.3

0 /2π π /2π3 π2

>=0.1252<Q

[rad]ψ0 1 2 3 4 5 6

0

0.1

0.2

0.3

0 /2π π /2π3 π2

>=0.4052<Q

[rad]ψ0 1 2 3 4 5 6

0

0.1

0.2

0.3

0 /2π π /2π3 π2

>=1.062<Q

[rad]ψ0 1 2 3 4 5 6

0

0.1

0.2

0.3

0 /2π π /2π3 π2

>=3.312<Q

PRELIMINARY

ψ1/

N d

N/d

1/N

dN

/dco

s θ

.

.

2Q



Summary and Outlook

• Very broad physics programme.

• A lot of data to be analyzed

– collected data of 2002, 2003

– Long run in 2004 (150 days)

• Physics analysis shows first results:

– Photoproduction of vector mesons (ρ0, φ, J/ψ)

– Λ, Λ polarization

– Quark flavor separation

– ∆G/G from high pt and open charm

– Transversity

• Good perspective and upgrade of spectrometer after 2005.



Thank you for

your attention


Spin physics in COMPASS - uni-mainz.de · Spin physics in COMPASS RICH characteristics mirror wall...

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Transcript of Spin physics in COMPASS - uni-mainz.de · Spin physics in COMPASS RICH characteristics mirror wall...