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Measurement of the single transverse-spin asymmetry of forward neutrons in p-p

collisions at RHIC-PHENIX

Manabu Togawa

for the PHENIX collaboration

from Kyoto University

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Outline

• Motivation

• PHENIX neutron measurement– Setup– Neutron asymmetry at sqrt(s)=200GeV

• Simulation study• Estimation of asymmetry error at 2005 RUN

– First measurement of neutron asymmetry at sqrt(s)=410GeV

• Summary

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Motivation• In the forward neutron production, there are some inte

resting behaviors.– In ISR experiment, pp sqrt(s)=30~63GeV, cross section of forward

neutron production at low pT was measured to be larger than at high pT. [left pict.]

– In the RHIC IP12 experiment, pp sqrt(s)=200GeV. We found large single transverse-spin asymmetry AN (-10%) [right pict.]

– PHENIX data can shed new light to understand the production mechanism.

<AN>=-0.1090.0072

Kinematics ±2.8mrad

Forward region

xFNucl. Phys. B109 (1976) 347-356

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• Does Feynman scaling hold when going to RHIC energy?

• From the ISR result, C.S. are well scaling by xF at 30<sqrt(s)<63 GeV..

• Forward neutron C.S. has peak structure and it is well described by pion exchange model.

• What is the mechanism of neutron asymmetry ?– pion exchange model

• Asymmetry can be appeared by interference of spin flip amplitude with other’s one.

– Twist-3 model• It can explain asymmetries of forwar

d pion (E704).• Based on pQCD, does it work in suc

h forward kinematics?

Eur.Phys.J.A7:109-119,2000

xF

xF dependencesqrt(s) >= 200GeV - cross section - asymmetry

pT dependencesqrt(s) >= 200GeV - asymmetry

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Forward neutron measurement at PHENIX

～ 1800cm

10cm±2.8mrad

PHENIX Collision Point

Dx magnetblue beam yellow beam

LRRL

LRRLN

NNNN

NNNN

PA

1

AN ~1

P

1

N total

To calculate asymmetry, use square root formula.

Typical energy distribution

GeV

* Energy calibration was done by CuCu data. (1 neutron peak)

scintillator

100GeV

sqrt(s)=200GeV

At 2005 RUN, sqrt(s) = 200, 410GeV polarized proton beams.

Trigger set sqrt(s)=200 sqrt(s)=410

NORTH or SOUTH 12M 11M

(NORTH or SOUTH) & Minbias

90M 72M

NORTH & SOUTH 30M 20M

GeV

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ZDC (Zero Degree Calorimeter) and SMD (Shower Max Detector)

beam

beam

～ 1800cm

10cm±2.8mrad

PHENIX Collision Point

Dx magnetblue beam yellow beam

150

unit:mm

100

Hadronshower

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Hadron Calorimeter

(sampling by Tungsten and fiber plates.)

5.1λT 149X0 (3 ZDCs)Energy resolution ~20% @ 100GeV

Position resolution ~1cm @ 50GeV (sim).

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Neutron asymmetry at PHENIX from RUN5 data (200GeV)

LRRL

LRRLraw

NNNN

NNNNA

Using square root formula.

Detector

Forward

Raw

as

ymm

etry

Backward

Raw

as

ymm

etry

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Simulation study• ZDC and SMD have not been measured energy and

position resolution less than 100GeV.– In the test experiment, energy resolution was measured 100

and 160GeV by proton beam.

• It is necessary to study the response by simulation.– Asymmetry is smeared by position resolution

GeV GeV

~ 21% ~ 20%

Real data (1 neutron @ CuCu data)

Simulation out (neutron 100GeV input)

Simulation is based on geant3.

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Estimation of asymmetry error - energy dependence -

LRRL

LRRLraw

NNNN

NNNNA

)200(@100~

GeVs

energyxF

• A has energy dependence ?– Flat at IP12 experiment.– In this forward region,

rawN

total

raw AP

AN

A11

~

simulation studyError reach ~ 10-3 order

Asymmetry definition

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Estimation of asymmetry error - pT dependence -

222

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*ryx

yxEpT

simulation studyError reach ~ 10-3 order

pT distribution at sqrt(s) = 200GeV

pT(GeV)

r

(0,0)

(x,y)

n

detector

Max x and y : ~5 cm r ~ 1800 cm

0.1GeV

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Asymmetry at 410GeV

Forward

Raw

as

ymm

etry

Backward

Raw

as

ymm

etry

First measurement

• Asymmetry remains in higher energy region.– pT is larger compared with 200GeV.– Local polarimeter needs this asymmetry until 500GeV in RHI

C spin program. it is expected to remain at 500GeV !

pT(GeV)

Analysis cut effect is uncorrected.

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Summary• We took forward neutron data at sqrt(s)=200 an

d 410 GeV in 2005 RUN at RHIC-PHENIX.• The expected asymmetry error at 2005 RUN w

ere estimated.– Asymmetry will be obtained with 10-3 accuracy. – Energy and pT resolution were estimated by simulati

on.

• Neutron asymmetry at sqrt(s)=410GeV was measured for the first time.– It remains in such high energy.– It is expected that asymmetry will remain when go t

o sqrt(s) = 500GeV.

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Back up

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How to measure how longitudinal?

AL 1

P

AN 1

P L R

L R

Parity violation : allow by weak decay unconvincing

Basic idea is from FNAL704 (AN of forward pion)

The E704 experiment at Fermilab

pp XSqrt(s)=19.4GeVpT=0.2~2.0GeV/c

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Electro Magnetic Cal-based System

-performance-

Calibrated with the beam experiment at SLAC.

E/E ~ 10/sqrt(E) %Noise ~ 1.4GeVx = y ~ 0.15cm for x = y ~ 0.5cm for N

Particle ID logic

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EMCal based results

Succeed in 0 reconstructionM/M ~ 9.3%

Average beam pol. ~ 11%Calculate asymmetry using sqrt root formula.

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Hadron Cal-basedsystem (1 ZDC)

-performance-Energy is calibrated by using cosmic-ray data and simulation. Flat response E>20GeV

E/E ~ 40 to 50 % at E>20GeVx ~ 3 to 4cm(post shower)

Particle ID logic

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Shower MAX Detector

For x-pos : 7For y-pos : 8

150

Unit : mm

• For measuring neutron position, SMD (Shower Max Detector) was installed btw ZDC1 and ZDC2. – Arrays of plastic scintillators – Obtain the position by calculating the center of gravity of

shower generating in first ZDC.– Position resolution ~1cm @ 50GeV neutron (simulation

study).

1 ZDC

SMD

100

5

Hadronshower

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Looking neutron peak

After subtract pedestals and apply calibration constant. To match 1 neutron peak is 100. (should be 100 GeV)

SOUTH NORTH

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ZDC shape

GeV

BLUE : SOUTHRED : NORTH

Very agree both shape after calibration.

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Gluon polarization• To understand the spin structure in the nucleon• 　　　 -> 1/2(proton) = 1/2DS(quark) + DG + L

ALL ( ) ( )

( ) ( )

ALL ~g(x1)

g(x1)

ei2

i

qi(x2)

ei2

i

qi(x2)ˆ a LL (qg q)

γ

For ex. gq -> qγ

Measured in DIS Calculated by pQCD

The experiment of longitudinal polarized proton collide had been started from this year by introducing the spin rotator.

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Square root furmula

LRRL

LRRLN

NNNN

NNNN

PA

1

AN ~1

P

1

N total

where : N total N R N R N L N L

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• Calculate for each bunch.• As a fit function,

Fit for all bunch as D,AB,AY

variables. (55 bunch mode at RUN3)

ALR (i) NL (i) NR (i)

NL (i) NR (i)

ALR D AB PB AY PY

1 DAB PB DAY PY

NL(R) : Number of Left(Right) D : detector asymmetryAB : BLUE asymmetryAY : YELLOW asymmetryPB : BLUE polarizationPY : YELLOWpolarization

Measured by CNI polarimeter

Our measurement

Bunch Fittinig

Pol derection

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Neutron energy (sim)neutron put to center

Input energy

Input energy

Ou

tput

ene

rgy

En

ergy

reso

lution

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Xpos

Ypos

Neutron energy

Neutron energy* Position resolution is defined as RMS.

Input line

Position resolution (sim)neutron put to center

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Neutron energy (sim)neutron put to edge

* Resolution is defined as RMS/Energy

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Asymmetry as function of ZDC2 cut

beam

Gamma stop at ZDC1.Neutron hadron shower goes to ZDC2.

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Reducing asymmetry valueby energy and position smearing

Input : neutron with flat energy distribution, 10 ~ 100GeV input positions are flat for x and y.

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EMCal based results

Calibrated for the photon only.

<AN>=-0.1090.0072

<AN>=-0.1080.0087Detector

Hadron Cal based results

<AN>=-0.1090.0072

AN 1

P L R

L R

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As a local polarimeter system

PHOBOS BRAHMS

STARPHENIX

Spin Rotators

Siberian Snakes

RHIC （ Relativistic Heavy Ion Collider ）CNI polarimeter

• One of the main program of RHIC experiment is that the determination of the polarized parton distribution function.– It needs longitudinally polarized proton-proton collision.

• Polarimeter at the collision point is necessary to confirm “beam is longitudinal”. (Local polarimeter)

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RUN5(2005) result.

AAT

AL

2

1cos

A

A

A

A TL

<(RawAsymmetry)/(pol.)/A>

BLUE 9.5 ± 2.2 (%)

YELLOW 15.5 ± 2.4 (%)

Through the RUN5, longitudinal component : <pL/p> BLUE = 99.54 ± 0.12 ± 0.03 (%)<pL/p> YELL = 98.78 ± 0.24 ± 0.06 (%)

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Fill by fill analysisby sqrt formula

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Bunch shuffling : sqrt formula (sim with 1% asymmetry)

Forward LR Forward UD

Backward LR Backward UD

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Bunch shuffling : sqrt formula (forward region)

BLUE LR BLUE UD

YELLOW LR YELLOW UD

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Bunch shuffling : sqrt formula (backward region)

BLUE LR BLUE UD

YELLOW LR YELLOW UD

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