A Novel Shakedown of the Proton Spin Breakdown

A Novel Shakedown of the A Novel Shakedown of the Proton Spin BreakdownProton Spin Breakdown

How the Field has Become Wider How the Field has Become Wider with a Polarized Proton Colliderwith a Polarized Proton Collider

University of Massachusetts AmherstUniversity of Massachusetts AmherstChristine AidalaChristine Aidala

September 13, 2007September 13, 2007Old Dominion UniversityOld Dominion University

C. Aidala, ODU, September 13, 2007

2

Nucleon Structure: The Early Years• 1933: Estermann and Stern measure

the proton’s anomalous magnetic moment indicates proton not a pointlike particle!

• 1960’s: Quark structure of the nucleon– SLAC inelastic electron-nucleon

scattering experiments by Friedman, Kendall, Taylor Nobel Prize

– Theoretical development by Gell-Mann Nobel Prize


3

Deep-Inelastic Scattering: A Tool of the Trade

• Probe nucleon with an electron or muon beam• Interacts electromagnetically with (charged)

quarks and antiquarks• “Clean” process theoretically—quantum

electrodynamics well understood and easy to calculate!


4

(Unpolarized) Proton Structure and “Bjorken-x”

Halzen and Martin, “Quarks and Leptons”, p. 201

xBjorken

xBjorken

1

xBjorken11

1/3

1/3

xBjorken

1/3 1

Valence

Sea

A point particle

3 valence quarks

3 bound valence quarks

3 bound valence quarks and some slow sea quarks

Small x

What momentum fraction would the scattering particle carry if the proton were made of …


5

Decades of DIS Data

• Wealth of data largely thanks to proton-electron collider, HERA, in Hamburg, which shut down in July this year

• Rich structure at low x• Half proton’s linear

momentum carried by gluons!

PRD67, 012007 (2003)


6

And a (Relatively) Recent Surprise From p+p, p+d Collisions

• Fermilab Experiment 866 used proton-hydrogen and proton-deuterium collisions to probe nucleon structure via the Drell-Yan process

• Anti-up/anti-down asymmetry in the quark sea, with an unexpected x behavior!

PRD64, 052002 (2001)

qq

New techniques let us continue to find surprises in the rich linear momentum structure of the proton,

even after > 40 years!

What about the proton’s angular momentum structure?


7

The Quark Spin Contribution ΔΣ

SLAC: 0.10 < xSLAC <0.7 CERN: 0.01 < xCERN <0.5

0.1 < xSLAC < 0.7A1(x)

x-Bjorken

EMC (CERN), Phys.Lett.B206:364 (1988)1349 citations in SPIRES!

1.005.007.0 CERNSLAC

0.01 < xCERN < 0.5 “Proton Spin Crisis”

qGLG 2

1

2

1


8

Decades of Polarized DIS

2000

ongoing

2007

ongoing

Quark Spin – Gluon Spin – Transverse Spin – GPDs – Lz

SLAC E80-E155

CERN EMC,SMC COMPASS

DESY HERMES

JLAB Halls A, B, C

HERMES

PRL92, 012005 (2004)So far no polarized electron-proton collider. R&D for future

Electron-Ion Collider in current Long-Range Plan for Nuclear Physics.

But what could you learn from a polarized proton-proton collider?


9

A Polarized Proton Collider:Opportunities . . .

• Proton-proton collisions Direct access to the gluons via gluon-quark, gluon-gluon scattering

• High energy provided by a collider allows production of new probes– W bosons

• High energy allows use of new theoretical tools– Factorized perturbative quantum

chromodynamics (pQCD) (more on this later!)

up quarks

down quarks

sea quarks

gluon

IF— Can find a way to maintain polarization of proton beamsthrough acceleration to high energies (O(102 GeV)) and during storage in a ring. Can create tools to measure the degree of beam polarization at various points in the process of acceleration and storage.


10

“Siberian Snakes” To Maintain Polarization

STARPHENIX

AGSLINAC

RHIC

Effect of depolarizing resonances averaged out by rotating spin by 180 degrees on each turn

• 4 helical dipoles in each snake• 2 snakes in each ring

- axes orthogonal to each other


11

Hydrogen-Jet Polarimeter for Beams at Full Energy

• Use transversely polarized hydrogen target and take advantage of transverse single-spin asymmetry in elastic proton-proton scattering

• Only consider single polarization at a time. Symmetric process! – Know polarization of your target– Measure analyzing power in

scattering– Then use analyzing power to

measure polarization of beam

recoil proton

scatteredproton

polarizedproton target

(polarized)proton beam

N

N

AP

PA

1

1

beam

target


12

The Relativistic Heavy Ion Collider at Brookhaven National Laboratory


13

AGSLINACBOOSTER

Polarized Source

Spin Rotators

200 MeV Polarimeter

AGS Internal Polarimeter

Rf Dipole

RHIC pC Polarimeters Absolute Polarimeter (H jet)

PHENIX

PHOBOS BRAHMS & PP2PP

STAR

AGS pC Polarimeter

Partial Snake

Siberian Snakes

Siberian Snakes

Helical Partial SnakeStrong Snake

Spin Flipper

RHIC as a Polarized p+p Collider

Various equipment to maintain and measure beam polarization through acceleration and storage


14

News to Celebrate


15

RHIC Physics

Broadest possible study of QCD in A+A, p+A, p+p collisions

• Heavy ion physics– Investigate nuclear matter under extreme conditions– Examine systematic variations with species and

energy

• Proton spin structure– Gluon polarization (G)– Sea-quark polarization – Transverse spin structure

du ,

Most versatile hadronic collider in the world!Nearly any species can be collided with any other, thanks

to separate rings with independent steering magnets.


16

)(),( xqxq

Flavor-Separated Sea Quark Polarizations Through W Production

)0( , ),(

),(

)0( , ),(

),(

2121

21

2121

21

WW

WWL

WW

WWL

yxxMxd

MxdA

yxxMxu

MxuA

Parity violation of the weakinteraction in combination withcontrol over the proton spin

orientation gives access to theflavor spin structure in the proton!

For W- interchange

u and d.


17

Spin Physics at RHIC

• Three experiments: STAR, PHENIX, BRAHMS

• Future running only with STAR and PHENIX

Transverse spin only

(No rotators)

Longitudinal or transverse spin

Longitudinal or transverse spin

2006 accelerator performance: Avg. pol 62% at 200 GeV (design 70%).

Achieved 3.5x1031 cm-2 s-1 lumi (design ~5x this).


18

Single-Spin Asymmetries for Local Polarimetry:Confirmation of Longitudinal Polarization

Blue Yellow

Spin Rotators OFFVertical polarization

Blue Yellow

Spin Rotators ONCorrect CurrentLongitudinal polarization!

Blue Yellow

Spin Rotators ONCurrent Reversed!Radial polarization

AN


19

A Polarized Proton Collider:Opportunities . . . and Challenges

• Proton-proton collisions Direct access to the gluons via gluon-quark, gluon-gluon scattering

• High energy provided by a collider allows production of new probes– W bosons

• High energy allows use of new theoretical tools– Factorized perturbative quantum chromodynamics (pQCD)

• Challenge: No longer dealing with the “clean” processes of QED!


20

RHIC Spin: Proton Structure with Quark and Gluon Probes

At ultra-relativistic energiesthe proton represents a jetof quark and gluon probes

Need QCD now, not QED!


21

Hard Scattering Process

2P2 2x P

1P

1 1x P

s

qgqg

)(0

zDq

X

q(x1)

g(x2)

Hard Scattering in p+p:Factorization and Universality

“Hard” probes have predictable rates given:– Parton distribution functions (need experimental input)

– pQCD hard scattering rates (calculable in pQCD)

– Fragmentation functions (need experimental input)

Un

iversa

lityU

niv

ersa

lity

)(ˆˆ0

210 zDsxgxqXpp q

qgqg

)(0

zDq


2P2 2x P

g 2f x

q 1f x1P

1 1x P

s

1ps

2ps qgqg X

)()ˆ(ˆ)()()(0

210 zDsxgxqXpp q

qgqg


22

pQCD in Action at s=200 GeV

Fraction pions produced ~0

PRL 95, 202001 (2005)

|| < 0.35

2

hh

STARSTAR

0

STARSTAR


23

Prompt Production at s=200 GeV

• Gluon Compton scattering dominates– At LO no fragmentation

function– Small contribution from

annihilation

2P 2 2x P

1P

1 1x P

qqgq

)(ˆ)(

)(

)(

)(

2

2

1

1 qgqaxq

xq

xg

xgA LLLL

Will be an important channel for G!


24

Probing the Gluon Polarization at RHIC

++ same helicity+- opposite helicity

N: yield

R: luminosity++/luminosity+-

RNN

RNN

PPALL ||

1

21

)()ˆ(ˆ)()()(0

210 zDsxgxqXpp q

qgqg

With factorized pQCD in hand as a theoretical tool, can probe G via double-helicity asymmetry measurements

Now for some results . . .

DIS pQCD e+e-?


25

Fraction pions produced

The Pion Isospin Triplet, ALL and G

• At transverse momenta > ~5 GeV/c, midrapidity pions dominantly produced via qg scattering

• Tendency of + to fragment from an up quark and - from a down quark and fact that u and d have opposite signs make ALL of + and - differ measurably

• Order of asymmetries of pion species can allow us to determine the sign of G

)( du)( ud

LLLLLL AAAG

0

0


26

ALL of at s=200 GeV

STARSTAR

STARSTARPHENIX Run-06 charged pions coming soon!


27

2005 data

Gluon Polarization from Inclusive Hadrons and Jets in Polarized p+p


28

Calc. by W.Vogelsang and M.Stratmann

“std” scenario, G(Q2=1GeV2)=0.4, is excluded by data on >3 sigma level: 2(std)2

min>9 Only exp. stat. uncertainties included (syst.

uncertainties expected to be small) Uncertainties from functional form Δg(x)

not included.

Update on ΔG vs ALL

0

in PHENIX from 2006 DataSasha Bazilevsky, AGS&RHIC Users’ Meeting, June 2007


29

Transverse Momentum vs. xgluon

• Limited sensitivity to x-dependence of g(x)

• Currently no sensitivity to low x and little to x>~0.3

Log10(xgluon)NLO pQCD: 0 pT=29 GeV/c xgluon=0.020.3

GRSV model: G(xgluon=0.020.3) ~ 0.6G(xgluon =01 )

Each pT bin corresponds to a wide range in xgluon

Data is not sensitive to functional form of g(xgluon) Quantitative analysis must include impact of different function form g(xgluon) shape


30

Extending x Range is Crucial!Gehrmann-Stirling parameterizations

GSC: G(xgluon= 01) = 1 G(xgluon= 0.020.3) ~ 0

GRSV-0: G(xgluon= 01) = 0 G(xgluon= 0.020.3) ~ 0

GRSV-std: G(xgluon= 01) = 0.4 G(xgluon= 0.020.3) ~ 0.25

GSC: G(xgluon= 01) = 1

GRSV-0: G(xgluon= 01) = 0

GRSV-std: G(xgluon= 01) = 0.4

Current data is sensitive to G for xgluon= 0.020.3


31

Spin Crisis Came Out of a Low-x Measurement

SLAC: 0.10 < xSLAC <0.7 CERN: 0.01 < xCERN <0.5

0.1 < xSLAC < 0.7A1(x)

x-Bjorken

EMC (CERN), Phys.Lett.B206:364 (1988)1349 citations in SPIRES!

1.005.007.0 CERNSLAC

0.01 < xCERN < 0.5 “Proton Spin Crisis”

qGLG 2

1

2

1

0.6 ~ SLAC Quark-Parton Model expectation!

E130, Phys.Rev.Lett.51:1135 (1983) 415 citations

The history of the field teaches us that low x is important!


32

Extend x Range• Upgrade PHENIX and STAR detectors with forward

coverage (observe collisions of low-x parton in one beam with high-x parton in other)

• Measure at different center-of-mass energies lower x at s = 500 GeV. Expected to start in 2009. higher x at s = 62.4 GeV. Done in 2006!

present x-ranges = 200 GeV

Extend to lower x at s = 500 GeV

Extend to higher x at s = 62.4 GeV


33

0 ALL at s=62.4 GeV

Theory curves by W. Vogelsang

Converting to xT, we can see the significance of the s=62.4 GeV data set compared to the Run-5 200-GeV preliminary data. Not bad for 2 weeks of 62-GeV running!

arXiv:0708.3060pQCD is earning its keep down to lower energies! - Resummed calculation by de Florian, Vogelsang, and Wagner


34

2121

2;

2 21 ee

xxee

xx TT

0 10 3020pT (GeV)

xgluon

500200

GeV101

102

Inclusive 0

N.B. x-range sampled depends on g(x,Q2) ! -- M. Stratmann

Going Beyond Inclusive Measurements

• Inclusive channels suffer from integration over x model-dependent G extraction

• Improved accelerator and detector performance will allow jet-jet and -jet coincidence measurements, placing better constraints on partonic kinematics

Note: Jets and prompt photons both carry total energy of scattered parton.

Don’t need any fragmentation functions . . .


35

Reminder: Factorized pQCD

)(0

zDq


2P2 2x P

g 2f x

q 1f x1P

1 1x P

s

1ps

2ps qgqg X

)()ˆ(ˆ)()()(0

210 zDsxgxqXpp q

qgqg

“Hard” probes have predictable rates given:– Parton distribution functions (need experimental input)

– pQCD hard scattering rates (calculable in pQCD)

– Fragmentation functions (need experimental input)

Un

iversa

lityU

niv

ersa

lity


36

Fragmentation Functions (FF’s):Improving Our Input for Inclusive Hadronic Probes

• FF’s not directly calculable from theory—need to be measured and fitted experimentally

• The better we know the FF’s, the tighter constraints we can put on the polarized parton distribution functions!

• Traditionally from e+e- data—clean system!• Framework now developed to extract FF’s using all

available data from deep-inelastic scattering and hadronic collisions as well as e+e-– de Florian, Sassot, Stratmann: PRD75:114010 (2007) and

arXiv:0707.1506


37

An Example: Cross Section and ALL of Meson

g2 gq q22P 2 2x P

1P

1 1x P

fragmentation function not yet available! No theoretical comparisons currently possible . . .


38

Parameterizing the FF Using e+e- and PHENIX p+p Data

L3

MARK II

CA, J. Seele, M. Stratmann, W. Vogelsang

OPAL

HRS

Once FF is available, asymmetry data will provide additional constraint on G!


39

RHIC Data and Global Fits• Global fits of measurements sensitive to different

processes and different kinematics essential to place tight constraints on nucleon pdf’s

• World DIS results will be combined with RHIC measurements of various probes– Pions, jets, prompt photons, heavy flavor, gamma-jet

and jet-jet correlations, . . . • Two global fit papers published so far including

PHENIX 0 ALL data from RHIC– M. Hirai et al., Phys. Rev. D74, 014015 (2006)– Navarro and Sassot, Phys. Rev. D74, 011502 (2006)

Still a long road ahead of us . . .


40

Conclusions and Prospects

• RHIC has been an exciting place to study nucleon spin structure for the past six years!– Very stimulating place to be an accelerator physicist,

too!

• Availability of a polarized proton collider has expanded breadth of field, attracted new members to the nucleon structure community

• Moving into new era where RHIC will make decisive contributions to knowledge of polarized gluon distribution and more

Nearly 20 years later, the proton spin crisis remains unresolved!

But there’s a large and diverse community of people—at RHIC and complementary facilities—continuing to coax the secrets out of one

of the most fundamental building blocks of the world around us.


41

Extra Slides


42

RHIC Specifications• 3.83 km circumference• Two independent rings

– Up to 120 bunches/ring– 106 ns crossing time

• Energy: Up to 500 GeV for p+p Up to 200 GeV for Au+Au

(per N+N collision)

• Luminosity– Au+Au: 2 x 1026 cm-2 s-1

– p+p : 2 x 1032 cm-2 s-1 (70% polarized)


43

Polarized Collider DevelopmentParameter Unit 2002 2003 2004 2005 2006

No. of bunches -- 55 55 56 106 111

bunch intensity 1011 0.7 0.7 0.7 0.9 1.4

store energy GeV 100 100 100 100 100

* m 3 1 1 1 1

peak luminosity 1030cm-2s-1 2 6 6 10 35

average luminosity

1030cm-2s-1 1 4 4 6 20

Collision points -- 4 4 4 3 2

average polarization,

store% 15 35 46 47 60-65


44

RHIC Polarimetry

• Proton-carbon (pC) polarimeter– For fast measurements (< 10 s!) of beam polarization– Take several measurements during each fill

• Polarized hydrogen-jet polarimeter– Dedicated measurements (weeks) to calibrate the pC

polarimeter

• Three-fold purpose of polarimeters– Measurement of beam polarization to provide feedback to

accelerator physicists– Measurement of beam polarization as input for spin-dependent

measurements at the various experiments– Study of polarized elastic scattering


45

RHIC Performance, Longitudinal Polarization(PHENIX integrated luminosities shown)

** initial estimate

Year s [GeV] Recorded L Pol [%]FOM (P4L)

2003 (Run-3) 200 .35 pb-1 27 1.5 nb-1

2004 (Run-4) 200 .12 pb-1 40 3.3 nb-1

2005 (Run-5) 200 3.4 pb-1 46 150 nb-1

2006 (Run-6) 200 7.5 pb-1 62 1100 nb-1

2006 (Run-6) 62.4 .08 pb-1 ** 48 4.2 nb-1 **


46

PHENIX Polarized-Proton Runs: Transverse Polarization

Year s [GeV] Recorded L Pol [%]FOM (P2L)

2001 (Run-2) 200 .15 pb-1 15 3.4 nb-1

2005 (Run-5) 200 .16 pb-1 47 38 nb-1

2006 (Run-6) 200 2.7 pb-1 57 880 nb-1

2006 (Run-6) 62.4 .02 pb-1 ** 48 4.6 nb-1 **

** initial estimate


47

Improving Forward Coverage at RHICSTAR Forward Meson Spectrometer will

provide full azimuthal coverage for range 2.5 4.0

• Broad acceptance in xF-p

T plane for inclusive ,

etc…production in p+p and d(p)+Au

• Broad acceptance for 0 and 0-0 from forward jet pairs to probe low-x gluon density in p+p and d(p)+Au collisions

PHENIX Muon Piston Calorimeter will provide full azimuthal coverage for range 3.1 3.7 and 2 < E(0) < 25GeV

Raw AN() of 0 from Run-6 MPC, 62.4 GeV

STARSTAR


48

Prompt Photons with PHENIX Nosecone Calorimeter

• γ-jet is:– Very clean way to measure the gluon

– Measuring the angle of the jet gives you access to xgluon

x

Q2

x

Central arms prompt

200 GeV 200 GeV

10-110-210-310-410-510-1

1

10

102

103

104

1

NCC prompt NCC prompt


49

W

Z

W Production in Polarized p+p Collisions

Single Spin Asymmetry in the naive Quark Parton Model

GeV 20

for dominates

Tp

W

2121

21 ,

),(

),(xx

Mxu

MxuA

W

WWL

Experimental Requirements: tracking at high pT

event selection for muons difficult due to hadron decays and beam backgrounds

control of all backgrounds

Parity violation of the weakinteraction in combination withcontrol over the proton spin orientation gives access to theflavor spin structure in the proton!


50

Gluon Polarization from Photon-Gluon Fusion in DIS

Photon-Gluon Fusion (PGF)

• golden channel: charm productiongolden channel: charm production

• hadron production at high Phadron production at high PTT

Favors small ΔG(x≈0.1)


51

)(

)2/(

0

0

HERMES

(hadron pairs)

COMPASS(hadron pairs)

E708(direct photon)

RHIC(direct photon)

CDF(direct photon)

Scale Dependence at RHIC vs Spin-Dependent DIS

Scale dependencebenchmark:

Tevatron ~ 1.2RHIC ~ 1.3COMPASS ~ 2.5 – 3HERMES ~ 4 – 5

Scale dependence at RHIC is significantly reduced compared to fixed target polarized DIS.

Ch

an

ge in

pQ

CD

calc

ula

tion

of

cro

ss s

ecti

on

if

facto

riza

tion

scale

ch

an

ge b

y f

acto

r 2

Conclusion: Extraction of spin dependent parton distributions will be possible at RHIC using perturbative QCD ab initio: First spin structure experiments at Collider energies!


52

Forward Hadron Production at s=200 GeV

BRAHM

S

Preliminary

PRL 97 (2006) 152302Good agreement between data and NLO pQCD at s=200 GeV, even at

larger rapidities

STARSTAR

Fraction pions produced


53

xT Scaling• xT scaling—can parametrize cross

sections for particle production in hadronic collisions by:

• Lower energy has higher yield at fixed xT

constant,2

ns

px T

T

)(~3

3

T

nxFs

dp

dE

3.5

3

3

3

3

2

sL

dxs

dp

dELdp

dp

dEL TT

We can probe higher xT with better statistics even with a short run at 62.4 GeV!! (compared to 200 GeV)

GeV540~62s

xT


54

ALL of J/at s=200 GeV

gg QQ

g

g

g

g

J/


55

Attempting to Probe kT from Orbital Motion

• Spin-correlated transverse momentum (orbital angular momentum) may contribute to jet kT. (Meng Ta-chung et al., Phys. Rev. D40, 1989)

• Possible helicity dependence• Would depend on

(unmeasured) impact parameter, but may observe net effect after averaging over impact parameter

kT larger kT smaller

Same helicity

Opposite helicity

Run-5

Di-hadron kT asymmetry


56

Total PHENIX Raw Data VolumesWAN data transfer and p+p data production at CC-J in RIKEN, Wako Japan

» 60 MB/s sustained rate using Grid technology

» 570 TB transferred in Runs 5 & 6


57

STAR Detector

-1 < < 1

0 < < 1

1 < < 2

2.2 < || < 5

|| ~3.3/3.7/4.0

Philosophy:Extensive acceptance,

but low rate capability

STARSTAR


58

PHENIX Detector2 central spectrometers-Track charged particles and detect electromagnetic processes

2 forward spectrometers- Identify and track muons

Philosophy:High rate capability to measure rare

probes,

but limited acceptance.

35.0||

9090

azimuth


59

BRAHMS DetectorPhilosophy:

Small acceptance spectrometer

armsdesigned with good charged particle ID.

Documents

A Novel Shakedown of the Proton Spin Breakdown