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Forward Physics with Polarized proton-proton Collisions

at the experiment.

John KosterRIKEN2012/07/25

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Motivation: Structure of Matter

Structure of the proton

1955 Hofstadter: Radius 0,8 fm Nobel Prize 1961

1968 Friedman, Kendall, Taylor: quarks in the proton Nobel Prize 1990

Highest Q²: quarks, gluons elementary

Q² = negative momentum transfer squared18/ 10 mp

p, proton

e, electron

g, photon

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The three leading order, collinear PDFs

Parton Distribution Functions

q(x)

Dq(x)

DTq(x)

unpolarized PDFquark with momentum x=pquark/pproton in a nucleon

helicity PDFquark with spin parallel to the nucleon spin in a longitudinally polarized nucleon

transversity PDF quark with spin parallel to the nucleon spin in a transversely polarized nucleon

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Deep inelastic scattering (DIS) and Semi-inclusive DIS (SIDIS)

pp collisions

Probes to Study Polarized Proton Structure

+ Kinematics are “simple” (x,Q2)+ Underlying theory is well understood Each angular moment accesses different proton structure.- Indirect access to gluons- Highest scales not accessible with

existing facilities. Collider project (EIC) in design stage.

- Most probes integrate over x and Q2

+/- Theoretical interpretation of results often requires additional effort. Typically, several effects contribute to one measurement.+ Direct access to gluons+ High scales accessible with RHIC

(collider)Figures from DSSV: Prog.Part.Nucl.Phys. 67 (2012) 251-259 

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Current Status of Distribution Functions

MSTW 2008 NLO PDFsEur.Phys.J.C63:189-285,2009

Selected experimental inputs:

F2 from ZeusD0: Phys.Rev.Lett.101:062001,2008E866: Phys. Rev. D 64 (2001) 052002

What do we learn?• Proton momentum:

carried ½ by gluons, ½ by quarks

∫ x q(x) dx• Gluon distribution

continues to rise at low-x.• Sea is not symmetric

between u and d.

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Current status of helicity distributions

All plots from DSSV: PRD 80, 034030 (2009), experimental results from respective collaborations

What do we learn?Decompose proton spin:

zLG +Δ+Δ= ∑2

1

2

1

Quark Spin +

~0.24

Gluon Spin +

so far: small in limited xBj range

Orbital Angular Momentum

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Current Status of Transverse Spin

Right

LeftEarly measurements in transverse spin indicated deeper structure when proton transversely polarized.

RL

RL

N PA

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Early Theory Expectation: Small asymmetries at high energies

(Kane, Pumplin, Repko, PRL 41, 1689–1692 (1978) )

s

mA qN Vanishing asymmetry

h

Z.Phys., C56, 181 (1992) IP Conf. Proc., vol. 915 (2007) PRL 101, 222001 (2008)

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Possible AN Explanations: Transverse Momentum Dep. Distributions

),()( 221

kzHxq)(),( 2,1 zDkxf h

qPTqT

SPkT,p

p

p

SP

p

p

Sq kT,π

Sivers Effect:Introduce transverse momentum of parton relative to proton.

Collins Effect:Introduce transverse momentum of fragmenting hadron relative to parton.

Graphics from L. Nogach (2006 RHIC AGS Users Meeting)

Correlation between Proton spin (Sp) and quark spin (Sq) + spin dep. frag. function

Correlation between Proton spin (Sp) and parton transverse momentum kT,p

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Possible AN Explanations: Higher Twist Correlation Functions

No kT (collinear partons)

Additional interactions between proton and scattering partons

Goes beyond leading twist (two free colliding quarks)

Higher twist interaction contributions expected to drop like 1/pT

PB

PA↑

Graphic from Zhongbo Kang

pT=0 AN=0

What is expected AN dependence on pT?

pT large, AN ~ 1/pT

Low pT (TMD regime)

So far, 1/pT has not been observed in proton-proton collisions

Selected Extractions of Transverse StructureSivers

Collins

Sivers

Transversity

Torino09

Connection to Partons in pp Collisions

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Proton 1 Proton 2

Detectedhadron

DetectedhadronƟ

Mid-rapidity Forward-rapidity

η=− ln ( tan (Ɵ2 ))

Large contribution from gg scatteringSymmetric x1,x2 distribution

Forward rapidity:• Selects large-x1, small x2

• Dominant process: quark-gluon scattering.

Forward Rapidity Measurements

1. What can the large transverse single spin asymmetries tell us about the proton’s structure?

2. What is the gluon spin contribution to the proton?• Low-x behavior is

unconstrained by experiment

3. What is the sea quark polarization?

Experimental Setup

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Relativistic Heavy Ion Collider

2 counter-rotating packets of particles collide at 2 interactions points108 ns between proton packets. Each packet has independent spin orientation (up or down). Important for control of systematic effects in spin measurements.Typical pp collision rates: 2 MHz. DAQ bandwidth: 7 kHz Efficient triggering systems are essential for physics

Relativistic Heavy Ion Collider Performance

• Accelerator performance improves every year of operation.

• 2012 “Breakthrough” year for polarized proton performance.

• ~135pb-1 delivered to experiments

• 2013 PAC priority #1: “Running with polarized proton collisions at 500 GeV to provide an integrated luminosity of 750 pb-1 at an average polarization of 55%”

2012+2013 dataset will provide critical datasets for RHIC Spin

Program

2012 RHIC Running Review

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√s(GeV)

Species Spin Weeks Measurements

200 Proton-Proton

Transverse 4.4 Heavy ion comparison data to existing Gold-Gold dataset.High pT behavior of AN in forward region

510 Proton-Proton

Longitudinal 4.9 Gluon and sea-quark helicity distributions

193 Uranium-Uranium

- 2.9 Explore collision geometry in heavy ion collisions

200 Copper-Gold

- 5.5

PHENIX Experiment

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Muon Arms 1.2 < | η | < 2.4• High momentum muons • J/Psi• Unidentified charged hadrons• Heavy Flavor

Central Arms | η | < 0.35• Identified charged hadrons• Neutral Pions• Direct Photon• J/Psi• Heavy Flavor

MPC 3.1 < | η | < 3.9• Neutral Pion’s• Eta’s

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Design detector PbWO4 crystals

Crystal wrap “party” Detector shells FNAL test beam Drive to BNL Prep for install Install Take data

Forward Calorimetry: Muon Piston Calorimeter

MPC Performance

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Detector performance is excellent and behavior is well understood with both Monte-Carlo and data.

Rare probes can be studied by using high-energy triggering system.

Unpolarized pion cross-section agrees well with world-data.

MPC π0 and η meson Reconstruction

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Most interesting region:High Energy, High pT

Where possible reconstruct meson’s invariant mass:

Otherwise, measure high energy clusters & perform decomposition using Monte-Carlo

Decay photonπ0

Direct photonF

ract

ion

of c

lust

ers

0 AN at High xF, s=62.4 GeV

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p+p0+X at s=62.4 GeV/c2

• xF>0 Non-zero and large asymmetries• Suggests effect originates from valence quark effect• Complementary to BRAHMS data

Isospin Dependence, xF>0, s=62.4 GeV

+ (ud)

- (du)

• Sign of AN seems consistent with sign of tranversity• However, transversity larger for u, but AN is larger for - • Pythia claims that originating quarks for mesons is:

+: ~100%u -: 50/50% d/u 0: 25/75% d/u• u quark dominance in pion production over d’s.

+ (ud)

- (du)

0 (uu+dd)/2

(Preliminary)

s Dependence of 0 AN

• No strong dependence on s from 19.4 to 200 GeV• Varying experimental acceptance most likely causes spread in AN

• Unexpected that AN does not vary over huge range of energy• pQCD does not reproduce low energy unpolarized cross-sections

(Preliminary)

η meson AN Results, s=200 GeV

24Preliminary Conclusion:AN η meson > with π0

Conclusion:AN η meson consistent with π0

Conclusion:AN η meson consistent with π0

arXiv:1206.1928

Suggestive drop in AN at high pT

Statistical significance is not large enough Recently acquired dataset will boost the significance.

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Data between preliminary and published

Cluster AN, s=200 GeV

Cluster AN, s=200 GeV

®Hint that ANgamma is probably small.

Direct photons are not sensitive to Collins effect Suggests dominant mechanism not Sivers

...)(00

fAfAfAclusterA NNNN

STAR data from:Phys. Rev. Lett. 101 (2008) 222001

STAR 2γ methodPHENIX inclusive cluster

Helicity Measurements at RHIC

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Inclusive Jet/hadron production

𝐴𝐿𝐿=𝜎++ ¿−𝜎+−

𝜎++¿+𝜎+−= 1𝑃1𝑃2

𝑁++¿−𝑅𝑁+−  

𝑁++ ¿+𝑅𝑁 +−  ¿¿ ¿

¿

𝑅=𝐿++¿

𝐿+−¿

𝜎 ++¿−𝜎+− ∆ 𝑓 𝑎 (𝑥1 )∆ 𝑓 𝑏 (𝑥2) ∆𝜎 𝑎𝑏𝑐𝑑𝐷𝑐

h ( 𝑧 )¿

Measured

MeasuredSpin sorted relative luminosities

Fragmentation function from parton c to hadron h with momentum fraction z

a

b

d

c

h

Hard scattering cross-section (calculable)

Helicity distributions (to be extracted in global analysis)

Measuring ALL at RHIC

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𝐴𝐿𝐿=1

𝑃1 𝑃2

𝑁 ++¿−𝑅𝑁 +−  

𝑁 ++¿+𝑅𝑁+−  ¿¿

𝑅=𝐿++¿

𝐿+−¿

a

b

d

c

h

Requirements1. Longitudinal beam polarization

(Dedicated effort needed to setup longitudinal beams)

2. Luminosity monitorsNecessary to measure R, relative luminosity. Done using high-rate process and scalers.

3. Detectors for measuring hadrons or jets.

RHIC ALL measurements

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200520062009

PHENIX Mid-Rapidity| η | < 0.35

Hadron ALL precision reaches 10-3 but results are consistent with zero

Currently, measurement is systematics limited!• Dedicated studies performed in 2012

to address this

RHIC ALL Measurements

Preliminary results from the STAR collaboration using Jets at mid-rapidity.

First non-zero ALL

results from RHIC! First look from

DSSV collaboration: =0.13(global analysis needs to be redone)arXiv:1112.0904

Like PHENIX ALL,

measurement is performed at mid-rapidity 30

xT=pT / ( ½ √s )

Expected impact on ΔG

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Region probed by existing mid-rapidity measurements.Reminder:

With existing probes: higher statistics and slightly lower range in x (√s=200 500 GeV)

High-x region: ΔG(x) at high x is an interesting measurement

However, even if gluons are 100% polarized, the number of gluons dries up at high x small possible contribution.

Low-x region: Paucity of data. Large number of gluons make it possible for large spin contributions.

Phys.Rev.D80:034030,2009

)(1

0xGxdG

2-2.5 GeV/c4-5 GeV/c9-12 GeV/c

2-2.5 GeV/c4-5 GeV/c9-12 GeV/c

0 at ||<0.35: xg distribution vs pT bin

s=500 GeV

s=62 GeV

s=200 GeV

Measuring ΔG(x) at low-x

Strategy: Exploit forward kinematics to probe small-x

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Expected asymmetries have been simulated (C. McKinney)

First measurements performed using MPC (S. Wolin)

Simulation Measurement

Increasing precision on forward ALL

Three essential components to forward ALL success:

1. High RHIC polarization and luminosityFrom 2012+2013 we expect a huge dataset.

2. Reduce systematic errors.– Dominant contribution from relative luminosity– Monte-Carlo + special accelerator studies in

2012 were performed with encouraging results. Followup studies planned for 2013.

3. Increase purity of MPC triggering system– Pre-2012: high fake rate from low-energy

neutron backgrounds.– Post-2012: Electronics upgrade

• Fully digital triggering system with “smart” trigger algorithm to reject isolated high energy towers.

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Parton Helicity

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W-production

𝐴𝐿

𝑊 +¿=𝜎+¿− 𝜎−

𝜎+¿+𝜎−=1𝑃1

𝑁+¿−𝑅 𝑁−  

𝑁+¿+𝑅𝑁−

  ¿¿ ¿

¿ ¿

𝑅=𝐿+ ¿

𝐿−¿

Measured

MeasuredSpin sorted relative luminositiesW+

l+

u

d

νe

Similar expression for W-

Presented measurements measure leptons from decay of WKinematic smearing, nonetheless, at forward rapidity

𝐴𝐿

𝜇+¿≈−∆𝑢 (𝑥1 )𝑑 (𝑥2 )−∆ 𝑑 (𝑥1 )𝑢(𝑥2 )

𝑢 (𝑥1 )𝑑 (𝑥2 )+𝑑 (𝑥1)𝑢(𝑥2)¿

𝐴𝐿

𝑊 +¿=−∆𝑢 (𝑥1 )𝑑 (𝑥2 )−∆𝑑 (𝑥1 )𝑢(𝑥2)

𝑢 (𝑥1 )𝑑 (𝑥2 )+𝑑 (𝑥1 )𝑢(𝑥2 )¿

Suppressed at forward rapidity

W-boson AL Benefits:• “Clean” probe• High scale• u and d enter at same level• Simpler fragmentation from

single hadron case

Expected Lepton Asymmetries

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Mid-rapidity via We+/-

Forward-rapidity Wμ+/-

In both cases, experimental signature is high momentum lepton with small event rates

Wμ+/- Challenge

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Design Luminosity√s = 500 GeV σ=60mb L = 1.6 x1032/cm2/s

Total X-sec rate ＝ 9.6 MHz

Default PHENIX Trigger: Rejection=200 ~ 500

DAQ LIMIT=1-2 kHz

Required Rejection10,000

Run11 Muon Trigger Hardware

Muon Tracker

Muon ID

RPC3

Absorber

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Absorber+Removes backgrounds

Muon Tracker+Offline p measurement +Online trigger

Muon ID+Low-p momentum threshold trigger

RPC3+Additional tracking+Timing information

Run12 Muon Trigger Hardware

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Muon Tracker

RPC1 Muon ID

RPC3

Absorber

FVTX

FVTX Upgrade+Adds tracking

RPC1+Adds acceptance +Adds trigger rejection

Additional Absorber+Shields detector from in-time backgrounds

Wμ: Trigger Commissioning

Keep-out region where trigger will take too much DAQ bandwidth

Trig

ger

Rej

ecti

on

Collision Rate [MHz]

Run13 Production Trigger

Run12 Production Trigger

Run11 Production Trigger

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Wμ: Trigger Commissioning

Run12 Muon-like track turn on curve

Yield (Production Trigger) / Yield (Minimum Bias)

40p (GeV/c)

South

North

Trigger maintains high rejection and selects high momentum tracks

Wμ: 2011 Results

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μ+

μ-

• First measurement at forward rapidities.

• Results statistics limited.

• In 2012+2013 dataset will be greatly expanded.

Wμ: 2012+2013 Projected Statistical Errors

Experimental Challenges:

1. Improving the existing S/BG ratio.

2. Finishing all shutdown activities

3. Bringing triggering system online quickly at the start of Run 13.

4. Operating PHENIX experiment efficiently to sample as much of delivered luminosity as possible.

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Summary & Outlook: Transverse

Search for falloff of AN at high pT will extend to higher pT with 2012 dataset

First hints that direct γ AN small Hurts case for Sivers effect

In longer term:– With availability of high luminosity facilities: shift in hadron collisions

towards “cleaner” probes. – Drell-Yan process is currently a hot topic

Expected sign change in Sivers amplitude between DY and SIDIS– Possibilities at RHIC:

• ANDY experiment recently shuttered

• PHENIX: Spin running in near-term will be with longitudinal polarization.

– Possibility at COMPASS:• Effort well underway to perform measurements.

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Summary & Outlook

Gluon Helicity distributions– RHIC effort moving towards low-x ΔG measurements.– Forward region is essential for reaching lowest x

possible. Sea Quark Helicity Distributions

– First AL Wμ results from PHENIX

– Substantial dataset collected in 2012.

Decisive dataset coming in 2013.

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Backup

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