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1
Forward Physics with Polarized proton-proton Collisions
at the experiment.
John KosterRIKEN2012/07/25
2
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
3
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
4
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
5
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.
6
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
1
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)
8
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.
25
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
27
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
29
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
31
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
32
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
34
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
35
Mid-rapidity via We+/-
Forward-rapidity Wμ+/-
In both cases, experimental signature is high momentum lepton with small event rates
Wμ+/- Challenge
36
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
37
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
38
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
39
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
41
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.
42
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.
43
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.
44
Backup
45