Future Perspectives on Transverse Single Spin Asymmetries at RHIC L.C. Bland Brookhaven National Laboratory INT Workshop on 3D parton structure of the

Future Perspectives on Transverse Single Spin Asymmetries at RHIC

L.C. Bland

Brookhaven National Laboratory

INT Workshop on 3D parton structure of the nucleon

Seattle, September 2009

Disclaimer: the realities and perspectives to be presented are my own, although may be shared by others.

Forewarning: my perspective is that spin is best probed in a polarized p+p collider in the forward direction. (Pause before starting so you can decide…)

3D parton structure, INT 2

Conclusions and Summaryfrom Overview of Transverse Single Spin Asymmetry Measurements at RHIC

• Transverse spin asymmetries are present at RHIC energies

• Transverse spin asymmetries are present at large

• Particle production cross sections and correlations are consistent with pQCD expectations at large where transverse spin effects are observed

• Essential to go beyond inclusive (meson) production to disentangle dynamical origins


Going Beyond Inclusive Meson ProductionFuture Transverse Single Spin Asymmetry Measurements at RHIC

(pending additional forward instrumentation+run time)

Drell-Yan/virtual photon production at large y

Direct photon production at large (+ away-side jet)

Large jet production (+ correlation in forward jet)

Incr

easi

ng im

pact

Increasing experimental accessibility

polarization observables at large x


Comments About Present Realities…


RHIC is the First (Only) Polarized Proton Collider

STAR

PHENIX

AGS

LINACBOOSTER

Pol. H- Source

Spin Rotators(longitudinal polarization)

Siberian Snakes

200 MeV Polarimeter

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC PolarimeterStrong AGS Snake

Helical Partial Siberian Snake

Spin Rotators(longitudinal polarization)

Siberian Snakes

GOALS reference: RHIC Spin Plan (2008) http://spin.riken.bnl.gov/rsc/report/spinplan_2008/spinplan08.pdf

• Determination of polarized gluon distribution (G) using multiple probes

• Determination of flavor identified anti-quark polarization using parity violating production of W

• Transverse spin: connections to partonic orbital angular momentum (Ly) and transversity ()


RHIC is a Unique Collider…

Source: http://www.agsrhichome.bnl.gov/RHIC/Runs/

• …capable of colliding essentially all positive ions over a broad range of s

• …with a broad and diverse physics program aimed at important questions

o What is quark-gluon plasma? heavy-ion collisions

o How does the proton get its spin? polarized proton collisions

o Does the gluon density saturate in a heavy nucleus? d+Au/p+Au collisions


Plans for future runs at RHIC have been written…

Year Likely Beam Species Science Goal

FY10 Au+Au at 200, 62.4 GeV,

assorted lower energy

low-mass dilepton spectrum; early collision temp.; improved jet quenching studies; begin energy scan for critical point

FY11 Subinjection Au+Au;

500 GeV p+p;

short 200 GeV U+U

continue critical pt. search; gluon pol’n at low x + antiquark pol’n from W production; 1st characterization of deformation effects in U+U centrality distributions

FY12 Au+Au at 200 GeV;

500 GeV p+p

RHIC-II heavy ion goals: heavy flavor, jet, quarkonium, multi-particle correlations; antiquark polarization in proton

FY13 200 GeV p+p; further heavy ion running to complement earlier runs

continue RHIC-II heavy ion goals; transverse spin asymmetries for +jet; pp reference data for new subsystems

FY14 200 GeV Au+Au; low-E Au+Au dictated by Run10+11 results

continuepursuit of+jet; energy scale and identified heavy flavor

Reference: http://www.bnl.gov/npp/docs/RHIC%20Run%2010%20Plan_r1a.pdf

This is the only transverse spin science goal written in the plan for the next 5 years


LuminosityRun-9 performance

Source: RHIC Collider Projections, W. Fischer et al. (2009)

• Challenges remain to be overcome to realize the best-case scenarios

• Luminosity increases at s=500 GeV relative to s=200 GeV were realized

• Depolarizing resonances in RHIC will require new tunes to reduce their impact


LuminosityFuture Projections

Source: RHIC Collider Projections, W. Fischer et al. (2009)

• Luminosity projections for s = 500 GeV are sufficient for transverse-spin DY

• Improved polarization is important to achieve sufficient accuracy


STAR Detector

Forward Meson Spectrometer commissioned/operated in RHIC run 8.

Cluster-pair triggered readout of Forward Time Projection Chamber in RHIC run 9. (Spatial resolution and pileup suppression adequate?) FTPC will be removed before RHIC run 11.

• STAR and PHENIX are primarily instrumented near mid-rapidity

• Forward direction can be viewed at STAR, but present instrumentation is limited and not completely compatible with high luminosity polarized p+p collisions

• All experiments at RHIC are challenging, even with existing apparatus


Influence of STAR SolenoidImpact on charged particles produced in the forward direction

)BPBP( α P zrrz • Radial and Azimuthal fields impart impulses in the Φ direction

• These impulses are small and in opposite directions (they partially cancel each other)

Field effects on forward charged particles are small Determining charge sign will require additional instrumentation

Charges see increasing radial fringe field as pT increases

Interaction point

FMS poletip


Fit with Gaussian + Offset

Gaussian Fit Parameters:– μ = 3.080 ± 0.020 GeV/c2

– σ = 0.082 ± 0.026 GeV/c2

– χ2/d.o.f. = 20.83/26– Significance from fit

• 4.5 σ

Forward p+p J/ψ – 2-Cluster AnalysisRHIC Run-8 Result

Cuts Applied:– E_pair > 60.0 GeV– Zγγ < 0.7– Isolation Radius:

Reconstructed 2-cluster invariant mass / (~ 6 pb-1 Sampled Luminosity)

– 0.4

– pT_cluster > 1.0 GeV/c

C.Perkins, QM09 arXiv:0907.4396

• high-xF J/ may have implications for intrinsic charm at large Bjorken-x in proton

• use to benchmark simulations for future transverse-spin Drell-Yan experiment


Forward p+p J/ψ – 3-Cluster AnalysisRHIC Run 8 Result

• Reconstructed invariant mass of candidate χC → J/ψ + γ events

• Significance depends on background model

• 2.9 σ significance with currently estimated background.

• Peak Counts = 8.40 ± 2.88• 2.9 σ Significance

• μ = 2.97 ± 0.025 GeV• σ = 0.070 ± 0.025 GeV • χ2/d.o.f. = 0.7 with 14 points fit.

C.Perkins, QM09 arXiv:0907.4396


Attempts at realizing future transverse single-spin asymmetry

measurementsA bottoms-up approach


Future Physics Goal (I) p+p+X, s = 500 GeV

Motivations for measurement:

• Strong evidence that large-xF AN persists over a broad range of √s exploit existing capabilities to establish if this continues to √s = 500 GeV

• There are prospects for a transverse spin DY measurement at RHIC. Likely best done at √s = 500 GeV. Persistence of pion AN to √s = 500 GeV is one physics requirement for transverse-spin DY

Requirements:

• Capabilities to robustly identify production to >100 GeV. Existing shower maximum detector in east FPD enables this identification (see backup)

• Best estimates based on xF,pT scaling of , and limits on xT scaling, suggest precision comparable to largest pT measurements at √s=200 GeV can be achieved with Lint=7 pb-1 with Pbeam=55%

http://drupal.star.bnl.gov/STAR/system/files/20090203.3.pdf


Future Physics Goal (II) p+p jet + X, s = 500 and 200 GeV


• Expectation that jets, with their fragments, will enable separation of contributions from Collins+Sivers(+other?), by analogy to semi-inclusive DIS

• Published calculations suggest strong interest as a test of present understanding

Requirements:

• Measurement of jet energy (see below) addition of Forward Hadron Calorimeter behind existing FMS at STAR

• Addition of hadronic+electromagnetic energy at trigger level to eliminate bias

• Anticipate need for modest Lint, Pbeam concurrently achieved with other goals


Forward Upgrade (I)Proposed Forward Hadron Calorimeter


Forward Jets with FMS + FHCImportance of hadronic and EM jet fragments

• Detectable hadrons and photons within acceptance of FMS+FHC are used for summed-energy trigger and for cone-based jet reconstruction

• Fraction of energy of reconstructed jet is a nearly uniform distribution


Forward Jets with FMS + FHCMeasuring the Jet Energy

• Detectable hadrons and photons within acceptance of FMS+FHC are used for summed-energy trigger and for cone-based jet reconstruction. Results also checked via “trigger on scattered parton into finite solid angle”

• Photon-only jets do not measure the scattered parton energy.

• Combining hadronic + EM energy does measure the scattered parton energy, limited mostly by fragmentation effects.

• Many jets are not particularly “jetty”, meaning only few hadrons are within the acceptance. Jets with few hadrons do not give a good measure of the scattered parton energy. Invariant mass from the FMS+FHC can discriminate “jetty” versus “non-jetty” fragmentations.


Future Physics Goal (III) p+p (+ jet) + X, s = 200 GeV


• Test predictions that AN for forward photon production will be negative

• DOE milestone as an experimental test of theoretical understanding

Requirements:

• Without a SMD, must be done at √s = 200 GeV to ensure single /diphoton separation at large xF.

• Requires robust performance from FMS, to ensure sufficient acceptance to suppress backgrounds from , … decays

• Correlated +jet will require development of trigger, to handle the rates

• Lint=30 pb-1 with Pbeam=65% at √s = 200 GeV


Forward Direct Photons

• Suppress contributions from decays by requiring candidate direct photon in yellow-shaded annulus, and by requiring effective isolation of the candidate using the remainder of the FMS as an effective veto. This could be implemented at the trigger level via masks.

• Primary background remains fragmentation photons.

• For inclusive direct photons, it is expected that isolation can be improved with FHC behind the FMS. The need to separate the FMS complicates spin-dependent correlation measurements.

References: RHIC spin plan / STAR run-10 beam-use request


Future Physics Goal (IV) p+p + X, s = 500


• Lambda reveals its polarization through the weak interaction

• Induced polarization measurement would be ~10x higher in s than from ISR

• DNN is sensitive to transversity without transverse-momentum dependent fragmentation

Requirements:

• Addition of FHC behind FMS

• Trigger on hadronic cluster, tagged as neutral by BBC match

• Ability to detect soft photons from nn


Reconstructed versus simulated decay vertex for events with n

Can be reconstructed via n?

Reconstructed versus simulated vertex for triggered events

With the vertex, Mn can be reconstructed. Backgrounds mostly from final states.

• Forward n reconstruction appears feasible with FHC + FMS

• Yields are model dependent, and may require elimination of hadronic showering in FMS


Future Physics Goal (V) p+p e+e- + X, s = 500


• The world is waiting to see if there is a sign change relative to SIDIS…

• Most robust test of present understanding

Requirements:

• Charge-sign determination for DY daughters restoration of tracking in interval spanned by FTPC + FMS shower-maximum detector

• Robust understanding of forward dilepton spectrum at √s=500 GeV

• Establish that transverse spin effects persist to √s=500 GeV

• Lint ~ 250 pb-1 with Pbeam > 50% at √s=500 GeV


Rapidity and Collision Energy Transverse Spin Asymmetries for the DY Process

http://spin.riken.bnl.gov/rsc/write-up/dy-final.pdf

Large rapidity acceptance required to probe valence quark Sivers function, also where p+p+X transverse spin

asymmetries are found to be large at RHIC.

Light mass DY, M> 4 GeV/c2

Rapidity distributions for different s


Forward Upgrade (II)Shower Maximum Detector (SMD) for FMS

• FMS-SMD is required for direct photon physics at large xF for s=500 GeV p+p collisions. Scope can be limited to annular acceptance.

• DY requires good space point at FMS and track near vertex to get charge sign. Feasibility of DY needs to be established, and run-9 multi-cluster triggered slow events can help. If feasible, restoration of tracking coverage of FTPC is required. Larger area coverage of FMS would then be required by FMS-SMD.

• Fiber/scintillator-strip factories are mostly gone, and would need to be restored to build FMS-SMD.

• Scope of FMS-SMD must be established before proceeding.


, Jet, photon, DY Lint requirements

Probe s Lint (pb-1) Pbeam Physics

p+p+X 500 7 55 s dependence

p+pjet+X 200 ~5 55 Sivers/Collins

p+p(+jet)+X 200 30 65 AN sign change

p+pDY+X 500 250 55 AN sign change

• RHIC spin plan involved mix of longitudinal/transverse polarization

• FHC addition enables jet measurements, and could be done at s=500 GeV in run 11 during time to measure p+p+X with east FPD.

• Feasibility tests of p+p+X needed to establish Lint, Pbeam requirements


Summary

• Measurements of transverse single spin asymmetries beyond inclusive meson production in the forward direction will require additional instrumentation + run time

• The experiment with the greatest impact is transverse spin DY. Realizing such an experiment will require demonstrated accelerator performance, additional instrumentation and run time.


Backup


• Can intrinsic heavy flavor expectations be tested experimentally?

• Diffractive Higgs production at the LHC via QQ in proton– May provide a clear signal for Higgs production due to

small background

• How can high-xF intrinsic heavy flavor happen?

– Not from Gluon Splitting (extrinsic heavy flavor)– Heavy quarks are expected to be multi-connected to the

valence quarks within a proton and appear at large x via…

Why does high-xF intrinsic heavy flavor matter?

4

1αM 2

1αMgggg

Phys.Rev. D73 (2006) 113005

QED QCD


East FPD Events from run 9

Example of event identified as a diphoton by the matrix and only a single photon by the SMD

7x7 matrix of lead glass cells

Shower Maximum Detector

preshower(7 Pb-glass cells)

lead converter

event requirements

• >1 cluster

• E > 50 GeV

Existing east FPD layout…

• single /diphoton separation for matrix shown by GSTAR analysis to be robust to E~55 GeV

• SMD response enables single /diphoton separation to E>100 GeV

• Plan is to add this performance to FMS in the future for √s=500 GeV operation


Status/Plan of Large-xF DY• Large-xF J/ production has been observed from bare large-y calorimeter response

in RHIC run 8.

• Cluster-pair trigger is operational for acquiring large-y tracking data in RHIC run-9. Pending analysis, requirements for future DY can be established (e.g., fast-tracking inside solenoid, space points in front of FMS).

• Sufficient luminosity for p+p s=500 GeV collisions has been established; further development of polarization is required, as is measurement of AN(xF) for p+p+X at s=500 GeV and measurement of large-xF J/ and production at s=500 GeV, to bracket light-mass DY region.

• Technical solutions exist for fast tracking inside solenoid (GEM trackers) and space points in front of FMS (forward meson preshower). Construction to span 2.5<<4 region is required, and could be completed in ~2 years, pending approval.

• RHIC schedule is oversubscribed DY would be after RHIC run 11 (>2011).

• Run-10 will be Au+Au energy scan for deconfinement critical point search, and Au+Au at sNN=200 GeV.

• Run-11 is expected to be polarized p+p, with unknown mix of s=200,500 GeV and longitudinal/transverse polarization.

Documents

Future Perspectives on Transverse Single Spin Asymmetries at RHIC L.C. Bland Brookhaven National Laboratory INT Workshop on 3D parton structure of the