Forward Particle Production and Transverse Single Spin Asymmetries

Forward Particle Production and Transverse Single Spin Asymmetries OUTLINE • Transverse single spin effects in p+p collisions at s=200 Ge. V • Towards understanding forward cross sections • Plans for the future L. C. Bland Brookhaven National Laboratory RBRC Workshop on Parton Orbital Angular Momentum Albuquerque 25 February 2006

Installed and commissioned during run 4 To be commissioned Installed/commissioned in run 5 Developments for runs 2 (1/02), 3 (3/03 5/03) and 4 (4/04 5/04) • Helical dipole snake magnets • CNI polarimeters in RHIC, AGS fast feedback 2/25/2006 • b*=1 m operataion • spin rotators longitudinal polarization • polarized atomic hydrogen jet target L. C. Bland, RBRC Parton OAM 2

RHIC Spin Physics Program • Direct measurement of polarized gluon distribution using multiple probes • Direct measurement of anti-quark polarization using parity violating production of W • Transverse spin: Transversity & transverse spin effects: possible connections to orbital angular momentum? 2/25/2006 L. C. Bland, RBRC Parton OAM 3

STAR detector layout • TPC: -1. 0 < < 1. 0 • FTPC: 2. 8 < < 3. 8 • BBC : 2. 2 < < 5. 0 • EEMC: 1 < < 2 • BEMC: -1 < < 1 • FPD: | | ~ 4. 0 & ~3. 7 STAR characterized by azimuthally complete acceptance over broad range of pseudorapidity. 2/25/2006 L. C. Bland, RBRC Parton OAM 4

Single Spin Asymmetry Definitions Two measurements: • Single arm calorimeter: • Definition: • dσ↑(↓) – differential cross section of when incoming proton has spin up(down) Left π0, x. F<0 π0, x. F>0 p R – relative luminosity (by BBC) Pbeam – beam polarization • Two arms (left-right) calorimeter: p No relative luminosity needed Right 2/25/2006 positive AN: more 0 going left to polarized beam L. C. Bland, RBRC Parton OAM 5

First AN Measurement at STAR prototype FPD results STAR collaboration Phys. Rev. Lett. 92 (2004) 171801 Similar to result from E 704 experiment (√s=20 Ge. V, 0. 5 < p. T < 2. 0 Ge. V/c) Can be described by several models available as predictions: Sivers: spin and k correlation in parton distribution functions (initial state) Collins: spin and k correlation in fragmentation function (final state) Qiu and Sterman (initial state) / Koike (final state): twist-3 p. QCD calculations, multi-parton correlations √s=200 Ge. V, <η> = 3. 8 2/25/2006 L. C. Bland, RBRC Parton OAM 6

Dynamical Origins of Transverse SSA p +p→ +Х • Sivers effect [Phys Rev D 41 (1990) 83; 43 (1991) 261]: Flavor dependent correlation between the proton spin (Sp), momentum (Pp) and transverse momentum (k. T) of the unpolarized partons inside: • Collins effect [Nucl Phys B 396 (1993) 161]: Correlation between the quark spin (sq), momentum (pq) and transverse momentum (k. T) of the pion. The fragmentation function of transversely polarized quark q takes the form: 2/25/2006 L. C. Bland, RBRC Parton OAM 7

Present Status Uses online beam polarization values Run 3 Preliminary Result: -more Forward angles -final FPD Detectors Run 3 Preliminary Backward Angle Data: -No significant asymmetry seen. A. Ogawa, for STAR: [hep-ex/0502040] Run 3 + Run 5 Preliminary < >=3. 7, 4. 0 D. Morozov, for STAR [hep-ex/0512013] 2/25/2006 L. C. Bland, RBRC Parton OAM 8

STAR x. F and p. T range of FPD data 2/25/2006 L. C. Bland, RBRC Parton OAM 9

AN(p. T) from run 3+run 5 at √s=200 Ge. V Uses online beam polarization values • Combined statistics from run 3 and run 5 with x. F>0. 4 • There is evidence that analyzing power at x. F>0. 4 decreases with increasing p. T • To do: systematics study 2/25/2006 L. C. Bland, RBRC Parton OAM 10

Forward production in hadron collider p E d N qq xqp qg E q xgp EN p Au (collinear approx. ) • Large rapidity production ( >4) probes asymmetric partonic collisions • Mostly high-x valence quark + low-x gluon <z> • 0. 3 < xq< 0. 7 <xq> NLO p. QCD • 0. 001< xg < 0. 1 Jaeger, Stratmann, Vogelsang, Kretzer • <z> nearly constant and high 0. 7 ~ 0. 8 <xg> • Large-x quark polarization is known to be large from DIS • Directly couple to gluons probe of low x gluons 2/25/2006 L. C. Bland, RBRC Parton OAM 11

But, do we understand forward 0 production in p + p? At s < 200 Ge. V, not really… √s=23. 3 Ge. V √s=52. 8 Ge. V Ed 3 /dp 3[mb/Ge. V 3] Data-p. QCD difference at p. T=1. 5 Ge. V q 15 o 2 NLO collinear calculations with different scale: q 6 o q 1 o p. T and p. T/2 q 53 o q 22 o x. F Bourrely and Soffer (hep-ph/0311110, Data references therein): NLO p. QCD calculations underpredict the data at s < 200 Ge. V (ISR and fixed target) 2/25/2006 L. C. Bland, RBRC Parton OAM data/ p. QCD appears to be function of q, √s in addition to p. T 12

pp X cross sections at 200 Ge. V The error bars are point-to-point systematic and statistical errors added in quadrature The inclusive differential cross section for 0 production is consistent with NLO p. QCD calculations at 3. 3 < η < 4. 0 The data at low p. T are more consistent with the Kretzer set of fragmentation functions, similar to what was observed by PHENIX for production at midrapidity. D. Morozov (IHEP), XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 2/25/2006 NLO p. QCD calculations by Vogelsang, et al. L. C. Bland, RBRC Parton OAM 13

STAR-FPD Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B 140 (1978) 189. Expect QCD scaling of form: Require s dependence to disentangle p. T and x. T dependence 2/25/2006 L. C. Bland, RBRC Parton OAM 14

PYTHIA: a guide to the physics Forward Inclusive Cross-Section: Subprocesses involved: q+g g+g and q+g+g STAR FPD Soft processes • PYTHIA prediction agrees well with the inclusive 0 cross section at 3 -4 • Dominant sources of large x. F production from: ● q + g (2 2) + X ● q + g + g (2 3) + X 2/25/2006 q g q L. C. Bland, RBRC Parton OAM g g 15

Plans for the Future • STAR Forward Pion Detector upgrade (FPD++) planned as an engineering test of the FMS during RHIC run 6 • STAR Forward Meson Spectrometer (FMS) planned for installation by RHIC run 7 Disentangle the dynamical origins to transverse SSA in p+p collisions via measurements of AN for Ø jet-like events Ø direct photon production 2/25/2006 L. C. Bland, RBRC Parton OAM 16

FPD++ Physics for Run 6 We intend to stage a large version of the FPD to prove our ability to detect jet-like events, direct photons, etc. Run-5 FPD Run-6 FPD++ The center annulus of the run-6 FPD++ is similar to arrays used to measure forward SSA. The FPD++ annulus is surrounded by additional calorimetry to increase the acceptance for jet-like events and direct events. 2/25/2006 L. C. Bland, RBRC Parton OAM 17

STAR Configuration for Run 6 2/25/2006 L. C. Bland, RBRC Parton OAM 18

Basic physics Goals Ideas to be tested using FPD++ in RHIC run 6 • Prototype for FMS (planned completion for RHIC run 7) • Discriminate dynamical origin of the forward AN – Measurement of jetlike events and AN for those • Similar to FPD (left/right symmetric) but with larger active area • Measure shape of forward jet – Measure direct photons cross section, possibly AN, requiring separation of and direct gamma • Continue the study of asymmetry in pp • other 2/25/2006 L. C. Bland, RBRC Parton OAM 19

New FMS Calorimeter Lead Glass From FNAL E 831 Loaded On a Rental Truck for Trip To BNL 804 cells of 5. 8 cm 60 cm Schott F 2 lead glass 2/25/2006 L. C. Bland, RBRC Parton OAM 20

Students prepare cells at test Lab at BNL Individual lead glass detectors are prepared and tested prior to installation in the calorimeter. In total, 13 students have been involved in this work since May, 2005. 2/25/2006 L. C. Bland, RBRC Parton OAM 21

Status report • Calorimeter cells for free thanks to FNAL / U. Col. and Protvino • Cells were refurbished and tested at BNL • South calorimeter in place on new FMS platform, readout electronics in place and tested • In situ cell-by-cell tests followed installation 2/25/2006 L. C. Bland, RBRC Parton OAM 22

Completed FPD++ Provides left/right symmetric calorimeters for detection of jet-like events 2/25/2006 L. C. Bland, RBRC Parton OAM 23

Jet spin asymmetry • Is the single spin asymmetry observed for also present for the jet the comes from? • Answer discriminates between Sivers and Collins contributions • Trigger on energy in small cells, reconstruct and measure the energy in the entire FPD++ • Average over the Collins angle and define a new x. F for the event, then measure analyzing power versus x. F 2/25/2006 Expect that jet-like events are ~15% L. C. Bland, RBRC Parton OAM 24 of events

Planned readout • Trigger on summed energy Etrig is energy sum from only the small cells of one calorimeter • Determine total energy for event Esum is the energy sum from all cells of one calorimeter • Photon and finding will be based on existing FPD software Reconstruct photon multiplicity (N ); , … invariant mass; etc. 2/25/2006 L. C. Bland, RBRC Parton OAM 25

Jet-Like Events L. C. Bland (hep-ex/0602012) • N >3 requirement should allow analysis • (upper left) for each event, examine PYTHIA record for final-state hard scattered partons event selection chooses jet-like events. • (upper right) event-averaged correlation between photon energy and distance in , space from thrust axis events are expected to exhibit similar jet characteristics as found at 0 • (middle) multi-photon final states enable reconstruction of parent parton kinematics via momentum sum of observed photons. • (bottom) projected statistical accuracy for data sample having 5 pb-1 and 50% beam polarization. Azimuthal symmetry of FPD++ around thrust axis, selected by Etrig condition, enables • integration over the Collins angle isolating the Sivers effect, or 2/25/2006 • dependence Partonthe OAM on the Collins. L. C. Bland, angle RBRC isolating Collins/Heppelmann effect 26

How do we detect direct photons? Isolate photons by having sensitivity to partner in decay of known particles: π0 M=0. 135 Ge. V BR=98. 8% K 0 π0π0 0. 497 31% 0. 547 39% π0 0. 782 8. 9% Detailed simulations underway 2/25/2006 L. C. Bland, RBRC Parton OAM 27

Where do decay partners go? di-photon parameters z = |E 1 -E 2|/(E 1+E 2) = opening angle Mm = 0. 135 Ge. V/c 2 ( ) Mm=0. 548 Ge. V/c 2 ( ) • Gain sensitivity to direct photons by ensuring we have high probability to catch decay partners • This means we need dynamic range, because photon energies get low (~0. 25 Ge. V), and sufficient area (typical opening angles are only. RBRC a few. Parton degrees at our ranges). 2/25/2006 L. C. Bland, OAM 28

Sample decays on FPD++ With FPD++ module size and electronic dynamic range, have >95% probability of detecting second photon from decay. 2/25/2006 L. C. Bland, RBRC Parton OAM 29

STAR Forward Meson Spectrometer [hep-ex/0502040] 2/25/2006 L. C. Bland, RBRC Parton OAM 30

Forward Meson Spectrometer for Run 7 • FMS will provide full azimuthal coverage for range 2. 5 4. 0 • broad acceptance in x. F-p. T plane for inclusive , , , K , … production in p+p and d(p)+Au collisions • broad acceptance for and from forward jet pairs to probe low -x gluon density in p+p and d(p)+Au collisions Run-7 FMS as seen from STAR interaction point 2/25/2006 L. C. Bland, RBRC Parton OAM 31

STAR detector layout with FMS TPC: -1. 0 < < 1. 0 FTPC: 2. 8 < < 3. 8 BBC : 2. 2 < < 5. 0 EEMC: 1 < < 2 BEMC: -1 < < 1 FPD: ~ 4. 0 & ~3. 7 FMS: | | 2. 5< < 2/25/2006 L. C. Bland, RBRC Parton OAM With FMS addition, STAR will have nearly contiguous electromagnetic calorimetry for 1 < < 4 32

Three Highlighted Objectives In FMS Proposal (not exclusive) 1. A d(p)+Au +X measurement of the parton model gluon density distributions xg(x) in gold nuclei for 0. 001< x <0. 1 For 0. 01<x<0. 1, this measurement tests the universality of the gluon distribution. 2. Characterization of correlated pion cross sections as a function of Q 2 (p. T 2) to search for the onset of gluon saturation effects associated with macroscopic gluon fields. (again d-Au) 3. Measurements with transversely polarized protons that are expected to resolve the origin of the large transverse spin asymmetries in reactions forward production. (polarized pp) 2/25/2006 L. C. Bland, RBRC Parton OAM 33

Frankfurt, Guzey and Strikman, J. Phys. G 27 (2001) R 23 [hep-ph/0010248]. • constrain x value of gluon probed by high-x quark by detection of second hadron serving as jet surrogate. • span broad pseudorapidity range (-1< <+4) for second hadron span broad range of xgluon • provide sensitivity to higher p. T forward reduce 2 3 (inelastic) parton process contributions thereby reducing uncorrelated background in D correlation. 2/25/2006 L. C. Bland, RBRC Parton OAM 34 Pythia Simulation

Timeline for the Baseline RHIC Spin Program Ongoing progress on developing luminosity and polarization Research Plan for Spin Physics at RHIC (2/05) Program divides into 2 phases: s=200 Ge. V with present detectors for gluon polarization ( g) at higher x & transverse asymmetries; s=500 Ge. V with detector upgrades for g at lower x & W production 2/25/2006 L. C. Bland, RBRC Parton OAM 35

Summary / Outlook • Large transverse single spin asymmetries are observed for large rapidity production for polarized p+p collisions at s = 200 Ge. V Ø AN grows with increasing x. F for x. F>0. 35 Ø AN is zero for negative x. F • Large rapidity cross sections for p+p collisions at s = 200 Ge. V is in agreement with NLO p. QCD, unlike at lower s. Particle correlations are consistent with expectations of LO p. QCD (+ parton showers). • Plan partial mapping of AN in x. F p. T plane for and measurement of AN for jet-like events in RHIC run-6 • Propose increase in forward calorimetry in STAR to probe low-x gluon densities and further studies of transverse SSA (complete upgrade by 11/06). 2/25/2006 L. C. Bland, RBRC Parton OAM 36

Backups 2/25/2006 L. C. Bland, RBRC Parton OAM 37

Time/luminosity dependent PMT Gain Matching Di-photon gain Massshift Reconstruction and calibration corrections Pb-glass reconstruction (no SMD) FTPC-FPD matching 0 reconstruction • Clustering and moment analysis Cluster categorization Photon conversion in beam pipe efficiency Luminosity • +Fitting shape p vs p. PMT with parametrized (+ X) shower (+ ) e+ e gain 2 photon • Number of photons found >= 2 cluster example MC & Data comparison Beam pipe Mass resolution ~ 20 Me. V • Fiducial volume > 1/2 cell width from edge D We understand ~2% level • Energy sharing z E 1 Egain 2 / (E 1+E 2) < 0. 7 • Absolute gain determined from peak Gain stability (before Limit with <0. 5 cutiscorrection) Efficiencies almost position forzggeach tower Try both purely • Energy dependent gain correction geometrically determined Energy Run/luminosity dependent gain correction from • reconstruction D D of MC(PYTHIA+GEANT) • Checking 1 g Cluster with MC (PYTHIA+GEANT) Gain stability (after correction) FPD position known Geometrical limit D relative to STAR 2 g Cluster High tower sorted mass distributions 2/25/2006 L. C. Bland, RBRC Parton OAM 2 nd moment of cluster (long axis) 38

Why Consider Forward Physics at a Collider? Deep inelastic scattering Kinematics Hard scattering hadroproduction Can Bjorken x values be selected in hard scattering? Assume: 1. Initial partons are collinear 2. Partonic interaction is elastic p. T, 1 p. T, 2 Studying pseudorapidity, =-ln(tanq/2), dependence of particle production probes parton distributions at different Bjorken x values and involves different admixtures of gg, qg and qq’ subprocesses. 2/25/2006 L. C. Bland, RBRC Parton OAM 39

Simple Kinematic Limits Mid-rapidity particle detection: NLO p. QCD (Vogelsang) 1. 0 1 0 and < 2> 0 p+p +X, s = 200 Ge. V, =0 xq xg x. T = 2 p. T / s fraction 0. 8 Large-rapidity particle detection: 1>> 2 qq 0. 6 qg 0. 4 0. 2 gg 0. 0 0 xq x. T e 1 x. F (Feynman x), and 10 20 30 p. T, (Ge. V/c) xg x. F e ( 1+ 2) Large rapidity particle production and correlations involving large rapidity particle probes low-x parton distributions using valence quarks 2/25/2006 L. C. Bland, RBRC Parton OAM 40

Constraining the x-values probed in hadronic scattering Guzey, Strikman, and Vogelsang, Phys. Lett. B 603, 173 (2004). Log 10(x. Gluon) For 2 2 processes TPC FPD Log 10(x. Gluon) Collinear partons: + + h 1 + h 2 ● x = p / s (e + e ) T h 1 h 2 ● x = p / s (e + e ) T CONCLUSION: Measure two particles in the final state to constrain the x-values probed 2/25/2006 FTPC Barrel EMC FPD Gluon • FPD: | | 4. 0 • TPC and Barrel EMC: | | < 1. 0 • Endcap EMC: 1. 0 < < 2. 0 • FTPC: 2. 8 < < 3. 8 L. C. Bland, RBRC Parton OAM 41

How can one infer the dynamics of particle production? Particle production and correlations near 0 in p+p collisions at s = 200 Ge. V Inclusive 0 cross section Two particle correlations (h ) STAR, Phys. Rev. Lett. 90 (2003), nucl-ex/0210033 At √s = 200 Ge. V and mid-rapidity, both NLO p. QCD and PYTHIA explains p+p data well, down to p. T~1 Ge. V/c, consistent with partonic origin Phys. Rev. Lett. 91, 241803 (2003) hep-ex/0304038 2/25/2006 Do they work forward rapidity? L. C. Bland, RBRC Parton OAM 42

Back-to-back Azimuthal Correlations with large Top View Fit LCP normalized distributions and with Gaussian+constant Trigger by ] forward • E > 25 Ge. V • 4 ] Midrapidity h tracks in TPC • -0. 75 < < +0. 75 Leading Charged Particle(LCP) Coicidence Probability [1/radian] Beam View LCP • p. T > 0. 5 Ge. V/c S = Probability of “correlated” event under Gaussian B = Probability of “un-correlated” event under constant s = Width of Gaussian 2/25/2006 L. C. Bland, RBRC Parton OAM 43

STAR PYTHIA (with detector effects) predicts • “S” grows with <x. F> and <p. T, > 25<E <35 Ge. V relimin P STAR ary • “ s” decrease with <x. F> and <p. T, > PYTHIA prediction agrees with p+p data 45<E <55 Ge. V y iminar Prel STAR Larger intrinsic k. T required to fit data Statistical errors only 2/25/2006 L. C. Bland, RBRC Parton OAM 44

New Physics at high gluon density 1. Shadowing. Gluons hiding behind other gluons. Modification of g(x) in nuclei. Modified distributions needed by codes that hope to calculate energy density after heavy ion collision. 2. Saturation Physics. New phenomena associated with large gluon density. • Coherent gluon contributions. • Macroscopic gluon fields. • Higher twist effects. • “Color Glass Condensate” Figure 3 Diagram showing the boundary between possible “phase” regions in the t=ln(1/x) vs ln Q 2 plane. , review for Quark Gluon Plasma 3, Edmond Iancu and Raju Venugopalan R. C. Hwa and X. -N. Wang (eds. ), World Scientific, 2003 [hep-ph/0303204]. 2/25/2006 L. C. Bland, RBRC Parton OAM 45

Dependence of Rd. Au y=0 As y grows G. Rakness (Penn State/BNL), XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005 Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , 094013 (2003) See also J. Jalilian-Marian, Nucl. Phys. A 739, 319 (2004) • From isospin considerations, p + p h is expected to be suppressed relative to d + nucleon h at large [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)] • Observe significant rapidity dependence similar to expectations from a “toy model” of Rp. A within the Color Glass Condensate framework. 2/25/2006 L. C. Bland, RBRC Parton OAM 46

Towards establishing consistency between FPD ( )/BRAHMS(h ) Extrapolate x. F dependence at p. T=2. 5 Ge. V/c to compare with BRAHMS h data. Issues to consider: • < > of BRAHMS data for 2. 3<p. T<2. 9 Ge. V/c bin. From Fig. 1 of PRL 94 (2005) 032301 take < >=3. 07 <x. F>=0. 27 • /h ratio? Results appear consistent but have insufficient accuracy to establish p+p / isospin effects 2/25/2006 L. C. Bland, RBRC Parton OAM 47

Systematics Measurements utilizing independent calorimeters consistent within uncertainties S y s t e m a t i c s: Normalization uncertainty = 16%: posit ion uncert aint y (dominant ) Energy dependent uncertainty = 13% - 27%: energy calibration to 1% (dominant) background/bin migration correction kinematical constraints 2/25/2006 L. C. Bland, RBRC Parton OAM 48

FPD Detector and º reconstruction • robust di-photon reconstructions with FPD in d+Au collisions on deuteron beam side. • average number of photons reconstructed increases by 0. 5 compared to p+p data. 2/25/2006 L. C. Bland, RBRC Parton OAM 49

d+Au + +X, pseudorapidity correlations with forward HIJIING 1. 381 Simulations • increased p. T forward over run-3 results is expected to reduce the background in D correlation • detection of in interval -1< <+1 correlated with forward (3< <4) is expected to probe 0. 01<xgluon<0. 1 provides a universality test of nuclear gluon distribution determined from DIS • detection of in interval 1< <4 correlated with forward (3< <4) is expected to probe 0. 001<xgluon<0. 01 smallest x range until e. RHIC • at d+Au interaction rates achieved at the end of run -3 (Rint~30 k. Hz), expect 9, 700 200 (5, 600 140) coincident events that probe 0. 001<xgluon<0. 01 for “no shadowing” (“shadowing”) scenarios. 2/25/2006 L. C. Bland, RBRC Parton OAM 50

STAR Forward Calorimetry Recent History and Plans • Prototype FPD proposal Dec 2000 – – • Approved March 2001 Run 2 polarized proton data (published 2004 spin asymmetry and cross section) FPD proposal June 2002 – – • Review July 2002 Run 3 data pp d. Au (Preliminary An Results) FMS Proposal: Complete Forward EM Coverage (hep-ex/0502040). 2/25/2006 L. C. Bland, RBRC Parton OAM 51

Disentangling Dynamics of Single Spin Asymmetries Spin-dependent particle correlations Collins/Hepplemann mechanism requires transversity and spindependent fragmentation Sivers mechanism asymmetry is present forward jet or Large acceptance of FMS will enable disentangling dynamics of spin asymmetries 2/25/2006 L. C. Bland, RBRC Parton OAM 52