1 Forward Di-Hadron Correlations in d+Au Collision at PHENIX UIUC IhnJea Choi (UIUC) 4/18/12

2 Outline The Relativistic Heavy Ion Collider PHENIX Experiment Motivation for Measurements Heavy-Ion Physics Nucleon Structure in Nuclei Gluon Saturation The New Forward Rapidity Muon Piston Calorimeters Selected Experimental Steps d+Au vs p+p Measurements of Azimuthal Angle Di-Hadron Measurements Mid (rapidity)-Forward (rapidity) Correlations Forward-Forward Correlations Conclusions/Discussion 4/18/12

Relativistic Heavy Ion(hadron) Collider Turned on in 2000 ~ 3.8 km storage ring for heavy ions and polarized protons Au+Au Collisions up to 200 GeV per nucleon d+Au Collisions p+p Collisions up to 500 GeV 34/18/12

STAR Four Major Experiments Completed as of /18/12

5 PHENIX Detector at RHIC Muon Arms 1.2 < | η | < 2.4 Heavy Flavor muons J/Psi Charged hadrons Central Arms | η | < 0.35 Charged hadrons Neutral pions / η-mesons Heavy Flavor electrons Direct Photon J/Psi Muon Piston Calorimeter (MPC) 3.1 < | η | < 3.8 Neutral pions / η-mesons 4/18/12

6 Outline The Relativistic Heavy Ion Collider PHENIX Experiment Motivation for Measurements Heavy-Ion Physics Nucleon Structure in Nuclei Gluon Saturation The New Forward Rapidity Muon Piston Calorimeters Selected Experimental Steps d+Au vs p+p Measurements of Single Hadron Yields Azimuthal Angle Di-Hadron Measurements Mid (rapidity)-Forward (rapidity) Correlations Forward-Forward Correlations Conclusions/Discussion 4/18/12

Hadronic Collisions Basics 7 Z, longitudinal Y X X-Y plane, Transversal 4/18/12

Fragmentation : Medium-Induced Energy Loss Final-State Interaction Heavy-Ion (HI) Motivation 8 Why heavy ions at RHIC? High energy-density collisions New, Plasma-like phase of deconfined quarks and gluons? Quark Gluon Plasma (QGP) Collide heavy nuclei at relativistic speeds (e.g c at  s NN = 200 GeV) 4/18/12 Au

Nuclear Modification Factor R AA 9 “Spectators” “Participants” N part, N coll are b-dependent N coll = number of nucleon-nucleon collisions N part = number of participants Au + Au p+p p T (GeV/c ) PHENIX Results 4/18/12

R AA (p T ) Peripheral Central Mid-Central Strong suppression of high p T yields in Au+Au Central Collisions Phenix: Phys.Rev. C69 (2004) (h + +h - )/2 00 Binary collision expectation Centrality % QGP Signature: High-p T Hadron Suppression 104/18/12

Cold Nuclear Matter Effect (CNM) 11 Strong suppression R AA observed in Au+Au collision Medium(Hot & Dense) Effect = Final state effect Does this Medium effect explain all suppression ? R AA Suppression <- Final state effect + Initial state effect Initial state effect (Cold Nuclear Matter effect) 4/18/12

Inside the Proton Fire electrons at protons. If proton “charge cloud”: If proton contains point charges, some of time see: e-e- u p d u e-e- e-e- e-e- p 4/18/1212

The Proton: A complex system of quarks, anti- quarks and gluons! 1 Fm ~ m valence quarks: 2 up-, 1 down-quark gluons, the force carriers of the strong nuclear force. “sea-quarks” : quark-anti-quark pairs that can be formed from a gluon for a short time and annihilate again. Partons : Quarks, Gluons 4/18/1213

DIS and Nucleon Structure Deep Inelastic Scattering ep collision Q 2 =-q 2 = 4-momentum transfer squared (or virtuality of the “photon”) x = fractional longitudinal momentum carried by the struck parton x =Q 2 /2Pq d 2 σ 2πα 2 dxdQ 2 xQ 4 [Y + F 2 (x,Q 2 )-y 2 F L (x,Q 2 )] = Gluons Mainly for quarks 144/18/12 Scattered electron angle and produced hadrons

The number density f(x) for the parton f to carry the fraction x of the proton momentum (as it turns out, it is also Q 2 -dependent) f(x) can be any parton flavor, e.g. Quark-Parton Model 1/3 Parton Distribution Functions(PDF) and F 2 u(x) : up quark distribution u(x) : up anti (or sea)-quark distribution 1/3 F 2 (x) x Three quarks with 1/3 of total proton momentum each. Three quarks with some momentum smearing. F 2 (x) x The three quarks radiate partons at low x, i.e. with anti-quarks and gluons Quark sea 154/18/12

Parton Distribution Functions (contd.) 16 F 2 (x,Q 2 ) DGLAP evolution used to extract PDFs Gluons Sea Quarks Valence quarks 4/18/12 Inside Proton Low-X

Ratio of Nuclear PDFs to Proton PDFs using DGLAP evolution Q 2 =100 GeV 2 Q 2 =1.69 GeV 2 R V Pb R S Pb R G Pb x x x Structure of Nucleons In Nuclei Non-unity behavior Modification of nucleon structure Competing phenomenological models 17 Nucleon PDFs in nuclei (nPDF) differ from those in free nucleons (i.e. neutron, proton) Perform eA  eX scattering Ratio of F 2 per nucleon in eA and ep Large uncertainty in low-x gluon distribution 4/18/12

Alternative Explanation for R AA Suppression 18 Au Beam Direction Rest Frame Boosted Frame What if the suppression originates in the initial state? Typical parton momentum fraction x of partons in p+p, Au+Au collisions is x ~ Low parton momentum fraction x  Large parton density Enhanced by Lorentz contraction in Au-nucleus Low-x partons overlap longitudinally (i.e. ~ 1/x) Transverse Overlap  Gluon Fusion With enough fusion, the gluon density saturates, as described by the Color Glass Condensate (CGC) model Reduces Au+Au cross section 4/18/12

19 Color Glass Condensate(CGC) lower x Proton Rest Frame Valence partons Static source for transient gluons Boost proton to higher energy Time dilation lengthens gluon time-scale Gluons become static source for other lower-x gluons Boost proton to higher energy More gluons produced Boost proton to higher energy Gluon overlap in transverse direction + Longitudinal Overlap  gluon fusion Parton 1 Parton 2 BFKL Evolution: Linear evolution, i.e. N G step ~ N G at previous step Gluon Saturation: Nonlinear evolution, i.e. Probability ~ N G 2 at previous step 4/18/12

d+A Collision : Why it needed? Provide baseline measurements for understanding Heavy Ion collision - Disentangle initial-state effects from final-state effects (Cold nuclear matter effects in nucleon structure ) - Study Low-x physics Deutron Parton x 1 Au Parton x 2 Deutron Parton x 1 Au Parton x 2 Forward rapidity (Deutron going direction) x 1 > x 2 (Low x) Backward rapidity (Au going direction) x 2 > x 1 Forward production : Access low-x in Au 4/18/12 Mid rapidity 20

Measure R dA in Deuteron-Gold Collisions (d+Au) Small energy density  QGP predicts no suppression High parton densities in Au  CGC expects suppression A Control Experiment: d+Au Collisions 21 What about forward angles, or rapidities? Central/Peripheral Mid-Central/Peripheral R cp BRAHMS Collaboration, Rapidity Dependence of R cp R dA No suppression observed at midrapidity!  QGP  0 meson Consistent with CGC Predictions!! Consistent with CGC Predictions!! Forward Scattering probes smaller x in the Au wavefunction Forward Rapidity R dA /R cp are Suppressed 4/18/12

22 CGC Effects at RHIC Existing Measurements: Brahms, STAR, PHENIX forward R dA, R cp suppression Explanations:  CGC  Shadowing (not large enough suppression)  E-Loss, Modification of Fragmentation 4/18/12

23 Outline The Relativistic Heavy Ion Collider PHENIX Experiment Motivation for Measurements Heavy-Ion Physics Nucleon Structure in Nuclei Gluon Saturation The New Forward Rapidity Muon Piston Calorimeters Selected Experimental Steps d+Au vs p+p Measurements of Single Hadron Yields Azimuthal Angle Di-Hadron Measurements Mid (rapidity)-Forward (rapidity) Correlations Forward-Forward Correlations Conclusions/Discussion 4/18/12

24 PHENIX Muon Piston Calorimeter Technology  ALICE(PHOS) PbWO 4 avalanche photo diode readout 2.20 x 2.2 x 18 cm 3 crystals Acceptance: 3.1 < η < 3.9, 0 < φ < 2π -3.7 < η < -3.1, 0 < φ < 2π Both detectors built, installed Usable for 2008 d+Au run. PbWO4 + APD + Preamp Assembly at UIUC MPC integrated in the piston of the muon spectrometer magnet. 4/18/12

25 North MPC MPC   Identification Two photon-candidate PID for  0’ s up to E ~ 25 GeV in MPC Limitations: tower separation and merging effects  p T max ~ 2 GeV/c Single Clusters at high energies Dominated by  0 (~ 80%) Access higher p T  0   Merged  0   4/18/12 η  

26 PHENIX Muon Piston Calorimeter Small cylindrical holes in Muon Magnet Pistons, Radius 22.5 cm and Depth 43.1 cm SOUTH PbWO 4 North Fwd-Fwd, x~(0.001,0.005) Mid-Fwd, x~(0.008,0.040) Mid-Bwd, x~(0.050,0.100) d(forward)Au(backward) 4/18/12

27 Outline The Relativistic Heavy Ion Collider PHENIX Experiment Motivation for Measurements Heavy-Ion Physics Nucleon Structure in Nuclei Gluon Saturation The New Forward Rapidity Muon Piston Calorimeters Selected Experimental Steps d+Au vs p+p Measurements of Azimuthal Angle Di-Hadron Measurements Mid (rapidity)-Forward (rapidity) Correlations Forward-Forward Correlations Conclusions/Discussion 4/18/12

28 Di-hadron  Correlations Measure  of all particle pairs trigger particle (usually leading p T ) associate particle (lower p T ) trigger associate  Beam view or transverse plane p+p, d+Au Central rapidity correlations are similar (RHIC Run3)  =0 is similar for d+Au (closed) and p+p (open) S.S Adler et al, Phys. Rev. C 73:054903,2006. Away-side Near-side  Nearside peak  Awayside Peak Transverse Plane 4/18/12

Accessing Low x with Di-Hadrons Guzey, Strikman, Vogelsang, PLB603, 173 Single Hadrons Di-Hadrons from Di-Jets  Narrow x-range  Smaller mean x 294/18/12

 Experimental Di-Hadron Signal Example CF CORRELATED N pair “Conditional Yield” Number of correlated particle pairs per trigger particle after corrections for efficiencies, combinatoric background, and subtracting off pedestal 30  Measure Gaussian width, CY for CF’s  Form I dA, J dA  Nuclear effects are signaled by  J dA < 1, I dA < 1  Angular decorrelation of widths 4/18/12

Original prediction by KLM for particles separated in rapidity Mid-forward correlations Expect decorrelation from CGC Recently, thought not to probe low enough x to see CGC effects Need both particles in forward direction, forward-forward correlations New predictions for forward-forward correlations CGC predicts significant b-dependence to suppression expected! Thesis results test both KLM predictions with measurements Predictions for Di-Hadron Correlations 31 Note: points offset from true to show p T dependence D. Kharzeev, E. Levin, and L. McLerran Nucl. Phys. A748 (2005) 627–640 J. L. Albacete and C. Marquet, PRL105 (2010) d Au Outgoing jets d Au Outgoing jets 4/18/12

32 KLM Mid-Forward Correlations  d Au PHENIX central spectrometer magnet Backward direction (South)  Forward direction (North)  Muon Piston Calorimeter (MPC) Side View d Au 0s0s  0 or h +/- x gluon ~ Nuclear enhancement increases density in d+Au Reminder: KLM correlations = forward di-hadron  correlations 4/18/12

Mid-Forward Per-Trigger KLM Correlations |  mid | < 0.35,  fwd = Mid-rapidity triggered     CFs Central d+Au shows suppression No broadening apparent 334/18/12

34 KLM Forward-Forward Correlations    d Au PHENIX central spectrometer magnet Backward direction (South)  Forward direction (North)  Muon Piston Calorimeter (MPC) Side View d Au Reminder: KLM Correlations = forward di-hadron azimuthal angle correlations Mostly Merged  0 s clusters 00 x gluon ~ Nuclear enhancement increases gluon density in d+Au 4/18/12

Fwd-Fwd Per-Trigger KLM Correlations  clus,  0 = Forward rapidity Cluster   CFs Use Zero-Yield at Minimum to subtract BG Central d+Au appears to show significant suppression Angular broadening possible in central d+Au 354/18/12

No significant broadening mid-forward rapidity azimuthal correlations (FMS-BEMC/FMS-TPC) Significant broadening for forward di-pion correlations (FMS-FMS) Strong suppression of away side peak for central forward-forward correlation with CGC prediction Di-hadron azimuthal correlation STAR arXiv: v1 Multiple soft scatterings de-correlate the away side peak 4/18/1236

J dA (Mid-Fwd, Fwd-Fwd) Suppression of J dA increases with N coll increase P T mid decrease P T fwd decrease Suppression Larger in fwd-fwd than mid-fwd Centrality dependent suppression Suppression of J dA increases with N coll increase Note: points offset from true to show p T dependence PRL107, (2011) 4/18/1237

Note: points for mid-fwd JdA are offset for visual clarity 60-88% (Peripheral ) Forward-ForwardMid-Forward % (Central) Suggests effect is due to low-x phenomena in the nucleus CGC? Initial State Energy Loss? More traditional Shadowing? Multi-parton Interactions? Theory work underway J dA versus x AU 4/18/12

J dA versus R G Au ? Low x, mostly gluons  J dA  High x, mostly quarks Weak effects expected ~ R G Au b=0-100% Q 2 = 4 GeV 2 x Au EPS09 NLO gluons Eskola, Paukkunen, Salgado, JHP04 (2009)065 R G Au Forward-Forward Mid-Forward arXiv: v1 4/18/1239

40 Summary RHIC crated Quark Gluon Plasma(QGP) state or hot and dense medium d+Au collision gives us information about Low-x and Initial state effect New MPC detector for reconstructing pi0 and eta meson Di-Hadron correlation -> access to lower x region Forward-forward di-hadron correlations are suppressed Small impact parameter  more suppression Suppression larger (J dA ) in forward-forward than mid-forward Seems to be more suppression as x decreases CGC? E-loss? Fragmentation? Angular broadening Mid-forward (none observed) Forward-forward (Inconclusive) 4/18/12

Backup Slides 414/18/12

Centrality Selection Charged particle track distribution representing 92% (+/- 2% systematic) of the 7.2 barn total Au+Au cross section. We then select event classes based on geometry (number of participating nucleons) using the Zero Degree Calorimeter and Beam-Beam Counter. 424/18/12

Nuclear Shadowing Nuclear shadowing is simply a decrease in R F2 A Common models include Leading twist-shadowing Ex: multiple scattering contributes negative value scattering amplitude Higher twist-shadowing Ex: multiple scattering with multiple gluons exchanged between parton and nucleus Twist refers to expansion in powers of 1/Q 2 Models that just fit data to nPDFs (e.g. eps09) Seems that both leading twist and higher twists are needed to describe R F2 A at low x, Q 2 43 See Frankfurt, Guzey, Strikman ph/ v4 4/18/12

44 Two forward pions can probe very low x g 1) Pythia simulation 2) Trigger on one  0 forward (3 2.5 GeV/c 3) Plot rapidity of all other pions between 1.5 GeV/c and p T of the trigger pion as function of x For plot: Forward- Forward probes lowest x Forward-Central Forward-Forward See Ermes Braidot, Quark Matter 2009 proceedings, arXiv: FMS-FMS FMS-Barrel calorimeter This region can be probed with FMS- endcap calorimeter   No Color for bins with 0-10 counts STAR’s plot 4/18/12

45 Why Forward in d+Au? Forward  Low x Probe low-x gluon distribution in Au nucleus Suppression of particle yields expected due to extremely high gluon density Gluon saturation Saturation momentum goes like Need jet p T ~ Q s to observe effects Shadowing effects Rapidity Separated Jets BFKL evolution, a.k.a Quantum Evolution, produces Mueller-Navelet Jets Larger rapidity gap between jets  larger probability for emitting gluons  decorrelation in 2 ptcl  distribution Experimental Program: Scan different  separations for jets and look at low-x d+Au: Mono-jet P T is balanced by many gluons Dilute parton system (deuteron) Dense gluon field (Au) shadowing Forward Direction 4/18/12

46 PbWO 4 The MPC Crystal Density8.28 g/cm 3 Size2.2 x 2.2 x 18 cm 3 Length 20 X 0, 0.92 Weight721.3 g Moliere radius2.0 cm Radiation Length0.89 cm Interaction Length22.4 cm Light Yield ≈10 25  C Temp. Coefficient -2% /  C Radiation Hardness1000 Gy Main Emission Lines , 500 nm Refractive Index2.16 about 50 years in PHENIX forward directions 4/18/12

Momentum transfer Q 2 = 0.1 GeV 2 Q 2 = 1.0 GeV 2 Q 2 = 20.0 GeV 2 Wavelength = h/p See the whole proton See the quark substructure See many partons (quarks and gluons) Proton Structure F 2 = x∑(q i (x) + q i (x)) e q 474/18/12