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Diffractive physics at STAR

Exclusive and diffractive processes at high energy. proton-proton and nucleus-nucleus collisions. Diffractive physics at STAR. results and future program. Andrzej Sandacz. National Centre for Nuclear Research, Warsaw. on behalf of the STAR Collaboration.

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Diffractive physics at STAR

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  1. Exclusive and diffractive processes at high energy proton-proton and nucleus-nucleus collisions Diffractive physics at STAR results and future program Andrzej Sandacz National Centre for Nuclear Research, Warsaw on behalf of the STAR Collaboration ECT*, Trento, Italy, February 27 – March 2, 2010

  2. Contents • diffraction in ultraperipheral collisions of heavy ions • elastic p-p scattering with spin • central production with forward protons • future of diffractive production with forward protons at STAR

  3. nucleus-nucleus (Au Au, Cu Cu, d Au …) Ebeam/nucleon up to 100 GeV polarised proton-proton Ebeam up to 250 GeV

  4. STAR detector in cross section TPC • Excellent particle identification capability: TPC dE/dx, ToF

  5. γγ → ρ0ρ0 , π+π‒ , l+l‒ , Q Q , … ¯ Ultraperipheral collissions of heavy ions • relativistic heavy ions as a source of intense photon fluxes • access to various γγ and γῬ interactions γῬ → ρ0 , π+π‒ , π+π‒π+π‒ , J/ψ … • separation of electromagnetic interactions from hadronic ones b > 2 RA i.e. pT < πħ / RAfor gold ( RA ≈ 7 fm) pT < 90 MeV/c • examples exclusive coherent (a) coherent with mutual nuclear excitation (b) de-excitation of Au* predominantly by emision of relativistic forward moving neutrons the convenient signature exploited at STAR for triggering

  6. STAR results on Au-Au ultraperipheral collisions • photoproduction of ρ0 ( and non-resonant π+π‒ pairs) √sNN = 130, 200, 62.4 GeV Phys. Rev. Lett. 89 (2002) 272302; Phys. Rev. C 77 (2008) 34910; Phys. Rev. C 85 (2012) 14910 • photoproduction of π+π‒π+π‒ ( ρ’) √sNN = 200 GeV Phys. Rev. C 81 (2010) 44901 • production of e+ e‒pairs Phys. Rev. C 70 (2004) 031902(R) • observation of two-source interference in photoproduction reaction Au Au → Au Au ρ0 Phys. Rev. Lett. 102 (2009) 112301; • new results on photoproduction of J/ψand φ in UPC of Au-Au √sNN = 200 GeV and diffractive scattering of ρ0on Au nuclei

  7. interference (Söding) ρ0BW direct π+π‒ Coherent ρ0 photoproduction in ultraperipheral Au-Au collisions Au Au → Au(*)Au(*)ρ0 Au Au → Au*Au*π+π‒ Au Au → Au*Au*ρ0 √sNN = 62.4 GeV √sNN = 62.4 GeV √sNN = 62.4 GeV pT < 150 MeV/c Au Au → Au(*)Au(*)ρ0 open – ρ0 candidates hatched – like-sign pairs hatched – combinatorial bkg. theoretical predictions for dσ/dy needed to estimate of σ in the full y range Klein and Nystrand model used √sNN = 200 GeV

  8. Au Au → Au(*)Au(*)ρ0 generalised VMD + QCD Gribov-Glauber color dipole + nuclear effects + parton saturation based on above, impact parameter dependence of color dipole cross section, DGLAP evolution VMD + Glauber for nuclear scattering Energy dependence best agreement with IIM-GM comparison of absolute values of measured σwith Gonsalves-Machado not straightforward because of different exptrapolation from |y| < 1 Au Au → Au(*)Au(*)ρ0√sNN= 200 GeV σtotal530 ± 19 ± 57 mb σ0n0n391 ± 18 ± 55 mb (74%) σ0nXn105 ± 5 ± 15 mb (20%) σXnXn31.9 ± 1.5 ± 4.5 mb ( 6%) σ1n1n2.4 ± 0.2 ± 0.4 mb(0.5%)

  9. ○lightest π+π‒ pair ●second π+π‒ pair M(ρ’) = 1540 ± 40 MeV/c2 Γ = 570 ± 60 MeV/c2 n (skewing factor) = 2.4 ± 0.7 χ2/NDF = 36/16 result of simulation assuming: Coherent π+π‒π+π‒photoproduction in ultraperipheral Au-Au collisions Au Au → Au*Au*π+π‒π+π‒ √sNN = 200 GeV ( 2007 ) relativistic S-wave BW modified by Ross-Stodolsky factor 2nd order polyn. fit to background. signal + background grey histogram: bkg. from 2+ or 2‒ four-prongs acceptance for |y| < 1 probably mixing of two resonances ρ0 (1450) and ρ0 (1700) their nature under debate; their decay patterns ρ’ →ρ0 f0(600) → [π+π‒]P-wave[π+π‒]S-wave do not match quark model predictions

  10. for reference, Au Au → Au*Au*ρ0from the same 2007 data set σcoh4π,XnXn = 4.3± 0.3 ± 1.5 mb σcoh4π,0n0n = 53± 4 ± 19 mb modif. S-wave BW of ρ’ tail of ρ0BW sum R < 2.5 % at 90 % CL Prospects for progress in studies of π+π‒π+π‒production 2010 data set ( Au Au at 200 GeV) 50-100 times larger than 2007 data set • study of ρ’ properties • search for coherent ρ0ρ0 production in UPC ρ0ρ0 signal could come either from γγor due to C-invariance, no signal of ρ0ρ0 in diffractive production ≈ 100 μb two independent γῬ reactions on the same ions ≈ 720 μb

  11. Coherent J/ψphotoproduction in ultraperipheral Au-Au collisions • exactly two tracks from the vertex • pair pT < 0.150 GeV • vector meson rapidity: 0.05 < y < 1 ρ0candidates J/ψcandidates (0.4 < minv< 1.1 GeV) (2.5 < minv< 3.5 GeV) Interest in heavy vector mesons • probe short distance scales • sensitivity to gluon distribution (x ≈ 0.015, Q2 ≈ mJ/ψ2) Ratio N(J/ψ) / N(ρ0) ● acceptance corrected data х background from like-sign pairs ■ data ■ Klein-Nystrand model(PR C60 (1999)14903) no other predictions are currently available ≈ 650 000 ρ0 and 125 J/ψ

  12. Illuminating Au nuclei with ρ0mesons coherent or incoherent scattering of a color dipole (vector meson) on the target nucleus renewed interest, part of the program for e – A colliders spatial distribution or density effects Au Au → Au*Au*ρ0 each Au* emits 1n STAR PRELIMINARY coh. incoh. analysis ongoing to identify backgrounds, and extract efficiency and acceptance corrections

  13. Spin Effects in Elastic Scattering at Collider Energies • Scientific interest Constraints from general principles of Field Theory e.g. allowed growth of single spin-flip / nonflip amplitude ~ ln s as s→ ∞ (at fixed t) Non-perturbative region of QCD ( | t | < 0.05 GeV2 ) but spin-flip probes smaller distances ( ~ 0.2 fm ) in nucleon than non-flip interaction ( ~ 1 fm ) Details of static constituent quark structure of nucleon non-zero spin-flip amplitude AN predicted by various models: compact diquark in the nucleon or anomalous color-magnetic moment of quarks or isoscalar magnetic moment of the nucleon properties of Pomeron, dominant exchange in elastic scattering at colliders Does it couple to nucleon spin? search for Odderon, C = -1 counterpart of Pomeron Double spin asymmetry ANN a sensitive probe of Odderon

  14. RHIC-Spin accelerator complex

  15. Helicity amplitudes for elastic scattering of spin ½ (identical) hadrons Observables cross sections and spin asymmetries spin non–flip double spin flip spin non–flip double spin flip single spin flip protons fully polarised along ň normal to scattering plane ASSanalogously, but parallel to vector š in scattering also ASL, ALL plane and perpendicular to the beam formalism well developed, however not much data ! at high energy only AN measured to some extent

  16. Single transverse spin asymmetry AN and Coulomb-nuclear interference the left-right scattering asymmetry AN arises from theinterference of thespin non-flipamplitude with thespin flipamplitude (Schwinger 1948) in absence of hadronic spin-flip contributions AN is exactly calculable (Kopeliovich & Lapidus) hadronic spin-flip modifies the ‘QEDpredictions’ µ(m-1)pµspptot AN (t) needed phenomenological input: σtot, ρ, δ (diff. of Coulomb-hadronic phases),also for nuclear targets em. and had. formfactors

  17. = Setup at STAR and principle of measurement of elastic p-p scattering • Elastically scattered protons have very small scattering angle θ*, hence beam transport magnets determine trajectory scattered protons • The optimal position for the detectors is where scattered protons are well separated from beam protons • Need Roman Pot to measure scattered protons close to the beam without breaking accelerator vacuum Beam transport equations relate measured position at the detector to scattering angle. x0,y0position at Interaction Point Θ*x ,Θ*yscattering angle at IP xD, yDposition at Detector ΘxD ,ΘyDangle at Detector The most significant matrix elements are Leff,so that approximately xD  LxeffΘx*yD  LyeffΘy*

  18. Data sample and selection of elastic events 4 dedicated RHIC stores with β*≈ 22m 2009, p-p at √sNN = 200 GeV to minimise angular divergence at IP ≈ 33 million elastic trigger events in order to allow measurements at small |t| 2π acceptance in ϕ provided by a pair of vertical and horizontal RPs on each side of IP with ≈ 30% overlap collinearity requirement θ*x,yE = θ*x,yW non-collinear background ≈ 0.5 – 1 %, σ = 58 μrad, mostly due to beam angular divergence after selection ≈ 21 mln elastic events for the analysis t= ‒ p2 ( 1 – cos θ ) p = 100.22GeV/c

  19. can be neglected wrt 1 ( < 0.03 ) Pb+ Py= 1.22 ± 0.04 Pb‒Py= 0.02 ± 0.04 Use Square-Root-Formulaeto calculate spin ( ,  ) and (,  ) asymmetries In this formulaeluminosities, apparatusasymmetriesandefficienciescancel Experimental Determination of AN examples for a selected t bin (with <t> = - 0.0077GeV/c2) ’N ~ (PB–PY) PRELIMINARY N ~ (PB+PY) ε’ consistent with 0 systematics check

  20. Results on ANand single spin-flip amplitude pp2pp (2003) result Re r5 = -0.033± 0.035 Im r5 = -0.43± 0.56 Only statistical errors shown NO account for polarization and –t uncertainties PRELIMINARY N. H. Buttimoreet. al. Phys. Rev. D59, 114010 (1999) tc = -8πα/ σtot; κ is anomalous magnetic moment of the proton;

  21. Transverse double spin asymmetries ANN and ASS • Cannot use square root formula – have to rely on normalized countsK+/– • Double spin effects are seen but very small PRELIMINARY All t-ranges • Both ANN and ASS are very small ~10–3 (except for the lowest t-range where larger systematic shifts may occur) • Need better systematic error studies – current normalization uncertainties are of the order of the effect PRELIMINARY Large systematic shift of 0-line is possible due to normalization PRELIMINARY Only statistical errors shown

  22. Central Exclusive Production in Double Pomeron Exchange large s, small| ti | , large rapidity gaps between p’iand Mx Pomeron – Pomeron interaction => gluon rich environement CP in DPE - a place to search for glueballs and hybrids Existance of glueballs predicted by QCD because of self-interaction of gluons with the lightest glueball being a scalar JPC = 0++ signatures • no place in q-qbar nonets • decay branching ratios incompatible with q-qbar states • enhanced production of gluon-rich channels of rad. decays until recently CP spectroscopy at lower √s • CERN Ω : WA76 (12.6 GeV), Wa91(23.7 GeV), WA102(29.1 GeV) • CERN GAMS (29.1 GeV) • FNAL E690 (38.8 GeV) • ISR AFS R807 (62 GeV) in this energy range non-negligible contribution of R-P and R-R contributions, complicate interpretations

  23. (1) orMxfrom central detector (2) Mx = √ ξ1 ξ2 s ‘large’ ≥ O( ΛQCD) smalldpT largedpT qq states: ρ0, η’, f2(1270), f1(1285), f2’(1525) suppressed as dpT -> 0 dpT = √ ( p’1T – p’2T) 2 glueball candidates fJ(1710), f0(1500), f2(1930) survive Kinematics of DPE and selections of glueball candidates ξi = (pi – pi’) / pi (1) vs. (2) needed for exclusivity selection unless full coverage inη kinematic ‘filter’ ( dpT ) for ‘gg’ (F. Close et al. / WA102)

  24. work in progress for identifying exclusivity of DPE events: rapidity gaps, PID, PT-balance, missing mass … Pilot DPE data from 2009 run in parallel with elastic data 4 dedicated RHIC stores with β* = 22m 2009, p-p at √s = 200 GeV • RP and multiplicity trigger using TOF barrel to select low multiplicity (0 < N < 6) central events about 700k CP triggers taken • reconstruction and selections • momenta of scattered protons using RPs and beam transport • STAR TPC tracking in |y| < 1 with TOF barrel and TPC PID • two tracks in RPs on opposite sides of IP • two (four) tracks in TPC from the vertex

  25. Phase II for DPE and elastic collisions • √s = 500 GeV • range of larger |t | 0.2 – 1.5 (GeV/c)2 compared to 0.003 – 0.035 in Phase I (2009 run) • wide rapidity gaps, beam rapidity at √s = 500 GeV y ~6.3 for MX < 3 GeV/c2rapidity gap > 4 units • higher reach in MX : MX max≈ 25 GeV/c2 (500 GeV) vs. ≈ 10 GeV/c2 (200 GeV) • no special beam optics required, high lumnosity • planned to be installed in 2013

  26. Phase II acceptance elastic at √s = 500 GeV at RPs full coverage in ϕ limited due to beam constraint significant overlap of acceptances of Si detectors at 15.3 and 17.3 m needed to determine momenta of scattered protons in DPE no significant dpT dependence of shape for acceptance of mass

  27. Expected yields Assumed: • total DPE cross section 140 μb and branching ratios as at ISR • expected trigger rate for DPE ≈ 100 Hz at L = 1·1031 cm-2s-1 • 2 years of running pp at 500 GeV

  28. Summary • diffraction in ultraperipheral collisions of heavy ions • coherent photoproduction of ρ0 in Au-Au collisions at √sNN range 64 - 200 GeV • observation of coherent photoproduction of π+π‒π+π‒( ρ’ ), J/ψ, φ at 200 GeV • large (50 – 100 x) set of data (2010) to be analysed • elastic polarised p-p scattering (200 GeV) • precise measurement of single transverse spin asymmetry AN in CNI region spin coupling of Pomeron • measurements of double transverse spin asymmetries ANN andASS search for Odderon • pilot run (2009) for DPE with RPs fully integrated into STAR experiment • exciting future program of DPE studies at STAR using central detector and detection of forward protons • not discussed here – future study of polarised and unpolarised elastic p p cross section in unexplored ranges of t and √s

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