Vector meson production in ultraperipheral collisions: accessing the small-x gluon

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1 Vector meson production in ultraperipheral collisions: accessing the small-x gluon Heikki Mäntysaari Brookhaven National Laboratory Probing QCD in Photon-Nucleus Interactions at RHIC and LHC: the Path to EIC, February 14, 2017 Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

2 Content 1 Background 2 Pb+Pb collisions 3 p+pb collisions Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

3 Content 1 Background 2 Pb+Pb collisions 3 p+pb collisions Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

4 Ultraperipheral collision as a deep inelastic scattering b 2R A : strong interactions are suppressed Z 1 e E 1 Nucleus creates a (real) photon flux n(ω) 2 Photon-nucleus scattering Z 2 e J. Nystrand et al, nucl-ex/ σ AA AA+V n(ω)σ γa VA (ω) Interesting QCD part: high-energy γ-nucleus or γ-proton scattering. Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

5 Diffractive vector meson production as a probe of small x Exclusive production of vector meson γp Vp γa VA Pocket formula for diffraction (2-gluon exchange) dσ γ H VH = 16π3 αs 2 Γ [ ee dt 3α em MV 5 xg(x, Q 2 ) ] 2 Diffraction is very sensitive to (small-x) gluons! UPC is γ p or γ A collision Nuclear DIS before the EIC era - and at very high energies! Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

6 QCD at high energy: Color Glass Condensate (BK) CGC = QCD at high energies x (energy) dependence: BK/JIMWLK (perturbative) Saturation of gluon density at small x Saturation scale Q 2 s (DGLAP) Natural framework to describe high energy scattering. Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

7 Deep inelastic scattering at high energy: dipole picture Optical theorem: σ γ p dipole amplitude σ γ p Vp dipole amplitude 2 Univesal dipole amplitude Same universal QCD evolved dipole amplitude N appears in calculations of DIS Diffraction Particle spectra in pp/pa... Non-perturbative input from a fit to HERA F 2 data. Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

8 IPsat model and DIS Use impact parameter dependent dipole amplitude (IPsat) fitted to HERA (Kowalski, Motyka, Watt, 2006; Rezaeian et al, 2012) F 2 (x,q 2 ) 2 n H1& ZEUS N(r, x, b) = 1 exp b-cgc IP-Sat x ] [ π2 α s xg(x, µ 2 )T p (b)r 2 2N c Q 2 [GeV 2 ], n 650, , , , , , , , 20 90, 19 70, 18 60, 17 45, 16 35, 15 27, 14 22, 13 18, 12 15, 11 12, 10 10, 9 8.5, 8 6.5, 7 4.5, 6 3.5, 5 2.7, 4 2.0, 3 1.5, 2 1.2, , 0 DGLAP evolved gluon distribution xg(x, µ 2 ) Proton profile T p Gaussian Very good agreement with structure function data Generalization for nuclei: S A (r, b, x) = A i=1 S p(r, b b i, x) Extremely good description of the precise HERA data Rezaeian et al, Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

9 Diffractive vector meson production 1 γ q q splitting, wave function Ψ γ (r, Q 2, z) 2 q q dipole scatters elastically 3 q q J/Ψ, wave function Ψ V (r, Q 2, z) Diffractive scattering amplitude A d 2 bdzd 2 rψ γ Ψ V (r, z, Q 2 )e ib N(r, x, b) Fourier transfer from impact parameter to transverse momentum access to spatial structure Still need to average over target configurations! Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

10 Coherent diffraction = target remains intact Target is at the same quantum state before and after the scattering (Miettinen, Pumplin, PRD 18, 1978,... ) with A dσ γ p Vp dt A(x, Q 2, t) 2 d 2 bdzd 2 rψ Ψ V (r, z, Q 2 )e ib N(r, x, b) Coherent t = 2 spectra is Fourier transfer of the average density Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

11 Incoherent diffraction = target breaks up Total diffractive cross section coherent cross section target breaks up with dσ γ p Vp A(x, Q 2, t) 2 A(x, Q 2, t) 2 dt A d 2 bdzd 2 rψ Ψ V (r, z, Q 2 )e ib N(r, x, b) Incoherent cross section is proportional to the amount of fluctuations in the impact parameter space Cross section for incoherent γa diffraction: T. Lappi, H.M, Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

12 Content 1 Background 2 Pb+Pb collisions 3 p+pb collisions Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

13 Going to small x with nuclei E EIC white paper Z 1 e Z 2 e Currently there is basically no small-x nuclear DIS data Momentum fraction x = M V e y / s Midrapidity J/Ψ at the LHC: x 10 3 Q 2 (GeV 2 ) Measurements with A 56 (Fe): ea/μa DIS (E-139, E-665, EMC, NMC) νa DIS (CCFR, CDHSW, CHORUS, NuTeV) DY (E772, E866) perturbative non-perturbative Forward J/Ψ at the LHC x 10 2 and x x EIC s = 90 GeV, 0.01 y 0.95 EIC s = 45 GeV, 0.01 y 0.95 Heikki Mäntysaari (BNL) UPC theory Feb 14, /

14 Signal of nuclear effects seen in γa diffraction / dy [mb] dσ coh γa J/ΨA Pb+Pb Pb+Pb+J/ψ CMS µb (2.76 TeV) CMS data ALICE data Impulse approximation Leading twist approximation Impulse approximation = scaled γp Clear nuclear effects seen (e.g. saturation, shadowing) y CMS, Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

15 Coherent diffraction, model comparison dσ/dy (mb) Pb+Pb Pb+Pb+J/ψ AB MSTW08 CSS AB HKN07 STARLIGHT GM LM fipsat AB EPS09 RSZ LTA AB EPS08 s NN = 2.76 TeV a) ALICE Coherent J/ψ Reflected y ALICE, Shadowing/saturation needed, compare e.g. AB-MSTW08 and AB-EPS09 (nuclear pdf) / LM-fIPsat (saturation) Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

16 Coherent and incoherent diffraction Coherent Incoherent dσ/dy (mb) Pb+Pb Pb+Pb+J/ψ AB MSTW08 CSS AB HKN07 STARLIGHT GM LM fipsat AB EPS09 RSZ LTA AB EPS08 s NN = 2.76 TeV a) ALICE Coherent J/ψ Reflected y dσ/dy (mb) Pb+Pb Pb+Pb+J/ψ s NN = 2.76 TeV b) ALICE Incoherent J/ψ STARLIGHT LM fipsat RSZ LTA Dipole model calculation (LM-fIPsat): ok simultaneous description More LHC data coming s = 5.02 TeV: x dependece of the gluon density Spectra differentially in t Geometric structure and fluctuations QM2017: Neutron tagging large x/small x separation y Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

17 This is becoming precision physics (linear scale!) dσ/dy [mb] Pb +Pb J/Ψ +Pb +Pb, s NN =2.76 TeV fipsat IIM ALICE y T. Lappi, H.M., Theory uncertainties are still large Dipole-nucleus amplitude No nuclear DIS data to fit But other data, e.g. R pa Vector meson wave function (thin-thick lines) Constrained mainly by the leptonic decay width = wave function at origin! Large phenomenological corrections Especially skewedness (2 gluons, x x ) is large, 50% NLO??? Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

18 This is becoming precision physics (linear scale!) dσ/dy [mb] Pb +Pb J/Ψ +Pb +Pb, s NN =2.76 TeV fipsat IIM ALICE y T. Lappi, H.M., Theory uncertainties are still large Dipole-nucleus amplitude No nuclear DIS data to fit But other data, e.g. R pa Vector meson wave function (thin-thick lines) Constrained mainly by the leptonic decay width = wave function at origin! Large phenomenological corrections Especially skewedness (2 gluons, x x ) is large, 50% NLO??? Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

19 This is becoming precision physics (linear scale!) dσ/dy [mb] Pb +Pb J/Ψ +Pb +Pb, s NN =2.76 TeV fipsat IIM ALICE y T. Lappi, H.M., Theory uncertainties are still large Dipole-nucleus amplitude No nuclear DIS data to fit But other data, e.g. R pa Vector meson wave function (thin-thick lines) Constrained mainly by the leptonic decay width = wave function at origin! Large phenomenological corrections Especially skewedness (2 gluons, x x ) is large, 50% NLO??? Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

20 This is becoming precision physics (linear scale!) dσ/dy [mb] Pb +Pb J/Ψ +Pb +Pb, s NN =2.76 TeV fipsat IIM ALICE y T. Lappi, H.M., Theory uncertainties are still large Dipole-nucleus amplitude No nuclear DIS data to fit But other data, e.g. R pa Vector meson wave function (thin-thick lines) Constrained mainly by the leptonic decay width = wave function at origin! Large phenomenological corrections Especially skewedness (2 gluons, x x ) is large, 50% NLO??? Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

21 This is becoming precision physics (linear scale!) dσ/dy [mb] Pb +Pb J/Ψ +Pb +Pb, s NN =2.76 TeV fipsat IIM ALICE y T. Lappi, H.M., Model uncertainties mainly affect normalization, not rapidity (Bjorken-x) dependence.(?) Theory uncertainties are still large Dipole-nucleus amplitude No nuclear DIS data to fit But other data, e.g. R pa Vector meson wave function (thin-thick lines) Constrained mainly by the leptonic decay width = wave function at origin! Large phenomenological corrections Especially skewedness (2 gluons, x x ) is large, 50% NLO??? Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

22 More differential measurements in the near future dσ A /dt [nb/gev 2 ] γa J/ΨA A =197, Q 2 =0 GeV 2, x =0.001 Quasielastic IPnonsat Coherent IPnonsat IPsat Coherent IPsat IIM Coherent IIM t [GeV 2 ] T. Lappi, H.M., Solid line: no saturation Dashed lines: with saturation Measure differentially in vector meson momentum t = 2 Incoherent: more sensitive to saturation effects QM2017, CMS: No/small incoherent cross section at small x! Qualitatively predicted: T. Lappi, H. M., Coherent: extract transverse density profile of the small-x gluons in the nucleus Toll, Ullrich, Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

23 Excited states: Ψ(2S)/J/Ψ Species dependence could test our understanding of the wave function ALICE, Qualitative agreement with dipole model: node effect is damped at large Q 2 s (contribution from large dipoles is suppressed). Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

24 Excited states: Ψ(2S)/J/Ψ Species dependence could test our understanding of the wave function ALICE, Qualitative agreement with dipole model: node effect is damped at large Q 2 s (contribution from large dipoles is suppressed). QM2017: σ(2s)/σ(j/ψ) = ± Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

25 Content 1 Background 2 Pb+Pb collisions 3 p+pb collisions Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

$Diffraction in pa collisions E Pb p Photon flux Z 2 Ultraperipheral pa collision γp collision Access small-x structure of the proton Collective phenomena seen in pp&pa Have$

26 Diffraction in pa collisions E Pb p Photon flux Z 2 Ultraperipheral pa collision γp collision Access small-x structure of the proton Collective phenomena seen in pp&pa Have to understand role of the initial state geometry Diffraction: average structure and fluctuations ATLAS, arxiv: Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

27 Diffraction at the TeV scale Total coherent diffractive cross section 10 3 IPsat H H ALICE 2014 σ γ p J/Ψp [nb] W [GeV] Total coherent cross section follows the same W γ power law as HERA No significant saturation effect expected, described by IPsat Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

28 Proton structure fluctuations Recall: Strategy Coherent diffraction probes average structure Incoherent diffraction is sensitive to amount of fluctuations Simultaneous description of HERA coherent and incoherent data allow us to constrain event-by-event proton structure fluctuations. Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

29 Constraining proton fluctuations Start with a simple constituent quark inspired picture: Sample quark positions from a Gaussian distribution, width B qc Small-x gluons are located around the valence quarks (width B q ). Combination of B qc and B q sets the degree of geometric fluctuations Dipole-target scattering: IPsat model fitted to F 2 data Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

Constraining proton fluctuations Start with a simple constituent quark inspired picture: Sample quark positions from a Gaussian distribution, width B qc Small-x gluons are located around the valence

30 Constraining proton fluctuations Start with a simple constituent quark inspired picture: Sample quark positions from a Gaussian distribution, width B qc Small-x gluons are located around the valence quarks (width B q ). Combination of B qc and B q sets the degree of geometric fluctuations Dipole-target scattering: IPsat model fitted to F 2 data Now proton = 3 overlapping hot spots. 3 T proton (b) = T q (b b i ) T q (b) e b2 /(2B q) i=1 Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

Lessons from the HERA data Lumpy: B qc = 3.3, B q = 0.7 10 3 Lumpy Smooth H1 Coherent Incoherent y[fm] 1 0 0.20 10 2 1 0.

31 Lessons from the HERA data Lumpy: B qc = 3.3, B q = Lumpy Smooth H1 Coherent Incoherent y[fm] dσ/dt [nb/gev 2 ] y[fm] x[fm] x[fm] IPsat Smooth: B qc = 1.0, B q = t [GeV 2 ] H.M, B. Schenke, PRL 117 (2016), and PRD94 (2016), H1 incoherent data requires large fluctuations Proton-photon center-of-mass energy x[fm] x[fm] W = 75 GeV, probing x 10 3 Units: GeV 2 Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30 y[fm] y[fm]

IP-Glasma and HERA data Include color charge fluctuation, parameters fitted to H1 data dσ/dt [nb/gev 2 ] 10 3 10 2

0 1 x[fm] 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.5 1.0 1.5 2.0 2.5 t [GeV 2 ] H.M., B.

32 IP-Glasma and HERA data Include color charge fluctuation, parameters fitted to H1 data dσ/dt [nb/gev 2 ] Round proton Geometric fluctuations H1 coherent H1 incoherent y[fm] y[fm] x[fm] x[fm] t [GeV 2 ] H.M., B. Schenke, PRD94 (2016), Initial condition for pa hydro, good description of v 2 and v 3 data! Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

33 Towards smaller x / larger W in γp ALICE measurement in γ + p J/Ψ + p(p ) collisions x Incoherent cross section not observed at small x Signature of smoothening at small x? Proton grows, diffractive slope B p : 4 GeV GeV 2 ALICE arxiv: Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

34 Proton smoothening at small x? Cepila, Contreras, Takaki ( ): Number of constituent quarks x a (1 + b x), parameters fitted to HERA data σ(γp JψX) (nb) H1 data Model 10 (GeV) W γp 3 Qualitatively expect incoherent cross section to decrease at high W Still expect to see significant incoherent contribution at 1 TeV Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

Evolution to small x (work in progress) HERA data constrains proton structure at x 10 3.

35 Evolution to small x (work in progress) HERA data constrains proton structure at x Evolve to smaller x by perturbative CGC evolution equation (JIMWLK) B. Schenke, S. Schlichting, Phys.Lett. B739 (2014) Proton grows Proton gets smoother Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

36 Energy evolution of diffractive J/Ψ production Work in progress / preliminary (qualitative results at this point) JIMWLK IPsat H incoherent/coherent W [GeV] Incoherent cross section grows more slowly Proton gets smoother Dipole must not scatter off other constituent quarks (included in IPsat calculation T. Lappi, H.M., ) We would expect to still see a large incoherent contribution at W 700 GeV Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

37 Conclusions UPC: diffractive vector meson production at very high energy CGC calculations compatible with both coherent and incoherent J/Ψ measurements form the LHC Signatures of saturation/shadowing in heavy ion collisions Still largish model dependence pa collisions: study proton structure at high energies Description of HERA data requires large proton structure fluctuations LHC pa data hints for smoothening at small x Fluctuations applied to hydro calculations of pa collisions: good description of the v n measurements (backup) Lots of new data coming! Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

38 BACKUPS Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

39 Fluctuating protons in pa collisions Hydro calculations with proton fluctuations from HERA Hydro numbers τ 0 = 0.4 fm T fo = 155 MeV Shear and bulk viscosity Initial π µν η/s = Large v 2 and v 3 at largest centrality bins reproduced well. In preparation with B. Schenke, C. Shen, P. Tribedy Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

40 Constrain evolution speed Work in progress / preliminary α s = JIMWLK H1&ZEUS Q 2 = 4.5 GeV 2 H1&ZEUS Q 2 = 10 GeV 2 H1&ZEUS Q 2 = 18 GeV F x F 2 data at Q 2 = GeV 2 M 2 J/Ψ constrain α s MV model does not give exactly correct Q 2 dependence Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

41 Adding color charge fluctuations: IP-Glasma Obtain saturation scale Q s (x T ) from IPsat (with fluctuations) MV-model: Sample color charges, density Q s (x T ) Solve Yang-Mills equations to obtain the Wilson lines ( V (x T ) = P exp ig dx ρ(x ), x T ) 2 + m 2 Dipole amplitude: N(x T, y T ) = 1 TrV (x T )V (y T )/N c Fix parameters B qc, B q and m with HERA data Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

42 Lumpiness matters, not details of the density profile 3 valence quarks that are connected by color flux tubes (Gaussian density profile, width B q ). Also good description of the data dσ/dt [nb/gev 2 ] Strigyn proton H1 Coherent Incoherent y[fm] y[fm] Example density profiles IPsat t [GeV 2 ] x[fm] x[fm] 0.00 H.M, B. Schenke, PRD Flux tubes implementation following results from hep-lat/ , used also e.g. in Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

43 Wave function overlap in J/Ψ production: (r/2) dz(ψ V Ψ) [GeV] Transversely polarized J/Ψ meson Q 2 =0.05 GeV 2 Q 2 =3.2 GeV 2 Q 2 =22.4 GeV 2 (r/2) dz(ψ V Ψ) [GeV] Longitudinally polarized J/Ψ meson Q 2 =0.05 GeV 2 Q 2 =3.2 GeV 2 Q 2 =22.4 GeV r[fm] r[fm] Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

44 Differential cross section dσ/dtdy [mb/gev 2 ] t [GeV 2 ] T. Lappi, H. Mäntysaari, fipsat IIM Assuming proton profile function T p (b) e b2 /(2B p) incoherent cross section e Bpt : probes spatial distribution of gluons in proton! Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

45 pa prediction dσ/dy [µb] fipsat IIM y T. Lappi, H. Mäntysaari, CMS frame As the photon flux Z 2, dominant process is the one where the nucleus emits the photon probes mostly proton structure. Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

46 Vector meson wave functions γ q q can be computed from QED, but q q vector meson requires some modelling, parameters fit to reproduce decay width. Excited states: Ψ(2S) wave function has a node (orthogonal to J/Ψ). Cross section d 2 r large suppression compared to J/Ψ (r/2) dz(ψ V Ψ) [GeV] Transversely polarized J/Ψ meson Q 2 =0.05 GeV 2 Q 2 =3.2 GeV 2 Q 2 =22.4 GeV 2 Radial overlap: r[fm] Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

47 Generalization for nuclei S matrix probability not to scatter [recall: S = 1 N]: A S A (r, b, x) = S p (r, b b i, x) Average over nucleon configurations O({b i }) N = i=1 A i=1 [ d 2 b i T A (b i ) ] O({b i }) Heikki Mäntysaari (BNL) UPC theory Feb 14, / 30

ALICE results on vector meson photoproduction in ultraperipheral p-pb and Pb-Pb collisions

ALICE results on vector meson photoproduction in ultraperipheral p-pb and Pb-Pb collisions Evgeny Kryshen (Petersburg Nuclear Physics Institute, Russia) for the ALICE collaboration INT workshop INT-17-65W