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A Better Understanding of the Pomeron Structure at the LHC

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A Better Understanding of the Pomeron Structure at the LHC 1 A better understanding of the Pomeron structure at the LHC Christophe Royon University of Kansas, Lawrence, USA 4th Elba Workshop on Forward Physics at LHC Energies La Biodola, Elba Island, Italy, 24-26 May 2018 QCD structure of Pomeron: quarks/gluons • BFKL resummation effects • Soft Color Interactions models • 2 Hard diffraction at the LHC Dijet production: dominated by gg exchanges; γ+jet production: • dominated by qg exchanges (C. Marquet, C. Royon, M. Saimpert, D. Werder, arXiv:1306.4901) Jet gap jet in diffraction: Probe BFKL (C. Marquet, C. Royon, M. • Trzebinski, R. Zlebcik, Phys. Rev. D 87 (2013) 034010; O. Kepka, C. Marquet, C. Royon, Phys. Rev. D79 (2009) 094019; Phys.Rev. D83 (2011) 034036 ) Three aims • – Is it the same object which explains diffraction in pp and ep? – Further constraints on the structure of the Pomeron as was determined at HERA – Survival probability: difficult to compute theoretically, needs to be measured, inclusive diffraction is optimal place for measurement 3 one aside: Parton densities in the pomeron (H1) Extraction of gluon and quark densities in pomeron: gluon dominated • Gluon density poorly constrained at high β • ) ) 2 2 Singlet2 Gluon Q [ 2] 0.2 0.5 GeV (z,Q 8.5 Σ 0.1 0.25 z z g(z,Q 0 0 0.2 0.5 20 0.1 0.25 0 0 0.2 0.5 90 0.1 0.25 0 0 0.2 0.5 800 0.1 0.25 0 0 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 z z H1 2006 DPDF Fit Fit B (exp. error) (exp.+theor. error) 4 Inclusive diffraction at the LHC: sensitivity to gluon density Predict DPE dijet cross section at the LHC in AFP acceptance, jets • with pT >20 GeV, reconstructed at particle level using anti-kT algorithm Sensitivity to gluon density in Pomeron especially the gluon density on • Pomeron at high β: multiply the gluon density by (1 β)ν with ν = 1,..., 1 − − Measurement possible with 10 pb−1, allows to test if gluon density is • similar between HERA and LHC (universality of Pomeron model) Dijet mass fraction: dijet mass divided by total diffractive mass • (√ξ1ξ2S) 5 10 ν gluons =-1 (pb) ν s) ξ =-0.5 2 0.015 < < 0.15 ξ 1 standard (HERA Fit PDF) ξ / 20% uncertainty bar dijet ν=0.5 104 /d(W ν=1 dijet σ d 103 102 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 mass fraction 5 Inclusive diffraction at the LHC: sensitivity to quark densities Predict DPE γ+jet divided by dijet cross section at the LHC • Sensitivity to universality of Pomeron model • Sensitivity to quark density in Pomeron, and of assumption: • u = d = s =u ¯ = d¯=s ¯ used in QCD fits at HERA Measurement of W asymmetry also sensitive to quark densities • ×10-3 d = s, u + d + s = const. d/u = 0.25 /dM 0.7 0.015 < ξ < 0.15 d/u = 0.5 dijet σ -1 d/u = 1 0.6 ∫ Ldt=300pb d/u = 2 /dM/d d/u = 4 jet γ 0.5 σ d 0.4 0.3 0.2 0.1 400 600 800 1000 1200 1400 1600 1800 2000 M(GeV) 6 W charge asymmetry: Sensitivity to quark densities Measure the average W charge asymmetry in ξ bins to probe the quark • content of the proton: A =(NW + NW − )/(NW + + NW − ) − Test if u/d is equal to 0.5, 1 or 2 as an example • A. Chuinard, C. R..,R. Staszewski, JHEP 1604 (2016) 092 • 7 Soft Colour Interaction models A completely different model to explain diffractive events: Soft Colour • Interaction (R.Enberg, G.Ingelman, N.Timneanu, hep-ph/0106246) Principle: Variation of colour string topologies, giving a unified • description of final states for diffractive and non-diffractive events No survival probability for SCI models • p qq qq qq q q q H H H q q q q q q q q q p 8 Inclusive diffraction at the LHC: sensitivity to soft colour interaction Predict DPE γ+jet divided by dijet cross section at the LHC for • pomeron like and SCI models In particular, the diffractive mass distribution (the measurement with • lowest systematics) allows to distinguish between the two sets of models: flat distribution for SCI d/u = 0.25 /dM d = s, u + d + s = const. dijet ξ d/u = 0.5 σ 0.0015 < < 0.02 d/u = 1 /dM/d d/u = 2 jet γ σ d d/u = 4 SCI model 10-3 60 80 100 120 140 160 180 200 220 240 M(GeV) 9 Looking for BFKL effects Dokshitzer Gribov Lipatov Altarelli Parisi (DGLAP): Evolution in Q2 • Balitski Fadin Kuraev Lipatov (BFKL): Evolution in x • 10 Jet gap jet events in diffraction Study BFKL dynamics using jet gap jet events in DPE • See: C. Marquet, C. Royon, M. Trzebinski, R. Zlebcik, Phys. Rev. D 87 • (2013) 034010 11 Exclusive diffraction p p γ γ p p Many exclusive channels can be studied: jets, χC , charmonium, J/Ψ.... • Possibility to reconstruct the properties of the object produced • exclusively (via photon and gluon exchanges) from the tagged proton CMS/TOTEM/ALFA have the possibility to discover/exclude glueballs • at low masses: Check the f0(1500) or f0(1710) glueball candidates 0 0 0 0 Simulation of signal f0(1710) ρ ρ and non resonant ρ ρ • → 12 Conclusion Better understanding of diffraction in QCD (pomeron structure) and • also BFKL resummation at low x Better constraints on gluon density in Pomeron: especially at high β • where it is poorly know Possible measurement of survival probability • Constrain quark densities in Pomeron and study dependence on quark • flavor: Compare Pomeron-like or soft models BFKL dynamics: Measure for instance jet gap jets • Exclusive production in QCD: Vector mesons, glueballs... •.
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