Recent ALICE Results on Ultra-Peripheral Collisions
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The 36th Winter Workshop on Nuclear Dynamics 1-7 March 2020 Puerto Vallarta, Mexico Recent ALICE results on ultra-peripheral collisions J. G. Contreras Czech Technical University in Prague On behalf of the ALICE CollaboraNon March 3, 2020, Puerto Vallarta 1 Contents ๏ IntroducAon: ⇨ The structure of hadrons at low x. ⇨ Vector meson photoproducAon. ๏ Photoproducon off protons ⇨ ALICE and vector meson photoproducAon. ⇨ Energy evoluon of exclusive J/ψ producon. ๏ Photoproducon off Pb. ⇨ Coherent J/ψ producon in UPC. ⇨ Coherent J/ψ producAon in peripheral collisions. ⇨ Coherent ρ0 producon in UPC. New! ๏ A look at the future ⇨ A new ALICE for Run 3+4 at the LHC. ⇨ Expectaons for photon-induced processes. J. G. Contreras, Recent ALICE results on UPC, WWND 2020 2 The structure of hadrons at low x 3 Low-x and the structure of hadrons CERNDiagram from CERN Courier Courier, July 20, 2010 Scale of interacAon J. G. Contreras, Recent ALICE results on UPC, WWND 2020 4 Low-x and the structure of hadrons The high-energy limit of pQCD corresponds to the low x limit. High energy CERNDiagram from CERN Courier Courier, July 20, 2010 Scale of interacAon J. G. Contreras, Recent ALICE results on UPC, WWND 2020 4 Low-x and the structure of hadrons The high-energy limit of pQCD corresponds to the low x limit. High energy In this limit, the structure of hadrons is expected to reach the saturaAon region. CERNDiagram from CERN Courier Courier, July 20, 2010 Scale of interacAon J. G. Contreras, Recent ALICE results on UPC, WWND 2020 4 Low-x and the structure of hadrons The high-energy limit of pQCD corresponds to the low x limit. High energy In this limit, the structure of hadrons is expected to reach the saturaAon region. One of the most interesAng quesAons nowadays in pQCD is the precise locaAon of the saturaAon region. CERNDiagram from CERN Courier Courier, July 20, 2010 Scale of interacAon J. G. Contreras, Recent ALICE results on UPC, WWND 2020 4 Low x and nuclear targets The boundary between the dilute and saturated regimes is expected to appear at larger x, smaller energy, in nuclei than in nucleons. Accardi et al, EPJA 52 (2016) 268 (2016) 52 al, EPJA et Accardi J. G. Contreras, Recent ALICE results on UPC, WWND 2020 5 Low x and nuclear targets The boundary between the dilute and saturated regimes is expected to appear at larger x, smaller energy, in nuclei than in nucleons. Accardi et al, EPJA 52 (2016) 268 (2016) 52 al, EPJA et Accardi How can we study the energy evoluAon of the QCD structure of hadron targets? PhotoproducAon processes at the LHC. J. G. Contreras, Recent ALICE results on UPC, WWND 2020 5 The LHC as a photon-hadron collider The EM field of hadrons circulaAng in the LHC can be viewed as a beam of quasi real photons. There are two potenal sources, correspondingly two potenAal targets. R2 b R1 J. G. Contreras, Recent ALICE results on UPC, WWND 2020 6 The LHC as a photon-hadron collider The EM field of hadrons circulaAng in the LHC can be viewed as a beam of quasi real photons. There are two potenal sources, correspondingly two potenAal targets. R 2 The photon is coherently emiced by the source and its b virtuality is restricted by the radius of the eming parAcle: Virtuality of photons from Pb: Q2 ≈ (30 MeV)2. R 1 The intensity of the photon beam is proporAonal to Z2. Heavy nuclei are intense sources of quasi-real photons! The maximum energy of the photons in the laboratory system is determined by the boost of the eming parAcle: Large photon energies are possible at the LHC! J. G. Contreras, Recent ALICE results on UPC, WWND 2020 6 The LHC as a photon-hadron collider The EM field of hadrons circulaAng in the LHC can be viewed as a beam of quasi real photons. There are two potenal sources, correspondingly two potenAal targets. R 2 The photon is coherently emiced by the source and its b virtuality is restricted by the radius of the eming parAcle: Virtuality of photons from Pb: Q2 ≈ (30 MeV)2. R 1 The intensity of the photon beam is proporAonal to Z2. Heavy nuclei are intense sources of quasi-real photons! The maximum energy of the photons in the laboratory system is determined by the boost of the eming parAcle: Large photon energies are possible at the LHC! The LHC is a photon-hadron and photon-photon collider! J. G. Contreras, Recent ALICE results on UPC, WWND 2020 6 The LHC as a photon-hadron collider The EM field of hadrons circulaAng in the LHC can be viewed as a beam of quasi real photons. There are two potenal sources, correspondingly two potenAal targets. R 2 The photon is coherently emiced by the source and its b virtuality is restricted by the radius of the eming parAcle: Virtuality of photons from Pb: Q2 ≈ (30 MeV)2. R 1 The intensity of the photon beam is proporAonal to Z2. Heavy nuclei are intense sources of quasi-real photons! The maximum energy of the photons in the laboratory system is determined by the boost of the eming parAcle: Large photon energies are possible at the LHC! for b>R1+R2 purely hadronic interacAons are strongly suppressed and only photon induced processes remain. The LHC is a photon-hadron and photon-photon collider! These are called ultra-peripheral collisions (UPC). J. G. Contreras, Recent ALICE results on UPC, WWND 2020 6 Exclusive photoproducon of vector mesons A A γ Vector meson (ρ0, J/ψ, ψ(2S), ...) W t p, A p, A J. G. Contreras, Recent ALICE results on UPC, WWND 2020 7 Exclusive photoproducon of vector mesons Only QED involved here A A The process can be factorised in two parts: ๏ Emission of the photon. γ Vector meson (ρ0, J/ψ, ψ(2S), ...) W t p, A p, A J. G. Contreras, Recent ALICE results on UPC, WWND 2020 7 Exclusive photoproducon of vector mesons Only QED involved here A A The process can be factorised in two parts: ๏ Emission of the photon. γ Vector meson (ρ0, J/ψ, ψ(2S), ...) ๏ InteracAon of the photon with the target. W t p, A p, A QCD is in here J. G. Contreras, Recent ALICE results on UPC, WWND 2020 7 Exclusive photoproducon of vector mesons Only QED involved here A A The process can be factorised in two parts: ๏ Emission of the photon. γ Vector meson (ρ0, J/ψ, ψ(2S), ...) ๏ InteracAon of the photon with the target. W t p, A p, A QCD is in here Cross secon depends on the square of the gluon distribuon. Ryskin: Z. Phys. C 57, 89-92 (1993) (When computed in the leading-log approximaAon of pQCD.) J. G. Contreras, Recent ALICE results on UPC, WWND 2020 7 kinemaAcs: energy and rapidity Centre-of-mass energy of the photon-target system: A A W = 2ω2Ebeam where ๏ Εbeam is the energy of the incoming beam and ๏ ω is the photon energy. γ Vector meson (ρ0, J/ψ, ψ(2S), ...) W t p, A p, A J. G. Contreras, Recent ALICE results on UPC, WWND 2020 8 kinemaAcs: energy and rapidity Centre-of-mass energy of the photon-target system: A A W = 2ω2Ebeam where ๏ Εbeam is the energy of the incoming beam and ๏ ω is the photon energy. γ Vector meson 0 (ρ , J/ψ, ψ(2S), ...) where 2ω = M exp(−y) W ๏ y is the rapidity of the vector meson measured with respect to the direcAon of the target and t ๏ M is the mass of the vector meson. p, A p, A ๏ All frame dependent variables are defined in the laboratory. J. G. Contreras, Recent ALICE results on UPC, WWND 2020 8 kinemaAcs: energy and rapidity Centre-of-mass energy of the photon-target system: A A W = 2ω2Ebeam where ๏ Εbeam is the energy of the incoming beam and ๏ ω is the photon energy. γ Vector meson 0 (ρ , J/ψ, ψ(2S), ...) where 2ω = M exp(−y) W ๏ y is the rapidity of the vector meson measured with respect to the direcAon of the target and t ๏ M is the mass of the vector meson. p, A p, A ๏ All frame dependent variables are defined in the laboratory. The rapidity of the vector meson determines the centre-of-mass energy of the photon-target interacAon J. G. Contreras, Recent ALICE results on UPC, WWND 2020 8 kinemaAcs: transverse momentum A A The square of the momentum transferred in the proton vertex -t is related to the transverse momentum of the J/ψ: 2 γ −t ≈ p⊥ Vector meson (ρ0, J/ψ, ψ(2S), ...) W t p, A p, A J. G. Contreras, Recent ALICE results on UPC, WWND 2020 9 kinemaAcs: transverse momentum A A The square of the momentum transferred in the proton vertex -t is related to the transverse momentum of the J/ψ: 2 γ −t ≈ p⊥ Vector meson (ρ0, J/ψ, ψ(2S), ...) W The -t distribuAon is determined by the nuclear form factor, which gives, through a fourier transform, the distribuAon of of gluons, in the t transverse plane. p, A p, A J. G. Contreras, Recent ALICE results on UPC, WWND 2020 9 kinemaAcs: transverse momentum A A The square of the momentum transferred in the proton vertex -t is related to the transverse momentum of the J/ψ: 2 γ −t ≈ p⊥ Vector meson (ρ0, J/ψ, ψ(2S), ...) W The -t distribuAon is determined by the nuclear form factor, which gives, through a fourier transform, the distribuAon of of gluons, in the t transverse plane.