Ultra-peripheral collisions in ALICE experiment Jaroslav Adam On - PowerPoint PPT Presentation

Ultra-peripheral collisions in ALICE experiment Jaroslav Adam On behalf of ALICE Collaboration Creighton University, Omaha Fermilab, November 1 – 3, 2017 2017 Annual US LHC Users Association Meeting Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 1 / 10

Ultra-peripheral collisions – making L γ HC and L γγ C Impact parameter is larger than the sum of nuclear radii Electromagnetic field of protons and ions behaves like a beam of quasi-real photons Photon beam intensity is proportional to Z 2 Photoproduction in γ p and γ Pb interactions QED processes in γγ interactions The LHC works as a photon-hadron and photon-photon collider Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 2 / 10

Photoproduction of coherent J /ψ in Pb-Pb 1.4 EPS09 HKN ) 1.2 2 =1.69 GeV nDS 1 0.8 2 0.6 (x,Q Pb 0.4 g R In pQCD, coherent cross section 0.2 is proportional to the square of 0 -5 -4 -3 -2 -1 10 10 10 10 10 1 nuclear gluon distribution: x α 2 � d σ ( γ A → J /ψ A ) s Γ ee � 2 16 π 3 � � xg A ( x , Q 2 ) = � 3 α em M 5 d t � t = 0 J /ψ g A ( x , Q 2 ) Amount of nuclear gluon shadowing is given by R A g ( x , Q 2 ) = Ag p ( x , Q 2 ) , the scale is Q 2 = M 2 J /ψ / 4 LHC probes Bjorken- x ∼ 10 − 2 − 10 − 5 , shadowing region with large uncertainties Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 3 / 10

Exclusive J /ψ photoproduction in p-Pb The J /ψ is produced in photon-proton interaction at CM energy W γ p According to HERA the cross section monotonically increases as σ ∝ W δ W γ p γ p � M J /ψ � 2 Bjorken- x of probed gluons is x = W γ p At higher W γ p energies we see more gluons at lower Bjorken- x At some high W γ p there should be enough gluons to interact with themselves, the density should saturate Measurements at different W γ p energies are sensitive to behavior of gluon dis- tribution in protons Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 4 / 10

The ALICE experiment In ALICE we measure J /ψ in decays to lepton pairs, three possible configurations: Leptons in central detector Muon spectrometer and central detector Both muons in muon spectrometer Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 5 / 10

Coherent J /ψ in muon spectrometer c ALICE Preliminary, PbPb s = 5.02 TeV Counts per 0.04 GeV/ NN -1 2 UPC, L 216 b 2.85 < m < 3.35 GeV/ c ≈ µ int µ µ -4.0 < y < -2.5 ALICE data Coherent J/ ψ 3 10 Incoherent J/ ψ Incoherent J/ with nucleon dissociation ψ Coherent J/ from (2s) decay ψ ψ Incoherent J/ from (2s) decay ψ ψ Continuum to γ γ µ µ 2 10 Sum 10 0 0.5 1 1.5 2 2.5 3 Dimuon p (GeV/ c ) T ALI-PREL-115897 ALI−PREL−117573 Clean, high statistics data sample ( ∼ 50 × more J /ψ than in Run 1) Also signal of ψ ( 2 S ) at 3 σ significance Fit to p T distribution around J /ψ mass allows extraction of coherent J /ψ signal Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 6 / 10

Cross section of coherent J /ψ in Pb-Pb Impulse approximation ◮ Assuming no nuclear effects to /dy (mb) ALICE Preliminary Pb+Pb Pb+Pb+J/ s = 5.02 TeV → ψ gluon density NN 10 ALICE data Impulse approximation STARLIGHT , VDM + Glauber σ STARLIGHT d 8 EPS09 LO (GKZ) approach LTA (GKZ) CGC (GM IIM BG) ◮ CPC 212 (2017) 258-268 CGC (GM BCGC LC) 6 CGC (LM IPsat) EPS09 LO (GKZ) , EPS09 gluon parametrization 4 ◮ PRC 93 (2016) 055206 2 LTA (GKZ) , Leading Twist Approximation 0 6 5 4 3 2 1 0 − − − − − − ◮ PRC 93 (2016) 055206 y ALI−DER−117542 CGC (GM)/BCGC , Color dipole model ◮ PRC 90 (2014) 015203, JPG 42 Calculations with moderate gluon shad- (2015) 105001 owing are consistent with the data CGC LM , Color dipole model + (EPS09 LO and CGC LM) IPsat ◮ PRC 83 (2011) 065202, PRC 87 (2013) 032201 Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 7 / 10

Exclusive J /ψ cross section in p-Pb 3 10 +p) (nb) ALICE (p-Pb) Photon-proton energy W γ p is ALICE (Pb-p) Power law fit to ALICE data given by the J /ψ rapidity y : H1 ψ J/ ZEUS W 2 γ p = 2 E p M J /ψ e − y → +p γ 2 ( 10 σ Power-law fit σ ∝ W δ γ p is JMRT LO compatible with HERA results JMRT NLO b-Sat (eikonalized) b-Sat (1-Pomeron) STARLIGHT parameterization pQCD and dipole models 10 2 3 describe the data 10 10 W (GeV) p γ ALI−PUB−89263 So far no change in cross section behavior from HERA to LHC energies (maxi- mum energy increased by more than a factor of 2 with the ALICE data) ALICE: PRL 113 , 232504 (2014) Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 8 / 10

Energy dependence of exclusive J /ψ photoproduction Data over three orders of magnitude in Bjorken- x of probed gluons Bjorken- x − − − − 2 3 4 5 Bjorken- x is given by W γ p 10 10 10 10 3 10 +p) (nb) energy: � 2 ALICE (PRL113 (2014) 232504) � M J /ψ ψ x = J/ W γ p → +p Recent calculations, γ ( σ describing the data: 2 10 NLO BFKL , PRD 94 (2016) 054002 NLO BFKL CGC (IP-Sat, b-CGC) CGC , PRD 90 (2014) × 2 2 3 20 30 40 50 10 2 10 10 054003 W (GeV) γ p ALICE: PRL 113 , 232504 (2014) Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 9 / 10

Summary and outlook Study of photon-hadron and photon-photon collisions – L γ HC and L γγ C Observed moderate gluon shadowing via coherent J /ψ photoproduction Energy dependence of photon-proton cross section measured to energies twice the HERA top energy Measurements on J /ψ in photon-proton interactions in other rapidity intervals from Run 1 close to publication Also UPC cross section measurements on light ρ 0 mesons and γγ to e + e − pairs New data from Run 2, currently being analyzed: ◮ Incoherent J /ψ , polarization studies ◮ J /ψ in p-Pb up to W γ p ∼ 1 . 5 TeV ◮ First UPC data on J /ψ → p ¯ p Stay tuned! Jaroslav Adam (Creighton University) Ultra-peripheral collisions in ALICE experiment Fermilab, November 1 – 3, 2017 10 / 10