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 LγHC and LγγC

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 γp and γPb 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

In pQCD, coherent cross section is proportional to the square of nuclear gluon distribution:

x

• 5

10

• 4

10

• 3

10

• 2

10

• 1

10 1 0.2 0.4 0.6 0.8 1 1.2 1.4 EPS09 HKN nDS

)

2

=1.69 GeV

2

(x,Q

Pb g

R

dσ(γA → J/ψA) dt

• t=0

= α2

sΓee

3αemM5

J/ψ

16π3 xgA(x, Q2) 2 Amount of nuclear gluon shadowing is given by RA

g (x, Q2) = gA(x,Q2) Agp(x,Q2),

the scale is Q2 = M2

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 Wγp According to HERA the cross section monotonically increases as σ ∝ W δ

γp

Bjorken-x of probed gluons is x = MJ/ψ

Wγp

2 At higher Wγp energies we see more gluons at lower Bjorken-x

Wγp

At some high Wγp there should be enough gluons to interact with themselves, the density should saturate Measurements at different Wγp 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

ALI-PREL-115897

) c (GeV/

T

p Dimuon 0.5 1 1.5 2 2.5 3 c Counts per 0.04 GeV/ 10

2

10

3

10

= 5.02 TeV

NN

s ALICE Preliminary, PbPb

• 1

b µ 216 ≈

int

L UPC,

2

c < 3.35 GeV/

µ µ

2.85 < m

• 4.0 < y < -2.5

ALICE data ψ Coherent J/ ψ Incoherent J/ with nucleon dissociation ψ Incoherent J/ (2s) decay ψ from ψ Coherent J/ (2s) decay ψ from ψ Incoherent J/ µ µ to γ γ Continuum Sum ALI−PREL−117573

Clean, high statistics data sample (∼50× more J/ψ than in Run 1) Also signal of ψ(2S) at 3σ significance Fit to pT 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

y

6 − 5 − 4 − 3 − 2 − 1 −

/dy (mb) σ d

2 4 6 8 10 STARLIGHT Impulse approximation EPS09 LO (GKZ) CGC (LM IPsat) LTA (GKZ) CGC (GM IIM BG) CGC (GM BCGC LC) ALICE data

= 5.02 TeV

NN

s ψ Pb+Pb+J/ → ALICE Preliminary Pb+Pb

ALI−DER−117542

Calculations with moderate gluon shad-

• wing are consistent with the data

(EPS09 LO and CGC LM) Impulse approximation

◮ Assuming no nuclear effects to

gluon density

STARLIGHT, VDM + Glauber approach

◮ CPC 212 (2017) 258-268

EPS09 LO (GKZ), EPS09 gluon parametrization

◮ PRC 93 (2016) 055206

LTA (GKZ), Leading Twist Approximation

◮ PRC 93 (2016) 055206

CGC (GM)/BCGC, Color dipole model

◮ PRC 90 (2014) 015203, JPG 42

(2015) 105001

CGC LM, Color dipole model + 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

(GeV)

p γ

W

2

10

3

10 +p) (nb) ψ J/ → +p γ ( σ 10

2

10

3

10 ALICE (p-Pb) ALICE (Pb-p) Power law fit to ALICE data H1 ZEUS JMRT LO JMRT NLO b-Sat (eikonalized) b-Sat (1-Pomeron) STARLIGHT parameterization

ALI−PUB−89263

Photon-proton energy Wγp is given by the J/ψ rapidity y: W 2

γp = 2EpMJ/ψe−y

Power-law fit σ ∝ W δ

γp is

compatible with HERA results pQCD and dipole models describe the data 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

(GeV)

p γ

W 20 30 40 50

2

10

2

10 × 2

3

10 +p) (nb) ψ J/ → +p γ ( σ

2

10

3

10

5 −

10

4 −

10

3 −

10

2 −

10 x Bjorken-

ALICE (PRL113 (2014) 232504) NLO BFKL CGC (IP-Sat, b-CGC)

Bjorken-x is given by Wγp energy: x = MJ/ψ Wγp 2 Recent calculations, describing the data: NLO BFKL, PRD 94 (2016) 054002 CGC, PRD 90 (2014) 054003

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 – LγHC and LγγC 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 ρ0 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 Wγp ∼ 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