Photon induced processes from semi-central to ultraperipheral - - PowerPoint PPT Presentation

SLIDE 1

Photon induced processes from semi-central to ultraperipheral collisions: Introduction

Wolfgang Schäfer 1

1 Institute of Nuclear Physics, PAN, Kraków

COST workshop on Interplay of hard and soft QCD probes for collectivity in heavy-ion collisions Lund University, Sweden, 25. February - 1. March 2019

SLIDE 2

Outline

Peripheral/ultraperipheral collisions Weizsäcker-Williams fluxes of equivalent photons electromagnetic dissociation of heavy nuclei “Soft to hard” in the diffractive photoproduction of vector mesons diffractive dissociation color dipole approach J/ψ photoproduction on the proton Diffractive processes on the nuclear target & multiple scattering expansion Coherent exclusive & incoherent diffraction with breakup of nucleus production in ultraperipheral HI collisions From ultraperipheral to semicentral collisions dileptons from γγ production vs thermal dileptons from plasma phase diffractive J/ψ in semi-central collisions

• A. Łuszczak and W. S., Phys. Rev. C 97 (2018) no.2, 024903 [arXiv:1712.04502 [hep-ph]].
• A. Łuszczak and W. S., arXiv:1901.07989 [hep-ph].
• M. Kłusek-Gawenda, R. Rapp, W. S. and A. Szczurek, Phys. Lett. B 790 (2019) 339

[arXiv:1809.07049 [nucl-th]].

SLIDE 3

Centrality

RA b

e.g. from optical limit of Glauber: dσin

AA

db = 2πb(1 − e−σin

NNTAA(b))

σin

AA ∼ 7 barn for Pb at LHC.

fraction of inelastic hadronic events contained in the centrality class C, fC = 1 σin

AA

bmax

bmin

db dσin

AA

db . experimentally, centrality is determined by binning in multiplicity and/or transverse energy. Probability of no inelastic interaction: Psurv(b) = exp[−σin

NNTAA(b)] ∼ θ(b − 2RA)

SLIDE 4

Fermi-Weizsäcker-Williams equivalent photons

Heavy nuclei Au, Pb have Z ∼ 80

v ∼ 0 v ∼ c equivalent photons

ion at rest: source of a Coulomb field, the highly boosted ion: sharp burst of field strength, with |E|2 ∼ |B|2 and E · B ∼ 0. (See e.g. J.D Jackson textbook). acts like a flux of “equivalent photons” (photons are collinear partons).

E(ω, b) = −i Z√4παem 2π b b2 ωb γ K1

ωb

γ

• ; N(ω, b) = 1

ω 1 π

• E(ω, b)

2

σ(AB) =

• dωd2b N(ω, b) σ(γB; ω)

SLIDE 5

Finite size of particle → charge form factor

bN(ω, b) [fm−1] ω = 1 GeV , γ = 100 2 4 6 8 10 12 14 16 18 20

b [fm]

1e-06 1e-05 1e-04 1e-03 1e-02 1e-01

with formfactor pointlike

E(ω, b) = Z

• 4παem
• d2q

(2π)2 exp[−ibq] q q2 + ω2/γ2 Fem(q2 + ω2/γ2) Fem(Q2) = exp[−R2

chQ2/6] , Q2 ≪

1 R2

ch

.

Seen from a large distance, the ion indeed acts like a pointlike charge. When we come closer, the finite-size charge distribution important. Sometimes its effect is simulated by a sharp lower cutoff in b.

SLIDE 6

Ultraperipheral collisions

some examples of ultraperipheral processes: A A

γ

A A

V γ

A A

V γ

A* A A

γ

A A

γ

A*

γ

photoabsorption on a nucleus diffractive photoproduction with and without breakup/excitation of a nucleus γγ-fusion. electromagnetic excitation/dissociation of nuclei. Excitation of Giant Dipole Resonances. the intact nuclei in the final state are not measured. Each of the photon exchanges is associated with a large rapidity gap. very small pT of the photoproduced system.

SLIDE 7

Absorption corrected flux of photons

A A

V γ

A A

V γ

A*

σ(A1A2 → A1A2V ; s) =

• dωNeff

A1 (ω) σ(γA2 → VA2; 2ω√s) + (1 ↔ 2)

Neff (ω) =

• d2b Psurv(b)N(ω, b)

survival probability: Psurv(b) = S2

el(b) = exp

• − σNNTA1A2(b)
• ∼ θ(|b| − (R1 + R2))

SLIDE 8

Electromagnetic excitation of heavy ions

E (MeV) 10

2

10

3

10

4

10

5

10 (mb)

Pb

208

γ

σ 10

2

10

3

10

A1 A2 A1 A∗

2(E∗ = E)

E A1 A2 A∗

1(E∗ 1 = E1)

A∗

2(E∗ 2 = E2)

E1 E2

¯ nA2(b) ≡

∞

Emin

dE NA1(E, b)σtot(γA2; E) . σtot(A1A2 → A1A∗

2; Emax) ≈

• d2bPsurv(b) exp[−¯

nA2(b)]

Emax

Emin

dE NA1(E, b)σtot(γA2; E) . Huge peak in the photoabsorption cross section – Giant Dipole Resonance.

SLIDE 9

Electromagnetic excitation of heavy ions

(MeV)

γ

E 5 10 15 20 25 30 (mb)

Pb

207

n → Pb

208

γ

σ 100 200 300 400 500 600 700 800

,n) γ ( 1972 1978 1985 1985 1991 1993 ,n+p) γ ,n)+( γ (

Livermore

1964,

Saclay

1970,

(MeV)

γ

E 15 20 25 30 35 (mb)

Pb

206

2n → Pb

208

γ

σ 50 100 150 200 250

,2n) γ ( 2003 2003 ,2n+p) γ ,2n)+( γ (

Livermore

1964,

Saclay

1970,

(MeV)

γ

E 20 25 30 35 40 45 (mb)

Pb

205

3n → Pb

208

γ

σ 5 10 15 20 25 30

,3n) γ ( 1970

Giant dipole resonance decays through emission of few neutrons. experimental data on excitation functions for the reactions γ208Pb → k neutrons + Pb allow us to calculate the fractions f (E, k) of a final state with k = 1, 2, 3 neutrons. we can calculate “topological cross sections” with given numbers of neutrons in the forward region of either ion. Monte Carlo Code “Gemini” for evaporation of neutrons based on Hauser-Feshbach Theory. σ(A1A2 → (mN, X)(kN, Y )) =

• d2b Psurv(b) Pexc

A1 (b, m) Pexc A2 (b, k) .

SLIDE 10

Electromagnetic excitation of heavy ions

(GeV)

NN

s

10

2

10

3

10

(b)

EMD

σ

1 10

2

10

3

10

total 1n 2n

(GeV)

NN

s

10

2

10

3

10

(b)

EMD

σ

1 10

2

10

3

10

total 1n 2n

electromagnetic dissociation cross section for 208Pb. Data from SPS and LHC (ALICE). calculations from M. Kłusek-Gawenda, M. Ciemala, W. S. and A. Szczurek, Phys. Rev. C 89 (2014) 054907. cross section at LHC ∼ 200 barn! these processes play an important role as “triggers” for ultraperipheral processes.

SLIDE 11

Inelastic diffraction: kinematics & t-channel exchanges

Ԃ To bridge a gap (say

✁y ✂ 3) : ✄(0) ✂ 1 (Pomeron, C= +1; Odderon(??), C

= -1).

Ԃ Exchange of secondary Reggeons:

(0)=0.5 for ☎, ✆,f2,a1; (0)=0 for pions

dies out exponentially with the gap size (no exchange of color or charge

• ver a large gap!).

Ԃ Pomeron/Odderon: multigluon exchanges; Reggeons: q q - exchange Ԃ Photons (J=1, C=-1) also qualify!

SLIDE 12

Total photoproduction cross sections

γ*

V

p p

From soft to hard diffraction in the photoproduction of vector mesons. Pomeron intercept depends on the meson...

SLIDE 13

Vector Meson Dominance

γ∗(Q2) V V p p

Extrapolate from the VM-pole to spacelike region: A(γ∗(Q2)p → Vp; W , t) =

• 3Γ(V 0 → e+e−)

MV αem M2

V

Q2 + M2

V

A(Vp → Vp; W , t)

SLIDE 14

Vector Meson Dominance

Extrapolate from the VM-pole to spacelike region: A(γ∗(Q2)p → Vp; W , t) =

• 3Γ(V 0 → e+e−)

MV αem M2

V

Q2 + M2

V

A(Vp → Vp; W , t) hadronic structure of the photon parameters of A(Vp → Vp; W , t) can be taken from πN elastic scattering ℑmA(Vp → Vp; W , t = 0) = s · σtot(Vp) σtot(ρ0p) = σtot(ωp) = 1 2(σtot(π+p) + σtot(π−p)) σtot(φp) = σtot(K +p) + σtot(K −n) − σtot(π+p) works well for photoproduction of ρ, ω, cannot be correct in the deeply spacelike region Q2 ≫ M2

V

connection to QCD degrees of freedom at large Q2 ? heavy flavours ?

SLIDE 15

Color dipole/ k⊥-factorization approach

γ∗(Q2) V r γ∗(Q2) V r Color dipole representation of forward amplitude: A(γ∗(Q2)p → Vp; W , t = 0) =

1

dz

• d2r ψV (z, r) ψγ∗(z, r, Q2) σ(x, r)

σ(x, r) = 4π 3 αS

• d2κ

κ4 ∂G(x, κ2) ∂ log(κ2)

• 1 − eiκr

, x = M2

V /W 2

impact parameters and helicities of high-energy q and ¯ q are conserved during the interaction. scattering matrix is “diagonal” in the color dipole representation. Color dipoles as “Good-Walker states”.

SLIDE 16

When do small dipoles dominate ?

the photon shrinks with Q2 - photon wavefunction at large r: ψγ∗(z, r, Q2) ∝ exp[−εr] , ε =

• m2

f + z(1 − z)Q2

the integrand receives its main contribution from r ∼ rS ≈ 6

• Q2 + M2

V

Kopeliovich, Nikolaev, Zakharov ’93 a large quark mass (bottom, charm) can be a hard scale even at Q2 → 0. for small dipoles we can approximate σ(x, r) = π2 3 r2αS(q2)xg(x, q2) , q2 ≈ 10 r2 for ε ≫ 1 we then obtain the asymptotics A(γ∗p → Vp) ∝ r2

Sσ(x, rS) ∝

1 Q2 + M2

V

× 1 Q2 + M2

V

xg(x, Q2 + M2

V )

probes the gluon distribution, which drives the energy dependence. From DGLAP fits: xg(x, µ2) = (1/x)λ(µ2) with λ(µ2) ∼ 0.1 ÷ 0.4 for µ2 = 1 ÷ 102GeV2.

SLIDE 17

Input to a calculation of J/ψ photoproduction

Overlap of light-cone wave functions Ψ∗

V (z, r)Ψγ(z, r)

= eQ √4παemNc 4π2z(1 − z)

• m2

QK0(mQr)ψ(z, r)

−[z2 + (1 − z)2]mQK1(mQr) ∂ψ(z, r) ∂r

• .

“boosted Gaussian” wave functions as in Nemchik et al. (’94) ψ(z, r) ∝ z(1 − z) exp

• −

M2

QR2

8z(1 − z) − 2z(1 − z)r2 R2

• parameters mQ, R & normalization as in Kowalski et al. (2006) for J/ψ and Cox et al. (2008)

for Υ. diffractive slope on a free nucleon: B = B0 + 4α′ log(W /W0) with W0 = 90 GeV, and α′ = 0.164 GeV−2 . We take B0 = 4.88 GeV−2 for J/ψ and B0 = 3.68 GeV−2 for Υ.

SLIDE 18

Dipole cross section from Xfitter

BGK-form of the dipole cross section σ(x, r) = σ0

• 1 − exp
• − π2r2αs(µ2)xg(x, µ2)

3σ0

• , µ2 = C/r2 + µ2

the soft ansatz, as used in the original BGK model xg(x, µ2

0) = Agx−λg (1 − x)Cg ,

the soft + hard ansatz xg(x, µ2

0) = Agx−λg (1 − x)Cg (1 + Dgx + Egx2),

fit I: BGK fit with fitted valence quarks for σr for H1ZEUS-NC data in the range Q2 ≥ 3.5 GeV2 and x ≤ 0.01. NLO fit. Soft gluon. fit II: BGK fit with valence quarks for σr for H1ZEUS-NC data in the range Q2 ≥ 0.35 GeV2 and x ≤ 0.01. NLO fit. Soft + hard gluon. fits from A. Łuszczak and H. Kowalski, Phys. Rev. D 95 (2017).

SLIDE 19

Corrections for real part and skewedness

numerically important corrections: real part of the diffractive amplitude: σ(x, r) → (1 − iρ(x))σ(x, r) , ρ(x) = tan

π∆I

P

2

• , ∆I

P =

∂ log

• V |σ(x, r)|γ
• ∂ log(1/x)

amplitude is non-forward also in the longitudinal momenta. Correction factor (Shuvaev et al. (1999)): Rskewed = 22∆I

P+3

√π · Γ(∆I

P + 5/2)

Γ(∆I

P + 4)

. apply K-factor to the cross section: K = (1 + ρ2(x)) · R2

skewed .

SLIDE 20

Exclusive diffractive J/ψ photoproduction on the proton

2

10

3

10

W [GeV]

10

2

10

3

10

[nb]

p ψ J/ → p γ

σ

GBW-S IIM BGK-I = 13 TeV) s LHCb ( = 7 TeV) s LHCb ( ALICE H1 ZEUS Fixed target exp.

besides the BGK-fit of Łuszczak & Kowalski, we show to other dipole cross section fits which incorporate heavy quarks:

1

‘IIM’ (Iancu, Itakura & Munier, which is a parametrization inspired by BFKL/BK-asymptotics).

2

a recent re-fit of the Golec-Biernat-Wüsthoff form of the dipole cross section obtained by Golec-Biernat & Sapeta (2018).

the data at high energies were in fact extracted from exclusive diffraction in pp-collisions by LHCb. note: for our applications on nuclear targets, the region of W ∼ 30 ÷ 100 GeV is the most relevant.

SLIDE 21

Diffractive processes on the nuclear target

A A γ* V A γ* V A A γ* V

*

A A

γ*

V

’

diffractive processes on nuclear targets: coherent diffraction – nucleus stays in the ground state complete breakup of the nucleus, final state free protons & neutrons intact nucleus, but an excited state partial breakup of the nucleus, a variety of possible fragments they all have in common: large rapidity gap between vector meson and nuclear fragments lack of production of additional particles

SLIDE 22

Off-forward amplitude

Amplitude at finite transverse momentum transfer ∆ A(γ∗Ai → VA∗

f ; W , ∆) = 2i

• d2B exp[−i∆B]V |A∗

f |ˆ

Γ(b+, b−)|Ai|γ = 2i

• d2B exp[−i∆B]

1

dz

• d2rΨ∗

V (z, r)Ψγ(z, r)A∗ f |ˆ

Γ(B − (1 − z)r, B + zr)|Ai. r = b+ − b−, b = (b+ + b−)/2 , B = zb+ + (1 − z)b− = b − (1 − 2z) r 2

z 1-z B b+

• b

r

SLIDE 23

Coherent diffraction – Glauber averages

A(γ∗Ai → VAi; W , ∆) = 2i

• d2b exp[−ib∆]
• d2rρV γ(r, ∆)Ai|ˆ

Γ(b + r 2, b − r 2)|Ai , ρV γ(r, ∆) =

1

dz exp[i(1 − 2z) r∆ 2 ]Ψ∗

V (z, r)Ψγ(z, r) .

ˆ Γ(b+, b−) = 1 −

A

• i=1

[1 − ˆ ΓNi (b+ − ci, b− − ci)] , in the limit of the dilute uncorrelated nucleus all we need are: M(b+, b−) =

• d2cTA(c)ΓN(b+ − c, b− − c) ≈ 1

2 σ(r)TA(b) Ai|ˆ Γ(b + r 2, b − r 2)|Ai = 1 −

• 1 − 1

A M(b+, b−)

A

≈ 1 − exp[− 1 2σ(r)TA(b)]

SLIDE 24

Incoherent diffraction: summing over nuclear states

dσincoh d∆2 =

• Af =A

dσ(γAi → VA∗

f )

d∆2 . Closure in the sum over nuclear final states:

• A=Af

|Af Af | = 1 − |AA|, dσincoh d∆2 = 1 4π

• d2rd2r′ρ∗

V γ(r′, ∆)ρV γ(r, ∆)Σincoh(r, r′, ∆) ,

Σincoh(r, r′, ∆) =

• d2bd2b′ exp[−i∆(b − b′)]C
• b′ + r′

2 , b′ − r′ 2 ; b + r 2 , b − r 2

• Only ground state nuclear averages:

C(b′

+, b′ −; b+, b−) = A|ˆ

Γ†(b′

+, b′ −)ˆ

Γ(b+, b−)|A − A|ˆ Γ(b′

+, b′ −)|A∗A|ˆ

Γ(b+, b−)|A .

SLIDE 25

Nuclear averages as in Glauber & Matthiae

ˆ Γ(b+, b−) = 1 −

A

• i=1

[1 − ˆ ΓNi (b+ − ci, b− − ci)] , in the limit of the dilute uncorrelated nucleus all we need are: M(b+, b−) =

• d2cTA(c)ΓN(b+ − c, b− − c)

Ω(b′

+, b′ −; b+, b−)

=

• d2cTA(c)Γ∗

N(b′ + − c, b′ − − c)ΓN(b+ − c, b− − c)

C(b′

+, b′ −; b+, b−)

=

• 1 − 1

A

• M∗(b′

+, b′ −) + M(b+, b−)

• + 1

A Ω(b′

+, b′ −; b+, b−)

A

−

• 1 − 1

A M∗(b′

+, b′ −)

• 1 − 1

A M(b+, b−)

A

SLIDE 26

Multiple scattering expansion of the incoherent cross section

Diffraction cone of the free nucleon: B ≪ R2

A

σ(x, r, ∆) = σ(x, r) exp[− 1 2B∆2] Multiple scattering expansion for ∆2R2

A ≫ 1

dσincoh d∆2 =

• n

dσ(n) d∆2 = 1 16π

• n

wn(∆)

• d2bT n

A(b)|In(x, b)|2 ,

wn(∆) = 1 n · n! ·

• 1

16πB

n−1

· exp

• − B

n ∆2 , and In(x, b) = V | σn(x, r) exp[− 1 2σ(x, r)TA(b)]|γ =

1

dz

• d2r Ψ∗

V (z, r)Ψγ(z, r) σn(x, r) exp[− 1

2σ(x, r)TA(b)]

• nuclear absorption

.

SLIDE 27

Diffractive incoherent photoproduction on the nuclear target

1 2 3 4 5 6

]

2

• t [GeV

9 −

10

8 −

10

7 −

10

6 −

10

5 −

10

4 −

10

3 −

10

2 −

10

1 −

10 1 10

2

10

]

• 2

b/GeV µ /dt [ σ d

n=1 n=2 n=3 n=4 n=5 total

ψ Pb, J/

208

(a) W=30 GeV,

1 2 3 4 5 6

]

2

• t [GeV

9 −

10

8 −

10

7 −

10

6 −

10

5 −

10

4 −

10

3 −

10

2 −

10

1 −

10 1 10

2

10

]

• 2

b/GeV µ /dt [ σ d

n=1 n=2 n=3 n=4 n=5 total

ψ Pb, J/

208

(b) W=100 GeV, −t = ∆2 , single scattering has the same diffractive slope as on the free nucleon, multiple scatterings have smaller slopes.

SLIDE 28

Incoherent diffraction at low ∆2

at low ∆2 the single scattering dominates, and one should rather use its exact form: dσincoh d∆2 = 1 16π

• w1(∆)
• d2bTA(b)|I1(x, b)|2 − 1

A

• d2b exp[−i∆b]TA(b)I1(x, b)
• 2
• vanishes for ∆2R2

A≫1

• .

I1(x, b) = V | σ(x, r) exp[− 1 2σ(x, r)TA(b)]

• nuclear absorption

|γ If we were to neglect intranuclear absorption, we would obtain for small ∆2: dσincoh d∆2 = A · dσ(γN → VN) d∆2

• ∆2=0

·

• 1 − F2

A(∆2)

• .

SLIDE 29

Diffractive processes on the nuclear target

0.01 0.02 0.03 0.04 0.05

]

2

• t [GeV

0.5 1 1.5 2 2.5 3

]

• 2

/dt [mb/GeV σ d

ψ Pb, J/

208

W=100 GeV, solid line: exact single scattering dashed: large |t|-limit of single scattering exact result merges into the large |t| limit quickly, the latter is a good approximation in a broad range of t. cross section dips, but does not vanish at t → 0. note: in the small to intermediate t region nuclear correlations may play a role.

SLIDE 30

Diffractive processes on the nuclear target

4 −

10

3 −

10

2 −

10

1 −

10

x

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

incoh

R

Pb

208

blue: Υ, red: J/ψ dashed line: dipole fit I (soft gluon), solid line: dipole fit II (soft+hard gluon) dependence on dipole cross section in its “applicability region” is rather small. nuclear absorption cannot be neglected, even for heavy vector mesons. Rincoh(x) = dσincoh/d∆2 A · dσ(γN → VN)/d∆2 =

• d2bTA(b)
• V |σ(x, r) exp[− 1

2σ(x, r)TA(b)]|γ

• 2

A ·

• V |σ(x, r)|γ
• 2

.

SLIDE 31

Incoherent diffraction in ultraperipheral heavy ion collisions

2 − 1 − 1 2

y

0.2 0.4 0.6 0.8 1 1.2 1.4

/dy [mb] σ d

= 2.76 TeV

NN

s (a)

2 − 1.5 − 1 − 0.5 − 0.5 1 1.5 2

y

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

/dy [mb] σ d

= 5.02 TeV

NN

s (b)

solid line: with skewedness/real part correction dashed line: without corr. data point from ALICE

• Eur. Phys. J. C 73 (2013)

Cross section for AA collision uses Weizsäcker-Williams photon fluxes: dσincoh(AA → VAX) dy = nγ/A(z+)σincoh(W+) + nγ/A(z−)σincoh(W−) , z± = mV √sNN e±y, W± = √z±sNN .

SLIDE 32

Coherent photoproduction of J/ψ in heavy ion collisions

A A

J/ψ J/ψ

A A

γ(ω ) γ(ω )

+

_

p1, p2 = transverse momenta of outgoing ions. Interference induces azimuthal correlation (p1 · p2)/(t1t2). the interference is concentrated at very low pT of J/ψ and can be neglected in rapidity distributions.

SLIDE 33

Energies available for photoproduction

√sNN = 2.76 TeV y W+[GeV] W−[GeV] x+ x− n(ω+) n(ω−) 0.0 92.5 92.5 1.12 · 10−3 1.12 · 10−3 69.4 69.4 1.0 152 56.1 4.13 · 10−4 3.05 · 10−3 39.5 100 2.0 251 34.0 1.52 · 10−4 8.29 · 10−3 14.5 132 3.0 414 20.6 5.59 · 10−5 2.25 · 10−2 1.68 163 3.8 618 13.8 2.51 · 10−5 5.02 · 10−2 0.03 188

Table: Subenergies W± and Bjorken-x values x± for √sNN = 2.76 TeV for a given rapidity y. Also shown are photon fluxes n(ω±).

√sNN = 5.02 TeV y W+[GeV] W−[GeV] x+ x− n(ω+) n(ω−) 0.0 125 125 6.17 · 10−4 6.17 · 10−4 87.9 87.9 1.0 206 75.6 2.27 · 10−4 1.68 · 10−3 57.2 119 2.0 339 45.9 8.35 · 10−5 4.56 · 10−3 28.5 150 3.0 559 27.8 3.07 · 10−5 1.24 · 10−2 7.5 181 4.0 921 16.9 1.13 · 10−5 3.37 · 10−2 0.35 213 4.8 1370 11.3 5.08 · 10−6 7.50 · 10−2 0.001 238

Table: Subenergies W± and Bjorken-x values x± for √sNN = 5.02 TeV for a given rapidity y.

SLIDE 34

Coherent photoproduction of J/ψ in heavy ion collisions

4 − 3 − 2 − 1 − 1 2 3 4

y

1 2 3 4 5 6 7 8

/dy [mb] σ d

= 2.76 TeV

NN

s

ALICE CMS GBW-S IIM BGK-I

reasonable description of experimental data. the highest γN energy at y = 0, about W = 100 GeV. explicit higher Fock states, c¯ cg, c¯ cgg...?

SLIDE 35

Coherent photoproduction of J/ψ in heavy ion collisions

5 − 4 − 3 − 2 − 1 −

y

1 2 3 4 5 6 7 8

/dy [mb] σ d

= 5.02 TeV

NN

s

ALICE preliminary GBW-S IIM BGK-I

1 2 3 4 5

y

1 2 3 4 5 6 7

/dy [mb] σ d

=5 TeV

NN

s Pb-Pb GBW-S IIM BGK-I

LHCb Preliminary

Preliminary data from ALICE & LHCb.

SLIDE 36

Dilepton production in semi-central collisions

A A

γ γ

Dileptons are a “classic” probe of the QGP. medium modifications of ρ, thermal dileptons dileptons from γγ fusion have peak at very low pair transverse momentum. can they be visible even in semi-central collisions?

SLIDE 37

Dilepton production in semi-central collisions

b b b 1 2 e+e- A A 1 2

dσll dξd2b =

• d2b1d2b2 δ(2)(b − b1 − b2)N(ω1, b1)N(ω2, b2) dσ(γγ → l+l−; ˆ

s) d(−ˆ t) , where the phase space element is dξ = dy+dy−dp2

t with y±, pt and ml the single-lepton rapidities,

transverse momentum and mass, respectively, and ω1 =

• p2

t + m2 l

2 (ey+ + ey−) , ω2 =

• p2

t + m2 l

2 (e−y+ + e−y−) , ˆ s = 4ω1ω2 . we adopt the impact parameter definition of centrality, of course... dNll[C] dM = 1 fC · σin

AA

bmax

bmin

db

• dξ δ(M − 2√ω1ω2) dσll

dξdb

• cuts

,

SLIDE 38

Dilepton production: impact parameter distribution

semi-central collisions are situated on the left side of the distribution, below b < 15fm. starting from RHIC energies, the contribution from coherent photons is practically energy-independent. also notice the long tails of the ultraperipheral part. Their importance rises with energy.

SLIDE 39

Dilepton production in semi-central collisions

1 2 3 4

(GeV)

• e

+

e

M

11 −

10

10 −

10

9 −

10

8 −

10

7 −

10

6 −

10

5 −

10

4 −

10

3 −

10

2 −

10

1 −

10 1 10

2

10

(1/GeV)

• e

+

e

/dM

• e

+

e

dN

STAR-200GeV Au: 60-80%

• 2

10 × 40-60%

• 4

10 × 10-40%

• e

+

e → γ γ + QGP ρ in-medium cocktail 0.2 0.4 0.6 0.8 1

(GeV)

T

P

11 −

10

10 −

10

9 −

10

8 −

10

7 −

10

6 −

10

5 −

10

4 −

10

3 −

10

2 −

10

1 −

10 1 10

2

10

3

10

(1/GeV)

T

/dP

• e

+

e

dN

STAR-200GeV Au: 0.40-0.76 GeV

• 2

10 × 0.76-1.2 GeV

• 4

10 × 1.20-2.6 GeV

• e

+

e → γ γ + QGP ρ in-medium cocktail

• M. Kłusek-Gawenda, R. Rapp, W.S. & A. Szczurek, Phys.Lett. B790 (2019)

electron pair PT < 150 MeV: dileptons from coherent photons dominate over a large range of centralities.

• ther mechanisms: medium modified ρ, thermal dileptons, Dalitz-decays (“cocktail”).

SLIDE 40

From ultraperipheral to peripheral nuclear collisions

Recently, the ALICE collaboration has observed a large enhancement of J/ψ mesons carrying very small pT < 300 MeV in the centrality classes corresponding to peripheral collisions. Centrality class 70 ÷ 90%: 13 fm < b < 15 fm, photon fluxes by Contreras Phys. Rev. C 96 (2017) dσincoh(AA → VX|70 ÷ 90%) dy = nγ/A(z+|70 ÷ 90%)σincoh(W+|pT < pcut

T

) + nγ/A(z−|70 ÷ 90%)σincoh(W−|pT < pcut

T

) ≈ 15 µb , The ALICE measurement is [Phys. Rev. Lett. 116 (2016)]: dσ(AA → VX|70 ÷ 90%; 2.5 < |y| < 4.0) dy = 59 ± 11 ± 8 µb . For an estimate of the coherent contribution, see: M. Kłusek-Gawenda and A. Szczurek, Phys.

• Rev. C 93 (2016), See talk by Antoni Szczurek on Friday.

SLIDE 41

Instead of a summary

Even when nuclei don’t touch each other, they have very large inelastic cross sections. EM dissociation ∼ 200 barn at LHC. Ultraperipheral heavy ion collisions give access to a lot of interesting processes. Photoproduction of J/ψ tells us about interaction of small dipoles with nuclear medium, potentially about the nuclear gluon distribution. Certain properties/phenomena can even carry over into the semi-central domain. Their exploration has just begun.