Electromagnetic fragmentation of nuclei at heavy-ion colliders Igor - - PowerPoint PPT Presentation

Igor Pshenichnov

Institute for Nuclear Research, Russian Academy of Sciences 117312 Moscow, Russia pshenich@inr.ru

Z

Electromagnetic fragmentation of nuclei at heavy-ion colliders

EMIN-2012 20-23 September 2012 Moscow

Ultraperipheral collisions at RHIC and LHC: the impact of strong Coulomb fields on nuclei

• Heavy nuclei Z2 ~ 802, strong Coulomb fields
• Tremendous Lorentz-contraction in colliders:
• for LHC:

“An Intercity train compressed to the thickness of a sheet of paper”

v

central ultra- peripheral peripheral

v v

Content

• Basics of the Weizsäcker-Williams (WW) method of equivalent

photons used to model ultraperipheral collisions of nuclei and its implementation in the RELDIS model (stands for Relativistic ELectromagnetic DISsociation)

• Single ElectroMagnetic Dissociation (EMD). Comparison with

experimental data from CERN SPS

• Mutual EMD: can be studied only at heavy-ion colliders and used to

study multiple excitations of nuclei. Comparison with first LHC data

• Practical issues: luminosity monitoring in collider and heating of LHC

components due to EMD

• Electromagnetic physics in pPb collisions at LHC in 2012-2013
• RELDIS predictions for CuAu and UU collisions at RHIC, AuAu

collisions at NICA

b>R1+R2 A1, Z1 A2, Z2

=

A2, Z2 b

v NZ1(E1,b)

=

A2, Z2 P1 P2

Weizsäcker-Williams method of equivalent photons

Enrico Fermi 1924: “äquivalente strahlung” C.F. Weizsäcker & E.J. Williams 1930s: pair production by high-energy photons from charge particles

I.P., Phys. Part. Nuclei 42(2011)215

Projectile nucleus: A1, Z1 Target nucleus: A2, Z2

small b, short pulse

E(t)

 t~b/ v

t

I(w)



Field strength Frequency spectrum

• f the pulse P1

max~ v /b

large b, long pulse

E(t)

 t~b/ v

t  max~ v /b

Soft photons Soft and hard photons

v v I(w)

Target nucleus: A2, Z2 Projectile nucleus: A1, Z1

=1−v

2/c 2 −1/2

How the spectrum of equivalent photons is build

I.P., Phys. Part. Nuclei 42(2011)215

Spectrum of Weizsäcker-Williams photons

Spectrum of equivalent photons from a nucleus , as seen by a nucleus in a collision with impact parameter :

• fine structure constant
• modified Bessel functions

Average number of photons absorbed by the nucleus

• total photoabsorption cross sections for the nucleus

Absorption of equivalent photons by nuclei

• C. Scheidenberger, I.P. et al., Phys. Rev. Lett. 88 (2002)042301

Giant resonances, (e.g. GDR), Eγ<30 MeV

• Quasideuteron absorption:

γ + (pn) -> p + n, Eγ<140 MeV

• Photoexcitation of single nucleons:

∆ and other baryonic resonances

• Multiple meson production

We need a good model to describe all that!

Photoabsorption on lead: a variety of processes

p n GDR p n γ+(np)-->n+p p π+ p π0 π− Eγ (MeV) GDR QD

multiple pions

∆ All such processes are under discussion at EMIN2012!

Nuclear excitations above the GDR region

channels with emission of

• nly protons “0n

channels” (~3% of the total single EMD) can be studied for the first time

p n γ+(np)-->n+p hadronic fragmentation Electromagnetic fragmentation dominates in ∆Z=-2, -1, 0, +1 channels Seen at SPS energies:

I.P. et al., Phys. Rev. С 70(2004)014902

PbPb at LHC

Kinematics of photon emission

Photon is emitted coherently by all charges in the nucleus, they are all inside the radius R. The nucleus is left in its ground state. Therefore, the square of 4-momentum is restricted: Photons are almost real compared to photons emitted in (e,e') reactions. The data from photonuclear experiments can be used. Photon 4-momentum: Assume that an ultrarelativitic nucleus is left in its ground state after emission and only a small part of nucleus' kinetic energy is taken away. Together with the coherence condition this gives:

LHC: RHIC:

Single electromagnetic dissociation

Single dissociation: only fragmentation of one beam is detected. The state of a counter-rotating nucleus is not traced: it may also fragment (mutual event) or left intact.

NLO contribution: 1-2% LO contribution: 98-99%

Single electromagnetic dissociation cross section for specific channels:

Leading order (LO) cross section of A2 dissociation into a channel i:

• total photoabsorption cross section for A2
• branching ratio for decay of nucleus A2 into the channel i

It is calculated by the Monte Carlo method Example follows: neutron emission in EMD

Photoneutron cross sections measured in different laboratories diverge. Evaluated nuclear data have to be used.

A 7% correction to the total cross section is proposed. It is adopted in RELDIS. Used as uncertainty estimation. Tables of total cross section tables used as an input, statistical model is employed for calculating 1n, 2n etc. rates.

Single EMD on various targets calculated by RELDIS and FLUKA for 30A GeV Pb and INR-Turin data

G.I. Smirnov, AMT workshop: Beam generated heat deposition and quench levels for LHC magnets, CERN, March 2005

• Points – ALICE-lumi experiment
• FLUKA – green lines
• RELDIS – red lines

Data: M.B. Golubeva, …, I.P. et al., Phys. Rev. C 71(2005)024905

Characteristic Z2

target

dependence of the cross section EM fragmentation ~ Z2

target

Only forward neutrons were detected: selection of EMD kinematics

Electromagnetic processes dominate in production of heavy secondary fragments in interactions of nuclei with γ>>10

hadronic fragmentation Electromagnetic fragmentation dominates in ∆Z=-2, -1, 0, +1 channels

Data: H. Dekhissi et al., NPA662 (2000) 207 I.P. et al., Phys. Rev. С 70(2004)014902

Emission of protons in EMD

Mutual electromagnetic dissociation

Mutual electromagnetic dissociation: both nuclei disintegrate in a single collision event

One photon absorbed on average in close collisions: b~bc~R1+R2 A subset of collision events: mutual excitation possible, LO: 3.75 b @1.38+1.38 A TeV Three and four photons as well, 1.4 b and 0.21 b

Mutual electromagnetic dissociation - LO

Mutual electromagnetic dissociation (LO) with fragmentation of A1 and A2 into channels i and j, respectively: Each photon emission can be considered independently of others : While the total energy of the nucleus which emits photon: So, the ratio Both the energy and momentum of emitter are not noticeably changed. The sequence of photon emission is not important (no time ordering). Photon energy is limited: is quite small for heavy nuclei!

Mutual electromagnetic dissociation: LO and NLO cross sections

Total cross sections of various orders and the total one.

A.J. Baltz, …, I.P. et al., Phys. Reports 458(2008)1

Contributions of multiple excitations to mutual dissociation

Triple excitations not yet discovered!

2.75+2.75 A TeV PbPb @ LHC Cross section (b) LO 3.92 NLO12+NLO21 1.50 NLO22 0.23 Triple excitations 0.56 Total 6.21

A.J. Baltz, …, I.A.P. et al.,

• Phys. Reports 458(2008)1

With ZDCs one can study nuclear structure effects: multiple collective excitations of colliding nuclei.

2.75+2.75 A TeV PbPb @ LHC Cross section (b) LO 3.92 NLO12+NLO21 1.50 NLO22 0.23 Triple excitations 0.56 Total 6.21

Multiple collective excitations of nuclei happen at relatively small b

A.J. Baltz, …, I.P. et al.,

• Phys. Reports 458(2008)1

I.P., Proc. EMIN-2003, p.234

Coulomb excitation of double Giant Dipole Resonances in low-energy nucleus-nucleus collisions: a conventional method

Example: the excitation of double GDR in

208Pb nucleus by 1A GeV 209Bi:

• nly a small structure in photon spectra.

It is very difficult to see triple excitations by this method . Double excitation in single electromagnetic dissociation ~1%, triple ~0.01%

Recent measurements at the LHC

Single and mutual EMD as seen by ALICE collaboration in ultraperipheral PbPb collisions

(0n,1n) (1n,0n) (2n,2n) (1n,2n) (1n,1n) (2n,0n) Event examples: Single EMD in green Mutual EMD in red ALICE Collaboration, arXiv:1203.2436v2 and PRL 2012, accepted 1.38A+1.38A TeV

Note: 1n+2n+3n seem to be overpredicted. Will 4n, 5n, 6n rates (not yet measured) be then underpredicted? Is it an indication for enhanced multiple excitations?

ALICE data for 1n, 2n and 3n yields in single EMD of Pb nuclei at 1.38+1.38 A TeV

ALICE Collaboration, arXiv:1203.2436v2 and PRL 2012, accepted

RELDIS agrees well with ALICE data for the total EMD sections for 1.38+1.38 A TeV PbPb

These cross sections (barns) were measured in a dedicated run by Van der Meer scan. ALICE Collaboration, arXiv:1203.2436v2 and PRL 2012, accepted (b) (b)

RELDIS describes data within six orders of magnitude of the Lorentz contraction of Coulomb field

SPS LHC More on the importance

• f SPS data for this

success in the talk by

• E. Karpechev

ALICE Collaboration, arXiv:1203.2436v2 and PRL 2012, accepted

Practical issues:

• LHC luminosity monitoring with mutual

dissociation events

• Estimations of heat load on LHC

components due to secondary nuclei resulting from electromagnetic interactions

Monitor LHC luminosity by detecting mutual evens

• A. Morsch & I.A. Pshenichnov, “LHC Experimental Conditions”,

ALICE Internal Note 2002-034 and ALICE PPR (results for nominal LHC energy)

Luminosity L=Rm

ED/σm ED

Rm

ED - mutual events rate measured with ZDCs

σm

ED - mutual dissociation cross section (theory) – should be reliable !

The sum of mutual 1n and 2n emission cs (LMN) is quite stable:

1.38 + 1.38 A TeV PbPb mutual dissociation cross sections (b) RELDIS

Pdirn = 0

(only statistical GDR decay)

RELDIS

Pdirn = 0.26

(statistical GDR decay + direct 1n emission)

(1nX|1nY ) 0.711 0.791 (1nX|2nY ) + (2nX|1nY ) 0.504 0.460 (2nX|2nY ) 0.089 0.068 Sum of 1n and 2n: 1.304 1.319

P.Cortese, et al., J. Phys. G Nucl. Part. Phys. 30(2004)1517, for 2.75A+2.75A TeV

Heat load exceeds acceptable limits by 40% at the maximum (design) luminosity. Additional collimators/masks may help.

Precise modeling of Pb beam loss in LHC: recent results from AB/ABP

Pb beam losses in LHC: BFPP, 1n and 2n electromagnetic fragmentation

Z1 + Z2 --> (Z1 + e-)1s1/2 + Z2 + e+ (~280 b) EM fragmentation 1n (EMD1) (~100 b) 2n (EMD2) (~30 b) 207Pb82+ are intercepted by existing collimators, but not 206Pb82+ Risk to induce quenches of a magnet after IP2 (ALICE) by impact of 206Pb82+ resulting from electromagnetic 2n loss.

Example of 206Pb created by 2-neutron EMD

J.M. Jowett, ALICE Forum, 20/7/2011

Green rays are ions that almost reach collimator Blue rays are 206Pb nuclei with rigidity change

Primary collimator

Beam pipe in IR7 (beam cleaning) of LHC

First pPb collisions are seen at LHC in September, 2012

• p (4 TeV) + Pb (1.577A TeV)
• Pb dissociation cross section scales as Z2 of the projectile,

but still noticeable, ~34 mb, about 1% of the total hadronic cross section

• Photoexcitation of proton by virtual photons can be also

seen ~390 mb, e.g. by detecting neutrons from γ p → π+ n

v p Pb

CuAu collisions at RHIC

100 A GeV + 100 A GeV beams AuAu CuAu (copper dissociation) AuCu (gold dissociation) Total single EMD cross section (barns) 95. 22.5 13.37 EMD cross sections are still much larger than the total hadronic cross sections.

A special case: UU collisions at RHIC

96.5 A GeV + 96.5 A GeV beams AuAu UU Total single EMD cross section (barns) 94.15 150.1 Total mutual EMD cross section (barns) 3.79 7.59

Electromagnetically induced fission of uranium beam nuclei is expected to be frequent

RELDIS predictions for NICA: single EMD

3.5+3.5 A GeV AuAu @ NICA Cross section (barns) Total 23.9

196Au + n

16.5

195Au + 2n

3.9

194Au + 3n

0.95

193Au + 4n

0.31

EMD cross section (~24 b) three times as large as hadronic interaction cross section (~7 барн) and affects the beam life-time in the collider. The probability to produce fragments similar in mass and charge to beam nuclei in forward direction (Pt< 200 MeV/c) is large. Where do they come? Will they hit ZDC, FS, beam pipe, collimators?

RELDIS predictions for NICA: mutual EMD

3.5+3.5 A GeV AuAu @ NICA Cross section (mb) LO 1374 NLO12+NLO21 357 NLO22 33 Triple excitations 62 Total 1826

Can be used to monitor collider luminosity. Multiple excitations of Au nuclei ~ 450 mb

Conclusions:

• RELDIS model describes well the first data on electromagnetic

fragmentation of lead nuclei at the LHC.

• Production of 206Pb nuclei via 2n emission in ultraperipheral

collisions have to be quantified: risks of LHC magnet quenching.

• It is expected that the mutual impact of highly Lorentz-contracted

Coulomb fields in ultraperipheral PbPb collisions leads to collective excitations of nuclei, including multiple GDR excitations.

• pPb at LHC, CuAu and UU collisions at RHIC, AuAu at NICA

can be also studied with RELDIS

• The contribution of multiple GDR excitations in mutual AuAu

EMD events at NICA is expected to be noticeable.

Many thanks to organizes for providing me the possibility to give this talk. Thank you for your attention!

Back-up slides

Total photoabsorption cross section on nuclei

M.V.Kossov, Eur. Phys. J. A 14, (2002) 377

RELDIS model developed at INR, NBI, GSI, FIAS (25+ papers published since 1995)

P.Golubev, …, I.P. et al.,

• Nucl. Phys. A 806(2008)216

A.S. Iljinov, …, I.P. et al.,

• Nucl. Phys. A 616(1997)575

π0 π+ π- π+

I.P. et al.,

• Eur. J. Phys. A

24(2005)69

• C. Scheidenberger,

I.P. et al.,

• Phys. Rev. С

70(2004)014902

pion photoproduction fission of heavy nuclei by photons Electromagnetic dissociation at SIS (GSI), AGS (BNL), CERN SPS

Pion production by WW photons: examples

p π+ p π0 π−

Meson photoproduction on nucleons

Contribution of electromagnetic fragmentation is shown in red. Protons are lost – nuclear charge decreases Also nuclei with ∆Z=+1 are seen: 83Bi produced from 82Pb. Proton pickup, charge-exchange ore

• ther mechanism?
• C. Scheidenberger, I.P. et al., Phys. Rev. С 70(2004)014902

EM vs hadronic

Hadronic and electromagnetic contributions to Pb fragmentation

• n collimators in LHC

copper as collimator material EM contribution is important for ∆Ζ=0,−1,−2

Single electromagnetic dissociation for 1.38+1.38 A TeV PbPb

Cross section (barns) electromagnetic RELDIS hadronic Abrasion- ablation model em+hadronic A.J. Baltz, S.N. White em+hadronic

σ(1n)

94.1 0.45 94.9 97.

σ(1nX)

98. 0.76 98.8

σ(2n)

18.3 0.14 18.4

σ(2nX)

23.1 0.41 23.4

σ(xn)

185.2 7.7 192.9 202.6

Note: “hadronic” struck neutrons are less forward peaked – easily missed by ZDCs Note: Baltz's model did not use a correction factor of 0.93 proposed by photonuclear data evaluators, see I.P. et al., PRC 64(2001)024903

Mutual electromagnetic dissociation of 1.38+1.38 A TeV PbPb

Cross section barns electromagnetic RELDIS hadronic Abrasion- ablation model em+hadronic A.J. Baltz, S.N. White em+hadronic

σ(1n|1n)

0.559

σ(1nX|1nY)

0.71

σ(1n|xn)

1.86 1.87

σ(xn|xn)

5.9 7.7 13.6 14.0

In this table: xn = 0n, 1n, 2n, ... etc.

Dependence of ∆Z=+1 cross section on target charge at various energies

Cross section ~Z2

target

at 158A GeV (CERN SPS): electromagnetic interactions Weak dependence on Ztarget at 10.6A GeV (AGS): peripheral hadronic collisions, hadronic charge-exchange process

• C. Scheidenberger, I.P. et al.,
• Phys. Rev. Lett. 88 (2002)042301

Creation of 83Bi on 208

82Pb by impact of

energetic equivalent photons

Calculations with RELDIS for Eγ=190, 260, 520 and 960 MeV γ n -> p π− π− photoproduction, Proton is captured by the nucleus, but π− leaves it γ

p n

π−

• C. Scheidenberger, I.P. et al.,
• Phys. Rev. С 70(2004)014902

With EM ∆Z=+1 process former approximations have to be revisited

Tsao, Silberberg, Barghouty,

• Astrophys. Journ. 501 (1998) 920

EM process ∆Z=+1 have to be taken into acount in interactions of medium-weight and heavy nuclei with γ >> 10

Data: Geer'95, Cummings'90, Waddington'00

• C. Scheidenberger, I.P. et al.,
• Phys. Rev. С 70(2004)014902

Evolution of the average excitation energy of the residual nucleus

• <E*> increases as Eγ

increases

• But the fraction of Eγ

converted to <E*> goes down

• <E*>/ARN < 1.5 MeV,

mostly evaporation and fission for heavy nuclei

I.P. et al., Eur. J. Phys. A 24(2005)69

Comparison to RHIC data

A.J. Baltz, …, I.P. et al., Phys. Reports 458(2008)1

RELDIS Baltz et al. Experimental data

RELDIS and STARLIGHT-DPMJET: charged particles in mid-rapidity

STARLIGHT-DPMJET Eγ > 6 GeV

I.P. et al., PRC 60 (1999) 044901 Djuvsland & Nystrand PRC 83 (2011) 041901(R)

RELDIS

π+, π-, p

all charged Model comparison in progress Expected to work better with hard photons

Low neutron multiplicities due to soft photons with Eγ<5 GeV

Large neutron multiplicities result from absorption of high-energy photons. However, 1n-5n are mostly due to soft photons: safe to use RELDIS

A.J. Baltz, …, I.P. et al.,

• Phys. Reports 458(2008)1

Heat load in LHC at IR3 (beam cleaning system) calculated with different fragmentation models, R. Bruce et al. RELDIS + abr-abl: <2.5 W/m FLUKA <2 W/m

RELDIS vs FLUKA. EM at LHC collision energy on carbon (collimator material)