Electromagnetic fragmentation of nuclei at heavy-ion colliders Igor Pshenichnov Institute for Nuclear Research, Russian Academy of Sciences 117312 Moscow, Russia Z pshenich@inr.ru EMIN-2012 20-23 September 2012 Moscow
Ultraperipheral collisions at RHIC and LHC: the impact of strong Coulomb fields on nuclei v v v ultra- peripheral central peripheral ● Heavy nuclei Z 2 ~ 80 2 , strong Coulomb fields ● Tremendous Lorentz-contraction in colliders: “An Intercity train compressed ● for LHC: to the thickness of a sheet of paper”
Content ● Basics of the W eizsäcker- W illiams ( WW ) method of equivalent photons used to model ultraperipheral collisions of nuclei and its implementation in the RELDIS model (stands for R elativistic EL ectromagnetic DIS sociation) ● Single E lectro M agnetic D issociation ( 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
Weizsäcker-Williams method of equivalent photons Enrico Fermi 1924: “äquivalente strahlung” C.F. Weizsäcker & A 1 , Z 1 E.J. Williams 1930s: b>R 1 +R 2 pair production by v high-energy photons = = from charge particles P 2 b N Z1 (E 1 ,b) A 2 , Z 2 P 1 A 2 , Z 2 A 2 , Z 2 I.P., Phys. Part. Nuclei 42(2011)215
How the spectrum of equivalent photons is build Frequency spectrum Projectile nucleus: Field strength of the pulse P 1 A 1 , Z 1 v Soft and hard I(w) E(t) small b , photons short pulse t ~ b / v 2 / c 2 − 1 / 2 = 1 − v t max ~ v / b Target nucleus: A 2 , Z 2 Projectile nucleus: A 1 , Z 1 v I(w) E(t) Soft photons t ~ b / v large b , long pulse t max ~ v / b Target nucleus: A 2 , Z 2 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 Giant resonances, (e.g. GDR), E γ <30 MeV - Quasideuteron absorption: γ + (pn) -> p + n , E γ <140 MeV - Photoexcitation of single nucleons: ∆ and other baryonic resonances - Multiple meson production We need a good model to describe all that! C. Scheidenberger, I.P. et al., Phys. Rev. Lett. 88 (2002)042301
Photoabsorption on lead: a variety of processes ∆ multiple pions QD GDR E γ (MeV) GDR n γ +(np)-->n+p π + n p π 0 p π − p p All such processes are under discussion at EMIN2012!
Nuclear excitations above the GDR region Seen at SPS energies: Electromagnetic PbPb at LHC fragmentation dominates in channels with emission of ∆ Z=-2, -1, 0, +1 only protons “0n channels channels” (~3% of the total single EMD) can be studied for the first time γ +(np)-->n+p n p hadronic fragmentation I.P. et al., Phys. Rev. С 70(2004)014902
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 A 2 dissociation into a channel i : - total photoabsorption cross section for A 2 - branching ratio for decay of nucleus A 2 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 EM fragmentation ~ Z 2 target ● Points – ALICE-lumi experiment ● FLUKA – green lines ● RELDIS – red lines G.I. Smirnov, AMT workshop: Beam generated heat deposition and quench levels for LHC magnets, CERN, March 2005 Characteristic Z 2 target dependence of the cross section Only forward neutrons were detected: selection of EMD kinematics Data: M.B. Golubeva, …, I.P. et al., Phys. Rev. C 71(2005)024905
Electromagnetic processes dominate in production of heavy secondary fragments in interactions of nuclei with γ >>10 Electromagnetic fragmentation dominates in ∆ Z=-2, -1, 0, +1 channels hadronic fragmentation I.P. et al., Phys. Rev. С 70(2004)014902 Data: H. Dekhissi et al., Emission of protons in EMD NPA662 (2000) 207
Mutual electromagnetic dissociation
Mutual electromagnetic dissociation: both nuclei disintegrate in a single collision event One photon absorbed on A subset of collision Three and four average in close collisions: events: mutual excitation photons as well, possible, LO: 3.75 b b~b c ~R 1 +R 2 1.4 b and 0.21 b @1.38+1.38 A TeV
Mutual electromagnetic dissociation - LO Mutual electromagnetic dissociation (LO) with fragmentation of A 1 and A 2 into channels i and j, respectively: Each photon emission can be considered independently of others : Photon energy is limited: While the total energy of the nucleus which emits photon: is quite small for heavy nuclei! 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).
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 2.75+2.75 A TeV Cross section PbPb @ LHC (b) LO 3.92 NLO 12 +NLO 21 1.50 0.23 NLO 22 Triple excitations 0.56 Total 6.21 Triple excitations not yet discovered! A.J. Baltz, …, I.A.P. et al., Phys. Reports 458(2008)1
Multiple collective excitations of nuclei happen at relatively small b 2.75+2.75 A TeV Cross section PbPb @ LHC (b) LO 3.92 NLO 12 +NLO 21 1.50 NLO 22 0.23 Triple excitations 0.56 Total 6.21 With ZDCs one can study nuclear structure A.J. Baltz, …, I.P. et al., effects: Phys. Reports 458(2008)1 multiple collective excitations of colliding nuclei. I.P., Proc. EMIN-2003, p.234
Coulomb excitation of double Giant Dipole Resonances in low-energy nucleus-nucleus collisions: a conventional method Double excitation in single electromagnetic dissociation ~1%, triple ~0.01% Example: the excitation of double GDR in 208 Pb nucleus by 1A GeV 209 Bi: only a small structure in photon spectra. It is very difficult to see triple excitations by this method .
Recent measurements at the LHC
Single and mutual EMD as seen by ALICE collaboration in ultraperipheral PbPb collisions Event examples: Single EMD in green Mutual EMD in red (2n,2n) (2n,0n) 1.38A+1.38A TeV (1n,0n) ALICE Collaboration, arXiv:1203.2436v2 and PRL 2012, (1n,1n) (0n,1n) (1n,2n) accepted
ALICE data for 1n, 2n and 3n yields in single EMD of Pb nuclei at 1.38+1.38 A 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 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 (b) (b) These cross sections (barns) were measured in a dedicated run by Van der Meer scan. ALICE Collaboration, arXiv:1203.2436v2 and PRL 2012, accepted
RELDIS describes data within six orders of magnitude of the Lorentz contraction of Coulomb field More on the importance of SPS data for this success in the talk by E. Karpechev LHC SPS 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
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