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Event Generators for Ultraperipheral Heavy Ion Collisions

Explore ultra-peripheral processes in heavy ion collisions, utilizing two-photon and e+e- pair production in Equivalent Photon Approximation. Implementations in Fortran77 and ALIROOT framework for accurate event generation and trigger rate estimations.

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Event Generators for Ultraperipheral Heavy Ion Collisions

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  1. Ultra-peripheral processes in heavy ion collisions • At impact parameters b>R1+R2 ions interact electromagnetically. • Processes of massive particle production can be described in Equivalent Photon Approximation; • Processes of e+e- pair production should be described by exact cross section calculation AA  AA e+e- AA  AA   AA X [G.Baur et al, Phys.Rep. 364 (2002) 359-450]

  2. AA  AA   AA X 5 two-photon processes are implemented:   X   quarkonium (C=+1)   f+f-   W+W-   V1V2 (V1=, , , J/) Event generator for UPC TPHIC

  3. TPHICunderlying machinery Cross section of 2-photon processes in HIC is connected with cross section of elementary 2-photon processes: L(W,Y) - 2-photon luminosity function is a core of TPHIC. L(W,Y) is calculated in EPA. Calculated once on the grid (W,Y), L is to be used for generating 2-photon events by random picking W and Y. Two-photon events are generated according to their cross sections in leading order except X which is performed by PYTHIA All particle decays are performed by PYTHIA

  4. TPHICimplementation Core routines for 2-photon luminosity calculation, event generation, interfacing with PYTHIA were written in Fortran77 in 1994-1998 [K.Hencken et al, IHEP Preprint 96-38] TPHIC in ALIROOT Interface between f77 and c++ is implemented as a class TPHICgen Class AliGenTPHIC is derived from basic event generator classes

  5. TPHIC: use case Config.C AliGenTPHIC *gener = new AliGenTPHIC(); gener->SetProcess(process); if (process == 1) { // gamma gamma -> X gener->SetMggRange(3.,100.); gener->SetYggRange(-8.,8.); gener->SetLumFunName("lum_ca_3_100.dat"); gener->SetLumFunFlag(-1); } else if (process == 2) { // gamma gamma -> eta_c gener->SetKfOnium(441); gener->SetGGwidthOnium(7.4e-06); } else if (process == 3) // gamma gamma -> mu+ mu- gener->SetKfFermion = 13; else if (process == 4) { // gamma gamma -> W+ W- gener->SetMggRange(70.,200.); gener->SetYggRange(-5.,5.); gener->SetLumFunName("lum_ca_70_200.dat"); gener->SetLumFunFlag(-1); } else if (process == 6) gener->SetKfVmesons(113,113); // gamma gamma -> rho0 rho0

  6. Event generator for UPC EPEMGEN AA  AA N(e+e-) The cross section at N=1 is several times higher than at N>1, so only N=1 case is implemented. The process is described by 5-dim. differential cross section d5/dpT+dpT-dy+dy-d12 which is hard to use as a probability distribution function for event generation. Simple 1-dimensional parameterizations are used to generate events. Events are assigned by the weight which is a ratio of the exact cross section and the parameterized function values. The weight sum gives a cross section of detected events.

  7. EPEMGENimplementation Core routines for event generation according to parameterized functions and for event weight calculation are written in Fortran77 [K.Hencken et al, ALICE Note 2002-27] EPEMGEN in ALIROOT Interface between f77 and c++ is implemented as a class TEpEmGen Class AliGenEpEmv1 is derived from basic event generator classes

  8. EPEMGEN: use case Config.C AliGenEpEmv1 *gener = new AliGenEpEmv1(); gener->SetPtRange(1.e-3,1.0); // pt[GeV] range of each e+- gener->SetYRange(-2.5,2.5); // y range of each e+- gener->Init(); Analysis: Double_t xsDet = 0; Bool_t detected; // Loop over events for (Int_t iEvent=0; iEvent<maxEvent; iEvent++) { gAlice->GetEvent(iEvent); // event weight can be retrieved from the first or second tracks: Float_t weight=gAlice->Particle(0)->GetWeight(); // Accumulate cross section if detector has a hit detected = … expression defining if there are any detected particle… if (detected) xsDet += weight; } // Calculate detected cross section xsDet /= maxEvent;

  9. e+e- pairs production is the main source of background for the gg physics in Ultraperipheral Heavy Ions Collisions. • SiPixel detector of the ITS (SPD) can be used to reject these events. • EPEMGEN is the good tool to estimate the detection cross sections and trigger rates.

  10. EPEMGEN: estimation of the e+e- background andtwo-photon trigger rates • L3 field 0.5T L=1027sm­2 s­1 e+e- in SPD1: 920 barn 920 kHz e+e- in SPD2: 130 barn 130 kHz • L3 field 0.2T L=1027sm­2 s­1 e+e- in SPD1: 3200 barn 3.2 MHz e+e- in SPD2: 600 barn 0.6 MHz

  11. Conclusion • Event generators for Ultraperipheral Heavy Ion Collisions are implemented in the Aliroot framework and stored into cvs: $ALICE_ROOT/TPHIC and $ALICE_ROOT/EPEMGEN. • The estimations based on the EPEMGEN gives the reasonable two-photon trigger rates when it’s required for both e+e- to hit SPD2.

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