Fitting fix target and ion collisions in the

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Fitting fix target and ion collisions in the LHCb Gauss simulation framework Patrick Robbe

Fitting fix target and ion collisions in the LHCb Gauss simulation framework Patrick Robbe (LAL Orsay), for the LHCb Collaboration, 10 July 2018

[IJMPA 30 (2015) 1530022] [JINST 3 (2008) S 08005] The LHCb experiment • LHCb

[IJMPA 30 (2015) 1530022] [JINST 3 (2008) S 08005] The LHCb experiment • LHCb can operate in various configurations beyond the initially foreseen pp collisions, in particular in fixed target mode, injecting gas (He, Ne, Ar) in the interaction region: all of the configurations must be simulated 5 to 8 Te. V 110 Ge. V 5 Te. V 70 Ge. V 2

LHCb Monte Carlo Simulation • Generation, simulation of detector and digitization of detector response

LHCb Monte Carlo Simulation • Generation, simulation of detector and digitization of detector response to mimic what happens in the detector • Processing chain is then applied exactly as for real data: • Online processing (hardware and software trigger) • Offline reconstruction and data analysis 3

Gauss simulation software • Gauss is the LHCb software project developed for the simulation:

Gauss simulation software • Gauss is the LHCb software project developed for the simulation: • Implemented in the Gaudi framework (using algorithms and tools) using LHCb specialized software (event model, …). • Interfaced to Geant 4 for detector simulation • Accesses several generator packages widely used in the HEP community (mainly Pythia 8, Evt. Gen and Hep. MC) • Gauss provides a framework to add interface to event generators in a flexible manner: • Coherent and automated configuration with Python scripts (configurables) • Configuration modules to ease usage by users. For example Gauss is started (schematically) as • gaudirun. py Gauss. py Star. Light. py Beam_pp_13 Te. V. py Jpsi. To. Mu. py • Where Beam_pp_13 Te. V. py (beam parameters) or Jpsi. To. Mu. py (type of event) are mostly generator independent and can be reused in various setups. 4

Gauss simulation software • Runs in two phases 8 5

Gauss simulation software • Runs in two phases 8 5

Gauss Generator Part • The generation phase takes care of: • Beam parameters (direction

Gauss Generator Part • The generation phase takes care of: • Beam parameters (direction and momentum) • Interaction region size • Number of pile-up interactions • Production of particles (Pythia 8, for Heavy Ion: EPOS, Star. Light) • Decay of particles and their time evolution • This can be easily adapted to the fixed target and ion case 6

Beam parameters • Beam tools configure the beam properties (momentum), following what happened during

Beam parameters • Beam tools configure the beam properties (momentum), following what happened during the data taking • Available tools: • Colliding beams: 2 colliding beams with a crossing angle smeared according to beam emittance and b*. • Fixed target: one single beam 7

Primary vertex smearing • Profile of collision region (size and position) also determined from

Primary vertex smearing • Profile of collision region (size and position) also determined from data • Available tools: • Beam. Spot. Smear. Vertex: Gaussian profiles in x, y, z adapted to collider mode • Flat. ZSmear. Vertex: Gaussian profile in x and y but flat in z, for fixed target mode 8

Evt. Gen: [NIMA 462 (2001) 152] Event Types and Configuration • All particle decays

Evt. Gen: [NIMA 462 (2001) 152] Event Types and Configuration • All particle decays are handled by Evt. Gen, for all production generator (Pythia 8, EPOS, …): • Ensures coherence of various samples: in addition all particle properties are forced to be exactly the same in all software used • All particles are then seen as stable particles in the production generator • Decays of all known particles are listed in a big and detailed table (DECAY. DEC) which must be as precise as possible: • Users can force given particles to a specific “signal” decay mode to study it • Many types of events (~4000) are produced: since the decay and production are decoupled in Gauss, these event types can be produced easily in different ways: • for example J/y➝m+m- can be produced from pp collisions with Pythia 8 and from p. Ar fixed target collisions with EPOS. 9

Event Types and Configuration User Input Generated Automatically • To speed up generation (and

Event Types and Configuration User Input Generated Automatically • To speed up generation (and then simulation), cuts are applied. The minimal one is to require that stable decay products fall in the LHCb acceptance. 10

Pythia 8: [CPC 178 (2008) 852] EPOS: [PRC 92 (2015) 034906] Production Generator Interface

Pythia 8: [CPC 178 (2008) 852] EPOS: [PRC 92 (2015) 034906] Production Generator Interface • Main generator: Pythia 8, with specific LHCb tune • Will also be used soon for heavy ion in LHCb (Angantyr model) • Alternatives for Heavy-Ion collisions: EPOS • FORTRAN generator which takes into account core-corona effects of the density profile • To generate signal events, the bare signal process (J/y➝m+m- for example) is generated with Pythia 8 and merged with underlying EPOS event (generated in Center of mass frame and then boosted to LHCb frame: p. Pb and fixed target collisions are not symmetric) 11

Star. Light: [ar. Xiv: 0706: 3356] Other Heavy Ion Generators • HIJING and Star.

Star. Light: [ar. Xiv: 0706: 3356] Other Heavy Ion Generators • HIJING and Star. Light (simulation of ultra-peripheral Pb. Pb collisions in particular) integrated, and can be tuned also from python options 12

Data-MC comparisons • Star. Light is used to compute the number of signal events

Data-MC comparisons • Star. Light is used to compute the number of signal events in data and cross-section • p. He @ 110 Ge. V: Track multiplicity well reproduced • Anti-proton cross-section in p. He collisions at 110 Ge. V: data (dots) and EPOS (lines) 13

Conclusions • Flexibility of Gauss simulation software used to implement new heavy ion generators

Conclusions • Flexibility of Gauss simulation software used to implement new heavy ion generators which are mandatory to describe the new collision systems that LHCb is now analysing. • Heavy ion simulations (both signal and minimum bias) have been used for publications already • Future developments: • • Use Pythia 8 also for p. Pb collisions Investigate alternatives to simulate high multiplicity Pb. Pb collisions Correlate event activity with signal process multiplicity Document all LHCb tunings 14