Generators, Calorimeter Trigger and J/  Production at LHCb Habilitation à diriger des recherches, Patrick Robbe, LAL Orsay, 12 March 2012

2 Introduction The LHCb experiment is one of the four large experiments installed at the Large Hadron Collider (LHC) at CERN. It exploits B and D hadrons produced by the pp collisions to study CP violation and rare B and D decays to test the Standard Model. Experimental validation of the description of CP violation in the Standard Model (via the quark mixing CKM matrix) is well established through global fits of “unitarity triangles” from several measurements: UTFIT Collaboration, JHEP 507 (2005), 28.

3 Introduction No inconsistency so far but need for new measurements at LHCb. Majority of measurements done at B factories (B A B AR and BELLE), LHCb should contribute with: – Increase statistics thanks to large bb cross-section and luminosity at the LHC: Improve knowledge of the  angle which is the least well known, using hadronic decay modes such as B→D (*) K (*), Study CP violation in charm decays. – Study B s 0 (and b-baryons or B c + ): Measurement of  s in B s 0 →J/  . Search for rare decays, with low branching fractions predicted in the Standard Model, that could be enhanced by New Physics contributions: – Search for B s 0 →  +  -.

4 LHCb Detector Acceptance for charged particles: 1.9 <  < 4.9 y x ⊗ beam1 beam2

LHCb started taking data end of 2009, and recorded up to now: – 1.1 fb -1 of pp collisions at √s = 7 TeV, – 70 nb -1 of pp collisions at √s = 2.76 TeV, – 6.8  b -1 of pp collisions at √s = 900 GeV. This happened after more than 10 years of preparation, construction, installation and commissioning. I will present activites I was responsible for, that took place during the preparation of the experiment for data taking: – Development of the generator software, – Installation and commissioning of the calorimeter trigger, – Measurement of the J/  production cross-section with the first data of the experiment. 5 Outline

A detailed simulation is needed for the data analysis, which must reproduce as accurately as possible: – Multiplicity and transverse momentum spectra of particles, in minimum bias and D/B events (trigger, reconstruction) – Production and decay properties of B events (tagging) – Time evolution of D/B hadrons, mixing, CP violation. 6 Generators in LHCb Simulation software: C++ application. It is composed of two distinct steps: The generation part is also composed of two main components: – Production of particles from the pp collision: P YTHIA – Decay of hadrons produced by the collision: E VT G EN

Widely used FORTRAN generator in High Energy Physics. Events generated at LHCb extracted from « minimum bias » events. In LHCb, tune the charged particle multiplicity, linked to the event structure at low p T. This is governed by the « multiple interaction model », ie each hadronic collision is the sum of a varying number of individual parton- parton interactions. The number of parton-parton interactions per event (then the particle multiplicity) is adjusted by the parameter: – p Tmin, cut-off below which the parton-parton cross-sections are set to 0 7 Production - P YTHIA CTEQ6L ~20 charged particles in the LHCb acceptance per interaction.

B (and D) production processes at lowest order in  S are the pair creation processes. At the LHC energy, higher order processes dominate. Gluon splitting causes the bb pair to be produced forward or backward. 8 B Production in P YTHIA

B flavour tagging in hadron colliders is based on the properties of the other B decay in the event, but also on the fragmentation characteristics of the signal B.   (1  w) 2 = – (3.2 ± 0.8) % (Opposite Side), – (1.3 ± 0.4) % (Same Side). 9 Excited B states tuning in P YTHIA Measured at LEP + Spin counting

The decay of all hadrons is delegated to E VT G EN, C++ generator developped by A. Ryd and D. Lange at CLEO and B A B AR. It contains a very detailed description of B 0 and B + decays, including polarization, decays with low branching fractions, kinematics, … (tuned from B A B AR measurements) Adaptations needed to move from the Y(4S) to the hadronic environment: – Add decay modes for B s 0, B c +,  b, … – B hadrons are produced and evolve incoherently – B 0 /B s 0 mixing and CP violation have to be described differently. Changes used by ATLAS/CMS and merged by A. Ryd and D. Lange in the new E VT G EN versions. 10 B decays - E VT G EN Nucl. Instrum. Meth. 462 (2001), 152

The problem is illustrated in time dependent CP violation in B 0 →J/  K S 0 11 CP Violation - E VT G EN  t = time between tag and signal decays 0 Y(4S)→B 0 B 0 A realistic Monte Carlo signal sample contains N(B 0 tags)=N(B 0 tags) Time distribution is generated following the probabilities, in E VT G EN : B 0 tags

The problem is illustrated in time dependent CP violation in B 0 →J/  K S 0 12 CP Violation - E VT G EN t = time between production and decay pp→B 0 BX A realistic signal sample contains N(B tags) ≠ N(B tags), but P YTHIA produces equal numbers of B 0 and B 0. In E VT G EN, generate time and B flavour according to and keep only events where P YTHIA and E VT G EN agree. B tags 0

Implementation of generator software allows CP and production asymmetry: Detector and trigger asymmetry 13 Asymmetries Physics CP asymmetryProduction asymmetryDetection asymmetry

 (pp→X, visible in LHCb) = 58.8±2.0 mb ⇒ F visible ~ 11 MHz (for 2011 conditions: L = 3.5x10 32 cm -2.s -1, N bunches = 1296) Two side problem: – Visible cross-section of interesting processes for CP violation are very small: F(B→  ) ~ 70 mHz (with  bb =288  b). – Charm cross-section is very large: F(pp→ccX) ~ 2 MHz (with  cc = 6 mb). Need for a software trigger, containing already analysis-like cuts. Trigger realized in 3 levels: 14 LHCb Trigger Level -0 L0 e,  40 MHz L0 had L0  Max 1 MHz High-Level Trigger Partial reconstruction HLT1 Global reconstruction 30 kHz HLT2 Inclusive selections ,  +track,  topological, charm, ϕ & Exclusive selections 3-4 kHz Storage: event size ~50kB hardware software

15 LHCb Calorimeters System of 4 detectors, located ~13m from the interaction point Scintillating Pad Detector (SPD) Preshower Detector (PRS) Electromagnetic Calorimeter (ECAL) Hadronic Calorimeter (HCAL)

16 LHCb Calorimeters Longitudinal segmentation allows to distinguish between photons, electrons and hadrons. Transverse segmentation in square cells to measure energy of individual particles SPD, PRS, ECAL (projective geometry) HCAL 7.7 m

The 4 subdetectors give information for the trigger. They use the same principle: light from scintillation in plastic scintillator is collected by wave-lenght shifting fiber, and transported to PMTs. 17 Calorimeters in the trigger SPDPRSECALHCAL Cell sizes and numbersInner: 3.96 cm (1536) Middle: 5.95 cm (1792) Outer: 11.9 cm (2688) Inner: 3.98 cm (1536) Middle: 5.98 cm (1792) Outer: cm (2688) Inner: 4.04 cm (1536) Middle: 6.06 cm (1792) Outer: cm (2688) Inner: 13.1 cm (880) Outer: 26.2 cm (608) Sampling materialLeadIron Information for the trigger (for each cell) Binary (1 if energy deposited above threshold) Transverse energy, E T =E sin , with maximum 5 GeV, coded on 8 bits (20 MeV precision)

Compute E T of clusters of size 2 cells x 2 cells. Identify 3 types of candidates: – Photon: ECAL cluster with 1 or 2 hit (or 3 or 4 in the inner area) in front in the PRS and no corresponding hit in the SPD. – Electron: ECAL cluster with 1 or 2 hit (or 3 or 4 in the inner area) in front in the PRS and at least one corresponding hit in SPD. – Hadron: HCAL cluster, with E T (hadron)=E T (HCAL)+E T (ECAL), adding the ECAL energy in front. Select the candidates with highest E T and accept the event if this E T is larger than (in 2011): – E T (photon)>2.5 GeV (60 kHz) – E T (electron)>2.5 GeV (120 kHz) – E T (hadron)>3.5 GeV (400 kHz) 18 Calorimeters in the trigger SPD PRS ECAL HCAL

Digitized detector cell information is processed in several custom electronic boards, at the LHC clock frequency (40 MHz), reducing the amount of information at each step: 19 Data flow HCAL FEB: compute highest 2x2 E T in 8x4 area ECAL FEB: compute highest 2x2 E T in 8x4 area PRS FEB: compute SPD/PRS hit pattern in front of ECAL 2x2 Trigger Validation Board (TVB): Combine informations to compute hadron, electron, photon candidates Selection Board (SB): Compute global maximum E T hadron, electron, photon candidate Decision Unit (L0DU): Compute final decision (comparison to threshold)

Computations realized in FPGA (Field Programmable Gate Array) – HCAL/ECAL FEB: anti-fuse Altera FPGA (non reprogrammable) – PRS FEB and TVB: flash memory based Actel FPGA (reprogrammable with difficulties) – SB and L0DU: « normal » reprogrammable Xilinx FPGA 20 Technologies HCAL FEB: LAL Orsay ECAL FEB: LAL Orsay PRS FEB: LPC Clermont- Ferrand TVB: LAPP Annecy SB: Bologna L0DU: LPC Clermont- Ferrand Radiation tolerant

21 Localisation Selection Boards L0DU Racks with: ECAL and PRS FEB TVB Racks with: HCAL FEB

Data need to be exchanged between different Front-End boards (neighbours, up to 4 different boards to make 1 cluster) 22 Connections (1) Area covered by one board

ECAL FEB Copper cable: category 6 ethernet cable (between 70 cm and m) Optical cable: multimode fiber (100 m) 23 Connections (2) HCAL FEB ECAL FEB PRS FEB TVB SB L0DU HCAL FEB TVB HCAL FEB PRS FEB ECAL FEB x4 x8 x28 x7 Copper cable Optical cable Shielding wall

Reading correctly and aligning data transmitted by the cables is one of the crucial point of the system Copper: Ser: DS90CR215 (National) or FIN1215 (Fairchild) / Des: DS90CR216 (National) – 21 bits serialized on 3 data pairs + 1 clock pair (ie 280 Mbits/s) – To sample the data correctly at the end of the cable, the sampling clock is adjusted in 1ns steps using a « delay chip » – Has to be done for each bit and each input, with usually one single parameter (which can vary between boards) 24 Data exchange and timing

Optical: – 32 bits serialized at 1.6 GHz – ser: GOL (CERN) / des: TLK2501 (Texas Instrument) – Deserializer takes care automatically of the correct data sampling, thanks to predefined synchronization patterns sent each LHC turn (during the empty crossings of the LHC cycle) – However, initialization of the link decoding by the deserializer takes a variable time (between 2 and 3 clock cycles): gymnastic needed to ensure fixed latency. 25 Data exchange and timing

26 Integration in global LHCb system Cadenced at 40 MHz Total L0 latency: 4  s Maximum L0 rate: 1 MHz Possible to read consecutive events

Total latency: 4  s. 27 Latency HCAL FEB: 552 ns ECAL FEB: 552 ns PRS FEB: ECAL + 68 ns TVB: 368 ns/518 ns SB: 525 ns/475 ns L0DU: 485 ns 158 ns 139 ns 78 ns 505 ns TFC: 735 ns 505 ns 71 ns 15 ns

PhD thesis of Alexandra Martín Sánchez (2013) 28 Calorimeter Trigger Performances and Asymmetries

Large cross-section: enough events to perform precise cross-section measurement with early data with decay mode J/  →  +  -. Detector performance: some of the most important first year LHCb measurements use di-muon in the final state: B s 0 →  +  - or B s 0 →J/  . One way to obtain the bb cross-section, mandatory for branching fraction measurements. Investigation of production mechanisms of J/  in hadron collisions: powerful test of QCD. 29 Measurement of J/  production at LHCb

30 Measurement of J/  production at the LHC One of the few measurement done by all the 4 LHC experiments, in different rapidity ranges. In pp collisions at the LHC energies, 3 sources of J/  (1 -- cc state) production: – Direct J/  production – J/  from decays of heavier charmonium states – J/  from decays of B hadrons. Prompt

31 J/  hadro-production “puzzle” Comparison of direct p T differential J/  production cross-section measured by CDF with Color Singlet Leading Order process (most natural process to consider). Fails both in shape and magnitude. CDF, Phys. Rev. Lett 79 (2997), 572 R. BAIER and R. RUECKL, Z. Phys C 19 (1983), 251

32 J/  hadro-production “puzzle” Add gluon and quark fragmentation processes (NLO Color Singlet processes) Better shape but magnitude is factor 30 too low. + E. BRAATEN, M. A. DONCHESKI, S. FLEMING and M. L. MANGANO, Phys. Lett. B 333 (1994), 548

Two different scales for quarkonium production: – qq formation is a hard process, – Binding and evolution of qq at softer scales. Models assume factorization between the two steps: Color singlet model: color of the qq pair neutralizes in the hard process, soft part is wave function at origin (only input). NRQDC: color neutralizes in the long distance part, the hard process can produce singlet or octet qq systems. No predictions of the LDME which are fitted from data (p T cross-sections) 33 NRQCD Short distance: perturbative cross-sections + pdf for the production of a qq pair Long distance matrix elements (LDME): non-perturbative part Universal coefficients

34 J/  hadro-production “puzzle” Add LO Color-Octet processes from NRQCD LDME fitted on the same data P. L. CHO and A. K. LEIBOVICH, Phys. Rev. D 53 (1996), 150

35 J/  hadro-production “puzzle” Perfect agreement when summing all contributions, with Color- Octet terms being dominant + + +

36 J/  hadro-production “puzzle” However at high p T : Color-octet dominant diagram forces J/  to be transversely polarized Gluon almost on shell (massless), and polarization is not modified by the long distance coefficients. This is a prediction of NRQCD, not a result of the LDME fit to the data

37 J/  polarization at CDF Experimental data (for prompt J/  ) exclude transverse J/  polarization (  =+1) Doubts that Color-octet process dominates at high p T. However, experimental situation not very clear either.

38 J/  hadro-production “puzzle” Re-inforced interest for Color-Singlet processes, with computations at higher orders (NNLO*), closer to CDF data. + Polarization not incompatible with data but huge uncertainties … P. ARTOISENET, J. P. LANSBERG and F. MALTONI, Phys. Lett. B 653 (2007), 6.

Need for new data at the LHC ! First measurement at LHCb uses 5.2 pb -1 of early data, recorded in September 2010, in pp collisions at √s=7 TeV. 39 J/  production at LHCb Double differential cross-section measurement, as a function of the transverse momentum, p T, and of the rapidity, y= – 14 p T bins: 0 < p T < 14 GeV/c – 5 y bins: 2.0 < y < 4.5 Separating the contributions of: – Prompt J/ , – J/  from B decays. PhD Thesis, Wenbin Qian

40 Trigger and Selection Selection: L0 Trigger: Single Muon: Di-Muon: p T >1.4 GeV/c p T,1 >0.56 GeV/c, p T,2 >0.48 GeV/c HLT1 Trigger: Single Muon: Di-Muon: Confirm L0 single Muon and p T >1.8 GeV/c (Pre-scaled by 0.2 in 3 pb -1 of data) Confirm L0 Di-Muon and M μμ >2.5 GeV/c 2 HLT2 Trigger: Di-Muon: M μμ >2.9 GeV/c 2 μ tracks: well reconstructed tracks identified as muons in muon detector, p T > 0.7 GeV/c, Track fit quality (χ 2 /ndf < 4). Reconstructed J/  : mass window: 0.15 GeV/c 2, vertex fit quality (p(χ 2 )>0.5%). Event: at least one reconstructed Primary Vertex (PV): to compute proper-time Global Event Cuts (GEC): reject events with very large multiplicities (93% efficiency), number of visible interactions per bunch crossing is 1.8.

41 J/  Sample N = ± 924 Each mass distributions obtained in the 70 bins are fit with: A Crystal Ball function for the signal to take the radiative tail into account, An exponential function for the background. Crystal Ball function:  M = 12.3 ± 0.1 MeV/c 2  = ± 0.1 MeV/c 2 (stat error only)

Fraction of J/  from b is given by fit of t z : Signal: – Convolved by resolution function: Sum of 2 Gaussians Background: fit with function describing the shape seen in the J/  mass sidebands: 3 positive exponentials and 2 negative exponentials. 42 Separation prompt J/  / J/  from b PV

43 t z fit Origin of the tail are signal J/  associated with the wrong Primary Vertex. The shape of the tail contribution is determined directly from data, simulating an uncorrelated PV using the one of the next event: Unbinned maximum likelihood fit realized in each of the 70 analysis bins. sideband subtracted with “next event” method t z resolution: 53 fs

Efficiencies are computed from Monte Carlo and are extensively checked on data, with control samples. Efficiencies are checked in Monte Carlo to be equal for prompt J/  and J/  from b in each (p T,y) bin. Small differences are treated as systematic uncertainties. 44 Efficiencies and systematics LHCb Simulation unprescaled trigger prescaled trigger

45 Polarization J/  are not polarized in the LHCb simulation, but efficiency depends strongly on polarization (which is unknown), through anisotropies in angular distributions of the muons. 3 extreme polarization cases studied, in the helicity frame where the angular distribution of J/  muons is: Differences of 3% to 30% between polarized and unpolarized efficiencies, depending on the bin: quote 3 different results of the prompt J/  cross-section, one for each polarization case (  and   assumed equal to 0). 45  =+1  =0  =-1 J/  momentum direction in pp frame   = -1   = 0   W(cos  )

Integrated over the acceptance: 46 Results Unpolarized prompt J/  (stat) (syst) (polar) LHCb, Eur. Phys. J. C 71 (2011), 1645

Results: bb cross-section From the J/  from b cross-section, extrapolate to the total bb cross-section in 4  :  4  =5.88: extrapolation factor computed from P YTHIA 6.4, no uncertainty associated to it [equal to 5.21 for FONLL]. B(b ➝ J/  X  (1.16±0.10)%  measured at LEP, with 9% uncertainty. With (old) hadronization fractions measured at Tevatron, we estimated B(b ➝ J/  X  (1.08±0.05)%: assign 2% uncertainty due to hadronization fractions. Result: LHCb published value from b ➝ D 0  X (Phys.Lett.B694 (2010) 209) 47

48 Comparison with other LHC experiments Inclusive J/  p T cross-section Fraction of J/  from B decays ALICE, Phys. Lett. B 704 (2011), 442 ATLAS, Nucl. Phys. B 850 (2011), 387 CMS, Subitted to JHEP

49 Comparison with theory J.-P. Lansberg [Eur. Phys. J. C 61 (2009) 693, arXiv: [hep-ph]] K. T. Chao et al. [Phys. Rev. Lett. 106 (2011) , arXiv: [hep-ph]] P. Artoisenet [PoS ICHEP 2010 (2010) 192] M. Butenschön and B. Kniehl [Phys. Rev. Lett. 106 (2011) , arXiv: [hep-ph]] M. Cacciari et al. [JHEP 0103:006 (2001)]

LHC data quite well described by theoretical models: huge progresses since first Tevatron data. Color Singlet Model: inclusion of some NNLO processes [J.-P. Lansberg et al.] NRQCD: full calculations of NLO processes in Fit of the 3 universal LDME using PHENIX, CDF, ALICE, ATLAS, CMS, LHCb, BELLE, DELPHI, ZEUS and H1 p T cross-sections. 50 LHC data and J/  production puzzle

Satisfactory description of p T spectrum, LDME can be used to predict polarization. Comparison with first polarization measurement at the LHC (ALICE) still leaves question opened (importance of feeddown). 51 LHC data and J/  production puzzle ALICE, PhD Thesis of L. Bianchi Phys. Rev. Lett. 108 (2012)

52 Conclusions Implementation of generator framework describing with accuracy the B production environment seen at LHCb. Move to modern production generators in the near future: Pythia8 for example (C++ instead of FORTRAN) Calorimeter trigger providing first level to the LHCb trigger system: Fundamental system to record hadronic B decays which will constitute the main physics program of LHCb in the next years (measurement of the  angle and charm physics) First LHCb data used to measure J/  cross-section: Illustration of excellent performances of the detector, Provides precise data as a first step towards better understanding of quarkonium hadro-production. More important results in this area will come soon: Production cross-sections of higher mass charmonium (  c,  (2S)) and bottomonium states (Y(1S), Y(2S), Y(3S),  b ). Polarization measurements (J/ ,  (2S), Y(nS)). Study of the exotic charmonium-like states (X(3872), Y, Z+, …)