Generators, Calorimeter Trigger and J/y Production at LHCb

Habilitation à diriger des recherches, Patrick Robbe, LAL Orsay, 12 March 2012

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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.

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Introduction• No inconsistency so far but need for new measurements at

LHCb.• Majority of measurements done at B factories (BABAR and

BELLE), LHCb should contribute with:– Increase statistics thanks to large bb cross-section and luminosity at the

LHC: • Improve knowledge of the g angle which is the least well known, using hadronic decay

modes such as B→D(*)K(*) ,• Study CP violation in charm decays.

– Study Bs0 (and b-baryons or Bc

+):• Measurement of fs in Bs

0→J/y f.

• Search for rare decays, with low branching fractions predicted in the Standard Model, that could be enhanced by New Physics contributions:– Search for Bs

0 → m+m-.

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LHCb Detector

• Acceptance for charged particles: 1.9 < h < 4.9

y

x ⊗

beam1 beam2

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• 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 mb-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/y production cross-section with the first data of

the experiment.

Outline

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• 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.

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: PYTHIA– Decay of hadrons produced by the collision: EVTGEN

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• 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 pT.

• 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:

– pTmin , cut-off below which the parton-parton cross-sections are set to 0

Production - PYTHIA

p Tmin

UA5 U

A5

UA5 CD

F

CDF

UA5

CTEQ6L

~20 charged particles in the LHCbacceptance per interaction.

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• B (and D) production processes at lowest order in aS 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.

B Production in PYTHIA

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• 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.

• e (1-w)2 = – (3.2 ± 0.8) % (Opposite Side),– (1.3 ± 0.4) % (Same Side).

Excited B states tuning in PYTHIA

Measured at LEP +Spin counting

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• The decay of all hadrons is delegated to EVTGEN, C++ generator developped by A. Ryd and D. Lange at CLEO and BABAR.

• It contains a very detailed description of B0 and B+ decays, including polarization, decays with low branching fractions, kinematics, … (tuned from BABAR measurements)

• Adaptations needed to move from the Y(4S) to the hadronic environment:– Add decay modes for Bs

0, Bc+, Lb, …

– B hadrons are produced and evolve incoherently – B0/Bs0 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 EVTGEN versions.

B decays - EVTGEN

Nucl. Instrum. Meth. 462 (2001), 152

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• The problem is illustrated in time dependent CP violation in B0→J/y KS0

CP Violation - EVTGEN

Dt = time between tag and signal decays

0

Y(4S)→B0B0

A realistic Monte Carlo signal sample contains N(B0 tags)=N(B0 tags)

Time distribution is generated following the probabilities, in EVTGEN:

B0 tags

B0 tags

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• The problem is illustrated in time dependent CP violation in B0→J/y KS0

CP Violation - EVTGEN

t = time between production and decay

pp→B0BX

A realistic signal sample contains N(B tags) ≠ N(B tags), but PYTHIAproduces equal numbers of B0 and B0.In EVTGEN, generate time and B flavour according to

and keep only events where PYTHIA and EVTGEN agree.

B tagsB tags

0

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• Implementation of generator software allows CP and production asymmetry:

• Detector and trigger asymmetry

Asymmetries

Physics CP asymmetry Production asymmetry Detection asymmetry

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• s(pp→X, visible in LHCb) = 58.8±2.0 mb F⇒ visible ~ 11 MHz (for 2011 conditions: L = 3.5x1032 cm-2.s-1, Nbunches = 1296)

• Two side problem:– Visible cross-section of interesting processes

for CP violation are very small: F(B→pp) ~ 70 mHz (with sbb=288 mb).

– Charm cross-section is very large: F(pp→ccX) ~ 2 MHz (with scc = 6 mb).

• Need for a software trigger, containing already analysis-like cuts.

• Trigger realized in 3 levels:

LHCb Trigger

Level -

0

L0 e, g

40 MHz

L0 had

L0 m

Max 1 MHz

Hig

h-L

evel

Tri

gger

Partial reconstruction

HLT

1

Global reconstruction30 kHz

HLT

2

Inclusive selectionsm, m+track, , mm

topological, charm, ϕ & Exclusive selections

3-4 kHz

Storage: event size ~50kB

hard

ware

software

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LHCb Calorimeters

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

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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

6.3

m

7.7 m

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• 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.

Calorimeters in the trigger

SPD PRS ECAL HCAL

Cell sizes and numbers Inner: 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: 11.95 cm (2688)

Inner: 4.04 cm (1536)Middle: 6.06 cm (1792)Outer: 12.12 cm (2688)

Inner: 13.1 cm (880)Outer: 26.2 cm (608)

Sampling material Lead Iron

Information for the trigger (for each cell)

Binary (1 if energy deposited above threshold)

Binary (1 if energy deposited above threshold)

Transverse energy, ET=E sinq, with maximum 5 GeV, coded on 8 bits (20 MeV precision)

Transverse energy, ET=E sinq, with maximum 5 GeV, coded on 8 bits (20 MeV precision)

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• Compute ET 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 ET(hadron)=ET(HCAL)+ET(ECAL), adding the ECAL energy in front.

• Select the candidates with highest ET and accept the event if this ET is larger than (in 2011):– ET(photon)>2.5 GeV (60 kHz)

– ET(electron)>2.5 GeV (120 kHz)

– ET(hadron)>3.5 GeV (400 kHz)

Calorimeters in the trigger

SPD PRS ECAL HCAL

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• 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:

Data flow

HCAL FEB: compute highest 2x2 ET in 8x4 area

ECAL FEB: compute highest 2x2 ET 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 ET

hadron, electron, photon candidate

Decision Unit (L0DU):

Compute final decision

(comparison to threshold)

8 128

50

188

100

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• 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

TechnologiesHCAL FEB:

LAL Orsay

ECAL FEB: LAL Orsay

PRS FEB: LPC Clermont-

Ferrand

TVB:LAPP Annecy

SB:Bologna

L0DU:LPC Clermont-

Ferrand

Radiation tolerant

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Localisation

Selection BoardsL0DU

Racks with:• ECAL and PRS FEB• TVB

Racks with:• HCAL FEB

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• Data need to be exchanged between different Front-End boards (neighbours, up to 4 different boards to make 1 cluster)

Connections (1)

Area covered by one board

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ECAL FEB

• Copper cable: category 6 ethernet cable (between 70 cm and 15.50 m)• Optical cable: multimode fiber (100 m)

Connections (2)

HCAL FEB

ECAL FEB

PRS FEB

TVB SB L0DU

HCAL FEB

TVB

HCAL FEB

HCAL FEB

PRS FEB

PRS FEB PRS FEB

ECAL FEB

ECAL FEB

ECAL FEB

ECAL FEB

ECAL FEB

ECAL FEB

x4

x8x8

x28 x7

Copper cable

Optical cable

Shielding wall

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• 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)

Data exchange and timing

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• 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.

Data exchange and timing

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Integration in global LHCb system

• Cadenced at 40 MHz• Total L0 latency: 4ms• Maximum L0 rate: 1

MHz• Possible to read

consecutive events

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• Total latency: 4 ms.

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

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• PhD thesis of Alexandra Martín Sánchez (2013)

Calorimeter Trigger Performances and Asymmetries

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• Large cross-section: enough events to perform precise cross-section measurement with early data with decay mode J/y →m+m-.

• Detector performance: some of the most important first year LHCb measurements use di-muon in the final state: Bs

0→m+m- or Bs0→J/y f.

• One way to obtain the bb cross-section, mandatory for branching fraction measurements.

• Investigation of production mechanisms of J/y in hadron collisions: powerful test of QCD.

Measurement of J/y production at LHCb

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Measurement of J/y 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/y (1-- cc state) production:

– Direct J/y production– J/y from decays of heavier

charmonium states– J/y from decays of B hadrons.

Prompt

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J/y hadro-production “puzzle”• Comparison of direct pT differential J/y production cross-sectionmeasured 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), 572R. BAIER and R. RUECKL, Z. Phys C 19 (1983), 251

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J/y hadro-production “puzzle”

• Add gluon and quark fragmentation processes (NLO Color Singletprocesses)• 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

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• 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 (pT cross-sections)

NRQCD

Short distance: perturbativecross-sections + pdf for theproduction of a qq pair

Long distance matrix elements (LDME): non-perturbative partUniversal coefficients

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J/y 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

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J/y hadro-production “puzzle”• Perfect agreement when summing all contributions, with Color-

Octet terms being dominant

+

+

+

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J/y hadro-production “puzzle”• However at high pT:

• Color-octet dominant diagram forces J/y 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.

+

+

+

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J/y polarization at CDF

• Experimental data (for prompt J/y) exclude transverse J/y polarization (lq=+1)

• Doubts that Color-octet process dominates at high pT.

• However, experimental situation not very clear either.

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J/y 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.

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• 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.

J/y production at LHCb

• Double differential cross-section measurement, as a function of the transverse momentum, pT, and of the rapidity, y=

– 14 pT bins: 0 < pT < 14 GeV/c– 5 y bins: 2.0 < y < 4.5

• Separating the contributions of:– Prompt J/y,– J/y from B decays.

PhD Thesis, Wenbin Qian

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Trigger and Selection

Selection:

L0 Trigger:Single Muon:

Di-Muon:

pT>1.4 GeV/c

pT,1>0.56 GeV/c, pT,2>0.48 GeV/c

HLT1 Trigger:Single Muon:

Di-Muon:

Confirm L0 single Muon and pT>1.8 GeV/c(Pre-scaled by 0.2 in 3 pb-1 of data)Confirm L0 Di-Muon and Mμμ>2.5 GeV/c2

HLT2 Trigger: Di-Muon: Mμμ>2.9 GeV/c2

μ tracks:• well reconstructed tracks identified as

muons in muon detector,• pT > 0.7 GeV/c,• Track fit quality (χ2/ndf < 4).

Reconstructed J/y:• mass window: 0.15 GeV/c2,• vertex fit quality (p(χ2)>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.

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J/y Sample

N = 564603 ± 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:

sM = 12.3 ± 0.1 MeV/c2

m = 3095.3 ± 0.1 MeV/c2 (stat error only)

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• Fraction of J/y from b is given by fit of tz:

• Signal:– Convolved by resolution function: Sum of 2

Gaussians

• Background: fit with function describing the shape seen in the J/y mass sidebands: 3 positive exponentials and 2 negative exponentials.

Separation prompt J/y / J/y from b

PV

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tz fit• Origin of the tail are signal J/y

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

tz resolution: 53 fs

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• 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/y and J/y from b in each (pT,y) bin. Small differences are treated as systematic uncertainties.

Efficiencies and systematics

LHCb Simulation

LHCb Simulation

unprescaled trigger

prescaled trigger

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Polarization• J/y 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/y muons is:

• Differences of 3% to 30% between polarized and unpolarized efficiencies, depending on the bin: quote 3 different results of the prompt J/y cross-section, one for each polarization case (l f and lqf assumed equal to 0).

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lq=+1

lq=0

lq=-1

J/y momentum direction in pp frame

= -1

= 0

+W

(cos)

• Integrated over the acceptance:

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Results

Unpolarized prompt J/y

(stat) (syst) (polar)

LHCb, Eur. Phys. J. C 71 (2011), 1645

Results: bb cross-section• From the J/y from b cross-section, extrapolate to the total bb

cross-section in 4p:

• a4p=5.88: extrapolation factor computed from PYTHIA 6.4, no uncertainty associated to it [equal to 5.21 for FONLL].

• B(b➝J/ y X)=(1.16±0.10)%: measured at LEP, with 9% uncertainty.• With (old) hadronization fractions measured at Tevatron, we estimated

B(b➝J/ y X)=(1.08±0.05)%: assign 2% uncertainty due to hadronization fractions.

• Result:

• LHCb published value from b➝D0mnX (Phys.Lett.B694 (2010) 209)

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Comparison with other LHC experimentsInclusive J/y pT cross-section Fraction of J/y from B decays

ALICE, Phys. Lett. B 704 (2011), 442ATLAS, Nucl. Phys. B 850 (2011), 387CMS, Subitted to JHEP

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Comparison with theory

J.-P. Lansberg[Eur. Phys.J. C 61 (2009) 693, arXiv:0811.4005 [hep-ph]]

K. T. Chao et al.[Phys. Rev. Lett. 106 (2011) 042002, arXiv:1009.3655 [hep-ph]]

P. Artoisenet [PoS ICHEP 2010(2010) 192]

M. Butenschön and B. Kniehl [Phys. Rev. Lett. 106 (2011)022301, arXiv:1009.5662 [hep-ph]]

M. Cacciari et al.[JHEP 0103:006 (2001)]

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• 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 2010. Fit of the 3 universal LDME using PHENIX, CDF, ALICE, ATLAS, CMS, LHCb, BELLE, DELPHI, ZEUS and H1 pT cross-sections.

LHC data and J/y production puzzle M

athi

as B

uten

scho

en, B

ernd

A. K

nieh

l, ar

Xiv:

1201

.386

2

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• Satisfactory description of pT spectrum, LDME can be used to predict polarization.

• Comparison with first polarization measurement at the LHC (ALICE) still leaves question opened (importance of feeddown).

LHC data and J/y production puzzle

ALICE, PhD Thesis of L. BianchiPhys. Rev. Lett. 108 (2012) 082001

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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 g angle and charm physics)

• First LHCb data used to measure J/y 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 (cc, y(2S)) and bottomonium states (Y(1S), Y(2S), Y(3S), cb).

• Polarization measurements (J/y, y(2S), Y(nS)).• Study of the exotic charmonium-like states (X(3872), Y, Z+, …)