lhcb: what to do with the first month of data taking · what to do with the first month of data...

CSN1, 16 Novembre 2004 Roberta Santacesaria, INFN Roma1 1

LHCb : what to do with the first

month of data taking


Premessa

LHCb plans to set up a detailedstrategy for the detector and trigger startup over the coming years.My report summarizes the current ideas being discussed with some detector and trigger experts and must be taken as a starting point


Outline

• detector and trigger general structure

• subsystems and calibration requirements

• trigger description and startup• a first look at physics


LHCb detector

p

~ 200 mrad~ 300 mrad(horizontal)

10 mradp

Inner acceptance 10 mrad from conical beryllium beam pipe


LHCb vertex region

VErtex LOcator around the interaction region, verticesreconstruction


LHCb tracking

Tracking system and dipole magnet: to measure angles and momenta (together with VeLo)


LHCb P-ID

Two RICH detectors for charged hadron identification


LHCb calorimeters

e

h

Calorimeter system to identify electrons,hadrons and neutralsand used in the L0 trigger


LHCb muon detection

µ

Muon system to identify muons and used in L0 trigger


Trigger strategy 40 MHz

HLT:Final state

reconstruction

LevelLevel--0:0:ppTT of of

µµ, e, h, , e, h, γγ

Level-1:Impact parameterRough pT: σ(p)/p~ 20%

•Level-0 : Use large B mass signaturehardware system with fixed latency (4µs)

•Level-1 : use B mass and lifetime signatureSoftware analysis on reduced data from only few detectors. Run on a PC farm in common with HLT(~ 1800 CPU total)

•High Level Trigger (HLT):Software.The complete event is reconstructed with almost final accuracySelection of interesting physics decays

CalorimeterMuon system

Pile-up system

1MHzVErtex LOcatorTrigger TrackerLevel 0 objects

(Muon)40kHz

Full detectorinformation

200Hz


VertexLocator- Silicon detector - 21 stations with

alternated R-φ segmentation

- sensitive area starts at 8mm from the beam

- retractable to 3cm during refill (the two halves open along the horizontal axis)

- used in L1 trigger: initiate tracking to compute IP and Pt

2 Pile-Up layers (only r sensors)to veto multiple interactions at L0

Interaction pointσz=5.3cm

~ 1m

~10cm


Top view of the VeLo system


VeLo structure: R-φ sensors

• angular coverage of each half disc = 182o

• total # channel ~170.000 • analogue readout

Pitch 40-103µm

Stereo angle 10 ° -20 °y

xy

x45° sector

φ: R:

Pitch 35-97µm


VeLo alignment figures

- Silicon strips are positioned on disc halves with <5µm- relative position of disc halves, within a half VeLo,

surveyed before installation at σ(x), σ(y)~ 5µm, σ(z)~ 20µm precision in air and at room temperature.to be checked on beam with tracks since vacuum and cooling may affect itGoal is an hardware alignment of σ(x), σ(y)~ 20µm for L1 trigger to work

- since VeLo retracts at each machine refill the two parts are re-positioned with a precision of<~100µm , this can be checked with a few seconds of data taking thanks to small overlapping surfaces

- final transverse software alignment precision for event reconstruction ~ 5µm


VeLo test beam activity before installationA complete Half VeLo module should be tested on 120 Gev beam

- measure, and physically correct, misalignment among half discs

- full test of vertex reconstruction capability (target placed in front of the velo to simulate interactions),including software

- this is very important for its own survival: at LHC startup beam position can be estimated by VeLoitself reconstructing primary vertices at its retracted position before getting close to it(VeLo can also determine the beam position before collisions with beam-gas interactions)


Tracking system

• 1 station upstream the magnet and 3 downstream

• dipolar magnet: Bl=4Tm(*)• each station is composed

of 4 layers : x,+5o,-5o,x• TT: silicon technology • T1-T3 mixed technology

(according to occupancy)Inner Tracker: silicon

strips Outer Tracker: straw

tubes

(*) installed&switched on successfully


Tracking system: TT Station

• four layers: distance between 2 layers 30cm

• total silicon area 4x2m2

• 183µm pitch• Build with sensors each 9.6x9.4cm2 bonded to formstrips 10cm–40cm long

• 143,000 FE channels• used in L1


Tracking system: T1-T3

IT

• silicon sensor: area 11x8cm2

• strip pitch 198µm• Max occupancy 1,5% • 130,000 channels

OT

45 0

• Two straw tubes (5mm Ø) layers staggered by5.25mm for nodead space

• 56,000 channels

• Resolution ~ 200µm withdrift-time measurement

• Occupancy max < 7%


Trackers alignmentSilicon trackers

• strips positioned with ~5µm• positioning precision of detector boxes at survey~1mm • relative precision needed at L1 for VeLo-TT ~<200µm,

to be done with first tracks• final software alignment aims at about 10µm for

complete event reconstruction

Straw tubes• installation precision at survey ~1mm• working without drift time information at the beginning (σ∼400µm),more ghosts but pattern recognition with reasonable efficiency possible.Found tracks used for alignment and r-t calibration


RICH system

CF4 gas

Beam pipe

300 mrad

120 mrad

Flat mirror

Spherical mirror

Photodetectorhousing

2 Rich with 3 Cherenkov radiators

Need of PID in large p range 3-100 GeV

RICH1 RICH2

P threshold (GeV) Aerogel C4F10 CF4

π 0.62.0

2.6 4.4K 9.3 15.6


RICH mirrors alignment(1)

• RICH system is used in the offline analysis (its use in HLT is under development)

• Rich1 has 4 spherical+ 16 flat mirror segments• Rich2 has 58 spherical and 40 flat mirror segments

Mirror tilts can affect the resolution on the Cherenkov angle

Software corrections are possible with a precision of0.1 mrad provided a mechanical precision of 1 mrad is obtained during mounting (attainable with laser alignment). This limits the fraction of photons whichcannot be attributed unambiguously to one mirror (for which no correction possible) avoiding the deterioration of PID performance. Alignment is much more criticalfor Rich2


RICH mirrors alignment(2)

Need for a working tracking, ~1k well reconstructed tracks/mirror needed,<~100Kevents


Calorimeter system(1)

ScintillatingPadDetector&PreShower :

two 15 mm scintillating pad layers + 2.5X0 lead in between

Electromagnetic calorimeter: “shashlik” 2mm lead + 4mm scintillator , 25Xo

σ(E)/E=10%/√E+ 1.5%Hadronic calorimeter.:

iron-scintillator plates , 5.6 λΙ

σ(E)/E=80%/√E+ 10%


Calorimeter system(2)

5952 ch

1468 ch

SPD : multiplicity, e/γ separationPreShower: e/h separationECAL: e, γ (E,Et) measurement }

HCAL: hadrons (E,Et)

12 cm26 cm

6 cm

4 cm 13 cm


Calorimeter calibration

• Ecal modules pre-equalized with cosmics• Hcal modules pre-equalized with 137Ce• a sub-sample is energy-calibrated on test-

beam, calibration transported on the whole detector with momentum well reconstructed tracks

• PM gain stability monitored with pulsed LED ( checked with photodiodes). This serves also as signal time synchronization


Muon system(1)

• 5 Stations + 4 iron filters

• 1380 chambers:1368 MWPC + 12 GEM => 435m2

• trigger imposes high efficiency/station since 5 stations/5 are required(>99%) 4 (2 in M1) gas gap ORed (each gap has ε >~95%),


Muon system (2)

• Large (10x8x7 m3) system

• 120K FE channelsORed to form26k logical channelssent to L0/DAQ

• Up to max 48 physicalchannels make up a logical channel

• Granularity: Min 6.3x31.3 mm2 (M2R1) Max 250x310 mm2(M5R4)

One quadrant of Station 2

A high precision space alignment is not needed for the muon system but……


Muon system time alignment(1).... an accurate time alignment is mandatory to ensure

high L0 muon trigger efficiency

MWPC time spectrum of one physical channel (OR of 2 gaps) with cosmics.

25 ns

If not well centered significant losses arepossible


Muon system time alignment(2)

Muon electronics is designed to providesynchronization tools:

ASD chip(Carioca)

DIALOG(*) chip

FE board

SYNC chip

ODE boardThe SYNC chip accumulates hits from the FE - in BX bins (25ns) - inside the BX with a 1.5ns resolution TDCWith SYNC histogramming facilities the total signal shift can

be determined as M*25ns+K*1.5ns- M is corrected on the SYNC itself while K is corrected

with an adjustable delay present on the DIALOG chip

(*) DIALOG performs the logic OR to form logical channels


Muon system time alignment(3)Coarse alignment: hits in 16BX around the beginning of a full bunch crossing sequence whose timing is given by the accelerator (16 entries/LHC orbit)

16 cycles

BX identifiedResidual misalignment

BX number

Before FTO correction

BCn+2 BCn+3 BCn+2 BCn+3

Phasemeasurement(25 ns gate)

On-chip histogram

BCcycle

FTO determinationand correctionvia front-end

ProgrammableDelays

(DIALOG chip)

FT alignment

Statistics in ~1h

Statistics in <~1hFine alignment


Muon system time alignment(4)• no muon trigger needed (all hits are usable)• no DAQ needed, histogram analysis through ECS• alignment possible at the physical channel level (masking facility in DIALOG chip). In principle, in fact, each FE channel must be independently centered in 25ns. Cosmics test shows, however, that FE channels inside a logical channel are in time within < 5 ns

Peak position for 16 FE chforming 1 logical channel

5 ns25 ns

It might be not true for logical channels formed by signals coming from different chambers


Muon system time alignment(5)

After L0 has started (even with reduced efficiency)refinement can be done with offline analysis requiring channels with a muon hit (TDC information in DAQ)

All hits

Muon hitsMC


L0Trigger

Calo Muon•Select local maxima in ECAL and HCAL towers•Use SPD and PS to separate e/γ/h

µ

>90% π/K decay

Nominal threshold

HCAL clusters


L0 Trigger(2)

HCa

ldom

inat

es

Muo

n do

min

ates

ECal

dom

inat

es

ZB ZA

RB

RA

ZPV

2 silicon R-stations

Possible cut:retain >98% of single and reject ~60% of multipleGain of 30-40% of single bb-events at optimal luminosity

PileUp veto (to reject multiple interactions) L0 result.


L1 Trigger - track reconstruction in VELOVELO sensor design optimized for fast track finding:

B’s move predominantly along beam-line impact parameter in RZ-view

Velo 2D reconstructionPrimary vertex search

1. Straight line search in R-Z view, forward and backward tracks~ 58 (+ ~30 backward) tracks

1. Vertexing, σZ ~ 60 µm, σX,Y ~ 25 µm2. Select tracks with high impact parameter, 0.15 to 3 mm

about 8.5 per event3. Full space tracking for those tracks4. Extrapolate 3-D VELO-tracks to TT, use fringe field for P

estimation dp/p=20-40% , ε(B-tracks)=94%


L1 Trigger–match large IP with L0 candidates

Match with high Pt L0 calo and muon tracksStudy ongoing to include muon information

Signal/background separation by

Pt and impact parameter


HLT Trigger• Confirm Level-1 decision

– Complete Velo 3D tracking– Primary vertex search– Select from the 3D Velo the large IP tracks , measure accurately the momentum, confirm the decision (same variable)

• Extend Velo tracks across the magnet →σ(p)/p ~ 0.6 %

• Gain a factor 2-3 withoutsignificant loss

• Fraction of CPU time budget

Input95% confirmed

• Physics selection (including lepton–ID but not Rich ,yet)It has been checked for a few representative channels that a loose exclusive selection brings the rates down to 10-20 Hz per channel


Trigger startup(1)Assumptions:• the subsystems are commissioned prior to data taking with the

LHC beam ( noise, missing channels etc)• one circulating beam cannot be easily used since earlier beam is

most likely in the wrong direction but it can be helpful for VeLo• something can be done with beam-gas interactions when the two

beams circulate (alignment of the tracking elements). • we have assumed we will use the first collisions

LHC scenario: L~1032cm-2s-1

75 ns bunch spacingE beam=6 TeV~ 5-6 MHz with>=1 collisions

5

2

Ncoll/BX

R(MHz)

0 1 2 3 4 5


The transfer line with the right direction (TI2) will be installed in 2007


Trigger startup(2)

• Muon system time is aligned as described • VeLo system is retracted • Calo system is operational• Set-up an interaction trigger with

minimum energy released in HCAL and/ormultiplicity in SPD. (SPD can be used also to selectparticularly “clean” events)

Take minimum bias data O(106) and check of:- m.b. rate- measure beam position with VeLo retracted- check correct functioning of L0 triggers - measure Pt distributions at L0, rates.vs.Pt cut- when VeLo closed, pile-up veto efficiency can be checked - take data also with magnet off to perform relative of tracking elements : aim at an alignment sufficient to let L1 work


Trigger startup(3)

L1 switched on• check IP distributions• check capability to match high Pt tracks• check L1 and HLT rates

A large fraction of the data processing during this startup phase could be done by the L1/HLT CPU farm at the pit, assuming the ~full CPU power is available at t0

Finally, as soon as we have a reasonable trigger, we can start taking useful data and first signal to look for is obviously J/ψ µµ


Look for J/ψ µµ

J/ψ is selected with a dedicated dimuon trigger

We can have a ~70Hz of J/ψ signal after L0&L1&HLT(*) with B/S~2-3

2 VeLo tracks coming from a common vertex and connected to 2 L0muon candidates (Pt1+Pt2)>1.3GeV no IP cuts, Minv window around J/ψ mass

(*) J/ψ HLT


J/ψ µµ• If we are lucky a J/ψ µµ sample of ~3x107 could be

gathered in 2 weeks running with a J/ψ µµ trigger

• With it we can do a more precise tracks momentumcalibration using J/ψ mass and vertex

Once the detector is well aligned and calibrated we may start to look at physics:− σ(J/ψ−prompt)/σ(mb) - a lot of b’s J/ψ X

Pythia gives: J/ψ (b)/ J/ψ−prompt ~ 5% (CDF ∼20%, probably effect of cuts)

~1.5Mevts b J/ψ + X


J/ψ µµ

• Dimuon Trigger: Pt > 1.5 GeV per µ• ~2 million psi• ~80% prompt

~20% B


b J/ψ µµStatistics of some interesting B decay channels assumingnominal detector performance Channel J/ψ ident Full ev.rec.

B0 J/ψ(µµ)Ks0(ππ)

BRvis=1.9x10-515Kevts 5Kevts

B0 J/ψ(µµ)K*0 (πK)BRvis=5.9x10-5

45Kevts 14Kevts

B+ J/ψ(µµ)K+

BRvis=6.8x10-552Kevts 37Kevts

B+ c J/ψ(µµ)π+

BRvis <=6.8x10-4<=1Kevts <=500evt


B0 J/ψ(µµ)K*0 (πK)• Control process, no CP violation• B oscillation could be seen (with some tagging

efficiency, maybe only µ)

Stat.err. only, nominal tagging,

no Bg (B/S=0.37)


B+ J/ψ(µµ)K+

• Tagging check• Lifetime check (no IP bias in this

sample) B+

c J/ψ(µµ)π+

•Very clean peak at 6.4 GeV•Never observed


Conclusions

• A detailed startup procedure will be established in the coming years

• Trigger startup, sub-detectors calib. & align.(even if not final) feasible in the first month of data provided a lot of work is done before, also on test-beams

• Part of this work can be done without offline analysis, while the rest implies data taking and full reconstruction

• Some “not really physics results but showing our capability of doing physics can be achieved”


Backup


Trigger: possible improvement

• There is an on-going study to increase to ~ 2kHz the events rate to DAQ.

• 200Hz as baseline strategy + ~2kHz of events with a muon with high impact parameter

• On muon ”highway” triggered events b content ~ 50%• Gain a sample of unbiased b (the companion of triggering b->µ)

– useful for systematic study– Inclusion of b decay not easy to trigger on– Inclusion of b decay not of interest now but maybe in the

future…


VELO

0

200

400

600

800

1000

−0.05 −0.04 −0.03 −0.02 −0.01 0 0.01 0.02 0.03 0.04 0.05Primary vertex resolution in x, y [mm]

Num

ber

of e

vent

s

Mean = 0.4 ± 0.1 µmσ1 = 7.8 ± 0.2 µmσ2 = 18 µm (26.5%)

0

100

200

300

400

500

600

700

800

900

−0.5 −0.4 −0.3 −0.2 −0.1 0 0.1 0.2 0.3 0.4 0.5

Mean = 8.3 ± 0.8 µmσ1 = 43.9 ± 1.6 µm

Primary vertex resolution in z [mm]

Num

ber

of e

vent

s

σ2 = 124 µm (21.8%)

Primary vertex resolution in bb events:⊥ and || to the beams

σz=44µmσx-y=8µm

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

IP r

esol

utio

n [m

m]

1000

b)

1/pT [GeV/c]−10 0.5 1 1.5 2 2.5 3 3.5 4

σIP=14µ+35µ/pT

Impact parameter resolution vs 1/PT


Vertex Locator

0

200

400

600

800

1000

−0.05 −0.04 −0.03 −0.02 −0.01 0 0.01 0.02 0.03 0.04 0.05Primary vertex resolution in x, y [mm]

Num

ber

of e

vent

s

Mean = 0.4 ± 0.1 µmσ1 = 7.8 ± 0.2 µmσ2 = 18 µm (26.5%)

0

100

200

300

400

500

600

700

800

900

−0.5 −0.4 −0.3 −0.2 −0.1 0 0.1 0.2 0.3 0.4 0.5

Mean = 8.3 ± 0.8 µmσ1 = 43.9 ± 1.6 µm

Primary vertex resolution in z [mm]

Num

ber

of e

vent

s

σ2 = 124 µm (21.8%)

Primary vertex resolution in bb events:⊥ and || to the beams

σz=44µmσx-y=8µm

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

IP r

esol

utio

n [m

m]

1000

b)

1/pT [GeV/c]−10 0.5 1 1.5 2 2.5 3 3.5 4

σIP=14µ+35µ/pTImpact parameter resolution vs 1/PT


Track finding strategy

VELO seeds

Long track (forward)

Long track (matched)

T seeds

Upstream track

Downstream track

T track

VELO track

Long tracks ⇒ highest quality for physics (good IP & p resolution)Downstream tracks ⇒ needed for efficient KS finding (good p resolution)Upstream tracks ⇒ lower p, worse p resolution, but useful for RICH1 pattern recognition


Tracking performance

pT,cut [GeV/c]

Gho

st ra

te

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.5 1 1.5 2 2.5 3p [GeV/c]

Effic

ienc

y

0.7

0.75

0.8

0.85

0.9

0.95

1

0 20 40 60 80 100 120 140

Ghost rate < 4% Pt>0.5GeV

Eff=94%

P>10GeV

δp/p = 0.35% – 0.55%

Momentum resolution

Ghost rate

Track finding efficiency


Rich Particle IdentificationBs->KK decay

Pion misidentification

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100

Kaon identification

Momentum (GeV/c)

Effic

ienc

y

<ε (K->K)> = 88%<ε (π->K)> = 3%


KS reconstruction

DD ε=54%

LL ε=75%

LU ε=61%

Ks decays •inside Velo(25%) (LL+LU)•between Velo TT(50%)(DD)•behind TT (25%)

Only first 2 cases reconstructed

Low momentum track swept out by the magnet: (U type track) poor p resolution

When both tracks are reconstructed the selection is reasonably efficient


Tag εTag (%)

w (%)

εeff(%)

Muon 11 35 1.0Electron 5 36 0.4Kaon 17 31 2.4Vertex Charge

24 40 1.0

Frag. kaon(Bs)

18 33 2.1

Frag π (B) 0.7~4.7Combined B0 (decay

dependent:Combined Bs trigger + select.)

~6

Flavour tagging

sources for wrong tags:Bd-Bd mixing (opposite side)b → c → l (lepton tag) conversions…

effective efficiency:εeff = εtag (1-2wtag )2

lhcb: what to do with the first month of data taking · what to do with the first month of data...

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