The LHCb experiment

Walter Bonivento - INFN Cagliari

The LHCb experiment 1

The LHCb experiment

Walter Bonivento – I.N.F.N. Sezione di Cagliari - Italy



Why B physics at the LHC

If NP will be found at LHC in direct searches, B Physics measurements will allow to understand its nature and flavour structure.

At LHC start-up several precise measurements will be available from B-Factories and Tevatron to test the CKM paradigm of flavour structure and CP violation.

However New Physics could still be hidden in mixing, in box and in penguin diagrams, realm of indirect discoveries.



Unitarity triangles• At the level of precision that will be probed by LHCb, there are two

unitarity relations of the CKM matrix that are of interest:

• Possible situation of the measurements when LHCb starts to take data:

measurement of the angle will be crucial

Differ at the percent level

phase of Vts

~240

SM)~650

χ(SM)~10



χ

~Vub* ~Vtd

~Vts

00Sd KΨJB

)('0 , ΨJΦΨJBs , ρππBd

0

KDBπDB

ss

d

0

02χ

DDBKDBKDB

sc

d

d

0

000

0

KKBB sd00 and and

~Vub* ~Vtd

~Vcb

Which B decays to measure the angles?



Precise determinations including from processes only at tree-level, in order to disentangle possible NP contributions

Several other measurements of CP phases in different channels for over-constraining the Unitarity Triangles

BsDsK, B0D0K*0, B0BsKK,…

B0K* B0K*0l+l-, bsl+l-, Bs...

B0Ks, BsB0B0…

A complete program on B Physics includes:

BsDs, … BsJBsJ(’)

Precise measurement of B0s-B0

s mixing: ms, s and phase s.

Search for effects of NP appearing in rare exclusive and inclusive B decays



PYTHIA

Why a forward detector for B physics at LHC

K-

B0

s K+

p p

few mm

1) b and bbar are mostly produced at small angles wrt beam pipe AND correlated in one unit of rapidity forward spectrometer to measure b decays and TAG them

2) large Lorenz boost large B meson average momentum ~ 80 GeV large average mean flight path ~cm accurate measurement of proper time is possible (few % ) AND selection of B decays at TRIGGER level is possible

3) momentum distribution match particle ID capabilities of RICH detectors ROOM is available for the detectors, contrary to cylindrical geometry

4) relatively low pT muon triggering possible because iron penetration depends here on pL which is large



Detector requirements(I)Physics requirements(I)•Main constraint: the DELPHI cavern (20m)•Collision point in one side•Fixed target experiment design with dipole field magnet good analysing power for forward tracks•Acceptance: 250(300) mrad-10mrad

Efficient particle identification :- p/K separation (1-->100 GeV) --> RICH ; also for flavor tagging, …)- electron and muon ID --> CALO + MUON (for B0(s) --> J/ψ X, flavour tagging, …)

defines the momentum range for the spectrometerand for particle ID



to measure the fast Bs oscillations,where A(mix)α cos(ΔmS τ), if ΔmS=20ps-1, oscillation period is T=300fs need a proper time resolution at least of σ(τ) ~< T/2π σ(τ)/<τ>(1.5ps)~ few %

But L= γβcτ = p/m cτ σ(p)/<p> <few % and σ(L)/<L> <few %

But average decay length ~7mm need a vertex detector to measure it at the few % level (~200μm)exercise: try to reconstruct the argument arguing the path length in the lab frame in one oscillation period

Background rejection mass (p and angular) resolution

Rare decays with many tracks (up to 5) efficient tracking with low X0 (m.s. and γ conversions)

- tracker and magnet

Physics requirements(II)

magnet


The LHCb experiment 9side view

Single arm forward spectrometer

pp collision

Acceptance

10 mrad

250/300 mrad v / h

The experiment



to avoid high number of interaction / bunch crossings :

L = 2 .1032 cm-2s-1 for LHCb

--> simpler events (one interaction per bunch crossing dominates) and less radiation damage

for the detectors

• σinelastic 80 mb and σ bb 0.5 mb

--> need an efficient trigger (also on fully hadronic channels)

trigger strategy:

a) first level, hardware: large B mass large pT of B decay products; and selection of single interaction events

b) second level, software: large B lifetime large impact parameters

Key issues



Comparison to other experiments• Enormous production rate at LHCb: ~ 1012

bb pairs per year much higher statistics than the current B factoriesBut more background from non-b events challenging triggerand high energy more primary tracks, tagging more difficult

• But in addition, all b-hadron species are produced: B0, B+, Bs, Bc , b …

• Only competition before LHC is from CDF+D0 (lower statistics, poorer PID)

• ATLAS and CMS will only have lepton trigger, poor hadron identification

230b100b

σ(B)

2003 2007

BdJ/KS

Bd

BsJ/ Bs DsK

LHCb 1y



VELO

Vertex locator around the interaction region

Reconstruction of decay vertexes of b and c hadrons and IP for flavor tagging + fast response for L1



VELO (II)

Design requirements and criteria:a) Impact parameter resolution b) L1 trigger fast stand alone patter recognition

MAIN IDEA: for B hadrons (IP)rz large but (IP)xy small the L1trigger first reconstructs in rzand then in 3d ONLY the tracks with large IP strips with constant r and (in other sensors) radial strips with stereo angle of 10-200

multiple scattering in RF foils and detectors

Δ01

Δ02

r1 r2

track

IP

intrinsic resolution of the sensors

to have an equal contribtution from the 2 measured R points: σ2= σ1· r2/r1 strip pitch increasing linearly with radius

small!!!

small extrapolation factor



~1m

Interaction region Downstre

am

21 stationsRetractable detector halves

VELO(III)

• 21 silicon tracking stations placed along the beam direction

• 2 retractable detector halves for beam injection periods

(up to 30 mm)

• an average track crosses 7 stations

while <0.1% crosses <4 stations

+ some geometrical constraints:primary vertex σ(z)~5.6cm ± 2 σeta coverage required: ( 15-250mrad) - maximum wafer sizes 100mm - minimum safe radius 8mm



VELO(IV)

x=5% of X0

σ=8μm

up to 3GeV/c it is multiple scattering dominated!lop p tracks limit the L1 performance!!

from simulation

30 μm



VELO Sensor design

• 2 sensor types: R and – R measuring gives radial position – measuring gives an approximate azimuthal

angle

• Varying strip pitch– 40 to 102 m (R – sensor)– 36 to 97 m ( – sensor)

• First active silicon strip is 8.2 mm from the beam line

• n+-on-n DOFZ silicon– minimises resolution and signal loss after type

inversion – the high field side is always on the strip side in

order to prevent loss of resolution and signal

• Double metal layer for detector readout

R-measuring sensor:

(concentric strips)

–measuring sensor:

(Radial strips with a stereo angle)



Double sided modules

(1 x R and 1 x sensor)

Cooling contacts

Carbon fibre paddle

TPG* substrate with carbon fibre frame

16 Beetle chips

Silicon Sensor

Secondary vacuum ChamberRetracting Detector Half

Silicon operating temperature -7oC

*Thermalised Pyrolytic Graphite

VELO in the Vacuum



VELO environment

• VELO sensors operate in a harsh non-uniform radiation environment

– fluence to inner regions 1.3 x 1014 neq./cm2

– fluence to outer regions 5 x 1012 neq./cm2

• Estimated to survive 3 years

Illustration of Vdep …

R/cm

Vdep



Tracking system

Tracking system and dipole magnet to measure angles and momenta



A particle of (pX, pY ,pZ) transversing (0,BY,0) receives a momentum kick of ΔpX=-e∫ BY dz and p= ΔpX/(sin α IN - sin α OUT)QUESTION: how to get pT?

Then σP/p = 2* (σX/L) ·p/ (e∫ BY dz ) with L the lever arm(Kleiknecht, Phys Rep,84, pp 85-161(1982)) .( σP/p )MS ~√x/X0, independent of pminimise material!!

To achieve σP/p ~0.5% at 100GeV/c, assuming some σX =100μ of detector point resolution, L~2.5ma bending power of ~4Tm is needed

warm magnet: 2 Al coils + iron yokeexcitation current : 2x2MApower dissipation: 4.2MWL(coil)=2H !!!

but easy ramp up and possibility to revert the field to check systematics on B asymmetries…

The spectrometer and the magnet

z

x

BY

z

y

BY

α IN



TTIT

OT

T1T2T3

The tracking chambers

4 layers/station (2 stereo)

4 layers/station (2 stereo)

1.3% of the area but20% of the particles!!!!occupancy<0.5%

type inversion NOT of concern here!!

occupancy<7%

Cdet~50pF

straw (=cannuccia) tubes; 5mm cell diameterAr/CO2; light matrix nomex; light wrapping (Al)



Track reconstruction (I)

In BJKs 25% of Ks decay in the VELO acceptance 50% before the TT

25% downstream of TT

reconstructed tracks72 on average in bb event: 26 long 11 upstream 4 downstream 26 VELO 5 T

VELO

TT

TT

T1-T3



Track reconstruction(II)

Example of reconstruction strategy: for Long tracks

A) FORWARD TRACKING (90% of long tracks)

1) start from a VELO seed (straight lines, low B field, NO p information)

2) combined with T-seed (parabola, B information)

3) search for TT hits

B) BACKWARD TRACKING

4) from remaining T hits extrapolate back to VELO

5) all tracks refitted with Kalman filter (dowstream to upstream)



Track reconstruction(III)Long tracks

13.3 VELO, 17(22) IT(OT), 4 TT 98.7% of hits correctly assigned!!



Track reconstruction(IV)Long tracks

Ks reconstruction in BJKs

DD

LL

LU

σ=4MeV

ε=55-75%multiple scattering dominated up to 100GeV

σ



RICH

Two RICH detectors for charged hadron identification



RICH (II)photon detectors

radiator gas (n)

beam pipe

mirrors



RICH (III)

charged particle

C

if v>c/n

or β>1/n



RICH (IV)

charged particle

C

2

111

cosp

m

nnC

particle mass!



2 RICH, 3 Radiators

RICH1upstream of the magnet

• Aerogel (2 - ~10 GeV/c); n=1.03

• C4F10 (10 -~60 GeV/c); n=1.0014

RICH2 downstream of the magnet

• CF4 (16 – 100 GeV/c); n=1.0005

2

2

)(

11sin)(

nN C

for low n needs a longer path for the charged particle

2m1m



Typical event

Question: what are the Aerogel rings?



Particle ID

3 radiators provide excellent pion/kaon separation !

2

11

cosp

m

nC



Particle ID

In BDsK

Momentum (GeV/c)

/ K

se

par

atio

n

Provide > 3–K separation for 3 < p < 80 GeV



Particle ID

In BDsK

and for kaon/proton…

it is possible to tune the PID cut (efficiency/purity) depending on the specific physics analysis



Calorimeter system

Calorimeter system to identify electrons, hadrons and neutrals

Important for the first level of the trigger

e

h



Muon system

Muon system to identify muons, also used in first level of trigger



Trigger (I)

The 3 levels of the LHCb Trigger – Level-0 hardware trigger (10 MHz 1MHz ; 4μs latency)

• Fully synchronous and pipe-lined (deadtime < 0.5%)• Pile-up System• Calorimeter and Muon high pT e, , 0,, or hadrons• Flexible L0 Decision unit

– Level-1 software trigger (1MHz 40kHz ; max latency 1ms)• Partial read-out: Vertex Detector (VeLo), Trigger Tracker (TT) and L0

summary p info thanks to magnet fringe field!!!– High Level software trigger (HLT)(40kHz200Hzstorage; 10ms)

• Full read-out: all detector data

Com

mon

h

ard

ware

In 10 Mhz of crossings with visibile pp interaction 100kHz of bb pairs; only 15% will have one Bwith all decay products in the accepatance; and BR for CP violation are at 10 -3 level!!!

At LHC energies bbar events very similar to minimum bias except for 2 things:1) high pT of decay products2) detached secondary (and tertiary) vertexes

The challenge:



~ O(1) kHz

How to determine the rejection level demanded by the L0?

1) Luminosity 2) L0 output rate

defines the minimum bias retention i.e. the rejection level

Trigger (II)



Muon system and trigger(I)

track finding (straight line to IP)and pT calculationquestion to students: how is pt calculated from muon system alone?

Triggering: OR of 5 stations minimum p of 5Gev (not pT!!!); rates varying from 100Hz/cm2 to 500kHz/cm2 higher than ATLAS or CMSMuon id. tagging and final state reconstruction

logical layout

high rate, high efficiency and ageing MWPCand Triple-GEM for M1R1 Ar/CO2/CF4 gas mixtures

with F.O.I. (few pads in the bending plane..)

each station has a pad segmentation



Muon system and trigger (II)

offline muon i.d.

standalone pT reconstruction ~ 20%

trigger performance

Main background: π and μ decays need to reduce by 50-100

less efficient at low p due to multiple scattering and decays in flight



Calorimeter system and trigger(I)

projective geometry: ECAL, SPD, PSD 4x4, 6x6 and 12x12 cm2

HCAL 13x13, 26x26 cm2

Preshower e from π± (introduces a longitudinal segmentation in the calo)SPD e from π0

In the muon trigger the signal dominates the only parameter to control the trigger rate is pT

For electron, completely different environment from the muons : background dominates!!!

irreduciblebackground

(suppressed at L1)

12% of λI



Calorimeter system and trigger(II)

Hadronic: iron-scintillating tiles with WLS fibers e.g. ATLAS

E 10% , 5.6λI

80%

performance of the hadron trigger(essentially a pt cut)less efficient of e and μ

cluster 2x2 cells

offline electron i.d.

E 1% ,

10%Electro-magnetic: Shashlik type

P.M.

25X0

e.g. DELPHI LumiMON

WLS fibers

performance of the electron trigger

BJ KR

pT

Bπ πR

pT

ε

ε



PILE-UP VETO IP (95% of lumi)

vs cut vs luminosity

Why is it useful?



HCAL trigger domina

tes

MUON trigger domina

tes

ECAL trigger domina

tes

L0 performance

OR

from the other B! typically in one unit of rapidity

less rejection



L1 ingredients

Makes use of the 1) VELO 2D tracks IP

2) VELO+TT pT

3) L0 information



Level-1 Decision Algorithm

Bandwidth division:

Overlaps are absorbed in this direction

Generic

Single-muon

Dimuon, general

Dimuon, J/PsiElectron

Photon

1) generic algorithm (IP+pT of PT1 and PT2) + specific (level 0 signatures+ 3D track reconstruction )



L0 efficiencyL1 efficiency

L0*L1 eff.

Combined efficiency of L0 and L1



Trigger Rates Overview

Level-0 Level-1HLT

L1-confirmationHLT

Full reconstruction

5.6%

1%



The Physics

We concentrate here on few benchmark measurements driving the experiment design

•B0s D-

sπ+ ΔmS

•B0d J/ S β

•B0s J/ χ and ΔΓS

•B0s Ds

K-+ γ



Time-dependent decay ratesB(BS) decay to a final state f:

q/p=exp(-iφ)=exp(2iχ)(phase of Bs-Bsbar mixing)



CP violating asymmetriesff and and

ff Any difference between or CP violation



The method for measuring the time dependent asymmetry: a case study

BSDSK(π)

1) reconstruct the signal B2) tag the flavor of the other b at production (always b and bbar produced)3) measure asymmetry vs time reconstruct the proper time



Event selection(I)

τ(Bs)=1.5psτ(Ds)=0.5ps

few cm

bachelor = s.m. celibe, scapolo

+<pB>~80GeV



Event selection(II)

Two types of background:

1) from other B decays similar to the one considered

2) combinatorial: the dominant contribution assumed from forward bb events107 generated(only few minutes of LHCb data taking)estimates statistically limited upper limits derived on S/B

sometimes some cuts relaxed (e.g. invariant massof B) to increase statistics it will be determined from the data using sidebands of mass distributions



Event selection(III)

track selection 1) some minimum pt (~ 300 MeV/track)+

primary vertex reconstruction quite good due to ~60 tracks even if it is boosted

ε=98%

2) some PID on tracks(this plot concerns the bachelor)



Event selection(IV)Ds vertex selection

+unconstrained vertex fit constrained vertex fit

+cuts on IP and D to PV



Event selection(V)Bs vertex selection

+unconstrained vertex fit constrained vertex fit

signed distance between B and D

better than for D due tolarge opening of bachelor and Ds (large B mass)

collinearity of p(B) and distance primary-secondary vertex



Event selection(VI)

Bs invariant mass

σ(M)~14MeV

B0

error dominated by p measurementin the spectrometer

the VELO provides the angle

after identification with RICH!!!!



Annual yields and backgrounds

geometrical and secondary interactions

track finding



Flavor tagging(I)Full reconstruction or even partial very difficult: small reconstructible BR, geometric acceptance,reconstruction efficiencyrely on charge correlations of decay leptons (bl) or kaons (bcs) large tagging efficiencybut sometimes erroneus tags

leptonspTkaonsIP

wrong tags: leptons from π and K decays, bcl, BDS+X give two kaons and flavor oscillationsfor neutral B’s



Flavor tagging(II)

Wrong tag fractions will be determined from the data: from control channels which are flavor-specificsuch as JK*0

to be compared to CDF/D0~1% and B-factories ~30%

wrong tag fraction



Proper time

proper time τ resolution~ 30-40 fs

dominated by the error on the decay length(the error on p accounts only for 8fs)

very important due to fast oscillations of Bsbrings anyway a 30% of dilution of the asymmetry!!

L= γβcτ = p/m cτ

(ATLAS and CMS ~50-70fs)



Mixing measurement : B0s D-

sπ+

Final state: flavour-specific (can be reached by the B and not by the Bbar), non CP eigenstate only one single tree diagram contributes (B0

s D-sπ+ does not exist)

it does not lead to CP violating observables but…

dilution factor: wrong tag fraction and experimental resolutionNot CP viol

sb

W

sc

du

0sB

sD

large branching fraction + large expected asymmetry whose amplitude we can calculate

those who have oscillated



Error on the amplitude vs ms

can make a 5 measurement in one year for ms up to 68 ps-1 (far beyond Standard Model expectation of 20 ps-1)

Once a Bs–Bs oscillation signal is seen, the frequency is precisely determined: ms ) ~ 0.01 ps-1

tme S

t

Dscos

2cosh~



CP asymmetry: B0d J/ S

In the S.M. Adir=0 (i.e. |λ|=1) and Amix=Im(λ)=sin(2β)

σ=12 MeV/c2 9MeV/c2

Ks downstream Ks long

Ksd d

md = 0.502 0.006 ps-1

Final state: flavor non specific, CP eigenstate

() ~ 0.60, () ~ 0.023 in one year



CP asymmetry: B0s J/

• Bs counterpart

of B0 J/ψ KS

• In Standard Model expected asymmetry sin 2very small ~ 0.04 sensitive probe for new physics

• Reconstruct J/ or ee, KK

• Final state is admixture of CP-even and odd contributions angular analysis of decay products required

() ~ 1.70, (s/s) ~ 0.02 in one year

σ(M)~15MeV



1. Bs->DsK (14-150)

2. B->, Bs->KK

(4-60)

3. B->DK*(7-80)

not affected by new physics in loop

diagrams

affected by possible new physics in

penguin

Determine the CKM parameters A, ,

independent of new physics

Extract the contribution of new physics to the

oscillations and penguins

affected by possible new physics in

D-D mixing

Measurements of γ

1 year sensitivity



CP asymmetry: Bs Ds K-+

large interference effects

2 asymmmetries 6 observables…

Final state: flavor non specific non CP eigenstate



CP asymmetry: Bs Ds K-+

• Very little theoretical uncertaintyInsensitive to new physics, which is expected to appear in loops

• Reconstruct using Ds KK

…which are functions of the parameters:(allow for possible strong phase difference δ between the two diagrams)

r=|λ|

-2χ

-2χ

-2χ-2χ



• Fit two time-dependent asymmetries: Ds

+Kasymmetry δ ( Ds

Kasymmetry δ (can extract bothδand(

will be determined using Bs J/ decays extract

Asymmetries for 5 years of simulated data

() ~ 14 in one year

ms=20 ps-1

data generated with



THE END

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The LHCb experiment

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