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The LHCb experiment. Walter Bonivento – I.N.F.N. Sezione di Cagliari - Italy. Why B physics at the LHC. 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.

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The LHCb experiment

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

The LHCb experiment

why b physics at the lhc
Why B physics at the LHC

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.

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

The LHCb experiment

unitarity triangles
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:

Differ at the percent levelphase of Vts




 measurement of the angle g will be crucial

The LHCb experiment

which b decays to measure the angles
Which B decays to measure the angles?














  • and g


The LHCb experiment


A complete program on B Physics includes:

  • Precise g 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

BsDsK, B0D0K*0, B0pp & BsKK,…

B0fKs, Bsff, ... B0rp, B0rr, …

  • Precise measurement of B0s-B0s mixing: Dms, DGs and phase fs.

BsDsp, … BsJ/yf, BsJ/yh(’)

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

B0K*g, B0K*0l+l-, bsl+l-, Bsm+m-...

The LHCb experiment



few mm






Why a forward detector for

B physics at LHC

  • 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
  • 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
  • momentum distribution match particle ID capabilities of RICH detectors  ROOM is available for the detectors, contrary to cylindrical geometry
  • relatively low pT muon triggering possible because iron penetration depends here on pL which is large

The LHCb experiment


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

defines the momentum

range for the spectrometer

and for particle ID

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, …)

The LHCb experiment



Physics requirements(II)

to measure the fast Bs oscillations,where A(mix)α cos(ΔmSτ), 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 γconversions)

- tracker and magnet

The LHCb experiment


The experiment

Single arm forward spectrometer

250/300 mrad

v / h


10 mrad

pp collision

side view

The LHCb experiment


Key issues

  • to avoid high number of interaction / bunch crossings :
  • L = 2 .1032cm-2s-1 for LHCb
  • --> simpler events (one interaction per bunch crossing dominates) and less radiation damage
  • for the detectors
  • σinelastic 80 mb and σbb 0.5 mb
  • --> need an efficient trigger (also on fully hadronic channels)
  • trigger strategy:
  • first level, hardware: large B mass large pT of B decay products; and selection of single interaction events
  • second level, software: large B lifetime  large impact parameters

The LHCb experiment

comparison to other experiments



Comparison to other experiments


LHCb 1y




  • 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 , Lb …
  • Only competition before LHC is from CDF+D0 (lower statistics, poorer PID)
  • ATLAS and CMS will only have lepton trigger, poor hadron identification




The LHCb experiment


Vertex locator around the interaction region

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

The LHCb experiment

velo ii







  • Design requirements and criteria:
  • Impact parameter resolution
  • L1 trigger fast stand alone patter recognition
  • MAIN IDEA: for B hadrons (IP)rz large but (IP)xy small the L1trigger first reconstructs in rz
  • and 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

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 extrapolation factor

The LHCb experiment

velo iii






+ some geometrical constraints:

primary vertex σ(z)~5.6cm  ± 2 σ

eta coverage required: ( 15-250mrad)

- maximum wafer sizes 100mm

- minimum safe radius 8mm

  • 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

21 stations

Retractable detector halves

The LHCb experiment

velo iv

from simulation

  • up to 3GeV/c it is
  • multiple scattering
  • dominated!
  • lop p tracks

limit the L1 performance!!

30 μm

x=5% of X0


The LHCb experiment

velo sensor design

R-measuring sensor:

(concentric strips)

F–measuring sensor:

(Radial strips with a stereo angle)

VELO Sensor design
  • 2 sensor types: R and F
    • R measuring gives radial position
    • F measuring gives an approximate azimuthal angle
  • Varying strip pitch
    • 40 to 102 mm (R – sensor)
    • 36 to 97 mm (F – 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

The LHCb experiment

velo in the vacuum
VELO in the Vacuum

Double sided modules

(1 x R and 1 x F sensor)

16 Beetle chips

Silicon Sensor

TPG* substrate with carbon fibre frame

Secondary vacuum Chamber

Retracting Detector Half

Cooling contacts

Carbon fibre paddle

Silicon operating temperature -7oC

The LHCb experiment

*Thermalised Pyrolytic Graphite

velo environment

Illustration of Vdep …



VELO environment
  • VELO sensors operate in a harsh non-uniform radiation environment
    • fluence to inner regions 1.3 x 1014neq./cm2
    • fluence to outer regions 5 x 1012neq./cm2
  • Estimated to survive 3 years

The LHCb experiment

tracking system
Tracking system

Tracking system and dipole magnet to measure angles and momenta

The LHCb experiment

the spectrometer and the magnet
The spectrometer and the magnet


A particle of (pX, pY ,pZ)

transversing (0,BY,0) receives a momentum kick of

ΔpX=-e∫ BY dz and p=ΔpX/(sin αIN - sin αOUT)

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 pminimise material!!

To achieve σP/p ~0.5% at 100GeV/c, assuming

some σX =100μ of detector point resolution, L~2.5m

a bending power of ~4Tm is needed

warm magnet: 2 Al coils + iron yoke

excitation current : 2x2MA

power dissipation: 4.2MW

L(coil)=2H !!!

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







The LHCb experiment

the tracking chambers







The tracking chambers

straw (=cannuccia) tubes;

5mm cell diameter

Ar/CO2; light matrix nomex;

light wrapping (Al)

4 layers/station (2 stereo)


type inversion NOT of concern here!!

4 layers/station (2 stereo)


1.3% of the area but

20% of the particles!!!!


The LHCb experiment

track reconstruction i
Track reconstruction (I)





  • reconstructed tracks
  • 72 on average in bb event

: 26 long

11 upstream

4 downstream


5 T

In BJKs 25% of Ks decay in the VELO acceptance

50% before the TT

25% downstream of TT

The LHCb experiment

track reconstruction ii
Track reconstruction(II)
  • Example of reconstruction strategy:
  • for Long tracks
  • FORWARD TRACKING (90% of long tracks)
  • start from a VELO seed (straight lines, low B field, NO p information)
  • combined with T-seed (parabola, B information)
  • search for TT hits
  • from remaining T hits extrapolate back to VELO
  • all tracks refitted with Kalman filter (dowstream to upstream)

The LHCb experiment

track reconstruction iii
Track reconstruction(III)

Long tracks

98.7% of hits correctly assigned!!

13.3 VELO, 17(22) IT(OT), 4 TT

The LHCb experiment

track reconstruction iv
Track reconstruction(IV)

Long tracks

Ks reconstruction in BJKs






multiple scattering dominated up to 100GeV


The LHCb experiment


Two RICH detectors for charged hadron identification

The LHCb experiment

rich ii

photon detectors

radiator gas (n)


beam pipe


The LHCb experiment

rich iii

charged particle



if v>c/n

or β>1/n

The LHCb experiment

rich iv

charged particle



particle mass!

The LHCb experiment

2 rich 3 radiators
2 RICH, 3 Radiators



  • RICH1upstream 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

for low n needs a longer path for the charged particle

The LHCb experiment

typical event
Typical event

Question: what are the Aerogel rings?

The LHCb experiment

particle id
Particle ID

3 radiators provide excellent pion/kaon separation !

The LHCb experiment

particle id1
Particle ID

In BDsK

Provide > 3s p–K separation

for 3 < p < 80 GeV

 / K separation

Momentum (GeV/c)

The LHCb experiment

particle id2
Particle ID

In BDsK

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

and for kaon/proton…

The LHCb experiment

calorimeter system
Calorimeter system



Calorimeter system to identify electrons, hadrons and neutrals

Important for the first level of the trigger

The LHCb experiment

muon system
Muon system


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

The LHCb experiment

trigger i
Trigger (I)
  • At LHC energies bbar events very similar to minimum bias except for 2 things:
  • high pT of decay products
  • detached secondary (and tertiary) vertexes

The challenge:

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 pTe, g, p0,m, 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)(40kHz200Hzstorage; 10ms)
    • Full read-out: all detector data

In 10 Mhz of crossings with visibile pp interaction 100kHz of bb pairs; only 15% will have one B

with all decay products in the accepatance; and BR for CP violation are at 10-3 level!!!



The LHCb experiment

trigger ii
Trigger (II)
  • How to determine the rejection level demanded by the L0?
  • Luminosity
  • L0 output rate
  • defines the minimum bias retention i.e. the rejection level

The LHCb experiment

~ O(1) kHz

muon system and trigger i
Muon system and trigger(I)

Triggering: OR of 5 stations minimum p of 5Gev (not pT!!!); rates varying from 100Hz/cm2 to 500kHz/cm2 higher than ATLAS or CMS

Muon id. tagging and final state reconstruction

high rate, high efficiency and ageing  MWPC

and Triple-GEM for M1R1

Ar/CO2/CF4 gas mixtures

track finding (straight line to IP)

and pT calculation

question to students: how is pt calculated from muon system alone?

each station has a pad segmentation

logical layout

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

The LHCb experiment

muon system and trigger ii
Muon system and trigger (II)

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

standalone pT reconstruction ~ 20%

trigger performance

offline muon i.d.

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

The LHCb experiment

calorimeter system and trigger i
Calorimeter system and trigger(I)

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!!!

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



12% of λI

(suppressed at L1)

The LHCb experiment

calorimeter system and trigger ii
Calorimeter system and trigger(II)





Electro-magnetic: Shashlik type

 1% ,



WLS fibers




performance of the electron trigger

e.g. ATLAS

Hadronic: iron-scintillating tiles with WLS fibers


 10% , 5.6λI



Bπ π


offline electron i.d.

cluster  2x2 cells


performance of the hadron trigger

(essentially a pt cut)less efficient of e and μ

The LHCb experiment

pile up veto

IP (95% of lumi)

Why is it useful?

vs cut

vs luminosity

The LHCb experiment


HCAL trigger


MUON trigger


ECAL trigger


L0 performance

less rejection


from the other B! typically in one unit of rapidity

The LHCb experiment


L1 ingredients

  • Makes use of the
  • VELO 2D tracks IP
  • VELO+TT pT
  • L0 information

The LHCb experiment

level 1 decision algorithm
Level-1 Decision Algorithm

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

Bandwidth division:










Overlaps are absorbed in this direction

The LHCb experiment

combined efficiency of l0 and l1
Combined efficiency of L0 and L1

L0 efficiency

L1 efficiency L0*L1 eff.

The LHCb experiment

trigger rates overview
Trigger Rates Overview




Full reconstruction





The LHCb experiment

the physics
The Physics

We concentrate here on few benchmark measurements driving the experiment design

  • B0s D-sπ+  ΔmS
  • B0d J/yKS  β
  • B0s J/yf χ and ΔΓS
  • B0s Ds K-+ γ

The LHCb experiment

time dependent decay rates
Time-dependent decay rates

B(BS) decay to a final state f:


(phase of Bs-Bsbar mixing)

The LHCb experiment

cp violating asymmetries
CP violating asymmetries

Any difference between or CP violation

The LHCb experiment

the method for measuring the time dependent asymmetry a case study b s d s k
The method for measuring the time dependent asymmetry: a case study BSDSK(π)
  • reconstruct the signal B
  • tag the flavor of the other b at production (always b and bbar produced)
  • measure asymmetry vs time reconstruct the proper time

The LHCb experiment

event selection i
Event selection(I)

bachelor = s.m. celibe, scapolo



few cm


The LHCb experiment

event selection ii
Event selection(II)
  • Two types of background:
  • from other B decays similar to the one considered
  • combinatorial: the dominant contribution assumed from forward bb events107 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

The LHCb experiment

event selection iii
Event selection(III)

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


track selection

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

2) some PID on tracks

(this plot concerns the bachelor)

The LHCb experiment

event selection iv
Event selection(IV)

Ds vertex selection

+unconstrained vertex fit constrained vertex fit

+cuts on IP and D to PV

The LHCb experiment

event selection v
Event selection(V)

better than for D due to

large opening of bachelor and

Ds (large B mass)

Bs vertex selection

+unconstrained vertex fit constrained vertex fit

signed distance between B and D

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

The LHCb experiment

event selection vi
Event selection(VI)

Bs invariant mass

the VELO provides the angle


after identification with RICH!!!!

B0 p+p-

error dominated by p measurement

in the spectrometer

The LHCb experiment

annual yields and backgrounds
Annual yields and backgrounds

geometrical and

secondary interactions

track finding

The LHCb experiment

flavor tagging i
Flavor tagging(I)

Full reconstruction or even partial very difficult: small reconstructible BR, geometric acceptance,

reconstruction efficiency

rely on charge correlations of decay leptons (bl) or kaons (bcs) large tagging efficiency

but sometimes erroneus tags



wrong tags: leptons from π and K decays, bcl, BDS+X give two kaons and flavor oscillations

for neutral B’s

The LHCb experiment

flavor tagging ii
Flavor tagging(II)

wrong tag fraction

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

Wrong tag fractions will be determined from the data: from control channels which are flavor-specific

such as JK*0

The LHCb experiment

proper time
Proper time

L= γβcτ = p/m cτ

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 Bs

brings anyway a 30% of dilution of the


(ATLAS and CMS ~50-70fs)

The LHCb experiment

mixing measurement b 0 s d s


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 (B0s D-sπ+ does not exist)
  • it does not lead to CP violating observables but…

those who have oscillated

Not CP viol

dilution factor: wrong tag fraction and experimental resolution

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

The LHCb experiment


Error on the amplitude vsDms can make a 5s measurement in one year for Dms 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: s (Dms ) ~ 0.01 ps-1

The LHCb experiment

cp asymmetry b 0 d j y k s
CP asymmetry: B0d J/yKS

Final state: flavor non specific, CP eigenstate




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

s(b) ~ 0.60, s(|l|) ~ 0.023 in one year

Dmd = 0.502  0.006 ps-1

Ks downstream

Ks long

σ=12 MeV/c2 9MeV/c2

The LHCb experiment

cp asymmetry b 0 s j y f
CP asymmetry: B0s J/yf


  • Bs counterpart

of B0 J/ψ KS

  • In Standard Model expected asymmetry  sin 2c = very small ~ 0.04  c=1.20 sensitive probe for new physics
  • Reconstruct J/y m+m- or e+e-, f  K+K-
  • Final state is admixture of CP-even and odd contributions angular analysis of decay products required

s(c) ~ 1.70, s(DGs/Gs) ~ 0.02 in one year

The LHCb experiment


gaffected by possible

new physics in

D-D mixing

Measurements of γ

2. B->pp, Bs->KK


3. B->DK*


1 year sensitivity

1. Bs->DsK


gnot affected by new physics in loop diagrams

gaffected by possible

new physics in penguin

Determine the CKM parameters A, r, h independent of new physics

Extract the contribution of new physics to the oscillations and penguins

The LHCb experiment

cp asymmetry b s d s k
CP asymmetry: Bs Ds K-+

large interference effects

Final state: flavor non specific non CP eigenstate

2 asymmmetries 6 observables…

The LHCb experiment

cp asymmetry b s d s k1






CP asymmetry: Bs Ds K-+

…which are functions of the parameters:

(allow for possible strong phase difference δ between the two diagrams)

  • Very little theoretical uncertaintyInsensitive to new physics, which is expected to appear in loops
  • Reconstruct using Ds- K-K+p-

The LHCb experiment

Fit two time-dependent asymmetries: Ds+K- asymmetry δ - (g - 2c) Ds-K+ asymmetry δ+ (g - 2c)can extract bothδand(g - 2c)

c will be determined using Bs J/yf decays  extract g

Asymmetries for 5 years of simulated data

s(g) ~ 14 in one year

data generated with

Dms=20 ps-1

The LHCb experiment

the end

The LHCb experiment