The Large Hadron Colliderbeauty Experiment

Precision measurements of loop-dominated B

decays as possible probe for New Physics

. – p.1/39

Outline of LHCb Lecture

date content

23.05. Introduction to B Physics & indirect Searches

The LHCb experiment

30.05. Review CKM Physics:

Neutral meson mixing, CP asymmetrie

06.06. Physics analysis techniques at LHCb;

Standard Model physics at LHCb

13.06. New Physics searches with the LHCb Experiment

Slides of this lecture:http://www.physi.uni-heidelberg.de/∼menzemer/LHC08.html

. – p.2/39

LiteratureGeneral Particle Physics:

Jonathan Allday: “Quarks, Leptons and the Big Bang”(entertaining, almost no formula, however informative)

David Griffiths: Introduction to elementary particle physics(very good introduction, “a bit old” no modern developments)

B Physics:

The BaBar Physics Book: http://www.slac.stanford.edu/pubs/slacreports/slacr-504.html

(good intro to CKM physics (1st chapter), however status of 2001)

Yosef Nir: CP violation a new area http://arxiv.org/abs/hep-ph/0109090

Helene Quinn: B Physics and CP violation http://arxiv.org/abs/hep-ph/0111177

LHCb specific:

LHCb Technical Design Report (TDR), especially TDR 9http://lhcb.web.cern.ch/lhcb/TDR/TDR.htm

. – p.3/39

15. June 1977Till 1977:

2 lepton families

(

e

νe

)

,

(

µ

νµ

)

& 2 quark families

(

u

d

)

,

(

c

s

)

(+ antiparticles)All sofar observed particles could be composed out of them.

Observation of Y(4S) = |bb̄ > atFermilab: proton beam on fixedtarget experiment.

. – p.4/39

Quark MassesTop quark (∼ 171 GeV) too heavy to form bounded states.Decays immediately dominantly in: t → bW

b heaviest quark which forms mesons (|qq̄ >) & baryons (|qqq >)Due to energy conversation, particle decay in lighter particles.→ B hadrons have large phase space for decay (many modes).

. – p.5/39

B Productionbb̄ pair production (> 99.9%) (only exception: top decays)

b

b

g b

bg

e

e

+

− q

qb

bEM interaction@ e+e− collidere.g. LEP, SLAC, KEK

strong interaction@ pp̄ or pp collidere.g. Tevatron (Fermilab), LHC

qq̄ → bb̄ dominant at Tevatron(√

s = 1.98 TeV)

gg → bb̄ dominant at LHC(√

s = 14 TeV)

. – p.6/39

B ProductionOne low and one very high momentum parton produce heavyparticles, however very high momentum partons are very rare.→ heavy particle production via two partons of ∼ same energy→ heavy particles have little boost → central region

For light particles a low and a medium momentum parton work.→ it is very unlikely that the two partons have same energy→ light particles are produced in forward direction.

2 × m(b) ≈ 10 GeV

m(Higgs) >114 GeV

2 × m(t) ≈ 350 GeV,

m(“New Physics”) ≈ 100 GeV - 1 TeV

In pp/pp̄ collisions optimal B detec-tors are forward spectrometers!

θ

. – p.7/39

bb̄ Production x-Section

∼ 1011 bb̄ pairs per year at LHCrunning a nominal luminosity!

B physics is high statisticsphysics!

. – p.8/39

B Hadronisation and DecayHadronisation:

B mesons: B+ = |b̄u >, B0d = |b̄d >, B0

s = |b̄s >, Bc = |b̄c >

B baryons: sofar discovered: Λb = |udb >, Ξb = |dsb >,

Σ+b = |usb >

heavier ones should exist not yet discovered.

Hadronisation ratio (if enough center of mass energy available):B+ : B0 : Bs : λb = 0.4 : 0.4 : 0.1 : 0.09Other baryons have very small x-section

Decay:

Average lifetime of B hadrons ∼ 1.5 ps

Decay into lighter quarks → transition to quarks of other families→ only possible via weak interaction

. – p.9/39

Decay of φ = |ss̄ >

strong IA weak IA EM IA

sus

us s

φK

K+

− sus

us s

φK

K+

−sus

us s

φK

K+

−

s

u

s

K−

K+

g Z0 γ

sφ

s

uW−

strong decay by far dominant!

quark sum identical in initial and final state!

. – p.10/39

Decay of B0= |b̄d >

bπ

π

d

u

u

d

+

−dB

W+

0

1 b quark in initial state, 0 b quarks in final state

→ only possible via weak interaction!

All B decays are weak decays, however large phase space→ long lifetime of 1.5 ps

. – p.11/39

Tons of Decay Modes ...

300 known B+

decay modes (alllisted in PDG)

decays for analysis of lifetime, CP violation, ...: BR ∼ 10−4 − 10−6

rare decays: BR ∼ 10−8, e.g. Bs → µ+µ−

very rare decays: BR ∼ 10−10

(additional have to BR of B daughters into account)

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Flavour Mixing

mass eigenstates 6= eigenstates of weak interaction

d′

s′

b′

=

Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

d

s

b

weak CKM matrix mass

u c t

d s b

u

d ’

c

s ’

t

b ’

CKM-Matrix fully determined via: 3 angles & 1 complex phase

⇒ 4 fundamental Standard Model parameters. – p.13/39

B Physics: Test of the CKM Mechanism

d′

s′

b′

=

Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

d

s

b

πBb

u

d

lν

Bb

d

lν

c D

b

bd

dt

tB Bdd

bs

st

tB Bss

b

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (Vtb ∼ 1). – p.14/39

B Physics

Standard Model Quark sector extremely well understood:

• Very precise theoretical predictions for weak IA

• High statistic B decays give experimentally high precisionmeasurements.

→ B physics: unique place to perform precision tests of theStandard Model (measurement of branching ratios, CP violation...)

Any deviation from Standard Model expectation:Unambiguous sign of “New Physics”

. – p.15/39

Motivation

Precision B physics as probe for New Physics in loop processes:

Box diagrams Penguin decays

Many New Physics models predict deviations from SM predictions.

Complementary to direct searches by ATLAS and CMS.

Direct searches sensitive to particles up to 1 TeV.

Indirect searches are sensitive to particles up to 10 TeV.

(new particles in the loop are virtual). – p.16/39

Reminder: Quantum Mechanics

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Examples from the Past

GIM Mechanism

Observed branching ratio K0 → µ+µ−

BR(KL→µ+µ−)BR(KL→all) = (7.2 ± 0.5) × 10−9

In contradiction with theoreticalexpectations in the 3 quark model

➡ Glashow, Iliopolus, Maiani (1970):

Prediction of a 2nd up type quark,additional Feynman graph cancelsthe “u box graph”

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Examples from the Past

Indirect measurement of the Top quark mass at LEP

. – p.19/39

Examples from the Past

1994 at LEP (√

s = 91 GeV): measurement of partial decay width

of Z: → m(t) = 179 +12/-9 GeV/c2

Top quark discovery 1995 at the Tevatron (√

s = 1.8 TeV)

m(t) = 178 ± 4.3 GeV/c2;

. – p.20/39

Examples from the Past

Precision electro-weak physics @ LEP and the Tevatron:

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B Production

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Advantage at pp collider

e+e− ppBabar/Belle e.g. LHCb

blablubblablubblablubblablub blablubblablubblablubblablub√

s=Y(4S)=10.6 GeV√

s=14 TeV

(limit energy due to synchrotron radiation!) heavy & excited B states

B0, B+ Bc, Bs, Λb, Ξb B∗, ...

decay length: ∼ 200 µm decay length: ∼ 2 cm

(produced almost at rest) → good proper time resolution

109 bb̄ pairs/y 1011 bb̄ pairs/y

cent

er-o

f-m

ass

ener

gypr

oduc

tion

rate

. – p.23/39

Challenge at Hadron-Collider

Babar/Belle e.g. LHCb

only BB̄ daughter tracks about 400 additional tracks

simple selection criteria σ(pp → X) = 1000 σ(pp → bb̄X)

isotrop bb̄ events Trigger ≡ key to B-Physics. – p.24/39

B Physics at LHC

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The LHCb Cavern

. – p.26/39

Typical B Event

decay length L typically ∼ 7 mmmomentum of decay products: 1-60 GeVsignal event very background like ...

⇒ high vertex resolution to identify PV & SV

⇒ good momentum resolution to measure proper time (cτ = Lmp

)

⇒ particle ID to select decay products(background mixture: 80% pions, 10% kaons, 5% protons)

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LHCb Experiment

. – p.28/39

Berilium Beam Pipe

Particles in forward direction traverse beam pipe under largeangle.→ Multiple scattering, imprecise vertex resolution.Replace “standard” beam pipe by ultra fine foil in IA region.Vertex detector (directly at beam pipe) as well in vacuum.

. – p.29/39

Sketch of Beam Pipe

Al bellows

25 mrad10 mrad SS bellows

Be

AlBe

UX85/1

UX85/2

UX85/3 UX85/4

2800

858

3224

7100

13100

14400

20088

IP

19696

SS bellows

SS

100

ID54

5000 10000 15000 20000All distances in mm

ID38

2

Al flange

10 mrad cone to guide particles out of LHCb detector w/o IA.25 mrad cone for particles which are reconstructed but not in VeloExtremely thin foil between both cones, where particles leavebeam pipe. → vertex detector is as well in vacuum.

Velo Vacuum box

. – p.30/39

Vertex Detector

2 halves, each 21 silicon discs; track resolution ≈ 30 µm.

Detector halves will be moved before each run (fill of proton

bunches in LHC) in open position (10 cm from beam) and be closed

(8 mm) once beam is stable. Change of position < 1 µm. – p.31/39

Magnet and Tracking

Angle of tracks before and after magnet → momentum.Plan half of the runs with positive and half with negative polarity.Important for asymmetry measurements!T stations important to guide to calorimeters & muon chambers.

. – p.32/39

RICH Detectors

Kaon, pion & proton ID for low momentum tracks (10-100 GeV);

unique to LHCb.. – p.33/39

RICH Detectors

Rich 1:

Aerogel: n = 1.03 (p: 1-10 GeV)

C4F10: n = 1.0014 (p: < 60 GeV)

Rich 2:

CF4: n = 1.0005 (p: < 100 GeV)

250 mrad

Track

Beam pipe

Photon Detectors

Aerogel

VELO exit window

Spherical Mirror

Plane Mirror

C4F10

0 100 200 z (cm) . – p.34/39

RICH Detectors

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Trigger System

. – p.36/39

High Level Trigger

Write out at 2 kHz:less read out channels and events less busy → less data/event(50 kB) compared to CMS and ATLAS

Select B daughter tracks by requiring large impact parameter (orsecondary vertex)

Secondary VertexPrimay Vertex B

.impactparameter

For rare B decays, largest background comes from other B decays→ full B reconstruction on trigger level for some decay modes.

. – p.37/39

The LHCb Experiment

. – p.38/39

B Physics & LHC Detectors

ATLAS/CMS:

• optimized for high-pT discovery

• central detectors, |η| <2.5

• B physics using muon triggers

• aim for highest possible luminosity:

• run at L → 2 × 1033cm−2s−1 n=5,

• later L → 1034cm−2s−1 n=25

LHCb:

• designed to maximize B acceptance

• excellent vertexing and particle ID

• forward spectrometer, 1.9 < η < 4.9

• “low” pT & impact parameter triggers

• Luminosity tuneable by adjusting beam focus:

• run at L → 2 × 1032cm−2s−1 n=0.5

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