Studying beauty with LHCb - PowerPoint Presentation

Slide1

Studying beauty with LHCb

First physics from the LHC

Monica Pepe Altarelli

CERN

AAAS, Washington, 20 February 2011

M.C. Escher

Slide2

Outline

LHCb: an experiment dedicated to the b quark

Context, motivations and goals

CKM theory and CP violationThe LHCb detector

Vertex LocatorRICH detectorsFirst results and prospects

Slide3

LHCb b stands for beauty!

Specialises in the study of B-particles (particles containing the

b

(eauty) quark) with the aim of

Measuring the slight asymmetry between matter and antimatter (CP violation) in B-particle decays.

CP symmetry: matter-antimatter symmetry C = charge conjugation (swapping particles & antiparticles)

P = parity (spatial inversion, like reflection in a mirror)

Explaining the observed imbalance between matter and antimatter in the Universe requires CP violation.

B and anti-B particles are unstable and short-lived, decaying rapidly into many other particles. B-particle decays have emerged as an optimal laboratory to study CP violation.

Slide4

From atoms to quarks

Protons and neutrons are particles made of quarks

Slide5

Light quarks: u,d,s

There are two classes of quark composites:

The ‘

-

’ indicates an antiquark

Slide6

Quarks

By now we know of six quark ‘flavours’

u

d

c

s

t

b

up

charm

down

strange

top

beauty

or

bottom

Q=2/3

mass

(Gev/c

2

)

Q=-1/3

mass

(Gev/c

2

)

0.0025

1.3

173

0.0048

0.1

4.5

Six flavours in three “families” or “generations” of increasing mass

Q

is the electric charge, in units of proton charge

Slide7

Heavy-quark composites

Mesons with c

Mesons with

b

-

Heavy quarks are unstable and decay via weak interactions to lighter quarks

V

cb

is a matrix element for quark-flavour mixing (Cabibbo-Kobayashi-Maskawa CKM matrix V

CKM

)

Slide8

Cabibbo-Kobayashi-Maskawa CKM matrix

The probability of the transition from flavour i to flavour j is ~ |V

ij

|

2

Probability of b

c decay ~ |V

cb

|

2

“

Strengths” of weak interactions between the six quarks. The intensities of the lines are determined by the CKM elements. Each quark has a preference to transform into a quark of its own generation.

Slide9

Cabibbo-Kobayashi-Maskawa CKM matrix

Cabibbo 1963, Kobayashi & Maskawa 1973

CKM theory specifies rates of different quark weak decays and predicts matter-antimatter asymmetries in these decays (CP violation)

In particular, large CP violating asymmetries are expected in b decays!

Slide10

Cabibbo-Kobayashi-Maskawa CKM matrix

Cabibbo 1963, Kobayashi & Maskawa 1973

CKM theory specifies rates of different quark weak decays and predicts matter-antimatter asymmetries in these decays (CP violation)

In particular, large CP violating asymmetries are expected in b decays!

2008 Nobel prize to K&M: Matter-antimatter asymmetry requires the existence of at least three families of quarks in nature

Slide11

Importance of testing the CKM theory

The CKM theory is a main building block of the Standard Model.

All experiments of flavour-changing decays have so far shown an overall good agreement with the CKM theory.

However, there are reasons to expect deviations from the Standard Model in the range of energies explored by LHC.

These New Physics effects presumably will also modify the CKM predictions.

Observing deviations from the CKM theory is one of the main goals of the LHCb experiment and would have important physical implications.

The precise study of b decays, the observation of very rare decay modes, and the accurate measurement of CP violation asymmetries in b decays is an essential tool for the identification of New Physics

Slide12

Heavy-quark lifetimes

The heavy quarks have a short lifetime t:

t

charm ~ 10-12

stbeauty

~ 1.5 10-12

st

top

~ 5 10

-25

s

While the t quark lifetime is too short, the b and c quarks live long enough so that we can study their production and decay sequence in detail. The b quark is ideal for experimental study of V

CKM

and CP violation:

relatively long lifetime

high mass (many possible decay final states)

larger CP asymmetries than for s and c

t

~

1/(m

5

|V

CKM

|

2

)

Slide13

Heavy-quark lifetimes

t

beauty

~ 1.5 10

-12 s

The b lifetime is long enough for it to propagate an observable distance D when produced at the LHC

: D = β

g c t

At the LHC:

b =

v/c

~ 1

g =

E/mc

2

~ 20

(E: b energy)

D =20•3•10

10

•1.5•10

-12

~ 1cm

Slide14

pp collision Point

Vertex Locator

VELO

Tracking System

Muon System

RICH Detectors

Calorimeters

~ 1 cm

B

Movable device

35 mm from beam out of physics /

7 mm from beam in physics

LHCb

Slide15

Slide16

Interaction

Point

Slide17

LHCC open session

17 February 2010

730 members

15 countries

54 institutes

Member countries of the LHCb Collaboration

Slide18

Why does LHCb look so different?

The B mesons formed by the colliding proton beams (and the particles they decay into) stay close to the line of the beam pipe, and this is reflected in the design of the detector.

Other LHC experiments surround the entire collision point with layers of sub-detectors, like an onion, but the LHCb detector stretches for 20 metres along the beam pipe, with its sub-detectors stacked behind each other like books on a shelf.

p

p

bb

p

p

Slide19

Specific features of LHCb

Particle detection in the forward region (down to the beam-pipe)

Excellent resolution for localisation of b decay vertices (Vertex Locator)

Excellent particle identification to distinguish p, k, π (RICH detectors)

Slide20

Vertex Locator (VELO)

The VELO is a precise particle tracking detector, which surrounds the pp collision point inside LHCb. It is composed of 21 stations, each made of two silicon half disks.

Slide21

Vertex Locator (VELO)

LHC proton beams pass through the full length of the detector, safely encased within a beryllium pipe. The only point where the beams collide, and B and anti-B particles are produced, is inside the VELO.

Slide22

Vertex Locator (VELO)

The VELO measures the distance between the point where protons collide (and where B particles are created) and the point where the B particles decay.

The B particles are therefore never measured directly - their presence is inferred from the separation between these two positions. VELO can locate the position of B particles to ~10

μ

m

B

s

K



K



K





+

D

s

B-production

at pp-collision



primary vertex

B-decay



displaced vertex

B

~1 cm

Slide23

B

s



J/y f event

B

s decay length is 20mm!

Bs

J/y f

J

/y



μ

+

μ

-

f

K

+

K

-

Slide24

Ring Imaging Cherenkov (RICH)

detectors

RICH detectors work by measuring emissions of Cherenkov radiation. This occurs when a charged particle passes through a medium faster than light does

(v >c/n, with

n

refractive index)

.

The particle emits light in a cone with cos

θ

c

=1/βn

, which the RICH detectors reflect onto an array of sensors using mirrors.

By measuring

θ

c

the velocity

β

of the particle is found. With knowledge of its momentum the mass of the particle can be found, which is unique for its identity.

cos

θ

c

=1/βn

RICH 2

kaon ring

Slide25

Ring Imaging Cherenkov (RICH)

detectors

Assembly of the high-precision mirrors used to focus the Cherenkov light onto the photon detectors

Slide26

Decoding LHCb event display: B

+→J/

ψK+

Top view

(24mx12m)

Face view(1mx0.5m)

Collision region(0.7mmx10mm)

Slide27

First results (with ~37 pb

-1

of luminosity)

Peak luminosity increased within ~1 month by

factor 100

!

(L~10

30

to

10

32

cm

-2

s

-1

)

With this small amount of data

LHCb

is already competitive

with B-factories and the

Tevatron

experiments

for many interesting decay modes

Slide28

Lots of B-particles already observed!

In 2011, we expect to produce

~10

11

(~109 at B-factories in their lifetime!)

@LHCb all species of particles containing a b-quark are produced:

First LHCb measurement @7 TeV

Slide29

RICH Particle Identification performance:

B

h+h’-

with h=p,k,

29

No RICH used

Slide30

RICH Particle Identification performance:

B

h+h’-

with h=p,k,

30

B

0

→

ππ

B

s

→KK

(BR = 5 x 10

-6

!)

B

0

→K

π

Λ

b

→pK

No RICH used

Deploy RICH to isolate each mode

Slide31

RICH Particle Identification performance:

B

h+h’-

with h=p,k,

31

B

0

→

ππ

B

s

→KK

(BR = 5 x 10

-6

!)

B

0

→K

π

Λ

b

→pK

No RICH used

Deploy RICH to isolate each mode

Slide32

Look in more detail at B

0

K

+-

~840 B

0

→K

+

π

-

Tighter selection



B

0

s

→K

+

π

-

Slide33

Look in more detail at B

0

K

+-

~840 B

0

→K

+

π

-



Separate into B

0

and B

0

Raw result shows clear evidence of CP-violation

Analysis being optimised & account being taken of some small corrections

2001: experimental proof of CP violation in B-system by B-factories (BELLE & BaBar)

Tighter selection



B

0

s

→K

π

-

Slide34

CP-Violation with

B

0

s system

mesons oscillate into their anti-matter particles at an astonishing 3 trillion times per second (3•10

12 /sec)!

Avenue opened by CDF experiment @Fermilab

Standard Model predicts CP violation effects at a few percent level

Use the decay

B

s



J

/

y

f

 ~900 events so far,

~20 times more in 2011!

CDF+D0 ~10000 events with 300 times more Luminosity

Slide35

CP-Violation with

B

0

s system

Another decay channel that can be used to study CP-violation in B0

s system: B

s→J/Ψ

f

0

First observation of this decay by LHCb!

Slide36

Probing New Physics in some very rare decays:

B

s

μ+

μ-

B

sμ

+

μ

-

is a very rare decay never observed so far

In the SM it has a relative probability of

3.2•10

-9

with respect to all other B

s

decays.

Since it is so rare in the SM, it provides us with a good chance to observe the effect of a new decay mechanism as is the case in some plausible New Physics Models (e.g. SUSY)

M. Aoki,

FPCP-2010

SM

Slide37

sensitive region

Probing New Physics in some very rare decays:

B

s

μ

+

μ

-

LHCb: Blind analysis of B

s

μ

+

μ

-

Look at mass vs variable based on decay topology

To avoid unconscious biases, data in sensitive region blinded

“Unblinding” only when analysis considered ready

Now almost time to open the box!

LHCb

competitive with

Tevatron

with first 37

pb

-1

!

Results available very soon!

Exclusion limit @ 90% C.L.

Slide38

Conclusion

Lots of beautiful data from LHCb!

The 2010 data already give LHCb the statistical precision for many competitive measurements

First cross-section measurements and first observations

Bs

 μμ and Bs

J/ψφ will reach a new sensitivity regime very soonExciting prospects and rich physics programme for 2011-2012!