The Physics potential of LHCb: CP violation and rare decays in the beauty sector at LHCb

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Submitted on 22 Mar 2002

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The Physics potential of LHCb: CP violation and rare

decays in the beauty sector at LHCb

R. Le Gac

To cite this version:

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Les Rencontre de Physique

February 27, 2000

de la Vallée d’Aoste

March 4, 2000

The Physics Potential of LHCb:

CP violation and rare decays

in the beauty sector

at LHC

R. Le Gac

†

on behalf of the LHCb Collaboration

1. Motivation

2. Methods

3. LHCb detector

4. Physics performances

5. Status and Conclusions

(3)

2 M

OTIVATION

● Standard Model accommodates CP violation through the CKM

matrix.This mechanism has to be validated.

Unitarity property of the matrix

→

Unitarity Triangles (UT):

● LHCb will measure:

– CP violation in the B

_s

system

– all phases:

β, γ, δγ, (β+γ),

…

– one side:

∆

m

_s

➥ Over-constrain the unitarity triangles.

Understand CP violation in the framework of the

Standard Model.

–

Currently the UT is weakly constrained

(

ε

, |V

_ub

|, |V

_cb

|,

∆

m

_d

)

–

Will be improved in ~2005:

• ∆

m

_s

?

(HERA-b, CDF/D0)

CP violation in the B

_d

system:

• β

(BaBar, Belle, CDF/D0, HERA-b)

• α

?

(BaBar, Belle, HERA-b)

–

Significant theoretical uncertainties on

the length of the sides

(4)

M

OTIVATION

…

●

2 nd

motivation is related to the evolution of the Universe.

The disappearance of the antimatter component requires CP violation.

But, the CP violation from the Standard Model is too small.

➥ Additional sources of CP violation are needed.

They might find their origin in supersymmetric models

● LHCb is a promising place to study new physics.

Complementary approach to ATLAS and CMS.

● LHCb will measure:

– The phase

β

,

γ

using different decay modes.

(5)

4

2. Experimental methods

to determine

(6)

T

IME DEPENDENT DECAY RATE ASYMMETRY

●

● ≡

≡

†

↓

Visible BR

2β

3.6×10

−5

2β

+

2γ

0.7×10

−5

2δγ

5.4×10

−5

†. When more than one amplitude contributes to the decay:

(7)

6 T

IME DEPENDENT DECAY RATE ASYMMETRY

…

● When the final state is not a CP eigenstate:

(8)

O

THER METHODS

…

● Dalitz plot analysis:

● Branching ratio measurements:

● New strategies are proposed to measured

γ

.

They are under evaluation.

(9)

8

3.

(10)

(11)

10 LHCb

DETECTOR PERFORMANCES

● Particle identification [RICH]:

– K/

π

separation

>

3 σ

in the momentum range of the experiment

● Resolutions [Vertex detector, Tracking chambers]:

(12)

R

UNNING CONDITIONS

Low Luminosity:

2×10

32 cm

−

2 s

−

1

• Tunable locally although ATLAS and CMS work at high luminosity

– Ensures long physics programme

– Guarantees physics results from the beginning of LHC

• Clean event:

– ~0.3 interactions/BC

• Highest statistics of B’s ever collected:

(13)

12

4.

(14)

B

_d

→ π

+

π

−

(β+γ)

Importance of particle identification

Branching ratio:

B

_d

→

π

+

π

−

=

0.7 ×

10

–5

,

→

K

±

π

+−

=

1.5 ×

10

–5

B

_s

→

K

+

K

−

=

1.5 ×

10

–5

,

→

K

±

π

+−

=

0.7 ×

10

–5

b)

a)

Without RICH

With RICH

(15)

14 B

_s

→

D

_s

K (

γ−2δγ

)

Major background: B

_s

→

D

_s

π

(No CP violation)

Importance of particle identification and mass resolution

0 20 40 60 80 100 120 140 160 180 5.2 5.3 5.4 5.5 5.6 5.7 GeV/c2 arbitrary scale

m(D

s

K)

m(D

s

K)

B

s

D

s

K

B

s

D

s

K

(

m

= 11 MeV/c

2 )

B

s

D

s

B

s

D

s

0 5 10 15 20 25 30 35 40 45 5.2 5.3 5.4 5.5 5.6 5.7 GeV/c2 arbitrary scale

With RICH

Without RICH

With RICH

σ

_im

= 11 MeV/c

2

(16)

B

_d

→ ρπ → π

+

π

−

π

0 (β+γ),

|P|, |T|,...

Importance of

π

0 reconstruction

M(

π

+

π

-

π

0

)

[

GeV/c

2

]

Events

B

_d

→

ρ

+

π

−

~1000

(

Br 4.4

×10

–5

)

B

_d

→

ρ

−

π

+

~ 200

(

Br 1.0

×10

–5

)

B

_d

→

ρ

0

π

0

_{~ 100}

(

_{Br 0.1}

×10

–5

)







S/B = 1.3

(17)

16 B

_s

→

D

_s

π

(

∆

_Ms

,

∆Γ

_s

)

Importance of decay-time resolution

➥ Measurement of

∆

m

_s

with a significance > 5, up to 48 ps

−

1 (x

_s

=75)

Standard Model expectation:

14.3 –

26 ps

−1

(18)

SUMMARY OF THE PERFORMANCES

Sensitivity per year of data taking:

LHCb

Lum

=

2 ×

10

32 σ

(sin2

β

)

0.02 when

β = 20°

σ

(A

dir

)

0.07 σ

(A

mix

)

0.09 σ(β+γ)

~2

°

If

_when

_α

₌₉₀

_°

_and

_δ

₌₃₀

_°

σ(β+γ)

2 °

–5

°

Preliminary:

Backg. not included in the fit

Depends on the value of

β+γ

σ(2β+γ)

~9

°

when

|2β+γ| < 25°

σ(δγ)

0.6 °

–

1.4°

depends on the value of

∆

m

s

SM expectation: ~1

°

σ(γ−2δγ)

6 °

–

13°

Depends on the value of:

• γ−2δγ

• strong phase shift difference

σ(γ)

4 °

–

19°

Depends on the value of:

• γ

• strong phase shift difference

(19)

18

5.

(20)

Brazil

France

Germany

Italy

PRC

Netherlands

_Romania

_Spain

(21)

20 S

TATUS AND PLANS

● Technical Proposal approved in September 98

● Preparing Technical Design Reports:

– Magnet

end 99

✓

– RICH

– Calorimeter

– Outer tracker

– Muon

}

from now on to 2002

– Vertex

– Inner tracker

– Trigger + DAQ

● Construction:

2001–2003

(22)

C

ONCLUSIONS

● LHCb is an opportunity to understand the CP violation

in the framework of the Standard Model

and BEYOND.