1. Studying beauty with LHCb
First physics from the LHC
Monica Pepe Altarelli
CERN
AAAS, Washington, 20 February 2011
M.C. Escher

2. LHCb: an experiment dedicated to the b quark
Outline
LHCb: an experiment dedicated to the b quark
Context, motivations and goals
CKM theory and CP violation
The LHCb detector
Vertex Locator
RICH detectors
First results and prospects

3. 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.

4. From atoms to quarks
Protons and neutrons are particles made of quarks

5. There are two classes of quark composites:
Light quarks: u,d,s
There are two classes of quark composites:
The ‘-’ indicates an antiquark

6. By now we know of six quark ‘flavours’
Quarks
By now we know of six quark ‘flavours’
Q is the electric charge, in units of proton charge up
charm
top u c t
Q=2/3
mass (Gev/c2)
0.0025
1.3
173
down
strange
beauty or bottom d s b
Q=-1/3
mass (Gev/c2)
0.0048
0.1
4.5
Six flavours in three “families” or “generations” of increasing mass

7. Heavy-quark composites
Mesons with c
Mesons with b -
Heavy quarks are unstable and decay via weak interactions to lighter quarks
Vcb is a matrix element for quark-flavour mixing (Cabibbo-Kobayashi-Maskawa CKM matrix VCKM)

8. Cabibbo-Kobayashi-Maskawa CKM matrix
“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.
The probability of the transition from flavour i to flavour j is ~ |Vij|2
Probability of bc decay ~ |Vcb|2

9. Cabibbo-Kobayashi-Maskawa CKM matrix
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!

10. Cabibbo-Kobayashi-Maskawa CKM matrix
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

11. 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

12. Heavy-quark lifetimes
The heavy quarks have a short lifetime t:
tcharm ~ s
tbeauty ~ s
ttop ~ 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 VCKM and CP violation:
relatively long lifetime
high mass (many possible decay final states)
larger CP asymmetries than for s and c
t ~ 1/(m5 |VCKM|2)

13. Heavy-quark lifetimes
tbeauty ~ 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/mc2 ~ 20
(E: b energy)
D =20•3•1010•1.5•10-12~ 1cm

14. LHCb Vertex Locator VELO Muon System RICH Detectors B Calorimeters
Movable device
35 mm from beam out of physics /
7 mm from beam in physics
pp collision Point
Collision rate - around 600 million per second - can’t record it all
Even so, information flow enormous -> data transmitted in 1s equiv information exchanged by world phone calls 100 million phone calls
Can well imagine huge computing challenges for recording, storing, analysing and making all this data accessible to the world’s physicists.
Looking for a needle in a haystack (Higgs into 4 muons 1 in 10^43 collisions)
~ 1 cm B
Calorimeters
Tracking System

16. Interaction
Point

17. Member countries of the LHCb Collaboration
730 members
15 countries
54 institutes
LHCC open session
17 February 2010
Member countries of the LHCb Collaboration
Fundamental research (pursuit of knowledge, not making money)
Pooling resources (money and intellectual)
training

18. 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. bb p p p p

19. 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)

20. 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.

21. 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.

22. 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 μm Bs K K + Ds
B-production at pp-collision  primary vertex
B-decay  displaced vertex B
~1 cm

23. BsJ/y f event
BsJ/y f
J/y μ+μ-
fK+K-
Bs decay length is 20mm!

24. 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 cosθc=1/βn, which the RICH detectors reflect onto an array of sensors using mirrors.
cosθc=1/βn
RICH 2
kaon ring
By measuring θc 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.

25. Ring Imaging Cherenkov (RICH) detectors
Assembly of the high-precision mirrors used to focus the Cherenkov light onto the photon detectors

26. Decoding LHCb event display: B+→J/ψK+
Top view
(24mx12m)
Face view
(1mx0.5m)
Collision region
(0.7mmx10mm)

27. First results (with ~37 pb-1 of luminosity)
Peak luminosity increased within ~1 month by factor 100! (L~1030 to 1032 cm-2s-1)
With this small amount of data LHCb is already competitive with B-factories and the Tevatron experiments for many interesting decay modes

28. Lots of B-particles already observed!
First LHCb TeV
In 2011, we expect to produce ~1011
(~109 at B-factories in their lifetime!)
@LHCb all species of particles containing a b-quark are produced:

29. RICH Particle Identification performance: Bh+h’- with h=p,k,
No RICH used 29

30. RICH Particle Identification performance: Bh+h’- with h=p,k,
No RICH used
Deploy RICH to isolate each mode
B0→ππ
(BR = 5 x 10-6 !)
B0→Kπ
Bs→KK
Λb→pK 30

31. RICH Particle Identification performance: Bh+h’- with h=p,k,
No RICH used
Deploy RICH to isolate each mode
B0→ππ
(BR = 5 x 10-6 !)
B0→Kπ
Bs→KK
Λb→pK 31

32. Look in more detail at B0K+-
Tighter selection 
B0s→K+π-

33. Look in more detail at B0K+-
Separate into B0 and B0
~840 B0→K+π-
Tighter selection 
Raw result shows clear evidence of CP-violation
Analysis being optimised & account being taken of some small corrections
B0s→Kπ
2001: experimental proof of CP violation in B-system by B-factories (BELLE & BaBar)

34. CP-Violation with B0s system
mesons oscillate into their anti-matter particles at an astonishing 3 trillion times per second (3•1012 /sec)! Avenue opened by CDF Standard Model predicts CP violation effects at a few percent level
Use the decay BsJ/y f
 ~900 events so far,
~20 times more in 2011!
CDF+D0 ~10000 events with 300 times more Luminosity

35. CP-Violation with B0s system
Another decay channel that can be used to study CP-violation in B0s system: Bs→J/Ψ f0 First observation of this decay by LHCb!

36. Probing New Physics in some very rare decays: Bsμ+μ-
Bs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 Bs 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

37. Probing New Physics in some very rare decays: Bsμ+μ-
LHCb: Blind analysis of Bs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!
sensitive region
LHCb competitive with Tevatron with first 37 pb -1!
Results available very soon!
Exclusion 90% C.L.

38. 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 soon
Exciting prospects and rich physics programme for !