1. Patrick Robbe, LAL Orsay, 20 Nov 2013
B physics at LHCb
Patrick Robbe, LAL Orsay, 20 Nov 2013

2. Introduction
LHCb main scientific goals are to search for New Physics through:
Measurement of rare B decays
CP violation in B decays
Compare Standard Model predictions with precise experimental measurements to look for discrepancies.

3. Rare decays For example, decay of Bs0 ➝ m+ m-
In the Standard Model, suppressed because of the small coupling between the t et s (Vts CKM matrix element) and because of helicity suppression.
The branching fraction can be increased if there are new particules that modify the coupling properties.
These particles are the same than those searched directly at ATLAS and CMS.
But here: virtual particles, a large range of mass values can be reached.
If they are directly discovered, B decays can help knowing their properties.

4. CP violation in the Standard Model
CP violation in the Standard Model occurs in weak interactions in the quark sector:
Quarks are not eigenstates of weak interactions.
The CKM (Cabibbo-Kobayashi-Maskawa) matrix describes the transition probabilities between different quark flavours

5. The CKM matrix The matrix is 3x3 and unitary
Fully described by 4 real parameters,
For example, in the Wolfenstein parametrization: l (~0.23), A, r and h.
Complex matrix elements: CP violation through quantum mechanics interference effects.

6. The Unitary Triangle
The unitarity condition of the CKM matrix can be represented graphically by triangles in the complex plane.
One of these triangles is called the Unitary Triangle and define the « angles »

7. State of the art
The CKM theory is very predictive: it governs a large variety of phenomena (K decays, t decays, …) with only 4 parameters.
The consistency of the experimental measurements is seen in global CKM fits:
Impressive agreement, but still room for precision improvement, in particular for the g angle, the least well known angle: inconsistencies between the measurement could be the sign of New Physics. CP violation measurements are a very powerful test of search for it.

8. g Measurements
The other motivation to measure g is that it is the theoretically cleanest CKM parameter:
It can be measured with decays proceeding only through tree diagrams (like B ➝ DK decays)
No loop: insensitive to New Physics effects
The parameter extracted from these measurements is g down to theoretical errors of 10-7 [arxiv: ]
Several modes can be combined
Reference measurement for a precise scrutiny of the validity of the CKM theory
Experimental g measurements:
BaBar: g = ˚ [PRD87, (2013)]
BELLE: g = ˚ [arXiv: ]

9. LHCb Detector
LHC collisions: ~1011 BB produced per year (1fb-1) in LHCb acceptance, at 7 TeV
Cross-section linear as a function of centre of mass energy

10. LHCb Detector
Signature of presence of a B: displaced vertex, B flies ~mm in the detector (t ~ 1.5 ps)
σefft = 45 fs

11. B mixing
These performances allow to see Bs mixing « easily » (fastest quantity in B physics)

12. B tagging
CP violation = difference between B and B, necessary to know in which state the meson was produced (if it decays to a CP eigenstate)

13. LHCb Detector

14. LHCb Trigger L0 Hardware Trigger:
Hadronic decays are mainly triggered with the HCAL: measurement of hadrons ET.
A fraction of the signal is also triggered through the decay products of the second B in the event

15. LHCb Trigger Software High Level Trigger Add tracking information:
Reconstruct primary vertex position from VELO tracks
Perform forward tracking and select tracks with large IP and large pT (>1.6 GeV)

16. LHCb Trigger Second level of software trigger:
Inclusive trigger on 2,3,4-body detached vertices
With BDT algorithm based on:
pT,
IP c2
Flight distance c2
Mass
Corrected mass

17. LHCb Upgrade
An upgrade of LHCb is planned at the end of the LHC Run II ( )
The main change is to suppress the 1MHz readout bottleneck
This increases trigger efficiencies (in particular for hadronic modes) by a large factor (up to 5)

18. Present and futur
LHC era
HL-LHC era
ATLAS & CMS
25 fb-1
100 fb-1
300 fb-1 →
3000 fb-1
LHCb
3 fb-1
8 fb-1
23 fb-1
46 fb-1
Belle II -
0.5 ab-1
25 ab-1
50 ab-1
The LHCb upgrade design is qualified for an integrated luminosity of 50fb−1 but it is anticipated that LHCb will continue to be operational throughout the HL-LHC era

19. Bd,s⟶μ+μ- SM : very rare (Vtq, helicity suppression)
Large sensitivity to NP, eg :

20. BR(Bd⟶μ+μ-) BR(Bs⟶μ+μ-)
CMS : Phys. Rev. Lett. 111 (2013) , arXiv:
LHCb : Phys. Rev. Lett. 111 (2013) , arXiv:
Combination : CMS-PAS-BPH ; LHCb-CONF
measured with 25% precision
From theory : BRs known to 10% (can be improved with refined lattice QCD calculations)

21. Bd,s⟶μ+μ- : what’s next ? σtheory ~5 %
2012
Measure Bd and Bs : NP effects can be different
σtheory ~5 %

22. Angular observables in Bd⟶K*0 μ+μ-
SM : μ+ μ- K- π+ Ф B θK θℓ
System described by
q2 =M2(ℓℓ)
3 angles
Distributions of the angular variables precisely predicted in the SM
Deviations expected from NP
Probe the NP structure if any effect observed :
eg in the low q2 region the Ф distribution is sensitive to the photon polarization

23. sensitive to RH-couplings (C’7) at low q2
AFB(q20) = 0
used as a reference
q20
Important phenomenological work to construct observables with low theoretical uncertainties
sensitive to RH-couplings (C’7) at low q2

24. Relative weights of the LHC experiments depend on the q2 bin
Current results :
ATLAS : ATLAS- CONF
CMS : arXiv:
LHCb : JHEP 08 (2013) 131
J/ψ
ψ(2S)
Relative weights of the LHC experiments depend on the q2 bin
‘cleanness’ varies with bins (resonances in M(μ+μ-))

25. σtheory ~7 % Expected precision on q20
K∗l+l−, where l = e or μ and K∗ includes both K∗0 and K∗+
σtheory ~7 %
triggering at low muon pT is mandatory
AFB is not the best variable (contains hadronic uncertainties)
much more than that in Bd⟶K*0 μ+μ- analyses

26. ϕs from Bs⟶J/ψΦ
CP violation due to the interference between mixing and decay
Requires :
initial state tagging
time measurement
angular analysis (VV final state) to disentangle CP-even and CP-odd contributions
measurement of ΔΓS

27. Bs⟶J/ψΦ current results
ΔΓs=ΓL-ΓH (ps-1)
ATLAS (2011) φs = 0.12 ± 0.25(stat.) ± 0.11(syst.) rad
LHCb(2011) φs = 0.01 ± 0.07(stat.) ± 0.01(syst.) rad
ATLAS : ATLAS-CONF
LHCb : Phys. Rev. D87 (2013)

28. Expected precision on ϕs (rad)
conservative
σΦΦ th ~0.02
σJ/ψΦ th ~0.003
can be quantified with data)
A crucial ingredient is the effective tagging efficiency and its knowledge
Importance of the J/ψ trigger

29. The CKM angle γ Interferences between b→c and b→u transitions
Time independent measurements
(B→DK(*) large family)
Tree diagrams
SM reference point

30. σsyst ~<1° σtheory negligible
Expected precision on γ from tree decays
σsyst ~<1°
σtheory negligible

31. Summary / conclusion
LHC experiments (mainly LHCb) and BELLE2 will explore with impressive precision flavour physics towards 2030, reaching theory errors in many of the important observables.
Many different measurements and approaches conducted: thorough exploration of flavour physics.

32. Polarized b-baryons Lb are produced mostly unpolarized at the LHC
Some analyses need polarized Lb (test of time reversal for example, with Lb ➝ J/y L decays)
Angular distributions:
Reference: Eric Conte PhD thesis [