BEYOND THE STANDARD MODEL WITH B-MESONS
BASED ON THE TALK “PROBING THE TEV SCALE WITH FLAVOUR TRANSITIONS AT THE LHC”
Koen Lambrechts
Kaushal Marthi
CONTENTS
 Short introduction to SM and BSM
 Why would we use B-mesons?
 Experimental probes
•
Rare decays
•
CP-violation
 Conclusions
INTRO TO SM
•
Describes 3/4 fundamental interactions
•
Leptons & Quarks – 3 generations
•
Gauge bosons – force carriers
EXCELLENCE OF SM
 Best model of physics
 Able to explain all established observations
 Unified EM and Weak forces – electro-weak theory
 Predicting particle existence
-
W and Z bosons
-
Quarks
UNEXPLAINED

Matter/ anti asymmetry

Dark matter/energy

Mass hierarchy

Origin of parity violation

3 generations of quarks/leptons

No photon mass

……
BEYOND THE STANDARD MODEL
 Supersymmetry:
-
Number of sub-theories
-
Bosons partnered to fermions
-
New particles at higher scale
 Extra dimensions
-
Inaccessible to us
-
Planck/electroweak scales maybe comparable
 String theory
 .......
B - MESONS
 Mesons formed from a quark and anti-quark
 b quarks form heaviest hadrons
 Many decay channels
-
Small branching fractions
-
Sensitive to small deviations
 Interested in 𝐵𝑠0 and 𝐵0 mesons
B - MESONS
 Mesons formed from a quark and anti-quark
 b quarks form heaviest hadrons
 Many decay channels
-
Small branching fractions
-
Sensitive to small deviations
 Interested in 𝐵𝑠0 and 𝐵0 mesons
LHCb – Large Hadron Collider beauty
SEARCH FOR NEW PHYSICS – DIRECT METHOD
 Attempt to physically produce new particles
 In particle collisions
 Proven to be successful – Higgs boson
SEARCH FOR NEW PHYSICS – INDIRECT METHODS
 Not discovering physical particle
 Observing possible effects of particle
 Virtual particles
 Two effects studied:
-
Rare decays
-
CP violation
RARE DECAYS
 Low branching ratio
 Some BSM models predict higher BR than SM
 Test for BSM
DECAY DIAGRAMS – TREE AND LOOP
Tree
Loop
DECAY DIAGRAMS – TREE AND LOOP
• Simple
• Fewer vertices
• Less suppression
• Large decay proportion
DECAY DIAGRAMS – TREE AND LOOP
• Virtual particles in loop
• Opens up new decays
• Increased vertices
• Far more suppressed
• Less frequent
0
𝐵𝑠,𝑑
→ 𝜇+ 𝜇−
Tree diagram: forbidden
‘Penguin’ loop diagram
0
𝐵𝑠,𝑑
→ 𝜇+ 𝜇−
Why only loop?
 Flavour changing neutral current
 Z boson interaction conserves flavour
 Diagonal transition required
 GIM mechanism
0
𝐵𝑠,𝑑
→ 𝜇+ 𝜇− - SUPPRESSIONS
 Must decay through loop
 More vertices
 CKM
 Helicity / Chirality
0
𝐵𝑠,𝑑
→ 𝜇+ 𝜇− - USEFULNESS FOR BSM
 Low branching ratio in SM
-
More sensitive to deviations
 Virtual particles in loop
-
Sensitive to higher mass particles
 Useful for new particle search
-
New virtual particles in loop
BRANCHING RATIO
 Decay width of one mode:
Γ∝
ℎ
∝ℎ𝑀
𝜏
2
 BR proportion of total:
𝐵𝑅 Γ1 𝑚𝑜𝑑𝑒 =
Γ1 𝑚𝑜𝑑𝑒
𝑖 Γ𝑖
SM BR PREDICTIONS

𝐵𝑠0 → 𝜇+ 𝜇−
𝐵𝑅 = 3.66 ± 0.23 × 10−9
 𝐵𝑑0 → 𝜇 + 𝜇−
𝐵𝑅 = 1.06 ± 0.09 × 10−10
(theoretical estimates from CMS, LHCb collaborations paper)
EXPERIMENT
 Joint CMS and LHCb venture
 Measured decays - 𝐵𝑠0 → 𝜇+ 𝜇− and 𝐵𝑑0 → 𝜇+ 𝜇−
 Results released in 2015
RESULTS
RESULTS
−9 @ 6.2σ
 𝐵𝑅 𝐵𝑠0 → 𝜇+ 𝜇− = (2.8+0.7
−0.6 ) × 10
−10
 𝐵𝑅(𝐵𝑑0 → 𝜇+ 𝜇− ) = 3.9+1.6
@ 3.2𝜎
−1.4 × 10
 Consistent with SM
CONSEQUENCE FOR BSM
 Observed BRs constrain shown BSM theories further
MORE RECENT RESULT ON B DECAY
0
 Release of 𝐵𝑠,𝑑
→ 𝜇 + 𝜇− decay results in 2017
 Performed at LHCb,
 Integrated luminosity 4.4 fb-1
 Results still in agreement with SM expectations
ATTEMPTS TO SOLVE THE MATTER/ANTI-MATTER ASYMMETRY
 CP violating Sakharov’s model
 CPT violating model
 ... Ideas?
SAKHAROV’S MODEL
 Requires 3 adjusted parameters:

CP violation: more than we know of.

Baryon number violation

The universe is not in thermal equilibrium
 Implies a stable asymmetry
CPT-VIOLATING MODEL
Pros
Cons
 Requires only a small amount of CPT violation
 No CPT violation has been observed
 No other parameters need to change
 CPT violation would imply new particles
 Would explain anti-matter disappearing today
CP-VIOLATION
 CP-violation is allowed by the standard model
 Described by the CKM matrix
CKM MATRIX AND CP VIOLATION
 Hermitian matrix
 Real diagonal elements
 Leaves 4 parameters for the off diagonal elements

3 parameters determine the absolute values

1 complex phase 𝜑 𝑠
V
CKM
V ud

 V cd

 V td
V V
V V
V V
us
cs
ts



cb

tb 
ub
𝐵𝑠0 AND 𝐵𝑠0 MIXING

Mass eigenstates: |𝐵𝐿

Flavor eigenstates: |𝐵𝑠0

𝐵𝑠0 (𝑏𝑠) and 𝐵𝑠0 (𝑏𝑠) are in a superposition
|𝐵𝐻

𝐵𝐿 = 𝑝 𝐵𝑠0 + 𝑞 |𝐵𝑠0

𝐵𝐻 = 𝑝 𝐵𝑠0 − 𝑞 |𝐵𝑠0
 λ=
𝑞𝐴
𝑝𝐴
|𝐵𝑠0
𝐵𝑠0 / 𝐵𝑠0 → J/Ψ + 𝐾 + + 𝐾 −
 Both states can decay via this decay channel
 Quarks: 𝑏𝑠 / 𝑏𝑠 → c𝑐 + u𝑠 + 𝑢𝑠
𝐾+
 Different decay rates for 𝐵𝑠0 and 𝐵𝑠0
𝑨=
𝐴𝑖
𝑨=
λ 𝒆−𝒊𝝋𝒔 𝐴𝑖
𝐾−
𝜇+
𝐽/ψ
𝜇−
DETECTION AND TRIGGERS: 𝐾 + 𝐾 −
 Look for specific Kaon pairs

Single PT > 250 MeV

Combined PT > 900 MeV

Invariant Mass > 1.05 GeV
 Quick reconstruction to vertex that is not a primary vertex
 Look for complementary particles to construct the complete decay
DETECTION AND TRIGGERS: 𝜇+ 𝜇−
 Look for specific muon pairs

Combined PT > 550 MeV

Combined invariant Mass within [-48, +43] MeV of the J/Psi
 Track the 4 particles to 1 Primary vertex
DETECTION AND TRIGGERS: 𝐵𝑠0 / 𝐵𝑠0
 See if the primary vertex was a B-meson decay
 Flavour tag the B-meson
𝐾+
B meson?
𝐾−
 We’re left with ≈ 3% of candidate events
𝜇+
𝐽/ψ
𝜇−
RESULTS
RESULTS - COMPARISON
 Single experiment: 𝜑𝑠 = 119 ± 107 ± 34 mrad
 Combined results: 𝜑𝑠 = −25 ± 45 ± 8 mrad
 Standard model:
𝜑𝑠𝑆𝑀 = −36.5+1.3
−1.2
CONCLUSION
 Standard Model is incomplete
 Many candidate beyond standard models
 However:
o
No new CP-violation
o
No decay excess observed
o
Combined with recent EDM results
 Definitive BSM theory is still not within reach
REFERENCES
1.
The CMS and LHCb collaborations, “Observation of the rare 𝐵𝑠0 → 𝜇+ 𝜇− decay from the combined analysis of CMS
and LHCb data”, 4 June 2015
2.
Carolyn M. Bertsche, “SELECTED RARE DECAYS as indication of physics beyond the standard model” 6 November
2012
3.
The LHCb collaboration “Measurement of the 𝐵𝑠0 → 𝜇+ 𝜇− branching fraction and effective lifetime and search for
𝐵0 → 𝜇+ 𝜇− decays”, 16 March 2017
4.
The LHCb collaboration, “Resonances and CP violation in 𝐵𝑠0 and 𝐵𝑠0 → J/Ψ + 𝐾 + + 𝐾 − decays in the mass region
above the 𝜑 (1020),” April 2017.
5.
http://lhcb-public.web.cern.ch/lhcb-public/
6.
http://www.hephy.at/user/friedl/diss/html/node8.html
7.
http://lhcb.web.cern.ch/lhcb/