Flavour in the era of the LHC
1st LHCb Upgrade Workshop
Edinburgh, Jan 11-12 2007
Michelangelo L. Mangano
Theoretical Physics Unit
Physics Department
CERN, Geneva
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Exposing the mechanism of EW symmetry breaking
(EWSB) and identifying the Higgs boson or its
alternatives are the top priorities of our field
When that’s done, we’ll be cleared to move on to the
next layer of deep questions in HEP:
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what is Dark Matter ?
what is the origin of neutrino masses?
what is the origin of the Baryon Asymmetry of the
Universe?
......
why SU(3)xSU(2)xU(1)?
why 3 generations, why their properties?
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mass spectra
mixing patterns
questions driven by
experimental facts
questions driven by
theoretical curiosity
.....
• In most of these questions, “flavour” is at
the center stage
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What is “flavour physics” ?
• In the SM, flavour is what deals with the fermion sector
(family replicas, spectra and mixings):
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all flavour phenomena are encoded in the fermion Yukawa
matrices.
Beyond the SM, “flavour” phenomena cover a wider
landscape. E.g.
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Flavour changing processes, both with ΔQ=1 and ΔQ=0, can be
mediated by
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gauge-sector particles, like charged higgses, gauginos, new gauge
bosons, or by
SUSY scalar partners
New sources of CP violation (phases in gluino, Higgs, etc. couplings)
New flavours may exist in the form of new generations, exotic
partners of standard quarks (e.g. Kaluza Klein excitations, mirror
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EWSB and flavour
• EWSB is intimately related to flavour:
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No EWSB
fermions degenerate
Why mtop = g/√2 mW (
no visible flavour effect
ytop = 1) ?
In most EWSB models flavour plays a key role. E.g.:
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Technicolor: killed by too large FCNC?
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Little Higgs theories
Supersymmetry: large value of top mass drives radiative EWSB
In several extra-dim models the structure of extra dimensions -driven by the need to explain the hierarchy problem of EWSB -determines the fermionic mass spectrum
top quark partners, mirror fermions, ...
The large top mass is responsible for the “small hierarchy”
problem .....
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The hierarchy problem(s)
LEP’s heritage is a strong confirmation of the SM, and at
the same time an apparent paradox:
SM fits: m(H)=98+52-36; on the other, SM radiative corrections give
How can counterterms artificially conspire to ensure a
cancellation of their contribution to the Higgs mass?
hierarchy problem
The existence of new phenomena at a scale not much larger than 400
GeV appears necessary to enforce such a cancellation in a natural way!
The accuracy of the EW precision tests at LEP, on the other hand, sets
the scale for “generic new physics” (parameterized in terms of dim-5
and dim-6 effective operators) at the level of few-to-several TeV.
small
hierarchy
problem
Very strong constraints on the nature of this possible new physics: must leave unaffected
the SM EW predictions, and at the same time to play a major role in the Higgs sector. 5
Most proposed solutions to this problem include “top-like” objects, whose
couplings to the Higgs can precisely cancel the top contributions
Supersymmetry is one example, via the scalar partner of the top
In Supersymmetry the radiative corrections to the Higgs mass are not quadratic in
the cutoff, but logarithmic in the size of SUSY breaking (in this case Mstop/Mtop):
antitop
H
top
antistop
H
+
H
stop
H
with
For Msusy< 2TeV
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Ex: Little Higgs models
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Embed SM in larger group
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New additional gauge bosons and heavy Higgses:
Higgs as pseudo-Goldstone boson
Cancel top loop with a new heavy top-like quark, T
Typically within reach of
ATLAS/CMS
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Flavour effects in Little Higgs models
Large mass scale, and MVF structure of new couplings, limit impact on flavour observables
(rare decays, FCNC, etc) to O(15%) at most
Buras et al, hep-ph/0607189
Can reduce impact of LH new states on EW observables by introducing discrete
symmetry, T-parity, decoupling them from tree-level vertices. This gives acceptable models
with lighter spectra
● Mirror spectrum of SM fermions
● Mirror fermion mixing
● NMFV
NMFV
Hubisz et al, hep-ph/0512169
Buras et al, hep-ph/0610298, 0605214
Rai Choudhury et al, hep-ph/0612327
potentially large contributions to flavour observables:
● ACP(Bd→ψKS)
● ΔmBs (either sign)
● ACP(Bs→ψφ) and AqSL up to 10 x SM
● BR(Bs→μμ) up to 1.5 x SM
● BR(l’→ l γ) as large as current limits
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Side remark
• The special role played by the 3rd generation is not
limited to the top
• Neutrino mixing is maximal in the 3rd-2nd generation,
something which most likely will find an explanation in a
complete theory of flavour linking quark and leptons
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What is the “flavour problem” ?
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Suppression of FCNC is built into the SM to reproduce the
data, and guaranteed by the following facts:
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Quark sector:
- unitarity of CKM (GIM mechanism)
- small mixings between heavy and light generations
Vki
di
uk
dj
∑
k=u,c,t
di
di
V∗kj
Δi j ∼
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dj
W
Vki Vk∗j
f (mk /mW ) ∼
∑
k=c,t
Vki
dj
Vki Vk∗j
2
m2k /mW
uk
V∗kj
∼
Vci Vc∗j
W Zo
W
m2c
+ Vti Vt∗j
2
mW
Lepton sector:
- mv=0
all phases and angles absorbed by field redefinitions, no
mixings/CPV at all
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Beyond the SM:
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There is absolutely
no guarantee that
these properties
be maintained in
extensions of the
SM
As soon as these
are released,
effects are
devastating!
S.Geer
MX >
MX >
MX >
MX >
MX >
MX >
Compare the to O(10 TeV) sensitivity
w.r.t. modifications of the gauge/EW sector
Flavour problem:
How can the new physics we need to understand
the open problems of HEP leave no trace of FCNC?
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summarizing ....
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The underlying problems of EWSB are the primary
theoretical motivation for expecting the existence of new
physics
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Of the three experimental proofs of the incompleteness of
the SM:
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Dark Matter
neutrino mass
baryon asymmetry of the Universe
at least 2 are explicitly linked to flavour
➡
There is an immense potential for synergy and
complementarity between the LHC (directly addressing EWSB
and the search for new phenomena) and flavour factories
(directly looking for new flavour phenomena)
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Studies of top properties, the LHC potential reach
1) tbW coupling
Probe anomalous couplings by measuring lepton FB
asymmetry in the top rest frame (A.Onofre et al, ATLAS)
1σ limits:
VR
gL
gR
min
-0.02
max
0.09
-0.01
0.01
-0.02
0.02
implications for
various BSM
models are being
studied
2) FCNC decays
BR
t→qZ
t→qγ
SM
10-13
10-13
2-Higgs SUSY RPV exotic Qs
Today
ATLAS+CMS 100 fb–1
≤ 10-6
≤ 10-4
≤ 10-2
≤ 0.08 (LEP)
≤ 4.7 x10-5
≤ 10-7
≤ 10-5
≤ 10-5 ≤ 0.003 (HERA)
≤ 1.8 x10-5
t→qg
10-11
≤ 10-5
≤ 10-3
≤ 10-4
≤ 0.29 (CDF)
≤ 4.3 x10-4
(Benucci&Castro, CMS/ATLAS, to appear in FlavLHC procs)
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Single top production
Electroweak process (contrary to standard t-tbar production)
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sensitive to the tbW vertex, possible anomalous couplings
Tait Yuan
FCNC Ztc
4-Q generations
top-flavour,
1 TeV Z’
SM
charged top
pion, 450 GeV
t-channel production rate
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s-channel production rate
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Inclusive Supersymmetry searches
Expected reach in the
overall mass scale for
gluinos and squarks:
1fb-1
1-1.5 TeV
10fb-1
1.5-2 TeV
100fb-1
2.5 TeV
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Reconstructing individual
SUSY states at the LHC
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Example, light stop
Consistent with mH, DM, EW scale baryogenesis, EDMs, etc
After ttbar and W+jets bg subtraction:
ATLAS
m(stop)-m(χ0)
Points: simulated data
Histo: MC truth
ATLAS
M(bjj)
1.8 fb-1
79 GeV
Mass spectrum (GeV)
t1
137
t2
1510
g
948
u,e
~10000
0
χ1
58
0
χ2
112
+
χ1
111
h
116
M(bl)
1.8 fb-1
GeV
Lari/Polesello, ATLAS17
Example, sbottom from gluino decays
~ ~
~
~*
_
_
pp → q g , g → b b → b b χ02 → b b l+ l- χ01
m(bbl)~m(gluino) - m(χ0)
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Example, slepton reconstruction
p
p
~
g
q
~
q
q
~
χ02
χ02 → !˜± !∓ → χ01!+!−
~
χ01
~
l
l
l
max (m(!+!−)) = m(χ2)
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!
˜
m2(χ2) − m2(l)
m2(χ2)
!
˜ − m2(χ1)
m2(!)
m2(!)
Example, LFV in neutralino-slepton decay chains
In SUSY GUTs the large νμ ντ mixing leads to mixings in the charged scalar lepton
sector. These can be probed via τ→μγ , but also via LFV neutralino decays
0 + −
χ02 → !˜±2 !∓
→
χ
1!2 !3
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5σ sensitivity on BR(χ2 →τμχ2) is 2.3%
δ ≈ 0.1
BR(τ→μγ) 10-9
Question: What’s the sensitivity of the LHCb upgrade to LFV τ decays such as τ→μee,
τ→μμμ? Signal dominated by Ds→τν, B→τνX, ...
large enough rates only at small pT not for ATLAS/CMS
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Example, LFV in neutralino-slepton decay chains
Sensitivity to:
10fb-1
Meμ
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Note
The presence of new gauge interactions (e.g. Z’)
above the production threshold of SUSY
particles could largely enhance the ability of the
LHC to study the SUSY spectrum
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Whether it will be SUSY or extra-dim or Little Higgs, the
information extracted from high-energy observables will
need to be supported, validated and extended by
independent low-energy measurements whose results,
whether positive or negative, will contribute to the
determination of the parameters of the new physics
What is the relation between the
discovery potential for new particles at
the ATLAS/CMS and the sensitivity of
flavour observables to their virtual
effects?
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Acp(BφKs) vs SUSY Models
280fb-1
Acpmix
5ab-1
mSUGURA
tanβ=30
SU(5)+νR
tanβ=30
degenerate
U(2)
tanβ=30
50ab-1
SU(5)+νR
tanβ=30
non-degenerate
 T. Goto, Y.Okada, Y.Shimizu,T.Shindou,
M.Tanaka, hep-ph/0306093, also in 24
SuperKEKB LoI
Address this question by exploring some
benchmark points in parameter space. E.g.
Presentations by S.Heinemeyer and by
M Schmitt et al, Flav@LHC Wshop,
Oct 9-11, WG1
μ=0.5 TeV, AU=1 TeV
Isidori, Paradisi, hep-ph/0605012
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μ+3jets+MET
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What will be the main driving theme of the
exploration of new physics ?
the gauge sector
(Higgs, EWSB)
The High Energy Frontier
LHC
SLHC
VLHC
LC
CLIC
....
the flavour sector
(ν mixings, CPV, FCNC, EDM, LFV)
The High Precision Frontier
Neutrinos:
Charged leptons
super beams
stopped μ
beta-beams
l →l’ conversion
ν factory
Quarks:
e/μ EDM
B factories
K factories
n EDM
This is an open question, which we need to keep
open, in view of possible, likely, surprises!
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