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Kaons@LHCb
Diego Martínez Santos
(Universidade de Santiago de Compostela & The European Research Council)
On behalf of LHCb
Diego Martinez Santos, HQL,山形 2018
(A strange program for the LHC)
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Introduction
Diego Martinez Santos, HQL,山形 2018
• LHCb experiment at LHC • Designed mostly for b and c decays
low trigger efficiency otherwise
• But there is also an ~infinite strangeness production at LHC
• Infinite production times zero efficiency requires L’Hopital • We still manage to get world best
results
• Major improvements in the trigger for s decays are being done
• The results I’m going to show correspond to Run –I data • 2011: 1fb-1 @ 7 TeV • 2012: 2fb-1 @ 8 TeV
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Trigger system: status and prospects
L0 (Hardware)
HLT1 (Software)
HLT2 (Software)
Main bottleneck for K. Can’t be changed
Not designed for K, but flexible. K triggers being implemented
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Trigger system: status and prospects
L0 (Hardware)
HLT1 (Software)
HLT2 (Software)
Main bottleneck for K. Can’t be changed
Not designed for K, but flexible. K triggers being implemented
ε(2011-2012) ~ 1-2% ε(Run-II) improved HLT1,2 ~ 18% Maximum possible ~30% (L0 won’t allow more)
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Trigger system: status and prospects
L0 (Hardware)
HLT1 (Software)
HLT2 (Software)
LHCb Upgrade
HLT (Software)
Main bottleneck for K. Can’t be changed
Not designed for K, but flexible. K triggers being implemented
(Note: This logo may not be official)
ε(2011-2012) ~ 1-2% ε(Run-II) improved HLT1,2 ~ 18% Maximum possible ~30% (L0 won’t allow more)
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Trigger system: status and prospects
L0 (Hardware)
HLT1 (Software)
HLT2 (Software)
HLT (Software)
Main bottleneck for K. Can’t be changed
Not designed for K, but flexible. K triggers being implemented
ε(Upgrade) ~ 80-100%? Simulation studies show that rate would be under control
V. Chobanova et al, CERN-LHCb-PUB-2016-017
(Note: This logo may not be official)
LHCb Upgrade
ε(2011-2012) ~ 1-2% ε(Run-II) improved HLT1,2 ~ 18% Maximum possible ~30% (L0 won’t allow more)
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SM CCKM X C? X BSM + SM CCKM X ( BSM + )
MFV paradigm Non-MFV paradigm
(small) (small)
(SM dynamics has the strongest suppression of all quark transitions)
SM CCKM X C? X BSM + eg, for c~ 1, bounds from kaon physics go up to a scale of 105 TeV
In a scenario in with BSM > few TeV, s-d transitions are central
Strangeness transitions
Diego Martinez Santos, HQL,山形 2018
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KS→μμ full Run-I analysis EPJC, 77 10 (2017) 678
Diego Martinez Santos, HQL,山形 2018
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• SM prediction: BR(KS → µµ) = (5.18 ± 1.50LD ± 0.02SD )x10-12 JHEP05(2018) 024 , JHEP 0401 (2004) 009, NPB 366 (1991) 189
• KS → µµ sensitive to different physics than KL → µµ, NP can be bigger than SM by ~1 order of magnitude or even more
KS→μμ: motivation
Diego Martinez Santos, HQL,山形 2018
Example of a SUSY scenario from
V.Chobanova et al., JHEP05(2018) 024 Leptoquark scenarios from Bobeth & Buras, JHEP02(2018)101
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KS→μμ full Run-I analysis
• Event yield extracted from a maximum likelihood fit in BDT categories
• Yield translated to BR via normalization to KSππ decays
• KSππ is also the main background: muon identification BDT trained against it
• Upper limit obtained integrating the posterior probability, combined 2011+2012
BR< 8 (10)x10-10
@90(95)%CL
EPJC, 77 10 (2017) 678
KSππ combinatorial
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KS→μμ prospects
• Extrapolating from Run-I result
• The most interesting region can be achieved by LHCb upgrade with trigger improvements
• Starting to investigate tagged decays, which would allow to access NP in the KS-KL interference
[D’Ambrosio&Kitahara PRL 119, 201802 (2017)] LHCb-upgrade Phase-II-upgrade?
Diego Martinez Santos, HQL,山形 2018
0 50 100 150 200 250 300
1E-3
0.01
0.1
1
BR
(K
S->)
limit a
t 95
%C
L [x10
-9]
TRIG
L [fb-1]
SM
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Σ→pμμ full Run-I analysis
LHCb-PAPER-2017-049 (arXiv:1712.08606)
Diego Martinez Santos, HQL,山形 2018
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The HyperCP evidence
• The HyperCP collaboration found evidence for Σ→pμμ decays, and provided a BR:
• Consistent w/ SM: 1.6 < BR[x10-8] < 9 X G He et al, PRD 72 (2005) 074003
• This evidence had wide relevance since all 3
observed events had the same dimuon invariant mass (214 MeV)
• Suggested the existence of a new
neutral particle at that mass
Diego Martinez Santos, HQL,山形 2018
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Σ→pμμ full Run-I analysis
• A Boosted Decision Tree (BDT) is trained against combinatorial background:
• Two main control samples • Σ+→pπ0 : Used for normalization of the
signal yield (translate yields into BR’s)
• Κ→ππ+π- : used to calibrate mass resolution and peak position
LHCb-PAPER-2017-049
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Σ→pμμ full Run-I analysis
• X→ μμ : Found no evidence for the 214 MeV particle. Upper limit for the resonant channel: 1.2x10-8 @95% CL
• Σ→pμμ : Found 4σ evidence
• BR(Σ→pμμ) : 2.1−1.2
+1.6 x 10-8
LHCb-PAPER-2017-049
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KS →π0μμ sensitivity study
CERN-LHCb-PUB-2016-017
Diego Martinez Santos, HQL,山形 2018
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Randall-Sundrum model
M. Bauer et al, JHEP 1009:017,2010
Why?
~ 1 order of magnitude variation of the BR in models with Extra Dimensions
Diego Martinez Santos, HQL,山形 2018
Another important s-d transition is KL π0μμ
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Another important s-d transition is KL π0μμ
Randall-Sundrum model
M. Bauer et al, JHEP 1009:017,2010
~ 1 order of magnitude variation of the BR in models with Extra Dimensions
Problem: The SM prediction is not too precise, which makes hard to distinguish small BSM effects from SM uncertainties
Why?
Diego Martinez Santos, HQL,山形 2018
It comes from the experimental uncertainty on BR(KS π0μμ) measured by NA48
M. Bauer et al, JHEP 1009:017,2010
PLB 599 (2004) 197-211
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V. Chobanova et al, LHCb-PUB-2016-017
• KS →π0μμ is searched for in two channels: • π0 (γγ) μμ: hereafter FULL (low eff.)
• π0 μμ: hereafter PARTIAL
p p
π0
In order to reconstruct the kaon mass for PARTIAL: • π0 mass PDG
• 𝑝𝑇𝜋 = −𝑝𝑇
𝜇𝜇
The only unknown is 𝑝𝐿
𝜋. But statistically is ≈10GeV
μμ
Diego Martinez Santos, HQL,山形 2018
KS →π0μμ sensitivity study
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KS →π0μμ sensitivity study V. Chobanova et al, LHCb-PUB-2016-017
Normalize yields to KS →ππ sensitivity to BR
0 20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
3.0
NA48(-)
S
TA
T [
10-9
]
TRIGL [fb
-1]
NA48(+)
PARTIAL channel
0 20 40 60 80 1000.0
0.5
1.0
1.5
2.0
2.5
3.0
NA48(-)
S
TA
T [
10-9
]
TRIG
L [fb-1
]
NA48(+)
FULL channel
LHCb-upgrade
Phase-II-upgrade?
LHCb-upgrade
Phase-II-upgrade?
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KS →π+π-ee sensitivity study C.Marin et al, LHCb-PUB-2016-016
Based on simulation: Expected a signal yield of
𝑁 = 120−100+280
For the full Run-I dataset Expected background yield is not well known yet
Diego Martinez Santos, HQL,山形 2018
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Ongoing/future studies
• K+ mass
• Semileptonic hyperon decays
• Kaons from (1020) decays (tagged kaons)
• KSX0μμ, (X whatever neutral system: eg γ) • KSX0πμ (X whatever neutral system: eg sterile neutrino)
• Workshop in 2013 and 2017 to discuss with TH community https://indico.cern.ch/event/280883/overview https://indico.cern.ch/event/590880/timetable/#20170426
• For info in future workshops you can subscribe to
rare-N-strange_at_cern_dot_ch
Diego Martinez Santos, HQL,山形 2018
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Conclusions
• There is an LHCs community in the LHCb village • Trigger is constantly improving • We aim for LHCb upgrade to reach efficiencies s as high as for b’s
• Available measurements for: Σ→pμμ, BR(KS → µµ)
• Published prospects for KS →π0μμ , KS →π+π-ee
• Some more channels in our TODO list
• Workshops to discuss with the TH community
Diego Martinez Santos, HQL,山形 2018
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Backup
Diego Martinez Santos, HQL,山形 2018
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Lifetime acceptance and KL→μμ background
KL and KS are distinguishable only by the decaytime… … and that is in theory. In practice, LHCb decaytime acceptance is not great for kaons
𝜖 𝑡 ~𝑒−𝛽𝑡 With β ≳ 5xΓs (>> ΓL).
This makes the two lifetime distributions to look similar
But the overall efficiency ratio is of course different
And makes KL μμ to become a negligible background for the current level of precision But can be relevant when we approach the 10-11 level
= O(10-3)
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Converting a signal yield into a branching ratio
Ν 𝐾𝑠0 → 𝜋𝜇𝜇 = 𝜎 𝐾𝑠
0 Β𝑅 𝐾𝑠0 → 𝜋𝜇𝜇 𝜺𝐿
𝐾𝑠0 production crossection Absolute efficiency
Integrated luminosity
Normalization of event yield
Diego Martinez Santos, HQL,山形 2018
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How? (normalization of event yield)
Converting a signal yield into a branching ratio
Ν 𝐾𝑠0 → 𝜋𝜇𝜇 = 𝜎 𝐾𝑠
0 Β𝑅 𝐾𝑠0 → 𝜋𝜇𝜇 𝜺𝐿
𝐾𝑠0 production crossection Absolute efficiency
Integrated luminosity
Ν 𝐾𝑠0 → 𝜋𝜇𝜇
Ν 𝐾𝑠0 → 𝜋𝜋
=𝜎 𝐾𝑠
0 Β𝑅 𝐾𝑠0 → 𝜋𝜇𝜇 𝜺𝐿
𝜎 𝐾𝑠0 Β𝑅 𝐾𝑠
0 → 𝜋𝜋 𝜺′𝐿
Introduce in the ntuples a 𝐾𝑠0 → 𝜋𝜋 decays counter Very well known
(69.20±0.05)%
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Dilepton mass distribution
Take formulae from hep-ph/9808289
z= m2 dΓ/dm = 2m dΓ/dz
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Remind of Bmm sensitivity
Diego Martinez Santos, HQL,山形 2018
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B mesons
We check that we get right the expected increase of B meson yields (i.e, a factor ~2)
B0s
B0d
B+/-
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D mesons
For D mesons the increase is slightly smaller (~1.6- 1.7)
D+
D-
D0
Ds
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Strange particles
K0s, K0L ,K+/-/0, φ(1020), K*
As well as anti baryons ,
Λ
Σ
Increase for most of them is ~40% A bit less for baryons (note: baryons, not anti-baryons) However, the momentum is also different w.r.t 7 TeV.
In particular, for the K0s decaying in the VELO the increase is “only” ~30% This is the number we really care for Ks μμ studies 34 Diego Martinez Santos, HQL,山形 2018
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Leptons
Electrons, muons
τ+
τ-
Increase in tau yiled consistent with ~ 2 , expected by the fact that most of them come from b’s and c’s Check with more stats if the asymmetry +/- is still there
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KS →π0μμ sensitivity study V. Chobanova et al, CERN-LHCb-PUB-2016-017
• As usual: BDT trained against combinatorial background • Specific backgrounds: KS →ππ, KL→πππ, KS/L →μμγγ (negligible)
Don’t affect the sensitivity estimate
The background discrimination
Diego Martinez Santos, HQL,山形 2018
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KS →π0μμ sensitivity study V. Chobanova et al, CERN-LHCb-PUB-2016-017
Fit, FULL
Diego Martinez Santos, HQL,山形 2018
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KS →π0μμ sensitivity study V. Chobanova et al, CERN-LHCb-PUB-2016-017
Fit, PARTIAL
Diego Martinez Santos, HQL,山形 2018
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The LHCb experiment
Diego Martinez Santos, HQL,山形 2018
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2 < η < 5
Best LHC detector for: • Vertex resolution (~100 microns)
• Invariant mass resolution
• Access to moderately low p Particularly good a (di) muons
Forward spectrometer at P8 of LHC
The LHCb experiment
Recorded luminosity: 3 fb-1 1 fb-1 at 7 TeV (2011) 2 fb-1 at 8 TeV (2012) ~2 fb-1 at 13 TeV (2015, 2016)
Diego Martinez Santos, HQL,山形 2018
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2 < η < 5
Best LHC detector for: • Vertex resolution (~100 microns)
• Invariant mass resolution
• Access to moderately low p Particularly good a (di) muons Less good at reconstructing charged hadrons
Forward spectrometer at P8 of LHC
The LHCb experiment
Recorded luminosity: 3 fb-1 1 fb-1 at 7 TeV (2011) 2 fb-1 at 8 TeV (2012) ~2 fb-1 at 13 TeV (2015, 2016)
Diego Martinez Santos, HQL,山形 2018
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2 < η < 5
Best LHC detector for: • Vertex resolution (~100 microns)
• Invariant mass resolution
• Access to moderately low p Particularly good a (di) muons Less good at reconstructing charged hadrons Even less good at photons
Forward spectrometer at P8 of LHC
The LHCb experiment
Recorded luminosity: 3 fb-1 1 fb-1 at 7 TeV (2011) 2 fb-1 at 8 TeV (2012) ~2 fb-1 at 13 TeV (2015, 2016)
Diego Martinez Santos, HQL,山形 2018
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2 < η < 5
Best LHC detector for: • Vertex resolution (~100 microns)
• Invariant mass resolution
• Access to moderately low p Particularly good a (di) muons Less good at reconstructing charged hadrons Even less good at photons Doesn’t detect neutrinos
Forward spectrometer at P8 of LHC
The LHCb experiment
Recorded luminosity: 3 fb-1 1 fb-1 at 7 TeV (2011) 2 fb-1 at 8 TeV (2012) ~2 fb-1 at 13 TeV (2015, 2016)
Diego Martinez Santos, HQL,山形 2018
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Rare decays of strange particles
LHCb collects world leading samples of b and c quark decays. Everything else is rejected because of bandwidth reasons
Making a new experiment inside LHCb
However, if BSM above few TeV (LHC energy scale), s-hadron decays are very important (very sensitive to non-MFV). But most of them are lost !
rest
b and c
s
b , c , s etc…
LHC LHCb
(used to be)
Diego Martinez Santos, HQL,山形 2018
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• SM prediction: BR(KS → µµ) = (5.1±1.5)x10-12 JHEP 0401 (2004) 009
• KS → µµ sensitive to different physics than KL → µµ (see JHEP 0401 (2004) 009)
• LHCb performed a search using 1fb-1 despite low
trigger performance:
BR(KS → µµ ) < 9(11)x10-9 @90(95)%CLs
JHEP 01 (2013) 090
We manage to get world best destpite low trigger efficiency
Rare decays of strange particles Proof of principle
JHEP 01 (2013) 090
Diego Martinez Santos, HQL,山形 2018
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Strangeness production/detection at LHCb
KL , KS produced in equal amounts. Acceptance ratio is ~2x10-3
(for Long Tracks)
• The pp collisions @ LHC produce a ‘kaon flux’ of 1013 KS
per fb-1 of luminosity in the LHCb acceptance
• Charged decay products can be reconstructed using Long Tracks or Downstream Tracks
• We use Long Tracks for RnS
• Downstream will be investigated (extra yield, but worse reconstruction quality)
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Ongoing stuff
mX
LHCb Fictitious
90% CL exclusion
γ π0
X
Diego Martinez Santos, HQL,山形 2018
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K+ studies
Large samples of charged kaon decays are available K+ mass is not very well known K+πμμ ?
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KS →X0μμ
• The KS →π0μμ PARTIAL analysis can be recasted for general/inclusive KS
→X0μμ. With X being whatever neutral system: • KS →γμμ. Can also be completed with photon reconstruction
• KS →(l+l-)μμ. Some of them are also being searched for explicitly
• Some exotic, eg, 17 MeV neutral boson of Phys. Rev. Lett. 116, 042501 (2016)
Limits can be provided as a function of X0 mass
mX
LHCb Fictitious
90% CL exclusion
γ π0
X
Diego Martinez Santos, HQL,山形 2018
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B and L violation (very low priority)
CLAS collaboration (Jefferson Lab): Limits on B and L violation
arXiv:1507.03859 [hep-ex]
We can easily do many of CLAS’ decays …as well as others: • Σ 3μ • Λ π3μ …and many other crazy (J conserving) combinations.
Currently very low priority, since we assume that BSM contributions can only be as much as BR ~10-56
Diego Martinez Santos, HQL,山形 2018
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Semileptonic decays?
• Semileptonic Hyperon decays • Camalich et al point out BSM is allowed on these decays
• KS →X0πμ , in a similar way as KS →X0μμ. In this case the X0 could be some (sterile) neutrino.
Actually current world best limits come from kaon decays in bubble chambers
Diego Martinez Santos, HQL,山形 2018
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KS→μμ full Run-I analysis
KL→μμ negligible: (down to 10-11
precision) K→πμν : negligible Λ pπ removed by a cut in the Armenteros-Podolanski plot. • Combinatorial background • KSππ double misid
Background
arXiv:1706.00758 [hep-ex]
• Analysed full Run-I (2011-2012) data
• Events classified using a BDT trained against combinatorial background
• Dedicated muon identification algorithm trained against KSππ
• Mass resolution 4 MeV
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KS →π+π-ee sensitivity study C.Marin et al, CERN-LHCb-PUB-2016-016
Based on simulation: Expected a signal yield of
𝑁 = 120−100+280
For the full Run-I dataset Expected background yield is not well known yet
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Dominant uncertainty, that makes difficult potential BSM interpretation of KL π0μμ It comes from the experimental uncertainty on BR(KS π0μμ) measured by NA48
NA48
~50% relative error
Improved measurements of BR(KS π0μμ) will translate into improved BSM constraints from KL π0μμ
Why? (KS π0μμ and SM errors on KL π0μμ)
Diego Martinez Santos, HQL,山形 2018