Studio di decadimenti rari a LHCb · rari a LHCb Matteo Palutan INFN-LNF 100° Congresso SIF, Pisa...

Studio di decadimenti rari a LHCb

Matteo PalutanINFN-LNF

100° Congresso SIF, Pisa 2014

Matteo Palutan, INFN-LNF

Indirect searches for New Physics

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- Photon polarization in b→sγ - Branching fractions and angular distributions of b→sμ+μ− - The very rare decays B0(s) → μ+μ−

• Recent LHCb results on FCNC b decays:

• Lepton Flavour Violating decays

OUTLINE

• High energy“real” new particles can be produced and discovered via their decays

Direct and indirect approach complement each other

• High precision“virtual” new particles can be discovered in loop processes, such as Flavor Changing Neutral Current (FCNC)


FCNC decays

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• Contribution from New Physics as correction to the SM

• In the Standard Model FCNC decays are forbidden at tree level, and occur at loop level only → strongly suppressed!

• SM diagrams for b→sμ+μ−: “ electroweak penguins”

What is the scale of ΛNP?What is its coupling cNP? loop tree


Three impersonations of the EW penguin

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αQED suppression

helicitysuppression

“Photon penguin” “Electroweak penguin” “Higgs penguin”

BR~3x10-4 BR~10-6 BR~3.6x10-9

angular distributions BR

BR,γ polarization


FCNC processes in effective field theory

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• Effective Hamiltonian for b→s FCNC transistions

• Wilson coefficients Ci encode short-distance physics and possible NP effects, computed perturbatively

• Local operators Oi with different Lorentz structure absorbe long distance effects

• O´i helicity flipped operators, ms/mb suppressed in SM

photon polarization in b→sγ


Photon polarization in b→sγ decays

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• The SM predicts that the photon emitted in b→sγ decays is predominantly left-handed (up to order ms2/mb2), since the recoil s-quark that couples to a W-boson is left-handed

• Several models beyond the SM predict the photon to acquire a significant right-handed component due to the exchange of heavy fermions in the electroweak penguin loop

[Atwood et al PRL 79 (1997) 185]

[Gronau and Pirjol PRD 66 (2002) 054008]

[Yu et al. JHEP12 (2013) 102]


Photon polarization in B+ → K+π−π+γ

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• Can infer the photon polarization from the up-down asymmetry of the photon direction in the K+π−π+ rest-frame. Unpolarized photons would have no asymmetry (conceptually similar to Mme. Wu experiment)

The up-down asymmetry is defined as:

Where sij = (pi+pj)2 and the ai contain the information about the K+π−π+ resonances and their interference: the odd terms is cosθ carry the photon polarization: if λγ≠0 the photon is polarized. In SM λγ ≈-1 (+1) for radiative B (anti-B) decays, up to O(ms2/mb2) corrections

[Gronau and Pirjol PR

D 66 (2002) 054008]

Matteo Palutan, INFN-LNF9

• There are a large number of overlapping resonances in the m(K+π−π+) mass spectrum. No attempt is made to separate these in the analysis, we simply bin in 4 bins of m(K+π−π+)


• At LHCb we look at B+ → K+π−π+γ decays using calorimeter photons, observe ~13k signal candidates in 3 fb-1


Up-down asymmetry

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• Combining the four m(K+π−π+) bins, the up-down asymmetry Aud is different from zero at 5.2σ

• To extract the polarization value from the up-down asymmetry theory input is needed about the amplitude composition of the hadronic system (ongoing)

• This is the first observation of photon polarization in b→sγ decays

[PRL 112 (2014) 161801]

• Alternative approach is being finalized at LHCb: extraction of λγ

from mixing-induced CP violation in B0s → φγ

b→s μ+μ− decays


Experimental data on b→sμ+μ−

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great interest, big effort...


Angular analysis of B0→ K*0[→K+π− ]μ+μ−

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• B0→ K*μ+μ− is the golden mode to test new vector

(-axial) couplings in b→s transitions: sensitivity to O7 , O9 and O10 and their primed counterparts.

• K*→Kπ is self tagged, hence angular analysis ideal to test helicity structure

• Decay described by 3 helicity anges and q2 = m(μ+μ− )

folding technique (φ→φ+π for φ<0 ) applied to reduce the number of fit parameters

FL: fraction of K* longitudinal polarization

AFB: Forward-Backward lepton asymmetry

S3: Asymmetry in K* transverse polariz.

AIm: T-odd CP asymmetry


B0→ K*0μ+μ− angular observables (1fb-1)

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[JHEP 08 (2013) 131]

• SM prediction from [C. Bobeth et al. JHEP 07 (2011) 067]

• Zero crossing point of AFB free from leading FF uncertainties

we measure

• Results based on 900 signal events from 1fb-1 at 7 TeV (angular analysis on 3 fb-1 being finalized)

to be compared with



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• And fortunately also ATLAS and CMS with ~0.4k candidates in 5 fb-1 start to contribute to this analysis. They particularly competitive at large q2



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• SM prediction from [Decotes-Genon et al. JHEP 05 (2013) 137]

• Can also apply different angular foldings to access different angular terms: focus on observables where leading form-factor uncertainties cancel, e.g.

• In 1 fb-1, LHCb observes a local (4.3<q2<8.7 GeV2) discrepancy of 3.7σ in P´5 (the probability that at least one bin varies by this much is 0.5%)

[PRL 111 (2013) 191801]

eagerly await for LHCb 3 fb-1 analysis!


Understanding the P´5 anomaly?

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• Several global fits to the available b→sγ and b→sl+l−

have been attempted, which include the observed anomaly => discrepancy with SM is 3-4.5 σ. Fits favour C9NP ~ -1.5 (non-SM vector current), and can also be explained by introducing a flavour-changing Z’ boson ~1-7 TeV

[Decotes-Genon, Matias, Virto ’13Altmannshofer, Straub ’13Gaul, Goertz, Haisch ’14

Beaujean, Bobeth, van Dyk ’13

Jaeger, Camalich ’13Lyon, Zwicky ’14Hofer, Matias ‘14

• By introducing form-factor uncertainties as nuisance parameters in the global fit, the discrepancy wrt SM is reduced to 2σ

• Form-factor uncertainties are being rediscussed, especially in the large recoil region, with appreciable impact on the comparison with data, and different opinions, of course!

(4.3<q2<8.7 GeV2)



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have been attempted, which include the observed anomaly => discrepancy with SM is 3-4.5 σ. Fits favour C9NP ~ -1.5 (non-SM vector current), and can also be explained by introducing a flavour-changing Z’ boson ~1-7 TeV





• Form-factor uncertainties are being rediscussed, especially in the large recoil region, with appreciable impact on the comparison with data, and different opinions, of course!

(4.3<q2<8.7 GeV2)

looks interesting but we need more data, and better understanding of theoretical uncertainties...

B0s,d → μ+μ−


B0(s) → μ+μ− decays

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• B0(s) → μ+μ− decays are a particular interesting case of EW penguin. The helicity suppression of the vector(-axial) terms, makes these decays very sensitive to new (pseudo-)scalar interactions → Higgs penguin!

e.g. in MSSM, branching fraction scales as

• Predicted precisely in the SM:

BR(B0s → μ+μ−) error budget

• The B0/B0s ratio is also powerful to discriminate among NP models: a deviation from the value predicted by SM, ~(Vtd/Vts)2, would indeed also imply the breaking of the Minimal Flavour Violation hypothesis

[Bobeth et al. PRL 112 (2014) 101801]


It’s a long history...with great contributors!

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CMS vs LHCb

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• Good trigger and muon ID• No hadron PID• Excellent silicon tracking to resolve signal decays in the high pile-up environment• Di-muon mass resolution 32-75 MeV/c2

• CMS: 5+20 fb-1 at 7 and 8 TeV

• Efficient muon trigger• Good muon and hadron PID• Track impact parameter resolution ≤20μm• Luminosity levelling at 4x1032 cm-2s-1

• Di-muon mass resolution 25 MeV/c2

• LHCb: 1+2 fb-1 at 7 and 8 TeV

~1 fb-1 at LHCb is equivalent to ~10 fb-1 at CMS


B0(s) → μ+μ− results at EPS2013

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• During EPS 2013 CMS and LHCb collaborations presented their results based on 25 fb-1 and 3fb-1, respectively


Combined CMS and LHCb results at CKM2014

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• Full simultaneous fit of CMS and LHCb data

• Statistical significance (Wilks’ theorem ):

likelihood profile scan for Bs

6 best S/B bins


Comparison with SM

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• 1D LL scan of the B0/B0s ratio: • 2D LL scan performed for the signal strength BR/BRSM :

theoretical errors included in the fits

• Compatibility with SM:

1.2σ for B0s 2.2σ for B0

• Compatibility with SM (and MFV):

2.3σ for B0/B0s ratio

B0/B0s ratioSignal strength


Implications

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• Latest results on B0(s) → μ+μ− strongly constrain the parameter space for many NP models, complementing direct searches from ATLAS/CMS: in particular, large tanβ with light pseudo-scalar Higgs in CMSSM is strongly disfavored

• Start to explore less constrained (and more “natural”) SUSY models: “split-family” SUSY, pMSSM

[Mamhoudi arXiv:1310.2556]

Barbieri et al. ’14Althmanshofer et al. ’13Mamhoudi ’13


More implications

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B0/B0s ratio fixed by MFV to SM value: it is therefore very relevant to clarify the experimental picture on B0

• Test the MFV hypothesis: flavour breaking in NP model described by CKM matrix → naturally small effects in FCNC observables

Chivukula, Georgi ’87D’Ambrosio, Giudice, Isidori, Strumia ’02

• Model independent constraints:the precision achieved now is such that B0(s) → μ+μ− sensitivity to (Z,γ) penguin cannot longer be considered sub-leading, and starts to compete with the golden mode B0→ K*μ+μ−

Althmanshofer et al. 1308.1501

Charles et al. 1309.2293

Lepton Flavour Violationand exotic searches


The quest for lepton flavour violation

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• The discovery of neutrino oscillations implies charged-LFV at some level. Many extensions of the SM introduce large cLFV effects.

• Important advantage wrt the quark sector: the reach for NP energy scale is not much affected by QCD uncertainties in the SM predictions

• The MEG collaboration at PSI using 3.6x1014 stopped muons reached an amazing sensitivity to μ→ eγ :

[PRL 110 (2014) 201801]

• In principle, τ are more sensitive per event than μ since mass typically decreases GIM suppression, >500;however production rate is much lower.


Search for lepton flavour violation in τ→μμμ

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• Possible as penguin with neutrino oscillation; SM prediction ~10-40, beyond experimental reach

• (some) NP predictions: SUSY ~10-10, mSUGRA+seesaw ~10-9, non universal Z´ ~10-8

• With ~1.4x109 τ at the B-factories the current limits are:

arXiv:1001.3221

• At the LHC τ are copiously produced (mainly from charm decays, Ds→ τν): ~1011 τ/fb-1 (~5x1014 at HL-LHC!).

BR(τ→μμμ)<4.6x10-8 at 90% CL

Belle: BR(τ→μμμ) <2.1x10-8 at 90%CL

BaBar: BR(τ→μμμ) <3.3x10-8 at 90%CL arXiv:1002.4550

Few days ago, LHCb presented at TAU2014 the search based on 3 fb-1


Search for Majorana neutrinos

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• Limit on BR(B- → π+μ−μ−) from 3fb-1 can be translated (with a model-dependent assumption on the decay width) to an upper limit on the coupling between muon and fourth generation neutrino

• In LHCb, heavy Majorana neutrinos can be sought in B- → π+μ−μ− decay, which is forbidden in SM but can proceed via production of on-shell massive neutrinos

B- → π+μ−μ−

• Observation of neutrino oscillations is a strong theoretical motivation for Majorana neutrinos to exist

[PRL 112 (2014) 131802]

decay width fromAtre et al. JHEP 05 (2009) 030+ S. Stone, Z. Xing ’13


Search for Majorana neutrinos - implications

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• Limit on BR can be translated to a model-dependent upper limit on the coupling between muon and fourth generation neutrino

[JHEP 05 (2009) 030]

• With this result LHC join the search for Heavy Neutral Leptons performed all around the world, both at colliders and fixed target experiments.

Conclusions


Conclusions

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• Flavor-changing transitions represent a unique window on physics beyond the SM: there is still a lot to learn and explore

• There are few interesting anomalies, but in general the agreement with the SM is excellent: large NP contributions, O(SM), ruled out in many cases

LHC (and LHCb) is acting as a fantastic flavour-factory

need combined th+exp precision at the few % level

• Interplay between low energy precision measurements and direct searches as strong as ever

• Ambitious program for upgrading integrated luminosity in the next years at LHC: LHCb aims at ~50fb-1


Conclusions

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• Flavor-changing transitions represent a unique window on physics beyond the SM: there is still a lot to learn and explore

• There are few interesting anomalies, but in general the agreement with the SM is excellent: large NP contributions, O(SM), ruled out in many cases

- LHC & LHCb are acting as a fantastic flavour-factory

- need combined th+exp precision at the few % level

• Interplay between low energy precision measurements and direct searches as strong as ever

• Ambitious program for upgrading integrated luminosity in the next years at LHC: LHCb aims at ~50fb-1

We don’t know yet what is the scale of NP: cast a wide net!

SPARES


Photon polarization in B+ → K+π−π+γ[PRL 112 (2014) 161801]


B0 → K*0μ−μ+ angular distribution

• in [PRL 111 (2013) 191801] fit P‘4,5,6,8=S4,5,7,8/sqrt(FL(1-FL)) with different angular foldings; e.g. for P’5: φ→-φ for φ<0; θl→ π-θl for θl> π/2- 24 bins in total

amplitudes depend on Wilson coefficient and form factors



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have been attempted, which include the observed anomaly => discrepancy with SM is 3-4.5 σ. Fits favour slightly negative C9NP (non-SM vector current), and can also be explained by introducing a flavour-changing Z’ boson ~1-7 TeV




G. Isidori at ICHEP 2014

(4.3<q2<8.7 GeV2)

• Form-factor uncertainties are being rediscussed, especially in the large recoil region, with appreciable impact on the comparison with data, and different opinons, of course!



Differential BRs at high q2 (>14GeV2)

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B → K(*)μ−μ+ isospin asymmetry


B+ → K+μ−μ+ angular distributionJHEP 05 (2014) 082


Lepton universality in B+ → K+l+l−


B0(s) → μ+μ− decays

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SM

NP


B0(s) → μ+μ− time-integrated BR

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Time-integrated BR vs CP-averaged BR

Lifetime bias in the analysis efficiency

a residual dependence vs analysis lifetime-dependent cuts is also corrected


LHCb

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~1011 b-bbar couples per fb-1

~ 3 fb-1 accumulated so far


B0(s) → μ+μ− at LHCb

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~ B0s → μ+μ− 12 SM evts/fb-1


Lepton flavour violation in τ→μμμ

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Search for Majorana neutrinos

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• Limit on BR can be translated to a model-dependent upper limit on the coupling between muon and fourth generation neutrino

• BR upper limits as a function of mass and lifetime in [1-1000]ps

decay width fromAtre et al. JHEP 05 (2009) 030+ S. Stone, Z. Xing ’13


Search for lepton flavour violation in B(s)→μe

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• Decays of the type B(s)→μe are allowed in models with a local gauge symmetry between quarks and leptons, with lepto-quark linking different quark/lepton generations

[Pati, Salam PRD 10 (1974) 275]

• With 1 fb-1 LHCb has put limits x20 more stingent than the previous best limits set by CDF

[Valencia, Willenbrock arXiv:hep-ph/9409201v1]

[PRL 111 (2013) 141801]

• These limits can be translated into limits on the value of the lepto-quark mass in the framework of the Pati-Salam model:

mLQ(Bs→μe)>101 TeV/c2 at 95% CL

mLQ(B→μe) >126 TeV/c2 at 95% CL

BR(B0s→μe)< 1.1 x 10-8 at 90% CL

BR(B0→μe) < 2.8 x 10-9 at 90% CL


Projections

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2012: LHCb Upgrade Framework TDR http://cdsweb.cern.ch/record/1443882/files/LHCB-TDR-012.pdf

http://cdsweb.cern.ch/record/1443882/files/LHCB-TDR-012.pdf

http://cdsweb.cern.ch/record/1443882/files/LHCB-TDR-012.pdf

Studio di decadimenti rari a LHCb · rari a LHCb Matteo Palutan INFN-LNF 100° Congresso SIF, Pisa...

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