Rare kaon decays at NA62 · 2019-06-18 · Data taking will resume after LS2 (in 2021). Developing...

Rare kaon decays at NA62

0 CLFV 2019 conference

Fukuoka, Japan 18 June 2019

Evgueni Goudzovski (University of Birmingham)

on behalf of the CERN-NA62 collaboration

Outline:

1) The NA62 experiment at CERN

2) Measurement of the K++ decay

3) Searches for lepton number/flavour violating K+ decays

4) Searches for HNL production in K+ decays

5) Summary

Kaon programme at CERN

SPS

NA48/NA62: ECN3 hall

Jura mountains

Geneva airport

France

Switzerland

LHC

N

NA48

1997: ’/: KL+KS

1998: KL+KS

1999: KL+KS KS HI

2000: KL only KS HI

2001: KL+KS KS HI

2002: KS/hyperons

2003: K+/K−

2004: K+/K−

tests

NA62

2007: Ke2/K

2

2008: Ke2/K

2

discovery

of direct

CPV

Earlier: NA31

1

NA62 RK run

NA48/1

NA48/2

tests

2015: commissioning

E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018

201618: physics run

Main NA62 goal: K++ measurement to 10% precision

with a novel decay-in-flight technique.

Currently ~200 participants from 31 institutions.

Beamline & detector

2

Un-separated hadron (p/+/K+) beam.

SPS protons: 400 GeV, nominally 3.3×1012/spill.

K+: 75 GeV/c (±1%), divergence < 100 rad.

Nominal beam rate: 750 MHz, K+ rate 45 MHz;

~5 MHz K+ decays in fiducial volume

KTAG: Cherenkov

kaon tagger, t=70ps

Anti-counters

GTK: beam

tracker

Spectrometer:

straw chambers

LAV: large-angle

photon veto (12 stations) Muon

detector

(MUV)

Small-angle photon veto LKr EM

calorimeter

Dump

Z [m]

NA62 collaboration,

JINST 12 (2017) P05025

t=70ps

Hadronic

Calorimeter

(HAC)

O(106) mbar


Currently, 1 year of operation 2×1018 protons on target; 3×1012 K+ decays.

Single event sensitivities for K+ decays: down to BR~1012.

Kinematic rejection factors: 1×103 for K++0, 3×104 for K+.

Hermetic photon veto: 0 decay suppression (for E0>40 GeV) = 3×108.

Particle ID (RICH+LKr+HAC+MUV): ~108 muon suppression.

300 m3

3

Commissioning run 2015: minimum bias data (~3×1010 protons/pulse).

Physics run 2016 (30 days, ~1.3×1012 ppp): 1011 useful K+ decays.

Physics run 2017 (161 days, ~2.0×1012 ppp): ~2×1012 useful K+ decays.

Physics run 2018 (217 days, ~2.3×1012 ppp): expect 3×1012 useful K+ decays.

Resuming data taking after Long Shutdown 2 in 2021.

NA62 data collection


4

K++ measurement: first result and prospects


Result based on the 2016 data:

Phys. Lett. B791 (2019) 156

Rare kaon decays: K

5

Theoretically clean, almost unexplored,

sensitive to new physics.

Mode BRSM1011

K++() 8.41.0

KL0 3.40.6 SM precision surpasses any other

FCNC process involving quarks.

Measurement of |Vtd| complementary

to those from e.g. BB mixing.

Main focus of kaon physics: measurement

of both K++ and KL0 decays.

The uncertainties are largely

parametric (CKM)

SM branching ratios

Buras et al., JHEP 1511 (2015) 033

SM: box and penguin diagrams

Ultra-rare decays with

the highest CKM suppression:

A ~ (mt/mW)2|V tsVtd| ~ 5 *


CKM unitarity triangle with kaons

Kaon measurements alone

can fully constrain the unitarity

triangle.

Complementary to B physics

in the description of NP flavour

dynamics.

NP with

CKM-like

flavour

structure

BR(KL0) vs BR(K++)

Current experimental

uncertainty (1)

6

Status of theory and experiment


K signal region definition mmiss

2=(PKP)2 vs track momentum

Region II

Region I

K+++

K++00

K++0

K++ Further background

suppression:

PID (calorimeters &

Cherenkov detectors):

suppression 108.

Hermetic photon veto:

suppression of

0 decays 3×108.

Main K+ decay modes

(>90% of BR) rejected

kinematically.

Resolution on mmiss2:

=1.0103 GeV4/c2.

Measured kinematical

background suppression:

K++0: 1×10−3;

K++: 3×10−4.

DATA 2016

E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018 7

8

Blinded Region 2

Blinded Region 1

Data sample: 30 days at

1.3×1012 ppp.

Number of kaon decays:

NK = (1.210.02syst)×1011.

Background estimates

are mostly data-driven.

Signal acceptance:

A = (4.00.1)%.

Single-event sensitivity:

SES = (3.150.24)×1010.

Data 2016 K+++

region

K++0

region

K++ region

2016 data


PLB791 (2019) 156

9

One K++ candidate observed:

BR(K++) < 11×1010 at 90% CL.

BNL-E949 (K+ decay at rest): BR(K++) = (1.73 )1010

SM prediction: BR(K++) = (0.840.10)×1010

+1.15 1.05

The NA62 decay-in-flight technique works.

A non-trivial result obtained with 2% of the total statistics collected.

Result 2016

x [mm]

y [

mm

] The RICH ring for the track

e


2016 data

PLB791 (2019) 156

K+++

region

K++0

region

K++ region

K++: next steps

10 E. Goudzovski / CLFV 2019, Fukuoka, 18 June 2018

Analysis of the 2017 data

Data sample: 161 days at 2.0×1012 ppp.

Analysis procedure is similar to the 2016 one.

Number of kaon decays: NK = (1.30.1)×1012.

Single-event sensitivity: SES = (3.40.4)×1011.

Expected signal: 2.50.4 SM K++ events.

Expected background: 0.760.10 events, excluding upstream decays.

Result expected later this year, surpassing present best sensitivity.

Analysis of the 2018 data

The largest sample collected so far: 217 days at 2.3×1012 ppp.

Analysis procedure being optimized to improve acceptance.

Further data collection necessary to reach the 10% precision

Data taking will resume after LS2 (in 2021).

Developing a strategy to collect 100 SM events by 2024.

Possible beam dump operation (3 months of data taking = 1018 pot):

competitive searches for hidden sector (long-lived HNL, DP, ALP).

(CERN SPSC open session, 2 April 2019)

KL0 vs K++: prospects

11

NA62 precision by 2025

(with 100 SM events)

KOTO sensitivity

goal following

beam & detector

upgrades (~2025?)


KOTO 2015 data: BR<3.0×109 @90% CL

PRL122 (2019) 021802

KOTO sensitivity

with 201518 data. Expect result in 2019.

12

Searches for lepton flavour and lepton number violation


Downscaled di-lepton trigger chains are in operation.

First results based on 80% of the 2017 data:

CERN-EP-2019-104, arXiv:1905.07770, submitted to PLB.

Analyses completed so far: searches for K+ℓ+ℓ+ decays.

Backgrounds and PID


Major background for 3-track decays:

the K+++ decay (BR=5.6%).

Studied with data-driven methods

and dedicated simulations.

Background to decays into leptons:

1) via decays in flight;

2) via misidentification as e.

Pion/electron identification:

1) by energy deposit in LKr (E/p);

2) by the RICH signal pattern.

Pion mis-ID probability vs momentum

13

K+e+e+: auxiliary selection


LNV selection: m(e+e+) SM selection: m(+e+e)

Signal region K+++ K+++

K++e+ K++e+

Auxiliary selection: LKr only is used for pion/electron ID

Validation of the background estimates using control mass regions.

Sensitivity is limited by K++0D background.

Therefore RICH is used for positron ID (10% loss of SES, 6 times lower background)…

K++0D

15

K+e+e+: main selection


LNV selection: m(e+e+) SM selection: m(+e+e)

Signal region

K++e+

Candidates observed: 2484

BR(K++e+e) = (3.000.09)×107

K+ decays in FV: (2.140.07)×1011

SES = (0.940.03)×1010

Expected background: 0.160.03 evt


K+[e+e]0e+

Result: BR(K+e+e+)<2.2×1010 at 90% CL

16

Search for K+++ decay


LNV selection: m(++) SM selection: m(++)


Background: 0.07%

BR(K+++) = (0.9620.025)×107

K+ decays in FV: (7.940.23)×1011

K+++ K+++

K+++ K+++

SES = (1.280.04)×1011

Expected background: 0.910.41 evt


Signal region

Signal region

17

K+++ decay: results


BR(K+++)<4.2×1011 at 90% CL

Backgrounds to K+++ decay

Data-driven estimates (inversion of PID criteria)

Biased fast simulations

with forced pion decays:

dominated by pion decays in the spectrometer

Full MC simulations tuned using

measured misID probabilities and measured pileup rates

Final result with the 2017 data set:

One candidate observed

18

LFV/LNV: results and prospects


Single-event sensitivities achieved with the 2017 data

For K++e+ [LNV] and K++e+ [LFV]:

SES5×1011 (factor ~5 improvement on BNL-E865).

For K+e++ [LFV], SES5×1011

(the first search for this mode);

For K+e+e+ [LFV], SES1×1010

(factor 100 improvement on PDG).

The full 201618 dataset is ~3 times the size of 2017 dataset.

Not competitive yet for K+++e and 0e∓.

Six analyses are in progress, none is limited by background

ULs obtained with 80% of the 2017 data set: (arXiv:1905.07770)

BR(K+e+e+)<2.2×1010

BR(K+++)<4.2×1011

(at 90% CL, assuming uniform phase space distribution).

Factor 23 improvement on the earlier results (BNL-E865 and NA48/2).

19

Searches for heavy neutral lepton production in K+ decays

Results based on the pilot 2015 data:

Phys. Lett. B778 (2018) 137


Heavy neutral leptons in MSM

Neutrino minimal SM (MSM) =

SM + 3 right-handed neutral heavy leptons. [Asaka et al., PLB631 (2005) 151]

Masses: m1~10 keV [DM candidate]; m2,3~1 GeV.

HNLs observable via production and decay.

Shaposhnikov, JHEP 0808 (2008) 008

Boyarsky et al., Ann.Rev.Nucl.Part.Sci.59 (2009) 191

|Uℓ4|2

Baryon asymmetry of the Universe

Big-Bang

nucleosynthesis

Accessible in

K+ℓ+N decays

R. Shrock PLB96(1980)159

m1 [keV]

m2,3 [GeV]

Astrophysical & cosmological

constraints on m1, m2,3

(K+ℓ+N) = (K+ℓ+) ℓ(mN)|Uℓ4|2

(mN)

RK = (K+e+)/

(K++)

2.5×105

e(mN)×RK

Kin

em

ati

c e

nhancem

ent

facto

r


K+ℓ+N data samples

Minimum bias data (1% intensity); 12k SPS spills (=5 days) in 2015.

Numbers of K+ decays in fiducial volume:

NK=(3.010.11)×108 in positron case; NK=(1.060.12)×108 in muon case.

Beam tracker not available: beam average kaon momentum is used.

HNL production signal: a spike above continuous missing mass spectrum.

Squared missing mass: (PKPe)2

K++, +e+

K+e+,

BR=1.6×105: 1.7k candidates

HNL search region:

low background due to

photon veto and kaon ID

Squared missing mass: (PKP)2

K++,

BR=64%: 24M candidates

HNL search

region


K+ℓ+N: resolution & acceptance

HNL mass resolution m vs mass

22

Selection for each HNL mass hypothesis (mHNL ) includes the “mass window”

condition: |mmHNL|<1.5m: background is proportional to mass resolution.

Also, resolution is crucial to resolve possible HNL mass splitting. [Baryogenesis: 2 quasi-degenerate mass states; Canetti et al., PRD87 (2013) 093006]

Signal selection acceptance vs mass

Relaxed

selection

at high mN

Standard

selection


Statistical analysis

23

Expected background (and stat.error) estimated from fits to the sidebands.

Numbers of observed and expected events converted into limits for the signal.

Background simulations used to certify the absence of peaking structures.

Full MC background estimate would allow searches for K+ℓ+.

K+e+N

K++N


HNL production search: results

24

Upper limits on BR(K+ℓ+N)

90%CL

Local signal significance never exceeds 2.2: no HNL signal is observed.

Reached 106107 limits for |Uℓ4|2 in the 170448 MeV/c2 mass range.

|Uℓ4|2 limits from production searches

Phys. Lett. B778 (2018) 137

Expected

Observed

K+e+N

K++N


HNLs: prospects with full dataset

25

Data sample 201618 in comparison to data sample 2015:

Beam tracker (GTK) in operation:

a factor ~2 improved HNL mass resolution m,

therefore lower background and broader mass range accessible;

a factor ~3 lower background in the K+e+N mode

(K++, +e+: muon decays in flight rejected geometrically);

lower background from upstream decays in the K++N mode.

Much larger datasets:

In the K+e+N mode, the main K++ trigger is used

(with reduced signal acceptance: max calorimetric energy = 30 GeV):

expect at O(106) K+e+ events, i.e. a factor ~1000 improvement.

In the K++N mode, downscaled control trigger (D=400):

expect O(109) K++ events, i.e. a factor ~100 improvement.

Expected sensitivities to |Uℓ4|2 with 201618 data:

at least 108 for both |Ue4|2 and|U4|

2


Summary

26

CERN-NA62 collected a sample of ~5×1012 K+ decays in flight

during the first physics run in 201618.

Focused on the K measurement (currently SES~1011).

A broader programme of rare decay measurements,

LF/LN violation and hidden sector mediator searches is pursued.

Analysis of the 2016 data (~2% of the main data set):

BR(K++) <11×1010 at 90% CL. [PLB791 (2019) 156]

Analysis of the 2017 data (~30% of di-lepton data sets):

BR(K+e+e+)<2.2×1010, BR(K+++)<4.2×1011 at 90% CL.

[arXiv:1905.07770]

Analysis of the 2015 minimum bias data: sub-106 limits on |Uℓ4|2

from HNL production searches. [PLB778 (2018) 137]

All results to be updated/expanded soon with larger data sets.


27

Spares


K acceptance & backgrounds

Events

Events

K++0() background vs + momentum

K++() background vs + momentum

K++()

K++0()

K++

K++

K++ acceptance

vs + momentum

+/+

separation

(RICH)

+/+

separation (calorimeters)

Rare kaon decays at NA62 · 2019-06-18 · Data taking will resume after LS2 (in 2021). Developing...

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Transcript of Rare kaon decays at NA62 · 2019-06-18 · Data taking will resume after LS2 (in 2021). Developing...