“Precision tests of the Standard

Model with Kaon decays at CERN”

Gianluca Lamanna (INFN) On behalf of NA62 collaboration

ICHEP 2014

Valencia 4.7.2014

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Introduction

Higgs found! Great success of the Standard Model!

(Still) No evidence of new particles

the Standard Model may be a self-consistent weakly coupled effective field theory up to very high scales without adding new particles up to the Plank mass scale (!).

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[Example from ATLAS before ICHEP2014]

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The end of the history? No!

Experimental “evidence” of new physics • Neutrino oscillations: tiny masses and flavor mixing;

• Barion asymmetry of the universe: CKM is not enough to explain all the CP violation;

• Dark matter;

• Dark Energy;

Theoretical “evidence” of new physics • SM flavour structure;

• Hierarchy problem;

• Strong CP problem;

• Gravity;

• …

Precision tests of SM indirect search of

new physics.

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Kaons @ CERN

Kaons:

Theoretical cleanliness unmatched

The SM at E~MK is remarkably simple (at least the EW part)

Breaks of many symmetries: C, P, CP, flavor

Extreme hard-GIM SM-suppressed FCNC decays

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1997 1998

1999 2000

2001 2002

2003 2004

2005

2006

2007 2008

2009 2010

2011 2012

2013 2014

2015

NA48 (1997-2001):

search for direct

CP violation in

neutral kaon

NA48/1 (2002):

Rare Ks and

hyperon decays

NA48/2 (2003-

2004): Direct CP

violation in

charged kaon and

rare decays

NA62-phase RK

(2007-2008): LFV in

leptonic charged

kaon decays

NA62 (2014-…):

BR(K+ pnn ̅ )

Simple event topologies gives clean experimental signature

Few decay channels

Availability of high intensity beams.

[A.Sergi – Prospects for K+ ->pi+ nu nu

observation at CERN in NA62 - 5.7.2014 at 9.00

– Flavour Physics]

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Outline

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New Physics effects in flavour physics

MFV: helicity suppressed observables are sensitive to SUSY (B→ll, B→ln, K→ln).

LFV decays forbidden in SM.

Non-MFV: FCNC decays with high suppression in the standard model and clean theoretical prediction.

Easier in K: the B decays are suppressed (Vub<<Vus) while RK is 10-5. Cleaner environment to search for forbidden decays.

The K→pnn ̅ has a very

clean SM prediction (l5 suppression).

NA62 & NA48/2 @CERN

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NA48/2 (2003-2004) and NA62-RK (2007-2008)

~100 m long decay region in vacuum

Triggers based on LKr peaks and energy, CHOD hits and DCH multiplicity in NA62-RK or DCH tracks in NA48/2

Simultaneous K+ and K- beams in NA48/2, alternate K+ and K- beams in NA62-RK

6 6

Spectrometer:

σp/p = 1.0% + 0.044% p [p in

GeV/c] in NA48/2

σp/p = 0.48% + 0.009% p [p in

GeV/c] in NA62-RK

LKR calorimeter:

σE/E = 3.2%/√E + 9%/E +

0.42% [E in GeV]

CHOD, HAC, MUV

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NA62 (2014- …)

SPS proton beam @400 GeV/c,

1.1x1012 protons on beryllium

target.

75 GeV/c (+1%) hadron beam

(6% kaons).

~750 MHz on beam spectrometer,

~ 10 MHz kaon decays on main

detectors downstream.

60 m decay volume. 7

Kinematics rejection factor:

5x103 K+→p+p0, 1.5x104

K+→mn .

Photon veto: 108 on p0→gg

(LAV, LKR, SAC, IRC).

Muon ID: 107 (RICH, MUV).

10% acceptance on K+→pnn ̅ .

Goal: O(100) events of

K+→pnn ̅ in 2 years.

[A.Sergi – Prospects for K+ ->pi+ nu nu

observation at CERN in NA62 - 5.7.2014 at 9.00

– Flavour Physics]

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LFNV in Kaon Decays

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Process Violates 90% C.L. limit Experiment Year

K+→p+m+e- LF < 1.3 x 10-11

E865

2005

K+→p+m-e+ LF < 5.2 x 10-10 2000

K+→p-m+e+ LF , LN < 5.0 x 10-10

2000

K+→p-e+e+ LN < 6.4 x 10-10 2000

K+→p-m+m+ LN < 1.1 x 10-9 NA48/2 2011

K+→m- ne+e+ LN < 2.0 x 10-8 Geneva-Saclay 1976

p0→m-e+ LF < 3.4 x 10-9

KTEV

2000

p0→m+e- LF < 3.8 x 10-10 2000

p+→m-e+e+n LF <1.6x10-6 JINR-SPEC 1991

Γ𝐿𝐹𝑉

Γ𝐿𝐹𝐶~𝑂

𝑔2𝑋

𝑔2𝑊

∙𝑀2

𝑊

𝑀2𝑋

For: gx~gw and BR~10-12

MX ~ 100 TeV

Example (with

dimensional argument):

NA62 will collect: ~1013 K

decays and ~ 2.5x1012 p0

decays in two years of

data taking

Single event sensitivity:

~10-12 for K+, ~10-11 for p0

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K+→p-m+m+

LFV possible at tree level;

|DL|=2 transition mediated by Majorana neutrino;

Unique opportunity to study neutrino-less double beta decay in second generation;

For some LFV SM extension the foreseen rate is close to the present experimental limit [Phys.

Lett. B 491 (2000) 285]

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-

W + W -

u

s -

u

d

n

u

s - u -

n

d

W +

W +

Upper limit before NA48/2 (E867):

Br(K+→p-m+m+) < 3.0 × 10−9 (90%C.L.) [Phys. Rev. Lett. 85 (2000) 2877]

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K+→p-m+m+ in NA48/2

Three tracks trigger based on charged hodoscope;

Signal (K+→p-m+m+ ) and normalization (K+→p+p+p-) selected concurrently with similar cuts to equalize the acceptance;

Kinematical cuts studied on right sign (K+→p+m+m-) analysis

52 candidates in signal region with respect to an expected background of 52.6±19.8

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BR (K±→p∓m±m± ) < 1.1×10–9 @ 90% CL [Phys. Lett. B 697(2011)107]

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K+→p-m+m+ in NA62

In NA48/2 the main background was due to p→mn decay in flight in the spectrometer;

The mass resolution on pmm was 2.6 MeV/c2;

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K-

>pmumu:

prospettiv

e in na62

In NA62 negligible background is expected in the signal region, defined by a smaller invariant mass resolution (1.1 MeV/c2);

A factor of x100 or x1000 improvement could be achieved in NA62.

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RK in the SM

The ratio of kaon leptonic modes is extremely well predicted in the SM

Helicity suppression;

Hadronic uncertainties cancel in RK ratio;

Radiative correction are included in the RK definition and well computed in SM;

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𝑹𝑲 =𝜞(𝑲+ → 𝒆+𝝂)

𝜞(𝑲+ → 𝝁+𝝂)=

𝒎𝒆𝟐

𝒎𝝁𝟐

𝒎𝑲𝟐 − 𝒎𝒆

𝟐

𝒎𝑲𝟐 − 𝒎𝝁

𝟐

𝟐

𝟏 + 𝜹𝑹𝑲𝒓𝒂𝒅.𝒄𝒐𝒓𝒓.

K+

e+

n

RK = (2.477 ± 0.001)x10-5 [V.Cirigliano, I.Rosell, Phys. Rev. Lett. 99

(2007) 231801]

A precise measurement of RK is a stringent test of the SM!

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RK BSM

In several SM extensions the presence of LFV terms enhance the decay rate with sizeable effect:

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𝑹𝑲𝑳𝑭𝑽 =

𝜞𝑺𝑴 𝑲 → 𝒆𝝂𝒆 + 𝜞𝑳𝑭𝑽 𝑲 → 𝒆𝝂𝝉

𝜞𝑺𝑴 𝑲 → 𝝁𝝂𝝁

𝑹𝑲𝑳𝑭𝑽 ≈ 𝑹𝑲

𝑺𝑴 𝟏 +𝒎𝑲

𝟒

𝒎𝑯±𝟒

𝒎𝝉𝟐

𝒎𝒆𝟐

|∆𝟏𝟑|𝟐𝒕𝒂𝒏𝟔𝜷

In model with 2 Higgs doublets (e.g. 2HDM-II MSSM) and LFV sources, for large tanb contribution at the level of % is expected:

[Masiero et al., PRD 74 (2006) 011701]

[Masiero et al., JHEP 0811 (2008) 042]

[Girrbach, Nierste, arXiv:1202.4906]

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Ke2 and Km2 selection

Km2 and Ke2 are collected simultaneously

1 track in the spectrometer and in the acceptance of the electromagnetic calorimeter and muon counter;

Decay vertex well reconstructed in the decay region;

Forward photon veto using the electromagnetic calorimeter;

Track momentum: 13<p<65 GeV/c.

Km2 and Ke2 are separated by:

Different E/p: • electron if (0.9 to 0.95)<E/p<1.1,

• muon if E/p<0.85;

Missing mass: M2miss=(PK-Pi)

2 with i=e or m.

The analysis is done in 10 momentum bins:

The selection criteria are tuned in each bin

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Ke3 Km2

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Data set

Ke2:

145958 events selected (more than

a factor 10 with respect to the best previous measurement)

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Background sources (in %):

Km2 5.64±0.20

Km2 (m decay) 0.26±0.03

Ke2g (SD+) 2.60±0.11

Beam halo 2.11±0.09

Ke3 0.18±0.09

K2p 0.12±0.06

Wrong sign K 0.04±0.02

Total 10.95±0.27

Km2:

42.817 M events (with pre-scaled

trigger)

Main background: beam halo muons (0.50±0.01)%

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Main background

The main background is given by Km2 in the Ke2 sample

m catastrophic energy loss in LKr: Pme=3∙10-6

Direct measurement of Pme selecting a pure m sample;

Lead plate in front of LKr (9.2X0, 20% of total area);

Uncertainty on the evaluated Km2 background 3 times smaller with respect to MC only.

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Final result

Analysis in 10 momentum bins and 4 data samples ;

Fit over 40 measurements: c2 = 47/39 ;

0.4% precision reached;

Theoretical error still one order of magnitude better.

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RK=(2.488±0.007stat±0.007syst)x10-5

=(2.488±0.010)x10-5

Source dRKx105

Statistical 0.007

Km2 0.004

BR(Ke2gSD+) 0.002

Beam halo 0.002

Ke3, Kp2 0.003

Material 0.002

Acceptance 0.002

DHC alignment 0.001

Electron ID 0.001

LKr read. ineff. 0.001

Trigger 0.001

Total 0.010

[Phys. Lett. B 719 (2013) 326]

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World Average

RK= (2.488±0.009)x10-5

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Precision of 0.4%. In agreement with SM expectation within 1.2s

[our own average]

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Forbidden and exotic decays

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B865 E787 E949 NA62

Energy p 25.5 GeV 25.5 GeV 25.5 GeV 400 GeV

Protons f 1013 in

2.8s/5.1s

~1013 /1.6s

~1013 /2.2 s ~1012 in 4.6s/16.8s

Energy K 6 GeV 700 MeV (stopped) 710 MeV

(stopped)

75 GeV

Beam 71 MHz K, 1.4

GHz p, 0.7 GHz

p

(2.5-4) MHz K with

p/K = 0.25

6 MHz K with

K/p~3

45 MHz K, 525 MHz p, 173

MHz p

Decay

region

5 m (6% kaon

decay)

Stopped (20%

kaon decay)

Stopped (20%

kaon decay)

60 m (10% kaon decay)

NA62 will collect an unprecedented

amount of K+ decays giving the

possibility to re-measure rare

decays properties and look for

forbidden and exotic decays.

The NA62 trigger systems is

flexible and fully reconfigurable

(based on FPGA).

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Example: forbidden and exotic decays

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Process motivation 90% C.L. limit Experiment Year

K+→p+X0 New particle < 5.9 x 10-11 mX0=0 E787, E949 2002

K+→p+ cc New particles (several) Example: E949 2008

p0→ee(g) Dark force (several) (several) --

p0→eeee T viol. in SI and EI C=-0.77+-0.53 Samios et. al. 1962

p0→ggg C violation < 3.1 x 10-8 Crystal Box 1988

p0→gggg light scalar < 2 x 10-8 Crystal Box 1988

p0→nn ̅ RH neutrino <2.7x10-7 E949 2005

sgoldstino New particle < 9 x 10-6 Hyper-CP, ISTRA+ 2004

K+→p+p+e-n DS!=DQ < 1.2 x 10-8 Geneva-Saclay 1976

K+→p+p+m-n DS!=DQ < 3.0 x 10-6 Birge et al. 1965

K+→p+g Angular momentum < 2.3 x 10-9 E949 2005

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Sterile neutrinos

Adding three right handed sterile Majorana neutrinos to SM it is possible to explain some of the open questions in particle physics:

Matter-antimatter asymmetry (leptogenesis induces baryogenesis)

Neutrino mixing

Dark matter

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In the Asaka-Shaposhinikov model nMSM [Phys. Lett. B 620 (2005) 17]

N1 is the lighter O(keV) dark matter candidate

N2,N3 are nearly degenerate (100 MeV to few GeV) to tune CPV-phases as extra-CKM sources of baryon asymmetry. N2,3 produce standard neutrino masses through seesaw with a Yukawa coupling of the order of 10-8

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Sterile neutrinos in K

The meson factories are an ideal place to produce and study heavy neutrinos (N2,3);

In principle several LFV decay modes accessible;

The neutrinos mass accessible in kaon decays is limited by MK

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W +

N

s -

u

U2

W +

N

d -

u

U2

production

decay

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Sterile neutrinos analysis

The ongoing analysis in NA62-RK is done assuming long lived heavy neutrinos

Missing mass analysis

In NA62 higher statistics, better resolution on the invariant mass, better photon veto to control Km3 and K2p, will increase significantly the sensitivity;

The kaons will help to cover the full parameters space in the regions allowed by general constraints.

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Kaons

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Dark photons

The search for the U boson (aka dark photon) is becoming interesting as possible explanation of several SM anomalies:

PAMELA e+ excess

Dama/Libra dark matter signals [R. Bernabei

et al., Eur. Phys. J. C56, 333 (2008)]

3.6s anomalies in (g-2)m

Several dedicated experiments

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Search in p0→Ug decays (with U→e+e-) NA62 will collect 108 p0→e+e-g decays/year

Mee resolution of 1 MeV

Sensitive to MU<100 MeV with e2~10-6

Analysis on the NA48/2 data ongoing

NA62 will improve the NA48/2 upper limit.

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Conclusions

The Kaons have written several chapters in particle physics:

Strangeness

Parity violation

Oscillation

Regenaration

CP violation

GIM mechanism

The Kaons have still arrows in their quiver: The K→pnn̅ is an unique laboratory to test new physics

scenarios;

Other LFV, forbidden and exotics decays may shed new light on some obscure point of the SM;

NA62 will profit from high intensity hadron beam, flexible trigger configuration and ~10% acceptance on several decays.

The new generation of K experiments are on stage:

NA62@CERN will start to take data next autumn.

KOTO is taking data (talk by T.Nomura – July,5- Flavour Physics).

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Starting with beam in October 2014!

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K→pnn ̅: the kaon’s «golden modes»

FCNC process:

Forbidden at tree level.

SD contribution: by

penguins and boxes.

Small BR due to small

CKM top coupling → l5

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Very clean theoretical environment:

LD corrections (mainly due to charm).

2-loops EW corrections below 1%

Hadronic matrix element related to well

measured Ke3 (K+→p0e+n).

SM predictions for the BR*:

BR(K+→p+nn ̅)=(7.81±0.75±0.29)10-11

Parametric error: dominated by Vcb, r

measurement.

Pure theoretical error dominated by LD

corrections.

*[Brod, Gorbahn, Stamou, Phys. Rev. D 83, 034030 (2011)]

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Other examples of exotics

The p0→nn ̅ is very sensitive to RH neutrinos.

The BR is 0 in case of massless neutrinos, otherwise (k=1 for Dirac,

k=2 for Majorana):

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From the nt mass measurement BR<5x10-10

Present experimental limit (90% C.L.): 2.7x10-7

NA62 can reach the level of ~10-10, as by product of the main

analysis.

The K+→p+g is completelly forbidden in SM.

Angular momentum violation: 0->0 spin transition with real

photon

Simple extension of the SM can’t accomodate the

presence of this decay

Present experimental limit (90% C.L.): 2.3x10-9

NA62 can reach the level of ~10-12

K

0

0

1

g

p

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Sterile neutrino

A simple extension of the SM is the introduction of sterile

RH neutrinos.

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NA62 will be sensible to sterile neutrinos in the mass

range [100:350] MeV

Km2: small dedicated runs

High acceptance

Order of 20M events per burst (before the trigger)10kevents

per burst

Few hours to superseed the statistics of other experiments.

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Technical run 2012

Test of readout electronics,

infrastructure, detector.

Partial setup: LKr, CEDAR, CHOD,

MUV2, MUV3

Data analysis based on K+->p+p0

reconstruction (p+ identified using

the missing mass).

Clean sample after selection based

on timing and spatial correlation

between detectors

Preliminary measurement of

detectors time resolution, CEDAR

efficiency, punch-through

probability.

Developped technique for detectors

time allignment.

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Technical run 2012

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Installation

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Installation

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GTK

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13.2 m 9.6 m

60

mm

GTK2

GTK3 GTK1

Momentum reconstruction on high intensity

beam → three stations in an achromath dipole

system

Thin detector → 200 mm pixel sensor+100 mm

readout chip (<0.5% X/X0 per station)

18000 pixels, 150 kHz rate per single pixel in the

central part

Excellent time resolution to match the

measurement with the other detectors → <200 ps

achieved in test beam

Cooling system to control the leakage current given by the radiation damage

→ microchannel cooling.

s(PK)/PK~ 0.2% s(dX/dZ)/(dX/dZ)~16 mrad

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LAV

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12 rings along the decay region

(in vacuum)

Fully angular coverage in the

8.5-50 mrad range

OPAL calorimeter lead glass

reused: 5 staggered rings per

station

more than 2500 cristals in total

Block tested at BTF@Frascati: inefficiency <

10-4 for 476 MeV e+

Time resolution: 700 ps

Large dinamic range → double threshold

readout with clamping diode and ToT technique

3 rings built and tested.

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Straws tracker

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Spectrometer in vacuum to decrease MS effects

4 chambers with 4 redundant views each (4

staggered layer per view)

Magnet with Ptkick = 256 MeV/c 2.1 m long straws, 9.6 mm mylar tubes

<0.1% X/X0 per view

Central “hole” (6 cm radius) for the beam

obtained removing straw tubes in the central

region

Full length prototype built and tested in vacuum

at SPS@CERN in 2007 and 2010

s(Pp)/Pp~ 0.3% 0.007%*Pp (GeV/c)

s(dX/dZ)/(dX/dZ)~ 45-15 mrad

Rejection power: 104 (K→pp0), 105 (K→mn)

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LKr

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LKr : old NA48 electromagnetic calorimeter,

cryogenic liquid kripton

Quasi homogeneous ionization chamber

more than 13000 channels

27 X0

Excellent energy resolution

very good time resolution: 100 ps

New readout electronics: 14 bits 40 MHz ADC

with large buffers

The performances as photon veto has been

measured in a special run with 75 GeV kaon

decays: Energy(GeV) Inefficiency

2.5-5.5 10-3

5.5-7.5 10-4

7.5-10 5x10-5

>10 8x10-6

p g

g

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CHANTI,SAC & IRC

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Small angle calorimeters:

IRC & SAC

IRC: to increase the

acceptance for small angle g

in the region not covered by

the LKr

SAC: to detect g in the

beam pipe region

The IRC is located around the beam pipe in front of the LKr, the SAC is

located in the beam dump, at the very end of the experiment.

The CHANTI is located after the GTK station three in order to detect

interaction in the beam spectrometer

IRC, SAC and CHANTI are in advanced R&D status

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CEDAR

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Positive identification of the kaon

in the hadron beam

Purpose: tagging the kaon to

decrease the requirements for the

vacuum in the decay region (10-5

mbar).

Technique: differential H2

cherenkov detector.

Old detector built at CERN in the

’70

New readout (PMs and

electronics)

New deflecting mirrors system to

decrease the rate per single channel

on the readout.

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RICH

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p m

e

p = 15 GeV/c

17 m long, 3 m in diameter

Filled with 1 atm Neon

p-m separation in the 15-35

GeV/c range

Cherenkov light collected in

two spots: 1000 PM each

Full length prototype tested in

2009

Average time resolution: 70 ps

Integrated mis-indentification

probability: ~5x10-3

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MUVs

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LKr MUV 1-2

MUV 3

MUV1-2 : to reach a factor of 106 in muon

rejection (combined with the RICH)

Partially reused the NA48 Hadron

calorimeter

Iron and scintillator

MUV3: fast muon identification plane for trigger

modules of 22x22 cm2 with 5 cm thick

scintillator readout with 2 PMs

<1ns time resolution achieved in test beam

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Trigger

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L0 trigger

Trigger primitives

Data

CDR

O(KHz)

EB

GigaEth SWITCH

L1/L2

PC

RICH MUV CEDAR LKR STRAWS LAV

L0TP

L0

1 MHz

1 MHz

10 MHz

10 MHz

L1/L2

PC

L1/L2

PC

L1/L2

PC

L1/L2

PC

L1/L2

PC

L1/L2

PC

100 kHz

L1 trigger

L1

/2

L0: Hardware

synchronous

level. 10 MHz

to 1 MHz. Max

latency 1 ms.

L1: Software

level. “Single

detector”. 1

MHz to 100

kHz

L2: Software

level.

“Complete

information

level”. 100

kHz to few

kHz.

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Trigger

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L0 selection:

RICH+!LKR+!MUV3

RICH: hit multiplicity

positive signal

!LKR: no 2 clusters more

than 30 cm apart

!MUV3: no signal in MUV3

Initial rate

(MHz)

After L 0

(MHz)

pp0 1.9 0.22

mn 5.7 0.04

ppp 0.5 0.1

ppp0 0.16 0.002

p0en 0.3 0.05

p0mn 0.2 0.002

TOT 6.7 0.4

pnn (eff.) 82%

Very good time resolution is

required to avoid random veto

At the software levels a more

complete analysis will be performed

(missing mass, Z vertex,…)

Input (max) Output (max) latency

L0 hw,sync ~10 MHz ~ 1 MHz 1 ms

L1 soft,async ~ 1 MHz ~ 100 kHz undefined

L2 soft,async ~ 100 kHz O(kHz) undefined

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GPU trigger

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GPUs 1 Tesla GPU

Single Precision

Performance 3.95 TeraFlops

Double Precision

Performance 1.31 TeraFlops

Memory 6GB DDR5

Cores 2688

Bandwidth 250 GB/s

The idea: exploit GPUs

(standard video card processors)

to perform high quality analysis at

trigger level

GPU architecture: massive

parallel processor SIMD

Easy at L1/2, challenging at L0

Real benefits: increase the

physics potential of the

experiment at very low cost!

Profit from continous develops in

technology for free (Video

Games,…)

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GPU trigger

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Natively built for pattern

recognition problems

First attempt: ultra-fast ring

reconstruction in RICH

detector.

Several algorithms tested →

best result 50 ns/per ring .

Long latency in data transfer

from PC to Video Card →

avoided transfering a packet of

1000 events.

Total processing time well

below 1 ms (per 1000 events) in

a single PC!

Problems with jitter due to CPU

should be avoided using real-

time OS (to be done)

Pilot project, very promising

R&D.

“Fast online triggering in high-energy physics

experiments using GPUs”

Nucl.Instrum.Meth.A662:49-54,2012

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GPU trigger

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INTEL PRO/1000

QUAD GBE

PCI-E

x16

4 GB/s

(20 MHz)*

8 GB/s

(40 MHz)*

PCI-E gen2

x16

100GB/s

(500 MHz)*

TESLA

GPU

V

R

A

M

R

A

M

CPU

CPU

30 GB/s

(150 MHz)*

FE Digitization + buffer +

(trigger primitives) PCs+GPU

PCs+GPU L0

L1

“quasi-triggerless” with GPUs