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Searches for new physics at NA62
14th Lomonosov Conference on Elementary Particle Physics
On Behalf of NA62 Collaboration

Vito Palladino


NA62 @ CERN

NA62 Collaboration: Bern ITP, Birmingham, CERN, Dubna, Fairfax,
Ferrara, Florence, Frascati, IHEP, INR, Louvain, Mainz, Merced, Naples, Perugia, Pisa, Rome I, Rome II, San Luis PotosÌ, SLAC, Sofia, Triumf, Turin


The Standard Model describes many phenomena in a very accurate way. Any deviation from these predictions could be an alternative way (with respect to high energy experiments) to search for New Physics.

Overview

The NA62 experiment follows this approach and aims to search for New Physics using precise measurements of rare Kaon decays.
· 2007/2008 Rk

PhaseI

· 2008/2011 construction PhaseII · 2011 Start K->
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Rk Standard Model
RkSM ( K eve ) me2 2 ( K v ) m
2 mK me2 r (1 RKa 2 m m2 K d.co r r

)

Helicity Suppression (V-A couplings)

Radiative correction (few%) due to IB part of the radiative K->e process, by definition INCLUDED in Rk

-> No direct use of (K->ll) due to the hadronic uncertainties => Rk -> Excellent sub-permill theoretical prediction -> Strong helicity suppression of electronic channel enhance sensitivity to non-SM effects

RKSM = (2.477 ± 0.001) x 10
V. Cirigliano and I. Rossell, Phys. Lett.99 (2007) 231801
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-5
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Rk Beyond Standard Model
In MSSM and large tanú scenario, a charged Higgs mediate a SUSY LFV contribution to the branching ratio with emission of .
L RK FV

2

SM ( K eve ) LFV ( K ev ) SM ( K v )
SM RK

1

4 4 2 mK m 2 m m 13 t an H e
A.Masiero, P.Paradisi, R.Petronzio,

PRD76 (2006) 011701 and JHEP 0811(2008) 042

Sizeable effects are predicted for reasonable SUSY parameters.:
13

= 5·10-4, tan = 40, mH = 500 GeV => RKLVF RKSM (1 + 0.013)
-3

Analogous effects in Pion decays are suppressed of a factor (mp/mK)4 ~ 6·10
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Rk Experimental Status
-> The PDG08 value is based on 3 measurements in 70s RK = (2.45 ± 0.11) · 10
-5 (4.5% error)

-> Preliminary results by KLOE and NA48/2 RK = (2.457 ± 0.032) · 10
-5 (1.3% error)

-> Final result by KLOE (LaThuile09) RK = (2.493 ± 0.025 ± 0.019) · 10
(1.3% with ~13.8k K
e2

-5

candidates, 16% background)

World average
RK = (2.468 ± 0.025) · 10
-5 (1% error)

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Magnetic spectrometer -> 4 view / DCH -> high efficiency -> p/p = 1.0% + 0.044%·p [GeV/c]

Apparatus and Trigger Logic

Hodoscope

-> Fast trigger -> t = 150ps

Electromagnetic calorimeter
-> -> -> ->

~10 m3 liquid krypton (~27 X0) High granularity, quasi-homogeneous E/E = 3.2%/E + 9%/E + 0.42% [GeV] x,y ~1 mm (@ 20 GeV)

K

l
Minimum Bias Hardware Trigger: -> Ke2 condition: 1TRK Q1 ELKr -> K2 condition: 1TRK Q1 Software Trigger: -> PDCH < 90 GeV/c -> ELKr/PDCH > 0.6 (K

e2

only)
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Measurement Strategy
K
e2

and K2 candidates collected simultaneously: -> Many systematic effects reduced, -> Measurement independent to the Kaon flux.

MC simulations used to limited extent: -> Acceptance correction (only for geometry), -> Simulation of "catastrophic" bremsstrahlung by muons.

Analysis in 10 track momentum bins.
Signal events Particle ID efficiency

Trigger Efficiency
(>99.9%)

RK =

1 D

N(Ke2) - NB(Ke2) N(Km2) - NB(Km2)
(Main source of systematic errors)

f f

m· e·

A(K2) · (K2) A(Ke2) · (Ke2) f

1
LKR

K2 downscaling

Background Events

Geometrical acceptance

Global LKr readout eff (0.998)

GOAL: Collect 150k Ke2 events => measure RK with a better than 0.5% accuracy
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Signals Selection
Common reconstruction:
-> 1 Reconstructed Track, -> Geometrical acceptance cuts, -> Limit on LKr extra energy deposition, -> Track momentum 15 GeV/c < p < 65 GeV/c -> Decay vertex defined as closest approach of track & nominal Kaon axis.
2

Electron Mass Hypothesis

Kinematical separation => Excellent Ke2/K separation at p<25 GeV/c:
-> Missing mass M2 = (pK - pl)2 -> PK : Average measured with K
3

decays

Particle Identification => Muon suppression ~10-6 -> E/p = (LKr energy deposit/track momentum) 0.95 Vito Palladino 14th Lomonosov Conference 9


Backgrounds (Muons)
The main background is due to "catastrophic" muon Bremsstrahlung events (E/pmuon>0.95) . The expected probability is P(->e)~3x10-6 (and momentum dependent), that corresponds to:

P(->e)/RK~10%
This impose a direct P(->e) measure to validate theoretical models in a specific E region. MC validation Used for Background subtraction

Solution: a ~10Xo deep lead wall, in order to have a

pure muon sample (electron contamination <10-7), was installed for 50% of running time and on about 20% of HOD area. This wall allowed to measure P(->e) and a very good Data/MC agreement has been found.
Analysis region
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A new special run has been scheduled in 2008 to collect a muon sample twice than 2007.
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(6.28±0.17)%


Backgrounds Summary
BKG Source K K K K x5 x5 x25 K
2 2

B/(S+B) (%) 6.28±0.17 0.23±0.01 1.02±0.15 0.45±0.04 0.03 0.03

(->e) (SD+)

e2

Beam Halo
e3 2

Tot

8.03±0.23

Selection criteria has been optimized individually in each track momentum bin. (e.g. Zvertex and Mmis2)
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Ke2: 40% of data set
Ke2 candidates

LogScale

NA62 estimate total Ke2 events: ~120k K+ & ~15k. In NA62 proposal the goal was fixed at ~150k (CERN-SPSC-2006-033). The present statistic gives 51 089 Ke2 candidates events. B/(S+B)=(8.0±0.2)%.
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K2: 40% of data set
The K2 is downscaled by a factor of 150 The K2 main background is the Beam Halo. 15.56M are the K2 candidates with low background B/(S+B)=0.25%

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RK Preliminary Result (40% data set)
RK = (2.500 ± 0.012stat ± 0.011syst) x 10-5 = (2.500 ± 0.016) x 10-5
Source
Statistical K K
2

RKx10
0.012

-5

0.004 0.001 0.004 0.001 0.007 0.002 0.007 (SD+)

Beam Halo
e2

Electron ID IB simulation Acceptance Trigger Timing

Tot

0.016
Precision 0.64%
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The whole sample will decrease the statistical uncertainty down to ~0.3% and a total uncertainty of 0.4-0.5%


March 2009

World Comparison
June 2009

> (2.467 ± 0.024) x 10
(precision 0.97%)
Vito Palladino

(2.498 ± 0.014) x 10-5
(precision 0.56%)
NA48/2 preliminary results excluded from new average: they are supersided by NA62

-5

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PhaseII: K+->+
The theoretical prediction within the SM is (8.5±0.7)x10-11, thus a precise measurement will be sensitive to: Precise Measurement of CKM matrix element Vtd. Evidence of Physics Beyond the SM if deviation to expectations.

Experimental status (E787/E949 experiment)

(1.47

+1.30

-1.05

) â 10

-10

Minimal SUSY Prediction BR(K+)SM / BR(K+)SUSY (0.65, 1.03)
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Kaon decay in flight to avoid scattering and backgrounds induced by the stopping target (long decay region). High momentum to improve background rejection (unseparated hadron beam). Precise timing to associate the decay to the correct incoming parent particle (K+) in a ~800 MHz beam (beam tracker with t ~ 100 ps). High photon detection efficiency ( 10-4 50MeV < E < 1GeV )
LAV

Apparatus and Measurement Strategy

Signature
-> Incoming high momentum (75 GeV/c) K+ -> Outgoing low momentum (< 35 GeV/c) p+

Unseparated Beam: -> Momentum 75GeV -> Kaon beam percentage ~6% -> Expected Signal 55 evs/y -> # Kaons decays 4.8x1012 -> S/B 10

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Conclusions & future plans
-> Rk measurement is a SM stringent test because of his helicity suppression enhancement. -> NA62 runs in 2007/2008 is optimized for Rk measurement provided a Ke2 sample ~10 times larger than world integrated statistic up to now. -> Preliminary results allow a 0.7% accuracy and no discrepancy from SM is observed. -> With the whole NA62 2007/08 data sample, a precision better than 0.5% is expected. -> In parallel the works for NA62 PhaseII are ongoing, the development for many detectors is complete and the final construction has started.
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Spares

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Backgrounds
}
Cos E (GeV)

Ee (GeV)

Only energetic (E > 230MeV) elactrons are compatible to Ke2 kinematic ID

Ke2 experimental knowledge is in 70s: BR = (1.52 ± 0.23)x10-5 Theoretically espec is model dependent: BR = (1.52 ± 0.23)x10-5 B/(S+B) = (1.02 ± 0.15)%

x = Ee/Em

ax

Uncertainty to be improved by KLOE and NA62 Vito Palladino

For BG due to in flight muons decays, according to Michel distribution in NA62 only energetic forward electrons are selected as Ke2 candidate => Highly suppressed B/(S+B) = (0.23 ± 0.01)%
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