Belarus activity in COMET experiment

Dz. Shoukavy, IP NAS of Belarus

Gomel, Belarus, July 27 - August 7, 2015

Context Introduction

Muon to electron conversion

COMET experiment

COMET preparation status

Belarus activity

Introduction The discovery of a Higgs boson at the LHC in 2012 provided the missing

piece in the Standard Model (SM) to explain electroweak symmetry breaking. However there remain many shortcomings in the SM's description of nature, notably: the lack of a dark-matter candidate, no explanation for the observed matter antimatter asymmetry in the universe, no quantum theory of gravity or explanation for neutrino masses.

So perhaps Standart Model is only a part of a bigger picture that includes new physics hidden deep in the subatomic world or in the dark recesses of the universe.

New information from different experiments will help us to find more of these missing pieces.

All these phenomena highlight the need for physics beyond the SM (BSM) and many of these models predict charged lepton flavour violation (CLFV).

Muon to electron conversion One of the most important muon LFV

processes is coherent neutrinoless conversion of muons to electrons so called μ- – e- conversion.

When a negative muon is stopped by some material, it is trapped by an atom, and a muonic atom is formed. After it cascades down energy levels in the muonic atom, the muon is bound in its 1s ground state.

Further, in SM the following processes are possible: muon decay in orbit μ− → e− νμνe

or

muon capture by a nucleus of mass number A and atomic number Z , i.e. μ− + N (A, Z ) → νμ + N (A, Z − 1).

Muon to electron conversion

However, in the context of physics beyond the Standard Model, exotic process of neutrinoless muon capture is expectedThis process is called μ- – e- conversion in a muonic atom. This process violates the conservation of lepton flavor numbers, Le and Lμ, by one unit, but the total lepton number L =Le+Lμ+Lτ is conserved.

Lμ +1 0 0 0Le 0 0 +1 0

Muon to electron conversion

μ- + (Z,A)->e- + (Z,A)

Backgrounds Radiative π/μ captureDeacay in Flight Cosmic rays

•Decay in orbit (DIO)

Bμ - binding energy

Muon to electron conversion. Hstory

Past experiments on μ- - e- conversion

Muon to electron conversion The size of the observable flavour effect for a variety of BSM SUSYand non-SUSY models. large effect

visible, but small

W. Altmannshofer et.al, Nucl. Phys. B 830 (2010) 17

unobservable

• AC - Abelian U(1) flavour symmetry model by Agashe and Carone

• RVV2- non-Abelian model by Ross, Velasco-Sevilla and Vives

• AKM- Antush, King andMalinsky SU(3) flavour• FBMSSM - Flavour-blind MSSM

• LHT- Littlest Higgs with T-Parity

• δLL- flavour models• RS - Randll-Sundrum model with custodial protection.

Muon to electron conversion. Summary

μ- + (Z,A) -> e- + (Z,A)

Undiscovered process

Branching ratio (BR) at Standard Model (SM)~O(-54)

BR at beyond SM ~O(-15)

Recent upper limit : 7 x 10-13 (SINDRUM-II at PSI)

Observation of muon to electron conversion would indicate

a clear signal of physics beyond the SM

COMET experiment COherent Muon to Electron Transition - COMET.

Experimental goal search for μ−e conversion in the field of an

aluminium nucleus, μ−N → e− N , with a single event sensitivity of 2.6×10−17

COMET experiment Powerful proton source at J-PARC (Japan)

Comet experiment

High efficiency π-capture systemThe pion capture solenoid is of critical importance for capturing pions from the proton target with a large solid angle, and transporting muons from pion decays to the muon stopping target efficiently. In the pion capture solenoid section, a magnetic field of 5 T along the direction of solenoid axis is required at the location of the proton target to capture as many low energy pions as possible. The magnetic field is adiabatically reduced down to 3 T in the matching solenoid section.

R - radius of the inner bore of the solenoid magnet

Comet experimentProton target

The proton target is required to generate pions which will decay to muons as they are transported to the muon stopping target in the detector solenoid. The target will be installed within the bore of the capture solenoid with a configuration designed to capture low energy negative momentum pions generated proton beam from the J-PARC MR synchrotron.

COMET PHASE-I

Comet experment

The selection of the electric charge and momentum of beam particles can be performed by using curved solenoids. In curved solenoids, the center of the helical trajectory of a charged particle is shifted(D[m])

This drift can be compensated by an auxiliary field parallel to the drift direction given by

COMET experiment

The muon-stopping target is placed in the centre of the detector solenoid. The muon-stopping targetis designed to: maximise the muon-stopping efficiency and the acceptance for the μ− N → e− N conversion electrons to arrive at the spectrometer and to minimise the energy loss of the conversion electrons as they exit the muon-stopping target in order to minimise the momentum spread of the electrons.

Muon Stopping Target

COMET experiment. StrEcal

StrEcal high resolution electron detector

COMET erxperiment

StrEcal detector •placed in vacuum and magnetic field of 1T

Straw tracker • 5 station, diameter 5 mm, gas mixture 50% Ar and 50% - C2H6

• thickness 20/12 μm• Detection of particle track in magnetic field • Momentum resolution : σp< 200 keV/c for 105 Mev/c electon •Spatial resolution < 200 μm

Ecal LYSO crystals (Lu1.8Y.2SiO5:Ce)• Measure the electron energy with resolution < 5% for 105 Mev/c electon•Provide particle indentification (E/p) with tracker•Ecal will also provide trigger signals•Time resolution < 100 ns•Cluster position resolution < 1 cm

Comet experiment Ecal structure

Ecal will consist of crystal modules which have a 2 × 2 cm2 cross-section and whose length is 12 cm for LYSO crystals.

R =55 cm

Basic unit: 2x2 crystals -> 1 module

COMET experiment. Staged approach

COMET Pahse-I COMET Pahse-II

Goal of COMET PHASE-I

1. Background study for the full COMET (Phase-II)

Direct measurement of potential background for the full COMET experiment using same detector as Phase-II.

2. Search for μ-e conversion

a search for μ-e conversion at the intermediate sensitivity which would be 100-times better than the present limit (SINDRUM-II)

COMET PHASE-I

μ-e conversion search in the Phase I

6x109 muon/sec

COMET PHASE-I

Physics Sensitivity for COMET phase-I

8 GeV, 3,2 KW proton beam is assumed

2,5•1012 protons/sec

running time is 110 days (9,5 •106 sec)

Expected single event sensitivity

Upper limit at 90% C.L.

COMET PHASE-I

Measure almost all the particles

Same detector system for Phase-II

•Straw tube tracker

•Crystal calorimeter

•Particle ID with dE/dx from tracker E/p from ECAL

Background study

COMET PHASE-I Cylindrical detector (CyDet) for the μ-e search

CDC - cylindrical drift chamber

Summary of COMET phase-I/II

COMET preparation status

•Construction of the COMET building is completed!

•Underground structure, beam line construction is ongoing

COMET preparation status

•Main part of COMET transport solenoid is already installed in the COMET hall

COMET preparation status

Straw mass production is oingoing at JINR group

COMET preparation status

COMET collaboration179 participants from 32 institutes as of July 2015

http://comet.kek.jp

COMET collaboration

CM15 at KEK/J-PARC,January 2015

Belarus activity

Beam Test in March 2014 @ Tohoku

Belarus activity

Beam Test in March 2014 @ Tohoku

2x2x12 cm

LYSO (Lu1.8Y.2SiO5:Ce)

2x2x15 cm

GSO (Lu1.8Y.2SiO5:Ce)

Physics Runs Momentum Scans65 Mev/c, 85 Mev/c, 105 Mev/c, 125 Mev/c, 145 Mev/c for GSO and LYSO.The incident beam is perpendicular to the center of the crystal array.

Beam

Belarus activity

Belarus activity

A scheme of analysis

1. Waveform fitting• Get deposit charges.

2. Conversion to energy

• Calibration factors obtained by cosmic runs

3. Clustering

• Sum up energy deposits

• Make energy spectra.

1.Energy resolution GSO (Gaussian fitting)

Belarus activity

Further we need to find the charge to energy conversion factors for every crystal so-called calibration factors using cosmic data. Example cosmic event.

Ru

n 11

3 E

vent

244

Bun

ch 0

GS

O

Belarus activityCalibration factors

Belarus activityCalibration factors

We must have a good criterion to exclude these ring noise events. We have found such one based on chi-square parameter.

chi2 = Fitting Function ->GetChisquare();

ring n

oise

even

t

Distribution of the total deposit charges in crystals for LYSO (p=105MeV/c)

Distribution of the total deposit charges in crystals for GSO (p=105MeV/c)

Thus in the first approximation to determine the resolution we can use the box 5x5 in the center of the crystal 24

Belarus activity.Clustering

• High energy resolution for both crystals without any additional cut to purify data.

• LYSO has an energy resolution less than 5% for 105 MeV/c !

Calibration factors obtained by fitting with Landau function

The comparison of energy resolution for GSO and LYSO

Stochastic terms GSO/LYSO:

is roughly

 

Belarus activity

Belarus activity

Japan group result (Kou Oishi)

Belarus activity

What cluster is better to use for ECAL pretrigger?

Array of 1x1, 2x2 or 3x3 modules ?

What algorithm for choice of a cluster is better for ECAL pretrigger?

What is the average energy deposited in modules ?

What the threshold do we need to use for better results?

We must know answers.

We have tried to answer these questions.

Simulation of an algorithm for ECAL pretrigger. First step

Belarus activity

Red - electrons; Green - photons

-20 < θ < 200

Uniform magnetic field: Bx=By=0,Bz=1TUsing Gaussian distribution of noise σ_noise_crystal is 0,4; 0,6 MeV.

BINP Algorithm for pretrigger

Another an algorithm for pretrigger

Deposited energy in 3x3 modules Generate 1 mln, electron with p =105 MeV/c

-200 < θ < 200

-50 < θ < 50 -10 < θ < 100

97,3775 MeV

97,4956 MeV

97,4991MeV

The distribution of total deposited energy in modules for 3x3 array

Max energetic module

-50 < θ < 50 -10 < θ < 100

-20 < θ < 200

Conlusion. One module can't use as the cluster

Conlusion. One module can't use as the cluster

for ECAL pretrigger

for ECAL pretrigger

Energy leak

Here the energy leak means difference between the total deposited energy in the setup and the total deposited energy in 3x3 & 2x2 modules

Why does the another algorithm look so bad?

Answer:Because this algorithm does not always correctly reсonstruct crystal with the largest energy deposit for max energetic module.

Here you can see the difference between the true value of line for crystal with the largest energy deposit and the reconstructing value. The same picture for raw too. As a result we correctly reconstruct the crystal with max energy deposit for ~60% events only.

Threshold

What is the optimal value for threshold energy?

We add the threshold energy for module in our model i.e we take into acount events when total deposited energy in module larger then threshold.

if (Energy_2x2_right_module < threshold) { Energy_2x2_right_module = 0;} and so on.

In the case σ_noise_crystal = 0,4 MeV we have

Also we consider the case when σ_noise_crystal =0,6 MeV

σ_noise_ module = 1,2 MeV

We have plotted the dependence of energy resolution on threshold energy (for threshold energy = 0, 0,5σ_noise_ module; 1σ_noise_module; 2σ_noise_

module and 3σ_noise_module)

Right Module

Threshold. Energy resolution.

0,5σ 1σ 3σ2σ0σ

2σ1σ0,5σ0σ 3σ

The optimal value for threshold energy is 0,5σ_noise_ module

-20 < θ < 200

• Red color corresponds σ_noise_ module=0.8 MeV

• Blue color - σ_noise_ module=1.2 MeV

Summary

CLFV would give the best opportunity to search for BSM. (So far, no BSM signals at the LHC.)

Muon to electron conversion could be one of the important CLFV processes in terms of theoretical and experimental points of view

The COMET is a search experiment for μ--e- conversion at J-PARC with sensitivity of O(-17) which is four orders of magnitudes better than the present limit.

We’re ready for the COMET Experiment phase-I !!!

Thank you !!!