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Impact of δ-rays on the performance of Kaon Aerogel Detector

Marco Carmignotto, Tanja Horn, Indra Sapkota

Hamlet Mkrtchyan, Simon Zhamkochyan, Vardan Tadevosyan, Arthur Mkrtchyan,

Mariana Khachatryan

Hall C Collaboration Meeting, 21 February 2014

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Outline- The Kaon Aerogel Detector

– Objectives and (brief) status update

- Delta rays and positioning of HMS and SHMS aerogel detectors

– Are there important effects of δ-rays on the

position of the Aerogel Detector in SHMS?

- Impact of delta rays on PID

– Effects in Cerenkov Threshold detectors

– Proton misidentification in aerogel

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Kaon Aerogel Cerenkov Detector

Threshold Cerenkov detector to be used in SHMS

SHMS base detector system provides particle identification for e, π, p over the full momentum range

• Noble gas Cerenkov: e/π• Heavy gas Cerenkov: π/K• Lead glass: e/π

Kaon x ProtonAEROGEL CERENKOV DETECTOR

But no K/p!

[JLab]

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Experiments (potentially) benefiting of the Aerogel detector

Approved 12 GeV experiments

E12-06-104 Measurement of the Ratio R=sigmaL/sigmaT in Semi-Inclusive Deep-Inelastic Scattering

E12-06-107 The Search for Color Transparency at 12 GeV

E12-09-011 Studies of the L-T Separated Kaon Electroproduction Cross Section from 5-11 GeV

E12-06-101 Measurement of the Charged Pion Form Factor to High Q2

E12-07-105 Scaling Study of the L-T Separated Pion Electroproduction Cross Section at 11 GeV

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Aerogel detector characteristics

PMTs to collect Cerenkov light radiated in the aerogel

4 exchangeable aerogel trays, possibility of different momentum ranges:

Inner surfaces covered with diffusive reflector

Refractive Index

π threshold (GeV/c)

K threshold (GeV/c)

P threshold (GeV/c)

1.030 0.57 2.00 3.80

1.020 0.67 2.46 4.67

1.015 0.81 2.84 5.40

1.011 0.94 3.32 6.31

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Status of detector construction

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(More details on these tests in Simon Zhamkochyan talk)

Machined at CUABrought to Jlab for assembly and tests

Ongoing tests with cosmic raysAerogel tiles stacking

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Delta rays and order of detectorsin HMS and SHMS

HMS SHMS

Detectors before the Aerogel

- Drift chambers - Heavy gas Cherenkov- Drift chambers- Hodoscope- Gas Cherenkov

Gas Cherenkov relative position

- After the aerogel: can we “detect” δ-rays produced in aerogel?

- In front of aerogel

Is the impact of δ-rays intensified due to the order of the detectors in the SHMS?

HMSSHMS

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Delta-rays in the Aerogel Detector

Example of a simulated event with delta-ray(images from the same event)

Also known as:

- Secondary electrons- Knock-on electrons- Delta electrons

Incident pion (above Cerenkov threshold)

Cone of Cerenkov radiation from pion

δ-ray

Cerenkov radiation from δ-ray

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Delta-rays in the Aerogel Detector- Charged particles, like delta electrons, radiate Cerenkov photons above the velocity threshold v

t :

vt = c / n

- A particle with momentum below the threshold (e.g. 3.0 GeV/c proton) may create delta-electrons in a material, e.g., the aerogel detector of the SHMS.

- This particle, e.g., a proton, was not supposed to create signal in the aerogel detector, but the created delta-electrons may be above threshold in the detector, and thus give a signal

- A particle with momentum below the threshold (e.g. 3.0 GeV/c proton) may create delta-electrons in a material, e.g., the aerogel detector of the SHMS.

- This particle, e.g., a proton, was not supposed to create signal in the aerogel detector, but the created delta-electrons may be above threshold in the detector, and thus give a signal

Electron threshold: 2.1 MeV/c (n=1.030)

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Analytical estimative of δ-rays in the Aerogel Detector

- Number of delta-electrons derived from Bethe-Block dE/dx (PDG, section 30.2.5):

where

F(T) = T – kinetic energy of delta-rayK – 4.π.N

A.r

e2.m

e.c2

Z – atomic number of absorberA – atomic mass of absorberz – charge of incident particleE – kinetic energy of incident particleβ – v/c of incident particle

- spin 0 particles

- spin ½ particles

and

is the maximum T of the delta-electrons

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δ-rays Kinematic Limits for Aerogel

- Lower limit: higher than threshold of Cerenkov radiation (1.6 MeV)We chose Tmin = 3.5 MeV (for n = 1.030) assuming:

* Photon yield is important already* dE/dx for electrons in aerogel

- Upper limit: the maximum T (depends on incident particle type and momentum)

- Range of delta-rays for the Aerogel Detector (according to their kinetic energy T):

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Range and Angular distribution of δ-rays in Aerogel Detector (from Protons)

Range of delta-rays in aerogel

These delta-rays can go through the entire layer of aerogel radiating Cerenkov photons

Angular distribution of delta-rays

They are created in a forward cone within the acceptance of the SHMS

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Analytical estimate of total number of δ-rays in SHMS Aerogel Detector

Total number is obtained by integrating the distribution of delta-rays:

– Assuming 10cm of aerogel n=1.030

– Only those above Tmin

δ-rays only from Aerogel itself

For 3 GeV protons, about 2% of the events will create δ-rays above Tmin

2% 2 : 100 1 : 50

Rejection factor:

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Total number of δ-rays in SHMS/HMSIn the HMS:

δ-rays in the Aerogel Detector are mainly created in the aerogel itself

[Hamlet]

In the SHMS:Number of δ-rays in the Aerogel

Detector has significative influence of detectors that comes before it

Detectors before the Aerogel double the total number of delta-rays in the Aerogel

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Geant-4 Simulation of δ-rays from protons in SHMS Aerogel Detector

Geant4 Simulation: (Simon) Including:

– Effects of all detectors– Cherenkov radiation yield– PMT quantum-efficiency– Detector light collection efficiency (light absorption) – NO electronics effects

[Simon]

- Effects of δ-rays are translated to number of photo-electrons in the Aerogel

- We can see how it affects PID efficiency

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Geant4: δ-rays in SHMS Aerogel due to detectors before Aerogel detector

Present configuration

Significative changes in Aerogel PID performance

[Simon]

If the hodoscope, heavy gas and gas Cherenkov were not

before the aerogel

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HMS Aerogel: estimative of Gas Cerenkov cut impact on δ-rays

Aim: try to estimate the number of misidentified protons from 6 GeV elastic runs (only protons)

Example: elastic run in FPI-2

Aerogel Signal

Proton rejection

Proton rejection

[Marco]

– Good agreement with analytical estimate of proton

rejection

– Difficult to improve rejection with Gas-Cherenkov software cut: analyzer may be rejecting

δ-rays already

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HMS: estimative of dependence on aerogel refractive index

1.030(Fpi-2 experiment)

Aerogel density vs. refractive index:

1.015(Meson duality

experiment)

(n - 1) = ρ . d

Number of accidental events (pions?) in this run is high

[Hamlet]

[Hamlet]

As seen in HMS Aerogel detector, PID performance for lower refractive

index can be lower.

(further detailed studies are coming for lower refractive indexes)

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Summary– δ-rays effects in the misidentification of particles should be taken into account in all experiments in the precision frontier.

– Order of detectors in the SHMS is important (from analytical calculations and simulations):

* With the present configuration, the total number of δ-rays in Aerogel is doubled due to detectors placed before it (heavy gas, drift chambers, hodoscope, gas Cherenkov).

– Delta electron contribution can affect PID and seems to depend on refractive index

* We showed results for aerogel n = 1.030. Preliminary analysis of HMS Aerogel shows the rejection is worse for lower refractive indexes (further studies needed to have better quantitative estimate for SHMS).

THANK YOU!

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Backup slides

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Light yield for different trays

(More details on Aerogel performance on Simon Zhamkochyan talk)

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Model of delta-ray emission

- Fast hadrons loses energy in material by three main processes:

* Bremsstrahlung* Excitation of atoms/molecules* Ionization

- Bethe-Block formula describes energy loss. For p/M in range [0.1, 1000], it has accuracy of few percent!

- Number of delta-electrons proportional to dE/dx.

[PDG]