Update – 29 jan. 2013

1. Tmax and effective angle in B-field: comparison between data and expectations;

2. Study of “doublet-mode” performance;3. “Efficiency” in magnetic field

1. Tmax and effective angle in B-field: comparison between data and expectations.

Electrons “slow-down” in magnetic field.If we compute the drift velocity in a magnetic field |vd| using the expressions:

we get

If q =90o (as in the case of H2 data) we have

so the drift velocity decreases with the magnetic field.This affects the maximum drift time Tmaxand the effective measured angle x since now

we have to take into account 2 effects:--> the trajectory is longer--> the velocity is lower

New expressions:

where q is the inclination angle and qL the Lorentz angle tanqL≈ 0.8 |B|.

Next slides: comparison with data

Tmax: data (red and green points – T3) compared to dashed line: expectations without “slow-down” effect solid line: expectations including “slow-down” effect

|B| (T)

Tmax (ns)

N.B. T3 has: 10 mm gap HVdrift = 600 VT0

max ≈ 200 ns

|B| (T)

x (deg.)

x: data (red and green points – T3) compared to dashed line: expectations without “slow-down”effect solid line: expectations based on “slow-down” effect

2. Study of “doublet-mode” performance

Measure the doublet middle-point xD corresponding to the track intercept at the plane y = 0. It can be applied to TPC and centroid

2 advantages: B offsets self-corrected (if B variations negligible at the

O(cm) scale); t0 jitter also self-corrected.

y

x

Resolution: July data, no magnetic field. T1 – T3 sC(xhalf), sC(xcent), sC(xcomb)(T1+T2)/2 – (T3+T4)/2 sD(xhalf), sD(xcent), sD(xcomb)

For xhalf and xcomb s = “score”. For xcent only 1 gaussian s.

Naifly I expect sD ≈ sC/√2 . But:

sC(xhalf)

sD(xhalf)sC(xcent)

sD(xcent)

Angle (deg.) Angle (deg.)

sD(xcomb)sC(xcomb)

Ratio of doublet / single chamber resolutionsR = sD / sC

Centroid:R ≈ 1/√2

mTPCR > 1/√2

9

Offset = average values of xD(1) - xD(2):The offset should be reduced to the the effect of the particle bending

Offsets are reduced to tipical slopes of 200÷250 mm/T. If p=150 GeV/c and l = 20 cm lower slopes are expected (d(m) ≈ 10-3l(cm)2B(T) = 40mm/T)

Doublet-mode operation more stringent requirements on chamber efficiency. It is interesting to see the effect of B on efficiency.

I have done the following test:Select “golden” events with:

a good mTPC position on T1 a good “doublet” on T3-T4:

Then look at T2.Three efficiencies:

e1 = at least 1 hit (whatever the charge) e2 = a good mTPC with extended cluster

definition (strip>2) e3 = a good mTPC with severe cluster

definition (MI recipe)“rough” space connection (can be improved)

First application to July data then to June data vs. B

T1 T2 T3 T4

3. “Efficiency” in magnetic field

July data: angular scan in standard operation

e1

e2

e3

In June data the chambers were operated at lower gain.Comparison btw run 7455 (July – blue lines) run 7340 (June – red lines)(both with B=0 and q = 10°)

inclusive strip charge strip multiplicity

Inefficiencies are higher inJune data.

Efficiencies vs. |B|

+10° data +20° data

|B| (T) |B| (T)

e1 e1

e2

e2

Very low efficiencies. But it is difficult to extrapolate to higher gain dataRelative e1 reduction: -(3 ÷ 4)% for B = 0.2 T

Summary

Evidence of electron “slow-down” effect: good description of data

Doublet-mode operation is ok, but: resolution score for mTPC doesn’t scale as expected in B, offsets are larger than expected

Sizeable reduction of efficiency in B (but data have low gain)