Beam Sizes measured by BaBar

Beam Sizes measured by BaBar

J. Thompson

SLAC

PEP-II MAC Review, 19 January 2006

http://www.slac.stanford.edu/BFROOT/www/Images/Babar54Lg.gif

J. Thompson, 19 Jan 06 2

Beam Sizes

HER: 9 GeV e- LER: 3.1 GeV e+

Bunch length z

(~RF) Transverse size x

2 = x x(z)

Angular spread x’

2 = x / x(z)

• Similarly for y

• HER and LER are independent

10 parameters so far need multiple measurements


In collision• BaBar sees the beams where they are colliding (use 2 prong events)

• Luminous region (“beamspot”) size observables:

• dL/dz – longitudinal luminosity distribution, online and offline

• measure effective *y

• measure z; get z,HER, z,LER when RF voltage changes

• Lx(z) – beamspot x size: good measurement online, great

measurement offline

• Ly(z) – beamspot y size: difficult measurement offline

• due to hourglass effect, measure effective *y

• beamspot xyx’y’ – coupling

• Lx’

• Ly’(z) – measure vertical emittances and effective *y

• Crossing angles and tilts


Ongoing Analyses

• B. Viaud, C. O’Grady: online monitoring of beamspot location and sizes; dL/dz fits

• J. Thompson, A. Roodman: measurement of transverse beamspot sizes

• M. Weaver, A. Cunha: angular distribution measurements


dL/dz

Run4 Run Run4 Run NumberNumber

ββy*y*

[cm[cm]]

Longitudinal luminosity distribution (assuming *y,HER = *y,LER):

2

00

2/2

1exp

zzyx

zzL

dz

dL

i2 = i,HER

2 + i,LER2 Hourglass effect gives a z dependence

to y:Fit the distribution to *y and z

Questions:

• *y ~ 1.3 -1.5 cm is higher than expected

• Clear change at ~run 45000 (February ’04)

This fit is now done online, with results updated once per BaBar run


dL/dz

Run4 Run Run4 Run NumberNumber

ββy*y*

[cm[cm]]

Longitudinal luminosity distribution (assuming b*yHER = b*yLER):

2

00

2/2

1exp

zzyx

zzL

dz

dL

i2 = i,HER

2 + i,LER2 Hourglass effect gives a z dependence

to y:Fit the distribution to b*y and Sz

Questions:

• *y = 1.3 -1.5 cm is higher than expected

• Clear change at ~run 45000 (February ’04)

This fit is now done online, with results updated once per BaBar run

Confirmed by independent fits I have done; true for both Run 4 and Run 5.

Results shown here are the best validated to date.


New Online Measurements: β*y & Σz

RFV(LER) = 4.5 -> 4.05 MV


New Online Plot: Phase Transient

1 Run !

Bucket number calculated with the “precise” t0 : σ=2 nsNot thoroughly tested though…

Abort gap


Transverse beamspot sizes

• Lx (~60m) is larger than resolution

(~30m)– can be extracted with good accuracy online

• Ly (~5m) is much harder

– Requires offline analysis:

• Offline analysis aims to measure Lx and

Ly with a method that removes resolution

from the measurement– Measure as a function of z


Lumi loss along minitrainsLate Run 4 Early Run 5

15% loss

5% loss

~Lu

min

osity

~Lu

min

osity

L x (m

m)

L x (m

m)


Lx(z), L

y(z)

2

*0

0, 1)(

y

Lzy

Ly

zzz

Lx in bins of z:

<Lx> = 65.5 ± 0.2 m

d(Lx)/dz = (7.2 ± 2.3)

x 10-5

Ly in bins of z:

Ly(z=0) = 5.2 ± 0.4 m

*y = 1.5 ± 0.4 cm

Sample Results:

2 days starting July 2

Global xy = -10 ± 2 mrad


Run 5: x Results

<L

x> (

cm)

• 2-day bins of time (some failed fits removed from plots)

• Non-zero dLx/dz

first noticed in late Run 4

• Origin is an open question

Date in Run 5

Date in Run 5


Run 5: y Results

Date

*y

(cm

)L

y,z=

0 (

cm)

• 2-day bins of time

• From fits to y(z)

No trend obvious;

Average for all points: *y = 1.40 ± 0.06 cm

Date


Bins of Specific Luminosity

Specific luminosity Specific luminosity

Specific luminosity Specific luminosity

Lx (cm)

*y

(cm

)L y,

z=0

(cm

)Lx(z) slope

Integrating over Run 5 through August


fH = EHER/(EHER-ELER) ~ 1.5

fL = ELER/(EHER-ELER) ~ 0.5

HER dominates measurement

z-dependence of angular spread


Sample angular spread fits


Boost method results

• Central values:

• *y = 14.6 mm

• y,HER = 3.2 nm rad

• y,LER ~ 9 nm rad

• sLy(z=0) ~ 5-6 m

y,LER bias is attributed to ISR (from simulation)

y,HER

y,HER

y,HER

y,LER

y,L

ER

z w (

mm

)z w

(m

m)

z w (

mm

)

*y (mm)

*y

(mm

)*

y (m

m)

4

14

2.6 3.8

14 15

Angular spread: Data Results


-22.4

-20.6

xz a

ngle

(m

rad)

-1

0

xz c

ross

ing

angl

e (m

rad)

HER IP x angle change, Aug 23 2005

Individual x beam sizes!

x beam size determination

This study done with quantities available online (BBR:BBRPEP:SVTDXDZ, BBR:BBRPEP:SVTXP)


Publications

• PAC05 papers:– BV, CO’G, WK, IN, AP:

http://accelconf.web.cern.ch/AccelConf/p05/PAPERS/TPAT076.PDF

– JT, AR, WK: http://accelconf.web.cern.ch/AccelConf/p05/PAPERS/MPPE057.PDF

• Plans for:– EPAC papers this year

• BV, CO’G, et al• MW et al

– NIM paper bringing all the results together


Plans• dL/dz:

• More bunch length measurements (December 9)

• Add hadronic events to increase statistics

• Transverse beam size fit:

• Why is dLx/dz non-zero? Does xy have a z dependence?

• Understand/reduce systematic error on Ly

• Boost analysis

• Work on bias due to ISR

• Measurement of x’y’

• Overall

• Compare to simulation

• How to best use the 3 methods together?

• Interpret results: why is the *y we observe so large? (Dispersion ruled out)


Extra Slides

1. Measurement methods:

1. Transverse beam sizes

2. Angular spread

2. Bunch length measurements

3. Additional transverse beam size results


y(z) method

1

2

An e+e- +- event as reconstructed

True production point

• Each muon track is reconstructed independently, with a point-of-closest-approach (poca) to the beam axis

• d measures production point

• m measures the reconstruction resolution

• Also use the estimated error on d, (from the track fit)

• 2 types of fit:

• To m distribution, using (determine resolution correction)

• To d distribution, using and corrected resolution:

• using , extract Lx, L

y

• Also measure z for each event


Angular studies: Boost method



Bunch Length Measurement

Fit determines:

z,HER = 11.1 ± 1.1 mm

z,LER = 11.1 ± 1.1 mm

Combine dL/dz fit with known scaling of bunch length with RF voltage


Bins of Beam Current: HER

mA mA

mA mA

Lx (cm)

*y

(cm

)L y,

z=0

(cm

)L

x(z) slope



Bins of Beam Current: LER

mA mA

mA mA

Lx (cm)

*y

(cm

)L y,

z=0

(cm

)L

x(z) slope


Documents

Beam Sizes measured by BaBar