Update on Large Angle Beamstrahlung detector for

SuperKEKBJ. Flanagan, K. Kanazawa, KEK

H. Farhat, R. Gillard, G. Bonvicini, Wayne State University

Goal: to build a 1% monitor of beam-beam interaction parameters

Admin. status

• 2 graduate students added

• Hosei Yosan $50,000 spent for first prototype hardware

• WSU in-kind contribution of $43,000 (graduate students salary)

• Nichibei $5,000 (not spent yet)

• NSF 2 proposals pending

What is beamstrahlung

• The radiation of the particles of one beam due to the bending force of the EM field of the other beam

• Many similarities with SR but

• Also some substantial differences due to very short “magnet” (L=z/2√2),very strong magnet (10T at KEKB). Short magnets produce a much broader angular distribution

• Discrimination against machine backgrounds done MOSTLY by angular collimation. At SuperKEKB, small leftover backgrounds to be further subtracted through spectral analysis

• Beamstrahlung POLARIZATION at specific azimuthal points provides unique information about the beam-beam geometry.

Some examples of Large Angle BMST pattern recognition

¼ CESR Set-up principal scheme

Transverse view Optic channel Mirrors PBS Chromatic

mirrors PMT

numeration

Set-up general view

• East side of CLEO

• Mirrors and optic port ~6m apart from I.P.

• Optic channel with wide band mirrors

On the top of set-up • Input optics

channel

• Radiation profile scanner

• Optics path extension volume

Main CESR results page

• Signal(x) strongly correlated to I+I-

2

• Signal strongly polarized according to ratios of vertical sigmas

• Total rates consistent with expectations at 10.3 mrad

DESIGN OF THE SuperKEKB DETECTOR

• Numerous changes compared to CESR device provide far better signal, signal stability, control of systematics, detector uniformity

• Current test bench aims at characterization of detector spectral response to 0.3%, and test bench measurement of angular acceptance

Most important change: much stronger beams at SuperKEKB. Comparison at =5mrad, =300-600nm,

0.5mrad2 acceptance)

Qty CESR-c S.KEKB Ratio

Sx(Hz) 6E4 3E11(L),1E11(H) (Prel.)

2-6E6

Sy(Hz) 6E4 6E10(L),2E10(H) (Prel.)

0.3-1E6

Bx(Hz)(N/) 2E6 (est.) 2E5(L),1E5(H) 0.05-0.1

By(Hz)(N/) 2E6 (est.) 2E6(L),1E6(H) 0.5-1

B(from beam) Very small Very small

Beam pipe insert

• View port location at ±90 degrees minimizes backgrounds, polarization measurement errors, and provides redundancy against beam orbit errors

• To be located anywhere between 5 and 10 mrad from the beam direction at the IP

• Suggested mirror and window sizes: 1.5X2mm2 and 1.7X1.7 mm2 (we could go lower at 10 mrad)

Beam transport and optics box

• Light is transported to optics boxes by means of simple (and replaceable) black-anodized pipes (2.5 cm ID) and mirrors

• Device consists of achromatic telescope with pinhole optics, pol. Splitter, and two gratings illuminating 4 PMT with filters (total system 32PMTs)

• Many adjustment screws throughout system

Current activities

Current activities (all measurements to 0.1% except absolute calibration of PMTs)

• Characterization of PMTs (nearly done)• Spectral characterization of all optical

components (mirrors, windows?, splitter, gratings, PMTs)

• Uniformity of all components• Build plywood optics box, check optics,

achromaticity and focus• Build and test optics box

Some results

Extra slides

CESR mirrors technical design

Check for alignment @ 4.2GeV

Directionality

• Scanning is routinely done to reconfirm the centroid of the luminous spot.

If the angle can be considered large and constant…

• Assuming (atan(z/)+atan((L-z)/ ) as the field profile, one gets (u=s,c=cos,sin())

€

d2P

dΩdω∝

(1− uc)2 + (su)2

(1+ u2)6

1

ω2exp(−ωδθ 2 /c)

Large angle beamstrahlung power

• Total energy for perfect collision by beam 1 is: P0=0.112re

3mc2N1N22/(x

2z)

• Wider angular distribution (compared to quadrupole SR) provides main background separation

• CESR regime: exponent is about 4.5

• ILC regime: exponent is very small

• KEKB: exponent is small

€

d2I

dΩdω=

3σ z4cπ π

P0

1

γ 4θ 4exp(

−ω2θ 4σ z2

16c2)

2nd major change: much better event record

• CESR record contained BMST data, bunch-by-bunch currents, luminosity monitors, independent measurements of vertical heights, energy, as well as other unused quantities. Beam length and beam horizontal size were computed by measuring size of luminous region using CLEO hadronic events.

• Need at least Beam Position Monitors near the IP to monitor beam shifts both in quads and in detector-beam axis angle

Properties of large angle radiation

• It corresponds to the near backward direction in electron rest frame (5 degrees at CESR, 2-4 degrees at KEKB/SuperKEKB, 7 degrees at DAPHNE)

• Lorentz transformation of EM field produces a 8-fold pattern, unpolarized as whole, but locally up to 100% polarized according to cos2(2), sin2(2) with respect to direction of bending force (Bassetti et al., 1983)

Beam-beam interaction (BBI) d.o.f. (gaussian approximation)