1. Other beam-induced background at the IP
B. Dalena (CEA-IRFU/SACM)
Contributions from:
D. Schulte and J. Esberg

2. Outline Beam-Beam backgrounds at CLIC 3 TeV CM energy
Expected Rates
Machine imperfections
Machine induced backgrounds
Synchrotron radiation from the final doublet
28 September 2011
B. Dalena, LCWS11

3. Beam-Beam interaction
Particles rates produced during beam-beam interaction are expected to be high at CLIC 3 TeV CM energy They impact the interaction region design
Spent beams particles
Beamstrahlung photons
Coherent pairs
Incoherent pairs
Bhabhas and radiative bhabhas
Hadrons from  collisions
Rates, energy and angles distributions computed with GUINEA-PIG
(+ PYTHIA for hadronization)
28 September 2011
B. Dalena, LCWS11

4. Spent beams and beamstrahlung photons
e+ e- strongly focused in the electromagnetic field of the opposite bunch emit photons
Average energy loss due to the radiation
ΔE/EBS ~29%
Average emitted photons per beam particle
n ~ 2.1 per beam particle
 Long tail at low energy of the spent beam
28 September 2011
B. Dalena, LCWS11

5. <  > ~ 1 quantum regime
Coherent processes
Conversion of a beamstrahlung photon into e+e- pairs in the strong electromagnetic field of the other bunch charge distribution
rate of e+e- pairs creation is small for Ecm < 1 TeV
at CLIC nominal center of mass energy it is important
<  > ~ 1 quantum regime
conversion of virtual photon (trident cascade) are also possible at CLIC energies
28 September 2011
B. Dalena, LCWS11

6. Incoherent pairs & hadrons
Incoherent e+e- pairs production from real-real, virtual-real and virtual-virtual photons scattering
Incoherent μ+μ- pairs production, through the same processes, is also possible but much smaller rate
e+ e-    hadrons
(hadronization from Pythia)
Cross sections increase with energy
Visible in the detector
28 September 2011
B. Dalena, LCWS11

7. Radiative Bhabhas e+ e-  e+ e- 
at lowest order and for leptons scattered at small angles the cross section can be factorized into two parts:
e+ e+  equivalent photon spectrum
e-   e- Compton scattering
Energy of the e- and photons up to quite all the beam energy
Confined at relative small angles
28 September 2011
B. Dalena, LCWS11

8. CLIC crossing angle c = 10 mrad
A cone of half aperture of 10 mrad contains most of the particles produced after collision
N.B.  is defined with respect to the beam axis in the collision reference system
c = 10 mrad
28 September 2011
B. Dalena, LCWS11

9. Luminosity spectrum 1% single bunch energy spread due to RF structures
beamstrahlung smears the center of mass energy spectrum
coherent processes contribute to the low energy tail of the spectrum ~4% total (1% from e- e- or e+ e+ collisions)
28 September 2011
B. Dalena, LCWS11

10. Beams parameters and Luminosity Beam-Beam products per BX [*]
Backgrounds rates
Beams parameters and Luminosity
Total Luminosity
[1034 cm-2 s-1]
5.9
Peak Luminosity
2.0 fr
[Hz] 50 Nb
312 t
[ns]
0.5 N
0.372e+10 z
[m] 44
x /y
[nm]/[nm]
660/20
*x /*y
45/1
Beam-Beam products per BX [*]
ΔE/EBS
29 % n
2.1 (x N)
Ncoherent
66e+7
Ntrident
67e+5
Nincoh_pairs
330e+3
Nincoh_muons
12.50
NHadrons
3.2
Nrad_bhabhas
110e+3
[*] GUINEA-PIG
The emittance values include the budgets for imperfections
The actual values depend on the single machine and can change during operation
28 September 2011
B. Dalena, LCWS11

11. Beam-Beam Jitter
the relative change in luminosity correspond roughly to the same change in the beam-beam background rates
(except for the coherent processes with vertical offsets)
B. D. and D. Schulte WEPE025, IPAC’10
28 September 2011
B. Dalena, LCWS11

12. Beam emittances
Beam-Beam products increase/decrease together with luminosity if the emittances of the two beams decrease/increase
(except for the coherent processes with vertical emittance change)
B. D. and D. Schulte WEPE025, IPAC’10
28 September 2011
B. Dalena, LCWS11

13. Machine imperfections and corrections
Due to the budget for imperfections forseen luminosity and background rate can be higher than nominal (up to 50% and 40% respectively)
example:
corrected machines (after BBA)
ground motion following ATL low (A = 0.5×10−6 (μm)2/(ms)) applied to main LINAC only[3]
luminosity and background values show a linear correlation
[3] V. Shiltsev IWAA 1995, vol. 4, p 352
B. D. and D. Schulte WEPE025, IPAC’10
28 September 2011
B. Dalena, LCWS11

14. Machine-induced background: photon fans
Spectrum of emitted photons peaks at few MeV
Maximum photon cone for an envelope covering 15x and 55y
arXiv: [physics.acc-ph]
28 September 2011
B. Dalena, LCWS11

15. Conclusion
At CLIC 3 TeV cm energy background rates from beam-beam interaction can be high
Their knowlegde is important for the interaction region design (quite important energy is deposited in the detector and extraction region)
They affect the luminosity (smearing the CM distribution and adding 4% to the total luminosity)
Nominal values account for imperfections  luminosity and background may increase ( up to 50% and 40% respectively)
Photon fans from the final doublet are contained in the extraction cone aperture
28 September 2011
B. Dalena, LCWS11

16. Beam charge Perfect machines but low intensity beam 28 September 2011
B. Dalena, LCWS11

17. Luminosity during operation
BBA FB FB FB
Emittance budget: 5 nm rad
(static imperfections)
Emittance budget: 5 nm rad
(dynamic imperfections)
28 September 2011
B. Dalena, LCWS11

18. Machine imperfections (static)
Inputs:
main LINAC “standard” magnets and cavities misalignments[1]
vertical plane only
Beam Based Alignment applied[2]
Beam parameters:
z = 44 m
N = 3.72 e+09
x = 660 nm rad
y = 10 nm rad
Results
~30% more luminosity on average with respect to nominal luminosity
~25% more background on average with respect to nominal background
fluctuation ~5% (rms/<value>) in the background values from one bunch to the other
[1] BD and D. Schulte WEPE025 IPAC’10
[2] D. Schulte TH6PFP045 PAC’09
28 September 2011
B. Dalena, LCWS11