Photon colliders: current and future projects
Valery Telnov
Institute of Nuclear Physics, 630090 Novosibirsk, Russia,
and DESY, Germany
Abstract
Photon colliders ( , e) are based on backward Compton scattering of laser light off the high energy electronsof linear colliders. This option is considered now for all
linear colliders projects. In this paper we present possible
parameters of the Photon Collider at TESLA. The
luminosity in the current design is determined by “geometricluminosity” of electron beams. Having beams with smaller
emittances one could obtain much higher luminosity. Especially attractive is the Higgs factory (if the Higgs bosonwith the mass of 100-200 GeV really exists). Problems of
multi-TeV photon colliders and possible solutions are also
discussed.

1 INTRODUCTION
The unique feature of the e+ e Linear Colliders (LC) withthe energy from hundreds GeV to several TeV which are
under development now [1, 3, 2, 4] is the possibility toconstruct on its basis a Photon Linear Collider using theprocess of the Compton backscattering of laser light off thehigh energy electrons [5, 6, 7] (see also proceedings of theworkshop on photon colliders held at Berkeley [9], 1995
and at Hamburg, 2000 [10]). The maximum energy of the

r

lase

electron
bunch

α

..C

e

γ (e)

IP

γ(e)

a)

αc

e

48

= 1 06= 0 82

= 250

=

48

08

=
=

2

(

01

08 )

2 PHYSICS+
Physics in e e and , e collisions is quite similar be-b)

Figure 1: Scheme of

1 17
=45

90 95%be

where E0 is the electron beam energy and !0 the energy ofthe laser photon. For example, for E0
GeV, !0: eV, i.e. : m (Nd:glass laser), we obtainx: and !m: E0.
The high energy photon spectrum becomes more peaked
for increasing values of x. It turns out that the value x : is the optimum choice for photon colliders, because forx > : the produced high energy photons create QED
e+ e pairs in collision with the laser photons, and theluminosity [6, 7, 11] will be reduced. Hence, the maximum
c.m.s. energy incollisions is about 80% (and 90% in ecollisions) of that in e + e collisions.
collisions is charA typical luminosity distribution inacterized by a high energy peak and a low energy part, seesection 3. The peak has a width at half maximum of about
15%. The photons in the peak can have a high degree ofcircular polarization. This peak region is most useful forexperimentation. When comparing event rates inand
e+ e collisions we will use the value of theluminosity
in the peak region z > : z m where z W = E0 (Wbeing theinvariant mass) and z m!m =E0. The
luminosity in this region is proportional to the geometric luminosity L geom of the electron beams: L z > : zm 
: Lee;geom.1 The geometric luminosity of electron beams
in acollision region can be made larger than the e + eluminosity because beamstrahlung and beam repulsion areabsent for photon beams.
The energy spectrum of high energy photons becomesmost strongly peaked if the initial electrons are longitudinally polarized and the laser photons are circularly polarized. This gives almost a factor of 3–4 increase of theluminosity in the high energy peak. The average degreeof circular polarization of photons within the high-energy
peak amounts to. The sign of the polarization caneasily be changed by changing the signs of both electron
and laser photon initial polarizations. Linear polarizationof high energy photons can be obtained by using linearlypolarized laser light [8], though it is not so high as the circular polarization.

, e collider.

scattered photons is [6]



4
E0!0
E0 h !0 i!m =E ; x  2 4 ' 15:3
;
x+1 0
m c
TeV eVx(1)cause the same particles can be produced. However, reactions are different and can give complementary information. Some phenomena can best be studied at photon colliders due to better accuracy (larger cross-sections) or larger
accessible masses (a single resonance (inand e) or apair of light and heavy particles (in e). A short list of
1 for a thickness of the laser target being equal to one collision lengthprocesses for the physics program of the photon collider ispresented in Table 1 [14]. Detail consideration of physics
program at photon colliders can be found elsewhere [14].
Table 1: Gold-plated processes at photon colliders
Reaction

Remarks! h0 ! bb! h0 ! W W (W W  )! h0 ! ZZ(ZZ )! H; A ! bb
! f~f~; ~+i ~i ; H + H! S [t~~t]e ! e~ ~01! W +W
e ! W e
! W W + W W (ZZ )! tt
e ! tbe! hadronse ! e X and eX
g ! qq; cc! J= J=

Mh0 < 160 GeV
140 < Mh0 < 190 GeV
180 < Mh0 < 350 GeVMSSM heavy Higgs

supersymmetric particles
tt stoponium~~Me~ < 0:9  2E0

M~01

anom. W inter., extra dim.
anom. W couplings
strong W W scatteering
anom. t-quark interactions
anom. W tb couplingtotalcross section
struct. functions
gluon distr. in the photon
QCD Pomeron

Table 2: Parameters of the photon collider at TESLA.

2E0

200

500

800

L [m]/xtL [scat]
N=1010z [mm]frep  nb [kHz]x=y =10 6 [mrad]1.06/1.8 1.06/4.5 1.06/7.2

x=y [mm] at IP

1.5/0.3 1.5/0.3 1.5/0.3

x=y [nm]140/6.8 88/4.3

1.35

1

1

2

2

2

0.3

0.3

0.3

14.1

14.1

14.1

2.5/0.03 2.5/0.03 2.5/0.03

b [mm]

Lee(geom) [1034 cm 2 s 1 ]Lee(z > 0:65)
W ; max (GeV)
L (z > 0:8zm; )W e; max (GeV)
L e (z > 0:8zm; e )Le+ e ; 1034cm 2 s 1

2.6

2.1

69/3.4
2.7

4.8

12

19

0.03

0.07

0.095

0.43

1.1

1.7

122

390

670

156

440

732

0.36

0.94

1.3

1.3

3.4

5.8

One can see that for the same energy

L

3

THE PHOTON COLLIDER AT TESLAThe TESLA team has published the Technical Design Report just two days before our conference. The Photon Collider is included in the project, though all technical aspects,
especially the laser system, should be developed in moredetail in the next 2–3 years.
The resulting parameters of the photon collider at
TESLA for the energy of electron beams E0 200, 500
and 800 GeV are presented in table 2. For comparison thee+ e luminosity at TESLA is also included. It is assumed
that the electron beams have 85% longitudinal polarizationand that the laser photons have 100% circular polarization.
The thickness of the laser target is one scattering length for
E0
500 and 800 GeV and 1.35 scattering length forE0 200 GeV (the Compton cross section is larger), sothat k 2  0.4 and 0.55, respectively (k is the e ! conversion coefficient). The laser wave length is 1.06 m forall energies. The conversion point is situated at a distanceb
y from the interaction point.
and e collisions given in
The maximum energies intable 2 are somewhat lower than follow from eq.1 due tothe nonlinear effects in Compton scattering. This lead todecrease of the effective value of x by a factor of2 ,2where the parameter : ; : ; : for E0;,
GeV, respectively.

2 =

2 =
2 =
=

500 800= 0 15 0 3 0 4(1 + )
2 = 200(z > 0:8zm)  13 Le+ e :(2)collisions are typically by
Note than cross sections in
one order of magnitude higher than in e + e collisions.
The relation (2) is valid only for the beam parametersconsidered. A more universal relation is (for k 2
: )

=04

L

(z > 0:8zm)  0:09Lee(geom):(3)

collisions there are also e colSimultaneously withlisions with somewhat lower luminosity, so one can study
both types of collisions simultaneously. Residual electronelectron luminosity is very small due to the beam repulsion.
The normalized
luminosity spectra for E 0
and
GeV are shown in Fig. 2 [12]. The luminosity
spectrum is decomposed into two parts with the total helicity of the two photons 0 and 2. We see that in the highenergy part of the luminosity spectra the photons have ahigh degree of polarization. In addition to the high energy
peak, there is a factor – higher luminosity at low energy.
It is produced mainly by photons after multiple Compton
scattering and beamstrahlung photons. These events have alarge boost and can be easily distinguished from the central
high energy events. Fig. 2 shows the same spectrum withan additional cut on the longitudinal momentum of the produced system, which suppresses the low energy luminosity
to a low level.

2

800

58

= 200

Figure 2:
luminosity spectra at TESLA(500). Solid line
for total helicity of the two photons 0 and dotted line fortotal helicity 2. Lower curves with cut on longitudinal momentum.

2

Figure 3:
Normalized e luminosity spectra at
TESLA(500) when the photon collider is optimizedforcollisions. Lower curves with cut on longitudinal
momentum.

= 500

The normalized e luminosity spectra for E 0
GeV are shown in Fig. 3. Again, besides the high energy peak there is a several times higher e luminosity at
low invariant masses. Note, that the e luminosity in thehigh energy peak is not a simple geometric characteristic of
the Compton scattering process (as it is incollisions).
For the case considered it is suppressed by a factor of –, mainly due to the repulsion of the electron beams andbeamstrahlung. The suppression factor depends strongly
on the electron beam parameters.
For dedicated e experiments one can convert only one
electron beam, increase the distance between the conversion and the interaction points and obtain a much moremonochromatic e luminosity spectrum.
luminosity distributions, including all their poThelarization characteristics, can be measured using processes! l+ l , where l e or . e luminosity can be measured using the process e ! e.

2

3

=

4

LUMINOSITY LIMITATIONS DUE TO
BEAM COLLISIONS EFFECTS ATTESLA ENERGIES
Beam collision effects in e+ e and , e collisions aredifferent. In particular, incollisions there are no beamstrahlung or beam instabilities. Therefore, it was of interest
to study limitations of the luminosity at the TESLA photon
collider due to beam collision effects. The simulation [12]was done for the TESLA beams and the horizontal size ofthe electron beams was varied.
Fig. 4 shows the dependence of the(solid curves) andthe e (dashed curves) luminosities on the horizontal beam
size for several energies. The horizontal beam size wasvaried by changing the horizontal beam emittance keeping

Figure 4: Dependence ofand e luminosities in thehigh energy peak on the horizontal beam size for TESLA
at various energies.
the horizontal beta function at the IP constant and equal to

1:5 mm.

One can see that all curves for the
luminosity follow
their natural behavior: L / = x (values of x <nm
are not considered because too small horizontal sizes may
introduce problems with the crab–crossing scheme). Notethat while in e + e collisions  x 
nm, incollisions the attainable  x with the planned injector (dampingring) is aboutnm.
In e collisions the luminosity at small  x is lower thanfollows from the geometric scaling due to beamstrahlung
and displacement of the electron beam during the beam col-

1

10

500

100

P

5

PHOTON COLLIDER FACTORIES

We have seen in previous section that at TESLA theluminosity in the high energy peak is about 1/3 of the e + eluminosity. The later is restricted by collision effects, but
theluminosity at the considered energies may be further increased by one order of magnitude. Moreover, thethe cross sections of most of processes at photon colliders are considerably larger. For example, already at current
TESLA parameters the rate of the neutral Higgs boson h0
incollisions will be higher by a factor of 1–10 for the
Higgs mass Mh 120–250 GeV [12]. The rate of charged
scalars (charged Higgs bosons or supersymmetric particles
expected in some theories) is higher by a factor of 8 not
far from the threshold. The rate of WW pairs will be also
higher approximately by the same factor. The list of examples could be continued. If methods to decrease electron
beam emittances were found (the laser cooling or something else) thenfactories of some particles would bepossible.

=

6

LASERS-OPTICS

The photon collider at TESLA requires the laser with wavelength about 1 m, the flash energy 5 J, duration 1.5 ps
and the repetition rate 14 kHz. The average power for two
beams should be about 140 kW. All parameters are reasonable for exception of the repetition rate (average power). To
overcome this problem each laser bunch in the scheme considered for TESLA, see Fig.5, is used for the e! conversion many times (12 in the current design) [12]. The laser
pulse is sent to the interaction region where it is trappedin an optical storage ring. This can be done using Pockels cells (P), thin film polarizers (TFP) and 1/4-wavelengthplates (= ). The same laser can be used for the whole
range of the TESLA energies.

4

TFP∆ T(loop)= 337 ns

11
00

1
0
1
1
0
0
1
0
1
0
1
0
1
0
1

e

e
12

2

in

1
0

λ/4

10

out

lision.
So, we can conclude that forcollisions at TESLA
one can use beams with a horizontal beam size down to
nm (maybe even smaller) which is much smaller thanthat in e+ e collisions. Note, that the vertical beam size
could also be additionally decreased by a factor of two (foreven smaller electron beam size the effective photon beam
size will be determined by the Compton scattering contribution). As a result, having beams with very smaller emittances, theluminosity in the high energy peak can be,in principle, several times higher than the e + e luminosity.
Production of the polarized electron beams with emittances lower than those possible with damping rings is achallenging problem. There is one method, laser cooling[15, 16] which allows, in principle, the required emittances
to be reached. However this method requires a laser power
one order of magnitude higher than is needed for e !
conversion. This is not excluded, may be for the second
generation of photon colliders.

11
00
00
11
00
11

∆ T(loop)= 337 ns

2 ps

1
0
0
1

1
0
0
1
0
1

11
00

λ/4

Figure 5: Optical trap (storage ring)

7

MULTI-TEV PHOTON COLLIDERS

The energy region of TESLA, JLC, NLC colliders (c.m.s.
energy below about one TeV) is perfectly suited for photon
colliders because the required laser wave length lies in theregion of most powerful solid state lasers, the flash energy
is quite reasonable and collision effects do not restrict theluminosity.
At multi-TeV linear colliders, such as CLIC, with thec.m.s. energy of 5 TeV there are many additional problems:
1. the required laser wavelength is about 10 m;
2. to avoid nonlinear effects in Compton scattering (theybecome more important with increase of the wavelength) one has to use a laser with several time larger
flash energy;
3. the electron bunch train is too short for multiple use of
the laser bunches;
4. collisions effects (conversion of the high energy photon into the e + e pair in the field of the opposite electron beam) leads to restriction of theluminosity.
All these problems have been analyzed recently in
ref.[13]. The following solutions were suggested:
1. to avoid the problem of nonlinear effects in Compton scattering the “traveling laser focus” can be used,
see Fig.6. In this case electrons are collided with laser
photons at a long distance and the density of laser photons in the laser focus can be lower. The required flash
energy may be lower by one order of magnitude. Thiscan be done using the chirped laser pulses.
2. to reduce the beam field at the interaction point (wherehigh energy photons are collided) it is suggested to
split the electron beams as it is shown in Fig.7.
Electron beams are tilted around the collision axis bysome relative angle   O : y =x. Having initial displacement each of the electron beams will be split during
the collision in two parts. If the transverse deflection during the beam collision, >  x, then the maximum field in
the region of high energy photons (at x   x ) is

(0 1)



Br = Bb (x =)2 / x:

So, with the decrease of x, theluminosity grows but
the field at the interaction region decreases! This effectworks better when beams are longer. The idea is interesting
and worth more detailed consideration. It may not work
due to necessity of very good stabilization of the verticalbeam position, that is very difficult for the CLIC with shortbunch train.
Other method is to use the plasma lens between the conversion and interaction points. Electron beams after conversion will be defocused to a large spot size and the beam
field near the axis (where high energy photons travel will
be suppressed). Such idea was considered for e colliders
in ref.[17].
Note, that if the multi-TeV photon collider uses thebeams with same transverse sizes as for e+ e collider thenthe problem with coherent pair creation is not important,that is because for any beam field the probability of beamstrahlung is several times higher than the probability of coherent pair creation. For
collisions one can use electron
beams with the horizontal beam size at least 3 times lower
than for e+ e collisions. The corresponding
luminosity in the high energy peak will be about 20-30% of e + eluminosity, this is acceptable because the cross sections inthecollisions are typically by one order of magnitude
higher than in e + e collisions.

In conclusion, photon colliders is a very promising direction of the particles physics. This option is included in
the projects of the next linear colliders. At the energies ofluminosity may be very high,several hundreds GeV theespecially if some methods of electron beams production
with very small emittance were found, and factories of various particles can be considered in future. For multi-TeV
energies there are several problems, especially with lasers,
which can be overcome, in principle, but need special developments.

8

REFERENCES[1] Zeroth-Order Design Report for the Next Linear Collider
LBNL-PUB-5424, SLAC Report 474, May 1996.
[2] Technical Design Report, DESY 2001-011, ECFA-97-182.
[3] JLC Design Study, KEK-REP-97-1, April 1997.
[4] R.W. Assmann et al., CERN-2000-008, 2000.
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34 (1981) 514; JETP Lett. 34 (1982) 491. (Prep. INP 81-50,
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Nucl.Instr.&Meth. 219 (1984) 5.
[9] Proc.of Workshop on Gamma-Gamma Colliders, Berkeley
CA, USA, 1994, Nucl. Instr. &Meth. A 355(1995).

θ
d"

"re

e

e"

lu

"b

Figure 6: Traveling laser focus at the conversion region.
The laser focus follows position of the electron beams.

[10] Intern. Workshop on High Energy Photon Colliders, To be published in Nucl. Instrum. Meth. A,
http://www.desy.de/˜ gg2000.
[11] V. Telnov, Nucl. Instr. &Meth. A 355 (1995) 3.
[12] V.I. Telnov, Proc. Intern. Workshop on High-Energy Photon
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[13] V.I. Telnov, Proc. Intern. Workshop on High-Energy Photon
Colliders, Hamburg, Germany, 14-17 Jun 2000. Submitted toNucl. Instr. Meth. A, hep-ex/0012047.
[14] E. Boos et al., Proc. Intern. Workshop on High-Energy Photon Colliders, Hamburg, Germany, 14-17 Jun 2000. Submitted to Nucl. Instr. Meth. A, hep-ph/0103090.

∆[15] V. Telnov, SLAC-PUB-7337, Phys. Rev. Lett., 78 (1997)
4757, erratum ibid 80 (1998) 2747, e-print: hep-ex/9610008.

σx

Figure 7: Idea of suppression of coherent pair creation at
photon colliders.

[16] V. Telnov, Nucl. Instr. and Meth. A 450 (2000) 63, hepex/0001029.
[17] S. Rajagopalan, D.B. Cline, P. Chen. 1995. Nucl. Inst. Meth.
A357 (1995) 1.