Photon-Photon Physics with Heavy Ions at CMS

2
In addition, if one is producing strong interacting nal states, one has to exclude also the region inside both nuclei. The nal state interaction of these states with the ions makes it impossible to detect them as (pure) two-photon events. Now the integration in Eq. (1) over b1 and b2 start from R. N (!; b) denotes the impact parameter dependent equivalent photon spectrum (see e.g. 5, 6, 7, 8]). For bi larger than R, the point nucleus approximation is very good whereas for b less than the nuclear radius the nuclear charge form factor has to be introduced. 4 Z dY Z 1 b db Z 1 b db Z 2 d N ( M eY ; b )N ( M e?Y ; b ) dL =dM = M 1 2 1 1 2 2 2 2 R R 0 (b2 + b2 ? 2b1b2 cos( ) ? (2R)2): 1 2 (1)
1 Overview: Using heavy ion collisions for photon-photon physics
Z
b> 2R
Z
Figure 1: In peripheral heavy ion collisions very strong elds interact for a very short time. They give rise to photon-photon collisions with unprecedented ux and energy.
The -luminosity in ion-ion collisions has in the last years been studied by a number of authors and is by now quite well understood. The luminosities can be calculated in the (semiclassical) impact parameter dependent \double equivalent photon approximation" (DEPA) 5, 6]. As already mentioned in the introduction, central collisions with their strong interaction, have to be excluded. In the black-disc approximation this is done by restricting the impact parameter in the calculation to b > 2R, where R is the radius of each ion (see also Fig. 1, an extension to the case of asymmetric collisions can be done easily.) 1) .
The main reason for using heavy ion beams in the LHC are the central collisions and the study of the quark gluon plasma. Peripheral collisions, that is, collisions where the ions do not interact strongly with each other, are of interest too. In recent years the electromagnetic interactions of these ions have been studied theoretically. Interesting applications and processes have been found in a wide range including particle, nuclear, and atomic physics. Especially the use of these beams for the photon-photon physics seems to be very promising. The high charge of the ions and the large collision energies lead to -luminosities and invariant masses not accessible with existing colliders. This allows to search for new particles or for detailed studies of , e.g., meson spectroscopy. Up to now hadronic accelerators have not been used for photon-photon physics, as the hadronic interactions dominate. An exception worth noticing is the observation of + ? pairs at the ISR 3]. In this special class of events, no hadrons are seen associated with the muon pair in a large solid angle vertex detector. It was also emphasized there that this process becomes increasingly interesting at higher energy hadron colliders. In the following we are going to describe some of the features of these processes. First we study the luminosities that can be achieved in these collisions. We discuss the need to have a trigger for peripheral collisions in order to trigger on -events and how such a trigger might be realized. Finally we show some interesting processes that can be studied in these collisions. Several proposals have been made in the mean time for peripheral collision programs at RHIC (STAR) and LHC (FELIX). Readers interested in more detailed studies are referred to 1, 2]. Our main aim is to give an overview of the speci c features of these collisions and how they could be used in connection with the CMS detector, for a topical review see 4].
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CMS NOTE 1998/009
The Compact Muon Solenoid Experiment
Mailing address: CMS CERN, CH-1211 GENEVA 23, Switzerland
CMS Note
January 26, 1998
Photon-Photon Physics with Heavy Ions at CMS
G. Baur
Forschungszentrum Julich, Julich, Germany
K. Hencken, D. Trautmann
Universitat Basel, Basel, Switzerland
S. Sadovsky and , Russia
Abstract Relativistic heavy ion beams are an intense source of (quasireal) photons with a spectrum extending up to several hundred GeV, due to the high charge Z and the large energy of the ions. In this note we discuss the possibility of using this in peripheral collisions for photonphoton physics. We discuss the total luminosities that can be achieved with di erent ion species. In order to suppress central collisions an e cient trigger for peripheral collisions is needed, that is, for the two ions to remain intact during the collision. We discuss the physical potential of these collisions especially for the search for new physics up to invariant masses of about 100 GeV and the hadron spectroscopy. Some aspect of this were discussed in detail in connection with the Letter of Intent for FELIX (CERN/LHCC 97-45, LHCC/I10) and we refer the reader to it for more details.