ACFA JointLinear Collider Physics and Detector Working Group


This page introduces you to physics working group activities of ACFA Joint Linear Collider studies

Statement of the Mission

Clearly our goal is to build a linear electron-poistiron collider and to open up a new era of high energy physics through the experiments there-at. As a necessary step toward this goal, the ACFA requested us to prepare a written report on the physics and detectors at the collider by the beginning of the year 2000.

The document should identify important physics targets and, by doing so, clarify required machine parameters such as beam energy and its spread, beamstrahlung, luminosity, background, etc. and detector parameters such as momentum resolution for tracking, energy resolutiion for calorimetry, impact parameter resolution for vertexing, minimum veto angle, and so on.

Now, what are the main targets? The best map of the world for the linear collider to explore is called the standard model Lagrangian, consisting of three sectors: gauge, Higgs, and Yukawa sectors. It should be emphasized that we know almost nothing about the Higgs and Yukawa sector,reflecting the fact that there are two particles, Higgs and Top, of which we do not know very much yet. The situation was more or less the same back in 1992 with the exception that there was no top quark discovered at that time.

The most important parameters that determine the overall scale of the project are naturally the masses of these two particles, since they decide the required beam energy for their direct productions. Although we had, already at that time, a fairly good estimate of the top quark mass, its recent direct measurement at Tevatron is far much better and gives us confidence to set our initial target machine energy to cover 350 to 400 GeV in the center of mass frame.  As for the Higgs mass, we now have an indirect measurement, M_h < 262 GeV at the 95% confidence level, thanks to the direct top mass measurement at Tevatron and various precision electroweak measurements at LEP and SLC, in particular. This again points us to the energy range: 350 to 400 GeV. It is thus very important for us to elucidate all the conceivable physics in this initial energy range of the project.
The next key issue to constrain the machine is the luminosity requirement.
The  figure plots production cross sections for various reactions of interest as a function of the center of mass energy. The inspection of the figure tells us that a luminosity of 5 x 1032 cm-2sec-1 is enough for most discovery physics, and at least 5 x 1033 cm-2sec-1 is needed for precision studies. This statement is still valid, but we definitely want more if we are to define our machine as a Higgs/Top/W/Z factory. How much more should be answered in the report.

As a working assumption, any physics simulatoins in the ACFA studies may assume

Detector Concepts

The detector parameters should be decided so as to make maximal use of the potential of the collider. It is thus very important to understand new features expected for the future linear collider experiments.
Firstly, since jets become jettier and calorimetric resolutions get better with energy, we will be able to identify heavy partons such as W/Z bosons and t-quarks by reconstructing their hadronic decays with jet-invariant-mass method. Full reconstruction of final states in this way is possible only in the clean environment of an e+e- collider and will allow us to measure the four-momenta of final-state partons, where partons include light quarks, charm, bottom, and top quarks, charged leptons, neutrinos as missing four-momenta, photons, W, Z, and gluons. For the heavy partons which involve decays, we may even be able to measure their spin polarizations. This is perhaps the most important new feature which makes unique the future linear collider experiments:  the Feynman diagrams behind a reaction become almost directly observable. The detector must take advantage of this and, therefore, has to be equipped with high resolution tracking and calorimetry for the jet-invariant-mass method, high resolution vertexing for heavy-flavor tagging, and hermeticity for indirect neutrino detection as missing momenta.

Another remarkable advantage of the future linear collider experiments is the availability of a highly polarized electron beam. For instance, let us consider the reaction: e+e- to  W+W-. In the symmetry limit where the gauge boson masses are negligible, we can treat this process in terms of weak eigen states. This process then involves only two diagrams: an s-channel diagram with a triple gauge boson coupling and a t-channel diagram with a neutrino exchange. Notice that only W0 (the neutral member of SU(2)L gauge bosons) can appear in the s-channel since B, belonging to U(1)Y, has no self-coupling. Because of this the cross section for this process will be highly suppressed at high energies, when the electron beam is polarized right-handed. We can say that Feynman diagrams are not only observable, but also selectable.

So far, so good. There are, however, some drawbacks inherent in linear collider experiments. Since the collider operates in a single pass mode, one has to squeeze the transverse size of its beam bunches to a nano-meter level to achieve the luminosity we required above. Such high density beam bunches produces an electro-magnetic field which is strong enough to significantly bend the particles in the opposing bunches and thereby generating breamstrahlung photons. This phenomenon is called beamstrahlung. Because of this, electrons or positrons in the beam bunches lose part of their energies before collision, resulting in a long tail towards the lower energy region in the effective center of mass energy distribution. Figure shows a typical differential luminosity distribution, demonstrating this effect. Notice that there is a sharp peak (delta-function part) at the nominal center of mass energy, corresponding to collisions without beamstrahlung.

The existence of this sharp peak implies that for most physics programs we can benefit from the well-defined initial state energy, a good tradition of e+e- colliders. In some particular cases, however, the finite width of the delta-function part, which is determined by the natural beam energy spread in the main linacs, and the beamstrahlung tail can be potential problem. Top pair procution at threshold is a typical example.

There is yet another potentially serious problem inherent in linear collider experiments which is new kinds of background induced by beam-beam interactions. The beam-beam background includes low energy e+e- pairs and so called mini-jets. In the ACFA studies, we need to  carefully design background mask system and detectors with good time resolution.

Any detector design should take these new features into account. We can summarize the performance goals as:

The first of these requirements sets a performance goal for a vertex detector, while the second imposes the most stringent constraint on the tracking system. It should be emphasized that the third point not only requires a good calorimeter, but also a good track-cluster matching capability to enable good energy flow measurements.

Figure is an example of possible detector systems designed to satisfy the above requirements, where both the central tracking chamber (CDC) and the calorimeter (CAL) are put in a solenoidal magnetic field of 2 Tesla to achieve good resolution and hermeticity. Notice that final focus quadrupole magnets and a background mask system are supported by a support cylinder which is installed in the detector and they should, thus, be considered as part of the detector system. Although there is no immediate need to change the design principle of the detector system, this design is almost 8 years old now and parameters of each detector component should be reexamined carefully, taking into account achievements in the past detector R&D's. 3 Tesla  option is definitely one of the most important study items for the ACFA stduies.

In what follows, we will examine some reference reactions to benchmark any detector design to be proposed in our ACFA studies, trying to elucidate typical measurements to be considered in the design process.

Reference Reactions and Score Sheet

The following is a list of representative reference reactions, chosen to illuminate typical relations among individual physics topics, new experimental features we discussed above, and performance requirements to individual detector components.
In contrast to what is listed in "Reference Reactions for the World-wide LC Study", the list here is made by trying to choose the most demanding ones in terms of detector performance. Consult the above link for more physics-oriented choice of reference reactions.
The table comprises a score sheet summarizing the representative reference reactions and their relation to various experimental features to be taken into design considerations. The future ACFA studies should not only expand this score sheet in both horizontal and vertical directions, but also fill individual boxes with numbers instead of circles with beam-related backgrounds realistically taken into account. Each detector subgroup should then identify most stringent requirements on their detector component and design it accordingly.


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Last update: December 14, 1999