ILC: Thoughts, Issues and R&D

Abstract

There appears to be a bewildering variety of vertex detector technologies being developed for the ILC. The technologies aim to meet the stringent goals of low mass, precise resolution, and good time resolution. We describe some of the design goals required by the physics, detector constraints imposed by the machine, and how various candidate technologies address these problems. Physics RequirementsThe ILC is intended to perform precise measurements of new particles produced with the low cross sections characteristic of high energy e + e − collisions. Figure 1 shows cross sections and event yields for a 500 −1 f b data sample for various established and hypothetical processes. Cross sections are low, typical event yields for supersymmetric particles are a few thousand per channel. The angular distributions due to s-channel spin 1 exchange are forward peaked, putting a premium on good coverage of the forward direction.Although the range of new physics to be studied at the ILC is not yet clear, there are a few flagship measurements which can drive detector design. One example is the set of measurements of Higgs couplings to quarks and bosons. These measurements, which will span more than two orders of magnitude in mass and coupling strength, require excellent separation of b, c, and light quark vertices. A related measurement is the self-coupling of the Higgs. Here the signal reaction, e + e − → Z 0 H 0 H 0 → qqbbbb with four b-jets must be separated from backgrounds like tt → bbcscs, ZZZ, and ZZH. This physics requires efficient B tagging and excellent b/c separation in complex events. Different constraints on the vertex detector come from measurements like heavy quark forward-backward asymmetry. Here the emphasis is on forward tracking with flavor tagging and determination of the charge of the parent b quark. Whether the ultimate focus is on Higgs, supersymmetry, or other new physics phenomena, it is likely that precise measurements of heavy quark jets and their decay vertices will play a crucial role [1] in ILC physics. Detector Goals.The relatively low event yield motivates a design that optimizes the vertex information for each event. This has to be done within the constraints imposed by beam backgrounds, ILC bunch structure and integration with other components of the detector. The successful SLC CCD-based VXD3 vertex detector, with < 5µ resolution and 0.4% radiation length per layer [2], provides a basis for extrapolating performance of thin, finely pixelated detectors to the ILC.An set of goals has been formulated taking these opportunities and constraints into account:• Good angular coverage with many layers close to the interaction point

Theoretical status of Bs-mixing and lifetimes of heavy hadrons

“…Subleading effects of O(1/m b ) turned out to be negligible [7]. Updating the result from [6] with matrix elements from [8] and the values V cb = 0.0415, m b = 4.63 GeV and f B = 216 MeV [9] we obtain a value, which is in excellent agreement with the experimental number [10,11]:…”

“…One expects a number that is close to one [5,6,12,13]. The experimental number [10,11] is slightly smaller…”

“…The non-perturbative matrix element is parameterized by the bag parameter B and the decay constant f B . Using the conservative estimate f Bs = 240 ± 40 MeV [18] and the bag parameter B from JLQCD [22] we obtain in units of ps −1 (experiment from [10,11,23]) ∆M Theo s = 19.3 ± 6.4 ± 1.9, ∆M Exp s = 17.77 ± 0.12 The first error in the theory prediction stems from the uncertainty in f Bs and the second error summarizes the remaining theoretical uncertainties. The determination of ∆M d is affected by even larger uncertainties because here one has to extrapolate the decay constant to the small mass of the down-quark.…”

“…The width difference ∆Γ s /Γ s was investigated in [29,11]. The semi-leptonic CP asymmetry in the B s system has been determined in [11,30] (see [18] for more details). Therefore we use as experimental input ∆Γ s = 0.17 ± 0.09 ps −1 , φ s = −0.79 ± 0.56.…”

Vertexing at the ILC

“…Increased circuit density due to multiple tiers of electronics allows advanced in-pixel data processing in very small pixels, which is one of the key ingredients for an ILC Vertex detector. A basic demonstrator chip, the VIP2b [31] has recently been developed. It features an array of 192 × 192 pixels with a pitch of 24 × 24 µm 2 pixels with an 8 bit digital time stamp.…”