Status of PEP-II and BaBar

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Nuclear PhysicsB (Proc. Suppl.) 66 (1998) 541-544

,immmlq PROCEEDINGS SUPPLEMENTS

Status of PEP-II and BABAR Luca Listaa* ~INFN Sezione di Napoli, Mostra d'Oltremare pad. 20, 1-80125 Napoli, Italy The status of PEP-II, the SLAC asymmetric e+e - B-factory, and of the BABAR experiment is presented. The major features of the project are described in conjunction with the physics requirements from CP violation capability. The expected sensitivities on varoius C P channels are summarised.

1. I n t r o d u c t i o n CP violation [1], experimentally observed in the K°/~ ° system [2], is presently one of the most interesting topics of particle physics. The B°/~ ° system offers the opportunity to study the CP violation in relatively abundant decay channels, and within a theoretially clean environment. CP asymmetry measurements can be related to the phases of the CKM matrix elements and permit to test the Standard Model origin of the CP violation. The experiments BABAR [3], Belle [4], HERAB and CDF will provide, with different characteristics, the opportunity to study CP violation in the B°/~ ° system before the LHC era. At e+e colliders, B°/~° pairs are produced with low background via the decay of the T(4S) resonance in a coherent state. One of the foundamenta] requirements for the measurement of CP asymmetries is the determination of the difference of the decay times of the two B mesons. In the T(4S) rest frame, the B mesons are produced with small kinetic energy, and their average vertex separation is only 30 pm. In order to achieve a measurable vertex separation, at PEP-II [5] the T(4S) is boosted in the electron beam direction with 197 = 0.56. This boost increases the average B vertex separation to the measurable value of 250pm. BABAR will be able to reconstruct many CP B decays which will permit to determine al least two angles of the unitarity triangle, sin 23 and sin 219, *on behalf of the B A B A R collaboration.

0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0920-5632(98)00104-2

from the CP asymmetries; it will study other non CP B physics, including accurate measurement of Tfrub and ~cb and will study rare B decay. The high luminosity provides the opportunity to study charm, tau and "r'f pysics as well.

2. T h e P E P - H B - f a c t o r y The construction of PEP-II takes advantage of the existing SLC injector and the existing PEP storage ring for the realization of the high energy ring (HER) for the electron beam. The decton a~l positron beams have energies of 9 and 3.1 GeV respectively. In order to have head-on collisions, complex optics with permanent dipole magnets near the interaction point is need, well inside the experiment. Such optics limits the forward angular coverage of the BABAR experiment to 350 mrad. The high number of bunches (1658) in each beam toghether with a high current permits a luminosity of 3 x 1033 cm-2s -1 which will ultimately reach 1034 cm-2s -1 after two years of run. The first injection in the HER has been successfully completed in may 1997 with a beam trajectory well in agreement with the design prediction. A lifetime of about three hours has been reached, and multi-bunch tests have been successfull. The rest of the project is well on schedule, with expected collision tests starting in may 1998. The BABAR experiment will be completed and moved to the interaction region at the beginning of 1999, in order to start the physics program early in 1999.

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L. Lista/Nuclear Physics B (Proc. Suppl.) 66 (1998) 541-544

3. T h e BABAR d e t e c t o r BABAa is optimized to perform a systematic study of CP violation in the B meson system, and will study a wide range of B, charm, v and 77 physics. In a typical CP event, one of the B ° mesons decays into a CP eigenstate, and its decay is completely reconstructed; the decay products of the other B ° are used to determine its flavour (b-tagging). The fundamental requirements to determine the CP asymmetries from B ° decays are therefore an efficient reconstruction of a wide range of exclusive final states with low background, a good particle identification to tag the flavour of the non-CP B °, and a precise vertex measurement to determine the difference of the decay times of the two B. The subdetectors which constitute BABAa (fig. 2) are designed to meet such goals. Starting from the interaction point, a Silicon vertex Detector (SVT) which is made of 5 double sided silicon microstrip layers provides a vertex resolution of 80 #m, which contributes a loss of precision smaller than 10% in the asymmetry measurements. The SVT is the only tracking medium for charged particles with transverse m o m e n t u m smaller than 100 MeV/c. Many CP final state require precise and efficient tracking measurements, like B ° --~ ~'+Tr-, B ° --* l / ¢ g ° with gO ___, 7r+Tr- and J / ¢ --* t+~ - . The main tracking detector is the Drift Chamber, made of 40 stereo wire layers, with planar endplates. An helium-isobutane mixture has been chosen to reduce the multiple scattering. The expected transverse momentum resolution is ~rpt/pt = 0.21% ~B0.14% x Pt. Since the d E / d z measurement in the Drift Chamber is insufficient to achieve a large pion/kaon/proton separation, this particle identification capability is achieved my mean of the DIRC (Detector of Internally Reflected Cherenkov light), a Cherenkov detector m ~ l e of 156 quartz bars which run parallel to the beam axis. The Cherenkov photons produced by the passage of charged particles through the bars (few tens of photons, in average) are internally reflected in the

bars, and finally reach a water standoff region put in the backward of BABAR, where they are detected by a matrix of about 13000 photomultpliers. The reconstruction of the photons emission angle determines the /3 of the particle, and allows the discrimination of different mass hypteses. A *r/K separation larger than 4 standard deviations for momentum up to 4 G e V / c is achieved. Particle identification ( r / K ) capability provides a powerful rejection of the B ° --* lr+K - background to the CP channel B ° --* lr+zr-; moreover the identification of the kaons from the decay chain b --+ c --* s is foundamental in the B flavour tagging toghether with lepton identification. An Electromagnetic Calorimeter made of about 7000 CsI crystals provides photon and electron identification with a resolution a E / E = l%/~/-E @ 1.2%. Soft r ° reconstruction is relevant for the study of B ° ---* Y / ¢ g ~ with gO __. r07r0 and for D* ---* DI/decays. The outermost detector is obtained with the instrumentation of the iron of the magnet flux return with 18 or 19 layers, in the end-caps and the barrel respectively, of Resistive Plate Counters (.RPC). This Instrumented Fluz Return (IFR), which includes an RPC layer inside the solenoid, provides identification of muons with momentum down to -.~ 500 MeV/c and reconstruction of neutral hadrons, in particular K~ for the study of the CP channel B ° ~ J / ¢ K °. Low momentun muon identification is performed with the study of the differences of the detailed interaction patterns of muons and pions in the iron. This increases the performances especially at low momentum with respect to classic muon detectors with few layers of sensitive detector elements. All the subdetectors are in the production phase. In particular, the Drift Chamber endpates are ready and installed on the automatic wiring robot; moreover, since the RPCs of the IFR must be inserted into the magnet iron, the IFR is the first subdetector which needs to be installed, and will take cosmic data in 1998. The insertion of the final RPCs in the iron is well advanced, and performance of the installed chambers have been successfully tested.

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L. Lista/Nuclear Physics B (Proc. Suppl.) 66 (1998) 541-544

Table 1 CP reach expected in B x B x R after the first year of run CP state Br sinCb J/~bK ° 0.5 x 10 - s Y/~bK ° 0.5 x 10 - z J/~bK °* 1.6 x 10 -a D+D 6 × 10 -4 D * + D *7 x 10 -4 D * + D ~: 8 × 10 . 4 Combined sin2fl z-+~r1.2 x 10 -5 pz5.8 x 10 . 5 al~r 6 x 10 - s Combined sln2~

efficiency 0.41 0.33 0.39 0.25 0.16

0.80 0.56

4. P h y s i c s p e r f o r m a n c e s The channels of interest for the measurement of sin 2c~ and sin 213 with the expected measurement errors after the first year of run are shown in table 1. The cleanest channel for the deterruination of sin 2fl is B ° ---, J/~bK°; sin 2a determination from the channels B ° ---, lr+~r - , plr and a11r suffers from the contribution of penguin diagrams, whose evaluation can be done with an isospin analysis of the decays B ° --, z'+lr - , z'%r° and B + ---, 7r±z"° [6]. In order to update the expected physics results including more recent theoretical predictions and more accurate studies, a series of BABAR physics workshops is going on. After the last workshop in september 1997, a BABAR Phgsics Book will be published.

~r 0.10 0.16 0.19 0.21 0.15 0.15 0.06 0.20 0.1 0.24 0.09

2. 3. 4. 5. 6.

at the Twentieth Annual Summer Institute on Particle Physics, SLAC, (1992). H Christenson et al., Phys. Rev. Lett 13, 138 (1964). BABAR Collaboration, BABAR Technical Design" Report, SLAC-R-95-457 (1995). M. T. Cheng et al., Belle Technical Design Report, KEK report 3-1995 (1995). P E P - I f conceptual Design Report, SLAC-418 (1993). M. Gronau, D. London, Phys. Rev. Lctt. 65, 3381 (1990).

5. C o n c l u s i o n s BABAR and P E P - I I will offer the opportunity to make an extensive study of CP violation in the B°/~ ° system. PEP-II project is well on schedule; injection in the HER was successful, and the first test collisions are expected by m a y 1998. BABAR construction has started, and physics runs are expected to begin early in 1999.

REPERENCES 1. Y. Nit, SLAC-PUB-5874, Lecture presented

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Figure 1. The iron of the magnet flux return.

L. Lista/Nuclear Physics B (Proc. Suppl.) 66 (1998) 541-544

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