The SuperB Project

Abstract

The SuperB project for a next generation asymmetric e+e- flavor factory to be built in the Rome area with a baseline luminosity of 10^{36}cm-2s-1 is discussed. Some explicit examples are given to elucidate how such a facility can provide a uniquely sensitive probe of physics beyond the Standard Model. The basic accelerator concepts allowing luminosities 50-100 times larger than the existing B factories are briefly discussed, along with the main characteristics of the SuperB detector. Comment: 7 pages, 2 figures. Prepared for the proceedings of the HQL2010 conference (Frascati, 11-15 October 2010)

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The SuperB Project
G. Finocchiaro∗†
INFN - Laboratori Nazionali di Frascati
E-mail: giuseppe.finocchiaro@lnf.infn.it
The SuperB project for a next generation asymmetric e+e−flavor factory to be built in the Rome
area with a baseline luminosity of 1036cm−2s−1is discussed. Some explicit examples are given
to elucidate how such a facility can provide a uniquely sensitive probe of physics beyond the
Standard Model. The basic accelerator concepts allowing luminosities 50-100 times larger than
the existing B factories are briefly discussed, along with the main characteristics of the SuperB
detector.
The Xth Nicola Cabibbo International Conference on Heavy Quarks and Leptons,
October 11-15, 2010
Frascati (Rome) Italy
∗Speaker.
†On behalf of the SuperB Collaboration
c ? Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
http://pos.sissa.it/
arXiv:1012.2449v1 [hep-ex] 11 Dec 2010

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HQL 2010 G. Finocchiaro
1. Introduction
New physics (NP) is generally expected beyond the Standard Model of High Energy Physics
(SM), although the energy scale Λ of the supposed new interactions is not univocally predicted.
The theoretically well-motivated possibility that Λ is around 1TeV could make the NP accessible
to the Large Hadron Collider (LHC). In such a case SuperB could study the flavor structure of NP,
measure the flavor couplings and search for still heavier mass states. Alternatively, the NP energy
scale could lie above the direct reach of LHC. In such a scenario SuperB can look for indirect NP
signals, understand where they may come from, and exclude regions in the multi-dimensional pa-
rameter space of NP models up to Λ ∼ 10TeV or more. In addition to probing energy scales higher
than LHC via virtual processes, SuperB can, thanks to its clean e+e−environment, experimentally
access several physics channels precluded to hadronic machines as LHC. Those channels include
decays with neutrinos or neutral particles in the final state, or whenever an inclusive analysis is
required, as for example BF(B→K(∗)νν), BF(B→Xsγ), and the measurement of the |Vub| or |Vcb|
CKM matrix elements.
In order to achieve the sensitivity goals of the project (some relevant examples will be dis-
cussed in the next section), a dataset of about 75ab−1(80×109BB pairs) is required, which could
be collected at the ϒ(4S) in 5 years of data taking if L = 1036cm−2s−1, assuming that the accel-
erator and detector efficiency are kept at the very high levels of the B factories. It is important to
remark that SuperB will also produce large samples of D meson (100×109) and τ lepton (70×109)
pairs, allowing for NP searches in the up-type quark and lepton sectors with unprecedented pre-
cision. The ability of SuperB to vary the center-of-mass energy from the charm and τ production
threshold up to the ϒ(5S), and to have at least one beam with 80% longitudinal polarization, will
further boost the physics potential of the experiment.
As demonstrated by the several hundred papers published by the B-factory experiments, a
wide range of important measurements can be performed in the clean e+e−environment at ϒ(4S).
Most of these are statistics-limited, and would therefore improve substantially with a data sample
of 75ab−1. In many cases, large control samples can be used to further reduce systematic and
theoretical errors. Control of the theoretical errors (e.g. those related to lattice calculations) is
particularly relevant to fully exploit the statistical power of the experimental measurements. New
developments in unquenched lattice calculation techniques, together with the expected increase in
computing power, will allow, by the time the data will be collected, to match the SuperB experi-
mental precision[1]. With 75ab−1SuperB will be able to substantially improve on the precision
of the CKM Unitarity Triangle (UT) parameters. For example, the present error on the ¯ ρ and ¯ η
parameters of the UT (±0.028 and ±0.016 respectively) could be reduced[2] to δ ¯ ρ = ±0.0028,
δ ¯ η = ±0.0024. The possibility to study a very large numbers of physical observables, and the
correlations among them, is a particularly important tool to elucidate the nature of new physics,
should deviations with respect to SM predictions be observed.
2. SuperB Physics Highlights
In this section a few examples of physics channels in which the SuperB can give significant
contributions are briefly reviewed. For an extensive discussion, we refer the reader to[1].
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2.1 B → K(∗)νν
The rare B → K(∗)νν decays are interesting probes of new physics from Z0penguins[3].
Moreover, since the neutrinos escape the detector unmeasured, the b → s+Emisschannel could
be affected by exotic sources of missing energy, such as e.g. light dark matter[4]. The B → Kνν
branching fraction, with an experimental upper bound only a factor of three above the SM predic-
tion, currently provides the most stringent constraint on NP. In the B → K∗νν decay one can also
measure the longitudinal polarization fraction fL(q2), which is theoretically very clean since form
factor uncertainties cancel to a large extent[5]. A detailed analysis based on the recoil technique
was performed to extrapolate the SuperB reach on the B → K(∗)νν decays from the BABAR mea-
surement. In the recoil technique one B meson is fully reconstructed, yielding a high purity sample
with known kinematics, flavor and charge. The improved SuperB hermeticity (see later) is crucial
in background-dominated, very rare channels. In SuperB an increase of about 25% with respect to
BABAR in the S/B ratio is expected.
In a general NP model BF(B → Kνν), BF(B → K∗νν), fL(q2) and the inclusive branching
fraction BF(B→Xsνν) can be expressed in terms of the two real parameters ε and η, which can be
over-constrained by the four measurements (ε = 1,η = 0 in the SM). Fig.1 shows the correlation
among the observables in the (ε,η) plane (left), and how the constraint in the plane could be
improved at SuperB (right).
Figure 1: Left: constraints on the ε−η plane, where the bands for BF(B→Kνν) (green), BF(B→K∗νν)
(black), fL(q2) (orange) and BF(B → Xsνν) (red) only account for theoretical errors[5]. The grey area is
excluded at 90% CL by present data. Right: expected constraint on the (ε,η) plane, from the measurement
of the branching fractions of B → K(∗)νν decays and the angular analysis of B0→ K∗0νν with 75ab−1.
2.2 LFV in τ Decays
Lepton flavor violation (LVF) in τ decays, which is negligibly small in the SM but is enhanced
in several SM extensions, is another very sensitive NP probe. The search for LFV has already been
actively pursued at the B factories, which pushed the 90% CL branching fraction limits in almost
50 different decay modes down to a few 10−8. From an extrapolation of the BABAR analysis, the
SuperB sensitivities in the “golden” LFV channels τ → µγ and τ → µµµγ are 2.4×10−9and
2.3×10−9respectively. The ratio of branching fractions to these modes could distinguish between
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the SUSY and Little Higgs models[2]. In addition to this, the limit on τ → µγ, when combined
with constraints on µ → eγ from MEG and ϑ13from accelerator and reactor neutrino experiments,
can be used to distinguish among different NP scenarios[2].
As a key feature of its baseline design, SuperB incorporates an 80% polarized electron beam
(polarization of the e+beam is more difficult to realize and will be considered as an upgrade). The
angular distribution of the decay products of polarized τ’s depends on the particular decay, and can
be used to effectively suppress backgrounds (a typical example being τ→µννγ for the LVF decay
τ→µγ). The polarization is also instrumental in improving the sensitivity on the τ EDM and g−2.
2.3 Charm at Threshold
The measurement of D0D0oscillations has opened a new window to search forCP violation in
charm which, if observed, would provide unequivocal NP signals. With 75 ab−1at ϒ(4S) SuperB
will dramatically improve (by a factor 12) the precision in the determination of the D0D0mixing
parameters. The strong phase difference δf between the produced D0and D0cannot however
be measured at ϒ(4S), but only at the charm production threshold, where the DD pairs exhibit
quantum coherence. With 0.5ab−1at the ψ(3770) SuperB can measure δfto ±1◦, several flavor-
changing neural current modes with sensitivities of the order of 10−8, and finally strongly reduce
the Dalitz-plot model uncertainty in the γ angle measurement.
3. SuperB Accelerator Highlights
The SuperB collider exploits a novel collision scheme[6, 7], based on very small beam dimen-
sions and betatron function at the interaction point, on large crossing and Piwinsky angle1and on
the “crab waist” scheme. This approach allows to reach the required luminosity of 1036cm−2s−1
and at the same time overcome the difficulties of early super e+e−collider designs, most notably
very high beam currents and very short bunch lengths. The wall-plug power and the beam-related
background rates in the detector are therefore kept within affordable levels[8].
The crab waist transformation consists in moving the waist of each beam onto the axis of the
other beam with a pair of sextupole up- and down-stream the IP. In this way all particles from
both beams collide in the minimum β∗
significantly) the x/y betatron resonances are naturally suppressed. The principle of the innovative
IRdesignsketchedabovehasbeenexperimentallydemonstratedattheFrascatiDaφNEcollider[9].
Very importantly, this test also validated the simulations used to calculate the IR optics.
Several other accelerator-physics ideas critical to the success of the SuperB project have also
been realized in practice, either in past colliders or as part of accelerator R&D activities such
as those for the Linear Collider: KEK-B and PEP-II have worked very well with asymmetric
interaction regions, storing 2-3A of beam currents and performing continuous injection with live
detectors; polarizedbeamshavebeensuccessfullyproducedattheSLC,andspinmanipulationtests
have been performed at Novosibirsk; finally, ultra-low emittance lattices were tested for the ILC
damping rings. This long list of achievements, and the fact that the luminosity of 1036cm−2s−1is
yregion, with a net luminosity gain. Moreover (and most
1The luminosity formula for e+e−beams colliding with an horizontal crossing angle ϑ can be expressed[7] in
terms of the vertical tune shift parameter ξy, the vertical beta function at the IP βyand the number of particles per bunch
N (proportional to the beam current) as L ∝Nξy/βy, with ξy∝Nβy/(σxσy
?1+φ2). The Piwinsky angle φ ?ϑσx/σy.
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not a singularity in the parameter space but can be obtained with different settings of the accelerator
parameters, as e.g. the x and y emittances and β∗
currents, adds confidence that this unprecedented luminosity can indeed be obtained in practice.
The SuperB design is based on recycling as much as possible existing PEP-II hardware, with
a significant reduction of costs. The optimal beam energy choice for accelerator design, including
polarization, is 4.18GeV positron beam on 6.71GeV electron beam.
The low currents, ultra-small emittance approach has been adopted recently also by the KEKB
accelerator team, which defined a new set of parameters very similar to that of the Italian SuperB.
yin both rings, the vertical tune shift, the beam
4. SuperB Detector Highlights
Most of the general requirements for the SuperB detector are common to those of the present B
factories, including large solid angle coverage, good particle identification (PID) capabilities over
a wide momentum range (π/K separation to over 4GeV/c), measurement of the relative decay
times of the B mesons, good resolution on the momentum of charged tracks and on the photon
energy, particularly in the sub-GeV part of the spectrum, relevant at the ϒ(4S) environment. The
SuperBdetectorconceptisthereforebasedontheBABARdetector, withthemodificationsrequiredto
operate at a much higher luminosity (and luminosity-scaling background rates), and with a reduced
center-of-mass boost[10].
The BABAR detector is composed by a tracking system – a five layer double-sided silicon
strip vertex tracker (SVT) and a 40 layer drift chamber (DCH) immersed in a 1.5T magnetic field
– a Cherenkov detector with fused silica bar radiators (DIRC), a homogeneous electromagnetic
calorimeter made of CsI(Tl) crystals (EMC), and a detector for muon identification and K0
tection (IFR) realized instrumenting the iron flux return with resistive plate chambers and limited
streamer tubes. SuperB is designed to reuse a number of BABAR components: the DIRC quartz bars,
the CsI(Tl) crystals of the barrel EMC, the flux-return steel, the superconducting coil.
The center-of-mass boost at SuperB is smaller than in BABAR (βγ = 0.24 vs. 0.56). While this
effectively improves the angular coverage of the detector, it also reduces the ∆z separation of the
decay vertices. The ∆t sensitivity in time-dependent measurements is maintained by improving the
vertex resolution: the SuperB vertex detector replicates the five-layer BABAR SVT, but exploits the
reduced dimensions of the beam pipe made possible by the ultra-low emittance SuperB beams to
add a very thin and precise measurement layer at a radius of only 1.5cm. The baseline technology
forthis “Layer0”usesshortdouble-sided silicon stripdetectors(“striplets”), whileotheroptionsare
being considered as possible upgrades. The SuperB DCH concept is derived from the BABAR one,
with several improvements: the mechanical structure entirely in Carbon-Fiber composite, resulting
in a 4-fold reduction of the endplate material; the gas mixture and the wire cell layout optimized
to minimize the multiple-scattering contribution to momentum resolution; the readout electronics
redesigned to cope with the higher trigger rate, and minimize the FEE material. The hadron PID
system will use the radiator quartz bars of the BABAR DIRC, read-out by fast multi-anode PMTs,
and with the imaging region considerably reduced in size to improve performance and reduce the
impact of backgrounds. The forward EMC will feature cerium-doped LYSO crystals, which have
a much shorter scintillation time constant, a smaller Molière radius and better radiation hardness
than the current CsI(Tl) crystals, for reduced sensitivity to beam backgrounds and better position
Lde-
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