The Super Flavor Factory

0.19 MB·

by  A. J. Bevan

The Super Flavor Factory
7
0 A. J. Bevana∗
0
2 aPhysics Department, Queen Mary, University of London, E1 4NS, UK.

n
a The main physics goals of a high luminosity e+e− flavor factory are discussed, including the possibilities to
J performdetailedstudiesoftheCKMmechanismofquarkmixing,andconstrain virtualHiggsandNon-Standard

3 ModelparticlecontributionstothedynamicsofrareBu,d,s decays. ThelargesamplesofDmesonsandτ leptons
2 producedataflavorfactorywillresultinimprovedsensitivitiesonDmixingandleptonflavorviolation searches,
respectively. One can also test fundamental concepts such as lepton universality to much greater precision than
2 existing constraints and improve the precision on tests of CPT from B meson decays. Recent developments
v
in accelerator physics have demonstrated the feasibility to build an accelerator that can achieve luminosities of
1 O(1036 cm−2s−1).
3
0
1
1
6 1. INTRODUCTION 2. PRECISION CKM METROLOGY
0
/ Recent developments in accelerator physics Violationofthecombinedsymmetryofcharge-
x
e show that it is feasible to construct an e+e− col- conjugation and parity (CP) was first seen in
- lider with a luminosity of 1036 cm−2s−1, which the decay of neutral kaons [8]. All CP violation
p
is a factor of fifty increase relative to the cur- (CPV) in the Standard Model (SM) is the re-
e
h rent B-factories [1,2]. This paper discusses the sultofasinglecomplexphaseintheCKMquark-
: physicspotentialofaSuperFlavorFactory(SFF) mixing matrix. It was shown some time ago that
v
i associated with such a collider. The physics po- CPV is a necessary but insufficient constraint in
X
tential of a SFF comes from vast samples of ordertogenerateanetbaryonanti-baryonasym-
r B , D mesons and τ leptons that can be pro- metryintheuniverse[9]. Theotherrequirements
a u,d,s
duced, in addition to the flexibility of operating forthisasymmetryareanon-thermalequilibrium
at different center of mass energies (√s). One intheexpansionoftheuniverseandbaryonnum-
can study Υ(nS) decays where n = 1,2,3,4,5, berviolation. Intheensuingyearstherehasbeen
and perform precision measurements of the ra- a tremendous amount of activity to elucidate the
tio R = σ(e+e− hadrons)/σ(e+e− µ+µ−). role of CPV in the SM. All measurements of
Detailed reports→have been compiled o→n the po- CPV produced by the BABAR [10] and Belle [11]
tential of a SFF [3–5]. experiments are consistent with the CKM de-
Theremainderoftheseproceedingsdiscusspre- scription of CPV, and insufficient to explain the
cision tests of the Cabibbo-Kobayashi-Maskawa matter-antimatter asymmetry of the universe.
(CKM) quark-mixing matrix [6,7], new physics The SM description of CPV for B decays is
u,d
constraints from loop-dominated processes, rare manifest in the form of a triangle in a complex
charmless B decays, tests of the combined sym- plane. This triangle has three non-trivial angles,
metry of charge-conjugation, parity and time re- α, β, and γ, and two non-trivial sides with mag-
versal(CPT), charmandτ physicsopportunities nitudes V V∗ /V V∗ and V V∗ /V V∗ ,
| us ub| | cd cb| | td tb| | cd cb|
andthephysicspotentialfromanalysingdataac- where V are elements of the 3 3 CKM ma-
ij
×
cumulated at the Υ(1S,2S,3S,5S) resonances. trix. The limiting factor in the determination of
the sides of the triangle are the magnitudes of
the CKM matrix elements V and V . The
ub td
∗This workis supported by PPARC and the DOE under first step toward understand|ing|any n|ew p|hysics
contractDE-AC02-76SF00515.
1
2
weak phase or amplitude contribution to the fla- quired to constrain penguins in b uud transi-
→
vor sectoris to precisely understandthe SM con- tions will only be accessible to a SFF.
tributions. To this end, one has to overconstrain A precision measurement of γ will require
the parameters describing the unitarity triangle a systematic study of the many methods pro-
before embarking on a quest to find deviations posed in the literature [27]. One of the most
from SM behavior. promising channels to extract γ is B DK,
→
The angle β is determined from a time- where D0 K0π+π−, and the structure of the
→ s
dependent analysis of b ccs decays [12]. This K0π+π− Dalitzplotisusedinthefit[28]. ASFF
→ s
determination is from tree-dominated processes with 50ab−1 should be able to measure γ at the
that are theoretically clean and provides a base- level of 2◦ with this method [29]. The precision
linetocompareagainsttheresultsfrommeasure- oftheconstraintonγ usingtheAtwood-Dunietz-
ments of b s and ccd transitions. The current Soni[30]andGronau-London-Wyler[31]methods
→
resultsfromtheB-factoriesonthisparameterare isexpectedtobedominatedbyDalitz-plotmodel
Refs. [13,14]. As the precision of these measure- uncertainties at a SFF.
ments increaseitwillbecomeincreasinglyimpor- A SFF will be able to test the closure of the
tanttoimproveourunderstandingofpossibleSM unitarity triangle to a few degrees with a data-
pollution [15–17]. Open charm decays such as set of 50ab−1. The projected sensitivities for α,
B0 J/ψπ0 [18,19]can be used to estimate this β andγ are2◦,0.2◦ and2◦,respectivelyasshown
→
SMpollution[17]. ASFFwillbeableto improve in Figure 1. This level of sensitivity is compa-
on existing measurements of this angle and pro- rable to the expectations of an upgraded LHCb
vide competitive results in the LHC era. experiment [32]. In addition to performing the
The measurement of α is more complicated primary measurements of the unitarity triangle
than that of β [20,21]. The parameters of the angles, a SFF has the ability to perform mea-
time-dependent analysis of b uud transitions surements that will enable better determination
→
do not provide a clean measurement of α, but of the SM theoreticalpollutionto the angle mea-
measureaneffectiveparameterdependentontree surements. This is a critical aspect of searching
and loop (penguin) contributions to the over- for manifestations of a NP weak phase.
all weak phase. The most stringent method for
extracting α currently comes from the study of
3. NEW PHYSICS FROM LOOPS
B ρρ decays [22–24]. This result has an 8-fold
→
ambiguity,withdegeneratesolutions. Thedegen- The Higgs particle and supersymmetry are in-
eracy can be resolved using the result of a time- troduced to the SM as the standard way to elu-
dependent Dalitz-plot analysis of B0 π+π−π0 cidate the mass generation mechanism [34] and
→
decays [25,26]. The precision on α measured us- resolve the hierarchy problem [35,36]. The en-
ing B ππ decays is limited by the ability to ergy scale of the Higgs and NP contributions are
→
measure B0 π0π0. A SFF will enable us to widely expected to be introduced below 1TeV.
→ ∼
perform a precision measurement of this mode. When extending the SM to accommodate new
Therewillbesufficientstatisticstomeasuretime- particles at this scale, one introduces couplings
dependent asymmetry parameters through π0π0 in the flavor sector that will impinge on low en-
decays with photon conversions, and a precision ergy measurements of flavour-changing neutral
studyofB0 π0π0 meansthatanisospinanaly- currents (FCNCs) and processes dominated by
→
sisofB ππ decayswillbecomeanincreasingly penguin amplitudes at a B-factory. Most calcu-
→
important contribution to the overall constraint lations with a NP contribution to the effective
onα. TheconstraintonαobtainedataSFFwill Lagrangian introduce a fine-tuning problem by
beacombinationofresultsfromallofthesechan- setting the NP flavor parameters to zero. The
nels. The hadronic environment of LHCb results rest of this section highlights a few specific ex-
in difficulties in studying channels with neutral amplesofprocessesthatcanbeusedtoconstrain
particles in the final state and some channels re- the effects of NP.
3
0.6
0.5
0.4
η 0.3
α
0.2
0.1
γ β
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
ρ
Figure1. A predictionofthe constraintonthe unitarity triangleobtainedataSFF usinga 50ab−1 data
sample. Thecontoursrepresent68.3%confidencelevelintervalsintheρ η plane(the parameterswhich
−
are defined in the Wolfenstein expansion of the CKM matrix [33]).
3.1. Measurements of ∆S signsofNP[3–5]. Theexpected50ab−1SFFpre-
TheB-factorieshaverecentlyobservedCPV in cision on ∆S for K+K−K0, which contains the
B η′K0 decays [37,38]. These b s penguin mode φK0, is 0.017.
→ → ∼
processes are probes of NP, and have the most
precisely measured time-dependent CP asymme- 3.2. b → sγ
try parameters of all of the penguin modes. Any The b sγ penguin decays provide one of
→
deviation∆S ofthemeasuredasymmetryparam- the most sensitive constraints on possible NP
eter Sη′K0 fromsin2β is anindicationofNP (for (for example, see Ref. [45]). The existing exper-
example, see Refs. [3,4]). The anticipated pre- imental measurements and interpretation of re-
cision on ∆S for η′K0 is 0.015 with a data sults has been widely debated (for example, see
∼
sample of 50ab−1. In addition to relying on the- Ref. [46]). Of interest are both inclusive and
oretical calculations of the SM pollution to these exclusive decays where the rate measurement is
decays [39–41], it is possible to experimentally usedto constrainthe massofHiggsparticles(see
constrain the SM pollution using SU(3) symme- Refs. [45,46]). However in addition to this, it is
try [42]. This requiresprecisionknowledgeof the possible to measure time integratedand time de-
branching fractions of the B meson decays to pendent CP asymmetry parameters. These mea-
the following pseudo-scalar pseudo-scalar (PP) surements also provide stringent constraints on
final states π0π0,π0η,π0η′,ηη,η′η,η′η′ [43,44]. NP models, and the expected precision on the
As these PP channels have severalneutral parti- chargeasymmetrywith10ab−1ataSFFis 1%.
∼
cles in eachof their finalstates, they will be very
difficult to study in a hadronic collider environ- 3.3. b → sll
imcaelnlyt.cTlehane dbecaysBpen→guφinKSc0hiasnnanelotthoesretahrcehorfeotr- sulTthoef FbC→NCssl.l (Tlh=e foer,wµ)arpdr-bocaecskswesaradreastyhmemree--
→
4
try of these FCNC processes are sensitive to NP 4. CPT
contributions. Recently the BABAR and Belle ex-
The combined symmetry CPT is conserved
perimentsstartedtostudytheasymmetry[47,48]
in locally gauge invariant quantum field the-
and a high statistics evaluation of the forward-
ory [58–61]. Itis possible to constructtheoriesof
backwardasymmetry is required to elucidate the
quantumgravitywhereLorentzsymmetrybreaks
nature of any NP contribution to these decays.
The SFF will be able to study both e+e− and downandthequantumcoherenceoftheBBstate
µ+µ− final states. The physics reach of a SFF produced in Υ(4S) decays is broken (for example
see [62,63]). A test of CPT is one of the funda-
withthesedecayswillbecompetitivewithanyre-
mental tests of nature that should be performed
sults from an upgraded LHCb experiment. With
50ab−1 it is expected that the effective parame- to increasingly greater precision. Current exper-
tersrelatedtotheWilsoncoefficientsC canbe imental constraints on CPT in correlated P0P0
9,10
measured to 10-15% from the forward backward systemhavebeenperformedforneutralK andB
asymmetry in B K∗l+l−. mesons where the most stringent limit on CPT
→ violation from B decays is discussed in Ref. [64].
3.4. B → VV decays
TheangularanalysisofB VV decays(where 5. CHARM DECAYS
→
V is a vector meson) provides eleven observables
(six amplitudesandfiverelativephases)thatcan Several reviews of charm physics [65] have re-
be used to test theoretical calculations [49]. The cently highlighted the motivation to revisit stud-
hierarchy of A , A , and A amplitudes ob- ies of charm meson decays at much higher lu-
0 + −
tained from a helicity (or A , A , and A in minosities. The proceedings of the talks within
0 k ⊥
the transversitybasis)analysis of such decays al- this conference give an overview of the state of
lows one to search for possible right handed cur- the art measurements in charm decays [66]. The
rents in any NP contribution to the total ampli- motivation for studying charm decays at a SFF
tude. For low statistics studies, a simplified an- includes the continued search for D-mixing and
gular analysis is performed where one measures CPV. Oneoftenneglectedfactis thatthe multi-
the fractionoflongitudinally polarisedeventsde- tudeofprecisioncharmmeasurementsareinstru-
finedasf = A 2/P A 2,wherei= 1,0,+1. mental to honing theoretical calculations used in
L 0 i
| | | | −
Current data for penguin-dominated processes the study of B meson decays. Charm physics is
(φK∗(892)[50,51],K∗(892)ρ[52,53])thatareob- an integralpart of the wider programpursued at
servedto have non-trivialvalues of f can be ac- a SFF.
L
commodated in the SM. A SFF with a 10ab−1
data-set would be able to provide sub 1% mea-
6. STUDY OF τ LEPTONS
surements of A in φK∗. In addition to this, one
i
cansearchforT-oddCP violatingasymmetriesin One of the most promising channels to experi-
triple products constructedfromthe angular dis- mentally constrain lepton flavor violation (LFV)
tributions [54]. It has also been suggested that inτ leptondecayistheprocessτ µγ. Thecur-
→
non-SM effects could be manifest in a number rent experimental branching ratio limits on this
of other observables [55]. The measured rates of process are (10−7) [67,68] using approximately
O
electroweak penguin-dominated B decays to fi- 1.5 109 τ pairs. An estimated 10 109 τ pairs
× ×
nal states involving a φ meson are also probes of will be produced each year at a SFF. The large
NP [56]. The study of B AV decays (where A number of recorded decays would enable one to
→
is an axial-vector meson) also provides this rich push experimental sensitivities of LFV down to
set of observables to study, however current re- the 10−9 to10−10 level. Suchastringentlimiton
sults only yield an upper limit on B0 a±ρ∓ LFV would impose serious constraints on many
decays [57]. BABAR have recently studied→the1an- models of NP [69]. In addition to LFV, one can
gular distribution for B φK∗(1430) [50]. search for CPV in τ decay.
→
5
7. Υ DECAYS BELOW THE 4S RESO- 9. ACCELERATOR DESIGNS
NANCE
The PEP-II and KEK-B asymmetric energy
Samples of Υ(1S,2S) decays can be obtained e+e− accelerators have outperformed expecta-
by operating a SFF at the Υ(3S) resonance and tionstointegrateacombinedluminosityof1ab−1
tagging the final state π+π−Υ(1S,2S), or via ra- since the B-factory operation started in 1999.
diative return from the Υ(4S) resonance. Therearecurrentlytwodesignsbeingentertained
The decays Υ(3S) π+π−Υ(1S,2S) with for a 1036 cm−2s−1 collider that would integrate
→
Υ(1S,2S) l+l− for l = e,µ,τ, have been pro- 50ab−1 of data during their lifetimes. One is
→
posed for testing lepton universality (LU) at the anupgradedKEK-Baccelerator[1](SuperKEK-
percent level using the existing B-factories [70]. B) that benefits from ILC technological devel-
TheCLEOcollaborationhaverecentlyperformed opments, and the other is the result of more
such measurements for τ and µ dilepton decays recent developments in trying to harness more
of Υ(1S,2S,3S) concluding that LU holds within ILC-related technology [2] (low emittance de-
the (10%) precision of the measurement [71]. sign). Highlights of the proposed parameter sets
O
CLEO analysed (1.1fb−1) of data accumulated of these machines are summarised in Table 1.
O
ateachoftheΥ(1S),Υ(2S)andΥ(3S)resonances. The luminosity of an e+e− collider is propor-
L
The data currently show a 2.6σ deviation from tional to Ie+/e−ξe+/e−,y/βy∗ where Ie+/e− is the
theexpectationofLU.VariousNPscenariosexist beam current,ξe+/e−,y is the beam-beam param-
where light CP-odd non-SM Higgs bosons could eter and β∗ is the vertical beta-function ampli-
y
breakLU[72–74]. AprecisiontestofLUcouldbe tude at the interaction point. In addition to
performed at a SFF by operating the accelerator this, there is a luminosity reduction factor, the
at √s = 10.355GeV corresponding to the Υ(3S) so-called hourglass effect [79], to consider when
resonance. simulating the delivered from a SFF. This re-
L
Most dark matter scenarios require a SM- duction factor is approximately 6% in the case
dark matter coupling, and studies of the decays of PEP-II. The luminosity increase of the Su-
Υ(1S) invisiblehavebeenproposedinorderto per KEK-B design is achieved by increasing the
→
study such couplings [75]. beam-beam term, the beam currents and reduc-
ing β∗. In order to reach the predicted luminos-
y
ity of 0.8 1036 cm−2s−1, the KEK-based de-
×
sign incorporates a number of upgrades includ-
ing the use ofcrabcavities to rotatethe colliding
8. ACCUMULATING DATA AT THE
bunches of electrons and positrons. This tech-
Υ(5S) RESONANCE
nology is expected to reduce the geometric re-
duction factor of the luminosity. The “low emit-
Recent work has shown that it is possible to
studyB decaysproducedattheΥ(5S)resonance tance design” achieves its luminosity increase to
s
with an asymmetric energy e+e− collider [76]. It 1.0 1036 cm−2s−1throughalowemittanceoper-
×
ationoftheaccelerator. Whilemostofthestudies
will be possible to measure ∆Γ/Γ for the B sys-
s
tem using B D(∗)D(∗) decays [77], and an forthisdesignarefocussedonapossiblesitenear
s s s
→ Frascati,thisisasite-independentdesign. Anim-
e+e− collider provides a clean environment to
portant aspect of achieving 1036 cm−2s−1 is the
search for NP in the B K∗γ and B φγ
s s
→ → use of so called ‘crabbed waist” scheme [80].
loop processes [78]. One can also constrain NP
There are pros and cons for both of the accel-
parameter space through measurements of semi-
erator concepts under study, however as it is un-
leptonic B decays. The large mixing frequency
s
likely that there will be more than one SFF built
of B mesons makes time-dependent CP asym-
s
intheworld,thenextstepforaSFFistocoalesce
metrymeasurementschallenging,andstudies are
the best of both designs to a common proposal
underwaytoelucidatethe prospectsofsuchmea-
on a timescale of the next year or two. Both de-
surements with a SFF.
6
Table 1
for CPV in the lepton sector.
Parameters of the accelerator configurations un- It is impossible to predict a priori what mea-
der consideration.
surements are needed to constrain NP contribu-
Parameter Super KEK-B low emittance
tions, so one has to perform as many different
ǫ (nm) 9.0 0.8
x measurements as possible. A SFF would provide
ǫ (nm) 0.045 0.002
y a base to perform a wide range of such measure-
β∗ (cm) 200.0 20.0
x ments.
β∗ (cm) 3.0 0.2
y
σ (mm) 3.0 7.0
z
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