The SuperB factory: Physics Prospects and Project Status
aa r X i v : . [ h e p - e x ] D ec Proceedings of the PIC 2012, ˇStrbsk´e Pleso, Slovakia
THE SUPER-B FACTORY:PHYSICS PROSPECTS AND PROJECT STATUS
LUIS ALEJANDRO P´EREZ P´EREZON BEHALF OF THE SUPERB COLLABORATION
INFN - Sezione di PisaLargo Bruno Pontecorvo 3, 56127 Pisa - ItalyE-mail: [email protected]
I will briefly review of some highlights of the SuperB physics programme, the statusof the accelerator and detector studies, and the future plans.
The LHC brought the high energy particle physics community to a new era in NewPhysics (NP) searches beyond the Standard Model (SM). Direct searches of NP upto the 1 TeV scale are possible with the ATLAS and CMS experiments, where theproduction of new particles and theirs decays can be studied to constraint the NPLagrangian. However, another complementary approach, where high energies aresubstituted by high statistics, can be made by performing precision measurementson the flavor sector of the SM. This approach is essential for two reasons: 1) Newparticles can manifest themselves through small virtual effects in decays of the SMparticles such as B and D mesons, and τ leptons. These contributions depend onthe energy scale of the NP, hence sufficiently precise measurements can allow toextend the NP searches beyond the 1 TeV scale. 2) These kind of searches canbe used to explore the parameter space of the NP weak complex couplings and todetermine its flavor structure.During the last decade the SM description of the heavy flavor sector has sur-passed all expectations. The first generation of B factory experiments, BABARand Belle, together with CDF and D0 at TEVATRON, have provided a plethoraof measurements that substantially confirm the CKM picture of the SM, reducingstrongly the parameter space of possible NP flavor mixing scenarios. Given thehigh expected number of B and D mesons to be produced at the LHC, it is evidentthe role of the LHCb in the NP searches program, where will be performed precisemeasurements of CP-violating/CP-conserving observables from the study of rarecharm and beauty decays.The recent approval of the SuperB project open new possibilities for the flavorphysics program. The new SuperB final focusing scheme for the e + e collider willincrease the instantaneous luminosity by two orders of magnitude, passing from10 to 10 cm − s − . SuperB intends to search for indirect and some direct signsof NP at low energy, while at the same time, enabling precision tests of the SM.The main focus of the physics programme rests in the study of so-called GoldenModes ( e.g. B ± → ℓ ± ν ℓ , B → K ( ∗ ) ν ¯ ν , B → K ( ∗ ) ℓℓ , τ → ℓ , τ → µγ and many c (cid:13) Institute of Experimental Physics SAS, Koˇsice, Slovakia A. P´erez P´erez others), these are decay channels that provide access to measurements of theoret-ically clean observables that can provide both stringent constraints on models ofNP, and precision tests of the SM. SuperB will accumulate 75 ab − of data over aperiod of five years of nominal data taking at the Υ(4 S ). In addition to operatingat the center of mass energy of the Υ(4 S ), this experiment will also run at otherenergies ranging from charm threshold, at the ψ (3770), up to the Υ(5 S ). The NPsensitive observables that SuperB will measure are complementary, and in manycases competitive, to those accessible by LHCb. Only by measuring the full set ofobservables at e + e − and hadron colliders (LHC) will be able to optimally elucidatedetails of the flavor structure at high energies. SuperB will play a crucial role indefining the landscape of flavour physics over the next 20 years.The physics potential [1,2], and the detector [4] and accelerator [5] plans havebeen extensively documented. The accelerator parameters have been defined foroperating in the ψ (3770) to Υ(5 S ) energy range and the accelerator will reuse largeparts of the SLAC PEP-II hardware. The project Technical Design Report (TDR)is expected to be released by the end of 2012. This experiment will be built at anew laboratory on the Tor Vergata campus near Rome, Italy named after NicolaCabibbo. Data taking should begin five to six years after construction begins. τ Physics
The intrinsic level of charged lepton flavour violation in the τ sector arising fromneutrino oscillations is expected to occur at the level of 10 − . Given that bothquark and neutral lepton number conservation is known to be violated at a smalllevel, it is natural to presume that there may ultimately be non-conservation ofcharged lepton number. Indeed many NP scenarios predict large (up to ∼ − )levels of charged lepton flavour violation (LFV). These predictions are model de-pendent: some models favour large µ → e transitions over other possibilities, andother models prefer large τ → µ or τ → e . While the quest for a discovery of LFVcontinues, it is clear that all three sets of transitions need to be measured or wellconstrained in order to understand the underlying dynamics. SuperB will be able toimprove existing limits from the B factories by between one and two orders of mag-nitude. Channels such as τ → ℓγ will see a factor of ten improvement as these haveirreducible SM backgrounds that one will have to contend with. Other channelssuch as τ → ℓ , which are free of SM backgrounds, will see a factor of one hundredimprovement. The e − beam at SuperB will be 80% polarized, enabling one to sep-arate contributions from SM-like LFV channels and other irreducible backgroundsas one can use the polarization of the final state τ lepton to suppress background.This works well for improving limits, or indeed searching for left handed sources ofNP. One can verify if there is a right handed component to any underlying NP bycomparing results with and without polarized beams. uperB project Concerning the NP search potential of SuperB, it is based on the use of indirectconstraints on rare processes to infer the existence to the corresponding energyscale Λ NP of the NP. The correlation between measurement of the rare decays(a branching fraction or other observable) and the energy scale is non trivial. Ifone considers the minimal super-symmetric model (MSSM) in the mass insertionhypothesis then for example the measurement of the inclusive branching fractionsof b → sγ and b → sℓℓ , along with the CP asymmetry in b → sγ can be used toconstrain the mass insertion parameter ( δ d ) LR . The magnitude of this parametercan be used to infer an upper limit on Λ NP to complement the null results obtainedso far from the LHC. If the generic MSSM is a realistic description of nature thenthe fact that the LHC has failed to find a low mass gluino implies that there is anon-trivial coupling ( δ d ) LR , and hence in turn SuperB should be able to observea non-trivial deviation from the SM when studying the inclusive decays b → sγ and b → sℓℓ . The magnitude of the observed deviation will benefit the SLHCcommunity as the inferred upper bound on the energy scale obtained will provideuseful information on the integrated luminosity required to yield positive resultsvia direct searches. For example if one measured | ( δ d ) LR | = 0 .
05, then the impliedupper limit on Λ NP is 3 . tanβ as can be seen from Ref. [3].There are numerous golden rare B decay channels at SuperB, including B → ℓν ,where ℓ = τ, µ, e . In the SM this decay is known up to uncertainties related tothe value of V ub and f B . The rate of these processes can be modified by theexistence of charged Higgs particles predicted in a number of extensions of the SMfor example two-Higgs Doublet models (2HDM) or SUSY extensions of the SM.Hence the measured rate of these decays can be used to place limits on the inferredmass of any H + particle, and such constraints are dependent on tanβ . The existingconstraints from the B factories from inclusive b → sγ decays exclude masses below295 GeV /c , and the constraint from B → τ ν excludes higher masses for large tanβ scenarios. With 75 ab − of data SuperB will be able to exclude, or detect, a H + with a mass 13 TeV, for tanβ between 40 and 100. This constraint results from acombination of B → τ ν and B → µν . Ref. [3] discusses the physics potential ofseveral other interesting rare B decays.Many of the CP asymmetry observables of B u,d decays available at SuperB aredominated by loop contributions and are sensitive to the same sources of NP thatcan affect many of the interesting rare decays discussed above. The golden modesto be used for measuring the angle β of the unitarity triangle are B decays to char-monium ( c ¯ c ), η ′ or ψ , and a neutral kaon. SuperB will be able to measure the CPasymmetries in these decays with precisions of 0.002, 0.008, and 0.021, respectively,using a data sample of 75 ab − . Both tree ( c ¯ cK ) and penguin dominated decayscan be affected by the presence of NP. To complement the B u,d programme at Su-perB, there will be a dedicated run at the Υ(5 S ) resonance which enables the studyof a number of B s related observables that may be affected by physics beyond theSM. These include the semi-leptonic asymmetry and branching fraction B s → γγ . A. P´erez P´erez
Charm mixing has been established by the B factories and is parameterized by twosmall numbers: x = ∆ m D / Γ and y = ∆Γ / x = (0 . +0 . − . )%, and y = (0 . ± . D → K S h + h − decays ( h = π, K ). At large integrated luminosities one of the limiting factorsof this analysis will be the knowledge of the strong phase variation across the K S h + h − Dalitz plot. This phase can be measured using data collected at thecharm threshold, where e + e − → ψ (3770) → D ¯ D transitions result in pairs ofquantum correlated neutral D mesons. These correlated mesons can be used toprecisely determine the required map of the strong phase difference required for thecharm mixing measurements. With this input from a data sample of 500 fb − themixing measurements at SuperB will still be statistics limited, and one should beable to achieve precisions of 0 .
02% and 0 .
01% on x and y , respectively. The strongphase difference map measured at the charm threshold will also be an importantinput used for the determination of the unitarity triangle angle γ for SuperB, BelleII and LHCb.In the framework of the SM one expects very small CP-violation (CPV) on thecharm sector, so any large measured deviation from zero would be a clear sign ofNP. Just like the B u,d system, the charm sector has a unitarity triangle that needsto be tested. The physics potential of SuperB in this area has recently been outlinedin Ref. [9]. In the months following the Lomonosov conference an intriguing hintof CPV in charm decays was produced by the LHCb experiment [10]. This relatesto a difference in direct CP asymmetry parameters measured in D → KK and D → ππ . If this is a real effect one will have to perform the measurements outlinedin [9] in order to understand the underlying physics. The power of SuperB comes from its ability to study a diverse set of modes thatare sensitive to different types of NP. Through the pattern of deviations from theSM expectations for the sensitive observables one will be able to identify viableNP scenarios and reject those that are not compatible with the data. This goesbeyond the motivation of simply discovering some sign of NP and is a step towarddeveloping a detailed understanding of NP. If no significant deviations are uncoveredthen this in turn can be used to constrain parameter space and reject models thatare no longer viable. Given that many of the observables that SuperB will measureare not accessible directly at the LHC, these results will complement the direct andindirect searches being performed at LHC. Detailed discussions on the interplayproblem can be found in Refs. [1,3].
The SuperB collider exploits a novel collision scheme [6], based on very small beamdimensions and betatron function at the interaction point, on large crossing andPiwinsky angle and on the crab waist scheme. This approach allows to reach the uperB project cm − s − and at the same time overcome the difficultiesof early super e + e − collider designs, most notably very high beam currents andvery short bunch lengths. The wall-plug power and the beam-related backgroundrates in the detector are therefore kept within affordable levels [5]. The crab waisttransformation consists in moving the waist of each beam onto the axis of the otherbeam with a pair of sextupole up- and down-stream the interaction point. In thisway all particles from both beams collide in the minimum β ∗ y region, with a netluminosity gain. Moreover (and most significantly) the x/y betatron resonances arenaturally suppressed. The principle of the innovative interaction region (IR) designsketched above has been experimentally demonstrated at the Frascati DAΦNE col-lider [7]. It is very importantly that, this test also validated the simulations usedto calculate the IR optics. The SuperB design is based on recycling as much aspossible the existing PEP-II hardware, with a significant reduction of costs. Theoptimal beam energy choice for the accelerator design, is 4 .
18 GeV electron beam(polarization of 80%) and 6 .
71 GeV positron beam. The low currents, ultra-smallemittance 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.
Most of the general requirements for the SuperB detector are common to those of thepresent B factories, including large solid angle coverage, good particle identification(PID) capabilities over a wide momentum range ( π/K separation to over 4 GeV / c),measurement of the relative decay times of the B mesons, good resolution of thecharged track momentum and of the photon energy, particularly in the sub-GeVpart of the spectrum, relevant at the Υ(4 S ) environment. The SuperB detectorconcept is therefore based on the BABAR detector, with the modifications requiredto operate at a much higher luminosity (and luminosity-scaling background rates),and with a reduced center-of-mass boost [4].The BABAR detector is composed by a tracking system a five layer double-sidedsilicon strip vertex tracker (SVT) and a 40 layer drift chamber (DCH) immersed ina 1 . K L detection (IFR) realized instrumenting theiron flux return with resistive plate chambers and limited streamer tubes. SuperBis designed to reuse a number of BABAR components: the DIRC quartz bars, theCsI(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 . . z separation of the decay vertices. The ∆ t sensitivity intime-dependent measurements is maintained by improving the vertex resolution:the SuperB vertex detector replicates the five-layer BABAR SVT, but exploits thereduced dimensions of the beam pipe made possible by the ultra-low emittanceSuperB beams to add a very thin and precise measurement layer at a radius of only1 . A. P´erez P´erez as possible upgrades. The SuperB DCH concept is derived from the BABAR one,with several improvements. The hadron PID system will use the radiator quartzbars of the BABAR DIRC, read-out by fast multi-anode PMTs, and with theimaging region considerably reduced in size to improve performance and reducethe impact of backgrounds. The forward EMC will feature cerium-doped LYSOcrystals, which have a much shorter scintillation time constant, a smaller Molireradius and better radiation hardness than the current CsI(Tl) crystals, for reducedsensitivity to beam backgrounds and better position resolution. The thickness ofthe flux-return iron will be increased with an additional absorber to bring to about 7the number of interaction lengths for muons, while the gas detectors will be replacedby extruded plastic scintillator bars to cope with the expected background rates.Finally, the Collaboration is considering to improve the detector hermeticity byinserting a veto-quality lead-scintillator EMC calorimeter in the backward direction,and to add a particle identification device in front of the forward calorimeter.
The physics programme at SuperB is varied, and the unique features of the facility:polarized electron beams and a dedicated charm threshold run add to its strengthsvia versatility. The charm threshold run in particular, in addition to facilitatinga number of NP searches, will provide several measurements required to controlsystematic uncertainties for measurements of charm mixing and the unitarity tri-angle angle γ . Results from SuperB will surely play a role in elucidating any NPdiscovered at the LHC and indirectly probe to higher energy than the LHC will beable to directly access. References
1. B. Meadows et al. , [SuperB Collaboation],
The impact of SuperB on flavourphysics , [arXiv: hep-ex/1109.5028] (2011).2. D.G. Hitlin et al. , [arXiv:0810.1312.1541]; M. Bona et al. , [arXiv:0709.0451]3. B. O’Leary et et al., arXiv:1008.1541.4. E. Grauges et al. , SuperB Progress Report Detector , [arXiv:1007.4241].5. M.E. Biagini et al. , SuperB Progress Report Accelerator
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