Status and prospects of Belle II at SuperKEKB

Ilya Komarov on behalf of Belle II ilya.komarov@desy.de

2

SuperKEKBStatus and improvements since KEKB

e+e- accelerator located in Tsukuba, Japan

Built in tunnels of KEKB, but is almost entirely new machine:

• x20 smaller beam focus at interaction region

• Doubled beam currents

This yields x40 higher peak luminosity (8x1034 cm-2s-1)

First beams in 2016 First collisions: next month

KEKB SuperKEKB

“nano-beam”

3

The Belle II detectorDesign and key performance numbers

Drift chamber Tracking

𝜎r𝜙 ~ 100 𝜇m𝜎dE/dx ~ 5%

Time of Propagation and RICH detectorsK/𝜋 separation:

𝜀~96% @ 1% fake

EM Calorimeter𝜎E/E = 2%

(for E=1GeV)

KL and Muon detector𝛥𝜙 = 𝛥𝜃 ~ 10-20 mrad (for KL)

𝜇/𝜋 separation: 𝜀~90% @ 1.5% fake

7GeV e-

4GeV e+

Vertex detector Tracking

𝜎z0 ~ 15 𝜇m(2x better than Belle)

4

Belle II schedule

Phase 1: first beams Goal: Main ring commissioning

Phase 2: first collisions Goal: Establish nano-beam scheme and reach KEKB luminosity Goal: Understand backgrounds

Phase 3 luminosity milestones: 1ab-1 after one year of data taking 5ab-1 mid 2020 50ab-1 by 2025

SuperKEKB luminosity projection

Goal of Be!e II/SuperKEKB

9 months/year 20 days/month

Inte

grat

ed lu

min

osity

(a

b-1)

Peak

lum

inos

ity

(cm

-2s-1

)

Calendar Year

SuperKEKB luminosity projection

Goal of Be!e II/SuperKEKB

9 months/year 20 days/month

Inte

grat

ed lu

min

osity

(a

b-1)

Peak

lum

inos

ity

(cm

-2s-1

)

Calendar Year

SuperKEKB luminosity projection

Goal of Be!e II/SuperKEKB

9 months/year 20 days/month

Inte

grat

ed lu

min

osity

(a

b-1)

Peak

lum

inos

ity

(cm

-2s-1

)

Calendar Year 2016 2017

SuperKEKB luminosity projection

Goal of Be!e II/SuperKEKB

9 months/year 20 days/month

Inte

grat

ed lu

min

osity

(a

b-1)

Peak

lum

inos

ity

(cm

-2s-1

)

Calendar Year

SuperKEKB luminosity projection

Goal of Be!e II/SuperKEKB

9 months/year 20 days/month

Inte

grat

ed lu

min

osity

(a

b-1)

Peak

lum

inos

ity

(cm

-2s-1

)

Calendar Year

Calendar Year

9 months/year20 days/month

Inte

grat

ed L

umin

osity

[ab-

1 ]Pe

ak L

umin

osity

[103

5 cm

-2s-

1 ]

Phase 1 w/o QCS/Belle II

Phase 2 BEAST II, no VXD

Phase 3 Physics run w/ VXD

Start of accelerator operations (this week)

Global Cosmic Run with the whole detector in operation

First Collisions (next month)

5

Belle II at Phase 2Special conditions and unique opportunities

• Belle II and SuperKEKB will soon start collecting data of the first collisions during the Phase 2 of commissioning.

• We aim to get 20-40 fb-1 of data in e+e- collisions

• Phase II special conditions:

‣ Dedicated detector that includes VXD sector and radiation monitors (BEAST II) is installed to measure radiation and backgrounds levels. Will be replaced by VXD in Phase 3.

‣ To demonstrate the nano-beam scheme, we will reach 1x1034cm-2s-1 instantaneous luminosity, 1.25% of the SuperKEKB design.

‣ Low initial luminosity allows to open up triggers for low-multiplicity events Placeholder for event display of real event

Time for physics searches going hand in hand with detector studies!

BEAST II detector after assembly on the central beam pipe

[MeV]recE0 1 2 3

310×

Entri

es /

Bin

0

2

4

6

8

310×

New Clustering, | t | < 125.0 ns

Barrel

0.39) MeV± = (2863.55 fit peakE

0.30) MeV± = (52.09 fitσ

0.01) %± = (1.82 fit peak / E fitσ

/ ndf = 35.4 / 152χ

= 1.000)ε145227 entries (

), p = 3000 MeV, BGx1.0γParticle gun (

U/O

-flow

: non

e / n

one

[MeV]recE0 1 2 3

310×

σR

/

5−05

Signal (example)

Belle II MCExample: mA = 7 GeV

6

An example of Physics at Phase IIDark photon search

Dark matter manifests itself in numerous cosmological and astrophysical observations, but yet is not discovered in laboratory environment.

ϵ

γ A′

χ1

χ2

γ

Introduction• Dark Photon A’ motivated by Dark Matter, g-2, ..

• Minimal Dark Matter model: Dark Matter particle χ and a new scalar or gauge Boson A’ as s-channel annihilation mediator (mA’ > 2mχ)

• Additional U(1)’ symmetry → Kinetic mixing* of massive Dark Photon with the SM photon

313/28

Towards First Physics: Dark Photon.

>Dark Photon motivated by dark matter, g-2 anomaly...

>Minimal dark matter model: Dark matter particle N and a new scalar or gauge boson A' as s-channel annihilation mediator (mA' > 2mN)

>Additional U(1)' symmetry ? “Kinetic Mixing”* of massive dark photon A' with the SM photon

*Holdom, Phys. Lett B166, 1986

Eγ=s−M A'

2

2√ s

13/28

Towards First Physics: Dark Photon.

>Dark Photon motivated by dark matter, g-2 anomaly...

>Minimal dark matter model: Dark matter particle N and a new scalar or gauge boson A' as s-channel annihilation mediator (mA' > 2mN)

>Additional U(1)' symmetry ? “Kinetic Mixing”* of massive dark photon A' with the SM photon

*Holdom, Phys. Lett B166, 1986

Eγ=s−M A'

2

2√ s

Beam background

Signal photonPeak at

recoil mass

• Dark sector can be connected to the SM through the Dark Photon A’: it has kinetic mixing with 𝜸 of strength 𝛆.

• One of experimental signatures for this signal is a single photon in the detector with the recoil mass peaking at the mass of A’.

• Biggest challenge: performance study of the photon detection.

• Improvement of BaBar results already at 20 fb-1!

7

An example of Physics at Phase IIDark photon search

Dark matter manifests itself in numerous cosmological and astrophysical observations, but yet is not discovered in laboratory environment.

• Dark sector can be connected to the SM through the Dark Photon A’: it has kinetic mixing with 𝜸 of strength 𝛆.

• One of experimental signatures for this signal is a single photon in the detector with the recoil mass peaking at the mass of A’.

• Biggest challenge: performance study of the photon detection.

• Improvement of BaBar results already at 20 fb-1!

(GeV)A'm2−10 1−10 1 10

ε4−10

3−10

2−10

BaBar 2017

E787, E949

NA64

σ 2±µ

(g-2)

α vs e(g-2)

(simulation)-1Expected sensitivity Belle II 20 fb

𝜺 (s

treng

th o

f kin

etic

mix

ing

with

𝜸)

8

Belle II physics programme

Belle II has a rich physics programme that is being summarised in a single document (664 pages now):

• Leptonic and semileptonic B decays

• Radiative and EWP B decays

• Precise measurements of CKM parameters

• Charm physics

• Quarkonium physics

• Tau physics

• BSM searches

Impossible to cover everything in short talk, but lets discuss some planned measurements showing key features of Belle II.

Link to the current status

𝓁

9

Belle II Key techniquesFull event interpretation

At Belle II, B-mesons are produced in pairs during decay of the 𝚼(4S).This is just above the bb̅ threshold, i.e. only the two B-mesons are produced in the collision.

If we fully reconstruct one B-meson decay than we can study final states with missing energy because the initial state is well known.

Useful variable here is missed 4-momentum:

pmiss = (pbeam - pBtag - pSignal)

Fully interpreted event at Belle II

10

Some of the Phase III key measurements

Belle II R(D) measurement strategy:

• Using the Full Event Interpretation (FEI) algorithm: reconstruct both B-mesons to missed momentum of signal candidate.

• Discriminate signal (B→D(*)𝜏𝜈)̅, normalisation (B→D(*)𝓁𝜈,̅ 𝓁=μ,𝑒) and backgrounds events in 2D fit to (mmiss2; |pl|) plane

Current World Average precision for R(D(*)) is 12%(6%). With only 5ab-1 Belle II will measure both values twice more precise according to simulations study

R(D) and R(D*) measurement

Combined R(D)/R(D*) measurement is 4𝜎 away from the SM prediction

Schematic location of signal, normalisation and background components on (mmiss2; |pl*|) plane

mmiss2

|pl*|

B→D(*)𝜏𝜈̅

B→D(*)𝓁𝜈̅

Backgrounds

R(D)0.2 0.3 0.4 0.5 0.6

R(D

*)

0.2

0.25

0.3

0.35

0.4

0.45

0.5 BaBar, PRL109,101802(2012)Belle, PRD92,072014(2015)LHCb, PRL115,111803(2015)Belle, PRD94,072007(2016)Belle, PRL118,211801(2017)LHCb, FPCP2017Average

SM Predictions

= 1.0 contours2χΔ

R(D)=0.300(8) HPQCD (2015)R(D)=0.299(11) FNAL/MILC (2015)R(D*)=0.252(3) S. Fajfer et al. (2012)

HFLAV

FPCP 2017

) = 71.6%2χP(

σ4

σ2

HFLAVFPCP 2017

Expected Belle 2 precision at 5 ab-1

11

Belle II Key techniquesTime-dependent CP violation

As with the FEI technique, measurement of the TD CPV requires reconstruction of the both mesons.

Unlike FEI, we don’t need to fully reconstruct the tag B: we only need to find its decay vertex and flavour.

𝛥t and Acp distributions for CP-odd (left) and CP-even (right) B→(cc)K0 modes[Phys. Rev. Lett. 108 171802]

Even

ts /

0.5

ps

0

50

100

150

200

250

t (ps)∆-6 -4 -2 0 2 4 6

Asy

mm

etry

-0.6-0.4-0.2

00.20.40.6

Even

ts /

0.5

ps

050

100150200250300350400

t (ps)∆-6 -4 -2 0 2 4 6

Asy

mm

etry

-0.6-0.4-0.2

00.20.40.6

—Btag

Btag

0

0

af (�t) ⌘ �B̄0!f (�t)��B0!f (�t)

�B̄0!f (�t)+�B0!f (�t) = Sf sin(�m�t) +Af cos(�m�t)

af (�t) ⌘ �B̄0!f (�t)��B0!f (�t)

�B̄0!f (�t)+�B0!f (�t) = Sf sin(�m�t) +Af cos(�m�t)

12

Some of the Phase III key measurementsTime-dependent CP violation in penguin-dominated decays

Simulation of Acp distributions for reference and signal channelsTheory gives clean constraints on 𝞓Sf = Sf - (sin 𝜙1)ccs for penguin-dominated b→qqs (q = u, d, s) processes, while the experiment is behind in precision.

B→η’K0 has among the strictest predictions here: 𝞓SQCDF = 0.01±0.01 𝞓SData = -0.05±0.06

• Belle II will test several combinations for the final states: • η’→η(𝜸𝜸)𝛑+𝛑-; η’→η(𝛑+𝛑-𝛑0)𝛑+𝛑-; η’→𝝆𝜸; • KS→𝛑+𝛑-; KS→𝛑0𝛑0; KL

• Key components of the measurement:

• Tagging: Effective tagging efficiency is ~37%

• Vertexing: signal 𝞓t resolution is 20% better than for Belle

• Sensitivity to neutrals: 23% selection efficiency for B→η’(→η(𝜸𝜸)𝛑+𝛑-)K0s final state

t (ps)Δ10− 5− 0 5 10

Asym

met

ry

0.4−

0.2−

0

0.2

0.4Belle II -1 L = 50 ab∫

(S = 0.70)S

KψJ/ (S = 0.55)

S' Kη

η’K0 η’K0

t (ps)Δ10− 5− 0 5 10

Asym

met

ry

0.4−

0.2−

0

0.2

0.4Belle II -1 L = 50 ab∫

(S = 0.70)S

KψJ/ (S = 0.55)

S' Kη

(sin 𝜙1)ccs

Sη’K0

t (ps)Δ10− 5− 0 5 10

Asym

met

ry

0.4−

0.2−

0

0.2

0.4Belle II -1 L = 50 ab∫

(S = 0.70)S

KψJ/ (S = 0.55)

S' Kη

Current World Average precision for Sη’K0 is 10%. With only 5ab-1 Belle II will measure it twice as precise according to simulation

Simulation

13

Belle II Key features

Belle II is the best laboratory to study 𝝉 physics:

• High rate of the tau: 45x109 𝝉+ 𝝉 - pairs are expected in the full dataset. σ(ee → 𝝉𝝉) ≈ 0.91nb; σ(ee → bb̅) ≈ 1.05nb

• Clean environment: exclusive production of 𝝉 pairs

in e+e- → 𝝉+ 𝝉 -

Belle II reconstruction procedure for 𝝉: • For each reconstructed 𝝉 we calculate invariant mass and

• Use event shape variables to discriminate from non-tau backgrounds (e+e- →qq̅)

• For neutrinoless 𝝉 decays (CLFV searches), the missing momentum of the tag side can also be used as a discriminating variable

𝞓E = E𝝉 - Ebeam/2CMCM

𝝉+

𝝉-

e- e+

𝝅+

𝝅+

𝝅-

�̅�𝝉

𝝂𝝉�̅�𝝁

𝝁-Tag side

Signal side

Belle II as a tau factory

R.H. Bernstein and P.S. Cooper, Phys. Rept. 532 (2013) 27 w/updates

DeeMe

COMET I

Mu2e

MEG

MEG II

Mu3e

COMET II

R.H. Bernstein and P.S. Cooper, Phys. Rept. 532 (2013) 27 w/updates

DeeMe

COMET I

Mu2e

MEG

MEG II

Mu3e

COMET II

BABAR / Belle

LFV

41

Charged lepton flavour violation: SM-free signals!

Charged-Lepton Flavour Physics

μR eR

q q

0

μ e

~ QED

Figure 2. Possible supersymmetric contributions to the transition dipole diagramsmediating the LFV processes µ ! e� (left) and µN ! eN conversion (right).

concept of fermion generations was developed. Non-discovery of µ ! e� and ⌧ ! µ� established that µand ⌧ were indeed new elementary leptons, as opposed to excited states of composite lighter leptons. Inanalogy to the GIM mechanism, the absence of µ ! e� also required to introduce the muon neutrino, priorto the ⌫µ discovery in 1962 [7], to cancel FCNC amplitudes [8].

Radiative lepton decays `1 ! `2� proceed via dimension-five left and right-handed radiative transitionamplitudes. The branching fraction can be written in the form [5]

B(`1 ! `2�) =3↵

32⇡

�|AL|

2 + |AR|2�· B(`1 ! `2⌫⌫) . (1)

For generic new physics at mass scale ⇤ one can parametrise the left and right-handed dipole amplitudesby AL = AR = 16

p2⇡2

/GF⇤2, where GF is the Fermi constant and ⇤ the scale of the LFV interaction.The upper limit of B(µ ! e�) < 1.2 · 10�11, obtained by the MEGA experiment at the Los AlamosMeson Physics Facility in 2001 [9], thus translates into the stringent bound ⇤ > 340TeV [5], which is wellbeyond the LHC reach for direct detection. Decays involving virtual photons, such as `1 ! `2`2`2 and µ–econversion, have an additional rate suppression factor ↵QED, but also probe different physics processes.

Figure 2 depicts example graphs for R-parity conserving supersymmetric contributions to the chargedLFV processes µ ! e� (left) and µN ! eN conversion (right). The predicted rates depend on the value ofthe slepton mass mixing parameter involved (cf. [10, 11] and references therein). Lepton flavour violationis also naturally present in R-parity violating models, where the strength of the effects is governed by thesize of trilinear lepton number violating couplings involving sleptons and leptons (�), and squarks, leptonsand quarks (�0) in the supersymmetric superpotential [12].

2.1 A new limit on B(µ+! e

+�) by the MEG experiment

The MEG experiment [13, 14] uses the presently most powerful quasi-continuous muon beam produced atthe PSI (Switzerland) ⇡E5 beam line. Positive 29 MeV surface muons hit with 3 ·107Hz rate a thin stoppingtarget that is surrounded by the MEG detector. The muon decay rate measured by MEG effectively has notime structure, because the 2.2 µs muon lifetime is long compared to the 50 MHz radio-frequency structureof the proton cyclotron producing the muons. MEG consists of a positron spectrometer (drift chamber)immersed in a gradient magnetic field that sweeps the produced positrons out of the interaction region,a time-of-flight counter, and a 900 litre liquid-xenon (LXe) scintillation detector outside of the magnet,measuring the photon incidence, time and energy. The solid-angle acceptance around the target is 10%.

The µ+

! e+� signal events are characterised by back-to-back, in-time monoenergetic (52.8 MeV)

positron-photon pairs. Their measured energies, polar and zenith opening angles, and time difference areused to separate them from backgrounds, which are dominated by accidental coincidence of a positron fromstandard µ

+! e

+⌫⌫ decays and a photon from radiative µ+

! e+�⌫⌫ decays, bremsstrahlung or positron

annihilation in flight. The reliance on a precise back-to-back signature invalidates the use of negativemuons, which would form muonium atoms in the target that would smear out the two-body kinematics.

3

LFV signals are expected in many BSM scenarios, such as the MSSM or as a consequence of Seesaw modelsBelle II will be able to improve current limits by a factor of 100 for !→3l and a factor of >10 for !→l #

MoriondEW, Mar 19, 2016 Experimental Summary

Charged lepton flavour violation: SM-free signals!A very active field of BSM searches

ANRV358-NS58-12 ARI 18 September 2008 23:47

1940 1960 1980 2000 2020

Year

90%

–CL

boun

d

10–14

10–12

10–10

10–8

10–6

10–4

10–2

100

μ eγμ 3eμN eNτ μγτ 3μ

Figure 1Record of selected lepton flavor violation searches.

cascade down to 1S orbitals. There, they can undergo (a) ordinary decaywith a rate of 5× 105 s− 1,(b) weak capture, µ − p νµn (which exceeds the ordinary decay rate for nuclei with Z > 6), or(c) coherent flavor changing conversion, µ − N e− N . The last of these reactions has alreadybeen significantly constrained using various targets. Indeed, the ratio of conversions to capture,

Table 1 A sample of various charged lepton flavor violating reactions

Reaction Current bound Reference Expected PossibleB (µ+ e+ γ ) < 1.2 × 10− 11 28 2 × 10− 13 2 × 10− 14

B (µ± e± e+ e− ) < 1.0 × 10− 12 37 – 10− 14

B (µ± e± γ γ ) < 7.2 × 10− 11 92 – –R (µ− Au e− Au) < 7 × 10− 13 15 – –R (µ− Al e− Al) – 10− 16 10− 18

B (τ ± µ± γ ) < 5.9 × 10− 8 Table 2 O (10− 9)B (τ ± e± γ ) < 8.5 × 10− 8 Table 2 O (10− 9)B (τ ± µ± µ+ µ− ) < 2.0 × 10− 8 Table 2 O (10− 10)B (τ ± e± e+ e− ) < 2.6 × 10− 8 Table 2 O (10− 10)Z 0 e± µ < 1.7 × 10− 6 90Z0 e± τ < 9.8 × 10− 6 90Z0 µ± τ < 1.2 × 10− 5 91K 0L e± µ < 4.7 × 10− 12 74 10− 13

D0 e± µ < 8.1 × 10− 7 78 10− 8

B 0 e± µ < 9.2 × 10− 8 79 10− 9

Data from current experimental bounds, expected improvements from existing or fundedexperiments, and possible long-term advances.

318 Marciano · Mori · Roney

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Only null observation to date

MEG

SINDRUM-II

BABAR / Belle

MEGA

Mu2e / COMET PRISM / PRIME

Super-B (50 ab–1)

Spectacular perspectives:

Mar

cian

o et

al.,

Ann

u. R

ev.

Nuc

l. Pa

rt. S

ci.5

8, 3

15 (

2008

), up

date

d

MEG II

Belle IICLEO / BaBar / Belle

14

Some of the Phase III key measurementsCharged Lepton Flavour Violation in 𝝉 decays

)γµ→τSignal region (

)2 (GeV/cinvM1.65 1.7 1.75 1.8 1.85

E (G

eV)

Δ

0.6−

0.4−

0.2−

0

0.2

0.4τν µν µBackground:

τν πBackground:

)γ (-µ +µBackground:

qBackground: q

)γµ→τSignal region (

Signal 𝝁𝜸 Belle II simulation

𝞓E = E𝝁𝜸 - Ebeam/2CMCM

Belle II expected sensitivity at 50 ab-1 is Br(𝝉→𝝁𝜸)<10-9

𝝉→𝝁𝜸 decays are prohibited in SM (Br(𝝉→𝝁𝜸)~10-40) and among the most sensitive to loop-generated CLFV.

Signal events peak in (𝞓E; M𝝁𝜸) plane.

Model SourceSUSY+GUT 10-7 PRD 66(2002)11501SUSY SO(10) 10-8 PRD 68(2003)033012SM+ heavy 10-9 PRD 66(2002)034008Non-universal Z’ 10-9 PLB 547(2002)252Little Higgs 10-10 JHEP 0705, 013 (2007)SUSY Higgs 10-10 PLB 566(2003)217SM 10-40 EPJ C8 (1999) 513

Br(𝝉→𝝁𝜸)

𝝂R

History plot of charged lepton flavour violating measurements and planned improvements

310× ππ

(1S)

ϒR 00.5

11.5

22.5

33.5

4

310× ππ

(2S)

ϒR

1−01234567

310× ππ

(3S)

ϒR

0.5−

00.5

1

1.52

2.5

(GeV)s10.6 10.65 10.7 10.75 10.8 10.85 10.9 10.95 11 11.05

’ bR

0

0.1

0.2

0.3

0.4

310× ππ

(1S)

ϒR 00.5

11.5

22.5

33.5

4

310× ππ

(2S)

ϒR

1−01234567

310× ππ

(3S)

ϒR

0.5−

00.5

1

1.52

2.5

(GeV)s10.6 10.65 10.7 10.75 10.8 10.85 10.9 10.95 11 11.05

’ bR

0

0.1

0.2

0.3

0.4

𝚼(5S) 𝚼(6S)𝚼(4S) ?

15

Some of the Phase III key measurements

Inner structure of heavy hadrons above the open flavour limit is still unclear: are there XYZ states, analogous to charmonium case?

• Cross-sections around 10.75 have different behaviour for BB, hb(nP)𝝅𝝅 and 𝚼(nS)𝝅𝝅 states

• Belle II will make precise scan of the region and decompose cross-sections to different BB states, that are predicted to have rich structure

• Scans beyond 𝚼(6S) will investigate new resonances around new thresholds

Energy scan of heavy bottomonium

Sup

erK

EK

B li

mit:

11.

24 G

eV

No other experiment, running or planned, can address the open topics in bottomonium physics

Phys. Rev. D 93, 011101 (2016)

PRL 53 9 878 (1984)

Unitarized Quark Model Predictions

16

Summary

• Next month Belle II will start collect data from the first collisions (without the vertex detector)

• The goal for this year’s data taking is to understand the machine and backgrounds, but early physics program aimed at low multiplicity physics is also planned

• By the end of the year, vertex detector will be installed and Belle II will start data taking fully operational in early 2019

• Rich physics programme with plenty of unique measurements

• Looking forward to the first results