Prospects for R(pp̄)Workshop on B → D∗τν, Nagoya

Mark Smith on behalf of the LHCb collaboration

28 March 2017

B → D∗τν, Nagoya R(pp̄) 28 March 2017 1 / 11

Introduction

Tension with SM in R(D) vs R(D∗) ∼ 4σ

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

0.5BaBar, PRL109,101802(2012)Belle, PRD92,072014(2015)

LHCb, PRL115,111803(2015)

Belle, PRD94,072007(2016)

Belle, arXiv:1612.00529

Average

SM Predictions

= 1.0 contours2χ∆

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)

HFAG

Moriond 2017

) = 67.4%2χP(

HFAGMoriond EW 2017

Your favouriteNP model here

Assuming the discrepancypersists we want to understandthe cause.

Different modes → different experimental/theoretical challenges; different physics:Pseudo-scalar and vector final states: R(D∗) and R(D).

Mass of the spectator quarks: R(Ds).

Isospin: R(Λc) and R(Λ∗c ).

Heavy quark transition: D0 → K−l ν̄l .

Orbital angular momentum: R(D∗∗) (also a major feed-down for R(D∗)).

b → u transition.B → D

∗τν, Nagoya R(pp̄) 28 March 2017 2 / 11

b → u lepton universality

B+ → τ+ντSimplest theoretically.

B0 → π+τ−ντ

Simplest theoretically with a finalstate hadron.

Already calculated.

Λ0b → pτ−ν̄τ

Λb → p FF already calculated.

Λb → Λc FF already calculated.

Experimental considerations

Need a good b decay-vertex.

Would prefer low backgrounds.

Your favouriteNP model here?

B+ → π+π−τ+ντCould go via a light resonance (ρ?).

Already attempted theoretically.

B+ → pp̄τ+ντTheoretically difficult.

B+ → N∗+p̄τ+ντTheoretically difficult.

B → D∗τν, Nagoya R(pp̄) 28 March 2017 3 / 11

b → u lepton universality

B+ → τ+ντNot at LHCb

B0 → π+τ−ντ

Very difficult at LHCb

Λ0b → pτ−ν̄τ

Maybe with hadronic τ

Might look like the B0s → τ+τ−

analysis.arXiv:1703.02508

B+ → pp̄τ+ντExperimentally preferredoption

B+ → π+π−τ+ντLots of wide overlapping resonances.

Expect large backgrounds.

B0 → ψπ+π− : Phys.Lett. B742 (2015)

) [GeV]-π+πm(0.5 1 1.5 2

Com

bina

tions

/ (18

.6 M

eV)

0

200

400

600

800

1000

1200

1400DataFitSignalBackground(770)ρ(500)0f(1270)2f(782)ω(1450)ρ(1700)ρ

LHCb

B+ → N∗+p̄τ+ντ

Lots of wide overlapping resonances.

B → D∗τν, Nagoya R(pp̄) 28 March 2017 4 / 11

R(pp̄)

R(pp̄) =B(B+ → pp̄τ+ντ )

B(B+ → pp̄µ+νµ)

Two high momenta protons:

Good B-decay vertex

Low combinatorial background.

Target flat selection efficiency.

Belle – PRD 89, 011101 (2014)

B(B− → pp̄e−νe) =(8.2+3.7

−3.2 ± 0.6)× 10−6

B(B− → pp̄µ−νµ) =

(3.1+3.1−2.4 ± 0.7)× 10−6

0

5

10

15

20

25

-1 -0.5 0 0.5 1 1.5 2 2.5 3

Missing mass squared, GeV2/c

4

(a) B-→ p p

– e

- ν–

e

Even

ts/0

.20G

eV

2/c

4

+++: Data: Fit result: Signal: Background

B → D∗τν, Nagoya R(pp̄) 28 March 2017 5 / 11

Experimental challenges

Backgrounds with extra charge tracks:

B+ → pΛ−c µ

+νµ:

Λ−c → p̄K+π−.

B → N∗pµνµN∗ → pπ

As in baryonic Vub: NaturePhys. 11 (2015) 743-747

Backgrounds with other neutral particles:B+ → pΛ−

c µ+νµ:

Λ−c → p̄K 0π0.

R

B → D∗τν, Nagoya R(pp̄) 28 March 2017 6 / 11

Theoretical challengesWe need input from theory:

R(pp̄) prediction.

Need B → pp̄ form-factors.

Need B → Λcp form-factors.

Can we help?

Expect good statistics

Should we use particular kinematicregions?

Is it better to have pp̄collinear?

Outlandish?

Λb → p was calculated for Vub.Detmold et al. Phys. Rev. D 92,034503

B → π+π− has been calculatedwith LCSR. Cheng et al.arXiv:1701.01633

0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00

q2/q2max

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

f0(Λb → p)

B → D∗τν, Nagoya R(pp̄) 28 March 2017 7 / 11

Plan

)2Corrected Mass (MeV/c2000 3000 4000 5000 6000 7000

Entri

es

0

500

1000

1500

2000

2500 νµpp→B

νµppX)→cΛ(→B

νµppX)→( N* →B

p misID→h

LHCb Simulation

Subsequently:

Measure differential BF in q2.

Attempt B+ → Λ−c pµ

+ν.

Measure R(pp̄)

Initially measure theB → pp̄µν branchingfraction.

Reasonably expect O(1000)signal events.

B → D∗τν, Nagoya R(pp̄) 28 March 2017 8 / 11

Plan

We should have the statistics to measure differential branching fractions →extract form-factors.

Neutrino 4-momentum not fullyreconstructible:

Can calculate q2 with 2-foldambiguity.

Which solution to pick?

Help from a multivariatealgorithm? Ciezarek et al. J.High Energ. Phys. (2017):21

Flight distance and B angleare correlated tomomentum.

B momentum estimatehelps to pick the correct q2

solution. [%]2/q2q∆100− 50− 0 50 100 150 200

Nor

mal

ised

ent

ries

3−10

2−10

1−10Regression

Random

B → D∗τν, Nagoya R(pp̄) 28 March 2017 9 / 11

Plan

Measure R(pp):

Exploit kinematic differencesbetween τ and µ modes.

3-dimensional template fit to q2,m2

miss and E∗µ .

Avoid warping kinematicdistributions.

B → pp̄µν B → pp̄τν

4 2 0 2 4 6 8

m 2miss [GeV2 ]

0

2

4

6

8

10

q2 [

GeV

2]

LHCb Simulation

100

101

102

4 2 0 2 4 6 8

m 2miss [GeV2 ]

0

2

4

6

8

10

q2 [

GeV

2]

LHCb Simulation

100

101

102

B → D∗τν, Nagoya R(pp̄) 28 March 2017 10 / 11

Conclusions

A measurement of B(B → pp̄µνµ) is underway at LHCb.

The intention is to make a measurement of R(pp̄). Theory inputwould be appreciated.

Alternative b → u lepton universality measurements are possiblebut are experimentally more challenging.

Thank you

B → D∗τν, Nagoya R(pp̄) 28 March 2017 11 / 11

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