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