Monday, September 9, 2019

B Meson Decay Anomalies Getting Smaller With More Data

A global fit to bcτν¯ anomalies after Moriond 2019

At Moriond 2019, Belle collaboration announced their new measurements on RD and RD which are in agreement with their Standard Model (SM) predictions within 1.2σ. After inclusion of these measurements, the discrepancy between the world averages and the SM predictions of RD-RD comes down from 4.1σ to 3.1σ. Here we do a global fit by taking all measurements in bcτν¯ transition. We find that there are seven allowed new physics solutions, each with different Lorentz structure. Further we show that it is possible to distinguish between them by means of the five observables: the τ polarization fraction in the decay BDτν¯, the precision measurement of RD, the forward-backward asymmetries in the decays B(D,D)τν¯ and the branching ratio of Bcτν¯.
Comments:5 pages, Based on arXiv:1903.10486, Contribution to proceedings of EPS-HEP 2019, July 10-17, 2019, Ghent, Belgium
Subjects:High Energy Physics - Phenomenology (hep-ph)
Cite as:arXiv:1909.02840 [hep-ph]
(or arXiv:1909.02840v1 [hep-ph] for this version)

Submission history

From: Suman Kumbhakar [view email]
[v1] Fri, 6 Sep 2019 12:02:50 UTC (23 KB)
The abstract in the pre-print version of the earlier paper (1903.10486) upon which this one is based also makes a notable observation missing in the current abstract:
We find that a large number of previously allowed solutions become equivalent to the solution where the new physics operator has (V − A) form. 
"(V-A) form" means a key term in which a vector current is subtracted from an axial current.

Looking more and more like a fluke, although the authors argue that the evidence still points to new physics. The introduction to the earlier article is informative:
The flavor ratios RD(∗) = Γ(B → D(∗) τ ν¯)/Γ(B → D(∗) e/µ ν¯) were measured by BaBar, Belle and LHCb collaborations. The average values of these measurements differ from their respective Standard Model (SM) predictions by 3.9σ. In all these measurements, the τ lepton was not reconstructed but was identified through other kinematical information. LHCb collaboration attempted to reconstruct the τ lepton through its 3π decay mode, in making a seperate measurement of RD∗. Post this measurement, the discrepancy of RD-RD∗ data with SM predictions increased to 4.1σ. The observed values of RD and RD∗ are noticeably higher than their respective SM predictions in all these measurements. These measurements indicate the violation of lepton flavor universality. The higher values of RD and RD∗ are assumed to occur due to new physics (NP) contribution to the b → c τ ν¯ decay. New physics in b → c {e/µ} ν¯ is ruled out by other data. LHCb collaboration also measured the related flavor ratio RJ/ψ = Γ(Bc → J/ψ τ ν¯)/Γ(Bc → J/ψ µ ν¯) and found it to be 1.7σ higher than the SM prediction. In the SM, the charged current transition b → c τ ν¯ occurs at tree level. To account for the measured higher values of flavor ratios, the NP amplitudes are expected to be about 10% of the SM amplitude. The complete list of effective operators leading to b → c τ ν¯ decay are listed in ref. [13]. These operators can be classified by their Lorentz structure. Different Lorentz structures contribute differently to the flavor ratios. The coefficients of these operators are determined by fitting the theoretical predictions to the data. The purely leptonic decay Bc → τ ν¯ is also driven by these operators. This decay mode has not been observed yet but the total decay width of Bc meson has been measured. 
In the SM, the branching ratio for this mode is small because of helicity suppression. The constraint that the branching ratio for Bc → τ ν¯ should be less than 1 leads to useful constraints on a class of NP operators. In addition to the branching ratios, it is possible to measure various other quantities in B → D∗ τν¯ decay. The polarization fractions of the τ lepton (P D∗ τ ) and the D∗ meson (f D∗ L ) are two such quantities which can be measured even without the reconstruction of τ lepton. These observables can lead to discrimination between different NP operators. If the τ lepton is reconstructed and its momentum determined then it is possible to measure two more angular observables, the forward-backward asymmetry AD∗ F B and longitudinal-transverse asymmetry AD∗ LT. If these asymmetries are measured then it can lead to further discrimination between NP operators. 
The new physics can be parametrized in terms of five different operators Oi , with different Lorentz structures. They are 
OVL = (¯cγµPLb)(¯τ γµPLν) , OVR = (¯cγµPRb)(¯τ γµPLν) ,
OSL = (¯cPLb)(¯τPLν), OSR = (¯cPRb)(¯τPLν), OT = (¯cσµνPLb)(¯τσµνPLν). (1) 
In writing the above operators, we assumed that the neutrino is purely a left chiral fermion. These operators appear in the effective Hamiltonian with coefficients C˜ i , where we assume C˜ i are real. First we consider the effect of each individual Oi on RD-RD∗ anomaly. 
• The operator OVL has the same Lorentz structure as the SM operator. This amplitude adds to SM amplitude and hence RD and RD∗ become proportional to (1 + C˜ VL )^2 . A fit to data gives a solution for C˜ VL because the fractional increase in RD and RD∗ are roughly the same. 
• If the NP operator is OVR, RD is proportional to (1 + C˜ VR )^2 where as RD∗ depends to a large extent on (1 − C˜ VR )^2 . Given the data, it is not possible to find a common solution to both RD and RD∗ . 
• The operators OSL and OSR contain the pseudoscalar bilinear ¯cγ5b. Hence the amplitudes due to these operators are not subject to helicity suppression. These amplitudes predict large branching ratios for Bc → τ ν¯. Therefore, the constraint on this branching ratio restricts the solutions given by RD-RD∗ fit. 
• For the solution with the tensor operator OT, the predicion of f D∗ L is much smaller than those of other solutions. Hence an accurate measurement of this polarization fraction can distinguish this solution from others. 
Last year, Belle collaboration announced the first measurement of f D∗ L. This year, at Moriond, Belle collaboration announced a new measurement of RD and RD∗. A novel feature of this measurement is the tagging of the τ lepton in the B → D/D∗ τ ν¯ decays. This latest measurement is consistent with the SM prediction. Inclusion of this measurement in computing a new world average brings down the discrepancy with SM from 4.1σ to 3.1σ. This is still a substantial discrepancy. Moreover, the central values of the new measurement are also higher than the SM predictions. This has been the feature of all RD and RD∗ measurements no matter what the discrepancy is. Given that the measured deviation from the SM prediction is always positive, it is expected that there is indeed new physics present. In this letter, we study the effect of these two recent Belle measurements on various solutions to RD-RD∗ anomaly.
So, Belle's measurements are the ones reducing the size of the anomaly and reducing the number and variety of possible new physics explanations. The fact that the anomaly is restricted to b quark decays to a charm quark and a tau lepton (with the associated tau anti-neutrino), which involve comparatively small data sets, and can be tricker to measure, also casts doubt on the lepton universality violations observed being due to anything more than statistical flukes and systemic experimental errors (and/or theoretical conceptual errors). 

The across the board measurement of values in excess of the Standard Model prediction also points to comparable systemic experimental errors and/or theoretical conceptual errors as a likely source of the apparent discrepancies with the Standard Model (as opposed to new physics, as the authors suggest).

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