# Rejecting background in $B$-meson decay

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I want to reconstruct the $B$ mass from the decay $$B^0 \rightarrow K^{0*} \gamma \quad\text{ where }\quad K^{0*} \rightarrow K^{+} \pi^{-}$$ and the equivalent antiparticle decay. A key element in the reconstruction is to detect the relevant photon $\gamma$.

Unfortunately there are a lot of photons whizzing around and there is a particular decay that seems to be contaminating our data: $$B^0 \rightarrow K^{0*}\pi^0 \quad\text{ where }\quad \pi^0 \rightarrow \gamma \gamma$$ One of these two photons misses the detector, and the other (detected) photon, together with the $K^{0*}$, is recorded as a $B^0 \rightarrow K^{0*} \gamma$ decay. But this will lead to a wrong mass reconstruction for the $B$ mass because of the energy carried away by the missed photon.

How can I discard the photons from the pion decay background?

My initial approach was: in the rest frame of the $B$, there is a 2 body decay from rest which means that $E_{K^{0*}}$ and $E_{\pi}$ ($E_{\gamma}$) are fixed. Conservation of energy and momentum lead to $$E_{K^{0*}} = \frac{1}{2}\frac{m_B^2-m^2+m_{K^{*}}}{m_B}c^2$$ where $m$ is $m_{\pi}$ in the case of $B^0 \rightarrow K^{0*}\pi^0$, or $0$ ($m_{\gamma}$) for $B^0 \rightarrow K^{0*}\gamma$ .

Starting from the $E_{K^{0*}}$ in the lab frame and transforming it into the $B$ frame (feasible), I could check whether this is equal to the above formula with $m = m_{\pi}$ or $0$.

But the calorimeter resolution (ECAL) in most of the CERN experiments is about $\sim 100 MeV$ so it wouldn't be able to distinguish between a $135 MeV/c^2$ pion and a massless photon. I guess I could impose a cut to disregard all events with reconstructed $m>m_{\pi}$? Any ideas?

This post imported from StackExchange Physics at 2015-02-11 11:55 (UTC), posted by SE-user SuperCiocia
retagged Feb 11, 2015
Why are you not satisfied with a monte carlo background? If you plot K0gamma invariant mass the contamination channel gamma within the peak, should be estimated from mc easily

This post imported from StackExchange Physics at 2015-02-11 11:55 (UTC), posted by SE-user anna v
cont: same for angular distributions

This post imported from StackExchange Physics at 2015-02-11 11:55 (UTC), posted by SE-user anna v
i agree with @annav, the branching ratios for the two decays are the same orders of magnitude, but for the background a photon must escape the detector, so it's probably much smaller than the signal? why can't you reconstruct the $B$ mass? Are you going to very high precision?

This post imported from StackExchange Physics at 2015-02-11 11:55 (UTC), posted by SE-user innisfree

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