# Rejecting background in $B$-meson decay

+ 4 like - 0 dislike
91 views

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

 Please use answers only to (at least partly) answer questions. To comment, discuss, or ask for clarification, leave a comment instead. To mask links under text, please type your text, highlight it, and click the "link" button. You can then enter your link URL. Please consult the FAQ for as to how to format your post. This is the answer box; if you want to write a comment instead, please use the 'add comment' button. Live preview (may slow down editor)   Preview Your name to display (optional): Email me at this address if my answer is selected or commented on: Privacy: Your email address will only be used for sending these notifications. Anti-spam verification: If you are a human please identify the position of the character covered by the symbol $\varnothing$ in the following word:p$\hbar$ysicsOverfl$\varnothing$wThen drag the red bullet below over the corresponding character of our banner. When you drop it there, the bullet changes to green (on slow internet connections after a few seconds). To avoid this verification in future, please log in or register.