Today's Progress 29. Jan. 2007

n-&pi invariant mass from 2n-1&pi events for (&Sigma^+-,n) invariant mass study

On the n-&pi invariant mass spectrum under the nn+&pi on TC coincidence condition, we expect a difficulty of the &Sigma identification due to huge combinatorial background caused by both side of neutron and &pi:

1. &pi from &Lambda decay

2. &pi from quasi-free hyperon production

3. Accidental neutron BG

4. neutron from &Sigma-&Lambda conversion / primary neutron from nonmesonic absorption

Therefore, we do not expect to find out &Sigma component very clearly at the first stage on the invariant mass spectrum.

In order to see &Sigma component clearly, the following selections are available:

selection(1):Software threshold for neutron detection. This strongly affects the constant neutron BG suppression, while it also kills the signal as well by a certain fraction. Note that this affects the resulting n&Sigma mass spectrum moderately.

selection(2):vcavn selection - since we have two back-to-back neutrons, we have two vcavn values. As was shon in &Lambda p analysis, the nucleon with larger VcaVN value is likely to originate from hyperon decay.

selection(3):fast&pi/slow&pi selection - fast&pi selection is originary introduced to tagg the &Sigma events. Hence, the selection is considered to purify the &Sigma^+- weak decay in the reaction final state.

selection(4):neutron momentum - back-to-back coincidence of low energy neutron is mainly due to QF hyperon production followed by &Sigma-&Lambda conversion, in which no &Sigma exists in the final state. Hence, we expect an enhancement of &Sigma component by cutting off low-energy neutron. However, this selection strongly deforms the resulting n&Sigma invariant mass spectrum, hence it should be considered very carefully.

From the following, we start to fix these 4 degree of freedom for the event selection for n&Sigma invariant mass investigation.

Determination of neutron detection threshold - definition of selection(1)

First of all, we just define the software threshold value for neutron detection from the BG level of the 1/&beta spectra. Below, arm-by-arm 1/&beta spectra are shown for 3, 5, 7 MeVee threshold. Here, note that nn coincidence events are defined only if both neutrons have light outputs beyond the threshold value simultaneously. Considering a larger contamination of &gamma component for high-momentum neutron region, threshold value larger than 5 MeVee may be indispensable. Physical neutron gates are now taken as 1/&beta (1.6,9.0/7.0/6.0) i.e. (105.0/135.6/158.8,752.3) for neutron momentum by MeV/c unit for 3/5/7 MeVee threshold values, respectively. Hereafter, the invariant mass analysis is examined only if two back-to-back neutrons have the momentum in the region at the same time.
Arm-by-arm 1/&beta spectra and the definition of "physical region" for nn back-to-back + &pi on TC events for 3/5/7 MeVee threshold values.

fast&pi/slow&pi selected n&pi invariant mass spectrum

Below, the n&pi invariant mass spectra are shown for 3/5/7 software threshold values under &pi/fast&pi/slow&pi selection.
n&pi invariant mass spectra. The red is taken from the combination of &pi and n with larger VcaVn value, while the black is that with smaller VcaVn value of two neutrons in coincidence Fig. 1 Fig. 2 Fig. 3.

From the plots, we find three points, namely,

1. The &Sigma signal is found only if &pi in combined with the neutron with larger VcaVn value.

2. Almost no &Sigma signal is found for slow&pi-tagged event set.

3. The S/N ratio is too poor to be adopted as it is, even under fast&pi selection and the decay neutron is selected as the one with larger VcaVn.

It is so clear that we do need additional event selection with neutron momentum even if we apply fast&pi selection, and identify the decay neutron properly as the one with larger VcaVn. In order to improve this poor S/N ratio, we cannot introduce further event selection with neutron momentum values.

neutron-momentum-selected n&pi invariant mass spectra and the definition of &Sigma

Now, we consider further event selection by detected neutron momentum to search for optimum condition for n&Sigma analysis. The n&pi invariant mass spectra from nn+&pi events, for 3(1st)/5(2nd)/7(3rd) MeVee threshold values. the sectra on the left colmun are constructed by combinning the &pi with decay-like neutron, while those on the right are by combinning that with reaction-like neutron. The Black/Red/Green color represents the spectra under the &pi/fast&pi/slow&pi event selection, respectively.
The n&pi invariant mass spectra under the condition described in the text Fig. 1 Fig. 2 Fig. 3.

Now, it is obvious that the &Sigma signal can be found only for fast&pi-selected events only if the neutron with larger vcavn is combined with &pi. Hereafter in this report, the fast&pi selection is always applied, and a neutron with larger VcaVn value is everytimes combined with &pi. Now, let me call a neutron with larger VcaVn as decay neutron, while the pother as primary neutron. Under these default selections, we search for possible optimum further more, by lowering the momentum cut-off value for the decay/reaction neutron. The candidates of the cut-off pattern are as follows:

1(black):decay neutron momentum is larger than 300 MeV/c. No requirement for the reaction neutron.

2(red):no requirement for the decay neutron. Reaction neutron momentum is larger than 300 MeV/c.

3(magenta):decay neutron momentum is larger than 350 MeV/c. No requirement for the reaction neutron.

4(sky-blue):no requirement for the decay neutron. Reaction neutron momentum is larger than 350 MeV/c.

5(green):decay neutron momentum is larger than 300 MeV/c, and reaction neutron momentum is larger than 300 MeV/c.

6(black-shaded):decay neutron momentum is larger than 350 MeV/c, and reaction neutron momentum is larger than 350 MeV/c.

We adopt cut-off pattern 5 or 6, and identify the &Sigma by invariant mass within (1150.,1225.).

The n&pi invariant mass spectra under the condition described in the text Fig. 1 Fig. 2 Fig. 3.

The final form of n&pi invariant mass spectra are show below for 3/5/7 MeVee threshold values.

n&pi invariant mass spectra under selection 5(black)/6(red) for 3/5/7 MeVee threshold values for neutron detection.

n&Sigma invariant mass spectra

Now, identified number of physical n&Sigma pair is tabulatted below. From those event sets, we obtain 6 n&Sigma invariant mass spectra.

n&Sigma^- pair number found under various requirements

deltaE threshold/momentum threshold300 MeV/c 350 MeV/c
3 MeVee 562 325
5 MeVee 351 218
7 MeVee 269 175

n&Sigma invariant mass spectra. (1st) n&Sigma invariant mass spectra under 3/5/7 MeVee neutron detection threshold for selection 5(black)/6(red). (2nd) n&Sigma invariant mass spectra for selection 5(top panel)/6(bottom panel) for 3/5/7 MeVee detection threshold value. In the 1st/2nd comparison, the lower mass component is not due to combinatorial BG, but phyical one.

Hereafter, we impose fast&pi, 5MeVee threshold and 300 MeV/c momentum cutoff for neutron detection for &Sigma identification, and show the correlation between

1. n_3-momentum VS &Sigma^-_3-momentum

2. n&Sigma invariant mass vs n_3-momentum

3. n&Sigma invariant mass vs &Sigma_3-momentum

4. n&Sigma invariant mass vs cos(n&Sigma)

5. n&Sigma invariant mass vs n&Sigma total 3-momentum

2D-correlations between various quantities from n&Sigma events Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5.

1D plots of cos(n&Sigma), cos(n&Sigma)-selected n&Sigma invariant mass, &Sigma momentum and n&Sigma total 3-momentum are shown below.

Fig. 1 Fig. 2 Fig. 3 Fig. 4

Lastly, we directly compare all p&Sigma^-/n&Sigma invariant mass spectra with openning angle selection.

p&Sigma^-/n&Sigma invariant mass spectra. On both, only the "2"-nucleon component is seen.