Today's Progress 13. Jan. 2007
p-&pi and p&Lambda/n&Lambda invariant mass spectra (1)
Method
Here, we examine p-&pi invariant mass analysis. Here, we require
1. (At least) 1 proton on PA/PDC/PB/NT. Proton energy-loss is corrected.
2. (At least) 1 charged&pi on VDC/TC system. &pi-momentum is determined by T0-TC_B TOF method as already intensively described, and the energy-loss inside the target system is not corrected.
Since we have two TC/VDC and PA/PDC/PB/NT arms, the p-&pi combination is oftenly not unique. Firstly, we examine the event set in which only 1 p and 1 &pi have been detected.
p-&pi invariant mass from 1p-1&pi events for &Lambda reconstruction
As was shown by Monte-Carlo, now &Lambda peak is very clearly seen on the p-&pi invariant mass spectrum exhibitted below. In order to avoid possible combinatorial ambiguity, we have selected 1p events only to construct the spectrum below. The peak center shows negative shift, as was already studied. The peak is not a fake one from p on TC events, as is compared below.
(Top)p&pi invariant mass distribution, for &pi on TC(black)/ p on TC(red)/ fast&pi on TC(green)/ slow&pi on TC(yellow) events. &Lambda peak is clearly seen at around 1108 MeV/c^2. (Bottom) 2D correlation between p&pi invariant mass and &pi momentum. Left/right strength of the &Lambda peak on the invariant mass spectrum originate from lower/higher momentum &pi, respectively.
In order to consider possible reduction of the combinatorial background, we study the 2D correlation between VcaVp and Minv for 1p-1&pi events. The result is shown below. It is so clear that the &Lambda component is clustered at VcaVp positive side.
2D correlation between p&pi invariant mass and VcaVp.
VcaVp-selected p&pi invariant mass spectra. On the top, the spectrum is just classified with VcaVp sign (Black:positive/Red:negative), and the classification is done more finely on the middle. On the bottom, VcaVp distribution is classified and compared with respect to invariant mass value. The black is with no mass selection, the red is with Minv within (1100.,1116.), and the green is with .NOT.(1100.,1116.). As discussed, the strength below 0. of the red spectrum is almost fake(S:N ~ 1:4).
&Lambda momentum spectrum is shown below.
&Lambda momentum spectrum , on which &Lambda is identified by Minv within (1110.,1116.). Its classification with VcaVp value is shown together.
Now, &pi-tagged proton momentum spectrum is classified by p&pi invariant mass value, and compared.
&pi-tagged proton momentum classified by p&pi invariant mass. The black one comes from &Lambda -> p&pi^- decay, while the red one comes from primary reaction if the process if non-mesonic.
p-&pi invariant mass from 2p-1&pi events and p&Lambda invariant mass study
Now, we select pp back-to-back+&pi on TC events, to construct p&Lambda invariant mass specrum.
It is obvious that the p&pi combination is not unique, then.
To solve this combinatorial ambiguity, we compare 2 VcaVp values, and the proton with larger/smaller VcaVp value is systematically selected to construct p&pi invariant mass, and compare two results. Below, they are shown and compared, and it is obvious that we should combine with a proton with LARGER VcaVp value, as easily expected from the result just developed above.
p&pi invariant mass spectra from pp+&pi onTC events. The black is that if proton with larger VcaVp is combined with &pi, and the red is when proton with smaller VcaVp is combined. We have almost no &Lambda intensity on the red.
From now, p&pi invariant mass is always taken with the proton with larger VcaVp value for pp+&pi events. Now, let we define p&Lambda events by Minv within (1100.,1120.). Then, we find 1400 p&Lambda back-to-back coincidence events in which about 200 combinatorial background is found, out of 2377 pp back-to-back+&pi on TC coincidence events, for E549 100% statistics (i.e. from run25 up to run306).
In order to study the origin of pp+&pi onTC events, proton momentum/pp invariant mass spectra from pp+&pi events are classified by the invariant mass value.
Top:comparison of proton momentum spectra bewteen &Lambda -like events (i.e. Minv is within (1100.,1120.)) and the others. Bottom:same comparison of pp invariant mass spectra. Now, it is very clear that the peak-like structure around 2060 MeV/c^2 comes from quasi-free hyperon production (i.e. 1 pion - 140 MeV/c^2 - is missing).
Now, p&Lambda correlation is studied. Essentially, we have two ways to define the p&Lambda mass -
1. p_3momentum VS &Lambda_3momentum
2. p&Lambda invariant mass vs p_3momentum
3. p&Lambda invariant mass vs &Lambda_3momentum
4. p&Lambda invariant mass vs cos(p&Lambda)
2D-plots are shown in order.
2D-correlations between various quantities from p&Lambda events. Fig. 1 Fig. 2 Fig. 3 Fig. 4
Finally, cos(p&Lambda) and cos(p&Lambda)-selected p&Lambda invariant mass plots are shown.
Top:cos(p&Lambda) distribution. Bottom:p&Lambda invariant mass selected by cos(p&Lambda) value. The black is the total spectrum, and red/green are cos(p&Lambda).ge/lt.-0.95, respectively.
p-&pi invariant mass from 1p1n-1&pi events and (&Lambda,n) invariant mass study
Now, we construct n&Lambda invariant mass spectrum from pn + &pi onTC events. The requirements are:
1. &pi on TC.
2. one proton on a arm, and one neutron with over 3 MeVee light output.
3. The &pi reach B_layer, to allow us the momentum analysis.
4. The neutron 1&beta is within (1.6,9.0).
The neutron 1/&beta spectrum under requirements 1 and 2 is shown below, with physical neutron gate - requirement 4. Now, the detected event number is 18241.
1/&beta spectrum of neutral particles under condition 1 and 2.
Then, p&pi invariant mass spectrum under requirements 1~4 and 2D-correlations between that and VcaVp/pion momentum are shown below. Note that now 13488 events are left, which is 5~6 times larger than the pp+&pi onTC case.
From now, p&pi invariant mass is always taken. Now, let we define n&Lambda events by Minv within (1100.,1120.) and VcaVp.ge.0. . The latter requirement is just taken to reduce the combinatorial background. Then, 4795 n&Lambda back-to-back events, in which about 1000 combinatorial background is found, are obtained.
Now, n&Lambda correlation is studied. Essentially, we have two different definition to define the &Lambda 4-momentum - and it is done as was done in the case of p&Lambda .
1. n_3momentum VS &Lambda_3momentum
2. n&Lambda invariant mass vs n_3momentum
3. n&Lambda invariant mass vs &Lambda_3momentum
4. n&Lambda invariant mass vs cos(n&Lambda)
2D-plots are shown in order.
2D-correlations between various quantities from n&Lambda events. Fig. 1 Fig. 2 Fig. 3 Fig. 4
cos(n&Lambda) and cos(n&Lambda)-selected n&Lambda invariant mass plots are shown now.
Top:cos(n&Lambda) distribution. Bottom:n&Lambda invariant mass selected by cos(n&Lambda) value. The black is the total spectrum, and red/green are cos(n&Lambda).ge/lt.-0.95, respectively.
Finally, p&Lambda and n&Lambda invariant mass spectra are compared with same scale.
Below, N&Lambda invariant-mass-selected &Lambda/N momentum spactra are shown.
proton/&Lambda momentum spectra selected by p&Lambda invariant mass value from p&Lambda coincidence events. Note that primary proton momentum is plotted (i.e. proton from &Lambda decay is not included) on the proton spectrum.
neutron/&Lambda momentum spectra selected by n&Lambda invariant mass value from n&Lambda coincidence events.
Total momentum VS Invariant Mass for p&Lambda/n&Lambda coincidence events.
proton+&Lambda total momentum spectra selected by p&Lambda invariant mass value (top), and 2D correlation between the invariant mass and the total momentum, from p&Lambda coincidence events (bottom).
neutron+&Lambda total momentum spectra selected by n&Lambda invariant mass value (top), and 2D correlation between the invariant mass and the total momentum, from n&Lambda coincidence events (bottom).
