Today's Progress 26. Jan. 2007

n&pi and &Sigma^- p invariant mass spectra from pn + &pi on TC events

Method

Here, we examine n&pi invariant mass analysis. Here, we require

1. (At least) 1 neutron on PA/PDC/PB/NT.

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 n&pi combination is oftenly not unique. Firstly, we examine the event set in which only 1p1n and 1 &pi have been detected and &pi from &Lambda-decay can be eliminated, to start width lower combinatorial background level.

n-&pi invariant mass from 1p1n-1&pi events for (&Sigma^-,p) invariant mass study

In this event set, the pion from &Lambda-decay events are safely eliminated from the &Sigma-candidate, and a neutron has back-to-back coincidence with relatively high-energy proton. Hence, a higer S/N ratio is expected. We apply event selection with neutron momentum and VcaVn.

n&pi invariant mass spectra under various condition Fig. 1 Fig. 2.

We define a &Sigma here, as

1. &pi is not from &Lambda decay, i.e. .NOT.(Minv(p&pi).ge.1100.AND.Minv(p&pi).le.1200.AND.Vcavp.ge.0.)

2. neutron momentum is larger than presetted threshold value.

3. VcaVn.gt.0.

4. Minv(n&pi).ge.1150. AND Minv(n&pi).le.1225.

Then, we obtain 1095 p&Sigma back-to-back coincidence events, for example 400MeV/c threshold value, out of 13488 pn+&pi events.

Definition of &Sigma, in case of &pi on TC and threshold = 400 MeV/c.

Since the neutron momentum threshold is considered to affect the resulting invariant mass spectrum strongly, we try to improve S/N ratio of the &Sigma identification by fast&pi selection on TC, and to set the neutron momentum threshold, which is not rigidly fixed at this moment, as low as possible. The invariant mass spectra under various neutron momentum threshold values and &pi/fast&pi selection are shown below.

Invariant mass spectra, under &pi(top panel)/fastt&pi(bottom panel) event selections and various neutron threshold values. S/N ratio is clearly better for fast &pi and higher threshold momentum case. The total number of p&Sigma pairs are tablulatted after the spectra.

p&Sigma^- pair number found under various requirements
neutron threshold/ selection on TC	&pi	fast &pi
200 MeV/c	2009	1621
300 MeV/c	1668	1327
400 MeV/c	1095	813

Neutron momentum spectra for &pi/fast&pi .AND.(.NOT.from &Lambda).AND.vcavn.ge.0. events are shown below.

Neutron momentum spectra from &pi(top panel)/fast&pi(bottom panel).AND.(.NOT.from &Lambda).AND.vcavn.ge.0. events. The red is the events from the n&pi invariant mass with (1150.,1225.), i.e. a spectrum from neutron produced by &Sigma decay. Note that at this moment neutron momentum threshold value for &Sigma identification is set to 0.

Proton momentum spectra from &pi(top panel)/fast&pi(bottom panel) .AND.(.NOT.from &Lambda) events. The red is the events from the n&pi invariant mass with (1150.,1225.), i.e. a spectrum from proton produced with &Sigma. Note that at this momentum neutron momentum threshold value for &Sigma identification is set to 0.

Resulting p&Sigma /pn invariant mass spectra are shown below.

Top:p&Sigma^- invariant mass spectra under various identification requirements of &Sigma^-. Bottom:pn invariant mass spectra from n&Lambda(black) and p&Sigma^- events(red). Note that now a &Sigma^- is identified under &pi on TC .AND. over 300 MeV/c neutron selection.

Hereafter, we impose fast&pi and 300 MeV/c threshold for neutron detection for &Sigma^- identification to get lower combinatorial BG, and show the correlation between

1. p_3-momentum VS &Sigma^-_3-momentum

2. p&Sigma^- invariant mass vs p_3-momentum

3. p&Sigma^- invariant mass vs &Sigma^-_3-momentum

4. p&Sigma^- invariant mass vs cos(p&Sigma^-)

5. p&Sigma^- invariant mass vs p&Sigma^- total 3-momentum

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

1D plots of cos(p&Sigma^-), cos(p&Sigma^-)-selected p&Sigma^- invariant mass, &Sigma^- momentum and p&Sigma^- total 3-momentum are shown below Fig. 1 Fig. 2 Fig. 3 Fig. 4.

Lastly, we directly compare all p&Lambda/n&Lambda/p&Sigma^- Invariant mass spectra without any openning angle selection.

p&Lambda / n&Lambda / p&Sigma^- invariant mass spectra. On p&Sigma^-, Only "2"-nucleon component is seen. p&Lambda is found to be especially peculiar now.