PQE: The search for Pentaquark partner states at Jefferson Lab Hall A, E04-012

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PQE: The search for Pentaquark partner states at Jefferson Lab Hall A, E04-012

An update to the Hall A Collaboration

Paul E. Reimer

What were we looking for?

How did we look?

What did we find?

(with help from all of my collaborators, especially Y. Qiang and O. Hanson and their talks at PANIC05 and Hadron05).

26 December 2005 Paul E. Reimer, Jefferson Laboratory Hall A Collaboration Meeting

Chiral Soliton Model

Physics Today, Sept 2003

Corners are manifestly exotic—with an unpaired antiquark!

Diakonov, Petrov and Polyakov, Z. Phys. A 359, 305 (1997)

All baryons are rotational excitations of a rigid object.

Reproduces mass splittings in lowest baryon octet and decuplet.

Apply to 3-flavor, 5-quark states. Anti-decuplet of states 1 “free” parameter—fixed by identifying the

Jp = (1/2)+ N(1710) explicitly with non-strange, non-exotic state in anti-decuplet

Predict mass splittings (equal) and widths.

M ¼ 1530 MeV < 15 MeV

PRL 91 (2003) 012002-1

SP

Rin

g-8 L

EP

S



+ partner states

E04-012 was approved to search for partner states to the + pentaquark.

Antidecuplet, non-exotic states– From Soliton Model, mass is set by

M = M+ + (1-s) £ 107 MeV/c2

– N* and 0


Isospin Partners (Capstick 2003)– Narrow width in terms of

isospin-violating strong decays– Predicts set of narrow, exotic

partners– ++

Narrow, Low mass, states of specific strangeness


Hall A Experiment E04-012 Reaction Mass Range

p(e,e0 K+)0 1550-1810 MeV/c2

p(e,e0 +)N* 1600-1830 MeV/c2

p(e,e0 K-)+

+

1500-1600 MeV/c2

Beam Energy: 5 GeV/c (Proposed 6 GeV/c)

Spectr. Angle: 6± (left and right w/septa) Spectr. Momenta: 1.8 to 2.5 GeV/c hQ2i ¼ 0.1 (GeV/c)2

In C-M K( )¼ 6± (7±) K()¼ 40 (30) msr


Kinematics

Calibration settingsKin E0 Spect. Mom. (GeV) Central Missing Mass Purpose

Left (K/) Right (e) K

1, 2 5.0 2.29 2.50 0.899 0.994 Neutron

3 5.0 2.22 2.50 1.123 1.14 1116)

4 5.0 2.10 2.00 1.523 1.585 1520)

14 5.0 0.55 1.85 2.094 2.359 RICH Eff.

++ settingsKin E0 Spect. Mom. (GeV) Central Missing Mass Purpose


8 5.0 2.10 2.00 1.523 1.585 ++

9 5.0 1.93 2.00 1.611 1.680 ++

17 5.0 2.06 2.00 1.550 1.613 ++


Kinematics0, N0 Settings

Kin E0 Spect. Mom. (GeV) Central Missing Mass Purpose


5 5.0 1.93 2.00 1.611 1.680 0, N*

6 5.0 1.93 1.93 1.648 1.718 0, N*

7 5.0 1.90 1.70 1.778 1.851 0, N*

10 5.0 1.93 1.83 1.700 1.770 0, N*

11 5.0 1.93 1.89 1.622 1.691 0, N*

12 5.0 1.93 2.02 1.600 1.669 0, N*

13 5.0 1.89 1.85 1.713 1.785 0, N*

Tasks Event identification (/K separation, random rejection) Acceptance correction between different separate

spectrometer settings Mass calibration Search for resonances (non-exotic 0, N*, and exotic ++)


PID and Coincidence System

Single Arm PID 2 Aerogel thres. Cerenkov

counters n = 1.015, 1.055 RICH n = 1.30 Single arm pion reject. 3£104

K/ ratio > 20

Coincidence Time ToF resolution,

FWHM ¼ 0.60 ns Coincidence time difference

¼ 2 ns

Reaction Vertex Z FWHM ¼ 2.5 cm 15 cm target reduces

background by factor of 2


Acceptance Correction

Missing Mass acceptance is proportional to the (diagonal) length in the 2-D momentum acceptance plot.– e + p ! e0 + K§ + X

– MX ¼ const – Ee0 – EK

p(e,e0+)Xaccidental

p(e,e0+)X


Acceptance Correction—Matching Spectrometer settings

Total and 4 of the 8 spectra, corrected for efficiencies, effective charge and acceptance


Missing Mass Calibration High resolution

missing mass = 1.5 MeV/c2

Missing Mass Uncertainty < 3 MeV/c2 (absolute)


Parameters of (1520)

M(1520) = 1519.8§ 0.6 MeV/c2

= 16.6 § 1.5 MeV/c2Measured cross section at forward angle


Within 50 MeV/c2 window, fit spectra twice

1. Linear, “background” only fit (2b)

2. Linear + resonance

Breit-Wigner (fixed width of = 1, 3, 5 MeV/c2) convoluted w/Gaussian, = 1.5 MeV/c2 detector resolution (2

b+s)

Test of significance (Where a is the integral of the diff. cross section of the hypothesized resonance)

Most significant peak, 2 ¼ -6

Resonance Search: 0

2 22

2 2

0,

0( )s b b

s b b

a

a


Peak Significance: A frequentist approach

Simulate smooth mass spectra (left) To achieve this, must consider acceptance/luminosity weight factors for

8 spectrometer settings, so randomly populate right distribution and weight events just as in analysis.


Repeat experiment 1000 timesSimulated background spectra

with actual experimental statistics—i.e. randomly populate missing mass spectra taking acceptance weights into account.

Apply peak search algorithm.

Peak Significance: A frequentist approach

2Find largest 2 improvement in each spectrumUse distribution of “greatest 2 improvement” to determine probability

such an improvement being a background fluctuation.For 0, =5 MeV, a 2 improvement of -6 corresponds to a < 55%

probability of not being a background fluctuation


Upper Limits—How small is too small to be observed (heard)?

Repeat experiment 1000 timesAdd small resonance.How large must resonance be for search

procedure to find beam at 90% CL

p(e,e0 K+)0

For 0, least restrictive upper limit at M=1.72 GeV/c2

0 90% CL upper limit:

8 to16 nb/sr for = 1 to 8 MeV/c2


N0 Upper Limits

p(e,e0 +)0

Probability of Real Peak < 50%For 0, least restrictive upper limit

at M=1.65, 1.68, 1.73, 1.86 GeV/c2

0 90% CL upper limit:

4 to 9 nb/sr for = 1 to 8 MeV/c2


++ Upper Limits

p(e,e0 K-)++

Low statistics—switch to log likelihood as estimator.

Probability of Real Peak < 80%For ++, least restrictive upper limit

at M=1.57 GeV/c2

++ 90% CL upper limit:

3 to 6 nb/sr for = 1 to 8 MeV/c2


Summary

PQE/E04-012 has completed a high resolution search for narrow partner states to the +.

No strong signal is observed for the ++, 0 or N0

All “bumps” are statistically consistent with the background.

Documents

PQE: The search for Pentaquark partner states at Jefferson Lab Hall A, E04-012