0

Cooler CSB

Direct or Extra Photons in d+d0

Andrew Bacherfor the CSB Cooler Collaboration

ECT Trento, June 2005

Outline of Talk

• Motivation and Overview• Near Threshold Considerations• Models of Continuum Processes• Results of Simulations• What Happens at Higher Energies?• Conclusion

Motivation and Overview To investigate the nature of the underlying continuum in our near-threshold measurements of dd

First, I will review why we think the events arise from d + d physical processes.

(instead of from accidental background processes)

Next, I will describe several models for these physical processes that might contribute to a continuum of events in the vicinity of the peak in the missing-mass distribution.

Near Threshold Considerations

• Magnetic channel and 4He parameters

• Pb Glass Arrays and parameters

• Results at 228.5 MeV and 231.8 MeV

How our apparatus is optimized for near-threshold measurements.

Target

D2 jet

Pb-glass array

256 detectorsfrom IUCF andANL (Spinka) +scintillators forcosmic trigger

228.5 or 231.8 MeVdeuteron beam

Separation Magnet

removes 4He at 12.5from beam at 6

20 Septum Magnet

FocussingQuads

MWPCs

Scintillator

E-1

Scintillators

E-2EVeto-1Veto-2

MWPC

COOLER-CSB MAGNETIC CHANNEL and Pb-GLASS ARRAYS•separate all 4He for total cross section measurement•determine 4He 4-momentum (using TOF and position)•detect one or both decay ’s from 0 in Pb-glass array

SINGLE AND DOUBLE GAMMA SIGNALSdata for all of July run

corr

ecte

d

time

cluster energy

A single may be difficult to extract.

But select on thesimilar locus on theother side of thebeam, and thesignal becomes clean.

Beam left-side array

Many ’s come frombeam halo hittingdownstream septum.

List of requirements: > correct PID position in channel scintillator energy > correct range of TOF values > correct Pb-glass cluster energies and corrected times

We will require two ’s.

keep above here

average

0 0.1 0.20

50

100

η = pπ/mπ

σTOT/η

RESULTS

231.8 MeV50 events

σTOT = 15.1 ± 3.1 pb

228.5 MeV66 events

σTOT = 12.7 ± 2.2 pb

missing mass (MeV)

Events in these spectra must satisfy: correct pulse height in channel scintillators usable wire chamber signals good Pb-glass pulse height and timing

Background shape based on calculateddouble radiative capture, corrected byempirical channel acceptance using 4He.

Cross sections are consistentwith S-wave pion production.

Systematic errors are6.6% in normalization.

Peaks give the correctπ0 mass with 60 keVerror.

Models for Continuum Processes via double radiative capture (“Gardestig model” where each n-p pair in the beam and target initiates an npd reaction and the two ds coalesce.)

via s-wave phase space (“Phase space model” where the matrix element is independent of energy and the directions of final state particles are uncorrelated.)

via a CS allowed process (We need to discuss the nature of this CS-allowed process and the effort required to estimate its magnitude.)

We have used Monte Carlo simulations based on the same GEANT model employed in the analysis of d + d 4He +

missing mass (MeV)

Cou

nts/

(0.1

MeV

)

missing mass (MeV)

Simulations for the Double Radiative Capture Model

Effi

cien

cy (%

)

Ed = 231.8 MeV

Calculated Efficiencies

Missing Mass Distributions of Events

Channel Efficiency

Gamma Efficiency

Starting Distribution

Events at End of Channel

Events with a two gamma condition

Simulations for the Phase Space Model

missing mass (MeV)

missing mass (MeV)

Effi

cien

cy (%

)

Cou

nts/

(0.1

MeV

)Ed = 231.8 MeV

Calculated Efficiencies

Missing Mass Distributions of Events

Channel Efficiency

Gamma Efficiency

Starting Distribution

Events at End of Channel

Events with a two gamma condition

Comparison of Radiative Capture and Phase Space

Starting Distributions Events thru Channel with 2 gammas

missing mass (MeV) missing mass (MeV)

Cou

nts/

(0.1

M

eV)

Cou

nts/

(0.1

MeV

)

Ed = 231.8 MeV

Phase Space

Gardestig

Phase Space

Gardestig

Result of Comparison: In our near-threshold measurements of d+d 4He + ,our efficiency for the extraction of events in the underlying continuum is independent of the starting distribution.

What Happens at Higher Energies?

• To determine how the cross-section ratio CSB/continuum varies with energy, we need to consider how the cross section for each process scales with energy.

In going from an energy near threshold, 230 MeV,up to an energy of 265 MeV, the s-wave cross section is predicted to increase by a factor of 3.

• Other experimental considerations at higher energies include:

Recoil alpha particles fill a larger cone and are harder to analyze with a magnetic channel.

Gamma measurements with improved angle and energy resolution may allow reconstruction of the ° mass as a way of separating CSB from the continuum.

Motivations for work at higher energies include:

(1) measuring the strength of p-wave CSB cross sections, and

(2) determining how CSB amplitudes depend on energy.

Conclusion• In addition to the observation of the CSB reaction, d + d 4He + , the

near-threshold measurements at IUCF have identified a continuum process that is probably due to the double radiative capture mechanism, d + d 4He + + suggested by Gardestig.

• Using Monte Carlo simulations similar to those developed to model the CSB reaction, we are able to reproduce the shape of the observed continuum, but we are not able to distinguish the double radiative capture process from a pure s-wave phase space distribution.

• Features of d + d measurements at higher energies (e.g., using the WASA detector at COSY) are discussed. Since both the CSB s-wave cross section and the double radiative capture process are expected to scale as the linear power of pp, we expect the ratio of CSB/continuum processes to be about the same.

SEPARATION OF 0 AND EVENTS

MWPC1 X-position (cm)

Y-p

ositi

on (

cm)

Time of Flight (ΔE1 - ΔE2) (ns)

needed TOFresolution

GAUSS = 100 ps

MWPCspacing= 2 mm

Calculate missing mass from the four-momentum measured in the magnetic channel alone, using TOF for z-axis momentum and MWPC X and Y for transverse momentum.

[Monte Carlo simulation for illustration. Experimental errors included.] 0 peak

TOT = 10 pb

background(16 pb)

predictionfrom Gårdestig

Difference is due to acceptance of channel.Acceptance widths are: angle = 70 mr (H and V) momentum = 10%

missing mass (MeV)

Cutoff controlledby availableenergy abovethreshold..

Major physicsbackground is from double radiative capture.

COMMISSIONING THE SYSTEM using p+d 3He+π0 at 199.4 MeV3He events readily identifiedby channel scintillators.

Recoil cone on first MWPC

Channel time of flight

Construction of missingmass from TOF andposition on MWPC.

FWHM = 240 keV

130 134 138

Pb-glass energy sumsnearest neighbors.

Response matched toGEANT model. Efficiency(~ 1/3) known to 3%.

data

Monte-Carlo

NOTE: Main losses inchannel from random veto,multiple scattering, andMWPC multiple hits.

It is important toidentify lossmechanisms.