Improving Reach at ~GeV (High) A’ Masses
Philip SchusterPerimeter Institute
with Natalia Toro
New Light Weakly Coupled Particle SessionCSS Conference
July, 2013
1
1Wednesday, 31 July, 13
0.001 0.01 0.1 1
10-5
10-4
10-3
10-2
mA' HGeVL
e
A' Æ Standard Model
APEXêMAMITest Runs
U70
E141
E774
am, 5s
am,±2s favored
ae
BaBar
KLOE
Orsay
2
muons+pionsacceptance
+ tolerance to high-current
Vertex precision(higher intensity?)
Directions for Improvement?
2Wednesday, 31 July, 13
0.001 0.01 0.1 1
10-5
10-4
10-3
10-2
mA' HGeVL
e
A' Æ Standard Model
APEXêMAMITest Runs
U70
E141
E774
am, 5s
am,±2s favored
ae
BaBar
KLOE
Orsay
Why does everyone lose reach at high mA′?
3
How do we improve high-mass
sensitivity?
Can we go here with fixed-target?
3Wednesday, 31 July, 13
4
This is the biggest effect, and may be avoidable w/ low-Z target
0.0 0.5 1.0 1.5 2.00.0
0.2
0.4
0.6
0.8
1.0
mA' HGeVL
Branchingratio
A' Branching Ratios to e and m
Why does everyone lose reach at high mA′?
• σ~1/mA′2 for point-like nucleus
• Falling B.R. for each species
• Acceptance cutoff
• Form factor
4Wednesday, 31 July, 13
q & qmin ⌘ m2A0
2Ebeam
Form factors at high A′ mass
5
0.01 0.02 0.05 0.1 0.2 0.5 1. 2.
0.2
0.5
1.
2.
5.
10.
20.
A' Mass HGeVL
cêZ2H"Lo
g"L
Gold Integrated Form Factors
6 GeV11.1 GeV
1.1 GeV2.2 GeV4.4 GeV
m�4A0
Aʹ′ production cross-section:[BEST]
d�
dx
⇡ 8↵3✏
2x
m
2A0
✓1 +
x
2
3(1� x)
◆Z
2Log
“Z2 Log” is ~ integral of nuclear form factor over γ momenta
Coherence lost for q > 0.4 GeVA�1/3
mA0 &p
GeV EbeamA�1/3
5Wednesday, 31 July, 13
What about Z-dependence?
• σA′ (low mass) ~Z2 Log
• σA′ (high mass) ~Z scattering off nucleons (i.e. Log ~1/Z)
• σbrem ~ Z2 Log ⇒ X0 ~ 1/Z2
6
Yield per e– per target thickness in r.l.
is independent of Z for low mA′ and ~1/Z for high mA′.
but
NA0
NeT⇠ Log(mA0)
↵
3✏
2
m
2A0
6Wednesday, 31 July, 13
What about Z-dependence?
70.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00
0.02
0.05
0.10
0.20
0.50
1.00
A' Mass HGeVL
RHGe
V2 L
R = cX0N0êA "A' production efficiency"
6.6 @11.1D GeV W6.6 @11.1D GeV Al6.6 @11.1D GeV Be
Yield = (Particle Physics Factor) x (Nuclear Physics Factor) WW effective photon cross-section ~ 1/m2
R=(eff. photon flux) x (column density)
How much does lower Z buy you?
R[anything]/R[W] for same mA′ & Ebeam
7Wednesday, 31 July, 13
8
R[X]/R[W] for same mA′, Ebeam.
At high mass:
0.01 0.02 0.05 0.10 0.20 0.50 1.00 2.00
0.5
1.0
2.0
5.0
10.0
20.0
A' Mass HGeVL
yieldHZLêyie
ldHWL
YieldHZLêYieldHWL @Dashed = 11.1 GeVD
6.6 @11.1D GeV W6.6 @11.1D GeV Al6.6 @11.1D GeV Be C or Be buys up to factor of
12 over W (~ ratio of Z’s)Al buys up to factor of 5
Low-Z better for high mass
What about Z-dependence?Yield = (Particle Physics Factor) x (Nuclear Physics Factor)
WW effective photon cross-section ~ 1/m2
R=(eff. photon flux) x (column density)
8Wednesday, 31 July, 13
• σA′ (low mass) ~Z2 Log
• σA′ (high mass) ~Z scattering off nucleons (i.e. Log ~1/Z)
• σbrem ~ Z2 Log ⇒ X0 ~ 1/Z2
9
Yield per e– per target thickness (in r.l.)NA0
NeT⇠ Log
↵
3✏
2
m
2A0
is independent of Z for low mA′, ~1/Z for high mA′.
Practical Issues with low Z?
e–/e+ singles and e+e– pair are mainly from Coulomb & trident processes with σ~Z2
⇒ yield per r.l. approximately indep. of Z
9Wednesday, 31 July, 13
• σA′ (low mass) ~Z2 Log
• σA′ (high mass) ~Z scattering off nucleons (i.e. Log ~1/Z)
• σbrem ~ Z2 Log ⇒ X0 ~ 1/Z2
• σπ ~ A ( σπ+π ~ ?? )
10
Yield per e– per target thickness (in r.l.)NA0
NeT⇠ Log
↵
3✏
2
m
2A0
is independent of Z for low mA′, ~1/Z for high mA′.
Yield of pions per r.l. ~ A/Z2 Switching to C, Be, (Al) would raise pion bkgs by factor of 10–15 (~5)
Practical Issues with low Z?
10Wednesday, 31 July, 13
Summary & Discussion• Low-Z target for 11 GeV beam (6 GeV?) may be
advantageous
• Low Z may increase statistics x 5–10 for mA′>0.5 GeV ⇒ 2–3 in α′/α
– Strategy will be limited by π backgrounds
• Need a better understanding of – pion contrib. to trigger, occupancy, etc. at 6, 11 GeV
π+e– fraction of fixed-target trigger rate?– Sources & effects of π+π– backgrounds– Gains vs. acceptance as function of mass– Engineering issues
11
11Wednesday, 31 July, 13
Top Related