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Mo#va#on • We want to es#mate the yield of ϒ states. – Cross sec#ons, Ra#os ϒ(3S)/ϒ(1S), ϒ(2S+3S)/ϒ(1S), etc.
• ϒ(3S) signal is small! – ϒ(1S) cross sec#on BRxdσ/dy ~ 680 pb at √s=1.8 TeV – Ra#os (BR x σ(nS))/(BR x σ(1S)):
– 2011 CMS DATA:ϒ(1S) signal in peripheral bins ~ 100 counts – Expect excited states to be suppressed more than ground state: 3S signal is ~ 10 counts.
– Aim: Es#mate upper limit on signals (and ra#os) for peripheral bin.
1/31/12 Manuel Calderón de la Barca Sánchez 2
Ra#o √s=38 GeV √s=1.8 TeV
ϒ(2S)/ϒ(1S) 0.28 0.26
ϒ(3S)/ϒ(1S) 0.18 0.14
Procedure • Create a toy Monte Carlo with Upsilon signal plus background.
– Model Upsilon 1S signal with Gaussian • mean mass = 9.46 GeV/c2, width = 0.092 GeV/c2
– Model Background using erf x exponen#al • Calculate upper limit bin-‐by-‐bin
– Use Bayesian confidence interval • Guillermo is studying CLs. Expect that both approaches will give similar results.
• Check behavior: – Does upper limit increase in 1S signal region? – Does it handle properly cases when background “fluctuated down” (less background than expected)?
– Do “1-‐sigma” intervals roughly correspond to sta#s#cal error bars on background in signal-‐free region?
– Do 90% and 95% intervals give progressively larger upper limits?
1/31/12 Manuel Calderón de la Barca Sánchez 3
ϒ mass distribu#on in peripheral bin
• From Guillermo’s analysis: – Peripheral bin: • 60-‐100% centrality.
– Sta#s#cs: ~654 counts. – erf parameters: • erf mean: 8.32 GeV/c2
• erf width: 1.14 GeV/c2
– Bin width: 0.07 GeV/c2 – Range: 7 – 14 GeV/c2 – Counts in highest bin ~ 60
1/31/12 Manuel Calderón de la Barca Sánchez 4
Signal and Background Func#ons
• Signal: Gaussian • Background: erf x exponen#al
1/31/12 Manuel Calderón de la Barca Sánchez 5
Signal and Background Histograms
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• Signal: Throw ~190 counts randomly taken from signal Gaussian • Background:Throw ~570 counts randomly taken from “erf x exp”
Toy MC signal+background
• Obtain histogram of signal + background • Roughly matches Guillermo’s peripheral bin stats.
1/31/12 Manuel Calderón de la Barca Sánchez 7
Calcula#on of Upper-‐Limit • Two approaches: “Frequen#st” vs. “Bayesian”
– PDG J. Phys. G. 37 (2010) 075021, Ch. 33 • Bayesian intervals for Poisson variables.
– Coun#ng experiment. – Observe n total counts in a given bin. – Expect a total of b counts due to known background
sources (e.g. combinatorics). – Likelihood that we observe n total counts is Poisson
distributed:
• Upper limit sup at confidence level 1-‐α is obtained by:
– Equa#on for α:
– Numerical solu#on.
– Note: the “prior”, π(s), in this approach simply restricts s to posi#ve values, and is regarded as providing an interval whose frequen#st proper#es can be studied.
1/31/12 Manuel Calderón de la Barca Sánchez 8
L(n | s)=s+b( )n
n!e− s+b( )
1−α =L(n | s)π (s)ds
−∞
sup∫L(n | s)π (s)ds
−∞
∞
∫
π (s)= 0 s < 01 s ≥ 0
#$%
α = e−sup
sup +b( )mm!
m=0
n
∑
bmm!
m=0
n
∑
Digression on calcula#ng factorials • What’s wrong with the following code?
unsigned long factorial(unsigned long n) { return (n == 1 || n == 0) ? 1 : factorial(n -‐ 1) * n; }
• Nothing... but runs into machine precision issues! – Can’t do 21!, computer runs out of digits.
• S#rling’s formula for approximate factorials:
– Valid for n>10 or so. • Difference at n=20 is 0.4%
• But also runs into machine precision (nn)... – Can’t do 143!, this number is > 10245
2/1/12 Manuel Calderón de la Barca Sánchez 9
n!≈ 2πnnne−n
α(s; n, b)
• Probability: – for a given n, b, alpha should decrease monotonically with increasing s.
– for a given n, s, alpha should decrease monotonically with increasing b.
• 1-‐σ : α = 0.3173 • 90% CL: α = 0.1 • 95% CL: α = 0.05
2/1/12 Manuel Calderón de la Barca Sánchez 10
Comparison, n=5 vs n=10
• For n=b (i.e. observe exactly the expected background), at a given α, sup is higher for higher n. – Larger fluctua#ons allow for a larger signal to be buried in them.
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Upper limits
• Le{: Invariant mass distrub#on • Right: Upper limit on a possible signal above background
expecta#on.
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Upper limits: α=0.3713, 0.1, 0.05
• Upper limits should get larger with decreasing α.
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Upper limits: α=0.3713, 0.1, 0.05
• Adding upper limit on top of background expecta#on (red line) • Upper limit for α=0.3173 should be similar to 1-‐σ error bars in region where there is no
signal. • Upper limits should get larger with decreasing α. • Does not give bad results when observe less counts than expected.
– Upper limit decreases, but is always posi#ve.
2/1/12 Manuel Calderón de la Barca Sánchez 14
Conclusions • Simple approach to upper limits – Based on Poisson sta#s#cs.
• Compare to approach following Frequen#st CLs. – Pursued by Guillermo. – Includes way to treat fluctua#ng background.
• Can give a cross check between methods. • Note: – D0 CL writeup: Expect both methods to give very similar results.
2/1/12 Manuel Calderón de la Barca Sánchez 15