Measurement of beauty
particles’ lifetimes at LHCb
Intern project proposal
Diego Tonelli (CERN)
Jan 29, 2013
1
Why
standard model of particle physics
Why is the universe made of matter
(and not antimatter)?
Why such matter only accounts for
about 5% of what we see?
[put your favorite question here]
....
Strong prejudice that there must be New Physics
2
How
Brute force
Smash particles as hard as
possible and seek new
particles in the debris
(ATLAS, CMS)
(Try to) outsmart nature
Look for tiny modifications in known
particle decays as clues of the
presence of new particles (LHCb,
NA62, many others...)
3
Large Hadron Collider beauty
40 million collision per second -- for 2 years
Few % of collisions yield pairs of “beauty” particles.
Beauty fly a few inches, then spontaneously disintegrate
(decay) into lighter daughters that LHCb “photograph”
Measuring the “lifetime” yields crucial information for the search
for BSM physics.
4
Events / 1.7 MeV
Lifetimes
16000
14000
+
µ12000
B"
sig. component
bkg. component
"pB"
µ-
10000
8000
6000
dB"
K+
4000
K-
2000
Measure the distance btw
the production point and the
decay point.
Measure the B momentum
as sum of the daughters’
momenta
Repeat for all decays
0
5150
Events / 0.14 ps
collision
LHCb preliminary
s = 7 TeV, L ≈ 1.03 fb-1
data
5200
5250
5300
LHCb preliminary
s = 7 TeV, L ≈ 1.03 fb-1
104
5350
5400
B+ mass [MeV]
data
sig. component
bkg. component
complete pdf
103
102
10
1
2
Determine the mean
lifetime through a fit
5
4
6
8
10
12
14
Proper time t [ps]
Figure 84: Fit projections of the B + mass (
the cF it method.
Lifetimes - a new idea
Use decays that involve invisible daughters
?
B"
collision
"pB"
dB"
Exploit a much larger number of decays ==> better
measurement
But, cannot measure lifetime because don’t know
the momentum of invisible daughter(s)...
6
Lifetime ratios
Measure the ratio of the observed lifetimes between
similar decay channels into invisible daughters
Bs → Ds(→KKπ) μ ν
B → D(→KKπ) μ ν
Bs
B
In the ratio, the effect of the undetected momentum
approximately cancels.
Event samples of O(1M) decays suggest 0.1%
resolution ==> world’s best
results
7
Take home message
This is an analysis project on real LHCb data
You will work daily with me and another coworker. Small
team, full control of all aspects of the project.
Not a side-project, it’s the real deal: you’ll show the status
at internal LHCb meetings and document it in an internal
note (and a physics paper - if we are successful)
You (probably) won’t find new physics nor get invited to a
party in Stockholm :-(
You’ll gain a strong background in basic data analysis:
make statistical distributions, fit them, study
backgrounds, determine systematic uncertainties, ...
Most importantly: we’ll have fun!
8
the end
9
Efficiency
Large Hadron Collider beauty
1.4
1.2
Δ LL(K - π) > 0
LHCb
s = 7 TeV Data
Δ LL(K - π) > 5
1
0.8
K→K
0.6
0.4
0.2
0
π→K
20
40
60
80
×103
100
Momentum (MeV/c)
Kaon from pions well
Kaon identification efficiency
and pion misidentification rate meas
distinguished
Figure 16:
a function of track momentum. Two different ∆logL(K − π) requirements hav
on the samples, resulting in the open and filled marker distributions, respectiv
Momentum
known within
≈0.5%
Efficiency
Transverse
decay position
known within
10-20 microns,
corresponding
to 0.1τ
1.4
1.2
s = 7 TeV Monte Carlo
1
0.8
0.6
10
LHCb
0.4
K→K
Δ LL(K - π) > 0
Δ LL(K - π) > 5