1. Rejection Of Background For the detection of Prompt Neutrinos
Newt Ganugapati
&
Paolo Desiati

2. Why?What? and Where?
The Conventional muon and neutrino fluxes come primarily from the decay of the relatively long lived pi and K mesons but with increasing energy the probability increases that such particles interact in the atmosphere before decaying.
This implies that even small fraction of short lived particles can give dominant contribution to the high energy Neutrino flux.These short lived ‘charmed’ particles or prompt muons and neutrinos arise through semi leptonic decays of hadrons containing heavy quarks most notably charm
At about 1 TeV the contribution of prompt neutrinos taking into account the LVD bound is as high as 10%.Between 1 TeV and 100 TeV these become the biggest source of uncertainty in the atmospheric neutrino flux and would limit the search for diffuse neutrinos.

3. UNCERTAINITY IN CHARM CROSS SECTIONS
There is a huge amount of uncertainty in the models for the prediction of prompt atmospheric fluxes as can be seen.Most of these models have been generated from QCD by modeling the interactions.
Note that the crossing between conventional to prompt muon fluxes happens between 40TeV and 3 PeV.
Out of these the most widely accepted models are the RPQM and QGSM model.(LVD bound)

4. PROMPT NEUTRINOS VS PROMPT MUONS
If we calculate the fluxes of prompt neutrinos (µ and e), we obtain essentially the same values as the prompt muon flux. The reason is that both the parent and the daughter particles are massive compared to the leptons and the decay kinematics becomes blind to lepton family number or flavor hence the fluxes are essentially the same until 100TeV.(The radiative decay mode of the muon into a neutrino and preferential energy transfer)
Decay length of Primary Nucleon
Interaction length of the hadron
Energy threshold
Charm production model from primary nucleons
Independence of lepton flavour,detection depth and zenith angle

5. ICECUBE AND HADRON INTERACTIONS
IceCube measurements with enhanced sensitivity may provide outstanding information on heavy quark interactions, just by discriminating atmospheric from cosmic neutrinos, at energies above tens of TeV.
Prompt muons are much easier to detect than prompt neutrinos since the later have to produce a charged particle inside the detector on the other hand we have lots of prompt muons.

6. ENERGY SPECTRA
The plot shows the energy spectra of conventional muons (single, multiple) and of the charm muons (RPQM) for zenith(2) greater than 71 degrees.
Multiple muons are the dominant background above energy of 15 TeV.Multiple muons become comparable to signal events at energies greater than 100 TeV and has a softer spectrum because of interaction.
The plot corresponds to a live time of 27.5 days MC.
Signal
Multiple(BG1)
Single(BG2)
Log10(ecpd)GeV

7. MUON MULTIPLICITY AT AMANDA DEPTH

8. QUALITY CUTS (degrees) FOR ZENITH(2)>71
Good quality tracks have high
Smoothness(evenly distributed
Hits),less chi square(fits the
Hypothesis well) and a good
Direct length(not too far of from center of detector).
To start with I try
Abs(smootallphit(2))<0.26
Jkrchi(2)<7.8
Ldirb(2)>120.(L1)
Which will be refined later.Also further they look more track like than cascade(sphericity in time distribution)
Jkrchi(8)-Jkrchi(2)>0.1(L2)
FOR ZENITH(2)>71
(degrees)

9. ANGULAR RESOLUTION (degrees) (degrees) SIGNAL MULTIPLE
MEAN (P1)
RMS
MEAN (P2)
RMS
MEAN (P3)
RMS
NUMBER OF EVENTS P P P
BEF ANY CUTS
L1 CUTS
L2 CUTS
(degrees)
(degrees)

10. ENERGY ESTIMATORS Any variable that is correlated
Well with the energy of the
Muons arriving at the center of
The detector could be used as
An observable for energy.I
Propose the following estimators
1)Nhits
2)Nch
3)Latehits(described later)
Signal
Multiples
Singles
NHITS

11. NHIT CORRELATION CHART
Some important points in this plot are
NHITS 200~30TeV
NHITS 300~60TeV
NHITS VS LOG10(ECPD)GeV

12. LATERAL SEPARATION Why does this plot flatten out in the middle?
As energy is going up clearly we have multiplicty going up but more a
RADIUS(m)VS LOG10(ECPD)GeV

13. EARLYHITS DUE TO MERGING OF CHERENKOV CONES
In the vicinity of the track where the muon bundle is dense the photons appear to be coming from a planewave unlike a cherenkov cone due to the merging of the fronts.Our reconstruction reconstructs the bundle of muons coming in as a single muon with a time delay described by cherenkov cone.
The Pandal Function doesn’t like early hits(<-15ns) in the vicinity of the track and does impose a large penality.

14. EARLY HITS
If a hit is the first hit in an OM in the vicinity of the track(0-50m) and has a negative time residual(less than –15ns) and occurs with a large adc (> 3.0p.e.)(in other words implying that it hasn’t got scattered much) then it means that it is more likely to be a multiple muon event by the method described previously.I call the number of such hits per each event as “earlyhits”.
The 3.0p.e. above is the expected adc in the vicinity of the track for hits produced by unscattered photons.

15. Method of Early Hits and some Limitations
If we see early hits it means that it is more likely a multiple muon event or a low quality single muon event.
The challenges to this method would be the
Random noise hits that occur due to the radioactive decay in the OM surface that look like early hits.
The TDC gate could have a timing error associated with it and a hit could seem early.
We could get Misreconstructed Events from single muons that might seem to have early hits or a terribly misreconsructed multiple muon event that could not have any early hits because its position is completely altered.
Most importantly the string spacing in AMANDA is ~30m so if there is no adequate lateral separation (~30m) than a multiple muon event would look like single muon event for all purposes due to the limited Detector Resolution

16. NHIT DISTRIBUTION AFTER CUT ON EARLY HITS
As can be seen by making a cut that the number of early hits be equal to zero we make the nhits(energy) distribution of multiple muon softer to increase our sensitivity to signal.
Why is it that the cut works better at large energies?
More muons and large zenith angle gives adequate lateral separation(detector spacing
problem conquered)
Signal
Multiples
Singles
NHITS

17. DELANG DISTRIBUTION OF MULTIPLES PASSING CUTS
Conventional single muon continue to be an irrestible background till nhits>200 and so I will be mostly interested in the region with nhit>200,where we get the maximum sensitivity.
At Nhit>200 I find 40 multiple muon events on a signal expectation of 5.6,so I want to find why these multiple muons pass the cuts on earlyhits.
Are they Misreconstructed Terribly?
Multiples
(degrees)

18. TIGHTER CUTS
At this stage I modify the early hits parameter to take care of noise hits and tighten it to count those hits with smaller adc’s and at largerdistances(<100m) .This reduces my background events from 40 to 19.While my signal events go down from 5.6 to 5.1
Multiples
(degrees)

19. REVISIT CHI SQUARE
BACKGROUND
SIGNAL
CUT
JKRCHI(2)
JKRCHI(2)

20. FINAL NHIT DISTRIBUTION
At this stage I am left with with a background of 5 multiple muon events on a signal expectation of 4.5 with a nhit>200.
The multiple muon spectrum
is going off at the same rate as the conventional single muon spectrum and will cut the signal spectrum at around TeV.
NHITS

21. DELANG DISTRIBUTION OF MULTIPLES PASSING ALL CUTS
The 5 multiple muon events passing the final cuts look reasonably well reconstructed as at least three of these look to be well with in the angular resolution we can get.
Why do these events still pass the cuts?
lateral separation,less than
the string spacing and the string spacing cannot resolve these muons.
(degrees)

22. LATERAL SEPARATION OF MULTIPLES PASSING ALL CUTS
The three muons shown in the previous slide agree with our intuition that they are not well separated laterally.
What abt the other two?
Since these look to be coming from a zenith angle of around 60 degrees it is possible that they escape the cuts in spite of good lateral separation.
(Vertical muons of same energy are known to give lesser number of hits than horizontal muons of the same energy).
These might have just escaped the time delay cut of –15ns.
RADIUS(m)

23. TIME DELAY DISTRIBUTION
This plot agrees with our intuition,it should be noticed that the two muons described above have early hits at –14.8ns that escaped the cut very narrowly(-15ns).It is also of some interest to see that there are many early hits.
TIME DELAY(NS)

24. TRUE ZENITH ANGLE OF MULTIPLES PASSING ALL CUTS
All the five muons that passed the cuts are from zenith angles less than 74 degrees,so may be by going to a more horizontal events we can get rid of the background better.(The flat spectrum of signal ensures that there are signal events left).
This will however require high angular resolution at large zenith angles which will be very difficult to attain.
TRUE ZENITH(degrees)

25. ZENITH DISTRIBUTION
The plot shows the zenith angle distributions after all the cuts for signal and B.G. events.There is no energy cut however.It can be seen why cutting on zenith also serves as an energy cut.
SIGNAL
MULTIPLE
SINGLE
ZENITH(2)

26. OPTIMIZE ZENITH BIN SIGNAL MULTIPLE SINGLE NHIT>150 ZENITH(2)
MRF VS ZENITH(2)

27. DATA DESCRIPTION AT L1
After the first level quality cuts the data is seen to agree nicely with the MC,note that they are going down with the same slope.
DATA
BG MC
SIGNAL MC
NHITS

28. DATA DESCRIPTION AT L2
After the second level quality cuts the data is seen to agree nicely with the MC,except that there is a small systematic shift.
DATA
BG MC
SIGNAL MC
NHITS

29. DATA DESCRIPTION AT L3
At this stage I have some unsystematic shifts due to a discrepancy in the description of time delays by the MC and the data
DATA
BG MC
SIGNAL MC
NHITS

30. TIME DELAY DISTRIBUTIONSMC
DATA
DATA MC
TIME DELAY(NS)
TIME DELAY(NS)