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

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.

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)

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

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.

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

MUON MULTIPLICITY AT AMANDA DEPTH

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)

ANGULAR RESOLUTION (degrees) (degrees) SIGNAL MULTIPLE MEAN 29.60 34.40 (P1) RMS 42.10 41.99 MEAN 4.06 6.74 (P2) RMS 3.91 7.27 MEAN 4.48 6.18 (P3) RMS 3.58 5.52 NUMBER OF EVENTS P1 15665 584986 P2 4990 104521 P3 3643 76395 BEF ANY CUTS L1 CUTS L2 CUTS (degrees) (degrees)

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

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

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

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.

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.

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

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

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)

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)

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

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 15-20 TeV. NHITS

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)

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)

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)

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)

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)

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

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

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

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

TIME DELAY DISTRIBUTIONS MC DATA DATA MC TIME DELAY(NS) TIME DELAY(NS)