High p T muons and Composition

1. High pT Muons in Air Showers with IceCube Spencer Klein and Dmitry Chirkin for the IceCube Collaboration (SRKLEIN@LBL.GOV & DCHIRKIN@LBL.GOV) Cosmic Ray IceTop Air Shower Muons in IceCube Air - target h ~30 km High energy muons in air showers come from p/K and charm/bottom decays; these are the conventional and prompt components respectively. A 1 PeV proton-induced air shower will typically produce a handful of muons with Em > 500 GeV; if the core of the shower passes through IceCube, this muon bundle will be detected. Most of these muons are produced with a transverse momentum (pT ) around a few hundred MeV/c, and will pass through IceCube in a narrow (radius 10-20 m) bundle. A few of the muons will have higher pT, and so have a larger separation from the bundle core. Normally, bundles are reconstructed as a single object; at small separations, the component muons are not individually reconstructible. However, at large enough separations, individual muons will be visible. e,g IceTop Low pT m bundle High pT m from c,b, & jets A schematic of an air shower + muon event in IceCube. The IceTop surface detector measures the shower energy and direction, while IceCube reconstructs muons with Em > 500 GeV. Higher pT muons further than ~ 100 m from other muons may be reconstructed separately from the muon bundle. Muon pT Determination An event from May 23, 2007. The air shower hits 11 surface stations. A total of 96 IceCube DOMs are hit; 84 DOMs on four strings near the extrapolated air shower direction, plus 12 DOMs on another string, about 400 m from the projection. Vertical muons with energies above about 500 GeV can be detected in IceCube. The muon energy is determined from it’s specific energy loss (dE/dx), with a resolution of roughly 30% in log10(Em). At lower energies, the muon range is also useful in determining energy. About 85% of high energy (above 500 GeV) muons come from particles produced in the first interaction. Muon pT is determined by the lateral distance from the muon to the core, d where h is distance from the cosmic-ray initial interaction to the surface. For vertical showers, h ~ 30 km. A full analysis will include the event-by-event variation in h The 125 m string spacing & 30 m scattering length set the scale for IceCube two-track resolution. Most of the light from two muons 100 m apart will appear in different strings. We use 100 m for an initial estimate of two-track separation. For Em ~ 1 TeV, 100 m corresponds to pT >3 GeV/c. The upper left shows an interesting event found in a search of 4 days of IceCube 22-string data from May, 2007. High pT muons in Air Showers High pT muons and Composition High energy muons are produced when a high-x parton from an incident nucleus strikes a low-x parton in the atmosphere. High pT muons are produced by high Q2 cosmic-ray interactions. Above pT ~ 2 GeV/c, production cross-sections are calculable using perturbative QCD. This figure shows that a 1017 eV proton has a very different gluon composition than a 1017 eV A=10 nucleus; the maximum gluon energy differs by a factor of 10; quarks and anti-quarks are similarly affected. The number of high Q2 interactions, leading to high pT muons, is much higher for protons than for heavier nuclei. The MACRO collaboration measured the decoherence function (lateral spread) of muons in muon bundles. Their simulations found an almost linear relationship between muon pair separation and muon pT, rising from 400 MeV/c at 0 m separation, to 1.2 GeV/c at 50 m separation. 1/D dN/dD Separation, D (m) Muon Decoherence (separation), as measured by MACRO. Gluon density Rates High pT muons come from two sources: p/K decay, and heavy-quark (c,b) decay; we treat these two sources separately. The complete 80-string IceCube + IceTop has a geometric acceptance of 0.3 km2 sr. We use a threshold of Em > 1 TeV/c, higher than IceCube’s actual threshold, roughly compensating for the fact that not every muon comes from a shower which triggers IceTop. The prompt muon flux is calculated with pQCD + transport equations to model the shower development (Thunman, Gondolo & Ingelman and by Pasquali, Reno and Sarcevic). These calculations agree around Em ~ 1 TeV, but differ by a factor of 6 at higher energies, mostly due to different structure functions. Both predict about 600,000 m/year with Em > 1 TeV from charm. 1-2% of the muons have pT > 3 GeV/c, or There is an additional contribution from bottom quarks. The bottom quark rate is small, but they have a harder pT spectrum so may dominate at sufficiently high pT. The non-prompt muon rate is known. IceCube will see more than 100 million muons/year associated with air showers. We assume that the muon spectrum matches the p0 spectrum measured by PHENIX in 200 GeV pp collisions; 1 in 200,000 muons has pT > 3 GeV/c. We expect a few thousand muons/year with pT> 3GeV/c; these are predominantly prompt muons. Gluon Energy Gluon density as a function of energy Air Showers in IceTop IceCube observes air showers with the IceTop surface array, 80 stations, each consisting of two 1.8 m diameter tanks, spread over 1 km2 surface area. IceTop has a threshold of about 300 TeV, and reconstructs shower direction with an accuracy about 20, and core position to within ~ 13 m. Conclusions IceCube is the first detector large enough to study high pT muon production in cosmic-ray air showers. A 100 m two-tracks separation would allow the study of muons with Em ~ 1 TeV, pT > 3 GeV/c; in the complete detector, a few thousand of these muons are expected each year. By measuring the energy and core separation of muons associated with air showers, the muon pT may be inferred. The muon pT spectrum may be related to the composition of the incident cosmic rays using perturbative QCD calculations. Two DOMs read out each IceTop tank. We thank the U.S. National Science Foundation and the Department of Energy, Office of Nuclear Physics. An IceTop tank.