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Quarkonium in the ALICE Muon Spectrometer

Quarkonium in the ALICE Muon Spectrometer. E. Scomparin (INFN Torino, Italy) for the ALICE Collaboration. EMMI Workshop "Quarkonium and deconfined matter in the LHC era" . Martina Franca (Italy) June 16-18 2010. Introduction. ALICE ( A L arge I on C ollider E xperiment):

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Quarkonium in the ALICE Muon Spectrometer

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  1. Quarkonium in the ALICE Muon Spectrometer E. Scomparin (INFN Torino, Italy) for the ALICE Collaboration EMMI Workshop "Quarkonium and deconfined matter in the LHC era"  Martina Franca (Italy) June 16-18 2010

  2. Introduction • ALICE (ALarge Ion Collider Experiment): • the dedicated heavy-ion experiment at the LHC • Main focus on Pb-Pb collisions  QGP studies • at the nominal LHC luminosity, 51026 cm-2s-1 • p-p collisions are a crucial aspect of the physics program • Reference for heavy-ion collision studies • Genuine p-p physics • Maximum luminosity limited to a few 1030 cm-2s-1 • due to pile-up in TPC • Faster detectors may stand a higher luminosity Running conditions appropriate for quarkonium studies (both charmonium and bottomonium)

  3. Quarkonia in the muon channel • Quarkonia measurement via muon pair decays • Well-known (and tested) detection technique • Hadron absorber(s) to filter out muons • Muon tracking in a magnetic spectrometer • Triggering on muon (pairs) to enrich the signal • Advantages • Fast detectors can be used  work at high luminosity • Soft background can be rejected at trigger level • Drawbacks • Careful design needed to have a satisfactory mass resolution •  Possibility of separating  states Concept of a muon arm for ALICE present (almost) from the beginning (TP)

  4. ALICE muon arm - tracking • 5 stations of two Cathode Pad Chambers ~ 100 m2 • 1.1106 channels, smallest pads 4.26.3 mm2 • (<5% occupancy in PbPb) • Chamber thickness ~3% X0 • Beam test results for spatial resolution  50 m • (<100 m required) • Measurement of detectors displacement with an • accuracy <50 m (GMS) St 3,4,5: 140 slats (max size 40280 cm2) St 1,2: 16 quadrants

  5. ALICE muon arm - trigger • 4 detector planes subdivided in 2 stations (16 and 17 m from IP) • 18 RPCs per plane, read on both sides with • orthogonal strips • Each plane ~5.56.5 m2 • 21k strips (1,2,4 cm pitch) and readout • channels • Projective geometry: different strip pitch • and length on each plane

  6. Muon trigger - principle • Muon pT cut helps reducing the background from light meson decays • Two programmable pT cuts • Latency time ~800ns  used as one • of the L0 triggers • 5 trigger signals: Single , UnLike and • Like-Sign dimuon high and low pT • Max muon trigger rate ~2 kHz Trigger principle pT cut using correlation between position and angle Deflection in dipole + vertex constraint

  7. Beam shield Front absorber Muon filter Absorber(s) • Front absorber: mainly carbon (also concrete, steel) (10 I) •  limit scattering and energy loss in the muon path • Muon filter: iron (7.2 I)  remove hadronic punch-through • Beam shield (along the pipe, tungsten): protect detectors Muon momentum cut: p = 4 GeV/c

  8. Acceptance • Rapidity coverage: 2.5<y<4 • Good transverse momentum coverage: down to pT=0 ! • Effect of muon trigger pT cut not too strong

  9. Physics performance, nominal LHC running conditions • Simulations for quarkonium physics performance study are based on • CEM calculations with MRST HO PDF • mc=1.2 GeV/c2, =2mc  for J/ • mb=4.5 GeV/c2, =2mb  for  CEM predictions, with these parameters, are in agreement with Tevatron data for the , but they underestimate by a factor ~2 the J/ J/ yields from these simulations may represent a pessimistic estimate Inclusive cross section, including higher resonances feed-down • ppJ/ = 31 b • pp = 0.50 b • ppJ/ = 53.4 b • pp = 1.12 b 5.5 TeV 14 TeV • PbPbJ/ obtained assuming • - binary scaling (Glauber model) • - nuclear shadowing (using EKS98 parametrization) • y, pT differential distributions • obtained from CEM predictions and from the extrapolation of the • CDF data at √s=2TeV, respectively (ALICE-INT-2006-029, ALICE-INT-2008-016)

  10. Other dimuon sources • Background consists of • Correlated dimuons •  both muons originate from the same heavy quark pair • Uncorrelated dimuons • combination of decay muons from uncorrelated sources • Muons from  and K decay (uncorrelated bck)  simulation based on HIJING assuming a pessimistic estimate of dNch/d|=0 ~ 8000 • muons produced after a first hadronic interaction in the absorber (secondary , K decays) <10% (after pT and vertex cut) From CEM  and PYTHIA simulation (tuned to reproduce NLO pQCD)

  11. Pb-Pb collisions, nominal LHC energy • Expected yields for the yearly ALICE Pb-Pb data taking period Time = 106 s L = 5 10 26 cm-2s-1 • Number of expected events (integrated over centrality) assuming no medium effects apart from shadowing and no enhancement in the quarkonium production due to statistical hadronization or cc recombination • With this statistics we can study • centrality and pT-dependence of J/ and  yields • (2S) more difficult  low significance • A measurement of J/ elliptic flow can also be carried out

  12. Pb-Pb collisions, mass spectrum J/ region strong background centrality dependence (uncorrelated bck dominates)  region weaker background centrality dependence (correlated bck dominates) central Mass resolution: J/~ 70MeV  ~ 100 MeV the  states can be clearly separated peripheral Uncorrelated background to be subtracted through event mixing techniques

  13. First heavy-ion run, end 2010 • Future difficult to predict, but for the moment 4 weeks of Pb beam • at √s=2.76 TeV/nucleon are foreseen, with maximum luminosity • Lmax = 5  1025 cm−2 s−1 Baseline scenario 1.2  106 s data taking (12 hours  28 days, L=Lmax) Lint = 6  10-2 nb-1 NJ/ ~ 8.5  104, N(1S) ~ 8.5  102 (Slightly more) pessimistic scenario 5  105 s data taking (6 hours  22 days, L=0.02Lmax) NJ/ ~ 7  102, N(1S) ~ 0 Lint = 5  10-4 nb-1 Could be enough to distinguish suppression vs enhancement scenarios ?! Warning: RAA estimate would profit a lot from a (long enough) pp run at 2.76 GeV

  14. p-p collisions, nominal LHC energy • Expected yields for the yearly ALICE Pb-Pb data taking period Time = 107 s L = 3 10 30 cm-2s-1 • It will be possible to study J/pT distribution with • reasonable statistics up to (at least) 20 GeV/c • (and down to pT= 0!) • The good  statistics will allow a study of its • differential distributions

  15. p-p collisions, mass spectrum MC, 107 srunning time, L=31030 • Contrary to Pb-Pb , the continuum is dominated by correlated • background (due to the low hadron multiplicity, the uncorrelated • contribution is small)

  16. Forward-y physics • Gluon PDF distributions have • large uncertainties at very low x, • since they rely on extrapolations • (no data available in this region) • LO CEM calculations show that • the shape of the quarkonium • rapidity distribution is strictly • related to the PDF. Since the • region 2.5<y<4 corresponds to • x < 10-5 •  it will be possible to put • constraints on the gluon PDF • at low x

  17. (J/ bck subtr) (J/ + bck)  = 0 Other physics topics: polarization J/ • Bias on the evaluation of the • J/ polarization due to the • background is not very large • (as expected) • With 200K J/, the error on • J/ is < 0.02  • With the  statistics collected in • one year we can evaluate the • polarization with a statistical error • between 0.05 – 0.11 • Statistical errors, for the pT • dependence of the polarization, vary • between 0.03 -0. 2 • ALICE expected statistics in 1 year ~ 3 times  CDF statistics (Run I, 3 yr)

  18. First LHC p-p run • First pp run at 7 TeV currently ongoing • Luminosity is increasing step by step • Depending on the maximum luminosity chosen for ALICE, • and assuming, tentatively, LHC=0.12 • L= 3 1029 cm-2s-1 (beginning)  104 J/ month-1 • L= 3 1030 cm-2s-1  105 J/ month-1 • Expected statistics at the end of 2011 similar to that expected • for a 1-year run at top LHC energy • See later for the statistics cumulated up to now....

  19. p-A collisions, too.... • They are in the program, but not • as first priority.... • Very important for our understanding • of A-A results, seen the large • uncertainties on shadowing Eskola et al., JHEP 0904:065 (2009) • p-Pb collisions, LHC single magnet ring with two beam apertures • imposes for p-Pb √s=8.8 TeV for p-Pb y=0.47 • Extrapolations needed when comparing p-p/p-Pb/Pb-Pb

  20. R. Vogt, PRC81(2010)044903 Shadowing and CGC in pA Large uncertainties on the ratio depending on the chosen PDF set J/ Inclusion of CGC-related effects gives systematically lower ratios at all y and a steeper variation of Rp-Pb as a function of pT (again with large uncertainties) EKS98 A measurement is mandatory CGC, kT kick power CGC, kT kick gauss.

  21. Moving from first signals..... • Spectrometer installed in 2007 and then commissioned step by • step during the 2008 cosmic run From the first cosmic muon....

  22. ..towards real data ..to the first muon pair in 900 GeV pp collisions Not yet a J/, anyway

  23. Trigger is alive... • Fine tuning of detector parameters Efficiencies • Data collected in May • Threshold 7 mV • All RPCs have efficiency >90% • on both cathodes • Mean value above 95%

  24. With trigger requirement No trigger requirement Muons Hadrons Total PYTHIA 7 TeV No trigger PYTHIA 7 TeV With trigger DCA(cm) DCA(cm) ...and working • For the moment (low luminosity) , use for data taking the lowest • possible trigger threshold pT = 0.5 GeV/c • Distance of closest approach (DCA) to the vertex for tracks • in the muon spectrometer • Muon tracking-trigger matching very effective in rejecting • Hadronic contribution • Soft (background related) component

  25. Alignment is important, too... • First J/ signal has started to pop out in the invariant mass • spectrum a few weeks after the beginning of the 7 TeV data • taking... ...but with a bad resolution, due to the absence of an alignment with straight tracks

  26. ...and of course very helpful • Resolution on the J/ peak in agreement with • expectations from Monte-Carlo (J/ ~ 80 MeV • for an alignment resolution 700-800 m)

  27. Towards the first physics results • Next step: produce physics results • On our list • Cross section • Differential distributions (pT, y) • Polarization being studied just now needs higher statistics • Obtain the cross section in the standard way • A · trig • High values down to pT =0 • Does not depend very • strongly on pT • track close to 100% Which is the main source of systematic error ?

  28. Effect of unknown polarization • The full angular distribution of decay muons is given by • Assuming =0 (as measured by • all previous experiments) we can • calculate the systematic error on •  due to our ignorance of  and  • The effect is rather strong +12.2, -13.5 % (Collins-Soper)  +10.6, -11.7 % (Helicity)  • Most of the effect is related to the uncertainty on , •  plays a weaker role

  29. Conclusions • ALICE is measuring quarkonium production in p-p collisions • at √s=7 TeV, at the LHC • The muon spectrometer, covering the rapidity region 2.5<y<4, • is currently taking data with satisfactory detector performance • First physics signal (J/) are popping out and will lead soon to • first physics publications • We are eagerly waiting for the first Pb-Pb run scheduled at • the end of 2010. A meaningful J/ signal seems within reach.... ...stay tuned!

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