New Physics with Proton Collisions in ALICE at LHC. Jean-Pierre Revol CERN Physics Department NIKHEF seminar March 3, 2006

Transcription

1 New Physics with Proton Collisions in ALICE at LHC Jean-Pierre Revol CERN Physics Department NIKHEF seminar March 3, 2006

2 A few words on the LHC project will have been preceded by more than 20 years of hard studies (1984 ECFA-CERN Workshop in Lausanne) and hard work to meet an unprecedented challenge in all areas of the project: Accelerator Detectors Computing Finances Organization The first collisions at LHC will therefore be an historical event. Physicists in the ALICE Collaboration are working hard to make sure that they will be actors of the event. J.-P. Revol / NIKHEF / March 3,

3 Next year our dream should become reality! Today (March 3), 82% of dipole magnets have been delivered to CERN (1011/1232), 964 equipped with cryostat, 920 cold-tested, 321 installed. Superconducting magnets (superfluid He at 1.8K ) Largest cryogenic station in the world! 1232 dipoles 8.4 Tesla, 15 m long, 35 ton each, over 27 km. There will be a final LHC schedule in the summer of no major change expected as the present local installation delays are of the order of 3 to 4 months, but with a plan to catch up. CERN is strongly committed to first collisions in the summer of J.-P. Revol / NIKHEF / March 3,

4 Solenoid magnet (0.5 T) 8000 tons [6900(Yoke) +1100(coil)] The ALICE Detector: 14 subdetectors more than the Eiffel tower (7300 tons) Cosmic-ray trigger 15 subdetectors Dipole magnet (3 T.m) J.-P. Revol / NIKHEF / March 3,

5 Inner Tracking System (ITS) Primary vertex & secondary vertices (resolution: 60 μm(pp); 10 μm(hi)) Strange, Charm, Beauty and Hyperon studies Tracking and identification of low P T (< 100 MeV/c ) particles. Complements TPC tracking Pixel (SPD) 50(rφ) 425(z) μm 2 9,830,400 channels Drift (SDD) 133,120 channels Strips (SSD) 2,608,128 channels Sate of the art technology, with an important contribution from the Netherland (NIKHEF/Utrecht) The ITS plays a crucial role in ALICE as we will see later! J.-P. Revol / NIKHEF / March 3,

6 Pixel J.-P. Revol / NIKHEF / March 3,

7 ALICE Time Projection Chamber The TPC plays a central role for tracking and PID. It provides information to all other detectors in the central region (-1.5 < η < 1.5). (most challenging TPC) Channels Gas Ne/CO 2 90/10 or + 5%N 2 Volume 88 m 3 Drift length Drift field Drift velocity Max drift time 2.5 m 400 V/cm 2.84 cm/μs 88 μs Diffusion D L = D T = 220 μm cm -1/2 X/X 0 3.5% at η = 0 ΔT 0.1K ΔB/B 10 4 J.-P. Revol / NIKHEF / March 3,

8 TPC J.-P. Revol / NIKHEF / March 3,

9 Proton-proton physics with ALICE ALICE is optimized for Heavy Ion physics, where low p T phenomena play a central role. Therefore, ALICE is ideal for studying low p T phenomena in proton-proton collisions at LHC. Indeed, proton-proton studies are a major part of the ALICE programme for several reasons: to provide reference data to understand heavy ion collisions. In a new energy domain, each signal in HI has to be compared to pp; For genuine proton-proton physics whenever ALICE is unique or competitive; note that ALICE can reach rather high p T, up to ~ 100 GeV/c, ensuring overlap with other LHC experiments. J.-P. Revol / NIKHEF / March 3,

10 Importance of low p T phenomena at LHC Complete understanding of the physics of colliding protons at LHC requires the study of all phenomena, especially those with large cross-sections, even though they were not the driving motivation for building the LHC. Those phenomena with cross-sections ranging probably between mb (σ inel. ), mb (σ el. ) and mb (σ diff. ) [cf. A. Kaidalov] represent the bulk of the events at LHC. They are generally characterized by low p T. As new kinematic domains are investigated, new physics could be revealed in low p T phenomena: they will contribute to our understanding of the strong interaction, at the frontier between perturbative and non-perturbative QCD (a most challenging domain) - Access to a much smaller x range => many more higher orders to calculate! they are inescapable at LHC, as they will be first to be observed in the commissioning phase of the machine and of the detectors, and will constitute the background for high p T rare events. In practice, ALICE commissioning will be carried out with pp collisions (simpler): the first ALICE physics result will come from proton-proton collisions. J.-P. Revol / NIKHEF / March 3,

11 LHC commissioning scenario (as envisaged today) T0 (= 1st of July 2007 as of today) One month to get the machine ready for beams (T0 + 1 month) Three months to commission the machine with beams (T0 + 4 months) => possibility for ALICE to collect the first pp data sample for first physics! One month of rather stable operations, interleaved with machine development with 43 and 156 bunches, with the possibility of collisions for physics during nights (~ 20 shifts of 10 hours each L ~ cm 2 s 1 ) (T0 + 5 months) => possibility for ALICE to collect the first large pp data sample! Perhaps first Pb-Pb collisions (T months) Shutdown (T to 9.9 months): today machine people talk about 3 to 4 months. The length will depend on requirements by experiments. If T0 = 1st of July, start of shutdown will coincide with the Christmas holidays. Preparation First collisions. Stable beams Shutdown 3 to 4 months? July Aug. Sept. Oct. Nov. Dec. Jan. Feb. Mar This is the period during which ALICE must collect the first few minutes of data J.-P. Revol / NIKHEF / March 3,

12 LHC commissioning scenario The first proton beam should be injected through TI8 starting in Nov. 2006, up to a beam dump just before Point 7: Effective start of LHC commissioning! First circulating p beam direction Z The first circulating beam will eventually reach ALICE from the muon arm side (perhaps good for shielding!) Y X (already commissioned!) J.-P. Revol / NIKHEF / March 3,

13 ALICE not designed for high luminosity! ALICE will have to reduce the luminosity by at least 3 orders of magnitude! J.-P. Revol / NIKHEF / March 3,

14 Obtaining low pp luminosity for ALICE How will ALICE get sufficiently low luminosity conditions? By using special beam optics at Point 2: The reduction factor is β*/β nom. = β* / 0.5 m. Under nominal conditions, maximum β* = m, which gives reduction-factor ; In special runs at low luminosity, to take data with negligible events overlap, and if possible a small enough β*, so that the transverse vertex spread is minimized, in view of heavy flavour physics; By displacing the beams, whenever the above conditions are not sufficient: σ Luminosity : LD ( )= L 0 exp D2 4σ 2 for D/σ = 4.5, the additional reduction factor is ~ 160. D L ALICE = cm -2 s -1 for L max = cm -2 s -1 By taking advantage of the LHC commissioning period J.-P. Revol / NIKHEF / March 3,

15 Commissioning beam characteristics (LHC-OP-BCP-0001 rev 1.) The very first collisions at LHC will be proton-proton collisions at a center of mass energy of 900 GeV. Then highest possible beam energy, but with a small number of bunches, and low intensity. Beam conditions will be ideal for ALICE. Beam Energy (TeV) to 7 6 to 7 6 to 7 Number of bunches β * [m] Crossing Angle [μrad] Transverse emittance [μm] Bunch spacing [ns] Bunch Intensity 1x x x x Luminosity [cm 2 s 1 ] 4x x x x10 30 Inelastic Rate [Hz] Time to produce 2x10 4 events 140s 5s 0.3s Only 3 minutes to collect sufficient sample N L = f N 1 N 2 b π 4σ x σ y J.-P. Revol / NIKHEF / March 3, ( )

16 Scenario for ALICE detector commissioning P-O 2 event (Matevz Tadel) Use, in addition to cosmic rays, beam-gas events for: detector commissioning trigger commissioning DAQ event building Offline reconstruction detector alignment J.-P. Revol / NIKHEF / March 3,

17 ALICE s initial goals (1) Catch the first few minutes of pp collisions for first physics at LHC (10 4 events). (2) Make use of large statistics (10 8 events) of pp data already available from the first month of LHC running! (3) Observe the first heavy ions collisions as soon as possible: HI collisions imply special additional effort from the machine crew. If proton commissioning goes well, Pb-Pb collisions could happen early (H.I. injector commissioned already, PS in 2006 and SPS in 2007 then it would take 2 weeks to commission LHC). Present ALICE status: Ready to take beams with a sufficient detector configuration on 1st July 2007! J.-P. Revol / NIKHEF / March 3,

18 A comment on pp and HI collisions Thin gluon walls Long non- Gaussian tail The real picture is in between [combined effect of the Lorentz boost (γ = 2750), of a distribution of low x partons and of the uncertainty principle]. Is there a difference between Pb ions and protons? Within a factor 2 at most, same structure functions; parton saturation (gg g~ A 1/6 ~ 2.4 ) is also present in pp. What changes really is the volume (350x) V Pb = 350 V p r 2 r 2 J.-P. Revol / NIKHEF / March 3, A lead fm proton fm

19 Proton-proton and Heavy Ion collisions pp collisions at LHC will reach initial energy densities comparable to those available in gold-gold collisions at RHIC. Therefore, they represent considerable interest for the study of high energy densities, going from small volumes with pp collisions to large volumes with Heavy Ion collisions. (In addition, it is important to measure pp and HI in the same detector!) ε i = 3 2 E πr N 2 A 2 3 dn Ch. dy 12 GeV/fm 3 (10 9 events) Can QGP be produced in pp collisions? These ideas have been already explored at the Tevatron, with intriguing results (T. Alexopoulos et al., E-735: Evidence for hadron deconfinement in proton-antiproton collisions at 1.8 TeV ). Why are pp system parameters thermal? Maximum entropy? J.-P. Revol / NIKHEF / March 3,

20 Comments on the minimum detector configuration to start physics It is likely that most of ALICE is ready by summer 2007, however, the bare minimum is one detector to measure charged tracks, one trigger detector, and of course a number of relevant systems: TPC (Tracking) V0 and T0 (Trigger) But to be safe there is a need for minimum redundancy: Detector redundancy: ITS (at least the pixels to measure multiplicity: e.g. PHOBOS at RHIC) Trigger redundancy at level 0: ITS pixels fast OR Systems: Trigger processor, DAQ, DCS, ECS, event display, offline (feedback) This redundancy to ensure readiness for first few minutes physics will actually bring significant additional pp physics, at the very start of LHC, at a time when ALICE will be competitive in pp physics. J.-P. Revol / NIKHEF / March 3,

21 First Physics It only takes a handful of events (UA1: 700) to measure a few important global event properties (dn/dη, dσ/dp T, etc.). In fact, it will take only a few minutes to collect a few tens of thousand events, sufficient for first physics. Pseudorapidity density dn/dη CDF: Phys. Rev. D41, 2330 (1990) Claus Jorgensen Multiplicity distribution p T spectrum unidentified hadrons CDF: Phys. Rev. Lett. 51, 1819 (1988) Mean p T vs multiplicity UA5: Z. Phys 43, 357 (1989) CDF: Phys. Rev. D65,72005(2002) J.-P. Revol / NIKHEF / March 3,

22 Note on data rates for first physics In the first month of stable running (November 2007, in the present LHC commissioning scenario), the average inelastic collision rate will be of order 70kHz (assuming a luminosity of cm 2 s 1 ) We can expect ~ 100 Million minimum bias events recorded in the central region, assuming 20 shifts of 10 hours each and 100 Hz (conservative) readout rate in the central region (depending on maximum TPC grid pulsing rate that can be achieved): This is already 10% of the total pp sample (10 9 events) ALICE wants to collect! A very significant sample, which will allow to explore most of the physics on the ALICE programme. but there is more we can do than just collect pp MB events! J.-P. Revol / NIKHEF / March 3,

23 The ITS will be equipped with a level zero trigger The Pixel fast OR signal provides a trigger, complementary to V0 & T0 and also a more intelligent trigger at level zero: High multiplicity selection to enrich pp data sample; Tracks pointing towards the HMPID; Eventually more sophisticated algorithms (two Jet topology trigger, etc.). Alex Kluge J.-P. Revol / NIKHEF / March 3,

24 Initial multiplicity reach At a collision rate of 70 khz, a multiplicity trigger could enrich data with events in the multiplicity tail with a statistics equivalent to 50 times the standard pp sample size required by ALICE (equivalent to 5x10 10 MB events). This is huge! i.e. asking for a multiplicity larger than 380 in the pixels ( η 2), could provide a data sample of 10 M events with multiplicity larger than 10 times the mean! This will undoubtedly open spectacular and unique new physics possibilities for ALICE. (major effort needed to simulate this with the initial ALICE configuration) Claus Jorgensen J.-P. Revol / NIKHEF / March 3,

25 First ALICE physics: Study of Global Event Properties Charged particle multiplicity: The Minimum Bias trigger is provided by a coincidence between V0 counters covering a pseudorapidity range from -1.7 to -3.7 and from 2.8 to 5.1. This corresponds to a visible non-elastic cross-section of ~ 65 mb. Pixel trigger will also be used (Trigger & Multiplicity). J.-P. Revol / NIKHEF / March 3,

26 Initial p T reach With MB evts, we can reach 10 GeV/c With 100 M MB events, we can reach 40 GeV/c With the pixel multiplicity trigger we can reach at least 80 GeV/c. => we need optimum momentum resolution, hence the ITS! J.-P. Revol / NIKHEF / March 3,

27 Importance of the ITS Much improved momentum resolution of charged tracks. The pixel are doing most of the job! (ΔMz/Mz ~ 3%) Marian Ivanov J.-P. Revol / NIKHEF / March 3,

28 Tracking in central region ALICE unique at low p T (small material budget, low B field). Magnetic field (T) Material thickness: X/X0 (%) P T cutoff (GeV/c) ALICE ATLAS CMS LHCb 4Tm * * Minimum momentum is 1 GeV, while for ALICE p ~ p T Number of events 10 3 ptave09 Nent = 8403 Mean = RMS = CMS Event average transverse momentum (GeV/c) Beam pipe Φ = 59.6 mm pushes the first Si pixel layer to 3.9 cm from the beam axis. Thickness = 0.8 mm Be (0.23% X 0 ). Competitive at LHC but not very daring! Improvement should be part of the upgrade (follow CDF! Be with R = cm; thickness = 0.58 mm) J.-P. Revol / NIKHEF / March 3,

29 Comparison with other LHC experiments Vertex detectors resolution CMS ATLAS, CMS ALICE Muon arm LHCb ALICE Very significant gain for ALICE Charged particles not including muons J.-P. Revol / NIKHEF / March 3,

30 Lead-lead collisions with ALICE The TPC was designed to work up to 8000 tracks per unit of rapidity, optimized for RHIC dn/dη ~ So we are probably safe! P T > 1 GeV/c Display is a challenge! J.-P. Revol / NIKHEF / March 3,

31 Proton-proton event in ALICE With dn/dη ~ 7, pp events will be studied under ideal conditions of momentum and de/dx resolution, thanks to low occupancy ~ few 10 4 P cut 1 TGeV/c PTPNo > 100 MeV/c T > 761 tracks J.-P. Revol / NIKHEF / March 3,

32 ALICE has unique particle identification capability J.-P. Revol / NIKHEF / March 3,

33 First data from the ALICE detector: HMPID at RHIC pions kaons Au-Au data collected at STAR both signs p > 1 GeV protons J.-P. Revol / NIKHEF / March 3,

34 First strange particle studies Based on Pythia, at LHC, we can predict significant samples of strange particles in 10 8 Minimum bias events: K s 0 Λ Ξ Ω P P Yield per event Statistics needed PP events needed K 0 : 10 7, Λ: 10 6, Ξ: 2x10 4, Ω: 10 3! Will already exceed the statistics of UA1! However, with the multiplicity triggered sample, the statistics will be much larger! Measurement of strangeness production vs multiplicity, and unique comparison with heavy ions! J.-P. Revol / NIKHEF / March 3,

35 Heavy flavour production at LHC Important test of pqcd requiring both B and C studies. Large uncertainties on NLO predictions for LHC. Probe of small x gluon dynamics. Sure to be important at LHC. It took many years to resolve B cross-section problem at CERN Collider and Tevatron! Cacciari, Frixione, Mangano, Nason and Ridolfi, hep-ph/ Pythia predicts that 10 8 MB pp evts contain ccbar and bbbar evts Beauty Charm HERA-LHC Workshop, Mangano, Nason, Ridolfi, NPB 373 (1992) CDF, hep-ex/ FONLL: Cacciari, Nason J.-P. Revol / NIKHEF / March 3,

36 First heavy flavour physics: Cham production D 0 K - π + in pp, down to P T =0! Andrea Dainese 30% GeV/c coverage with 10 8 evts (preliminary, cuts not optimized for pp) J.-P. Revol / NIKHEF / March 3,

37 First heavy flavour physics: Beauty production Semi-electronic beauty decay in pp, electron identified in TRD and TPC (de/dx). 1 TRD module C. Bombonati et al. J.-P. Revol / NIKHEF / March 3,

38 Baryon Production in Central Rapidity Region q Can gluons carry baryon number? (Rossi, Veneziano String Junction) J q q p {10} g g Baryon number can be transferred by specific configuration of the gluon field: (B. Z. Kopeliovitch and B. Zakharov Z. Phys. C43 (1989) 241). At LHC huge rapidity interval between incoming protons (y p = ±9.6) and central rapidity. Veneziano s model would result in substantial baryon production in central rapidity region. {10} g {8} g {3} q q J String junction q J String unction q q HERA point plotted at rapidity = 2.2 p {8} J.-P. Revol / NIKHEF / March 3,

39 Baryon Production in Central Rapidity Region (cont.) p ALICE will be able to measure p,,λ, Ξ, and Ω. The idea is to measure the asymmetry between baryons and anti-baryons (B. Kopeliovitch): A p 2 p p+ p ~5%at LHC p A 2 Λ Λ 30%at LHC Λ Λ+Λ p Systematics 2% (Beam-Gas, antiproton absorption, secondary protons) 10 9 MB events pions, , 10 7 Λ, Ξ and 10 4 Ω. Since baryon stopping implies more strings to exchange, it is expected that those events have higher particle multiplicities, hence a measurement of the asymmetry as a function of multiplicity is needed. Such measurements will be relevant to heavy ion collisions where baryon stopping should be dramatically enhanced. ALICE can also study heavy flavor baryons (Λ b, Ξ b, Ω b,...) which are poorly known. With Br.(Λ b J/ψΛ) = (4.7 ± 2.8) 10 4, 10 9 events, triggered on J/ψ using the TRD detector, should produce a few thousand Λ b s. What's more, there is a chance to observe Ξ b and Ω b J.-P. Revol / NIKHEF / March 3,

40 Advantage of the Pixels for quarkonia studies Necessary for sufficient momentum resolution of the muon arm (the resolution must be better than 100 MeV for Y, which implies knowing the vertex within 1 cm). [The pixels are needed since the TPC will not normally be read out.] Christophe Suire J.-P. Revol / NIKHEF / March 3,

41 Quarkonia physics Muon channel: (2.5 <y< 4): J/Psy and 2000 Y! (Gines Martinez et al.) The initial sample should be sufficient to measure production rates for J/Psi and Y, in the muon channel. Electron channel: it will depend on how much of the TRD is installed. E.Vercellin Example of muon channel E.Vercellin J.-P. Revol / NIKHEF / March 3,

42 Two main motivations: Photon Physics in ALICE Calibrate γ production in pp to subtract pqcd contribution in Heavy Ion collisions, in search for thermal photon signal from QGP, and study jet quenching (longer lifetime at LHC). At RHIC, QCD contribution observed, but not conclusive on thermal γ s, may be in γ => e + e? In pp, perform high P T QCD, LO, NLO tests (γ, π 0 cross-section, jet fragmentation, etc.) π 0 γγ, η γγ, ω π 0 γ... Results non-conclusive at RHIC Perturbative QCD contribution πρ πγ; QGP radiation Compton annihilation J.-P. Revol / NIKHEF / March 3,

43 First Photon Spectrometer (PHOS) physics Dimensions: η ±0.12, ϕ 100 (1 8 m 2 ) at radius R 4.6 m PbWO 4 crystals, X 0 = 0.89 cm, λ int = 19.5 cm, Moliere radius: 2.0 cm Granularity: cm 2 (Δη Δϕ ), length: 18 cm The granularity is the same as CMS but the distance from the interaction point is 3 times that of CMS; π 0 identified from 1 to 80 GeV/c; Energy resolution 2% above 3 GeV/c; Two modules at the start? Validation of pqcd calculations (NLO) γ/π 0 ratio Reference point for forthcoming AA collisions J.-P. Revol / NIKHEF / March 3,

44 Statistics and trigger rates 7x10 5 evt With TPC information 2 PHOS modules (Δφ = 40, Δy = 0.25). pp run: L = cm -2 s -1, Time = s LT= 8.6x10 8 mb -1 From Y. Schutz et al. p T, GeV/c dn γ dir /dp T, dn γ GeV -1 decay /dp T, dn π /dp T, GeV -1 GeV -1 Trigger rate, Hz 1 3x x10 8 3x x x x10 3 6x10 5 3x x x x x x x x x x In one month a decent measurement up to ~ 20 GeV Already reaching RHIC s limit! J.-P. Revol / NIKHEF / March 3,

45 Conclusion A very broad pp physics programme for ALICE. Many items not covered here (jets, etc.): ALICE Collaboration, J. Physics G: Nucl. Part. Phys. 30 (2004) Low P T proton-proton phenomena studies at LHC will be very exciting from the very start with ALICE: Necessity to explore fully and in detail a new domain of physics; Contribute to a better understanding of the strong interaction in a challenging kinematic domain; Needed for the study of heavy ion collisions; Some of it (Minimum Bias and underlying event properties) is relevant to the understanding of the background to high P T signals. Physics with ALICE will start with the first few minutes of collisions at LHC, but within a very short time, a significant data sample, taken in optimum beam conditions, will be available for new pp physics. J.-P. Revol / NIKHEF / March 3,