Physics of the PHENIX HBD

Physics of the PHENIX HBD Chiral Symmetry Restoration (I. Tserruya)** Correlated Charm Pairs (A. Dion, A. Toia, S. Campbell) Experimental Challenge (A. Toia, S. Campbell) Background Reduction Approach. (A. Drees) The FIRST HBD prototype (R. Pisani) Key design features of PHENIX HBD. (S. Milov, I Ravonivich, J. Kamin) NOTE: This talk will be followed next week by Jason Kamin on the HBD detailed design & construction **Slides stolen unabashedly from these authors

Motivation I: Chirality • Two fundamental properties of the QGP: • Deconfinement • Chiral symmetry restoration • What is chirality? • Comes from the greek word “” meaning hand • An object or a system has chirality if it differs from its mirror image. • Such objects then come in two forms, L and R, which are mirror images of each other. • Simple definition: • the chirality of a particle is determined by the projection of its spin along its momentum direction (this is in fact the definition of helicity. In the high energy limit chirality ≈ helicity)) Thomas K Hemmick

Chiral Symmetry • If a particle has mass both right- and left-handed components must exist. • The reason is that massive particles travel slower than the speed of light • and a particle that appears left-handed in a particular reference frame • will look right-handed from a reference frame moving faster than the • particle  chirality is not conserved Right-handed • In a massless world chirality is conserved • (sufficient but not necessary condition) Left-handed Thomas K Hemmick

Chirality formal definition: • the helicity operator gives the spin projection along its momentum: • the chirality operator PR = ½(1+5) and PL = ½(1-5) (5 is the product of the Dirac  matrices i0123) projects out the right and left handed chirality states of the wavefunction u: uR,L = PR,L u • in the high energy limit the chirality operator is equal to the helicity operator QCD and Chiral Symmetry • QCD, the theory of the strong interaction, is encoded in a one line Lagrangian: Free gluon field q interaction with gluon field Free quarks of mass mn at rest • mu ≈ 4 MeV md ≈ 7 MeV • proton = uud neutron = udd mNucleon ≈ 1 GeV • only ~5% of the nucleon mass is due to the current quark masses

Explicit and Spontaneous Chiral Symmetry Breaking • The mass term mnnnexplicitly breaks the chiral symmetry of the QCD Lagrangian • Chiral limit:mu = md = ms = 0 • In this idealized world, the interactions quark-gluon conserve the quark chirality. (left–handed u,d,s, quarks remain left-handed forever). Chiral symmetry of QCD means: all states have a chiral partner with opposite parity and equal mass • In reality: •  (JP = 1-) m=770 MeV chiral partner a1 (JP = 1+) m=1250 MeV ≈500 MeV • For the nucleons the splitting is even larger: N (1/2+) m=940 MeV chiral partner N* (1/2-) m=1535 MeV ≈600 MeV • The difference is too large to be explained by the small current quark masses Chiral symmetry is spontaneously (≡ dynamically) broken

X Origin of mass Constituent quark masses generated by spontaneous chiral symmetry breaking Origin of (our) mass: 95% of the (visible) mass is due to the spontaneous breaking of the chiral symmetry. Only 5% of the mass is due to the Higgs field. Current quark masses generated by spontaneous symmetry breaking (Higgs field) Thomas K Hemmick

Chiral Symmetry Restoration • Spontaneous breaking of a symmetry is marked by: • * a non-zero order parameter, the quark condensate in the case of QCD: • Many models link the hadron masses to the quark condensate. • At high temperatures (T>TC) or high baryon densities (>C), numerical QCD calculations on the lattice predict that the quark condensate vanishes: constituent mass  current mass chiral symmetry (approximately) restored

How does CSR manifest itself ? • What happens when chiral symmetry is restored? Meson properties (m,) expected to be modified but how? • Is there an explicit connection between the spectral properties of hadrons (masses,widths) and the value of the chiral condensate <qq> ? • From the QCD Lagrangian, the only requirement is that parity doublets should be degenerate in mass. • how is the degeneracy of chiral partners realized ? • do the masses drop to zero? • do the widths increase (melting resonances)? All very good questions with no good answer to any of them. Thomas K Hemmick

The theoretical picture is confusing Theoretical predictions for the in-medium modification of the -meson properties (compilation from S. Damjanovic) Lattice QCD coming to the rescue: hadron spectral functions are being studied on the lattice. Another example where experiment has the potential to guide the theory.

Low-mass dileptons and CSR • Low-massdileptons are the best probes to look for CSR effects: * Large mfp:  no final state interaction carry information from place of creation to detectors. m [MeV] Gtot [MeV]  [fm/c] r 770 150 1.3 w 782 8.6 23 f 1020 4.4 44 • Best candidate: the -meson It has a short lifetime compared to the medium lifetime ( ≈ 10 fm/c) and can decay and can be regenerated in the medium •  meson:a special probe for CSR, long lifetime but m(Ф) ≈ 2 m(K) simultaneous measurement of   e+ e- and  K+ K- could be a powerful tool to evidence in-medium effects. Thomas K Hemmick

NA60 nucl-ex/0605007 CERESPb-Au 158 GeV mee (GeV/c2) Results from SPS • Considerable theoretical thought was initiated by the CERES observation of significant low mass di-lepton yield. • Higher statistics from NA60 seem to favor collisions broadening over mass shift. • What happens at RHIC? e- e+ Thomas K Hemmick

Not enough statistics to get precision measure of low mass di-electron yields (chiral/collisional). The problem is not lack of Events…it is the uncertainty created by background subtraction. PHENIX Preliminary PHENIX Preliminary PHENIX: Au+Au -- MB Cocktail Comparison NOTE: At RHIC, there is a new significant component: Correlated Open Charm. Thomas K Hemmick

>100x Signal + Combinatorial BG Generated BG from Event Mixing Method Normalized Generated BG mee Motivation II: Combinatorial Background and Event Mixing • Combinatorial Background grows proportionally to multiplicity squared; signal grows proportionally to multiplicity • In high multiplicity RHIC collisions, signal-to-background is a key consideration • Event Mixing Method • Generate Combinatorial Background Shape • Mix across events with similar centralities, z-vertices • Background negligible contribution to statistical error PHENIX has the greatest challenge in di-electron combinatorics of any experiment. Thomas K Hemmick

K- p+ Open heavy-flavor at RHIC • reference for J/y suppression • medium probe in its own right • experimental approaches at RHIC • direct reconstruction of charm decays (e.g. D0→K+p-): STAR • difficult in HI environment • remaining residual background after subtraction of combinatorial BG • indirect measurement via semi-leptonic decays: PHENIX & STAR • requires good control of lepton background from other sources • CORRELATED Charm is “double” semi-leptonic decay. Thomas K Hemmick

HQ Energy Loss and Flow Radiative energy loss only fails to reproduce v2HF. Heavy quark transport model has reasonable agreement with both RAA and v2HF. Small relaxation time t or diffusion coefficient DHQinferred for charm. Measurement of CORRELATED charm should be sensitive to the energy loss mechanism. Thomas K Hemmick

Nuclear Modification in IMR PHENIX preliminary Thomas K Hemmick

PHENIX preliminary Nuclear Modification in IMR • Similar to high pT single electron suppression pattern • Attributed to charm suppression • Fukutaro Kajihara’s talk on 11/18 • Shape Comparison Only Thomas K Hemmick

PHENIX Preliminary Nuclear Modification in IMR • Similar suppression pattern as J/Psi • Taku Gunji’s talk on Sat Nov 18 • Shape Comparison Only Limitation is again the size of the background not the number of recorded events! Thomas K Hemmick

~12 m Why so much background? • Typically only 1 electron from the pair falls in the acceptance. • The magnetic field bends the pair in opposite directions. • Some curl up in the magnetic field and never come out • The most common sources are the biggest problem. • The new detector needs: • >90% electron ID • sit near the collision • sit in zero B-field • catch e+/- before they get lost Thomas K Hemmick

Normalization Overview • A Single Normalization Factor for all masses & pTs • 4 Normalization Methods • Event counting: Nevents / Nmixedevents • Independent Production: <N+-> = <N+><N-> • Ideal for jets • Pair Production: ∫BG+- = 2√∫FG++ ∫FG— • Ideal for leptons • Like-sign yield: ½(∫FG++/ ∫BG++ + ∫FG--/ ∫BG--) • Integrals excluded the region of 0-200 MeV • Avoid like-sign signal from π0 dalitz decays and photon conversions • Normalization Agreement • In Au+Au: within 0.5%, systematic error of 0.25% • In Cu+Cu: within 0.85%, systematic error of 0.47% Like-sign Signal Thomas K Hemmick

e+e- pair Background Normalization • Background Normalization Summary: • For correlated production, i.e. e+e- • For independent production • Event multiplicity correlations: “ξ-correction” • Needed for normalization via event counting and but NOT for !!! • Centrality bin width; non Poisson (e.g. negative binomial) multiplicity distribution; reaction plane etc • Pair cut bias: “post-field opening angle cut” • different probability to lose like and unlike sign pairs must be corrected when using Later true if Poisson distributed Thomas K Hemmick

Notations for pairs and single tracks • n+ -, n++, n- - • pairs in event with fixed # of true pairs N and reconstructed pairs • N+ -, N++, N- - • Average # of pairs in event with fixed N • F+ -, F++, F- - • Average # of foreground pairs in event • including background FB+ -, FB++, FB- - and signal S • With average number of tracks F+ and F- • B+ -, B++, B- - • Number of background pairs in mix events Thomas K Hemmick

Basic Statistics • Pair statistics: • N e+e- pairs are produced in an event with probability P(N) • Probability to reconstruct a pair ep • The number of pairs reconstructed npin event with N pairs follows a binomial distribution B(np; N, ep) with: • Single track statistics: • Of the remaining N’=(N-np) pairs single tracks are reconstructed • For one pair the probability to reconstruct a e+ is e+ and for e- it is e- • Total number of single tracks n+ and n- then follows a Multinomial distribution M(n+,n-; N’,e+,e-) with: Be patient, it’s going to get ugly! Thomas K Hemmick

Pairs in event of fixed N pairs and fixed reconstructed np pairs • Unlike sign pairs • Like sign pairs Thomas K Hemmick

Next sum over number of reconstructed np pairs • Unlike sign: • Like Sign Thomas K Hemmick

Last step sum over all possible N • F+- number of unlike sign foreground pairs • F++ and F– number of like sign foreground pairs • Number of background pairs independent of P(N) given by 2sqrt signal Thomas K Hemmick

Look at F+F- • First fixed N and np: • Sum over allnp: • And finally over allN: • For e+e- pair production F+F- is NOT equal to number of background pairs! Thomas K Hemmick

Background Normalization Summary: For correlated production, i.e. e+e- For independent production Event multiplicity correlations: “ξ-correction” Needed for normalization via event counting and but NOT for !!! Centrality bin width; non Poisson (e.g. negative binomial) multiplicity distribution; reaction plane etc Pair cut bias: “post-field opening angle cut” different probability to lose like and unlike sign pairs must be corrected when using Summary: e+e- pair Background Normalization Later true if Poisson distributed Thomas K Hemmick

Is it a π or a φ? A lot of particles have e+e- decay channels. How can we tell the Dalitz decays and photon conversions apart from the decays that we’re interested in?? Back to the basics (briefly)… relativistic electrons pads Dalitz decay electrons have apparent mass from 2mempwith highest probability near 2me φ π g Photon conversions EVEN MORE tightly peaked around 2me Background pairs have VERY low mass. Lighter pairs have smaller opening angles!! What detector rejects background but leaves signal? Thomas K Hemmick

Dalitz and  conversions: at least one track •  meson: both tracks For a 90% rejection need to cut up to 200 mrad Opening Angle Cut Thomas K Hemmick

Signal lost by random close hit cut Signal and Background - I: No e-ID • Inner detector: • no e-id, perfect spatial resolution • “Blind” close hit cut: • use all hits (280  + 9e) in inner det. Thomas K Hemmick

Signal lost by random close hit cut Signal and Background - II: No e-ID • Inner detector: • no e-id, perfect spatial resolution • Almost “Blind” close hit cut: • use ‘single’ hits (107  + 9e) not • belonging to tracks. Thomas K Hemmick

Cut up to 200 mrad keeping • ~50% of Signal • S/B limited by ~10% of tracks which have their partner outside fiducial acceptance Signal and Background - III: Perfect e-ID • Inner detector: • perfect e-id, perfect dhr • close hit cut using only e (9e) •  rejection = ,e-eff = 100% Thomas K Hemmick

Veto Area • Most widow tracks and their • partners sit at the boundaries • of the fiducial area “Widow “ tracks Location of tracks and their partners when partner is outside fiducial acceptance Thomas K Hemmick

Choice: Signal and Background - IV: Veto Area • Inner detector: veto area + • perfect e-id + perfect dhr • close hit cut using only e (9e) •  rejection = ,e-eff = 100% Thomas K Hemmick

Looking Closer… • Inner coil can cancel B-field at r < 60 cm • Not enough room for traditional optics… mirrors won’t work. • Just put the detector right in the middle of things! • Has potential, but… • must be thin • must detect a single UV photon and still be blind to all ionizing particles passing through it!!! Thomas K Hemmick

The original idea • The original idea by Y.Giomataris and G.Charpak 1991 (NIMA 310) • Electrons from Cherenkov light are produced at the photocathode and amplified full way. Mesh Cherenkov photon HV • Electrons from primary ionization are produced at random and are amplified much less (exponential law). Ionizing particle Photocathode Thomas K Hemmick

FIRST PHENIX HBD Detector Concept • We Borrowed our design concept from the original article:Y. Giomataris and G. Charpak, Nucl. Instr. And Meth. A310 (1991) 589. • There an unfocussed “blob” of Cherenkov light was measured by a PPAC which used a solid CsI photocathode and a common avalanche/radiator gas. • We made one simple modification to the design by adding a thin LiF window to separate the radiator and avalanche gasses for flexibility. • 0.5mm of LiF is only 0.3% of a radiation length and has TIR for Cherenkov light from a particle at normal incidence Original Concept Stony Brook Design Thomas K Hemmick

Detector Design • Rapid Assembly • Uniform gain • Protect window • Small X0 • Miss ionization • Single Photon Thomas K Hemmick

Beam Line Apparatus • The apparatus featured both a radiator/detector tube and a transparency monitor tube. • A movable light block (“Sail”) was used to limit the radiator length to 120, 80, and 40 cm. • The transparency monitor used a Deuterium lamp and MgF2 windows and was used to compare radiator transparency to vacuum at 150 and 130 nm. Thomas K Hemmick

Event Displays Hit pattern of the FIRST 25 events which were selected by beamline counters as electrons Hit pattern of the FIRST 25 events which were selected by beamline counters as pions Thomas K Hemmick

Electron ID Scattergram • At all lengths of radiator, electrons are clearly separated from pions. • The short radiator runs have low statistics due due near failure of the beam line transport for the last 24 hours of our run. • Separation of electrons from pions should be done using both number of pads and total charge. Thomas K Hemmick

Original idea modified • Problem: such setup requires a window. • Radiator gas must be transparent. • The avalanche in the gas contains as many photons as electrons • What happens if photons shine back on photocathode? • We need to separate the radiator and detector volume by a window HV • And get even more problems… • Windows are bulky • Window is a perfect source of the Cherenkov photons from any particle! • Let’s get rid of the window! Thomas K Hemmick

Gas Electron Multiplier (GEM) 150μ • The original idea by F.Sauli (mid 90s) US Patent 6,011,265 • HV creates very strong field such that the avalanche develops inside the holes • The same field is sufficient to pull electrons from the surface into holes • Photon feedback is not there: GEMs screen the photocathode Thomas K Hemmick

The concept. More Next Week! HV • Take a GEM • Put a photocathode on top • Electron from Cherenkov light goes into the hole and multiplies • Use more GEMs for larger signal • Pick up the signal on pads • What about ionizing particle? • Mesh with a reverse bias drifts ionization away from multiplication area • Sensitive to UV and blind to traversing ionizing particles Thomas K Hemmick

Summary • Di-electron measurements contain a rich physics content: • Vector Mesons. • J/Psi == deconfinement • r, w, f == Chrial Symmetry Restoration • Low Mass Continuum (Chiral Symmetry) • Intermediate Mass Continuum (Heavy Quark Dynamics). • Access to low mass V.M. and continuum limited by “photonic” background (Dalitz, conversion), not events. • HBD detector concept (windowless Cherenkov inside magnet) allows significant background reduction and gives access to all this physics. • Next week Jason Kamin will discuss the all the fascinating details of the real PHENIX HBD. Thomas K Hemmick

Physics of the PHENIX HBD